Seminars 2016

Title: Magneto-optical oxide thin films for integrated nonreciprocal photonic and magnetoplasmonic device applications
Speaker: Professor Lei Bi
Date:22 December 2016
Time:11am - 12pm 
Venue:Hilbert Space (SPMS-PAP-02-01)
Host:Professor Xiong Qihua 
Abstract: In this talk, I will present our recent progress on integration of magneto-optical oxide thin films for on-chip nonreciprocal photonic and magnetoplasmonic device applications. I will first introduce the growth of phase pure cerium doped yttrium iron garnet thin films on silicon substrates with superior magneto-optical properties compared to their single crystal counterparts. Using these magnetooptical thin films, we demonstrate two types of on-chip monolithically integrated optical isolators based on nonreciprocal silicon racetrack resonantors and multimode interferometers. By integrating low loss magneto-optical oxide thin films in plasmonic structures, we also fabricated magnetoplasmonic devices with high index sensitivity, high device figure of merit (FoM) and good chemical stability for ultrasensitive chemical/biomedical sensor applications. Our study demonstrates that magneto-optical oxide thin films featuring low optical loss and strong magneto-optical properties are attractive material candidates for nanophotonic device applications.

 

Title: A theoretician's approach to nematic liquid crystals, their modelling and applications
Speaker: Professor Apala Majumdar
Date:15 December 2016
Time:4pm - 5pm 
Venue:Hilbert Space (PAP-02-02)
Host:Lu Bing Sui 
Abstract: Nematic liquid crystals are classical examples of mesophases or partially ordered complex fluids that combine the fluidity of liquids with the orientational order of crystalline solids. In this talk, we review the commonly used macroscopic and molecular-level theories for nematic liquid crystals. We present a case study focussed on a bistable or multistable liquid crystal device, which can be modelled using molecular-level and conventional macroscopic approaches. We compare and contrast the different approaches and illustrate how mathematical modelling can make both qualitative, and in some cases quantitative predictions about experimentally observed states, their stability and dependence on geometry, boundary conditions, temperature and material properties.

 

Title: Tunable metamaterials and ultrafast (femtosecondspicoseconds) nonlinear optics with liquid crystals
Speaker: Professor Iam Choon Khoo
Date:15 December 2016
Time:2pm - 3pm 
Venue:Hilbert Space (SPMS-PAP-02-01)
Host:Associate Professor Cesare Soci
Abstract: Liquid crystals, particularly nematics and cholesterics (including blue phase) possess extraordinarily large electro-optical and nonlinear optical responses with response times spanning the conventional milliseconds speed to the ultrafast (subpicoseconds) time scale. Together with their unique physical attributes such as fluidity, tunable birefringence applicable throughout the visible-far infrared regime, they have become a favorite material for incorporation into various photonic structures including photonic crystal inverse opals, nanostructured plasmonic and metamaterials, channel waveguide and micro-ring resonator, etc. [1-3] In this talk, I will first present a critical review of some of the exemplary work reported previously in these contexts, and then delve into several recent studies of ultrafast laser beam modulation and holographic image processing using the nonlinear optical field induced refractive index changes in cholesteric and Blue-Phase liquid crystals (CLC, BPLC) [4-6]. In particular, we have conducted various coherent image processing operations such as edge enhancement, wavelength and contrast conversion, etc. with BPLC doped with appropriate dye to enhance their nonlinear optical response in the sub-milliseconds time scale. Using the ultrafast (sub-picoseconds) molecular electronic nonlinearity, we have demonstrated direct femtosecond laser pulse conversion as well as stretching and recompression operation of femto- and picoseconds laser pulses. New understanding of the underlying fundamental mechanisms as well as possible applications of these observations will be discussed.

References: [1] Physics Reports 471, 221-267 (2009); [2] Progress in Quantum Electronics 38, 77–117 (2014); [3] Optics Letters 39, 5435-5438 (2014); [4] Optics Letters 38, 5040-5042 (2013); [5] Optics Express 24, 10458-10465 (2016); [6] Sci. Rep. 6, 36148 (2016).

 

Title: Composite boson many-body physics
Speaker: Dr Shiue-Yuan Shiau
Date:9 December 2016
Time:3pm – 4pm
Venue:Hilbert Space (SPMS-PAP-02-02)
Host:Dr Ho Shen Yong
Abstract: I will present my recent works using composite boson many-body formalism, with topics related to trions and biexcitons, composite boson scattering lengths of semiconductor excitons and cold-atom dimers, as well as composite boson coherent states.

 

Title: Listening to Einstein’s Universe: How We Detected Gravitational Waves
Speaker: Professor Martin Hendry
Date:5 December 2016
Time:11.30am - 12.30pm 
Venue:Hilbert Space (SPMS-PAP-02-02)
Host:Tan Hai Siong
Abstract: On September 14th 2015 two giant laser interferometers known as LIGO, the most sensitive scientific instruments ever built, detected gravitational waves from the merger of a pair of massive black holes more than a billion light years from the Earth. LIGO estimated that the peak gravitational wave power radiated during the final moments of this merger was nearly fifty times greater than the combined light power from all the stars and galaxies in the observable Universe. Join Professor Martin Hendry from the University of Glasgow as he recounts the inside story of this remarkable discovery - hailed by many as the scientific breakthrough of the century. Learn about the amazing technology behind the LIGO detectors, which can measure the signatures of spacetime ripples less than a million millionth the width of a human hair, and explore the exciting future that lies ahead for gravitational-wave astronomy as we open an entirely new window on the Universe.

 

Title: The role of Quantum Measurement in Stochastic Thermodynamics
Speaker: Professor Alexia Auffeves
Date:1 December 2016
Time:11am - 12pm 
Venue:Hilbert Space (PAP-02-02)
Host:Assistant Professor Gu Mile, Dr Felix Binder
Abstract: I will present a new formalism to investigate stochastic thermodynamics in the quantum regime, where stochasticity and irreversibility primarily come from quantum measurement. In the absence of any bath, a purely quantum component to heat exchange, that corresponds to energy fluctuations caused by measurement backaction.

Energetic and entropic signatures of measurement induced irreversibility are then investigated for canonical experiments of quantum optics, and the energetic cost of counter-acting decoherence is characterized on a simple state-stabilizing protocol.

I will finally open on a new kind of genuinely quantum engines, where work is solely extracted from quantum measurement.

 

Title: 2D Materials - A New Platform for Strong Light-Matter Interactions
Speaker: Dr Ajit Srivastava
Date:23 November 2016
Time:12.30pm - 1.30pm
Venue:Office of Research and Graduate Studies Conference Room (SPMS-CBC-02-01)
Host:Assistant Professor Gao Weibo
Abstract: A New Platform for Strong Light-Matter Interactions A recent addition to low-dimensional materials are monolayer transition metal dichalcogenides (TMDs), such as WSe2, with an atomically thin, honeycomb lattice and optical band gaps. In addition to spin, charge carriers in TMDs exhibit a “valley” degree of freedom, which can be optically addressed using circularly polarized light, opening up exciting possibilities for “valleytronics". Another curious aspect of TMDs lies in the non-trivial geometry of their band structure which gives rise to equal but opposite Berry curvature, an effective magnetic field in the momentum space. Owing to unusually strong Coulomb interactions in truly 2D limit, optical spectra of monolayer TMDs is dominated by tightly bound excitons that are expected to strongly couple to light and form stable polaritons - half light, half matter excitations. In this talk, I will begin by presenting our recent results on valley Zeeman effect, where in analogy to spins, valleys shift in energy with magnetic field. Next, I will discuss our theoretical results on how the non-trivial geometry of Bloch bands modifies the excitonic fine structure of TMDs resulting in an orbital Zeeman effect in reciprocal space and a Lamb-like shift of levels. Finally, I will present our recent results on the observation of microcavity polaritons confirming the strong light-matter interactions in these materials. The presence of valley degree of freedom, nontrivial geometry of bands, and the possibility of introducing non-linearities in form of quantum emitters makes polaritons in TMDs particularly appealing for studying correlated many-body physics and topological states of matter.

[1]. A. Srivastava et al., Nature Phys. 11, 141-147 (2015).
[2]. A. Srivastava et al., Nature Nanotech. 10, 491-496 (2015).
[3]. A. Srivastava and A. Imamoglu, Phys. Rev. Lett. 115, 166803 (2015).

 

Title: Limitations and performance of Kerr nonlinear plasmonic nanostructures
Speaker: Dr Martijn de Sterke
Date:22 November 2016
Time:3pm - 4pm 
Venue:MAS Executive Classroom 1 (SPMS-MAS-03-06)
Host:Professor Xiong Qihua
Abstract: Photonic devices can be faster and more energy efficient than electronics, but this requires strong nonlinear light-matter interactions. By confining the light well beyond the diffraction limit, plasmonic (i.e., metal-based) nanostructures greatly enhance light intensities and hence the nonlinear interactions. The design of high performance nonlinear plasmonic devices is challenging because of optical losses and optical damage. We investigate the limitations of Kerr nonlinear plasmonic waveguides and find the counterintuitive result that the ultimate nonlinear performance depends more strongly on the linear than on the nonlinear properties of the materials, and that the interactions can be enhanced by strong confinement combined with broadband slow-light effects. We can thus identify the limitations and merits compared to conventional all-dielectric structures and point to alternative approaches for performance improvement.

 

Title: Fluctuation forces in confined systems
Speaker: Dr Lu Bing Sui
Date:17 November 2016
Time:11am - 12pm 
Venue:MAS Executive Classroom 1 (SPMS-MAS-03-06)
Host:-
Abstract: Fluctuation forces are forces which arise from thermal or quantum fluctuations of waves and/or matter, and are expected to be significant in the nanoscale world, especially in systems where the effect of the confining surfaces cannot be ignored. In this seminar we shall look at three such examples of confined systems. We start by looking at the case of fluid membranes in solution, where membranes interact via forces induced by membrane fluctuations as well as other types of interaction (e.g., electrostatic, van der Waals, and/or hydration). We examine in particular a variational field theory approach that allows one to self-consistently determine the effects of the fluctuations of interaction forces on the overall fluctuation pressure between pairs of membranes, and their fluctuation amplitudes. Following this, we consider Casimir/van der Waals-type interactions, which are forces induced between bodies by the correlations of charge fluctuations. In particular we consider the interaction between two hot, neutral planar bodies separated by a vacuum, and look at how the interaction behavior can be "tuned" by a temperature difference between the bodies, and/or applying an electric field to one of the bodies. For our third example, we consider a pair of two multi-layered, birefringent slabs (which can be clay) immersed in and interacting across a solvent medium. The optical anisotropy of the slabs means that the force between them induced by fluctuations of the electromagnetic vacuum will also depend on the orientations of the optic axes. In particular we study the character of the van der Waals torque that arises between such birefringent media, looking at how it varies with the relative orientations of the optic axes in the layers and the thicknesses of such layers.

 

Title: Tailoring the properties of NV color centers in diamond
Speaker: Dr Jean-Francois Roch
Date:11 November 2016
Time:4pm – 5pm
Venue:Hilbert Space (PAP-02-02)
Host:Associate Professor David Wilkowski / Assistant Professor Gao Weibo
Abstract: The Nitrogen-Vacancy (NV) color center is a point defect in diamond which behaves as an artificial atom hosted in a solid-state matrix. Due to its electron spin properties which can be read-out and manipulated as an elementary quantum system even at room temperature, the NV center has found a wide panel of applications as a qubit for quantum information and as a magnetic field sensor. However these applications require to control the properties of the NV centers and their localization. I will describe methods aimed to taylor the properties of NV centers by combining techniques for the implantation of nitrogen atoms then converted into NV centers and for the plasma-assisted (CVD) synthesis of diamond.

 

Title: Smeared-out ions at a charged surface
Speaker: Dr Derek Frydel
Date:10 November 2016
Time:4pm – 5pm
Venue:Hilbert Space (PAP-02-02)
Host:Dr Lu Bing Sui
Abstract: The focus of the talk are "soft" counterions, generated when a charge is smeared-out over a finite volume. It turns out that such ions overscreen a charged surface by overcompensation of a surface charge. This phenomenon is known as charge inversion and is connected to the related phenomenon of attraction between the same charged surfaces. In standard paradigm involving point-ions, correlations play a central role in the charge inversion mechanism. For smeared-out ions, on the other hand, correlations have no importance and charge inversion is captured by the mean-field. In addition, charge inversion of smeared-out ions may transform into charge layering when a charge density profile becomes oscillating. The point when this happens is completely determined by a bulk electrolyte.

 

Title: Singapore Quantum Materials Series
Speaker: Dr. Derek Ho and Dr. Ivan Verzhbitskiy
Date:4 November 2016
Time:11am - 12.30pm 
Venue:Hilbert Space (PAP-02-02)
Host:Assistant Professor Justin Song
Abstract: 

Seminar title: Effective Medium Theory for Coulomb Drag in Graphene
Speaker: Dr. Derek Ho (Shaffique Adam group, NUS) 

When two semiconductor sheets are held close together and a current driven through one of them, a small current gets pulled along in the other due to the Coulomb force coupling the charges in the two layers. This effect is known as the Coulomb drag and is a direct transport manifestation of electron-electron interactions. Such experiments now have a history of almost three decades. The last four years in particular have seen a sustained interest in Coulomb drag between graphene sheets both theoretically and experimentally for both fundamental science and applications-oriented reasons. Here, I report on our recent theoretical work on graphene Coulomb drag. It is known that in a single graphene sheet at low density, charge inhomogeneities play an important role in carrier transport. The effects of such inhomogeneity on measurements have previously been successfully understood using effective medium theory (EMT), a framework for predicting transport in single spatially inhomogeneous layers of material. Up till now however, there has been a conspicuous lack of an EMT in the literature for multi-layer transport problems such as Coulomb drag. We remedy this and extend the EMT for the first time to the double layer drag problem. We show that this new EMT resolves a glaring contradiction between existing (homogeneous) theory and experiments and also makes new predictions for future experiments at higher temperatures.

Seminar title: Superconductivity in Li-intercalated MoS2
Speaker: Dr. Ivan Verzhbitskiy (Goki Eda group, NUS) 

Interest in the superconducting properties of layered transition metal dichalcogenides has been recently renewed by several key discoveries such as electric-field-induced superconductivity and the role of spin-valley locking in MoS2 . Superconductivity in intercalation compounds of TMDs has been studied since the ‘60s but its connection to the recently observed electric-field-driven superconductivity remains elusive. We present a thorough experimental study of superconducting transition in Lix MoS2 grown by chemical vapor transport. We observe multiple transition temperatures ranging from 3 to 7 K. Interestingly, resistivity was found to show insulator-like temperature dependence above the transition temperature in contrast to typical superconducting materials including electrostatically doped MoS2 . This change in the sign of temperature coefficient near superconducting transition and its magnetic-field dependence are consistent with a superconductor-to-insulator transition driven by induced localization of Cooper pairs.

 

Title: Holography Principle: New Paradigm of Theoretical Physics
Speaker: Professor Bum-Hoon Lee
Date:3 November 2016
Time:3pm - 4pm
Venue:Hilbert Space (PAP-02-02)
Host:Associate Professor Pinaki Sengupta
Abstract: Physical phenomena of strongly interacting systems, which do not allow the traditional perturbative approach, have been a big challenge. Such phenomena occur in almost all branches of physics, such as nuclear and particle physics and strongly correlated phenomena in the condensed matter systems, etc. Holography principle, discovered from the recent progress in the string theory, may provide totally new paradigm for understanding many such systems. This principle transforms the question on the quantum physical system into that on the classical geometry by relating the strongly interacting quantum field theory with the corresponding effective classical gravity theory in one-dimension higher space-time. After introducing this principle, we present some explicit examples of application: Nuclear physics, explaining the spectra, phase structures, and the density effects among others. Application to the strongly correlated phenomena in condensed matter will also be briefly mentioned.

 

Title: Light-matter interaction in micro/nanostructures: hybrid perovskite polaritons and giant luminescent downshifting meta-structure.
Speaker: Dr Hai Son NGUYEN
Date:1 November 2016
Time:11am - 12pm 
Venue:Hilbert Space (PAP-02-02)
Host:Dr Carole Diederichs
Abstract: 

 

Title: Mass-energy-loss of an isolated gravitating system due to energy carried away by gravitational waves with a cosmological constant
Speaker: Mr Saw Vee-Liem
Date:28 October 2016
Time:3pm – 4pm
Venue:Hilbert Space (PAP-02-02)
Host:Dr Ho Sheng Yong 
Abstract: The theoretical basis for the energy carried away by gravitational waves that an isolated gravitating system emits was first formulated by Hermann Bondi during the 1960s. Recent findings from looking at distant supernovae revealed that the rate of expansion of our universe is accelerating (Nobel Prize in Physics, 2011), which may be well-explained by sticking in a positive cosmological constant into the Einstein field equations for general relativity. By solving the Newman-Penrose equations (which are equivalent to the Einstein field equations), we generalise this notion of Bondienergy and thereby provide a firm theoretical description of how an isolated gravitating system loses energy as it radiates gravitational waves, in a universe that expands at an accelerated rate.

[1, 2].This is in line with the observational front of LIGO's first announcement in February 2016 that gravitational waves from the merger of a binary black hole system have been detected. [1] V.-L. Saw, "Mass-loss of an isolated gravitating system due to energy carried away by gravitational waves with a cosmological constant", Physical Review D, accepted on 11th of October, 2016, https://arxiv.org/abs/1605.05151 [2] V.-L. Saw, "Behaviour of asymptotically electro-Λ spacetimes", http://arxiv.org/abs/1608.06886

 

Title: Tunable negative magnetoresistance in hydrogenated graphene
Speaker: Dr Jian-Hao Chen
Date:28 October 2016
Time:11am – 12pm
Venue:MAS Executive Classroom 2 (SPMS-MAS-03-07)
Host:Dr Carole Diederichs
Abstract: The problem of unconventional magnetism in materials without d and f electrons has attracted continuous attention. In particular, a lot of efforts have been devoted to understanding the origin and effects of magnetic moments induced in graphene with structure defects such as missing carbon atoms, absorption of light atoms such as hydrogen or fluorine. We have measured the magnetoresistance (MR) of graphene at low temperature with in-situ hydrogenation in ultra-high vacuum environment. The evolution of weak localization and weak anti-localization provide strong evidence that hydrogenation of graphene has introduced local magnetic moment in the electron system, and have substantially increase the spin-orbit interaction of the sample. Large and nonsaturating negative MR was also found in hydrogenated graphene which could be tuned by carrier density and sample temperature.

 

Title: Spatial squeezing of Terahertz light via graphene acoustic plasmons
Speaker: Professor Marco Polini
Date:26 October 2016
Time:11am – 12pm
Venue:Hilbert Space (PAP-02-02)
Host:Assistant Professor Justin Song
Abstract: Terahertz (THz) fields are widely used for sensing, communication, and quality control. In this talk I will discuss how THz light can be efficiently confined well below the diffraction limit through the excitation of graphene plasmons in van der Waals heterostructures comprising graphene, hexagonal boron nitride, and metal gates. In these vertical stacks, graphene plasmons exhibit an acoustic (linear) dispersion, implying greatly-improved field confinement capabilities. Electrical detection of graphene plasmons in these systems and accurate comparisons with microscopic theory reveal linear confinement factors on the order of 66, lifetimes on the order of 500 fs, and long-range scattering as main mechanism of plasmon losses.

Based on work one in collaboration with P. Alonso-González, A.Y. Nikitin, Y. Gao, A. Woessner, M.B. Lundeberg, A. Principi, N. Forcellini, W. Yan, S. Vélez, A.J. Huber, K. Watanabe, T. Taniguchi, F. Casanova, L.E. Hueso, J. Hone, F.H.L. Koppens, and R. Hillenbrand and largely sponsored by the Graphene Flagship.

 

Title: Fermiology in Solid State Sciences
Speaker: Professor Mukunda P Das
Date:25 October 2016
Time:4pm – 5pm
Venue:Hilbert Space (PAP-02-02)
Host:Associate Professor Pinaki Sengupta 
Abstract: The Fermi surface is an abstract object in the reciprocal space for a material lattice, enclosing the set of all electronic band states that are filled according to the Pauli principle. Its topology is dictated by the underlying material structure, and its volume is the carrier density in the material. The Fermi surface is central to predictions of thermal, electrical, magnetic, optical and superconducting properties in metallic systems. In this talk I shall discuss mainly complex correlated systems emphasising several key facts about Fermi surfaces, where a proper theoretical understanding is still lacking. We address some critical difficulties whether the Fermi surface is a ground state property and concerning its stability in strongly correlated systems.

 

Title: KERR EFFECT IN HYBRID PLASMONIC WAVEGUIDES
Speaker: Dr. Stefano Palomba
Date:24 October 2016
Time:2.30pm - 3.30pm 
Venue:Hilbert Space (PAP-02-02)
Host:Professor Xiong Qihua
Abstract: Optical nonlinearities have already enabled a range of capabilities, such as signal switching, de-multiplexing, wavelength conversion, light amplification and supercontinuum generation. Integrating these abilities in an on-chip photonic platform will lead to benefits in terms of cost, footprint, energy consumption and performance. These will in turn help meeting current demands of larger bandwidth, enabling all-optical signal processing vastly overcoming the electronic bottlenecks. The current leading platform for integrated nonlinear optics is silicon-on-insulator (SOI). However, silicon suffers from a fundamental material limit imposed by strong two-photon absorption (TPA) and the diffraction limit. Among complementary and alternative platforms, the hybrid plasmonic platform, has gained attention as it generates strongly compressed fields with moderate losses. The simplest structure of this kind is represented by a nonlinear dielectric material, sandwiched between a metallic layer and a Si nanowire, for which the first experimental evidence of third order nonlinearities we recently reported. Here the plasmonic and photonic modes are combined to generate a hybridized fundamental mode which has been shown to possess a high field confinement, associated with the plasmonic mode, and reduced propagation losses, associated with the photonic mode. The high confinement and modest loss is required for producing strong nonlinearities, which may be further increased through the incorporation of highly nonlinear polymers. Although the geometry with the largest theoretically reported nonlinear parameter is of this type, the origin for such a high nonlinearity remains somewhat unclear. In order to gain more insight we have analyzed the Kerr nonlinear parameter of HPWGs, which quantifies the strength of the nonlinear effect, and its dependence on the energy velocity, effective area and the average nonlinear refractive index, with and without the presence of the nonlinear polymer. In addition, in order to efficiently evaluate their Kerr nonlinear performance we propose a simple figure of merit (FOM) for Kerr nonlinear effects in plasmonic waveguides, including degenerate four wave mixing (DFWM). The effectiveness of the FOM is verified with an all-plasmonic waveguide and a hybrid-plasmonic waveguide configuration. Rigorous results show that the length for optimal DFMW efficiency is equal to the attenuation length, and that the FOM provides the obtainable upper limits of the DFWM efficiency and the nonlinear phase shift. These results provide fundamental theory and useful guidance in exploring plasmonic waveguides for nonlinear optical applications.

Acknowledgements. This work has been funded through the Australian Research Council (ARC) Discovery Project number DP150100779.

 

Title: Emerging photovoltaic materials: opportunities and challenges
Speaker: Associate Professor Silvija Gradečak
Date:22 August 2016
Time:11am – 12pm
Venue:Hilbert Space (PAP-02-02)
Host:Professor Xiong Qihua
Abstract: In contrast to the gradual development of mature wafer-scale silicon photovoltaic (PV) technologies, more recent development of nanostructured and mixed halide perovskite PV devices has experienced an explosive growth. These materials are solution-processable and as such can be deposited on flexible substrates, in a form of semi-transparent devices, and potentially at low cost. In this talk, we will review our present understanding of the basic mechanisms that govern operation of these devices by focusing on current limitations and future opportunities. Colloidal quantum dots (QDs) have been studied as promising candidates for single-junction and tandem solar cell applications due to their direct and tunable band gap in the visible and near-IR spectral regions. However, a mismatch between the optical absorption length and the carrier collection length prevents these devices from achieving optimal photocurrent generation. To enhance charge collection we combined (1) hydrothermally grown ZnO nanowire arrays to form an ordered bulk heterojunction and (2) a band alignment engineered PbS QD film that utilizes inorganic and organic ligands to generate cascaded energy level offsets. Champion devices achieve PCE up to 9.6% and short-circuit current density greater than 30 mA/cm2 . Furthermore, we have developed a simple method to grow high-quality ZnO nanowires on graphene electrodes resulting in flexible semi-transparent devices. In another example, the impact of microscale film inhomogeneities on performance of organic-inorganic perovskite solar cells (PSCs) remains poorly understood, despite the recent astronomical success of this class of PV devices. We show that localized regions with increased luminescence in CH3NH3PbI3 perovskite films correspond to iodide-enriched regions. These observations constitute direct evidence that nanoscale stoichiometric variations produce corresponding inhomogeneities in film luminescence intensity. Moreover, we observe the emergence of high-energy transitions attributed to beam induced iodide segregation, which may mirror the effects of ion migration during PSC operation. Our results demonstrate that such ion segregation can fundamentally change the local optical and microstructural properties of organic-inorganic perovskite films in the course of normal device operation and therefore address the observed complex behavior in PSC devices.

 

Title: Van der Waals interactions — their role in molecular recognition and self-assembly
Speaker: Dr Lu Bing-Sui
Date:19 August 2016
Time:11am – 12pm
Venue:Hilbert Space (PAP-02-02)
Host:-
Abstract: Van der Waals (vdW) interactions emerge from the correlations of fluctuating dipoles in polarizable media. They are expected to be significant in the nano- to micron-scale world. In this Seminar we examine two examples of practical relevance in which vdW interactions can play an important role. We first look at the case of short double-stranded (ds) DNA molecules in salt solution. We show how vdW interactions can result in a stronger attraction between a pair of such molecules, if the chemical sequences of the two molecules are identical and more heterogeneous, and/or the two molecules are oriented in parallel. For our second example, we consider two dielectrically anisotropic plane-layered media made of the same material interacting across a dielectrically isotropic solvent, the optic axes of the layers being perpendicular to the plane of the layers. The optic axes of the oppositely facing anisotropic layers of the two interacting slabs generally have an angular mismatch, and within each multilayered slab the optic axes may either be the same or undergo constant angular increments across the anisotropic layers. We show that a vdW torque is induced in addition to a vdW force. For the case of weak, uniaxial anisotropy, we examine how the behaviours of the van der Waals torque and force can be “tuned” by adjusting the layer thicknesses, the relative angular increment within each slab, and the angular mismatch between the slabs.

 

Title: Introduction to the C++ libSimEngine library for rapid development of physics simulation software, and application in a Grand-canonical Monte-Carlo simulation of DNA hexagonal bundles
Speaker: Dr Nguyen The Toan
Date:19 August 2016
Time:10am – 11am
Venue:Hilbert Space (PAP-02-02)
Host:Associate Professor Phan Anh Tuan
Abstract: The SimEngine library, a C++ library for rapid development of Molecular dynamics and Monte-Carlo simulations for application in material sciences, soft matter and biophysics developed in our group is introduced. The library employs multi-level parallelization for optimum speed: SSE3/AVX/AVX2/AVX512 vectorized codes on a single CPU core, OpenMP multi-core CPU parallelization, OpenCL GPGPU computing and OpenMPI multi-machine parallelization. A specific application of a Grand-canonical Monte-Carlo simulation of DNA hexagonal bundles is presented. The problem of DNA-DNA interaction mediated by divalent counterions in aqueous solution containing two salts is studied. Experimentally, it is known that divalent counterions have strong effect on the DNA condensation phenomenon, but the condensation varies qualitatively with different system, different coions. The variations among different divalent salts might be due to the ion-specific hydration effect. We try to understand this variation using a very simple parameter, the size of the divalent counterions. We investigate how divalent counterions with different sizes can lead to varying qualitative behaviors of DNA condensation in restricted environments.

 

Title: Disentangling Strongly Correlated Quantum Systems
Speaker: Professor Ulrich Schollwöck
Date:16 August 2016
Time:2.45pm – 3.45pm
Venue:Executive Classroom 1, MAS-03-06 School of Physical and Mathematical Sciences
Host:School of Physical and Mathematical Sciences
Abstract: Strongly correlated quantum systems, where more traditional methods of quantum many-body physics fail, have attracted enormous attention over the last decades but still provide formidable problems for our understanding: high-Tc superconductors, frustrated quantum magnets, transition metal oxide and rear earth materials, ultracold atomic gases in optical lattices. Key numerical advances have been made using so-called tensor network methods, the best known of which is the density matrix renormalization group (DMRG). After an introduction into the methodology, I want to present selected results from areas which in my view present particularly interesting challenges also in the future: non-equilibrium dynamics of correlated systems (here: ultracold atoms in lattices) and material properties of three-dimensional transition metal oxides.

 

Title: Ultrafast Growth of Graphene Single Crystal
Speaker: Professor Yu Dapeng
Date:8 August 2016
Time:3pm - 4pm 
Venue:Hilbert Space (PAP-02-02)
Host:Associate Professor Xiong Qihua
Abstract: Graphene single crystal has become as a promising material for next generation electronics and optoelectronics. Unfortunately we can’t grow 12-inch graphene single-crystal wafer as we can do in silicon industry. One of the main hamper is that we nowadays grow graphene single crystal very slow. In this talk I will introduce why we need to grow graphene faster and how fast we can achieve in our recently developed new technology [1]. Xiaozhi Xu, Zhihong Zhang, Dapeng Yu, Enge Wang, Feng Ding* , Hailin Peng* , Kaihui Liu* , et al. “Ultrafast Growth of Single-crystal Graphene Assisted by a Continuous Oxygen Supply”, Nature Nanotechnology 2016, in press.

 

Title: Biophysical Analysis of Molecular Interactions with switchSENSE
Speaker: Dr Ulrich Rant
Date:3 August 2016
Time:2pm – 3pm
Venue:MAS Executive Classroom 2 (MAS-03-07)
Host:Professor Michel-Beyerle
Abstract: switchSENSE® is an automated biosensor chip technology that employs electrically actuated DNA nanolevers for the real-time measurement of binding kinetics (kON, kOFF) and affinities (KD). Interactions between proteins, DNA/RNA, and small molecules can be detected with femto-molar sensitivity. At the same time, protein diameters (DH) are analyzed with Angstrom accuracy and conformational changes as well as melting transitions (TM) can be measured using minimal amounts of sample. The principles and applicability of three complementary measurement modalities provided by switchSENSE will be introduced in this talk: Fluorescence Proximity Sensing, Molecular Ruler Measurements, and Switching Dynamics Measurements. In addition to standard workflows we’ll discuss unique possibilities for the functionalization of the sensor surface, i.e. the electrical adjustment of ligand densities and the precise assembly of different ligands on bifunctional nanolevers.

Application examples from drug development, quality control, and fundamental research will be discussed, including:
- Small molecule binding and small molecule induced conformational changes in proteins
- Analysis of complex binders: high-affinity and bispecific antibody formats
- RNA/DNA binding proteins
- Enzymatic activity of polymerases and nucleases (CRISPR/Cas9)

 

Title: Electronic devices of two-dimensional materials: from atomic to molecular
Speaker: Dr Xinran Wang
Date:26 July 2016
Time:11am – 12pm
Venue:Hilbert Space (PAP-02-02)
Host:Associate Professor Xiong Qihua
Abstract: Two-dimensional (2D) layered materials represent a promising class of materials for electronic and photonic devices, benefiting from their unique properties such as high mobility and ultrathin body. In this talk, I will present our recent works on using 2D materials for electronic device applications. The first part focuses on inorganic transistion metal dichalcogenides (TMDs), which have a sizable bandgap of 1-2eV and are suitable for logic applications. However, current TMD transistors suffer from many extrinsic scatterings, showing much lower mobility than theoretical expectations. Using MoS2 and WS2 as examples, we show that charged impurities, traps and defects are main limiting factors in current TMD materials. We further carry out systematic interface engineering to significantly improve the carrier mobility. We achieve room-temperature mobility of 150cm2/Vs and 83cm2/Vs for monolayer MoS2 and WS2 respectively, which are among the highest values reported so far. The second part focuses on molecular materials. I will show that highly ordered few-layer molecular crystals can be epitaxially grown on graphene, BN and MoS2. This emerging class of 2D materials is not only capable of making high-performance organic field-effect transistors but also as a powerful platform to study intrinsic structure-property relationship in organic electronics. Precise control of epitaxy offers new possibilities in achieving well-defined heterostructures based on organic materials.

 

Title: Quantum Plasmonics and Hot Carrier Induced Processes
Speaker: Prof Peter Nordlander
Date:25 July 2016
Time:5pm - 6pm 
Venue:MAS Executive Classroom 2 (SPMS-MAS-03-07)
Host:Professor Nikolay Zheludev
Abstract: Plasmon resonances with their dramatically enhanced cross sections for light harvesting have found numerous applications in a variety of applications such as single particle spectroscopies, chemical and biosensing, subwavelength waveguiding and optical devices. Recently it has been demonstrated that quantum mechanical effects can have a pronounced influence on the physical properties of plasmons. Examples of such effects is the charge transfer plasmon enabled by conductive coupling (tunneling) between two nearby nanoparticles and nonlocal screening of the plasmonic response of small nanoparticles. One relatively recent discovery is that plasmons can serve as efficient generators of hot electrons and holes that can be harvested in applications. The physical mechanism for plasmon-induced hot carrier generation is plasmon decay. Plasmons can decay either radiatively or non-radiatively with a branching ratio that can be controlled by tuning the radiance of the plasmon mode. Non-radiative plasmon decay is a quantum mechanical process in which one plasmon quantum is transferred to the conduction electrons of the nanostructure by excitation of an electron below the Fermi level of the metal into a state above the Fermi level but below the vacuum level. In particular I will discuss external control of charge transfer plasmons for active plasmonic devices, molecular plasmonics, hot carrier generation, decay and fluorescence, and hot carrier induced processes and applications such as photodetection, photocatalysis, and phase changing of nearby media.

 

Title: Sustainable Plasmonics and Plasmonics for Sustainability
Speaker: Professor Naomi J. Halas
Date:25 July 2016
Time:4pm - 5pm 
Venue:MAS Executive Classroom 2 (SPMS-MAS-03-07)
Host:Professor Nikolay Zheludev
Abstract: The intense research activity of the past two decades focused on the collective electronic oscillations in highelectron-density media, known as surface plasmons, has led to multiple breakthroughs in fields ranging from chemical sensing and catalysis, to active optical devices, solar light harvesting, even nanomedicine. For many of these applications, the original focus on noble metals may ultimately limit their transition from the research laboratory to widely used commercial technologies. We will describe several research directions that, as they point towards more sustainable materials, open up new research opportunities. Aluminum, the most abundant metal on earth, opens the door to new colorimetric sensing applications and opportunities for active devices. In applications that directly address sustainability, we will discuss how plasmonic nanoparticles can be used for solar distillation of liquid mixtures, providing insight into the mechanism of nanoparticle-based distillation that in certain cases allows the distillation fraction to deviate dramatically from conventional thermal distillation processes

 

Title: Materials Challenges for Next-Generation, High-Density Magnetic Recording: Media and Read Heads
Speaker: Dr Kazuhiro Hono
Date:4  July 2016
Time:10am - 11.30am
Venue:Hilbert Space (PAP-02-02)
Host:Associate Professor S.N. Piramanayagam 
Abstract: The hard disk drive industry is making continuous efforts to increase the areal density of magnetic recording. To realize an areal density of higher than 2 Tbit/in2 in the future, both media and readers need technical breakthroughs. Since the bit size will be in the range of 20 nm, the magnetic grains in the recording media must be reduced to less than 6 nm, requiring the use of ferromagnetic materials with high magnetocrystalline anisotropy such as L10 FePt. The shield-to-shield spacing of read sensors must also be smaller than 20 nm with low device resistance, which is very difficult to achieve using MgO based tunneling magnetoresistance devices. In this talk, we will address the materials challenges to the realization of an ideal media nanostructure using L10 FePt for heat-assisted magnetic recording (HAMR) media and narrow readers for > 2 Tbit/in2 areal density. Recently significant progress has been made in current-perpendicular-to-plane giant magnetoresistive (CPP-GMR) devices using highly spin-polarized Heusler alloy ferromagnetic layers and new spacer materials. The very high magnetoresistance ratios achieved in CPP-GMR are encouraging for future read head applications of CPP-GMR, or its laterally extended version, lateral spin valves. The devices with high magnetoresistive output at low RA may open new applications in addition to disk read heads

 

Title: Enhanced Radiative Interactions at Meta Surfaces
Speaker: Professor Girish S Agarwal
Date:12 May 2016
Time:3pm - 4pm 
Venue:MAS Executive Classroom 2 (MAS-03-07)
Host:Assistant Professor Ranjan Singh
Abstract: We report several radiative effects involving exchange of single photons at meta surfaces. An important process involving such an exchange is the dipole dipole [dd] interaction. A large variety of applications like quantum entanglement; two photon absorption, Forster energy transfer require large dd interaction which can occur only for distances smaller than a wavelength. The meta surfaces are especially useful for this purpose. Perfect negative materials can yield perfect dd. This can be achieved by using hyperbolic materials. We present results on giant dd interaction and spin Hall effect of light. We discuss the possibility of strong entanglement between artificial atoms on meta surfaces. Meta surfaces make certain physical processes allowed which otherwise are forbidden. We also show how quantum interferences at meta surfaces can induce intra-atomic interaction and coherent population trapping with no laser fields.

 

Title: The Dawn of Time-Domain Quantum Optics
Speaker: Dr Denis V. Seletskiy
Date:29 April 2016
Time:11am - 12pm 
Venue:MAS Executive Classroom 1 (MAS-03-06)
Host:Associate Professor Xiong Qihua
Abstract: The ability to electro-optically sample time evolution of a field has proven to be invaluable for probing ultrafast dynamics of low-energy degrees of freedom in condensed matter. While some information on a quantum nature of many-body interactions can be obtained from classical probes, it is the nonclassical detection schemes that can provide direct access to nontrivial correlations of fundamental degrees of freedom in condensed matter. Thus, to develop a unified subcycle approach to quantum physics, the building blocks of field-resolved quantum sampling need to be addressed head on. I begin with a brief introduction to ultrafast fiber lasers and motivate their use for advanced timedomain experiments. In the middle part and following a quick reminder of fundamental aspects of frequency-domain view of quantum optics, I motivate a new regime of time-domain quantum optics, where fluctuations in the field can be probed on subcycle timescales. In particular, I will discuss first direct detection of vacuum fluctuations of an ultrabroadband electric field [1,2]. I will conclude by outlining advanced concepts of time-domain physics which we are currently working on at the University of Konstanz.
[1] C. Riek, D. V. Seletskiy, A. S. Moskalenko, J. F. Schmidt, P. Krauspe, S. Eckart, S. Eggert, G. Burkard, and A. Leitenstorfer, Science 350, 420 (2015).
[2] A. S. Moskalenko, C. Riek, D. Seletskiy, G. Burkard, and A. Leitenstorfer, Phys. Rev. Lett. 115, 263601 (2015).

 

Title: Universal Phase diagram for passive electromagnetic/quantum scatterers and hiding the interior region of core-shell Quantum particles with invisible cloaks
Speaker: Dr Jeng Yi Lee
Date:28 April 2016
Time:3pm - 4pm
Venue:MAS Executive Classroom 1 (MAS-03-06)
Host:Assistant Professor Chong Yidong 
Abstract: Following the exciting development of electromagnetic metamaterials, the problem of designing functional subwavelength-devices has attracted much recent attention. Invisible cloaks, compact resonators, coherent perfect absorbers, superscatterers and superabsorbers have been theoretically proposed. All of them are constructed with isotropic and homogeneous multi-layered structures.

In the first part of this talk, a phase diagram for electromagnetic scatterers is introduced based on the competitions among absorption and scattering cross sections. It can display all allowable solutions and physical boundaries for scattering coefficients regardless of any structures and materials. The characteristic eigen-solutions for anomalous scatterers reported in the present literatures is clearly illustrated in the phase diagram. A new inverse design method is also presented for designing light interactions with particles.

In the second part, we introduce an alternative method to achieve a quantum invisible cloak and create a hidden region by core-shell structures only, which is totally different from the transformation method. Based on the scattering cancellation method, we introduce concepts of Goos-Hanchen phase shift and management of conservation of probability flux flow into the shell layer by tuning the effective mass and potential in the shell. In this way, any materials embedded in the hidden region will not modify the scattering cross section, allowing this special is otropic-multi-layered quantum scatterer to have invisibility and cloaking simultaneously. Our methodology can be applied in other fields, such as electromagnetic and acoustic systems, etc.

 

Title: Polaritons in micro-assembled polaritonic crystal
Speaker: Professor  Zhanghai Chen
Date:19 April 2016
Time:11am - 12pm 
Venue:Hilbert Space (PAP-02-02)
Host:Associate Professor Xiong Qihua 
Abstract: Band engineering in strongly coupled light-matter systems opens new horizons of photonics and crystal physics. Recent reports on Dirac bosons in triangular and honeycomb exciton-polariton crystals show that new quasiparticles and new many-body phenomena can be found in semiconductor structures where a periodic potential acting on mixed light-matter excitations strongly modifies their dispersion relations. Till now, polariton crystals were realized by chemical etching or metal deposition on the top of planar microcavities. They provided photonic confinement, and were operating at liquid helium temperature. Here, we realize the room-temperature polaritonic crystals by simple micro assembling: laying the one-dimensional ZnO microrods on a silicon slice with periodic pattern. We clearly observe band folding and gap formation which manifests appearance of a super-band structure for exciton-polaritons. Most interestingly, in the non-linear regime, we observe polariton lasing from the edge of the Brillouin zone (π-state), which suggests a peculiar condensation mechanism in our system similar to "weak lasing" recently proposed.

References
1. Zhang, L.; Chen, ZH et al, Weak lasing in one-dimensional polariton superlattices, PNAS 1502666112 (2015)
2. Lai, C. W. et al. Coherent zero-state and π-state in an exciton-polariton condensate array. Nature 450, 529-533 (2007).
3. Kim, N. Y. et al, Dynamical d-wave condensation of exciton–polaritons in a two-dimensional square-lattice potential, Nature Physics, 7, 681-685 (2011).
4. Kim，N. Y. et al, Exciton-polariton condensates near the Dirac point in a triangular lattice, arXiv:1210.2153, (2012).
5. Kusudo, K. et al, Stochastic Formation of Polariton Condensates in Two Degenerate Orbital States, arXiv:1211.3833 (2012).
6. I.L. Aleiner, B.L. Altshuler and Y.G. Rubo, Radiative Coupling and Weak Lasing of Exciton-Polariton Condensates, Phys. Rev. B, 85, 121301 (2012).
7. Xie, W.; Chen, ZH et al, Phys. Rev. Lett. 108, 166401 (2012).

 

Title: Molecular Dynamics and Cluster Formation in Superfluid Helium Droplets
Speaker: Dr Wolfgang E. Ernst
Date:14 April 2016
Time:11am - 12pm
Venue:Hilbert Space (PAP-02-02)
Host:Associate Professor Claus-Dieter Ohl
Abstract: Superfluid droplets of 104 to 109 helium atoms (HeN) are doped with foreign atoms or molecules that move freely in or on the droplets and may form complexes in this cold environment. In this way, alkali and alkaline earth metal atoms were observed to react and form mixed diatomic molecules that in the future may also be produced from ultracold atoms in traps. By the same method, large Cu; Ag, Au, and Ni aggregates of different morphology are generated and their landing on a solid substrate was modelled in a molecular dynamics simulation. Nanowires and core-shell clusters with one metal surrounding a core of different kind were observed, deposited on solid substrates, and analyzed by high resolution electron microscopy and tomography. As it turns out, the temperature of the substrate and the doping rate have an important influence on the final cluster or wire structure. Our systematic studies help to provide recipes for the creation of tailored nanoparticles. A brief survey will be given on other projects in my group addressing electron-nuclear coupling phenomena in molecules and semimetals.

 

Title: Optical curl forces and beyond
Speaker: Professor Sir Michael Berry
Date:13 April 2016
Time:2pm - 3pm 
Venue:SPMS LT1, SPMS-04-07
Host:Assistant Professor Yidong Chong and Professor N. Zheludev
Abstract: A physical example of a force that depends on position but is not derivable from a potential, that is, a nonconservative force with non-zero curl, is the force on a dielectric particle in an optical field. The resulting dynamics need not be Hamiltonian or Lagrangian, yet is non-dissipative, with unusual chaotic dynamics. Noether’s theorem does not apply, so the link between symmetries and conservation laws is broken. Although unambiguous in optics, the physical existence of curl forces has been controversial among engineers. Motion under curl forces near optical vortices can be understood in detail, and the full series of ‘superadiabatic’ correction forces derived, leading to an exact slow manifold in which fast (internal) and slow (external) motion of the particle is separated. These classical optical forces have quantum effects

 

Title: Brillouin microscopy for measuring micromechanical properties of biological samples
Speaker: Professor Peter Török
Date:7 April 2016
Time:3pm - 4pm 
Venue:Hilbert Space (PAP-02-02)
Host:Associate Professor Claus-Dieter Ohl
Abstract: Optical elastography is a technique when samples are mechanically stimulated and simultaneously optically probed which can reveal mechanical properties of the sample under test. Due to the mechanical stimulation needed, this technique requires special access to the sample and so its use in vivo has been limited. Optical Coherence Tomography has recently been applied to elastography (Optical Coherence Elastography - OCE) yielding stiffness images from appreciable depths of the sample. Nevertheless, the spatial resolution of OCE is still limited, hitherto providing little evidence for subcellular imaging capability. Spectroscopy of light spontaneously scattered on lattice vibrations, or optical phonons, known as Brillouin spectroscopy, is an established technique to measure elastic properties of materials. A study published in 2005 described the combination of a Brillouin spectroscope and an optical microscope thus proposing the first Brillouin microscope (BM) with 20m spatial resolution. Subsequent studies have established BM as an alternative to OCE in optomechanical imaging. Our group has developed a Brillouin Confocal microscope that is capable of subcellular resolution thus permitting the study of micromechanical properties of single cells and bacteria. In this talk we describe optical elastography in general with special emphasis on Brillouin confocal microscopy. We discuss some aspects of equipment development, challenges and their solutions thus far. We also show typical Brillouin microscope images obtained from coronary artery walls, endothelial cells and biofilms.

 

Title: From Theory to Microgrid - Electrochemical Energy Storage Challenges
Speaker: Dr Shirley Meng
Date:28 March 2016
Time:11am - 12pm 
Venue:Hilbert Space (PAP-02-02)
Host:Associate Professor Fan Hongjin
Abstract: 

Energy storage in the electrochemical form is attractive because of its high efficiency and fast response time. New and improved materials for electrochemical energy storage are urgently required to enable the effective use of renewable energy sources. All electrochemical energy storage and conversion materials function as “living” systems (batteries and fuel cells), within which electrons and ions are moving during charge and discharge. These electronic and ionic motions often trigger defect generation and phase transformations, and consequently result in significant changes in energy density and rate capability of the materials. Establishing the fundamental basis for these dynamical mechanisms during electrochemical processes will accelerate the creation of new synthetic materials with superior energy storage and conversion properties.

In this seminar, I will talk about the recent efforts on “closing the gap” between the theoretical and practical energy density of the state-of-the-art lithium ion batteries. Furthermore I will discuss a few new perspectives for energy storage materials including atomistic modeling and design of novel energy storage materials and their micro and nanostructured electrodes. I hope to demonstrate how to combine knowledge-guided synthesis, advanced characterization and computational modeling to develop and optimize long-life high performance energy storage and conversion materials.

 

Title: Generation of highly indistinguishable photons with cw resonantly driven quantum dots
Speaker: Dr Carole Diederichs
Date:24 March 2016
Time:11am - 12pm
Venue:Hilbert Space (PAP-02-02) 
Host:Associate Professor Xiong Qihua 
Abstract: Quantum information protocols require light sources emitting single indistinguishable photons. In this context, the semiconductor quantum dots are good candidates for the development of such sources in the solid state, with the great advantage that they could be integrated into on-chip devices. However, their interaction with the solid state environment reduces the photon coherence time and thus broadens their emission spectrum which makes the emitted photons partially distinguishable over time. A characteristic time of the photon indistinguishability is thus tightly linked to the photon coherence time of these emitters which is typically of the order of few hundreds of picoseconds and which makes difficult the use of conventional photodetectors with nanosecond time resolutions. In this talk, I will show that by performing a cw resonant excitation of quantum dots in the resonant Rayleigh scattering regime where the coherently scattered single photons inherit the coherence time of the excitation laser [1,2], photon temporal indistinguishabilities of more than a dozen of nanoseconds can be reached and controlled by simply varying the spectral width of the excitation laser [3]. References [1] H. S. Nguyen, G. Sallen, C. Voisin, Ph. Roussignol, C. Diederichs, and G. Cassabois, “Optically Gated Resonant Emission of Single Quantum Dots”, Phys. Rev. Lett. 108, 057401 (2012). [2] H. S. Nguyen, G. Sallen, C. Voisin, Ph. Roussignol, C. Diederichs, and G. Cassabois, “Ultra-coherent single photon source”, Appl. Phys. Lett. 99, 261904 (2011). [3] R. Proux, M. Maragkou, E. Baudin, C. Voisin, Ph. Roussignol, and C. Diederichs, “Measuring the photon coalescence time window in the continuous-wave regime for resonantly driven semiconductor quantum dots”, Phys. Rev. Lett. 114, 067401 (2015).

 

Title: Quantum diagrammatic theory of the extrinsic spin Hall effect in Graphene
Speaker: Dr Mirco Milletari
Date:22 March 2016
Time:11am - 12pm 
Venue:Hilbert Space (PAP-02-02)
Host:Assistant Professor Chong Yidong
Abstract: Spintronics, the science that aims at utilising the spin degrees of freedom in addition to the charge of electrons for lowpower operation and novel device functionalities, has seen rapid developments in the past decade. In particular, the Spin Hall (SH) effect, i.e. the emergence of a transverse spin current in response to an applied longitudinal electric field, has attracted much interest for the possibility of building all-electric Spin manipulation devices. The efficiency of the Spin current generation is measured by the SH angle. While in semiconductors the SH angle is quite small (0.0001-0.001), it was shown that a “giant” SH conductivity can be achieved in Graphene decorated with small doses of resonant, Spin orbit active adatoms. It was argued that in this case, the effect is mostly due to the semiclassical Skew scattering mechanism, where electrons of different spins are scattered asymmetrically. In this talk, I will present a fully quantum mechanical analysis of the extrinsic SH effect in graphene in the strong scattering regime. This analysis allows us to consider other purely quantum effects such as the Quantum Side jump, in a self consistent and essentially exact way. In particular, I will focus on the transition between the semiclassical and the “quantum” regime, and show that the latter can be experimentally accessed in some regions of parameter space. In order to have a fully consistent treatment, we evaluate the SH conductivity within the T-Matrix approximation using Kubo formalism, that allows a resummation of all non-crossing contributions coming from the disorder averaging procedure. We then compare this procedure to the more common Gaussian white noise one, and show that this does not give the correct picture even in the weak scattering limit. Finally, I will discuss the effect of including a sub-class of crossing diagrams on the quantum to semiclassical transition and highlight their physical meaning. The importance of these diagrams was recently discussed in the context of the Anomalous Hall effect with massive Dirac fermions, albeit in the Gaussian approximation.

 

Title: Spin-valley coupling in monolayer transition metal dichalcogenides
Speaker: Professor Xiaodong Cui
Date:10 March 2020
Time:4.30pm - 5.30pm 
Venue:Hilbert Space (PAP-02-02)
Host:Associate Professor Xiong Qihua
Abstract: The monolayers of group VI transition metal dichalcogenides feature a valence band spin splitting with opposite sign in the two valleys located at corners of 1st Brillouin zone. This spin-valley coupling, particularly pronounced in tungsten dichalcogenides, can benefit potential spintronics and valleytronics with the important consequences of spin-valley interplay and the suppression of spin and valley relaxations. In this talk we discuss the spin-valley coupling in monolayers and multilayers WS2. We explored the interplay of spin and valley degrees of freedom with photocurrent experiments. We experimentally demonstrate that this giant spin-valley coupling, together with the valley dependent physical properties, may lead to rich possibilities for manipulating spin and valley degrees of freedom in these atomically thin 2D materials.

 

Title: Dynamical solitons in individual and mutually synchronized spin-torque and spin-hall effect driven nano-oscillators
Speaker: Professor Johan Åkerman
Date:4 March 2016
Time:2.30pm - 4PM
Venue:Hilbert Space (PAP-02-02)
Host:Associate Professor S.N. Piramanayagam
Abstract: Nano-contact spin-torque nano-oscillators (STNOs) and spin Hall effect nano-oscillators (SHNOs) provide excellent playgrounds for the study of highly non-linear and nano-scopic spin wave modes and phenomena. While originally studied for their potential as highly broadband microwave signal generators, these devices now attract a rapidly growing interest as spin wave generators in magnonic devices and as skyrmion injectors in magnetic nanowire based memories. In my talk I will give an overview of how magnetodynamical solitons, such as spin wave bullets, magnetic droplets, and dynamical skyrmions, can be nucleated and controlled in both STNOs and SHNOs and how these, as well as propagating spin waves, can be utilized to mutually synchronize a very large number of STNOs and SHNOs.

 

Title: Jumping on water
Speaker: Professor Ho-Young Kim
Date:26 February 2016
Time:3pm - 4pm 
Venue:Hilbert Space (PAP-02-02)
Host:Associate Professor Claus Dieter Ohl
Abstract: A water strider can jump on water to the height comparable to that on land without even making splash. Because this insect usually jumps to escape from predators, understandi ng how it exploits the water surface can shed light on the ultimate degree of semi-aquati c motility of living creatures made possible by millions of years of evolution. We observe d water striders′ leg movements during jumping with a high-speed camera and theoretic ally analyzed their dynamic interaction with deforming liquid-air interface. This enabled u s to obtain the optimal jumping condition for the strider to gain the maximum takeoff velo city by fully exploiting the capillary force without breaking the water surface. We applied t he combined results of biological and hydrodynamic study to developing a robotic water strider capable of water jumping as elegant and efficient as its natural counterpart. 1

1 J.-S. Koh, et al. “Jumping on water: Surface tension-dominated jumping of water strider s and robotic insects,” Science 349, 517-521 (2015).

 

Title: NANOPARTICLE FILM FORMATION BY ELECTROPHORETIC DEPOSITION
Speaker: Associate Professor James H. Dickerson
Date:26 February 2016
Time:10am - 11am 
Venue:Hilbert Space (PAP-02-02)
Host:Associate Professor Fan Hongjin
Abstract: For nanoparticles to be employed in an array of commercial and industrial applications, a technique for the facile, rapid, and site-selective assembly of homogeneous, densely packed, defect-free thin films must be realized. The most widely used methods for casting nanoparticle constituents into densely packed, thermally stable films, such as evaporation-driven self-assembly, and Langmuir-Blodgett casting, have some recognized limitations, including the inability to achieve both large-scale ordering of the nanoparticles as well as robust chemical and structural properties. Nanoparticle deposition schemes also require an understanding of both the nanoparticle dynamics in solution and the interactions that govern nanoparticle-substrate and nanoparticle-nanoparticle binding. The only nanoparticle deposition scheme that considers the primary physical characteristics of the nanoparticles in the film formation and incorporates the most favorable attributes of nanoparticle deposition is electrophoretic deposition.

Recent progress in the electrophoretic deposition of nanoparticles and other nanoscale materials will be reviewed in this presentation. Highlighted are the recent discoveries of the controllable fabrication of monolayers of nanoparticles with varying degrees of local order. Other achievements, such as the production of free-standing nanoparticle thin films, comprised solely of electrophoretically deposited iron oxide nanocrystals, cadmium selenide nanocrystals, carbon nanotubes, and graphene oxide nanosheets, will be featured.

 

Title: The Standard Model of Particle Physics and its completion (The Nobel Prize in Physics 2013)
Speaker: Professor Lars Brink
Date:11 February 2016
Time:4pm – 5pm
Venue:MAS Executive Classroom 1 (MAS-03-06)
Host:Associate Professor Pinaki Sengupta
Abstract: I will describe how the Standard Model of Physics emerged as the description of particle physics at the scales where we now can measure. Especially I will discuss the BEH effect which allows a gauge interaction to have short range as is observed in the weak interactions. I will also describe how the Higgs particle was discovered.

 

Title: Constructing vacuum spacetimes by generating manifolds of revolution around a curve
Speaker: Mr Saw Vee-Liem
Date:28 January 2016
Time:1pm - 2pm 
Venue:Hilbert Space (PAP-02-02)
Host:Associate Professor Chew Lock Yue
Abstract: We develop a general perturbative analysis on vacuum spacetimes which can be constructed by generating manifolds of revolution around a curve, and apply it to the Schwarzschild metric. The following different perturbations are carried out separately: 1) Non-rotating 2-spheres are added along a plane curve slightly deviated from the ``Schwarzschild line''; 2) General non-rotating topological 2-spheres are added along the ``Schwarzschild line''; 3) Slow-rotating 2-spheres are added along the ``Schwarzschild line''. For (1), we obtain the first order vacuum solution and show that no higher order solution exists. This linearised vacuum solution turns out however to be just a gauge transformation of the Schwarzschild metric. For (2), we solve the general linearised vacuum equations under several special cases. In particular, there exist linearised vacuum solutions with signature-changing metrics that contain closed timelike curves (though these do not correspond to adding topological 2-spheres). For (3), we find that the first order vacuum solution is equivalent to the slowly rotating Kerr metric. This is hence a much simpler and geometrically insightful derivation as compared to the gravitomagnetic one, where this rotating-shells construction is a direct manifestation of the frame-dragging phenomenon. We also show that the full Kerr however, cannot be obtained via adding rotating ellipsoids.

[1] V.-L. Saw, "Constructing vacuum spacetimes by generating manifolds of revolution around a curve", Classical and Quantum Gravity, accepted on 25th of November, 2015.

 

Title: de Broglie vortices – nano tornadoes in the lab!
Speaker: Professor Mohamed Babiker
Date:26 January 2016
Time:11am - 12pm 
Venue:Hilbert Space (PAP-02-02)
Host:Associate Professor Xiong Qihua
Abstract: The physics of optical vortex beams is now a well-established and a growing branch of optical physics with a number of inter-disciplinary connections and several important applications. Recent advances, however, have led to the prediction and generation of the electron vortex beam in an electron microscope. This is the first matter de Broglie vortex beam state with several key properties, additional to and distinct from those of the optical vortex. Is it possible that any de Broglie vortex beam can, in principle, be created from a particle beam, including those of elementary particles, neutral atoms and even large molecules? If so, there is scope for wide ranging variations in the scales and parameters as well as the physical principles involved in the vortex generation. This talk will first discuss the general concept of de Broglie vortices. It then presents the main features of the vortex beam states of both the optical and electron types and considers their respective interactions with matter, including their use in matter manipulation. Furthermore, experiments are described seeking to detect the dichroism associated with the interaction of the two types of vortex with atomic matter and the results are compared with the theoretical predictions. The talk will also highlight recent developments in work carried out by our group at York, together with collaborators, involving the properties and interactions of optical and matter vortices[1-12]. Fundamental vortex properties, namely linear and orbital angular momentum contents, spin and associated electromagnetic fields as well as mechanical and vortex multipolar interactions are discussed. Several features involving spin and orbital angular momentum exchange with matter are highlighted.

1. S. M. Lloyd, M. Babiker, J. Yuan and C. Kerr-Edwards, Phys. Rev. Lett. 109, 254801, (2012)
2. S. M. Lloyd, M. Babiker and J. Yuan, Phys. Rev. Lett. 108, 074802 (2012)
3. S. M. Lloyd, M. Babiker and J. Yuan, Phys. Rev. Lett. 110, 189502 (2013)
4. V. E. Lembessis and M. Babiker, Phys. Rev. Lett. 110, 083002 (2013)
5. S. M. Lloyd, M. Babiker and J. Yuan, Phys. Rev. A 86, 023816 (2012)
6. J. Yuan, S. M. Lloyd and M. Babiker, Phys. Rev. A 88, 031801 (2013)
7. S. M. Lloyd, M. Babiker and J. Yuan, Phys. Rev. A 88, 031802 (2013)
8. V. E. Lembessis, D. Ellinas and M. Babiker, Phys. Rev. A 84, 043422 (2011)
9. V. E. Lembessis and M. Babiker, Phys. Rev. A 81, 033811 (2010)
10. V. E. Lembessis, D. Ellinas, M. Babiker, and O. Al-Dossary, Phys.. Rev. A89, 013806 (2014)
11. M. Babiker, J. Yuan and V. E. Lembessis, Phys. Rev. A 91, 053616 (2015)
12. S. M. Lloyd, M. Babiker, G. Thirunavokkarasu and J. Yuan (Review Modern Physics 2015)

 

Title: Helicalised fractals
Speaker: Mr Saw Vee-Liem
Date:25 January 2016
Time:3pm - 4pm 
Venue:Hilbert Space (PAP-02-02)
Host:Associate Professor Chew Lock Yue
Abstract: We formulate the helicaliser, which replaces a given smooth curve by another curve that winds around it. In our analysis, we relate this formulation to the geometrical properties of the self-similar circular fractal (the discrete version of the curved helical fractal). Iterative applications of the helicaliser to a given curve yield a set of helicalisations, with the infinitely helicalised object being a fractal. We derive the Hausdorff dimension for the infinitely helicalised straight line and circle, showing that it takes the form of the self-similar dimension for a self-similar fractal, with lower bound of 1. Upper bounds to the Hausdorff dimension as functions of ω have been determined for the linear helical fractal, curved helical fractal and circular fractal, based on the no-self-intersection constraint. For large number of windings ω→∞, the upper bounds all have the limit of 2. This would suggest that carrying out a topological analysis on the structure of chromosomes by modelling it as a two-dimensional surface may be beneficial towards further understanding on the dynamics of DNA packaging.

[1] V.-L. Saw and L.Y. Chew, "Helicalised fractals", Chaos, Solitons & Fractals 75, 191 (2015).

 

Title: Quantitative Research in JPMorgan
Speaker: Dr Xiaolan Zhang
Date:25 January 2016
Time:3pm - 4pm 
Venue:LT28 (Basement 1, Near LKC-LT)
Host:Assistant Professor Cheong Siew Ann
Abstract: JPMorgan is a very large global financial institution and has a top ranking in the world. The Quantitative Research at JPMorgan has an unparalleled reputation for excellence in financial derivatives world and is at the forefront of innovation and control for products and risk management. The talk will give an introduction to JPMorgan and its Quantitative Research and then walk through a few examples of real life QR work including derivative pricing, algorithmic trading, counterparty risk management, high performance computing. It will explain all areas of quant work from asset classes to corporate level risk assessment and control. Model and product development will be a key part of QR jobs and the skills required will be explained. While mathematics and computing skills are basic pre-requisite for a quant the talk will also highlight the importance of communication and teamwork. The ability to apply models to real world problems is a key measure of quant strength. The talk will discuss how science/engineering postgraduates can transform him/herself to be a great quant at JPMorgan.

 

Title: Spin Orbit Torque Induced Magnetization Switching and Spin Hall Resistance in Ta/CoFeB/MgO structures
Speaker: Dr Jiangwei Cao
Date:20 January 2016
Time:2pm - 4pm 
Venue:MAS Executive Classroom 1 (MAS-03-06)
Host:Associate Professor Lew Wen Siang
Abstract: Controlling the magnetization direction via the interaction between spins and charges is crucial for spintronic memory and logic devices. Magnetization switching using conventional current-induced spin transfer torque (STT) requires a spin polarizer in a spin valve structure. Recently, the combination of STT with spin-orbit coupling has led to a new type of torque, namely spin-orbit torque (SOT), which originate mostly from the spin hall effect (SHE) in heavy metal layer with strong spin-orbit coupling. In heavy metal(HM)/ferromagnetic(FM)/insulator(I) structures, the spin current originating from the charge current in HM can exert a torque on the adjacent FM layer and induce a deterministic magnetization switching. The application of SOT induced switching in magnetic random access memory (MRAM) may avoid the damage to the insulating layer from the large writing current, which is the main issues in STT-MRAM. The reciprocal of the SHE is the inverse spin Hall effect (ISHE), i.e., the conversion of an injected spin current into a transverse electric current or voltage. In HM/FM structures, the reflected spin current by the FM layer, which depends on the magnetization direction of FM layer, may converts to a charge current via ISHE, and results in a resistance change of the device. This effect is called “spin Hall magnetoresistance (SMR)”. SMR shows a different angular dependence from normal AMR effect in ferromagnetic materials. This allows studying and quantifying spin Hall effects in paramagnetic metals as well as spin transfer to magnetic layers via simple dc magnetoresistance measurements. In this talk, I will review the recent progress on SOT induced magnetization switching and SMR effect in heavy metal(HM)/ferromagnetic(FM) structures. Finally, I will also present you some new results on this topic in our group.

 

Title: Crystalline whispering gallery mode resonators and their applications
Speaker: Dr Nan Yu
Date:19 January 2016
Time:3pm - 4pm 
Venue:MAS Executive Classroom 1 (MAS-03-06)
Host:Assistant Professor Lan Shau-Yu
Abstract: Taking advantage of some of unique optical properties of crystal materials, we explore use of crystalline whispering gallery mode resonators for lasers and nonlinear optics. In this talk, I will discuss some of the recent works in laser frequency stabilization, second harmonic generation, optical frequency comb source, and low threshold UV laser.

 

Title: NTU / UniLu Joint Workshop TOPOlOGICAL PHASES: NOVEL QUANTUM STATES OF MATTER
Speaker: 

Thomas Schmidt (UniLu)
Shengyuan Yang (SUTD)
Siu Zhuo Bin (NUS)
Ivan Shelykh (NTU)
Yidong Chong (NTU)
Giacomo Dolcetto (UniLu)
Su Ying Quek (NUS)
Elbert Chia (NTU)
Hyunsoo Yang (NUS)
Henrique Miranda (UniLu)
Alejandro Molina-Sanchez (UniLu)

Date:18 January 2016
Time:9am - 5pm 
Venue:PAP Hilbert Space (PAP-02-02)
Host:Associate Professor Pinaki Sengupta
Abstract: Ever since the theoretical prediction and subsequent experimental discovery of topological insulators, the study of topological phases has grown into one of the most active fields of research in contemporary Condensed Matter Physics. The rapid progress in theoretical studies has been complemented by experimental results and has resulted in a wealth of knowledge about topological states of matter. Till recently, bulk of the interesting results in topological insulators has been from combining non-interacting band theory with the notion of topology, ignoring the effects of electron interaction. Combining topological effects with electron interaction is expected to drive the emergence additional new phases. This workshop brings together speakers from the three Universities in Singapore (NTU, NUS and SUTD) and the groups of Prof. Thomas Schmidt and Prof. Ludger Wirtz of the University of Luxembourg who are working on different aspects of topological phases in condensed matter as well as photonic systems.

 

Title: Precision measurements with atomic sensors in space
Speaker: Dr Nan Yu
Date:18 January 2016
Time:4pm - 5pm 
Venue:MAS Executive Classroom 1 (MAS-03-06)
Host:Assistant Professor Lan Shau-Yu
Abstract: Space platforms provide unique experimental conditions to explore fundamental physics and quantum phenomena. The microgravity environment in space is especially favorable for cold atom-based sensors and experiments. Space also affords investigations of physical phenomena over large spatial, velocity, and gravitational variations that cannot be accessed in ground-based laboratories. In this talk, we will give an overview of JPL’s activities in mission concept and related technology developments. The main discussions will focus on using atom interferometers for Earth gravity measurements and for test of Einstein equivalence principle on the International Space Station.

 

Title: Homogenization of Periodic Electromagnetic Structures: Uncertainty Principles and a Fresh Look at Nonlocality
Speaker: Professor Igor Tsukerman
Date:8 January 2016
Time:3pm - 4pm 
Venue:Hilbert Space (PAP-02-02)
Host:Assistant Professor Chong Yidong
Abstract: An intriguing feature of periodic electromagnetic structures (photonic crystals and metamaterials) is their nontrivial magnetic response under certain conditions. We show that such artificial magnetism has limitations and is subject to at least two general uncertainty principles. First, the stronger the deviation of the effective magnetic permeability from unity, the less accurate (“certain”) the predictions of any local homogeneous model. Second, if the magnetic response is strong, then no local effective medium theory can accurately reproduce the transmission and reflection parameters and, simultaneously, power dissipation in the material. Our theoretical analysis is illustrated with numerical examples. The accuracy of homogenization can be significantly higher in nonlocal models. As an example, about an order-of magnitude improvement in the accuracy of transmission and reflection coefficients for a periodic layered structure is demonstrated. Nonlocal models have a wide range of applications beyond photonic crystals and metamaterials: in plasma physics, plasmonics, electrostatics of macromolecular and colloidal systems, and more. In all homogenization theories, we emphasize the importance of interface boundary conditions in addition to dispersion relations. This is particularly challenging for nonlocal models, where translational invariance of the integral kernel breaks down near interfaces. A fresh look at these issues favors real-space models over more traditional reciprocal-space ones.