arXiv subjects

N. H. Kwong

Publications and source records attributed to N. H. Kwong.

18 recordsLinked to original sources

arXiv2026

Universal Topology of Exceptional Points in Nonlinear Non-Hermitian Systems

Exceptional points (EPs) are non-Hermitian degeneracies where eigenvalues and eigenvectors coalesce, giving rise to unusual physical effects across scientific disciplines. The concept of EPs has recently been extended to nonlinear physical systems. We theoretically demonstrate a universal topology in the nonlinear parameter space for a large class of physical systems that support 2nd order EPs in the linear regime. Knowledge of this topology (called elliptic umbilic singularity in bifurcation theory) deepens our understanding of 2nd order linear EPs, which here emerge as coalescence of 4 nonlinear eigenvectors. This helps guide future experimental discovery of nonlinear EPs and their classification, establish rigorous bounds of sensitivity enhancement of EPs in nonlinear systems, and helps envision and optimize technological applications of nonlinear EPs. Our theoretical approach is general and can be extended to nonlinear perturbations of 3rd and higher-order EPs.

physics.optics↗

arXiv2023

Non-Hermitian dispersion sign reversal of radiative resonances in two dimensions

In a recent publication [Wurdack et al., Nat. Comm. 14:1026 (2023)], it was shown that in microcavities containing atomically thin semiconductors non-Hermitian quantum mechanics can lead to negative exciton polariton masses. We show that mass-sign reversal can occur generally in radiative resonances in two dimensions (without cavity) and derive conditions for it (critical dephasing threshold etc.). In monolayer transition-metal dichalcogenides, this phenomenon is not invalidated by the strong electron-hole exchange interaction, which is known to make the exciton massless.

cond-mat.mes-hall↗

arXiv2023

Gapless fluctuations and exceptional points in semiconductor lasers

We analyze the spectrum of spatially uniform, single-particle fluctuation modes in the linear electromagnetic response of a semiconductor laser. We show that if the decay rate of the interband polarization, $γ_p$, and the relaxation rate of the occupation distribution, $γ_f$, are different, a gapless regime exists in which the order parameter $Δ^{(0)} (k)$ (linear in the coherent photon field amplitude and the interband polarization) is finite but there is no gap in the real part of the single-particle fluctuation spectrum. The laser being a pumped-dissipative system, this regime may be considered a non-equilibrium analog of gapless superconductivity. We analyze the fluctuation spectrum in both the photon laser limit, where the interactions among the charged particles are ignored, and the more general model with interacting particles. In the photon laser model, the order parameter is reduced to a momentum-independent quantity, which we denote by $Δ$. We find that, immediately above the lasing threshold, the real part of the fluctuation spectrum remains gapless when $0 < | Δ| < \sqrt{2 / 27} \, | γ_f - γ_p |$ and becomes gapped when $| Δ|$ exceeds the upper bound of this range. Viewed as a complex function of $|Δ|$ and the electron-hole energy, the eigenvalue set displays some interesting exceptional point (EP) structure around the gapless-gapped transition. The transition point is a third-order EP, where three eigenvalues (and eigenvectors) coincide. Switching on the particle interactions in the full model modifies the spectrum of the photon laser model and, in particular, leads to a more elaborate EP structure. However, the overall spectral behavior of the continuous (non-collective) modes of the full model can be understood on the basis of the relevant results of the photon laser model.

cond-mat.mes-hall↗

arXiv2020

Metamorphosis of Goldstone and Soft Fluctuation Modes in Polariton Lasers

For a driven-dissipative quantum many-body system prepared in a spontaneous broken-symmetry steady state, in addition to the Goldstone mode the soft fluctuation modes provide important insight into the system's dynamics. Using a microscopic polariton laser theory, we find a rich transformation behavior of discrete and continuum soft modes in a two parameter (pump density and cavity dissipation rate) space. In addition to finding exceptional-point Goldstone companion modes, our theory yields a unified picture of a variety of seemingly disconnected physical concepts including Mott transition, Mollow spectra or relaxation oscillations, and polaritonic Bardeen-Cooper-Schrieffer gaps.

cond-mat.mes-hall↗

arXiv2017

Directional optical switching and transistor functionality using optical parametric oscillation in a spinor polariton fluid

Over the past decade, spontaneously emerging patterns in the density of polaritons in semiconductor microcavities were found to be a promising candidate for all-optical switching. But recent approaches were mostly restricted to scalar fields, did not benefit from the polariton's unique spin-dependent properties, and utilized switching based on hexagon far-field patterns with 60° beam switching (i.e. in the far field the beam propagation direction is switched by 60°). Since hexagon far-field patterns are challenging, we present here an approach for a linearly polarized spinor field, that allows for a transistor-like (e.g., crucial for cascadability) orthogonal beam switching, i.e. in the far field the beam is switched by 90°. We show that switching specifications such as amplification and speed can be adjusted using only optical means.

cond-mat.other↗

arXiv2017

Theory of optically controlled anisotropic polariton transport in semiconductor double microcavities

Exciton polaritons in semiconductor microcavities exhibit many fundamental physical effects, with some of them amenable to being controlled by external fields. The polariton transport is affected by the polaritonic spin-orbit interaction, which is caused by the splitting of transverse-electric and transverse-magnetic (TE-TM) modes. This is the basis for a polaritonic Hall effect, called optical spin Hall effect (OSHE), which is related to the formation of spin/polarization textures in momentum space, determining anisotropic ballistic transport, as well as related textures in real space. Owing to Coulombic interactions between the excitonic components of the polaritons, optical excitation of polaritons can affect the OSHE. We present a theoretical analysis of the OSHE and its optical control in semiconductor double microcavities, i.e. two optically coupled cavities, which are particularly well suited for the creation of polaritonic reservoirs that affect the spin-texture-forming polaritons. The theory is formulated in terms of a set of double-cavity spinor-polariton Gross-Pitaevskii equations. Numerical solutions feature, among other things, a controlled rotation of the spin texture in momentum space. The theory also allows for an identification of the effective magnetic field component that determines the optical control in phenomenological pseudo-spin models in terms of exciton interactions and the polariton density in the second lower polariton branch.

cond-mat.mes-hall↗

arXiv2016

Controlling the optical spin Hall effect with light

The optical spin Hall effect (OSHE) is a transport phenomenon of exciton polaritons in semiconductor microcavities, caused by the polaritonic spin-orbit interaction, that leads to the formation of spin textures. In the semiconductor cavity, the physical basis of the spin orbit coupling is an effective magnetic field caused by the splitting of transverse-electric and transverse-magnetic (TE-TM) modes. The spin textures can be observed in the near field (local spin distribution of polaritons), and as light polarization patterns in the more readily observable far field. For future applications in spinoptronic devices, a simple and robust control mechanism, which establishes a one-to-one correspondence between stationary incident light intensity and far-field polarization pattern, is needed. We present such a control scheme, which is made possible by a specific double-microcavity design.

physics.optics↗

arXiv2016

Real-time Kadanoff-Baym approach to nuclear response functions

Linear density response functions are calculated for symmetric nuclear matter of normal density by time-evolving two-time Green's functions in real time. Of particular interest is the effect of correlations. The system is therefore initially time-evolved with a collision term calculated in a direct Born approximation as well as with full (RPA) ring-summation until fully correlated. An external time-dependent potential is then applied. The ensuing density fluctuations are recorded to calculate the density response. This method was previously used by Kwong and Bonitz for studying plasma oscillations in a correlated electron gas. The energy-weighted sum-rule for the response function is guaranteed by using conserving self-energy insertions as the method then generates the full vertex-functions. These can alternatively be calculated by solving a Bethe -Salpeter equation as done in some previous works. The (first order) mean field is derived from a momentum-dependent (non-local) interaction while $2^{nd}$ order self-energies are calculated using a particle-hole two-body effective (or 'residual') interaction given by a gaussian \it local \rm potential. We present numerical results for the response function $S(ω,q_0)$ for $q_0=0.2,0.4$ and $0.8 {\rm fm}^{-1}$. Comparison is made with the nucleons being un-correlated i.e. with only the first order mean field included, the 'HF+RPA' approximation. We briefly discuss the relation of our work with the Landau quasi-particle theory as applied to nuclear systems by Sjöberg and followers using methods developped by Babu and Brown, with special emphasis on the 'induced' interaction.

nucl-th↗

arXiv2015

Polarization dependence of nonlinear wave mixing of spinor polaritons in semiconductor microcavities

The pseudo-spin dynamics of propagating exciton-polaritons in semiconductor microcavities are known to be strongly influenced by TE-TM splitting. As a vivid consequence, in the Rayleigh scattering regime, the TE-TM splitting gives rise to the optical spin Hall effect (OSHE). Much less is known about its role in the nonlinear optical regime in which four-wave mixing for example allows the formation of spatial patterns in the polariton density, such that hexagons and two-spot patterns are observable in the far field. Here we present a detailed analysis of spin-dependent four-wave mixing processes, by combining the (linear) physics of TE-TM splitting with spin-dependent nonlinear processes, i.e., exciton-exciton interaction and fermionic phase-space filling. Our combined theoretical and experimental study elucidates the complex physics of the four-wave mixing processes that govern polarization and orientation of off-axis modes.

physics.optics↗

arXiv2013

Real-Time Kadanoff-Baym Approach to Nuclear Response Functions

Linear response functions are calculated for symmetric nuclear matter of normal density by time-evolving two-time Green's functions with conserving self-energy insertions, thereby satisfying the energy-sum rule. Nucleons are regarded as moving in a mean field defined by an effective mass. A two-body effective (or residual) interaction, represented by a gaussian local interaction, is used to find the effect of correlations in a second order as well as a ring approximation. The response function S(e,q) is calculated for 0.2<q<1.2 fm^{-1}. Comparison is made with the nucleons being un-correlated, "RPA+HF" only.

nucl-th↗

arXiv2013

Transverse optical instability patterns in semiconductor microcavities: polariton scattering and low-intensity all-optical switching

We present a detailed theoretical study of transverse exciton-polariton patterns in semiconductor quantum-well microcavities. These patterns are initiated by directional instabilities in the uniform pump-generated polariton field and are measured as optical patterns in a transverse plane in the far field. Based on a microscopic many-particle theory, we investigate the spatio-temporal dynamics of the formation, selection, and optical control of these patterns. An emphasis is placed on a previously proposed low-intensity, all-optical switching scheme designed to exploit these instability-driven patterns. Simulations and detailed analyses of simplified and more physically transparent models are used. Two aspects are studied in detail. First we study the dependencies of the stability behaviors of various patterns, as well as transition time scales, on parameters relevant to the switching action(the degree of built-in azimuthal anisotropy in the system and the switching beam intensity). It is found that if the parameters are varied, the pattern system undergoes abrupt transitions at threshold parameter values which are accompanied by multiple-stability and hysteresis behaviors. Moreover, during a real-time switching action, the transient dynamics of the system may depend significantly on the proximity of unstable patterns. The second aspect is a classification and detailed analysis of the polariton scattering processes contributing to the pattern dynamics, giving us an understanding of the selection and control of patterns as results of these processes' intricate interplay. The crucial role played by the relative phases of the polariton field in determining the gains or losses of polariton densities in various momentum modes is highlighted, and interpretation of the actions of the various processes in terms of concepts commonly used in classical pattern-forming systems is given.

cond-mat.mes-hall↗

arXiv2007

All-optical switching in planar semiconductor microcavities

Using a microscopic many-particle theory, we propose all-optical switching in planar semiconductor microcavities where a weak beam switches a stronger signal. Based on four-wave-mixing instabilities, the general scheme is a semiconductor adaptation of a recently demonstrated switch in an atomic vapor [Dawes et al., Science 308, 672 (2005)].

cond-mat.other↗

arXiv2007

Large optical gain from four-wave mixing instabilities in semiconductor quantum wells

Based on a microscopic many-particle theory, we predict large optical gain in the probe and background-free four-wave mixing directions caused by excitonic instabilities in semiconductor quantum wells. For a single quantum well with radiative-decay limited dephasing in a typical pump-probe setup we discuss the microscopic driving mechanisms and polarization and frequency dependence of these instabilities.

cond-mat.other↗

arXiv2007

Polariton amplification in a multiple-quantum-well resonant photonic crystal

Based on a microscopic many-particle theory we study the amplification of polaritons in a multiple-quantum-well resonant photonic crystal. For the Bragg-spaced multiple quantum wells under investigation we predict that in a typical pump-probe setup four-wave mixing processes can lead to an unstable energy transfer from the pump into the probe and the background-free four-wave mixing directions. We find that under certain excitation conditions this phase-conjugate oscillation induced instability can lead to a large amplification of the weak probe pulse.

cond-mat.other↗

arXiv2007

Influence of exciton-exciton correlations on the polarization characteristics of the polariton amplification in semiconductor microcavities

Based on a microscopic many-particle theory we investigate the influence of excitonic correlations on the vectorial polarization state characteristics of the parametric amplification of polaritons in semiconductor microcavities. We study a microcavity with perfect in-plane isotropy. A linear stability analysis of the cavity polariton dynamics shows that in the co-linear (TE-TE or TM-TM) pump-probe polarization state configuration, excitonic correlations diminish the parametric scattering process whereas it is enhanced by excitonic correlations in the cross-linear (TE-TM or TM-TE) configuration. Without any free parameters, our microscopic theory gives a quantitative understanding how many-particle effects can lead to a rotation or change of the outgoing (amplified) probe signal's vectorial polarization state relative to the incoming one's.

cond-mat.other↗

arXiv2004

Inducing electron spin coherence in GaAs quantum well waveguides: Spin coherence without spin precession

Electron spin coherence is induced via light-hole transitions in a quantum well waveguide without either an external or internal DC magnetic field. In the absence of spin precession, the induced spin coherence is detected through effects of quantum interference in the spectral domain coherent nonlinear optical response. We interpret the experimental results qualitatively using a simple few-level model with only the optical transition selection rule as its basic ingredients.

cond-mat.mes-hall↗

arXiv2001

Dielectric properties of correlated quantum plasmas

Results for the dynamic structure factor of a one-component plasma are presented for the situations of strong coupling and weak degeneracy and strong degeneracy and weak coupling. Possibilities to obtain rigorous results when both, coupling and degeneracy are large are discussed.

cond-mat.stat-mech↗

arXiv1995

The Nucleon Spectral Function at Finite Temperature and the Onset of Superfluidity in Nuclear Matter

Nucleon selfenergies and spectral functions are calculated at the saturation density of symmetric nuclear matter at finite temperatures. In particular, the behaviour of these quantities at temperatures above and close to the critical temperature for the superfluid phase transition in nuclear matter is discussed. It is shown how the singularity in the thermodynamic T-matrix at the critical temperature for superfluidity (Thouless criterion) reflects in the selfenergy and correspondingly in the spectral function. The real part of the on-shell selfenergy (optical potential) shows an anomalous behaviour for momenta near the Fermi momentum and temperatures close to the critical temperature related to the pairing singularity in the imaginary part. For comparison the selfenergy derived from the K-matrix of Brueckner theory is also calculated. It is found, that there is no pairing singularity in the imaginary part of the selfenergy in this case, which is due to the neglect of hole-hole scattering in the K-matrix. From the selfenergy the spectral function and the occupation numbers for finite temperatures are calculated.

nucl-th↗