arXiv subjects

Li Ge

Publications and source records attributed to Li Ge.

At least 37 records · Page 2Linked to original sources

arXiv2023

Non-Hermitian gauged laser arrays with localized excitations: Anomalous threshold and generalized principle of selective pumping

We investigate non-Hermitian skin modes in laser arrays with spatially localized excitation. Intriguingly, we observe an unusual threshold behavior when selectively pumping either the head or the tail of these modes: both cases exhibit the same lasing threshold and hence defy the conventional principle of selective pumping, which aims to maximize the overlap between the pump profile and the target lasing mode. To shed light on this enigma, we reveal a previously overlooked phenomenon, i.e., energy exchange at non-Hermitian coupling junctions with the photonic environment, which does not occur with uniform gain or loss. Utilizing a transfer matrix approach, we elucidate the mechanism of this anomalous threshold behavior, which is determined by the specific physical realization of the non-Hermitian gauge field (i.e., using gain, loss, or their mixture). Finally, we derive a generalized principle of selective pumping in non-Hermitian arrays, which shows that the decisive spatial overlap is given by the tripartite product of the pump, the lasing mode, and its biorthogonal partner. Our study provides a glimpse into how the two forms of non-Hermiticity, i.e., asymmetric couplings and a complex onsite potential, interact synergetically in laser arrays, which may stimulate further explorations of their collective effects in photonics and related fields.

physics.optics↗

arXiv2023

Anomalous minimum and scaling behavior of localization length near an isolated flat band

Using one-dimensional tight-binding lattices and an analytical expression based on the Green's matrix, we show that anomalous minimum of the localization length near an isolated flat band, previously found for evanescent waves in a defect-free photonic crystal waveguide, is a generic feature and exists in the Anderson regime as well, i.e., in the presence of disorder. Our finding reveals a scaling behavior of the localization length in terms of the disorder strength, as well as a summation rule of the inverse localization length in terms of the density of states in different bands. Most interesting, the latter indicates the possibility of having two localization minima inside a band gap, if this band gap is formed by two flat bands such as in a double-sided Lieb lattice.

physics.optics↗

arXiv2023

Analysis of Dirac exceptional points and their isospectral Hermitian counterparts

Recently, a Dirac exceptional point (EP) was reported in a non-Hermitian system. Unlike a Dirac point in Hermitian systems, this Dirac EP has coalesced eigenstates in addition to the degenerate energy. Also different from a typical EP, the two energy levels connected at this Dirac EP remain real in its vicinity and display a linear instead of square root dispersion, forming a tilted Dirac cone in the hybrid space consisting of a momentum dimension and a synthetic dimension for the strength of non-Hermiticity. In this report, we first present simple three-band and two-band matrix models with a Dirac EP, where the linear dispersion of the tilted Dirac cone can be expressed analytically. Importantly, our analysis also reveals that there exist Hermitian and non-Hermitian systems that have the same (real-valued) energy spectrum in their entire parameter space, with the exception that one or more degeneracies in the former are replaced by Dirac EPs in the later. Finally, we show the existence of an imaginary Dirac cone with an EP at its center.

quant-ph↗

arXiv2022

Tracking exceptional points above laser threshold

Recent studies on non-Hermitian optical systems having exceptional points (EPs) have revealed a host of unique characteristics associated with these singularities, including unidirectional invisibility, chiral mode switching and laser self-termination, to mention just a few examples. The vast majority of these works focused either on passive systems or active structures where the EPs were accessed below the lasing threshold, i.e. when the system description is inherently linear. In this work, we experimentally demonstrate that EP singularities in coupled semiconductor nanolasers can be accessed and tracked above the lasing threshold, where they become branch points of a nonlinear dynamical system. Contrary to the common belief that unavoidable cavity detuning will impede the formation of an EP, here we demonstrate that this same detuning is necessary for compensating the carrier-induced frequency shift, hence restoring the nonlinear EP in the lasing regime. Furthermore, unlike linear non-Hermitian systems, we find that the spectral location of EPs above laser threshold varies as a function of total pump power and can therefore be continuously tracked. Our work is a first step towards the realization of lasing EPs in more complex laser geometries, and enabling the enhancement of photonic local density of states through non-Hermitian symmetries combined with nonlinear interactions in coupled laser arrays.

physics.optics↗

arXiv2022

Imaginary gauge transformation in momentum space and Dirac exceptional point

An imaginary gauge transformation is at the core of the non-Hermitian skin effect. Here we show that such a transformation can be performed in momentum space as well, which reveals that certain gain and loss modulated systems in their parity-time (PT) symmetric phases are equivalent to Hermitian systems with real potentials. Our analysis in momentum space also distinguishes two types of exceptional points (EPs) in the same system. Besides the conventional type that leads to a PT transition upon the continuous increase of gain and loss, we find real-valued energy bands connected at a Dirac EP in hybrid dimensions, consisting of a spatial dimension and a synthetic dimension for the gain and loss strength.

quant-ph↗

arXiv2022

Discriminative-Region Attention and Orthogonal-View Generation Model for Vehicle Re-Identification

Vehicle re-identification (Re-ID) is urgently demanded to alleviate thepressure caused by the increasingly onerous task of urban traffic management. Multiple challenges hamper the applications of vision-based vehicle Re-ID methods: (1) The appearances of different vehicles of the same brand/model are often similar; However, (2) the appearances of the same vehicle differ significantly from different viewpoints. Previous methods mainly use manually annotated multi-attribute datasets to assist the network in getting detailed cues and in inferencing multi-view to improve the vehicle Re-ID performance. However, finely labeled vehicle datasets are usually unattainable in real application scenarios. Hence, we propose a Discriminative-Region Attention and Orthogonal-View Generation (DRA-OVG) model, which only requires identity (ID) labels to conquer the multiple challenges of vehicle Re-ID.The proposed DRA model can automatically extract the discriminative region features, which can distinguish similar vehicles. And the OVG model can generate multi-view features based on the input view features to reduce the impact of viewpoint mismatches. Finally, the distance between vehicle appearances is presented by the discriminative region features and multi-view features together. Therefore, the significance of pairwise distance measure between vehicles is enhanced in acomplete feature space. Extensive experiments substantiate the effectiveness of each proposed ingredient, and experimental results indicate that our approach achieves remarkable improvements over the state- of-the-art vehicle Re-ID methods on VehicleID and VeRi-776 datasets.

cs.CV↗

arXiv2021

The damping and diffusion of atoms moving in the background electromagnetic environment

The interaction between an atom and the quantized electromagnetic field depends on the position of the atom. Then the atom experiences a force which is the minus gradient of this interaction. Through the Heisenberg equations of motion and the Born-Markov approximation, the mean and correlation of the force are obtained, showing that the center-of-mass motion of the atom is damped and diffused. This approach can be easily generalized to multi-level atoms, where the damping force and diffusion coefficients are just the weighted average of the contributions from all pairs of energy levels that have nonvanishing dipole elements. It is shown that these results are invariant under Galilean transformation, and in principle can be used to determine the velocity of the lab relative to the background radiation.

quant-ph↗

arXiv2021

Direct observation of zero modes in a non-Hermitian nanocavity array

Zero modes are symmetry protected ones whose energy eigenvalues have zero real parts. In Hermitian arrays, they arise as a consequence of the sublattice symmetry, implying that they are dark modes. In non-Hermitian systems, that naturally emerge in gain/loss optical cavities, particle-hole symmetry prevails instead; the resulting zero modes are no longer dark but feature $π/2$ phase jumps between adjacent cavities. Here we report on the direct observation of zero modes in a non-Hermitian three coupled photonic crystal nanocavity array containing quantum wells. Unlike the Hermitian counterparts, the non-Hermitian zero modes can only be observed for small sublattice detuning, and they can be identified through far-field imaging and spectral filtering of the photoluminescence at selected pump locations. We explain the zero mode coalescence as a parity-time phase transition for small coupling. These zero modes are robust against coupling disorder, and can be used for laser mode engineering and photonic computing.

physics.optics↗

arXiv2021

Insight of the Green's function as a defect state in a boundary value problem

A new perspective of the Green's function in a boundary value problem as the only eigenstate in an auxiliary formulation is introduced. In this treatment, the Green's function can be perceived as a defect state in the presence of a $δ$-function potential, the height of which depends on the Green's function itself. This approach is illustrated in one-dimensional and two-dimensional Helmholtz equation problems, with an emphasis on systems that are open and have a non-Hermitian potential. We then draw an analogy between the Green's function obtained this way and a chiral edge state circumventing a defect in a topological lattice, which shines light on the local minimum of the Green's function at the source position.

cond-mat.mes-hall↗

arXiv2021

Pseudo-chirality: a manifestation of Noether's theorem in non-Hermitian systems

Noether's theorem relates constants of motion to the symmetries of the system. Here we investigate a manifestation of Noether's theorem in non-Hermitian systems, where the inner product is defined differently from quantum mechanics. In this framework, a generalized symmetry which we term pseudo-chirality emerges naturally as the counterpart of symmetries defined by a commutation relation in quantum mechanics. Using this observation, we reveal previously unidentified constants of motion in non-Hermitian systems with parity-time and chiral symmetries. We further elaborate the disparate implications of pseudo-chirality induced constant of motion: It signals the pair excitation of a generalized "particle" and the corresponding "hole" but vanishes universally when the pseudo-chiral operator is anti-symmetric. This disparity, when manifested in a non-Hermitian topological lattice with the Landau gauge, depends on whether the lattice size is even or odd. We further discuss previously unidentified symmetries of this non-Hermitian topological system, and we reveal how its constant of motion due to pseudo-chirality can be used as an indicator of whether a pure chiral edge state is excited.

quant-ph↗

arXiv2020

Origin of robust exceptional points: a restricted bulk zero mode

Recently a type of robust exceptional points was found that is insensitive to the coupling disorder in the bulk. Here we show that a disparity emerges when the number of coupled cavities in this one-dimensional array changes from even to odd. The robust exceptional point only exists in the former case, whereas the location of the exceptional point in the latter depends inversely on the size of the cavity array and is subjected to coupling disorder in the bulk. We further show that the exceptional points in these two cases are second and third order, respectively. We elucidate the origin of the robust EP as a restricted bulk zero mode, which shares the same robustness against coupling disorder and has a finite amplitude adjacent to the boundary. This finding enables us to identify robust EPs in higher dimensional systems reliably, which can exist in the presence of either sublattice symmetry or the non-Hermitian particle-hole symmetry evolved from it.

cond-mat.mes-hall↗

arXiv2020

Chiral symmetry in non-Hermitian systems: product rule and Clifford algebra

Chiral symmetry provides the symmetry protection for a large class of topological edge states. It exists in non-Hermitian systems as well, and the same anti-commutation relation between the Hamiltonian and a linear chiral operator, i.e., $\{H,Π\}=0$, now warrants a symmetric spectrum about the origin of the complex energy plane. Here we show two general approaches to identify and generate chiral symmetry in non-Hermitian systems, with an emphasis on lattices with detuned on-site potentials that can vary in both their real and imaginary parts. One approach utilizes the Clifford algebra satisfied by the Dirac matrices, while the other relies on the simultaneous satisfaction of non-Hermitian particle-hole symmetry and bosonic anti-linear symmetry, extended beyond simple spatial transformations to include, for example, an imaginary gauge transformation.

quant-ph↗

arXiv2019

Synthetic parity-time symmetry breaking in a single microcavity

Non-Hermitian systems based on parity-time (PT) symmetry reveal rich physics beyond the Hermitian regime. So far, realizations of PT-symmetric systems have been limited to the spatial domain. Here we theoretically and experimentally demonstrate PT symmetry in a synthetic spectral dimension induced by nonlinear Brillouin scattering in a single optical microcavity, where electromagnetically induced transparency or absorption in two spectral resonances provides the optical gain and loss to observe a phase transition between two symmetry regimes. This scheme provides a new paradigm towards the investigation of non-Hermitian physics in a synthetic photonic dimension for all-optical signal processing and quantum information science.

physics.optics↗

arXiv2019

Time-reversal invariant scaling of light propagation in one-dimensional non-Hermitian systems

Light propagation through a normal medium is determined not only by the real part of the refractive index but also by its imaginary part, which represents optical gain and loss. Therefore, two media with different gain and loss landscapes can have very different transmission and reflection spectra, even when their real parts of the refractive index are identical. Here we show that while this observation is true for an arbitrary one-dimensional medium with refractive index $n(x)$ and its time-reversed partner with refractive index $n^*(x)$, there exists a universal scaling that gives identical transmittance and reflectance in these corresponding systems. Interestingly, these scaled transmittance and reflectance reduce to their standard, unscaled forms in a time-reversal invariant system, i.e., one without gain or loss.

physics.optics↗

arXiv2018

Cooling of the rotation of a nanodiamond via the interaction with the electron spin of the contained NV-center

We propose a way to cool the rotation of a nanodiamond, which contains a NV-center and is levitated by an optical tweezer. Following the rotation of the particle, the NV-center electron spin experiences varying external fields and so leads to spin-rotation coupling. By optically pumping the electrons from a higher energy level to a lower level, the rotation energy is dissipated. We give the analytical result for the damping torque exerted on the nanodiamond, and evaluate the final cooling temperature by the fluctuation-dissipation theorem. It's shown that the quantum regime of the rotation can be reached with our scheme.

quant-ph↗

arXiv2018

Chiral-reversing vortex radiation from a single emitter by eigenstates phase locking

The radiation of an emitter does not depend only on its intrinsic properties but also on the surrounding photonic environment, the notion of which is essential in the developments of lasers, quantum optics and other light-matter interaction related fields. However, in conventional wisdom, an emitter radiates into photonic eigenstates in the weak coupling regime and does not alter the property of the latter. Here, we report a counterintuitive phenomenon where the radiation field of a dipole in a parity-time symmetric ring resonator displays the opposite handedness to the eigenstates of the system. This chiral-reversing radiation takes place at an exception point of the underlying non-Hermitian system, where the singularity at the exceptional point forces a phase locking of the coalesced eigenstates when interacting with the dipole emitter. Such an intriguing phenomenon has been employed to construct vortex radiation with controllable topological charge from a single quantum dot embedded plasminic nanocavity with Purcell enhancement factor up to 1000. Our scheme enriches the interesting physics of an exception point in the quantum region and may open a new paradigm for chiral quantum optics and vortex lasers at nanoscale.

physics.optics↗

arXiv2018

Defect states emerging from a non-Hermitian flat band of photonic zero modes

We show the existence of a flat band consisting of photonic zero modes in a gain and loss modulated lattice system, as a result of the underlying non-Hermitian particle-hole symmetry. This general finding explains the previous observation in parity-time symmetric systems where non-Hermitian particle-hole symmetry is hidden. We further discuss the defect states in these systems, whose emergence can be viewed as an unconventional alignment of a pseudo-spin under the influence of a complex-valued pseudo-magnetic field. These defect states also behave as a chain with two types of links, one rigid in a unit cell and one soft between unit cells, as the defect states become increasingly localized with the gain and loss strength. A realistic photonic design is presented based on coupled InP/InGaAsP waveguides, and we also extend the discussion to two- and three-dimensional lattices.

physics.optics↗

arXiv2018

Non-Hermitian lattices with a flat band and polynomial power increase

In this work we first discuss systematically three general approaches to construct a non-Hermitian flat band, defined by its dispersionless real part. They resort to, respectively, spontaneous restoration of non-Hermitian particle-hole symmetry, a persisting flat band from the underlying Hermitian system, and a compact Wannier function that is an eigenstate of the entire system. For the last approach in particular, we show the simplest lattice structure where it can be applied, and we further identify a special case of such a flat band where every point in the Brillouin zone is an exceptional point of order 3. A localized excitation in this "EP3 flat band" can display either a conserved power, quadratic power increase, or even quartic power increase, depending on whether the localized eigenstate or one of the two generalized eigenvectors is initially excited. Nevertheless, the asymptotic wave function in the long time limit is always given by the eigenstate, and in this case, the compact Wannier function or its superposition in two or more unit cells.

physics.optics↗