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Muhan Choi

Publications and source records attributed to Muhan Choi.

17 recordsLinked to original sources
arXiv2026

Localization in microcavities revealed by phase-space non-Hermitian skin effect

Contrary to the semiclassical expectation for fully chaotic systems, localization of resonances is found to be a common feature in open microcavities. In spiral-shaped dielectric microcavities, a substantial fraction of resonances localize on polygonal patterns in real space, are chiral, and their momentum distributions accumulate near the critical line for total internal reflection. Despite the extensive investigation, the physical mechanism responsible for their remarkable abundance has remained a long-standing question. Addressing this, we reveal a physical correspondence between an inhomogeneous-loss Hatano-Nelson model and the dielectric phase space of a spiral microcavity. We show that the combination of geometry-induced momentum drift and refractive escape yields a generalized non-Hermitian skin effect in the phase space momentum. We identify this mechanism as the origin of the critical-line localization of resonances in open chaotic spiral microcavities, extending the skin-effect concept beyond nonreciprocal lattices to phase space and to open chaotic wave systems.

physics.optics↗
arXiv2024

Topological Switching via Exceptional Point Pairs in an Optical Microcavity Laser

Conventional mode switching mechanisms, which rely on dynamically encircling exceptional points (EPs) through non-adiabatic transitions (NATs), suffer from intrinsic nonlinear dynamics that hinder precise control and reproducibility in experimental settings. Additionally, these methods exhibit low transmission efficiencies due to path-dependent attenuation, limiting their effectiveness in optical switching and logic gate applications. To overcome these limitations, we propose a novel mode switching approach that leverages a pair of EPs configuration in an optical microcavity, characterized by superradiance and subradiance. This mechanism exploits the topological structure of the Riemann surface to enable robust mode switching control and tunable Q-factor through purely adiabatic encircling. Furthermore, topological protection validated via braid isotopy ensures robustness against noise and parametric perturbations, facilitating a compact, robust, and adaptive non-Hermitian system.

physics.optics↗
arXiv2020

Circle-shaped Transformation Cavity via Center-Shift Möbius Transfomation

Circle-shaped transformation cavities of shifted center are the most simple transformation cavities in the sense that they have only spatial index variation without boundary shape deformation. For these cavities, we analyze the characteristics of conformal whispering gallery modes according to center-shift coordinate transformation. For this study, we use an alternative scheme for designing a transformation cavity without using a shrinking parameter that is needed to satisfy total internal reflection condition. It is observed that the isotropic emission of whispering gallery modes in the uniform index circular cavity changes to the bi-directional emission of conformal whispering gallery modes as the center-shift increases. Also, using a purity factor that quantitatively measures in RV space the degree of intactness for the internal wave pattern of resonant modes in transformation cavities compared with corresponding whispering gallery mode in uniform disk cavity, we show that the conformal whispering gallery modes in the circle-shaped transformation cavity has not only directionality but also high persistence for characteristics of whispering gallery modes.

physics.optics↗
arXiv2019

Universal light-guiding geometry for high-nonlinear resonators having molecular-scale roughness

By providing an effective way to leverage nonlinear phenomena in chip-scale, high-Q optical resonators have induced the recent advances of on-chip photonics represented by micro-combs and ultra-narrow linewidth lasers. These achievements mainly relying on Si, SiO$_{2}$, and Si$_{3}$N$_{4}$ are expected to be further improved by introducing new materials having higher nonlinearity. However, establishing fabrication processes to shape a new material into the resonator geometries having extremely smooth surfaces on a chip has been a challenging task. Here we describe a universal method to implement high-Q resonators with any materials which can be deposited in high vacuum. This approach, by which light-guiding cores having surface roughness in molecular-scale is automatically defined along the prepatterned platform structures during the deposition, is verified with As$_{2}$S$_{3}$, a typical chalcogenide glass of high-nonlinearity. The Q-factor of the developed resonator is 14.4 million approaching the loss of chalcogenide fibers, which is measured in newly proposed tunable waveguide-to-resonator coupling scheme with high ideality. Lasing by stimulated Brillouin process is demonstrated with threshold power of 0.53 mW which is 100 times lower than the previous record based on chalcogenide glasses. This approach paves the way for bringing various materials of distinguished virtues to the on-chip domain while keeping the loss performance comparable to that of bulk form.

physics.optics↗
arXiv2019

Boundary integral equation method for resonances in gradient index cavities designed by conformal transformation optics

In the case of two-dimensional gradient index cavities designed by the conformal transformation optics, we propose a boundary integral equation method for the calculation of resonant mode functions by employing a fictitious space which is reciprocally equivalent to the physical space. Using the Green's function of the interior region of the uniform index cavity in the fictitious space, resonant mode functions and their far-field distributions in the physical space can be obtained. As a verification, resonant modes in limaçon-shaped transformation cavities were calculated and mode patterns and far-field intensity distributions were compared with those of the same modes obtained from the finite element method.

physics.optics↗
arXiv2019

Ultrathin, polarization-independent, and focus-tunable liquid crystal diffractive lens for augmented reality

Despite the recent advances in augmented reality (AR), which has shown the potential to significantly impact on our daily lives by offering a new way to manipulate and interact with virtual information, minimizing visual discomfort due to the vergence-accommodation conflict remains a challenge. Emerging AR technologies often exploit focus-tunable optics to address this problem. Although they demonstrated improved depth perception by enabling proper focus cues, a bulky form factor of focus-tunable optics prevents their use in the form of a pair of eyeglasses. Herein, we describe an ultrathin, focus-tunable liquid crystal (LC) diffractive lens with a large aperture, a low weight, and a low operating voltage. In addition, we show that the polarization dependence of the lens, which is an inherent optical property of LC lenses, can be eliminated using birefringent thin films as substrates and by aligning the optical axes of the birefringent substrates and LC at a specific angle. The polarization independence eliminates the need for a polarizer, thus further reducing the form factor of the optical system. Next, we demonstrate a prototype of AR glasses with addressable focal planes using the ultrathin lens. The prototype AR glasses can adjust the accommodation distance of the virtual image, mitigating the vergence-accommodation conflict without substantially compromising the form factor or image quality. This research on ultrathin lens technology shows promising potential for developing compact optical displays in various applications.

physics.optics↗
arXiv2019

Optimization of conformal whispering gallery modes in limaçon-shaped transformation cavities

In limaçon-shaped gradient index dielectric cavities designed by conformal transformation optics, the variation of Q-factors and emission directionality of resonant modes was traced in their system parameter space. For these cavities, their boundary shapes and refractive index profiles are determined in each case by a chosen conformal mapping which is taken as a coordinate transformation. Through the numerical exploration, we found that bidirectionality factors of generic high-Q resonant modes are not directly proportional to their Q-factors. The optimal system parameters for the coexistence of strong bidirectionality and a high Q-factor was obtained for anisotropic whispering gallery modes supported by total internal reflection.

physics.optics↗
arXiv2014

Abnormal high-$Q$ modes of coupled stadium-shaped microcavities

It is well known that the strongly deformed microcavity with fully chaotic ray dynamics cannot support high-Q modes due to its fast chaotic diffusion to the critical line of refractive emission. Here, we investigate how the Q factor is modified when two chaotic cavities are coupled, and show that some modes, whose Q factor is about 10 times higher than that of the corresponding single cavity, can exist. These abnormal high-Q modes are the result of an optimal combination of coupling and cavity geometry. As an example, in the coupled stadium-shaped microcavities, the mode pattern extends over both cavities such that it follows a whispering-gallery-type mode at both ends, whereas a big coupling spot forms at the closest contact of the two microcavities. The pattern of such a 'rounded bow tie' mode allows the mode to have a high-Q factor. This mode pattern minimizes the leakage of light at both ends of the microcavities as the pattern at both ends is similar to whispering gallery mode.

physics.optics↗
arXiv2014

Quantum Goos-Hänchen shift and tunneling transmission at a curved step potential

We study the quantum Goos-Hänchen (GH) shift and the tunneling transmission at a curved step potential by investigating the time evolution of a wave packet. An initial wave packet is expanded in terms of the eigenmodes of a circular step potential. Its time evolution is then given by the interference of their simple eigenmode oscillations. We show that the GH shift along the step boundary can be explained by the energy-dependent phase loss upon reflection, which is defined by modifying the one-dimensional (1D) effective potential derived from the 2D circular system. We also demonstrate that the tunneling transmission of the wave packet is characterized by a free-space image distant from the boundary. The tunneling transmission exhibits a rather wide angle divergence and the direction of maximum tunneling is slightly rotated from the tangent at the incident point, which is consistent with the time delay of the tunneling wave packet computed in the 1D modified effective potential.

quant-ph↗
arXiv2012

Switching teraherz waves with gate-controlled active graphene metamaterials

The extraordinary electronic properties of graphene, such as its continuously gate-variable ambipolar field effect and the resulting steep change in resistivity, provided the main thrusts for the rapid advance of graphene electronics. The gate-controllable electronic properties of graphene provide a route to efficiently manipulate the interaction of low-energy photons with massless Dirac fermions, which has recently sparked keen interest in graphene plasmonics. However, the electro-optic tuning capability of unpatterned graphene alone is still not strong enough for practical optoelectronic applications due to its nonresonant Drude-like behaviour. Here, we experimentally demonstrate that substantial gate-induced persistent switching and linear modulation of terahertz waves can be achieved in a two-dimensional artificial material, referred to as a metamaterial, into which an atomically thin, gated two-dimensional graphene layer is integrated. The gate-controllable light-matter interaction in the graphene layer can be greatly enhanced by the strong resonances and the corresponding field enhancement in the metamaterial. Although the thickness of the embedded single-layer graphene is more than 'six' orders of magnitude smaller than the wavelength (< λ/1,000,000), the one-atom-thick layer, in conjunction with the metamaterial, can modulate both the amplitude of the transmitted wave by up to 90 per cent and its phase by more than 40 degrees at room temperature. More interestingly, the gate-controlled active graphene metamaterials show hysteretic behaviour in the transmission of terahertz waves, especially when fabricated with multilayer graphene, which is indicative of persistent photonic memory effects.

physics.optics↗
arXiv2008

Dependence of far-field characteristics on the number of lasing modes in stadium-shaped InGaAsP microlasers

We study spectral and far-field characteristics of lasing emission from stadium-shaped semiconductor (InGaAsP) microlasers. We demonstrate that the correspondence between a lasing far-field emission pattern and the result of a ray simulation becomes better as the number of lasing modes increases. This phenomenon is reproduced in the wave calculation of the cavity modes.

physics.optics↗
arXiv2005

Chaos Synchronization of delayed systems in the presence of delay time modulation

We investigate synchronization in the presence of delay time modulation for application to communication. We have observed that the robust synchronization is established by a common delay signal and its threshold is presented using Lyapunov exponents analysis. The influence of the delay time modulation in chaotic oscillators is also discussed.

nlin.CD↗
arXiv2005

Hiding message in Delay Time: Encryption with Synchronized time-delayed systems

We propose a new communication scheme that uses time-delayed chaotic systems with delay time modulation. In this method, the transmitter encodes a message as an additional modulation of the delay timeand then the receiver decodes the message by tracking the delay time.We demonstrate our communication scheme in a system of coupled logistic maps.Also we discuss the error of the transferred message due to an external noiseand present its correction method.

nlin.CD↗
arXiv2004

Quasi-Scarred Resonances in a Spiral-Shaped Microcavity

We study resonance patterns of a spiral-shaped dielectric microcavity with chaotic ray dynamics. Many resonance patterns of this microcavity, with refractive indices $n=2$ and 3, exhibit strong localization of simple geometric shape, and we call them {\em quasi-scarred resonances} in the sense that there is, unlike the conventional scarring, no underlying periodic orbits. It is shown that the formation of quasi-scarred pattern can be understood in ter ms of ray dynamical probability distributions and wave properties like uncertainty and interference.

nlin.CD↗
arXiv2004

Synchronization of Chaotic Oscillators due to Common Delay Time Modulation

We have found a synchronization behavior between two identical chaotic systems^M when their delay times are modulated by a common irregular signal. ^M This phenomenon is demonstrated both in two identical chaotic maps whose delay times are driven by a common^M chaotic or random signal and in two identical chaotic oscillators whose delay times are driven by^M a signal of another chaotic oscillator. We analyze the phenomenon by using^M the Lyapunov exponents and discuss it in relation with generalized synchronization.^M

nlin.CD↗
arXiv2004

Characteristics of a Delayed System with Time-dependent Delay Time

The characteristics of a time-delayed system with time-dependent delay time is investigated. We demonstrate the nonlinearity characteristics of the time-delayed system are significantly changed depending on the properties of time-dependent delay time and especially that the reconstructed phase trajectory of the system is not collapsed into simple manifold, differently from the delayed system with fixed delay time. We discuss the possibility of a phase space reconstruction and its applications.

nlin.CD↗