Jung-Wan Ryu

Nonorthogonal pairs of copropagating optical modes in deformed microdisk cavities

Recently, it has been shown that spiral-shaped microdisk cavities support highly nonorthogonal pairs of copropagating modes with a preferred sense of rotation (spatial chirality) [ J. Wiersig et al. Phys. Rev. A 78 053809 (2008)]. Here, we provide numerical evidence which indicates that such pairs are a common feature of deformed microdisk cavities which lack mirror symmetries. In particular, we demonstrate that discontinuities of the cavity boundary such as the notch in the spiral cavity are not needed. We find a quantitative relation between the nonorthogonality and the chirality of the modes which agrees well with the predictions from an effective non-Hermitian Hamiltonian. A comparison to ray-tracing simulations is given.

Analysis of multiple exceptional points related to three interacting eigenmodes in a non-Hermitian Hamiltonian

Department of Physics Education, Pusan National University, Busan 609-735, South Korea(Dated: March 29, 2012)We have investigated the exceptional points (EPs) which are degeneracies of a non-HermitianHamiltonian, in the case that three modes are interacting with each other. Even though the para-metric evolution of the modes cannot be uniquely determined when encircling more than two EPsonce, we can recover the initial configuration of the modes by encircling two EPs three times or threeEPs twice. We confirm our expectation by numerically calculating the modes of an open quantumsystem, two dielectric microdisks, and 3×3 matrix model.

Designing coupled microcavity lasers for high-Q modes with unidirectional light emission

We design coupled optical microcavities and report directional light emission from high-Q modes for a broad range of refractive indices. The system consists of a circular cavity that provides a high-Q mode in the form of a whispering gallery mode, whereas an adjacent deformed microcavity plays the role of a waveguide or collimator of the light transmitted from the circular cavity. As a result of this very simple, yet robust, concept we obtain high-Q modes with promising directional emission characteristics. No information about phase space is required, and the proposed scheme can be easily realized in experiments.

Ray and wave dynamical properties of a spiral-shaped dielectric microcavity

We investigate ray dynamical properties and resonance patterns of a spiral-shaped dielectric microcavity in which quasiscarred resonances can be supported. The ray dynamical properties of this open system can be characterized by the steady probability distribution which contains information of the dynamics and the openness of the chaotic microcavity. It is shown that the quasiscarring phenomenon can be understood by considering the unique properties of wave propagation at the dielectric boundary. The bouncing positions of the quasiscarred resonances are explained through a semiclassical quantization condition with Maslov indices. We also show qualitative agreements between the ray dynamical distributions and the wave dynamical distributions obtained from the average over resonance modes.

Exceptional points in coupled dissipative dynamical systems.

We study the transient behavior in coupled dissipative dynamical systems based on the linear analysis around the steady state. We find that the transient time is minimized at a specific set of system parameters and show that at this parameter set, two eigenvalues and two eigenvectors of the Jacobian matrix coalesce at the same time; this degenerate point is called the exceptional point. For the case of coupled limit-cycle oscillators, we investigate the transient behavior into the amplitude death state, and clarify that the exceptional point is associated with a critical point of frequency locking, as well as the transition of the envelope oscillation.

Resonances in a circular dielectric cavity

Abstract We study resonance distributions in a circular dielectric cavity. It is shown that the decay-rate distribution has a peak structure and the details of the peak are consistent with the classical survival probability time distribution. We also investigate the behavior of the complex resonance positions at the small opening limit ( n → ∞ , n is the refractive index of the cavity). At the large n limit, the real part of complex resonance positions approaches the solutions with different m of Dirichlet problem with a scale n −2 and the imaginary part goes zero as n − 2 m for TM and n − 2 ( m + 1 ) for TE polarization, where m is the order of the resonance.

Quantum localization in open chaotic systems

We study a quasibound state of a delta -kicked rotor with absorbing boundaries focusing on the nature of the dynamical localization in open quantum systems. The localization lengths xi of lossy quasibound states located near the absorbing boundaries decrease as they approach the boundary while the corresponding decay rates Gamma are dramatically enhanced. We find the relation xi approximately Gamma(-1/2) and explain it based upon the finite time diffusion, which can also be applied to a random unitary operator model. We conjecture that this idea is valid for the system exhibiting both the diffusion in classical dynamics and the exponential localization in quantum mechanics.

Rotationally reconfigurable metamaterials based on moiré phenomenon

Exploiting moiré interference, we make a new type of reconfigurable metamaterials and study their transmission tunability for incident electromagnetic waves. The moiré pattern is formed by overlapping two transparent layers, each of which has a periodic metallic pattern, and the cluster size of the resulting moiré pattern can be varied by changing the relative superposition angle of the two layers. In our reconfigurable metamaterials, both the size and structural shape of the unit cell can be varied simultaneously through moiré interference. We show that the transmission of electromagnetic waves can be controlled from 90% to 10% at 11 GHz by experiments and numerical simulation. The reconfigurable metamaterial proposed here can be applied in bandpass filters and tunable modulation devices.

Terahertz beat frequency generation from two-mode lasing operation of coupled microdisk laser

We propose a coupled microdisk laser as a compact and tunable laser source for the generation of a coherent continuous-wave terahertz radiation by photomixing. Using the Schrödinger-Bloch model including the nonlinear effect of active medium, we find single-mode and two-mode lasings depending on the pumping strength. We explain the transitions of lasing modes in terms of resonant modes that are the solutions of the Schrödinger-Bloch model without active medium and nonlinear interaction. In particular, a two-mode lasing is shown to generate a terahertz oscillating frequency originating from the light beating of two nearly degenerated resonant modes with different symmetries.

Quasiscarred modes and their branching behavior at an exceptional point

We study quasiscarring phenomena and mode branching at an exceptional point (EP) in typically deformed microcavities. It is shown that quasiscarred (QS) modes are dominant in some mode group and their pattern can be understood by short-time ray dynamics near the critical line. As cavity deformation increases, high-Q and low-Q QS modes are branching in opposite ways, at an EP, into two robust mode types showing QS and diamond patterns, respectively. Similar branching behavior can also be found at another EP appearing at a higher deformation. This branching behavior of QS modes has its origin in the fact that an EP is a square-root branch point.