Younghoon Shin

Nonlinear resonance-assisted tunneling induced by microcavity deformation

Noncircular two-dimensional microcavities support directional output and strong confinement of light, making them suitable for various photonics applications. It is now of primary interest to control the interactions among the cavity modes since novel functionality and enhanced light-matter coupling can be realized through intermode interactions. However, the interaction Hamiltonian induced by cavity deformation is basically unknown, limiting practical utilization of intermode interactions. Here we present the first experimental observation of resonance-assisted tunneling in a deformed two-dimensional microcavity. It is this tunneling mechanism that induces strong inter-mode interactions in mixed phase space as their strength can be directly obtained from a separatrix area in the phase space of intracavity ray dynamics. A selection rule for strong interactions is also found in terms of angular quantum numbers. Our findings, applicable to other physical systems in mixed phase space, make the interaction control more accessible.We report the first experimental observation of the resonance-assisted dynamical tunneling (RADT) in the inter-mode interaction in an asymmetric-deformed microcavity. A selection rule for strong inter-mode coupling induced by RADT was observed on angular mode numbers as predicted by the RADT theory. In addition, the coupling strength was measured to be proportional to the square of the phase-space area associated with the nonlinear resonance involved in RADT. The proportionality constant was found to depend only on the nonlinear resonance, supporting the semiclassical nature of RADT.

Observation of an exceptional point in a two-dimensional ultrasonic cavity of concentric circular shells

We report observation of an exceptional point in circular shell ultrasonic cavities in both theory and experiment. In our theoretical analysis we first observe two interacting mode groups, fluid- and solid-based modes, in the acoustic cavities and then show the existence of an EP of these mode groups exhibiting a branch-point topological structure of eigenfrequencies around the EP. We then confirm the mode patterns as well as eigenfrequency structure around the EP in experiments employing the schlieren method, thereby demonstrating utility of ultrasound cavities as experimental platform for investigating non-Hermitian physics.

Experimental Observation of Bohr’s Nonlinear Fluidic Surface Oscillation

Niels Bohr in the early stage of his career developed a nonlinear theory of fluidic surface oscillation in order to study surface tension of liquids. His theory includes the nonlinear interaction between multipolar surface oscillation modes, surpassing the linear theory of Rayleigh and Lamb. It predicts a specific normalized magnitude of 0.416η2 for an octapolar component, nonlinearly induced by a quadrupolar one with a magnitude of η much less than unity. No experimental confirmation on this prediction has been reported. Nonetheless, accurate determination of multipolar components is important as in optical fiber spinning, film blowing and recently in optofluidic microcavities for ray and wave chaos studies and photonics applications. Here, we report experimental verification of his theory. By using optical forward diffraction, we measured the cross-sectional boundary profiles at extreme positions of a surface-oscillating liquid column ejected from a deformed microscopic orifice. We obtained a coefficient of 0.42 ± 0.08 consistently under various experimental conditions. We also measured the resonance mode spectrum of a two-dimensional cavity formed by the cross-sectional segment of the liquid jet. The observed spectra agree well with wave calculations assuming a coefficient of 0.414 ± 0.011. Our measurements establish the first experimental observation of Bohr’s hydrodynamic theory.

Unperturbed-basis mode interactions via resonance chains in a deformed microcavity

Summary form only given. We investigated the interactions among unperturbed-basis modes in an asymmetrical-deformed optical microcavity. We found that the coupling strength is enhanced when the angular mode number difference of interacting modes equals an integer multiple of the number of the chain islands. We could explain this phenomenon by applying the theory of resonance-assisted tunneling (RAT) to our microcavity system.

Mode interaction in a circular shell ultrasonic cavity

Summary form only given. We have studied circular shell ultrasonic cavities immersed in water in both theory and experiment. Calculations show that there exist two types of modes (mainly residing in the inner cavity and in the shell, respectively) and they interact with each other via the coupling across the inner boundary. We have experimentally confirmed this by using the schlieren method.

Reconstruction of tunnelling emission pattern by using Husimi function

Summary form only given. Output directionality of asymmetric optical microcavities has been widely studied revealing many interesting features. Especially the tunneling emission is now attracting many interests due to the unexpected features in the semiclassical model. We found that the tunneling emission pattern in the far-field region can be reconstructed from the outside-outgoing Husimi function corresponding to the critical line in the phase space. Using this Husimi-reconstruction model the tunnelling emission phenomenon can be studied even in the high quality factor regime in which both experimental and theoretical difficulties arise.

Resonance mode calculations in an acoustic cavity using the boundary element method

We have performed boundary-element-method (BEM) simulations in a ultrasonic cavity. To confirm the accuracy of our BEM calculation, the BEM results obtained for a circular cavity were compared with the analytic solutions for the same cavity. In the simulation results, we found that the imaginary part of the resonance frequency shows different aspects from that of a corresponding optical microcavity.

Observation of scar modes in a deformed ultrasonic cavity

We employ an ultrasonic acoustic cavity in which the pressure field satisfies the same form of Helmholtz equation as the optical field in an optical microcavity of much interest, where many interesting features of mode distributions are expected but difficult to observe directly. Mode patterns in an acoustic cavity are measured with the Schlieren method, not perturbing the mode distributions at all. In addition, we have performed numerical simulations based on the boundary element method and compared the results with the experiment.

Visualization of quasi-resonance modes in a deformed ultrasonic cavity

The spatial mode patterns in an optical microcavity reveal many interesting feature. However, it is not possible to measure these mode patterns directly in most cases. To supplement this limitation, we employ acoustic cavities equivalent to the optical cavities of interest. The pressure field distribution there can be measured using the Schlieren method. We observed chaotic and scar modes in a center-displaced annular cavity.