Chang-Hwan Yi

Lasing characteristics of a Limaçon-shaped microcavity laser

We investigate lasing characteristics of a Limacon-shaped InGaAsP semiconductor microcavity laser. When the cavity is pumped with dc current, the laser generates directional emissions. Near the threshold a series of modes with equidistantly spaced resonant frequencies emit. From the mode spacing we confirm the emission of whispering gallery-type resonance modes. As the injection current increases, a single resonance mode becomes dominant with directional emission. The quality factor of the single mode is obtained near the threshold, which is larger than 22 000.

Directional single mode emission in a microcavity laser

We report directional single mode emission in an InGaAsP semiconductor microcavity laser, which is composed of a circle and an isosceles trapezoid. When exciting a whole cavity, the laser generates a single mode without hopping in two directions over a wide range of continuous injection currents. In the emission spectrum, it is confirmed from the equidistant mode spacing that a scar mode becomes a single lasing mode above the lasing threshold. A numerical analysis of the boundary element method shows that the far-field pattern of the resonance agrees well with that of the experiment.

Directional and low-divergence emission in a rounded half-moon shaped microcavity

Emission characteristics of a rounded half-moon shaped InGaAsP semiconductor microcavity laser are studied. When excited by pulse current, the laser generates whispering gallery type modes (WGTMs) with equidistant mode spacing in two directions with a narrow divergence angle of 4°. It is confirmed from the mode spacing that the lasing modes are WGTMs. A numerical analysis by the boundary element method shows that the emission direction of resonances well coincides with the experimental result.

Resonant torus-assisted tunneling.

We report a new type of dynamical tunneling, which is mediated by a resonant torus, i.e., a nonisolated periodic orbit. To elucidate the phenomenon, we take an open elliptic cavity and show that a pair of resonances localized on two classically disconnected tori tunnel through a resonant torus when they interact with each other. This so-called resonant torus-assisted tunneling is verified by using Husimi functions, corresponding actions, Husimi function distributions, and the standard deviations of the actions.

Chaos-assisted tunneling in a deformed microcavity laser

We investigate the impact of local dynamics on chaos-assisted tunneling in a highly deformed microcavity whose classical ray dynamics exhibits a small measure of trapezoidal-shaped orbit (TSO) stability islands in a main chaotic sea. These two classically completely decomposed regions in phase space can support resonance modes of their own respectively. Using numerical ray and wave analyses, we show that the emission characteristics of the TSO resonance mode are determined by local ray dynamics near TSO islands. The emission characteristics of the other high-Q resonance modes, on the other hand, are governed by usual ray-wave correspondence. We experimentally demonstrate that the TSO emission mode can be lased without selective excitations by devising a half-moon shaped highly deformed cavity. And we also show that the emission characteristics of the TSO lasing modes are well explained by numerical ray and wave analyses.

Light emission of a scarlike mode with assistance of quasiperiodicity

In an elliptic In{sub x}Ga{sub 1-x}AsP microcavity laser, various scarlike modes are experimentally observed. Below the lasing threshold, a bouncing-ball, a triangle, a double, and a triple bow-tie mode spontaneously emit. Above the threshold, a bow-tie scarlike mode lases alone. Our numerical analysis reveals that the bow-tie scarlike mode is not caused by regular islands in phase space due to the Goos-Haenchen shift, but by unstable periodic orbits and that the light emission is assisted by a quasiperiodic orbit.

Analysis of broad emission direction in a spiral-shaped microcavity laser

Broad emission direction with two lobes is analyzed in a spiral-shaped microcavity laser. By pumping the boundary of the InGaAsP semiconductor laser with dc current injection, whispering-gallery-type modes emitting through a notch are obtained. By classifying the lasing modes into mode groups based on the equidistant mode spacing, it is shown that each mode group has its own emission direction with a narrow divergence angle and that a superposition of emission direction of dominant mode groups reproduces a far-field pattern of total intensity with two lobes, which is contrary to theoretical results.

Avoided level crossings in an elliptic billiard

In an elliptic billiard, we find avoided level crossings taking place over wide ranges, which are of a Demkov type for generations of eigenfunctions localized on an islands chain and its pair unstable periodic orbit. For a proof of the existence of avoided level crossings, first, we show that the quantized eigenvalue of the unstable periodic orbit, obtained by the Einstein-Brillouin-Keller quantization rule, passes the eigenvalues of bouncing-ball modes localized on the unstable periodic orbit after Demkov type avoided level crossings so that pairs of bouncing-ball modes are sequentially generated. Next, by using a perturbed Hamiltonian, we show that off-diagonal elements in Hamiltonian are nonzero, which give rise to an interaction between two eigenfunctions. Last, we verify that the observed phenomenon is Fermi resonance: that is, the quantum number difference of two normal modes equals the periodic orbits, where eigenfunctions are localized after an avoided level crossing.

Chirality of a resonance in the absence of backscatterings.

Chirality of a resonance localized on an islands chain is studied in a deformed Reuleaux triangular-shaped microcavity, where clockwise and counter clockwise traveling rays are classically separated. A resonance localized on a period-5 islands chain exhibits chiral emission due to the asymmetric cavity shape. Chirality is experimentally proved in a InGaAsP multi-quantum-well semiconductor laser by showing that the experimental emission characteristics well coincide with the wave dynamical ones.

Mechanism of Q-spoiling in deformed optical microcavities

It was reported that Q spoiling in a chaotic microcavity is caused by chaos [PRL, 75, 2682 (1995)] and chaos-assisted tunneling [Nature 385, 45 (1997)]. However, even when a cavity is slightly deformed not to exhibit a broad chaotic region in phase space, high Q modes are spoiled. We find that Q spoiling in this region is caused by the transition of a whispering gallery mode (WGM) to a scarred resonance when a WGM interacts with its pair quasi-normal mode through an avoided resonance crossing. We prove that this transition induces Q spoiling in a quadrupole dielectric microcavity by showing that Q factors obtained from the Husimi functions depending on resonance deformation during the transition agree well with those obtained from the complex eigenvalues.