Kyungwon An

Observation of an Exceptional Point in a Chaotic Optical Microcavity

We present spectroscopic observation of an exceptional point or the transition point between mode crossing and avoided mode crossing of neighboring quasieigenmodes in a chaotic optical microcavity of a large size parameter. The transition to the avoided mode crossing was impeded until the degree of deformation exceeded a threshold deformation owing to the systems openness also enhanced by the shape deformation. As a result, a singular topology was observed around the exceptional point on the eigenfrequency surfaces, resulting in fundamental inconsistency in mode labeling.

Observation of scarred modes in asymmetrically deformed microcylinder lasers.

We report observation of lasing in the scarred modes in an asymmetrically deformed microcavity made of liquid jet. The observed scarred modes correspond to morphology-dependent resonance of radial mode order 3 with their Q values in the range of 10(6). Emission directionality is also observed, corresponding to a hexagonal unstable periodic orbit.

Cavity ring-down technique and its application to the measurement of ultraslow velocities

We have developed a new ring-down technique that does not require a shutter to turn a probe laser on and off. With a rapid cavity scan we can measure a simple exponential cavity decay from which a cavity finesse can be found. When the cavity is scanned slowly, the cavity decay exhibits an amplitude modulation, and an analytic expression is derived for this modulation. With this new technique we measured the ultraslow relative velocity of the mirrors (of the order of micrometers per second) as well as the linewidth (~100 kHz) of the probe laser.

Ultraviolet laser induced fluorescence of human aorta

Abstract Laser induced fluorescence from normal human aorta is studied with u.v. excitations of 305 to 310 nm, observing emission from 320 to 500 nm. In this region LIF lineshapes are strongly dependent on the excitation wavelength, suggesting that at least two fluorophores are being observed. The short wavelength fluorophore, peaking at 34Onm, is identified as tryptophan, while the longer wavelength fluorophore, peaking at 387 nm, is associated with collagen and elastin. In addition, fluorescence time decays of each component are measured with a time correlated photon counting system. A four-exponential fit of each decay is necessary to extract fluorescence lifetimes, which range from 33 ps to 8.6 ns.

Cavity-Q-driven spectral shift in a cylindrical whispering-gallery-mode microcavity laser

Cavity-Q-driven spectral shift of lasing was observed in a cylindrical microcavity formed by rhodamine6G-doped quinoline in a capillary. The envelope of lasing spectrum showed a blueshift induced by the decreasing cavity Q of whispering gallery modes as the pump fluence increases. The thermally induced refractive index changes were measured from the shifts of individual lasing modes. The observed cavity-Q-driven spectral shift was well described by a simple dye laser model, which accounts for the dependence of cavity Q on the thermally induced refractive index change.

Waveguide mode lasing via evanescent-wave-coupled gain from a thin cylindrical shell resonator

We reported waveguide mode lasing in a thin dielectric cylindrical shell made on the inner wall of a circular capillary, in which the laser oscillation was achieved through the evanescent-wave-coupled gain from the interior dye-doped liquid. Periodically peaked spectra of waveguide modes showed blueshift when the refractive index of interior liquid was decreased. We could quantitatively estimate the occupation factor of inner evanescent-wave portion from the dependence of blueshift, which allowed the spectral analysis on the effective cavity quality factor Q. Typical Q of the lasing waveguide modes was about 2×10 6 .

Interferential coupling effect on the whispering-gallery mode lasing in a double-layered microcylinder

Enhanced mode selection through a strong interferential coupling effect was observed in a double-layered cylindrical microcavity laser. The coupling was induced by partial reflection and recombination with refracted light on the internal circular boundary of the fused silica capillary tube, filled with a dye-doped liquid having lower reflective index than that of fused silica. The observed interference period agreed well with the prediction by a ray-optic model.

Spatial coherence of forward-scattered light in a turbid medium.

We study spatially coherent forward-scattered light propagating in a turbid medium of moderate optical depth (0-9 mean free paths). Coherent detection was achieved by using a tilted heterodyne geometry, which desensitizes coherent detection of the attenuated incident light. We show that the degree of spatial coherence is significantly higher for light scattered only once in comparison with that for multiply scattered light and that it approaches a small constant value for large numbers of scattering events.

Optical bistability induced by mirror absorption: measurement of absorption coefficients at the sub-ppm level

We have observed optical bistability caused by absorption-induced thermal expansion of mirrors forming a Fabry-Perot interferometer. From the resulting anomalous transmission line shapes, absorption coefficients of the mirror coatings as low as 0.2parts in 10(6) (ppm) have been successfully measured.

Development of a deformation-tunable quadrupolar microcavity

We have developed a technique for realizing a two-dimensional quadrupolar microcavity with its deformation variable from 0% to 20% continuously. We employed a microjet ejected from a noncircular orifice in order to generate a stationary column with modulated quadrupolar deformation in its cross section. Wavelength redshifts of low-order cavity modes due to shape deformation were measured and were found to be in good agreement with the wave calculation for the same deformation, indicating that the observed deformation is quadrupolar in nature.