Kazuhiro Ema

All‐optical sub‐Tbits/s serial‐to‐parallel conversion using excitonic giant nonlinearity

An all‐optical serial‐to‐parallel conversion technique, which is the inverse process of the real‐time pulse shaping, is reported. A conversion of 0.14‐Tbits/s signals using excitonic‐giant nonlinearity of ZnSe film is reported. The response of this conversion system is faster than 13 ps with a conversion efficiency of 0.1%.

Raman study on silica optical fibers subjected to high tensile stress

For silica (pure and synthetic) optical fibers, on which tensile stresses from 0 to 4 GPa were imposed, Raman studies were carried out. It has become evident by deconvoluting the silica Raman peaks that the main peak intensity at 440 cm−1 decreases with increasing stress while the peaks at 490, 604, and 800 cm−1 remain practically unchanged. The fact that the 490 cm−1 defect line was not affected by the stress applied to fibers is contrary to the conclusion by G. E. Walrafen, P. N. Krishnan, and S. W. Freiman [J. Appl. Phys. 52, 2832 (1981)]. The results obtained in the present study are rather extensively discussed from a microscopic view of silica glass networks.

Excitonic n-string in linear chains

Abstract The bound states of n Frenkel excitons ( n = 2,3,…), which are called excitonic n-strings, are theoretically shown to exist in contrast with a system of Wannier excitons. The electronic structure and nonlinear optical responses of these excitonic n-strings are clarified. The first evidence for the excitonic 2- and 3-strings is provided from comparison of the calculated and observed differential transmission spectrum of an Anthracene-PMDA crystal pumped by femtosecond laser pulses.

Lasing and Intermode Correlation of Whispering Gallery Mode in Dye-Doped Polystyrene Microsphere

Abstract We study the lasing behavior and photo induced quenching of dye doped polystyrene microspheres with diameter ranging from 10 μ to 92μ. Lasing of the spherical cavity modes is clearly observed. Emission from individual sphere placed on a glass plate is examined under the pumping of nanosecond pulses which are tuned to the absorption band of the dye. Intensity of each lasing line is strictly controlled by additional irradiation of monochromatic laser which can couple to resonance modes of the cavity. The line intensity plummets down when the additional beam frequency matches to the resonances of the modes which have the same radial profile of relevant mode.

Free induction decay and quantum beat of excitons in ZnSe

Abstract We study coherent transient phenomena of excitons using femtosecond time-resolved four-wave-mixing (TR- FWM) in two types of high quality thin films of ZnSe; one is a homo-epitaxial film (1.2 μm thickness) and the other is a very thin (50nm) hetero-epitaxial film on GaAs substrate. Free induction decay (FID) behavior of the third-order polarization is clearly observed. We obtained the same values of exciton dephasing time from the temporal measurements, the decay time of FID, and the frequency domain measurements, analyses of reflection spectra. This implies that the excitons in ZnSe films are homogeneous. In the thin film sample, we observe a beat signal in time-integrated and time-resolved FWM. Based on the perturbational calculation, we conclude that the beat originates from the quantum interference of heavy- and light-hole excitons.

Optical compression using a monochromator and a concave mirror

Abstract An optical pulse compressing technique using a grating monochromator and a concave mirror is demonstrated. This technique uses the quadratic phase correction of the input pulse on the Fourier (spectral) domain. It is shown that a pulse longer than the full interference distance of the grating can be compressed.

Observation of coherent phonon-polariton modes in a bulk ZnSe using a noncollinear OPA

We observed coherent phonon-polariton modes in a bulk ZnSe by means of a transient grating method using a noncollinear optical parametric amplifier. We confirmed the generation mechanism of phonon-polariton and measured the dispersion relationship.

Raman Spectra of SiO2 Fibers at High Tensile Strain

Raman spectra were obtained for SiO2 fibers of 125 μm diameter at tensile strains ranging form 0 to 3.5.%. The spectrum only for the main peak at 440 cm-1 decreased its height with increase in the tensile strain. All the Raman spectra observed were decomposed into their Gaussian components, which are then assigned to normal vibration modes of either SiO4 or Si2O molecules. It is concluded that the stress-induced change in the Raman spectrum near 440 cm-1 is principally due to a change in the tetrahedral angle of the SiO4 molecules constituting silica networks and not to either a Si-0 bond stretching or a Si-0-Si bond angle broadening.