Hiroki Nakatsuka

Analysis of optical nonlinearity by defect states in one-dimensional photonic crystals

Enhancement of optical nonlinearity in one-dimensional photonic-crystal structures with a defect is considered theoretically. It is shown that a large enhancement can be obtained for absorption saturation and degenerate four-wave mixing efficiency as a result of large optical field amplitude of the localized photonic-defect mode at the defect layer. The figure of merit of the use of the photonic-crystal structure is derived especially for systems in which the concentration of the nonlinear optical material can be arbitrarily adjusted. Optical bistability is also predicted for optically dense samples. They can be applied in real photonic devices because of their simple structure and the large enhancement obtained.

Enhancement of two-photon excited fluorescence using one-dimensional photonic crystals

We fabricated a one-dimensional photonic crystal structure with a defect layer of a poly(vinyl alcohol) thin film doped with 2-aminopurine (2AP). The defect induced a transmission peak in the photonic band gap at 610 nm, to which ultrashort laser pulses were tuned. We observed enhanced two-photon fluorescence emission from 2AP in the photonic crystal structure with a factor of 120. The enhancement was attributed to the high local field of light generated by a photonic state localized at the defect layer. Furthermore, under the enhanced light intensity, we carried out photobleaching experiments, which gave useful information on the photochemistry of 2AP.

Enhancement of Nonlinear Optical Effect in One-Dimensional Photonic Crystal Structures

We observed the photonic band structure of one-dimensional photonic crystals using a white-light Michelson interferometer. In one-dimensional photonic crystals with a structural defect, there is a localized mode where the field is localized around the position of the defect. If the defect is made of nonlinear optical material, the effective nonlinearity of the photonic crystal is largely enhanced by the enhanced light intensity at the defect. In a one-dimensional photonic crystal with a defect, we observed a large enhancement of degenerate four-wave mixing efficiency. Since the total thickness of the whole structure is only a few micrometers, it was successfully used to form an optical phase-conjugated image.

Time Response of One-Dimensional Photonic Crystals with a Defect Layer Made of Semiconductor Quantum Dots

The effective optical nonlinearity of semiconductor quantum dots can be enhanced in one-dimensional photonic crystal structures. We have fabricated a one-dimensional photonic crystal with a structural defect which is made of CdSe quantum dots. Besides the enhancement of degenerate four-wave mixing efficiency, a fast response of transient population grating in an order of a picosecond was obtained in the one-dimensional photonic crystal. This structure can be used as a future photonic device with high nonlinearity and fast response.

Femtosecond Two-Photon Response Dynamics of Photomultiplier Tubes

An intermediate state of two-photon photoemission processes in a photomultiplier tube was found and relaxation with a time constant of 270 fs was observed by time-correlated measurements using 15 fs, 800 nm optical pulses. Optical Bloch equation analysis of the signal intensity was carried out using a perturbation method.

INTERFEROMETRIC OBSERVATION OF FEMTOSECOND FREE INDUCTION DECAY

Femtosecond free induction decay of cobalt–phthalocyanine in pyridine and of octaethyl porphine in pyridine was observed by use of a white-light interferometer with a resolution time of 6 fs. Also, frequency-domain complex optical constants of the samples were obtained by Fourier-transform analysis of the phase-sensitive interferograms.

Autocorrelation Measurement of Femtosecond Optical Pulses Based on Two-Photon Photoemission in a Photomultiplier Tube

The very sensitive intensity autocorrelation measurement of 15 fs, 800 nm pulses from a Ti:sapphire laser has been achieved using two-photon-induced photocurrent in a photomultiplier tube.

Enhancement of optical nonlinearity in one-dimensional photonic crystals

Local light intensity at a defect in a photonic crystal structure is greatly enhanced from the input light intensity by the existence of the photonic state localized around the defect. Enhancement of effective optical nonlinearity of photonic crystal structures with a defect which has optical nonlinearity was considered theoretically, and studied experimentally. In experiment, intensity dependence of the transmittance was measured, and reduction of the effective saturation intensity by factor of 20 was observed.

Observation of the waveform of accumulated photon echoes in a dye-doped polymer film by use of an interferometer

We used a Michelson-type interferometer to observe the waveform of the accumulated photon echoes in a 3,3′-diethylthiatricarbocyanine iodide–doped poly(vinyl alcohol) film. By Fourier analysis of the measured interferograms we obtained the shapes of the burned population grating and the phase grating in the sample. From the frequency distribution of the dye molecules, that is efficient for persistent spectral hole burning, we estimated the inhomogeneous frequency distribution of the 0–0 transition from S0 to S1 levels of the dye molecules in the poly(vinyl alcohol) film.

Femtosecond Interferometric Waveform Measurement of Photon Echoes Using a Collinear Geometry

A new method for waveform measurement of photon echoes using a modified Michelson interferometer has been developed. The experimental setup uses collinear geometry, and photon echo signals are detected using a double-phase modulation technique. The amplitude and phase of photon echoes and those of excitation pulses are simultaneously measured by this technique. The method has been applied to a photon echo experiment on a dye solution. The results are explained using a stochastic modulation model with an accumulation effect of population grating.