Noriaki Tsurumachi

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.

Wide-field optical coherence tomography: imaging of biological tissues

We describe a two-dimensional optical coherence tomography technique with which we were able to obtain multiple longitudinal slices of a biological sample directly in a single Z scan. The system is based on a femtosecond Cr4+:forsterite laser and an infrared camera for wide-field imaging of the sample with a depth resolution of 5 microm. With this imaging apparatus we were able to investigate human skin and mouse ear samples and to observe the different constitutive tissues.

Phase-stable sub-cycle mid-infrared conical emission from filamentation in gases

Sub-single-cycle pulses in the mid-infrared (MIR) region were generated through a conical emission from a laser-induced filament. Fundamental and second-harmonic pulses of 25-fs Ti:sapphire amplifier output were focused into argon to produce phase-stable broadband MIR pulses in a well-focusable ring-shaped beam. The beam profile and spectrum of the MIR field are accurately reproduced with a simple calculation based on a four-wave mixing process. The ring-shaped pattern of the MIR beam originates from a dramatic confocal-parameter mismatch between the MIR field and the laser beams.

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.

Real-time two-dimensional imaging in scattering media by use of a femtosecond Cr 4+ :forsterite laser

An original femtosecond Cr(4+):forsterite laser source associated with a nonlinear optical correlator was used for imaging through scattering media with 1220-nm light. The system, which operates as an ultrafast optical gate by sum-frequency generation in a nonlinear crystal, was able to detect the light reflected from a resolution chart hidden in a turbid medium, at an attenuation of as much as 15 mean free paths. When the object was illuminated with a collimated beam, real-time two-dimensional images were obtained, with a maximum transverse resolution of ~20 microm.

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.

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.

Observation of ultrashort pulse propagation anisotropy in a semiconductor quantum nanostructure optical waveguide by cross-correlation frequency resolved optical gating spectroscopy

Femtosecond optical pulse propagation in a quantum well (QW) waveguide and a quantum wire (QWR) waveguide was investigated by cross-correlation frequency resolved optical gating (XFROG) spectroscopy. An optical pulse transmitted through the GaAs QW waveguide was found to stretch greatly from 140 fs to almost 1 ps due to nonlinear dispersion around the heavy hole exciton resonance at transverse electric polarization in a near resonant experiment. In contrast, only slight chirping of the pulse transmitted was observed either at transverse magnetic polarization or off resonance for both polarizations. In the GaAs QWR waveguide, the polarization anisotropy of a crescent shaped QWR could also be observed in terms of dispersion by XFROG spectroscopy in spite of the small absorption compared with that in the QW.

Enhancement and suppression of terahertz emission by a Fabry-Perot cavity structure with a nonlinear optical crystal

We have fabricated Fabry-Perot (FP) cavities in the THz region with a ZnTe crystal as a cavity layer by a simple stacking method. We observed more than a three times enhancement of the THz emission intensity in the FP cavities compared with the bare ZnTe crystal at the frequencies of the resonant modes and stopband edges. On the other hand, suppression of the THz emission occurs at frequencies in the stopband. The enhancement and suppression of the THz emission are caused by the modification of the optical density of state in the FP cavities compared to the vacuum.