Masayuki Katsuragawa

Generation of a 10.6-THz ultrahigh-repetition-rate train by synthesizing phase-coherent Raman-sidebands.

A train of highly-stable, high-beam-quality ultrashort pulses is successfully produced by synthesizing phase-coherent rotational-Raman-sidebands in parahydrogen. The-intensity-waveform of this ultrashort-pulse-train is directly evaluated in time domain based on a sum-frequency-generation autocorrelation-technique. It is shown that a 10.6-THz ultrahigh-repetition-train of short pulses is formed with an effective-duration of 20 fs and a high peak-power of 2 MW.

Dual-wavelength injection-locked pulsed laser.

An injection-locked pulsed Ti:sapphire laser oscillating at dual wavelengths is demonstrated for the first time to our knowledge. By use of two feedback loops, seeds of two independent master lasers are locked on specific longitudinal modes of a power oscillator, leading to a stable dual-wavelength oscillation over a long time scale. The two injection-locked pulsed outputs completely overlap in time, with spectral purities reaching a Fourier-transform limit. The dual-wavelength oscillation is controlled by the master lasers only, allowing for flexible selectivity of the two wavelengths and full controllability of the relative two-wavelength pulsed energies.

Frequency comb generation at terahertz frequencies by coherent phonon excitation in silicon

By exciting few-cycle femtosecond laser pulses at 397 nm in near-resonance with the direct bandgap of silicon, researchers experimentally demonstrate coherent phonon generation in silicon at a fundamental frequency of 15.6 THz and all-optical >100 THz frequency comb generation.

SOLID HYDROGEN FOR NONLINEAR OPTICS

We recently developed a new method of preparing a parahydrogen crystal which is suitable for experiments on nonlinear optical processes. The crystal was grown slowly from pressurized liquid–phase in order to avoid internal stress from the thermal constriction, and was cooled down to 4.2 K. The obtained crystal was uncracked and perfectly transparent with a high damage threshold. To evaluate the quality, the vibrational coherence decay was measured using the time–resolved coherent anti–Stokes Raman spectroscopy (CARS) technique. The decay process showed a nonexponential behavior, with an asymptotic limit characterized by a single time constant of 2.6 μs, which corresponds to a linewidth of less than 0.1 MHz. This decay time is extremely slow compared to the previously published work.

The Current Trends in SBS and phase conjugation

The current trends in stimulated Brillouin scattering and optical phase conjugation are overviewed. This report is formed by the selected papers presented in the “Fifth International Workshop on stimulated Brillouin scattering and phase conjugation 2010” in Japan. The nonlinear properties of phase conjugation based on stimulated Brillouin scattering and photo-refraction can compensate phase distortions in the high power laser systems, and they will also open up potentially novel laser technologies, e.g., phase stabilization, beam combination, pulse compression, ultrafast pulse shaping, and arbitrary waveform generation.

Spectral phase measurements for broad Raman sidebands by using spectral interferometry

We report a method of spectral phase measurement for a femtosecond pulse train composed of a discrete Raman spectrum. The method is based on an idea of spectral interference. Making use of a small portion of two Raman pump laser radiations, we produce two sum-frequency spectra that spectrally interfere with each other. Based on this interfering sum-frequency spectra, we successfully derive the spectral phase and reconstruct the temporal profile. The reliability of this method is also confirmed by measuring the spectral phase changes caused by intentionally inserted silica plates of various thicknesses and comparing them to theoretical predictions.

Raman gain measurement in solid parahydrogen

We report a steady-state Raman gain measurement of the Q(1)(0) transition (v = 1 ? 0, J = 0 ? 0) in solid parahydrogen. We carry out measurements by pumping with a continuous-wave frequency-doubled YAG laser at 532 nm and observing the direct amplification of a probe-laser beam for the first Stokes transition at 683 nm. A large single-pass amplification coefficient of 2.3 +/- 0.2 is obtained at a pump intensity of 46 kW/cm(2), with an interaction length of 1 cm, giving a steady-state Raman gain coefficient of 18 +/- 3 cm/MW.

Dual-wavelength injection-locked pulsed laser with highly predictable performance

The characteristics of a dual-wavelength injection-locked pulsed laser are systematically studied. A simple and effective model is proposed to quantitatively study this type of laser system. It is shown that the model precisely predicts the performance of such a system over a wide spectral region and a full dynamic range. Furthermore, the results confirm the accuracy of the assumption regarding the homogeneous broadening in Ti:sapphire lasers, and also prove that competition between the two wavelength components does not induce instability.

Coherent phonon-induced optical modulation in semiconductors at terahertz frequencies

The coherent modulation of electronic and vibrational nonlinearities in atoms and molecular gases by intense few-cycle pulses has been used for high-harmonic generation in the soft x-ray and attosecond regime, as well as for Raman frequency combs that span multiple octaves from the terahertz to petahertz frequency regions. In principle, similar high-order nonlinear processes can be excited efficiently in solids and liquids on account of their high nonlinear polarizability densities. In this paper, we demonstrate the phononic modulation of the optical index of Si and GaAs for excitation and probing near their direct band gaps, respectively at 3.4 and 3.0eV. The large amplitude coherent longitudinal optical (LO) polarization due to the excitation of LO phonons of Si (001) and LO phonon-plasmon coupled modes in GaAs (001) excited by 10fs laser pulses induces effective amplitude and phase modulation of the reflected probe light. The combined action of the amplitude and phase modulation in Si and GaAs generates phonon frequency combs with more than 100 and 60THz bandwidth, respectively.

Stable confinement of nanosecond laser pulse in an enhancement cavity.

We present a technique that enhances the intensity of a nanosecond laser pulse by confining it in an enhancement cavity. The point of the technique is that a weak continuous-wave laser radiation, locked to the enhancement cavity, is injected into a nanosecond injection-locked pulsed laser as a seed. This leads to a stable confinement of the nanosecond pulse in the enhancement cavity. It is demonstrated that the pulsed intensity is enhanced by a factor of 120 for a 40-ns pulse, consistent with the theoretical prediction.