Yu Oishi

Generation of extreme ultraviolet continuum radiation driven by a sub-10-fs two-color field

We have proposed and demonstrated a novel approach for generating high-energy extreme-ultraviolet (XUV) continuum radiation. When a two-color laser field consisting of a sub-10-fs fundamental and its parallel-polarized second harmonic was applied to high-order harmonic generation in argon, a continuum spectrum centered at 30 nm was successfully obtained with an energy as high as 10 nJ. This broadband emission indicates the possibility of generating intense single attosecond pulses in the XUV region.

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.

Phase locking in a Nd:YVO 4 waveguide laser array using Talbot cavity

We demonstrated phase-locking in a laser-diode-array-pumped Nd:YVO₄ laser array (15 emitters) using a Talbot cavity. The Nd:YVO₄ slab crystal was coated by dielectric material for claddings and formed a planar waveguide for the vertical mode. To stabilize the horizontal array mode, periodical thermal lenses were generated by controlling the heat flow. The phase-locked waveguide array generated 1.65-W output power, while 2.02 W was available in a standard cavity. Two-peak supermode was demonstrated with the Talbot cavity and was converted to a single peak with a spatial light modulator. We also experimentally and numerically analyzed the characteristics of Talbot phase-locking.

Temporal control of local plasmon distribution on Au nanocrosses by ultra-broadband femtosecond laser pulses and its application for selective two-photon excitation of multiple fluorophores

We theoretically demonstrate spatiotemporal control of local plasmon distribution on Au nanocrosses, which have different aspect ratios, by chirped ultra-broadband femtosecond laser pulses. We also demonstrate selective excitation of fluorescence proteins using this spatiotemporal local plasmon control technique for applications to two-photon excited fluorescence microscopy.

Sub-10 fs, multi-mJ Ti:sapphire laser system with a pressure-gradient hollow fiber

The delivery of sub-10 fs multi-mJ pulses from a 1-kHz repetition-rate Ti:sapphire chirped-pulse amplification system with a pressure-gradient hollow fiber is reported. Anovel technique of hollow fiber pulse compression allows the generation of short pulses of less than 10 fs in the multi-mJ energy level.

Coherent Superposition of 800 and 400-nm Spectral Components in Supercontinuum Pulse Generated in Ar-Gas-Filled Hollow Core Fiber

800 and 400 nm broadband components in a supercontinuum pulse generated by phase modulation based on copropagation of fundamental and second-harmonic femtosecond pulses in an Ar-gas-filled hollow core fiber were separately compressed and coherently superposed.

Characterization and control of femtosecond localized plasmon using spectral interferometry with SNOM or fringe-resolved autocorrelation with dark-field microscopy

We apply two nova methods, a spectral interferometry with NSOM, and fringe-resolved-autocorrelation with dark-field microscopy in the spatio-temporal characterization of femtosecond localized plasmon at metal nanostructures. Spatio-temporal plasmon control is studied using these diagnostics.

Thermal-Birefringence-Induced Local Depolarization in Thin YAG Ceramic

The thermal-birefringence-induced spatially-distributed local depolarization in thin Nd:YAG ceramics were investigated. We found the variation of local depolarization was increased one order of magnitude when the thickness of ceramics was reduced near the grain size.

Continuum Harmonic Radiation in the Extreme Ultraviolet Region Using Synthesized Sub-10-fs Two-Color Field

When the laser field consisting of sub-10-fs fundamental and its second harmonic was used for high harmonic generation, a continuum centered at 30 nm with a spectral bandwidth of 8 nm was obtained with an energy of 10 nJ.

Adaptive Control of Two-Photon Excitation of Green Fluorescent Protein with Shaped Femtosecond Pulses

For many years, it has been believed that a Fourier-transform-limited (FTL) laser pulse is the most effective light source for the generation of nonlinear phenomena, since the FTL pulse has the shortest pulse duration, that is, the highest intensity, that can be limited by the spectral width due to the principle of uncertainty. Recently, many reports have been published on the adaptive control of nonlinear phenomena with shaped femtosecond excitation laser pulses [1, 2]. Their reports have shown that the modification of the spectral and temporal phases of excitation pulses can increase or decrease the probabilities and efficiencies of such nonlinear phenomena. This method has been widely applied to studies on the active control of molecular motions or chemical reactions [3,4]. Considering further novel biological applications, we focus on the two-photon excited fluorescence (TPEF) of the green fluorescent protein (GFP) from the jellyfish Aequorea victoria. GFP is spontaneously fluorescent and is relatively nontoxic compared with other organic dyes used as optical markers. Therefore, it has been widely used as a “tag” material for the fluorescence observation of living cells [5]. Two-photon excitation microscopy (TPEM) is a powerful tool for biological real-time observation due to its various advantages, such as a clear contrast, good S/N ratio, and high spatial resolution [7]. From a practical point of view, however, there is a serious problem with TPEM, which is the photobleaching of a dye. The intensity of a fluorescence signal decreases significantly during observation. One of the reasons for this is that the chromophore structure is degraded by intense excitation laser pulses that are required for efficient two-photon excitation. In this study, therefore, we attempted to determine the optimal phase for maximizing the fluorescence efficiency of a GFP variant with excitation laser pulses of minimal intensity. We considered that GFP can be an ideal dye for the investigation of the dependence of photobleaching on the phase of excitation pulses, because the GFP chromophore is located at the center of s-can, that is, it is far from the site of reaction with activated molecular oxygen included in the solution, which can also cause photobleaching. The suppression of the photobleaching of a GFP variant in two-photon excitation is demonstrated.