Keiichi Sueda

Laguerre-Gaussian beam generated with a multilevel spiral phase plate for high intensity laser pulses

We investigated an Laguerre-Gaussian (LG) beam that can carry an orbital angular momentum and has a doughnut-shaped intensity pattern. We developed a multilevel spiral phase plate (SPP) that generates an LG beam by applying the wave surface of a spiral structure directly to a Gaussian beam for application to microscopic laser material processing.We experimentally demonstrate, for the first time, that it is possible to generate an LG beam with the multilevel SPP that allows the use in high intensity laser pulses.

Intense longitudinal electric fields generated from transverse electromagnetic waves

We present a simple method for generating an intense longitudinal electric field from transverse electromagnetic waves (laser pulses) with radially symmetric polarization, to which a liquid crystal device converts linear polarization with energy efficiency of ∼99%. The laser-generated longitudinal electric field was observed in two dimensions and distinguished from the transverse component using the optical Kerr shutter method. The measured amplitude was 1.1 GV/m at the focus of a pulsed Nd:YAG laser beam of 0.25-MW peak-power.

Energetic Proton Generation in a Thin Plastic Foil Irradiated by Intense Femtosecond Lasers

Energetic protons generated in a thin plastic foil under the irradiation of ultra-intense femtosecond lasers have been experimentally and computationally investigated. More energetic protons were emitted forward in the direction of laser propagation than backward. The maximum energy of protons has been scaled via laser intensity I as 1 0.7, up to 2 MeV at 1018 W/cm2. The laser intensity scaling law for the maximum proton energy was reproduced by 2D-PIC simulation.

Experimental demonstration of spatially coherent beam combining using optical parametric amplification

We experimentally demonstrated coherent beam combining using optical parametric amplification with a nonlinear crystal pumped by random-phased multiple-beam array of the second harmonic of a Nd:YAG laser at 10-Hz repetition rate. In the proof-of-principle experiment, the phase jump between two pump beams was precisely controlled by a motorized actuator. For the demonstration of multiple-beam combining a random phase plate was used to create random-phased beamlets as a pump pulse. Far-field patterns of the pump, the signal, and the idler indicated that the spatially coherent signal beams were obtained on both cases. This approach allows scaling of the intensity of optical parametric chirped pulse amplification up to the exa-watt level while maintaining diffraction-limited beam quality.

High-efficiency laser-diodes-pumped microthickness Yb:Y3Al5O12 slab laser

A high-efficiency and high-power Yb:Y3Al5O12 oscillator was developed based on the microthickness slab structure with high aspect ratio to obtain one-dimensional temperature gradient and to reduce the temperature increase in the crystal. Laser output power of 257W was obtained in cw oscillation with 42% optical conversion efficiency at a pump power of 608W.

Ultrafast time-resolved pump-probe spectroscopy of PYP by a sub-8 fs pulse laser at 400 nm.

Impulsive excitation of molecular vibration is known to induce wave packets in both the ground state and excited state. Here, the ultrafast dynamics of PYP was studied by pump-probe spectroscopy using a sub-8 fs pulse laser at 400 nm. The broadband spectrum of the UV pulse allowed us to detect the pump-probe signal covering 360-440 nm. The dependence of the vibrational phase of the vibrational mode around 1155 cm(-1) on the probe photon energy was observed for the first time to our knowledge. The vibrational mode coupled to the electronic transition observed in the probe spectral ranges of 2.95-3.05 and 3.15-3.35 eV was attributed to the wave packets in the ground state and the excited state, respectively. The frequencies in the ground state and excited state were determined to be 1155 ± 1 and 1149 ± 1 cm(-1), respectively. The frequency difference is due to change after photoexcitation. This means a reduction of the bond strength associated with π-π* excitation, which is related to the molecular structure change associated with the primary isomerization process in the photocycle in PYP. Real-time vibrational modes at low frequency around 138, 179, 203, 260, and 317 cm(-1) were also observed and compared with the Raman spectrum for the assignment of the vibrational wave packet.

Controlling the phase matching conditions of optical parametric chirped-pulse amplification using partially deuterated KDP.

Using a partially deuterated KDP crystal for an optical parametric amplifier, we demonstrated ultrabroadband optical parametric chirped-pulse amplification of more than 250 nm bandwidth at a center wavelength of 1050 nm. We numerically show how to control the broadband phase matching conditions at different wavelengths to match center wavelengths of suitable broadband seed sources by adjusting the deuteration level in partially deuterated KDP.

Control of spatial polarization by use of a liquid crystal with an optically treated alignment layer and its application to beam apodization

We have investigated the alignment of a liquid crystal whose orientation is controlled by photoisomerization reaction for use in developing optical devices to improve beam quality. A glass window of a liquid-crystal cell that is coated with poly(vinyl alcohol) doped with azo dye was illuminated with a Hg lamp. We confirmed the dependence of the spatially controlled alignment direction of a liquid crystal on the irradiation time of this ultraviolet light. The new azo dye used in this study substantially reduced the illumination energy density required for aligning liquid-crystal molecules. We have demonstrated the control of polarization and successfully fabricated a serrated apodizing aperture and a soft aperture.

Speckle suppression of laser light using liquid crystals aligned by photoisomerization of dye molecules

For energy applications of high-power laser systems, uniform patterns are especially important in inertial confinement fusion research. We have demonstrated alignment control of liquid crystals using photoisomerization induced by polarized ultraviolet light to fabricate a polarization control plate. A glass substrate of a liquid-crystal cell, coated with polyvinyl alcohol doped with azo dye, was illuminated with a linearly polarized third harmonic of a Nd:YAG laser light. We confirmed the alignment of liquid crystals within the illuminated region dependent on the polarization direction of the ultraviolet light through the photoisomerization of azo dye molecules. Reduction of laser speckle pattern contrast by a polarization control plate was discussed.

Indirect monitoring shot-to-shot shock waves strength reproducibility during pump–probe experiments

We present an indirect method of estimating the strength of a shock wave, allowing on line monitoring of its reproducibility in each laser shot. This method is based on a shot-to-shot measurement of the X-ray emission from the ablated plasma by a high resolution, spatially resolved focusing spectrometer. An optical pump laser with energy of 1.0 J and pulse duration of similar to 660 ps was used to irradiate solid targets or foils with various thicknesses containing Oxygen, Aluminum, Iron, and Tantalum. The high sensitivity and resolving power of the X-ray spectrometer allowed spectra to be obtained on each laser shot and to control fluctuations of the spectral intensity emitted by different plasmas with an accuracy of similar to 2%, implying an accuracy in the derived electron plasma temperature of 5%-10% in pump-probe high energy density science experiments. At nano-and sub-nanosecond duration of laser pulse with relatively low laser intensities and ratio Z/A similar to 0.5, the electron temperature follows T-e similar to I-las(2/3). Thus, measurements of the electron plasma temperature allow indirect estimation of the laser flux on the target and control its shot-to-shot fluctuation. Knowing the laser flux intensity and its fluctuation gives us the possibility of monitoring shot-to-shot reproducibility of shock wave strength generation with high accuracy. Published by AIP Publishing.