Hiraku Matsukuma

Correlation between laser absorption and radiation conversion efficiency in laser produced tin plasma

The correlation between the laser absorption and the conversion efficiency (CE) for 13.5 nm extreme ultraviolet (EUV) light in a laser-produced tin plasma was investigated. The absorption rate α and the CE were measured simultaneously for a laser-pre-formed low-density tin target as a function of the time delay between the pre-pulse and the main laser pulse. A clear and positive correlation between α and CE was found with increasing delay time; however, the CE decreases rapidly at longer delay times. This result is partly attributed to a reduction in the absorption rate, but is mainly attributed to the self-absorption of EUV light in excessively long-scale plasmas.

A Liquid-Surface-Based Three-Axis Inclination Sensor for Measurement of Stage Tilt Motions

In this paper a new concept of a liquid-surface-based three-axis inclination sensor for evaluation of angular error motion of a precision linear slide, which is often used in the field of precision engineering such as ultra-precision machine tools, coordinate measuring machines (CMMs) and so on, is proposed. In the liquid-surface-based three-axis inclination sensor, a reference float mounting a line scale grating having periodic line grating structures is made to float over a liquid surface, while its three-axis angular motion is measured by using an optical sensor head based on the three-axis laser autocollimation capable of measuring three-axis angular motion of the scale grating. As the first step of research, in this paper, theoretical analysis on the angular motion of the reference float about each axis has been carried out based on simplified kinematic models to evaluate the possibility of realizing the proposed concept of a three-axis inclination sensor. In addition, based on the theoretical analyses results, a prototype three-axis inclination sensor has been designed and developed. Through some basic experiments with the prototype, the possibility of simultaneous three-axis inclination measurement by the proposed concept has been verified.

Disalignment rates of the neon 2p[5] and 2p[10] atoms due to helium atom collisions measured at temperatures between 77 and 294 K

Using a positive column of helium–neon glow discharge plasma at several temperatures between 77 and 294 K, the disalignment rates of excited neon atoms in the 2p5 and 2p10 (in Paschen notation) levels are measured by a polarization-resolved laser-induced fluorescence technique. For the 2p10 case, the disalignment rate due to radiation reabsorption is evaluated from the optical thickness of the plasma measured by a self-absorption method, and then is subtracted from the disalignment rate measured. From the slope of the obtained disalignment rate plotted against the helium atom density we determine the rate coefficients due to helium atom collisions. These rate coefficients are compared with the results of quantum multi-channel close-coupling calculations using the modified long-range potentials proposed by Bahrim and Khadilkar (2009 Phys. Rev. A 79 042715) from the original potentials of Hennecart and Masnou-Seeuws (1985 J. Phys. B: At. Mol. Phys. 18 657). Our present experiment agrees excellently with the theory for the 2p5 level at any temperatures between 77 and 294 K, and for the 2p10 state only at 294 K. Below 294 K, the experimental rate coefficients for the 2p10 state show a more rapid decrease with the decrease in temperature than the theory predicts.

The effects of microstructure on propagation of laser-driven radiative heat waves in under-dense high-Z plasma

This work was motivated by previous findings that the measured laser-driven heat front propagation velocity in under-dense TiO2/SiO2 foams is slower than the simulated one [Perez et al., Phys. Plasmas 21, 023102 (2014)]. In attempting to test the hypothesis that these differences result from effects of the foam microstructure, we designed and conducted an experiment on the GEKKO laser using an x-ray streak camera to compare the heat front propagation velocity in “equivalent” gas and foam targets, that is, targets that have the same initial density, atomic weight, and average ionization state. We first discuss the design and the results of this comparison experiment. To supplement the x-ray streak camera data, we designed and conducted an experiment on the Trident laser using a new high-resolution, time-integrated, spatially resolved crystal spectrometer to image the Ti K-shell spectrum along the laser-propagation axis in an under-dense TiO2/SiO2 foam cylinder. We discuss the details of the design of this ...

Actively Q-switched dual-wavelength pumped Er3+:ZBLAN fiber laser at 3.47 μm

We demonstrate the first actively Q-switched fiber laser operating in the 3.5xa0μm regime. The dual-wavelength pumped system makes use of an Er3+ doped ZBLAN fiber and a germanium acousto-optic modulator. Robust Q-switching saw a pulse energy of 7.8xa0μJ achieved at a repetition rate of 15xa0kHz, corresponding to a peak power of 14.5xa0W.

Q-switched dual-wavelength pumped 3.5-µm erbium-doped mid-infrared fiber laser

Short pulse operation of fiber lasers operating at wavelengths up 3 micron have been reported in recent years. At longer wavelengths, fiber lasers have only been demonstrated with a continuous operation mode. Short pulse operation in the mid-IR is necessary for utilizing such lasers in laser radars and for medical applications. Our previous numerical work suggested that Q-switching is possible on the 3.5 μm transition in erbium-doped ZBLAN in a similar manner to work demonstrated on the 2.8 μm transition in erbium. In this work we report on initial experimental results of a Q-switched, dualwavelength pumped fiber laser operating on the 3.5 μm transition in erbium-doped ZBLAN glass fibers. Using a hybrid fiber and open resonator configuration utilizing an acousto-optic modulator we demonstrated stable single pulse Q-switching while operating at repetition rates of 20 kHz and up to 120 kHz. The laser achieved a peak power of 8 W with pulse energy of 7 μJ while operating at 25 kHz. Long pulse widths on the order of 1 μs were obtained. The low peak power and long pulses are likely the result of both low gain of the transition and additional losses in the resonator which are currently being investigated. Our latest results will be presented.

Far-infrared-light shadowgraphy for high extraction efficiency of extreme ultraviolet light from a CO2-laser-generated tin plasma

The two-color, double-pulse method is an efficient scheme to generate extreme ultraviolet light for fabricating the next generation semiconductor microchips. In this method, a Nd:YAG laser pulse is used to expand a several-tens-of-micrometers-scale tin droplet, and a CO2 laser pulse is subsequently directed at the expanded tin vapor after an appropriate delay time. We propose the use of shadowgraphy with a CO2 laser probe-pulse scheme to optimize the CO2 main-drive laser. The distribution of absorption coefficients is derived from the experiment, and the results are converted to a practical absorption rate for the CO2 main-drive laser.

Microwave discharge plasma production with resonant cavity for EUV mask inspection tool

A microwave-discharge-produced plasma source was developed to generate 13.5 nm extreme ultraviolet (EUV) radiation for application as a mask inspection tool. The EUV radiation of a system with a high Q-factor (>3900) resonant cavity and a solid-state oscillator was studied. The gas pressure and microwave power dependences on the EUV radiation for transverse-magnetic mode TM010 and transverse-electric mode TE111 were determined. For the solid-state oscillator, the efficiency of the EUV radiation over the input power was 5.8 times higher than that for a magnetron. EUV radiation of 10 mW/(2πsr) was observed under a gas pressure of 5 Pa and microwave power of 400 W. We expect that more EUV power and a smaller plasma is generated when a magnetic field is applied to confirm the plasma and a facility is operated with an improved system to cool an entire cavity.

Perturber Dependence of Disalignment Cross Sections of the Argon 2p2 Atoms Measured at Temperatures between 77 and 295 K

In a temperature controlled glow gaseous discharge of pure argon, or a mixture of argon and helium or neon at 77, 135, 190, 240, and 295 K, a linearly polarized laser pulse produces polarized argon atoms on the 2p 2 (in Paschen notation) level from the 1s 3 level. We measure the direct fluorescence from the 2p 2 level to the 1s 2 level with its polarization components resolved. From the temporal evolution of the intensities for these components, we determine the disalignment rate. We also determine the disalignment rate coefficients due to helium, neon and argon atom collisions by separating the effect of radiation reabsorption, and then determine the effective cross section of disalignment from the rate coefficient divided by the mean relative velocity of colliding atoms. It is found that perturber rare gas atoms with larger atomic number have larger depolarization cross section. This effect can be attributed to larger values of the dipole polarizability for perturbers with larger atomic number.

Alignment relaxation of Ne * (2pi (J=1)) atoms in He-Ne * glow discharges

Alignment relaxation of the Ne*(2p5 3p; 2pi [J = 1]) atoms (where i = 2, 5, 7 or 10) induced by collisions with He atoms in glow discharges at 77 K < T < 1,000 K are reported. Close-coupling many-channel quantum calculations using a model potential for the Ne*(2p5 3p) – He system are compared with measurements of the alignment relaxation using the LIFS technique and the Hanle effect. The addition of the dipole polarization potential of the Ne*(2pi [J = 1]) atoms to the spin-orbit coupling and the electrostatic interaction between Ne* and He atoms leads to good agreement between theory and experiment.