Low-loss and high damage threshold mirrors are needed for laser interferometer-type gravitational-wave detectors.
Collaborative development with Japanese company of SIGMA KOKI CO., LTD., National astronomical
observatory of Japan and Institute for Laser Science, University of Electro-Communications was made for this
purpose. As a result, high reflectivity mirror of 99.99% for 1064nm has both low-scattering loss of less than
10ppm and high-damage threshold of over 400 J/cm2. Such mirrors can be applied for high finesse cavity of
more than 10000 with high laser input power of over 10 Watts. The mirror will offer great benefit for various
precise measurements with high power lasers.
In this work we report observation of ultra-fast switching of vacuum ultra-violet (VUV) light caused by saturable
absorption by a solid metal foil. A sub-picosecond VUV pulse from a free-electron laser located in SPring-8 is focused
on a metal target and transmission is measured as a function of input energy, thickness of the absorbing layer, and VUV
laser wavelength. As is well known, metals have a strong linear free electron response associated with the plasma
oscillation and collisional absorption (high-frequency resistivity). Due to the plasma screening and strong absorption, it
is difficult to use bulk metals for optical components. However, above the plasma frequency as in our experiments, a
metal can transmit light and shows phenomena related to the band gap structure, similar to the optical properties
observed in transparent materials for visible and infrared light.
We observe a strong gating of Sn transmission at energy fluences above 6J/cm2 at wavelength of 51nm. The ratio
of the transmission at high intensity to low intensity is typically greater than 100:1. The estimated saturated
transmittance is about 0.25. The mechanism of the switching phenomena is partially explained by the shift of Sn N shell
band edge, however, more details should be investigated with more exact physical models and precise measurements.
We think this is the first observation of such a strong nonlinear phenomena for VUV light and this result will
promote the development of new nonlinear photonic devices such as auto-correlator and pulse slicer for the VUV region.
We report a novel cooling technique for solid-state laser, which provides opportunity for liquid nitrogen temperature. The cooling holder does not have any windows on the laser beam path in order to minimize dispersion-related effects and nonlinear absorption under femtosecond laser operation. cw laser oscillation of 4% Yb3+-doped Y2O3 ceramics at 120 K temperature and 450 mW output power at 1030 nm, as obtained.
For the purpose pf determining optical properties of high-density plasmas created by ultra-short-pulse lasers, we have measured reflectivity with the pump-probe method. Nominal parameters of pump laser were 248nm, 300fs, and 1013~1014W/cm2 and those of probe beam were 745nm and 120fs. In the case of Al target and a normal incidence probe beam, the reflectivity decreased with time after arrival of the pump pulse. The duration of this change in reflectivity was apparently longer than pulse duration of pump beam at high intensities, while it was almost the same as pulse duration of pump beam for lower irradiance. A change of absorption during the pump pulse and non-linear dependence of resistivity on temperature could not explain these observations. A simple layered structure model was proposed in which heated layer penetrates into cold region with time and interferometric reflection occurs. This model could qualitatively explain the experimental results. Ellipsometric parameters were also measure with a single-shot based four-detector system. There were different time histories observed for the ratios of I(0,0), I(π/2,0) and I(π/4, π/2). This result also implies the plasma has some layered structure.