Plasma-mirror frequency-resolved optical gating for simultaneous retrieval of a chirped vacuum-ultraviolet waveform and time-dependent reflectivity

プラズマミラーを用いた波長分解オプティカルゲートによる真空紫外光の波形測定および時間分解反射率測定

板倉 隆二*; 熊田 高之   ; 中野 元善*; 赤木 浩*

Itakura, Ryuji*; Kumada, Takayuki; Nakano, Motoyoshi*; Akagi, Hiroshi*

周波数分解オプティカルゲート法(FROG)による真空紫外光の波形測定法を確立しその解析アルゴリズムを確立させた。高強度フェムト秒レーザー照射によって石英試料表面に生じたプラズマによりVUV反射率は急激な増大を示す。その反射VUV光の時間分解反射スペクトルを測定することによりVUV光の波形を決定することができる。

We demonstrate that the methodology of frequency-resolved optical gating (FROG) is applicable to time-resolved reflection spectroscopy of a plasma mirror in the vacuum-ultraviolet (VUV) region. Our recent study has shown that a VUV waveform can be retrieved from a VUV reflection spectrogram of a plasma mirror formed on a fused silica (FS) surface by irradiation with an intense femtosecond laser pulse. Simultaneously, the increase in the reflectivity with respect to the Fresnel reflection of the unexcited FS surface can be obtained as a time-dependent reflectivity of the plasma mirror. In this study, we update the FROG analysis procedure using the least-square generalized projections algorithm. This procedure can reach convergence much faster than the previous one and has no aliasing problem. It is demonstrated that a significantly chirped VUV pulse as long as 1 ps can be precisely characterized.