[14a-B3-1~10] CS.1 3.5&3.14 Code-sharing session

[14a-B3-7] Phase-matched sum frequency mixing below 150 nm in KBe₂BO₃F₂

〇(PC)Tomoharu Nakazato¹, Isao Ito², Yohei Kobayashi², Xiaoyang Wang³, Chuangtian Chen³, Shuntaro Watanabe¹ (1.Tokyo Univ. of Science, 2.Univ. of Tokyo, 3.Chinese Academy of Science)

Keywords:frequency conversion, vacuum ultraviolet

Narrow band light sources in the vacuum ultraviolet (VUV) region are attractive for photo lithography and high resolution photoelectron spectroscopy. Phase matching is essential to generate high power VUV lights by using a narrow band, low peak intensity, nanosecond (ns) pump source. In this research, sum frequency mixing has been demonstrated below 150 nm in KBeBO₃F₂ by using the fundamental with its fourth harmonic of a 6 kHz Ti:sapphire laser system. The laser system we have developed in this research, consists of a Ti:sapphire laser system and a frequency conversion stage. The former part was composed of an oscillator, a pulse slicer and a 2-stage multipass amplifier. The bandwidth of the fundamental was 1.4 GHz at 749.1 nm. The output power and pulse duration of the amplified fundamental were 8.3 W and 1.2 ns at 745.4 nm. The amplified fundamental (w) beam was split by a beam splitter (R: 90%, T: 10%). The 90% fundamental was used for the second harmonic generation (SHG) and the rest for the fifth harmonic generation. We employed a LiB₃O₅ (LBO) crystal for generating the SH (2w), a prism-coupled KBBF device both for the forth harmonic generation (FHG) and for the sum frequency mixing of FH (4w) with w to generate the fifth harmonic (5w). The wavelength of 149.8 nm is the shortest ever obtained to our knowledge by phase matching in nonlinear crystals. The output powers were 3.6 mW at 149.8 nm and 110 mW at 154.0 nm, respectively. The pulse duration was 0.4 ns. The phase matching angles measured from 149.8 to 158.1 nm are larger by 3-4 degrees than those expected from the existing Sellmeier equation. The optical transmission spectra of some KBBF crystals were measured by the spectrophotometer. The transmittance near the absorption edge support the generation of coherent radiation below 150 nm. The improvement of a prism-coupled device also contributed to the 5w generation and enabled the generation of coherent radiation below 150 nm. Another reason for the present break through to the shorter wavelength is the use of the short pulse (w: 1.2 ns, 5w: 0.4ns) driving source compared with our previous research (w:16 ns, 5w: 9.7 ns).