Femtosecond vacuum ultraviolet seed pulse generation at 126 nm by using harmonic radiation

Abstract

We report femtosecond seed pulse generation at 126 nm using the 7th harmonic radiation from a Ti:Sapphire laser at 882 nm. The 7th harmonic emission was optimized in a low pressure Xe as a nonlinear medium.The progress of an ultrashort-pulse high-intensity laser is mainly based on solid-state laser media as represented by a Ti:Sapphire laser. The lasing wavelengths are thus inevitably limited to the near infrared and visible spectral regions. In contrast, ultrashort-pulse lasers in the ultraviolet (UV) and vacuum ultraviolet (VUV) wavelength regions have not well been developed. Among existing VUV lasers, the discharge-pumped ArF (193 nm) and F 2 (157 nm) lasers have most widely been used as powerful laser sources for various applications. The pulse widths of these lasers are, however, usually in the nanosecond regime. In another approach, nonlinear wavelength conversion such as harmonic generation is widely used to generate ultrashort UV and VUV pulses. The conversion efficiency, however, becomes extremely low in the deep VUV region, since only available nonlinear media are low-density gases. As a result, an observed pulse energy is limited on the order of nJ [1], which may be too small to be used for applications. We have proposed a new concept of the femtosecond VUV laser system [2], where we have used an optical-field-induced-ionization (OFI) rare-gas excimer amplifier to amplify a weak VUV ultrashort seed pulse generated by harmonic generation. This system can be operated in a table-top size with a high repetition rate by using one commercial ultrashort pulse laser in the near infrared region. In this paper, we report the generation of femtosecond VUV seed pulses using harmonic conversion of an infrared Ti:Sapphire laser.The experimental apparatus consisted of three elements: a Ti:Sapphire laser chirped-pulse amplification (CPA) system to generate harmonic radiation, a gas chamber where a low-pressure rare gas as a nonlinear medium was filled, and a VUV detection system. The Ti:Sapphire laser at 882 nm produced an output energy of 1 mJ with a pulse width of 170 fs at a repetition rate of 1 kHz. The laser pulse was focused with a 50 cm focal-length lens into a gas-filled chamber. The laser intensity was controlled up to 10 14 Wcm -2 at the focus by using a half wave plate and a thin film polarizer. Harmonic radiation from the focal point was detected using a micro-channel plate (MCP) coupled with a flat-field VUV spectrometer. A linear CMOS sensor detected fluorescence intensity from a phosphor screen placed behind the MCP. Ionized rare-gas ions coming from the interaction area were detected by using a Faraday cup. All equipments were connected to a personal computer for data acquisition and processing.We observed the 5th and 7th harmonics at 176 and 126 nm in the VUV spectral region generated from a Ti:Sapphire laser at the wavelength of 882 nm, respectively. The 7th harmonic at 126 nm had a spectral bandwidth of 1.2 nm, which would be amplified in an OFI argon excimer at 126 nm with a spectral bandwidth of ...