Using nonlinear optical frequency conversion of ultrashort laser pulses in birefringent nonlinear optical crystals a wavelength range from about 160nm up to the far-IR (or THz) can be covered. Higher harmonic generation in gas jets, non-resonant four-wave mixing or hollow waveguides and filaments are new methods for the generation of femtosecond light pulses in the vacuum UV (VUV). Compared to near resonant four wave mixing in a gas cell the two latter techniques allows for higher conversion efficiency and supports generation of significantly shorter pulses (see fig. 1). For increase of the efficiency the OPCPA technique is introduced for avoiding ionization of the gas filled in the hollow waveguide. Additionally higher-order propagation modes in hollow waveguides are implemented supporting higher phase-matching pressures and a MOCVD chemical reactor is developed for coating of the the inner surface of the hollow waveguide with aluminum and thus increasing the waveguide optical transmission with more than three times. As a result chirped 200-fs pulses with energies up to 0.6 µJ and spectral bandwidth supporting sub-50-fs were demonstrated slightly tunable around 161 nm at 1 kHz repetition rate. Further extension of these technique for efficient generation and self-compression of continuously tunable sub-100-fs pulses in the VUV is feasible (see fig. 2). and flexibility. It can be regarded as a prerequisite for the generation of continuously tunable energetic pulses in the VUV at higher (kHz) repetition rates because the excimer modules for amplification of ultrashort UV-pulses can be substituted by an all-solid-state laser system.

In a second step compression of extremely short, isolated UV/VUV pulses by high-order stimulated Raman processes will be also investigated. Hollow waveguides filled with a Raman-active gas will be used in a pump-probe regime. A pump pulse at 800 nm creates a transient refractive index modulation in the gas and a delayed UV/VUV probe pulse with short input duration experiences spectral broadening (fig. 3). After compression by prisms or windows sub-30-fs pulses were demonstrated at 266nm (see fig. 4) and 200nm with µJ energies. Further extension of this technique toward compression of VUV pulses is expected.