High Harmonic Generation (HHG), the technique of choice to produce coherent photons in the ultraviolet and X-ray spectral ranges with good spatio-temporal quality, has greatly extended the wavelength range of traditional (low order) nonlinear optics, allowing for spectroscopic observations of fundamental physical processes. Unfortunately, HHG suffers at the moment of low conversion efficiency from the driver, which is typically a femtosecond pulse in the visible or infrared.
C. Homann et al. use a very appealing method based on traditional second-order nonlinear optics to produce pulses in the near and mid ultraviolet. The authors start from an amplified Ti:sapphire laser, build a visible optical parametric amplifier, double its frequency and generate sum frequency with a part of the Ti:sapphire output. This way, they generate tunable UV femtosecond pulses with wavelengths down to 190 nm (6.5 eV). The used second-order techniques are well-established, and the pulses produced in the past via these methods have been a workhorse for spectroscopy at the femtosecond time scales.
The authors demonstrate that the second-order techniques, where the light at new wavelengths is generated in crystals, produce photons with efficiencies well higher than HHG, where the nonlinear medium is typically a gas. The method is very simple, as it does not require bulky vacuum chambers and pressure control. The limitation is that the wavelengths that can be produced are longer than 190 nm, or the photon energies lower than 6.5 eV, due to the absorption of the used nonlinear crystal. For higher photon energies one needs HHG, a technique which is very promising but still under development.
The range of photons produced in this work, the broad tunability and the relative simplicity of the method make these pulses very attractive for many important applications discussed in the paper, in particular for photoelectron spectroscopy.
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