New publication in Optica: Generation of microjoule pulses in the deep ultraviolet at megahertz repetition rates
Ultraviolet (UV) laser pulses are an essential tool in many areas of science and technology, for applications like spectroscopy and femtosecond pump-probe measurements. Felix Köttig, Francesco Tani, Christian Martens Biersach, John Travers and Philip Russell from the Max Planck Institute for the Science of Light in Erlangen have now succeeded in generating wavelength-tunable UV laser pulses at high power by using a remarkable feature of optical solitons propagating in a gas-filled hollow-core photonic crystal fiber (PCF): the emission of so-called dispersive waves in the UV.
A standard way to generate UV laser pulses is by harmonic generation of near-infrared lasers in bulk crystals. With this approach, wavelength-tunability of the UV pulses requires complex wavelength-tunable pump lasers and high pump energies. As a result, most of these systems operate at maximum kilohertz repetition rate. Köttig et al. use a different approach: they make use of dispersive wave emission from self-compressed higher-order solitons. While a fundamental soliton is an electromagnetic wave that propagates in a medium without ever changing, higher-order solitons have the ability to self-compress during propagation, without any external optics.
In gas-filled hollow-core PCFs, it is straightforward to excite such higher-order solitons with microjoule-level pump energies. Upon propagation along the fiber, the spectrum of the self-compressed pulse reaches deep into the UV, where the soliton shakes off energy in the form of a dispersive wave. This UV emission can be tuned via the gas filling the fiber, within seconds and without realignment of the system. A key in this experiment was the use of one of the latest advancements in hollow-core PCF technology: the so-called single-ring PCF. This fiber provides exceptionally low loss and high power handling capabilities.
With only 20 μJ pump pulses from an ytterbium fiber laser, the system generated microjoule-level UV pulses at megahertz repetition rates and conversion efficiency up to 8%, making this approach accessible to laboratories around the world to perform next generation ultrafast UV pump-probe and spectroscopy experiments.