Overview: Applications of Coherent EUV and Soft-X-Ray Beams

Extreme ultraviolet (EUV) and soft X-ray (SXR) science has seen revolutionary advances over the past decade. First, small- and large-scale coherent sources, including high harmonic generation (HHG) sources, are under rapid development worldwide. Compared with previous generation sources, these new light sources have remarkable properties. Tabletop HHG sources bring a tunable X-laser to your laboratory, with selectable wavelength from 2 – 47 nm (26 – 700 eV), control over the linewidth from <100meV to a coherent supercontinuum, fully spatially coherent beams, linear and circular polarization, as well as the ability to generate attosecond pulses and pulsetrains that are perfectly synchronized to the driving laser pulses. These unique new capabilities are allowing scientists to make the most precise measurements to date – from capturing intrinsic sub-femtosecond electron dynamics in materials, to implementing the first sub-wavelength EUV/SXR microscopes. Both of these applications were enabled by the KMLabs XUUS – the first engineered commercial HHG source.

3-Photon Microscopy

Multi-photon microscopy is a widely deployed tool for rapid, in-vivo, volumetric imaging. For three-photon excitation fluorescence microscopy, the necessarily longer wavelength ultrafast pump pulse increases the image penetration depth. Of particular utility are ultrafast sources in the so-called biological imaging windows at 1300 nm and 1680 nm, where the combination of Mie scattering and water absorption create optimal windows of excitation wavelength for maximal image penetration [Horton2013]. As biological tissue is easily damaged with excessive micro-illumination average power, shorter pulse durations (<200 fs) at a set average power increases the output fluorescent signal light.

Mid-IR Supercontinuum Generation

The Y-Fi OPA MIR output pulse can be used to generate MIR continuum in a fluoride fiber. Coherent broadband light sources in the MIR are useful for a variety of imaging and spectroscopy applications. Having wavelengths across the so-called fingerprint region is useful for gathering chemical-specific information about samples. Such broadband, compressible MIR pulses also find application in time-resolved and nonlinear spectroscopy.

2D IR Spectroscopy

The MIR output of the Y-Fi OPA can be used for 2D IR spectroscopy, a nonlinear infrared spectroscopic technique that can measure structural information not available with standard linear spectroscopy. The use of short pulse durations enables high temporal resolution. The tunable OPA output can be used directly from 2300-4300 cm^-1 (4300-2300 nm), or could be used to drive MIR continuum generation, enabling a broader probe bandwidth.

Coherent Raman Scattering

Coherent Raman scattering, in all of its various incarnations (CARS, SRS, ISRS, RIKES, etc.) derives species specificity and concentration by probing the molecular structures present in the foci of an appropriately prepared laser(s) without the addition of any external labels. The capacity to generate supercontinuum with high power spectral density in the short-wave infrared via high pulse energy, high repetition rate, and short pulse duration enables the generation of a broad spectrum stokes pulse suitable for multiplexed Raman scattering experiments. The long wavelength of these excitation pulses in the short-wave infrared are ideally situated being in both eye-safe [Rasskakzov2017] and overlapping a biological imaging window for deep tissue penetration.