Femtosecond OPCPAs from XUV to MIR for ultrafast dynamics experiments – a brief overview

Ultrafast dynamics experiments in physics, chemistry and biology, in condensed matter or gas phase require different types of ultrafast laser sources. Class 5 Photonics builds high-power optical parametric chirped pulse amplifiers (OPCPA) which are a powerful toolbox to deliver femtosecond pulses from the XUV to MIR spectral range.

Generally, research laboratories focused on ultrafast phenomena require robust and flexible laser systems in order to drive a variety of experiments. Previously, laser sources were driven from Ti:Sapphire lasers at 800 nm with limited spectral bandwidth (Fourier limited pulse of ca. 20 fs), and more importantly limited power levels and repetition rates. Commonly available systems were achieving a few microjoules at 100 kHz repetition rate, or a few millijoules at 1 kHz repetition rate.

However, for many applications high repetition rates in the MHz range are needed to increase the signal-to-noise ratio. At the same time, driving nonlinear interactions requires sufficient pulse energies up to the millijoule range. The recent advances in Yb-based lasers brought a new generation of high-power scientific lasers to the community, however, also at limited spectral bandwidth (Fourierlimited pulses of 200-2000 fs).

Optical parametric chirped-pulse amplication (OPCPA) together with bulk crystal white-light-generation (WLG) opens up the possibility for ultrashort (< 10 fs), high-power lasers with average power up to 100 W, repetition rates of 100 kHz in the millijoule-range or up to 10 MHz in the microjoule-range. Further, a wide wavelength range is accessible from ultraviolet (UV) to mid-infrared (MIR), making OPCPA an ideal technology for realizing complex pump-probe architectures, including secondary sources, such as terahertz and extreme ultraviolet high-harmonic generation (HHG).

Example: PEEMS

The White Dwarf for ultrafast material science is a compact OPCPA system developed by Class 5 Photonics to provide two synchronized outputs that can be individually spectrally tuned at a repetition rate of 4 MHz. The complete system is realized in a single housing on a footprint of 800 mm x 1200 mm, including the pump laser (Coherent Monaco-1035-80-60). A first output is tunable between 650 and 1300 nm, with corresponding SHG output between 325 and 650 nm in order to achieve gap-less excitation from 325 to 1300 nm. A second output for probing is tunable between 650 and 950 nm (fundamental wavelength), with a second-harmonic generation stage (SHG) providing wavelengths between 325 and 475 nm, and a third-harmonic generation stage (THG) providing UV wavelengths between 250 and 316 nm.

The outputs achieve up to 3 W of average power in each of the fundamental outputs, 0.9 W in the SHG, and 0.3 W in the THG with >pulse durations between 30 and 50 fs.The system includes full monitoring and diagnostics to ensure reliability, long-term robustness and user friendliness.

Example: Pump-Probe System from THz to XUV

The White Dwarf HE is a high-power, femtosecond laser system customizable to span an ultra wide frequency range from Terahertz (THz) to extreme-ultraviolet (XUV). The system can be used for long-wavelength excitation of carrier dynamics and short-wavelength (XUV) probing of excited photoelectrons, with suficient probing photon energy of 21.6 eV to cover the full Brillouin-zone of a solid-state material. The White Dwarf HE OPCPA is based on a 350 W Yb-based picosecond laser, which pumps three synchronized outputs: a high-field THz generation source, two individually tunable optical parametric chirped-pulse ampliers (OPCPA 1+2, 3), a second- and fourth-harmonic generation (SHG and FHG) source, and a high-harmonic generation source (HHG).

Example: THz-generation in organic crystals

The excitation of low energy modes in matter plays a key role in advanced material studies. A commonly used generation process for high-field THz-pulses is optical rectification, which is a second-order nonlinear process. Recently developed organic nonlinear crystals, such as DAST and DSTMS feature a high transparency and a large nonlinear coefficient. Using driver laser pulses centered at 1.55 µm allows for an easier collinear phase-matching scheme.

The White Dwarf HE OPCPA concept was extended to a tunable, high power laser system centered at a wavelength of 1.55 µm providing e.g. 30 µJ, sub 36 fs at 350 kHz. In addition, an optically synchronized compressed probe pulse with pulse duration of < 15 fs at 850 nm, was made available as a second output. This probe-pulse can be used, e.g. for electro-optical sampling in a THz set-up. The White Dwarf HE OPCPA can be pumped by a standard industrial Yb-based laser system with up to 300 W.

Example: Ultrafast dynamics in the water window (200-550 eV)

High-harmonic generation (HHG) spectroscopy and associated photoionization spectroscopy in the soft-x-ray regime have been successfully proven as a technique to resolve fundamental strong-field phenomena and electron dynamics on the attosecond scale. To scale the generated photon energy cut off, mid-infrared laser drivers are beneficial. Additionally, the majority of the measurements are subject to long integration times due to the weak light-matter interaction cross-sections in the experimental target and the detection technologies. Hence, higher repetition rates are required.

Class 5 Photonics further develops the high-power Supernova OPCPA system to provide a CEP-stable HHG laser driver at a wavelength of 2 µm at a repetition rate of 50 kHz, and a pulse duration of < 30 fs. Currently, the system is under commissioning achieving more than 47 W (940 µJ at 50 kHz).

Class 5 Photonics sponsored the “Ultrafast Phenomena and Nanophotonics” session at Photonics West 2020. Parts of the results described above have been presented at the corresponding conference.