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Ultra-broadband near-infrared NOPAs based on the nonlinear crystals BiBO and YCOB

Ultrashort and broadband laser sources are formidable tools for a wide range of scientific areas. In the field of ultrafast science, laser pulses lasting only a few optical cycles are used to generate secondary sources employed in probing matter at atomic scales. Such sources are also widely adopted in applications in ultrafast spectroscopy, pump-probe in chemistry, and optical coherence tomography among many other fields.

Simultaneously, ultrashort, high-energy intense sources enable the study of astrophysical phenomena in laboratories and contribute to advancements in several high-field physics topics e.g. particle acceleration schemes. Over the past three decades, Ti: sapphire- and Nd: glass-based laser systems have been the regular workhorses for reliably generating energetic (up to 100s of J) ultrashort (sub-100 fs) pulses. The main drawbacks of these well-performing setups are their restricted tunability, limited spectral bandwidth, amplified spontaneous emission and low repetition rate (kHz for Ti: sapphire, sub-Hz for Nd: glass). These limitations are significant hindrances currently limiting their performance and preventing them from reaching the multi-petawatt regimes with high repeatability.

A widely used amplification technique that allows overcoming the mentioned issues is optical parametric chirped pulse amplification (OPCPA), mixing the chirped pulse amplification (CPA) and optical parametric amplification (OPA) techniques. OPCPA enables ultrashort (few-fs) pulse amplification with high single-pass gain, high contrast and no critical thermal effects, allowing the development of ultra-broadband, few-cycle sources and multi-PW peak power laser systems.

Besides the clear advantages of the concept, it also presents some challenges: the pump and signal pulses require precise and stable temporal matching and synchronization and the pump beam must possess spatial uniformity in order to enable homogeneous spatial gain. Furthermore, a very limited range of nonlinear crystals with sufficient aperture for high-energy stages that fulfil the phase-matching condition to a degree that enables broad bandwidth amplification are available.

For the low-energy amplification stages, beta-barium borate (BBO) and lithium triborate (LBO) are two of the most widely used crystals. They possess relatively high nonlinear coefficients (~1 pm/V) and broad gain bandwidths (100s of nm). Meanwhile, for high-energy stages there are only few crystals that can be implemented in view of the required large apertures and 10s of cm length. Among these we find KDP, K*DP or LBO. However, they have the drawback of a low nonlinear coefficient. In this context, alternative solutions for high-power, ultrashort pulse operation are highly desirable.

In the work "Ultra-broadband near-IR NOPAs based on the nonlinear crystals BiBO and YCOB", researchers from Universidade de Lisboa consider two alternative crystals for both low- and high-energy, few-cycle OPCPA: bismuth borate (BiBO) and yttrium calcium oxyborate (YCOB). The research results are published on High Power Laser Science and Engineering, Vol. 8, Issue 3, 2020.

The researchers have performed experimental measurements supported by numerical simulations to evaluate their performance for broadband amplification. They developed a noncollinear optical parametric amplification (NOPA) setup seeded by a continuum pulse generated from a sapphire plate, operating in the near-infrared region, in order to test each crystal. For a 5 mm YCOB crystal, they obtained an amplified bandwidth of 200 nm and a gain ~100, and for a 2.5 mm BiBO crystal they obtained an amplified bandwidth of 240 nm and a gain of ~1000. These results are supported by their theoretical and numerical analysis of the nonlinear process and the "magic" noncollinear angle, which also corroborates the measured spectral profile.

To the best of the researchers' knowledge, this is the first demonstration of both ultra-broadband OPA in BiBO in the near-IR wavelength range and the broadest bandwidth obtained using YCOB. These results are therefore of high interest to the laser developer community, highlighting promising alternatives to the commonly used nonlinear crystals. Due to its size, YCOB can be implemented in high-energy, long-pulse (nanosecond) regime NOPA stages with comparable performances, both in bandwidth and efficiency, to the commonly used LBO and KDP crystals in the near-IR regime where high-power laser systems usually operate. Meanwhile, BiBO can be advantageously adopted in the low-energy, high repetition rate, few-cycle or femtosecond/picosecond stages (within high-power systems) showing superior performances when compared to BBO or LBO crystals in the near-IR regime. This capability is also critical for multi-stage systems where preservation of the ultrabroad bandwidths involved is essential for further efficient high-power amplification.

In particular, the spectral range studied, covering wavelengths from 750 – 1050 nm, is currently of great interest for high energy, petawatt-level systems based on OPCPA, as well as for low-energy few-cycle systems. This work is part of a wider research program to develop novel laser sources, for applications and to be used as front end for multi-petawatt future facilities, where the requirement for the high-energy stages will imply strong limitation over the spectral region.

The results, highlighting the high gain and ultra-broadband spectrum achieved, is the first milestone of the all OPCPA-based laser systems project at Central Laser Facility (STFC, RAL) and of the upgrade of the Laboratory for Intense Lasers (L2I) at Instituto Superior Tecnico (Universidade de Lisboa) where a multi-TW high repetition rate laser system is expected to be able to explore a wide range of scientific areas for both low-energy few-cycle and high-energy fs-level communities, setting a standard for noncollinear OPCPA sources.

Gonçalo Figueira, head of L2I, affirms that "these results are remarkable and applicable to many facilities because they clearly show that there is room for improvement in high peak and average power OPCPA by choosing the right combination of nonlinear media and interaction geometry".

The experimental setup for the multi-J OPCPA amplification test stage for evaluate the performance of different type of crystal, operating in the Vulcan laser and the Central Laser Facility, STFC, UKRI.