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Femtosecond mid-IR optical vortex laser based on optical parametric chirped pulse amplification

An optical vortex refers to a beam whose wavefront shows a helical shape. Compared with the conventional Gaussian beam, since the optical vortex has doughnut intensity distribution and orbital angular momentum (OAM), it has acquired considerable interest in quantum information, optical trapping and manipulation, super-resolution microscopy and high-order harmonic generation (HHG). Spurred by these exciting technologies, widespread attention has been paid to the generation and manipulation of optical vortex. However, due to the low damage threshold and transmittance of optical components, it is difficult to directly generate high-energy mid-infrared (mid-IR) femtosecond optical vortex pulses using spiral phase plates (SPPs), spatial light modulators (SLMs), etc. Optical parametric chirped pulse amplification (OPCPA), combining optical parametric amplification (OPA) and chirped pulse amplification (CPA) technologies, displays unique advantages, such as high gain, high temporal contrast, less thermal effect and tunable wavelength, and it has been widely employed in recent high peak power laser systems. Therefore, OPCPA is able to be an effective technology to generate high energy femtosecond mid-IR optical vortices.

The scientists from State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences demonstrate a high energy femtosecond mid-IR optical vortex laser system based on OPCPA for the first time. The research results are published in Photonics Research, Vol. 8, Issue 3, 2020 (Junyu Qian, Yujie Peng, Yanyan Li, et al. Femtosecond mid-IR optical vortex laser based on optical parametric chirped pulse amplification[J]. Photonics Research, 2020, 8(3): 03000421), and was selected as Editors' Pick.

In the experiment, by using a two-stage OPCPA, the 4 μm optical vortices laser beam generated by an SPP is amplified. A 4 μm optical vortex laser with 20 Hz, 9.53 mJ, 119 fs and a topological charge of 1 is obtained. The optical vortex seed is generated from a femtosecond 4 μm laser by a silicon SPP with the step height of 1650 nm. Then the seed pulses are injected into the KTiOAsO4 (KTA) based two-stage OPCPA system which is preceded by a conventional Öffner-type stretcher and followed by a two-grating compressor. In addition, the researchers also verify that the topological charge of the amplified optical vortex pulse is still 1 by interference with plane wave, thereby confirming that OPCPA does not destroy the vortex characteristics of optical vortex pulse.

The researchers think that this high energy 4 μm optical vortex laser source provides a new tool in the area of high field laser physics, and can suit as a driver laser for HHG to extend cutoff energy to keV range with OAM, as well as other secondary radiations. Moreover, this method can be further extended to optical vortex lasers with other wavelengths and higher peak power.

Schematic of femtosecond mid-IR optical vortex laser based on OPCPA