Tunable Optical Parametric Amplifier for Visible Photocathode Excitation*

Traditional particle accelerators use high-power ultraviolet, high repetition laser systems for photocathode excitation [1], but new photocathode materials, e.g. bi-alkali antimonides [2], with excitation bands in the visible spectrum (500-600 nm) are being developed with the promise of overcoming practical and scientific challenges [3,4]. These wavelengths do not correspond well to the harmonics of commercial Yb- and Ti-based lasers in the near-infrared (IR). As such, there is a need for a tunable visible laser source for photocathode characterization. We present numerical simulations of a tunable optical parametric amplifier for narrowband amplification, using an IR Yb-based laser front-end. In Type-I BBO pumped by narrowband 515 nm pulses, we amplify signal wavelengths between 1-1.2 µm, which can be converted to the desired visible wavelengths with high efficiency through second-harmonic generation. Ease of tunability is realized by adjusting the pump group delay and phase-matching angle. The resulting signal intensity gains are on the order of 1000, with good spatiotemporal profiles. The output signal pulses are well-suited for subsequent OPA and pulse stretching stages for producing consistently amplified femtosecond and picosecond pulses in a broader gain spectral region not seen in common laser materials. 

[1]    S. Gilevich et al., The LCLS-II Photo-Injector Drive Laser System, in Conference on Lasers and Electro-Optics (Optica Publishing Group, Washington, D.C., 2020).

[2]    S. Schubert et al., Bi-Alkali Antimonide Photocathodes for High Brightness Accelerators, APL Mater. 1, 032119 (2013).

[3]    F. Liu, L. Guo, J. DeFazio, V. Pavlenko, M. Yamamoto, N. A. Moody, and H. Yamaguchi, Photoemission from Bialkali Photocathodes through an Atomically Thin Protection Layer, ACS Appl. Mater. Interfaces 14, 1710 (2022).

[4]    Y. Wang et al., Thermal Emittance and Lifetime of Alkali-Antimonide Photocathodes Grown on GaAs and Molybdenum Substrates Evaluated in a −300 kV Dc Photogun, Phys. Rev. Accel. Beams 23, (2020).