A tuneable frequency comb via dual-beam laser-solid harmonic generation

When a high-power laser pulse hits a plane solid target, odd harmonics of its frequency will be emitted [1]. However, the spacing of the harmonic lines in this configuration is fixed. Here, we study harmonic generation using two laser beams incident on a plane target at opposite angles to the target normal (see also Hickstein et al. [2]), via particle-in-cell simulations. When looking at the harmonic radiation in a specific direction via a narrow slit or pinhole, we select an angle-dependent subset of the harmonic spectrum. This way, we obtain a harmonic frequency comb that we control via the observation angle and the input laser frequency. The divergence of the harmonic radiation will be reduced by using wider laser spots, thus increasing the efficacy of the scheme. We will discuss extensions to this scheme, such as using beams with unequal frequencies, a slight tilt of the target, or employing more than two beams.

 [1] R. Lichters et al., Phys. Plasmas 3, 3425 (1996).

[2] D. Hickstein et al., Nature Photonics 9, 743 (2015).