Tunable harmonic frequency combs via dual-beam or structured-target laser-solid interactions

When an intense laser pulse strikes a flat solid target at normal incidence, only odd, equally spaced harmonics are typically generated. Using particle-in-cell simulations, we show that introducing transverse spatial modulation—either via two beams incident at small opposing angles or via a single beam reflecting from a corrugated surface—produces a two-dimensional harmonic spectrum. Selecting the emission through a narrow slit isolates an angle-dependent subset of harmonics, forming a tunable frequency comb controlled by both the viewing angle and the drive-laser frequency. Distinct combs arise for <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mi>s</a:mi> </a:math> - and <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:mi>p</b:mi> </b:math> -polarized emission, while increasing the focal spot size reduces divergence and improves efficiency. Additional control is achieved by varying beam frequencies, target tilt, or beam number. Replacing the dual-beam configuration with a single beam and structured surface provides equivalent functionality with significantly reduced experimental complexity.