Tunable, spatially separated twin beam deep-UV third harmonic generation via a transient Kerr grating in thin...

We report a directional, tunable third-harmonic generation (THG) in the deep-UV (DUV) (220-270 nm) from thin transparent dielectrics using a Kerr-induced transient grating (TG). Two noncollinear femtosecond pulses induce a transient Kerr grating whose wavevector K→_TG=k→₁-k→₂ provides a quasi-phase-matching contribution that compensates the phase mismatch Δk=k_3ω-3k_ω, enabling THG via degenerate four-wave mixing and yielding twin, spatially separated DUV beams. With a fixed crossing angle, the TH signal exhibits cubic intensity scaling and a zero-delay temporal gate, confirming its ultrafast χ⁽³⁾ origin. Across DUV-transparent solids, efficiency under this fixed geometry correlates with DUV dispersion, apart from the reported variations in χ⁽³⁾. In CaF₂, we measure a 0.14% conversion efficiency at 266 nm, which is ∼20× higher than measured in quartz under identical experimental conditions, despite a similar order of magnitude of χ⁽³⁾. TG-assisted THG thus offers a compact, simple-to-align route to directionally separated, femtosecond-pumped, continuously tunable DUV beams. We outline avenues to maximize efficiency, angle tuning for increasing effective coherence length, and material selection for a twin DUV source useful in ultrafast and photoemission spectroscopy.