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

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 <i>K</i>→_{<i>T</i><i>G</i>}=<i>k</i>→₁-<i>k</i>→₂ provides a quasi-phase-matching contribution that compensates the phase mismatch <i>Δ</i><i>k</i>=<i>k</i>_3<i>ω</i>-3<i>k</i>_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 <i>χ</i>⁽³⁾ origin. Across DUV-transparent solids, efficiency under this fixed geometry correlates with DUV dispersion, apart from the reported variations in <i>χ</i>⁽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 <i>χ</i>⁽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.