Fig. 3: THz pump-optical probe experiments with collinear pulses.
a Spectral content of narrowband pump pulses with different central frequencies ωp. b Phase-matched point in transmission geometry identified by the intersection between the PhP dispersion (gray) and ck/neV (blue dashed). Dotted horizontal lines are guides to the eye at ωp. Black arrow represents the momentum mismatch Δk for one of the pumps. c Transmitted signal \(| {{\bf{A}}}_{{\rm{tr}}}({\omega }_{pp})|\) for the different pump pulses shown in panel (a). The blue dot marks the phase-matched frequency. d Transmitted signal \(| {{\bf{A}}}_{{\rm{tr}}}({\omega }_{pp})|\) induced by a broadband pump pulse (black). \({n}_{{\rm{eV}}}/\sqrt{{\varepsilon }_{\infty }}\) is changed between 1.20 (light blue), 1.32 (blue) and 1.54 (dark blue) to move the phase-matched frequency (dots) along the dispersion (inset). Vertical dashed lines highlight ωTO and ωLO. e Reflected signal ∣Aref(ωpp)∣ induced by a broadband pump pulse (black). The phase-matched frequency (dot) is fixed by \({n}_{{\rm{eV}}}/\sqrt{{\varepsilon }_{\infty }}=1.41\) and falls on the high-momentum region of the dispersion (inset). Vertical dashed lines highlight ωTO and ωLO. In all panels, ωLO = 1.125ωTO and γ = 0.01ωTO. Sample thickness is \(d=50c/{\omega }_{{\rm{TO}}}\sqrt{{\varepsilon }_{\infty }}\) for transmission and d → ∞ for reflection geometries.
