Subcycle Extreme Nonlinearities in GaP Induced by an Ultrastrong Terahertz Field

We report on the experimental observation of extreme laser spectral broadening and a change in optical transmission in gallium phosphite induced by $\text{25\hspace{0.17em}\hspace{0.17em}}\mathrm{MV}/\mathrm{cm}$ terahertz (THz) single-cycle internal field. Such intense THz radiation leads to twofold transient modifications of the optical properties in the electro-optical crystal. First, the electric field provokes extensive cross-phase modulation via the ${\chi }^{(2)}$ and ${\chi }^{(3)}$ nonlinearities on a copropagating 50 fs near infrared laser pulse which turns into 500% spectral broadening. Second, we observe an instantaneous change of the optical transmission occurring at the THz field which is alleged to interband Zener tunneling and charge carrier density modification by impact ionization turning the semiconductor in a metal-like transient state. The presented scheme displays a pathway to coherently control the optical properties of semiconductors on an ultrafast time scale by a strong THz field.

Figure 1

Experimental setup: the 50 fs, 800 nm laser copropagates with a strong single-cycle THz of 40 $\mathrm{MV}/\mathrm{cm}$ in a $50\mu \mathrm{m}$ thin GaP crystal (110). The crystal is an electro-optical active semiconductor with a 2.33 eV band gap, larger than the probe (1.55 eV) and THz (0.4–20 meV) photon energy. The optical properties of the GaP are modified by the THz electric field. This turns into spectral broadening of the probe and a change of optical transmission. The temporal evolutions of these phenomena are visualized as a function of the delay of the optical probe.

Figure 2

(a) THz electric field reconstructed by electro-optical sampling. (b) Giant THz-induced XPM gives rise to strong spectral modification of a laser pulse. The spectrum of the probe pulse is shown as a function of the delay between the optical and THz field.

Figure 3

Laser spectral centroid and $-10\mathrm{dB}$ width are well approximated by the derivative and the amplitude of the THz waveform. (a) The central wavelength of the probe is blueshifted by more than 40 nm, while (b) the spectral width is broadened by almost a factor of 5 when overlapping with the maximum THz field.

Figure 4

(a) The original laser spectrum recorded at the THz peak field amplitude is progressively broadened (b)–(f) by XPM for increasing THz field strengths. (g) The $-10\mathrm{dB}$ spectral width (black curve) and centroid (blue curve) are shown as a function of the THz peak field.

Figure 5

Maximum spectral width at $-10\mathrm{dB}$ recorded as a function of the GaP orientations. The different curves refer to different THz field strengths.

Figure 6

Change in the optical transmission in $50\mu \mathrm{m}$ GaP in a strong THz field regime. (a) and (b) show the temporal evolution of the transmission on a long and fast time scale induced by the single-cycle THz field peaked at zero delay.