Ultrafast strong-field terahertz nonlinear nanometasurfaces

Abstract

Strong-field terahertz (THz)-matter interaction permits the investigation of nonequilibrium behaviors in the nonperturbative zone. However, the unavailability of a high-field free-space THz source with high repetition rates, excellent beam quality, and high stability hinders its development. In this work, we obtain the nonlinear modulation dynamics of a "THz-nano" metasurface on silicon substrates using a time-resolved strong-field THz-pump THz-probe (TPTP) with a thousand orders local field enhancement through confining THz waves into nano-gaps (15 nm, λ/33,000). By switching the THz field strength, we successfully realize a self-modulation ∼50 GHz frequency shift, which is further verified via the TPTP ultrafast time-resolution technique. The phenomenon is attributed to the impact ionization (IMI) of the silicon substrate under the excitation of extremely confined strong THz fields in nano-gaps. Both strong-field induced intervalley scattering (IVS) and IMI effects of photodoped silicon occurring in nano-gaps and large-area substrates were also observed by 800 nm optical injection of carriers. These aforementioned findings provide a robust research platform for the realization of ultrafast time resolution nanoscale strong-field THz-matter interaction and new ideas for nonextreme laboratories to realize extreme THz science, applications, and THz nonlinear modulation device development.

Keywords: THz-pump THz-probe; impact ionization; intervalley scattering; photodoping.

Figure 1:

Integrated strong THz pump and multispectral probing time-domain system. (a) The schematic of the system. (b) The generated strong-field THz temporal waveform and (c) its corresponding spectrum. The inset in (c) shows the focused THz beam profile with a diameter of ≈1.6 mm (1/e) measured at the focus of OAP 2.

Figure 2:

Carrier dynamics in the nano-gap via TPTP. (a) Schematic of the enlarged TPTP function. (b) and (c) Local schematic of the optical path setup for the validation experiment. (b) The sample is rotated by 45° along the z-axis, and a THz polarizer with the polarizing direction of 45° is placed behind the sample, allowing only the transmission of THz waves in TM polarization; (c) the polarizing direction of the THz polarizer is −45°, and only the THz waves in the TE polarization can transmit. (d) The TM polarization transmission spectra measured at the incident fields of 2.5 and 180 kV/cm in Figure 2(b), showing a clear nonlinear frequency modulation. (e) Corresponding TE polarization transmission spectra in Figure 2(c), showing no significant nonlinear frequency modulation. The combination of Supplementary Material shows that this setting can measure nonlinear frequency modulation and the necessity of using the THz polarizer.

Figure 3:

TPTP of THz-nano metasurface. (a) The transmission spectra of the THz probe with the resonance frequency at 0.764 THz. (b) Typical THz probe transmission spectra before and after the THz pump, showing a frequency shift of 21 GHz. (c) TPTP dynamic curve of resonant frequency-time delay for the THz-nano metasurface on the highly resistive silicon substrate.

Figure 4:

Nonlinear response of THz-nano metasurface induced by photodoping and THz field. (a) Photodoping induced transmission nonlinearity under an incident THz probe field strength of 1 kV/cm and (b) its numerical simulation results. (c) THz transmission with incident field strength $\le$ 64 kV/cm. (d) THz transmission of 64 kV/cm $\le$ incident field strength $\le$ 350 kV/cm. (e) Numerical simulation results corresponding to (c) and (d). (f) The resonance frequency variation before and after 800-nm laser photodoping as a function of the incident THz field strength.