Nonlinear ultrafast modulation of the optical absorption of intense few-cycle terahertz pulses in $n$-doped semiconductors

We use an open-aperture $Z$ scan technique to show how intense few-cycle terahertz pulses can experience a nonlinear bleaching of absorption in an $n$-doped semiconductor due to terahertz-electric-field-driven intervalley scattering of electrons in the conduction band. Coherent detection of the transmitted terahertz pulse wave form also allows the nonlinear conductivity dynamics to be followed with subpicosecond time resolution. Both the $Z$ scan and time-domain results are found to be in agreement with our theoretical analysis.

Figure 1

(Color online) Experimental setup. (a) Schematic of the experimental setup. (b) Electric field profile of the terahertz beam emitted by the ZnTe optical rectification source. Inset: power spectrum of the THz pulse. (c) Pyroelectric image of the terahertz spot size at the focus $\left(z=0\right)$, with a $1/e^2$ diameter of 1.6 mm.

Figure 2

(Color online) Experimental results. (a) $Z$ scan normalized transmission of the total THz pulse energy measured with a pyroelectric detector after the sample; black curve: InGaAs epilayer on an InP substrate. Red (gray) curve: InP substrate alone. (b) Transmitted THz pulse electric field for different positions of the $Z$ scan. (c) Normalized transmission of the time integral of the modulus squared of the transmitted electric field as a function of the $z$ position along the scan. (d) Normalized electric field differential transmission as a function of time for different $z$ position along the scan. Note that the initial positive slope is related to the THz pulse duration and the population rate, while the negative slope is indicative of the carriers decay time.

Figure 3

(Color online) Comparison to model. (a) Normalized electric field differential transmission as a function of time at the focus of the $Z$ scan. (Red (gray) line: model; black line: experiment). (b), Peak value $\left(t=2.2\text{ps}\right)$ of the normalized electric field differential transmission as a function of the $z$ position along the scan. (c) Comparison to data from Fig. 2c. (d) Incident electric field dependence of the normalized energy transmission.