Effective Mass Anisotropy of Hot Electrons in Nonparabolic Conduction Bands of $n$-Doped InGaAs Films Using Ultrafast Terahertz Pump-Probe Techniques

The anisotropic effective mass of energetic electrons in an isotropic, nonparabolic conduction band is revealed using ultrafast THz-pump–THz-probe techniques in a $n$-doped InGaAs semiconductor thin film. A microscopic theory is applied to identify the origin of the observed anisotropy and to show that the self-consistent light-matter coupling contributes significantly to the THz response.

Figure 1

(a) Schematic of an isotropic, nonparabolic conduction band with an electron excited in the $k_x\mathrm{\text{−}}{k}_y$ plane to ${\mathbf{k}}_0=k_0\widehat{\mathbf{x}}$. (b) Schematic dispersion relation for the electron along $k_x$ within plane A and (c) along $k_y$ within plane B. (d) The corresponding effective masses $m_x$ and $m_y$ together with the $\Gamma$-point mass $m_{\Gamma }$ (shaded area). (e) Polarization-dependent TPTP configuration. The large electric field of the THz-pump pulse drives electrons high in the band along the $x$ axis. The polarization of the THz-probe pulse can then be set to monitor carrier motion either parallel (colinear, CL) or perpendicular (cross-linear, XL) to the THz-pump pulse polarization.

Figure 2

(a) Electric field profile of the THz-pump beam emitted by the ZnTe optical rectification source. Inset: amplitude spectrum of the THz pulse. (b) Schematic of the experimental setup. (c) Electric field profile of the transmitted THz-probe beam at various delay times between the main positive peaks of the THz-pump and THz-probe pulses.

Figure 3

Measured (a) and calculated (b) normalized peak transmission of the THz-probe pulse as a function of the pump-probe delay time. The solid red lines and the blue shaded areas show the results for CL and XL TPTP configurations, respectively. The dashed red line and the light-blue shaded area in (b) represent corresponding calculations omitting radiative back-coupling effects.