Terahertz radiation and second-harmonic generation from InAs: Bulk versus surface electric-field-induced contributions

Polarized second-harmonic generation and terahertz radiation in reflection from (100), (110), and (111) faces of $n$-type InAs crystals are investigated as a function of the sample azimuthal orientation under excitation from femtosecond Ti:sapphire laser pulses. The expressions describing the second-order response (optical second-harmonic generation and optical rectification) in reflection from zinc-blende crystals, such as InAs, are calculated taking into account the bulk electric-dipole contribution and the first-order surface electric-field-induced contribution. It is shown that the two contributions can be separated based on rotation symmetry considerations. Moreover, a direct comparison of the second-harmonic generation and terahertz radiation emission indicates that the observed dominant surface electric-field-induced optical rectification component may be attributed to the large free-carrier contribution to the third-order susceptibility in InAs.

Figure 1

Geometry for polarizations and surface orientation of InAs wafers used in the calculations. The pump beam is linearly polarized and the $s$- and $p$-polarized radiated fields are examined.

Figure 2

Azimuthal dependencies of SHG from (100), (110), and (111) faces of $n$-InAs crystals at approximately $1\mathrm{mJ}∕{\mathrm{cm}}^2$ excitation fluence for $p$-polarized pump radiation. The solid curves represent fits to the data expected for the bulk electric-dipole contribution using the results in Table . Filled circles represent $p$-polarized SH intensity, and open circles represent $s$-polarized SH intensity. The azimuthal angle is defined as the angle that the projection of the pump beam in the surface plane of the crystal ($\widehat{k}$ in Fig. 1) makes with the $\left[01\overline{1}\right]$, $\left[1\overline{1}0\right]$, and $\left[\overline{2}11\right]$ crystal axes for the (100), (110), and (111) crystal faces, respectively. The plots are shown in the same scale.

Figure 3

Comparison of terahertz to SH emission for $p$-$p$ and $p$-$s$ polarization combinations, respectively. Filled circles represent THz emission (scale on right axis), and open circles represent SH intensity (scale on left axis). The solid curves are fits to the THz data based on the expectation of surface electric-field induced optical rectification (Table ), and the dotted curves are fits to the SH intensity based on the expectation of bulk dipole second-order response (Table ). In both measurements, the data is taken at a fluence of $\sim 1\mathrm{mJ}∕{\mathrm{cm}}^2$. The azimuthal angle is defined as the angle that the projection of the pump beam in the surface plane of the crystal ($\widehat{k}$ in Fig. 1) makes with the $\left[01\overline{1}\right]$, $\left[1\overline{1}0\right]$, and $\left[\overline{2}11\right]$ crystal axes for the (100), (110), and (111) crystal faces, respectively.