Interband Impact Ionization and Nonlinear Absorption of Terahertz Radiation in Semiconductor Heterostructures

We have theoretically investigated nonlinear free-carrier absorption of terahertz (THz) radiation in $\mathrm{I}\mathrm{n}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}\mathrm{S}\mathrm{b}$ heterojunctions. By considering multiple photon process and conduction-valence interband impact ionization (II), we have determined the field and frequency dependent absorption rate. It is shown that (i) electron-disorder scatterings are important at low to intermediate field, and (ii) most importantly, the high field absorption is dominated by II processes. Our theory can satisfactorily explain a long-standing experimental result on the nonlinear absorption in the THz regime.

Figure 1

(a) Electron-hole pair generation $N/N_0$ and (b) electron temperature $T_e/T$ vs $E_{\mathrm{a}\mathrm{c}}$ with and without the II process in $\mathrm{I}\mathrm{n}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}\mathrm{S}\mathrm{b}$ HJ at $f_{\mathrm{a}\mathrm{c}}=0.64$, 1, and 1.7 THz, respectively.

Figure 2

(a) Absorption percentage $\alpha$ vs $E_{\mathrm{a}\mathrm{c}}$ at $f_{\mathrm{a}\mathrm{c}}=0.64$, 1, and 1.7 THz, respectively. The circles are the experimental results from Ref. 3. (b) The corresponding phonon-induced ($S_p$), II-induced ($S_{\mathrm{I}\mathrm{I}}$), and total energy gains ($S_p+S_{\mathrm{I}\mathrm{I}}$) vs $E_{\mathrm{a}\mathrm{c}}$. The inset shows the $n$-photon absorption percentage ${\alpha }_n$ vs the order $n$ of the Bessel function at $f_{\mathrm{a}\mathrm{c}}=1.7\mathrm{T}\mathrm{H}\mathrm{z}$.

Figure 3

Calculated total absorption percentage $\alpha$ vs wavelength $\lambda$ at $E_{\mathrm{a}\mathrm{c}}=2\mathrm{k}\mathrm{V}/\mathrm{c}\mathrm{m}$ (triangles), $5.8\mathrm{k}\mathrm{V}/\mathrm{c}\mathrm{m}$ (circles), and $9\mathrm{k}\mathrm{V}/\mathrm{c}\mathrm{m}$ (squares), respectively. They are fitted by the function (lines): $\alpha (\lambda )={\alpha }_0{\lambda }^b$. The inset shows the first five order multiphoton contribution at $E_{\mathrm{a}\mathrm{c}}=2\mathrm{k}\mathrm{V}/\mathrm{c}\mathrm{m}$.