Extremely nondegenerate two-photon absorption in semiconductors

Degenerate two-photon absorption, 2PA, in direct-gap semiconductors has long been known to scale with the inverse third power of the band gap, resulting in very large 2PA coefficients for narrow-gap semiconductors. We show that for any given pair of photon energies, (sum =\textit{h$\omega $}$_{1}$\textit{ + h$\omega $}$_{2})$, the 2PA is smallest for the degenerate case, \textit{$\omega $}$_{1}$\textit{ = $\omega $}$_{2}$, and is enhanced by orders of magnitude in the extremely nondegenerate case \textit{($\omega $}$_{1}$\textit{/$\omega $}$_{2}$\textit{ $>>$ 1 or $\omega $}$_{1}$\textit{/$\omega $}$_{2}$\textit{ $<<$ 1}). We experimentally demonstrate that 2PA in direct-gap semiconductors (e.g. GaAs, CdTe, ZnSe, ZnO, GaN) is enhanced over the degenerate value by up to 3 orders of magnitude using extremely nondegenerate pairs of photons (energy ratios $\sim $10/1). These extremely nondegenerate 2PA coefficients are similar in magnitude to coefficients obtained in narrow-gap semiconductors such as InSb and make 2PA feasible for applications such as subfemtosecond gated detection, all-optical switching etc. We demonstrate gated detection in a GaN LED used in a reverse-biased detection mode with fs 5.6$\mu $m and 400nm pulses. We see nearly 4 orders of magnitude enhancement over the degenerate case and can also easily detect sub-nW of IR light using modulation methods.