Phonon-assisted two-photon absorption in the presence of a dc-field: the nonlinear Franz–Keldysh effect in indirect gap semiconductors

Hernando Garcia1 and Ramki Kalyanaraman2
1 Department of Physics, Southern Illinois University, Edwardsville, IL 62026, USA
2 Department of Physics, Washington University in St Louis, St Louis, MO 63130, USA

Hernando Garcia and Ramki Kalyanaraman 2006 J. Phys. B: At. Mol. Opt. Phys. 39 2737

The two-photon absorption coefficient of an indirect gap semiconductor (phonon-assisted two-photon absorption) in the presence of a strong dc-electric field applied perpendicular to the direction of propagation of the optical field is calculated using the formalism developed elsewhere (Aspnes 1996 Phys. Rev. B. 147 554). We show that depending on the type of transition (i.e., allowed–allowed, allowed–forbidden or forbidden–forbidden), the absorption coefficient followed different dispersion relations. In the limit of a weak electric field, we recovered results previously calculated using perturbation theory. In the strong dc-field regime, we found that below the rescaled energy gap given by Eg/N, where N is the number of photons, the tunnelling effect is present, but to our surprise, above the rescaled gap, the Franz–Keldysh oscillations are present only for the allowed–allowed transition. This absence of the oscillations in the allowed–forbidden and forbidden–forbidden transitions is possible due to the weak coupling of the tails of the electron and hole wavefunctions.




Compound figure of merit for photonic applications of metal nanocomposites
Hernando Garciaa
Department of Physics, Southern Illinois University, Edwardsville, Illinois 62026
Hare Krishna and Ramki Kalyanaraman
Department of Physics, Washington University in St. Louis, St. Louis, Missouri 63130

Selecting nanocomposites for photonic switching applications requires optimizing their thermal,
nonlinear, and two-photon absorption characteristics. The authors simplify this step by defining a
compound figure of merit FOMC for nanocomposites of noble metals in dielectric based on criteria
that limit these structures in photonic applications, i.e., thermal heating and two-photon absorption.
The device independent results predict extremely large values of FOMC for a specific combination
of the metal and insulator dielectric constants given by h= 1− 2 /2, where h is the dielectric
constant of the host and 1 and 2 are the real and imaginary parts for the metal.