Laser-Induced Above-Band-Gap Transparency in GaAs

We report the observation of large ($\sim 40\%$) laser-induced above-band-gap transparency in GaAs at room temperature. The induced transparency is present only during the pulse width of the driving midinfrared laser pulses and its spectral shape is consistent with a laser-induced blueshift of the band edge. Our simulations based on the dynamic Franz-Keldysh effect reproduce the salient features of the experimental results, demonstrating, in particular, that the amount of the band edge shift is approximately given by the ponderomotive potential.

Figure 1

Interband absorption in a typical direct-gap semiconductor near the band gap $E_g$ with (dashed line) and without (solid line) a strong driving field. The dynamic Franz-Keldysh effect predicts: (i) below-band-gap absorption (region I), (ii) blueshift of the band edge causing an induced transparency (region II), and (iii) oscillatory behavior above the band gap (region III). The magnitude of the blueshift is predicted to be equal to the ponderomotive potential $U_p$.

Figure 2

Transmission quenching below the band gap in the presence of strong MIR pump beams of different wavelengths. The effect increases with increasing wavelength despite the fact that the intensity of our MIR source decreases for longer wavelengths. This is because the ponderomotive potential is larger for longer wavelengths in our case.

Figure 3

(a) Normalized transmission spectra of the white light probe beam through a GaAs film for zero time delay (black) and a 3 ps time delay (gray). The pump wavelength was $9\mu \mathrm{m}$. A large ($\sim 40\%$) induced transparency is observed above the band gap (1.42 eV) only when the two beams overlap temporally. (b) The observed change in transmission spectra of the probe beam in the presence of the pump beam ($9\mu \mathrm{m}$) with different intensities. (c) Transmission spectra of the probe beam for different pump wavelengths. The intensity of the pump beam decreases with increasing wavelength. The observed effect is more pronounced at longer wavelengths, even though the intensity of the pump beam decreases with increasing wavelength.

Figure 4

Theoretical fit to the experimental data based on Yacoby’s theory (see text).