Direct measurement of the intersubband electro-optic coefficient in asymmetric multiple quantum wells

Abstract

An extremely large electro-optic coefficient in asymmetric quantum well systems has been inferred from optical rectification measurements. To date, however, a direct measurement of the electro-optic coefficient in these materials has not been performed. We present a method to directly measure the electro-optic coefficient in an asymmetric quantum well systemconsisting of 30 Å GaAs wells, 65 Å Al(0.20)Ga(0.80)As step barriers, and 500 Å Al(0.40)Ga(0.60)As barriers. The devices consist of short waveguides with asymmetric quantum well cores. The waveguide endfaces are polished to form Fabry–Perot cavities. Electrodes are deposited running parallel to the waveguides to apply electric fields. The waveguides are analyzed using a Fourier transform infrared spectrometer. Interference fringes are observed in the reflected spectra at room temperature. The fringe spacing yields the refractive index of the material, while plots of the refractive index versus applied electric field yield the electro-optic coefficient. Absorption measurements through these waveguides showed the characteristic polarization dependent intersubband absorption. Because the electrodes appeared to be Schottky like, a large voltage was applied across the device and the voltage across the quantum wells was estimated from the device leakage current and the current-voltage characteristics of a similar device structure after scaling for length, area, and doping. There may be some uncertainty in this approximation. The maximum applied electric field is estimated to be 2.3 kV cm −1. No anomalous effects were observed in the Fabry–Perot fringe pattern in the presence of this electric field, indicating the electro-optic coefficient in these waveguides is most likely less than 0.9 nm V −1over a wide wavelength range. Clearly, however, measurements need to be performed at higher electric fields to accurately measure the electro-optic coefficient.