DIRECT MEASUREMENT OF THE INTERSUBBAND ELECTROOPTIC COEFFICIENT IN ASYMMETRIC MULTIPLE-QUANTUM WELLS

An extremely large electro-optic coefficient in asymmetric quantum wel l systems has been inferred from optical rectification measurements. T o date, however, a direct measurement of the electro-optic coefficient in these materials has not been performed. We present a method the el ectro-optic coefficient in an asymmetric quantum well system consistin g of 30 Angstrom GaAs wells, 65 Angstrom Al(0.20)Ga(0.80)As step barri ers, and 500 Angstrom 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 de posited running parallel to the waveguides to apply electric fields. T he waveguides are analyzed using a Fourier transform infrared spectrom eter. Interference fringes are observed in the reflected spectra at ro om temperature. The fringe spacing yields the refractive index of the material, while plots of the refractive index versus applied electric held yield the electrooptic coefficient. Absorption measurements throu gh these waveguides showed the characteristic polarization dependent i ntersubband absorption. Because the electrodes appeared to be Schottky like, a large voltage was applied across the device and the voltage a cross 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 uncertai nty in this approximation. The maximum applied electric field is estim ated to be 2.3 kV cm(-1). No anomalous effects were observed in the Fa bry-Perot fringe pattern in the presence of this electric field, indic ating the electro-optic coefficient in these waveguides is most likely less than 0.9 nm V-1 over a wide wavelength range. Clearly, however, measurements need to be performed at higher electric fields to accurat ely measure the electro-optic coefficient. (C) 1996 Academic Press Lim ited