Experimental and theoretical electroabsorption in an InGaAsGaAs strained-layer superlattice, and the performance of a wave-guide modulator

Excitonic electroabsorption was investigated in an InGaAs–GaAs strained-layer superlattice (SLS) wave-guide structure. The structure consisted of a SLS region embedded in a 1.2 μm thick single-mode wave guide. The SLS formed the intrinsic region of a pin diode structure and contained 40 periods of 10 nm thick In 0.14 Ga 0.86 As wells alternating with 10 nm thick GaAs barrier layers. Zero-bias transmission and photocurrent spectra were recorded for light incident normal to the surface, showing a sharp exciton feature centred at 955 nm. With increasing reverse-bias voltage this absorption peak shifted to longer wavelength, and became broader and weaker. The electric-field-induced changes in the exciton position were modelled using two different exactly solvable methods. Theoretical absorption strengths and widths were also determined. Calculation and experiment agreed reasonably well, though some differences were noted. Slab wave-guide samples, 360 μm long, were prepared, and transmission spectra for various reverse-bias voltages were obtained. At 982 nm, a transverse electric-polarization extinction ratio of 25 dB was measured between 0 and 8 V bias, with a zero-bias absorption loss of 2.8 dB. At 993.5 nm the zero-bias absorption was reduced to 0.8 dB but the extinction ratio fell to 17 dB.