Refractive Index Modulation in Selectively Contacted GaAs N-I-P-I Structures

A GaAs Nipi with selective contacts was used for refractive modulation characterization; this Nipi device has the alternative n- and p-doped thin layers at a total thickness of ≅ 0.68 μm and a doping concentration of n ≅ p ≅ 4×1018 cm-3. The selective contactings were grown in situ with the n-i-p-i structure by MBE technology. The distance between two contact electrodes is ≅ 0.7 mm and each electrode is measured at ≅ 1 mm in length. The active area of this device is, therefore, approximately 0.7 mm × 1 mm. The I-V measurement of this n-i-p-i device shows good diode characteristics. The refractive changes at various current levels were determined by using a Mach-Zehnder interferometer. An Ar+-laser pumped ring dye laser, which has a useful wavelength range from 8000 to 9000 Å, was used as a light source. The refractive index change was determined by the fringe shift of the interference pattern generated by the signal beam (which passes through the n-i-p-i device) and the reference beam. A train of current pulses at a few hertz was used to drive the n-i-p-i modulator and also eliminate the slow fringe drift due to environmental noises of the interferometric system. A positive refractive index change of Δn > 1.5 was recorded at an injection current level I < 50 mA at λ = 9000 Å, as shown in Figure 1. The above result of refractive index modulation is more than two orders of magnitude larger than the prediction from our theoretical calculation.1 This refractive index change is observed in the wavelength range from 8900 Å to 9050 Å. The measurement wavelength range was limited by the strong interband absorption at λ < 8900 Å, and by the cutoff lasing wavelength of the sty-9 dye used in our ring dye laser beyond λ > 9050 Å.