B. Pezeshki

Vertical coupled-cavity microinterferometer on GaAs with deformable-membrane top mirror

Fabry-Perot microinterferometer is demonstrated that combines a GaAs-AlAs vertical cavity with a suspended movable membrane. Electrostatic displacement of the gold/silicon nitride membrane allows for broad and continuous wavelength tuning of the cavity resonance formed by the combination of the GaAs cavity and the air gap below the membrane. The device exhibits a 32-nm tuning range around the 920-nm center wavelength for 0-14 V applied bias and FWHM linewidths near 3 nm; this corresponds to membrane deflections of up to 0.27 /spl mu/m. Such structures provide the foundation for wavelength selective photodiodes, light emitters, and lasers in which the active wavelength is under voltage control.<<ETX>>

Optimization of modulation ratio and insertion loss in reflective electroabsorption modulators

We show theoretically that the maximum modulation ratio in Fabry–Perot reflective electroabsorption modulators for a given insertion loss is solely a function of the ratio of the maximum to minimum absorption. We increase this ratio by using thinner quantum wells than are conventionally used and obtain significantly improved performance. We obtain an insertion loss of 1.2 dB and a modulation ratio of 7.5 with a 5 V operating bias.

Molecular beam epitaxy growth of vertical cavity optical devices with in situ corrections

We demonstrate a novel approach to the molecular beam epitaxial (MBE) growth of vertical Fabry–Perot cavities with quarter‐wave mirrors. At two selected points, the growth is interrupted and the reflectivity spectrum is measured without removing the wafer from the vacuum system. Corrections are then made in the growth of subsequent layers. By making measurements on the incomplete structure, separate corrections can be made to center both the mirror reflectivity and the cavity resonance at the desired wavelength. We present theoretical and experimental data demonstrating the effectiveness of the approach, and estimate the effects on device performance.

Electroabsorptive modulators in InGaAs/AlGaAs

Theoretical and experimental work shows that electroabsorption improves and the quantum‐confined Stark effect becomes stronger in In0.2Ga0.8As/AlxGa1−xAs quantum wells as the aluminum concentration (x) is increased in the barriers. This improved electroabsorption was used in a reflection modulator that exhibited the largest reported reflectivity change of 77%.

GaAs/AlAs quantum wells for electroabsorption modulators

We demonstrate both theoretically and experimentally that the quantum‐confined Stark effect leads to larger absorption changes in GaAs/AlAs quantum wells compared to the more conventional GaAs/AlGaAs system due to the higher Γ confinement. The lower indirect valleys in AlAs do not degrade the performance and the exciton resonance is maintained at higher‐energy shifts. The improvement in exciton oscillator strength is nearly 50% with a 70‐meV shift. The greater exciton strength at high fields has important applications for optical modulators and switches that operate at the long‐wavelength side of the zero‐bias exciton.

Novel cavity design for high reflectivity changes in a normally off electroabsorption modulator

We demonstrate both theoretically and experimentally a novel Fabry–Perot electroabsorption modulator. Both a normally off reflectivity characteristic and negative differential conductivity were obtained by increasing the optical absorption coefficient with voltage. Using the large absorption change of the quantum confined Stark effect, we obtain excellent modulation characteristics with a change in absolute reflectivity of 47% and a contrast ratio greater than 15.

Optical phase modulator utilizing electroabsorption in a Fabry–Perot cavity

By changing the absorption coefficient in a Fabry–Perot cavity from a point below to a point above the critical matching condition, the phase of the reflected beam at resonance can be changed. At a constant refractive index, the phase switches by 180°. We demonstrate a device fabricated with GaAs/AlGaAs quantum wells that uses a combination of index and absorption modulation to produce a 90° phase change in a vertical Fabry–Perot cavity with an insertion loss of about 6 dB. Similar phase measurements on a conventional normally on absorption modulator yielded a chirp parameter, αc<0.4, a value superior to that of waveguide electroabsorption devices.

Large reflectivity modulation using InGaAs-GaAs

The first reflection modulator using electroabsorption in strained InGaAs-GaAs quantum wells is reported. With an insertion loss of 1.9 dB and a reflectivity change of 42% at room temperature, the modulator is comparable to the best devices reported in GaAs-Al-GaAs system. A framework is presented for examining the tradeoff between insertion loss and modulation ratio from a theoretical perspective and compare the predicted limits to reported devices.<<ETX>>

1.3 μm Exciton resonances in InGaAs quantum wells grown by molecular beam epitaxy using a slowly graded buffer layer

We achieve sharp excitonic resonances near 1.3 μm in InGaAs quantum wells grown on a GaAs substrate by molecular beam epitaxy using a slowly graded InGaAs buffer layer. Our results indicate that linear grading is preferable to step grading for high In composition InGaAs on GaAs. SIMS results confirm the linearity of the grading. Cross-section TEM analysis of the graded sample reveals dislocations within the buffer but none which are threading up to reach the quantum wells. In addition to a well-defined exciton at 1.25 μm under zero bias, we observe pronounced quantum confined Stark effect in the absorption spectrum of a sample including a graded buffer layer while the spectra from samples with step-graded buffers exhibit no excitonic features

High contrast asymmetric Fabry–Perot electro‐absorption modulator with zero phase change

By analyzing the exciton line shapes of quantum wells, we can determine the wavelengths and biases at which they exhibit large absorption changes and zero refractive index changes relative to zero bias. To use this effect, we placed GaAs/AlGaAs quantum wells in the intrinsic region of a reverse‐biased p‐i‐n diode asymmetric Fabry–Perot modulator. We then developed a sample that could be post‐growth processed to optimize all relevant parameters. We used computer simulation to determine the exact post‐growth correction required and produced reflection modulators with 90% reflection change and zero relative phase change.