W. Y. Jan

Quantum well carrier sweep out: relation to electroabsorption and exciton saturation

The authors studied the effects of changing the barrier design of GaAs-Al/sub x/Ga/sub 1-x/As quantum wells on the electroabsorption, exciton saturation, and carrier sweep-out times. Five samples with x values ranging from 0.2 to 0.4 and barrier thicknesses from 35 to 95 AA were studied. Within this range, the authors find that the electroabsorption is not very sensitive to the barrier thickness, but that the ionization field of the excitons approximately doubles for an increase of x from 0.2 to 0.4. The samples with high, thick barriers have lower internal quantum efficiencies than those with low, thin barriers. It was found that the exciton saturation intensity increases with increasing applied field, and decreasing barrier thickness or height. Time-resolved electroabsorption measurements confirm the variation in sweep-out rates between samples, and indicate that the escape mechanism at low field is probably a thermally-assisted tunneling process. >

Low-loss intracavity AlAs/AlGaAs saturable Bragg reflector for femtosecond mode locking in solid-state lasers

We introduce a new low-loss semiconductor structure for femtosecond intracavity mode locking in low-gain solidstate lasers. This monolithic device can be engineered to exhibit specif ic saturation characteristics desirable for mode locking solid-state lasers. Self-starting 90-fs pulses are obtained with Ti:sapphire and diode-pumped Cr:LiSAF lasers. We discuss mode-locking mechanisms in quantum-well passively mode-locked solid-state lasers.

Does luminescence show semiconductor interfaces to be atomically smooth

Luminescence spectra from quantum wells are routinely interpreted in terms of atomically smooth and atomically abrupt interfaces. Here we show that this interpretation is inconsistent with photoluminescence, photoluminescence excitation, and quantitative microscopic (chemical lattice imaging) results. We argue that the discussion of interfacial roughness in terms of ‘‘an island size’’ is too naive. A full characterization of an interface requires the description of a ‘‘roughness spectrum,’’ specifying the amplitude of the interfacial corrugation versus corrugation wavelength over the relevant length scale.

Progress in Low-Power Switched Optical Interconnects

Optical links have successfully displaced electrical links when their aggregated bandwidth-distance product exceeds ~100 Gb/s-m because their link energy per bit per unit distance is lower. Optical links will continue to be adopted at distances of 1 m and below if link power falls below 1 pJ/bit/m. Providing optical links directly to a switching/routing chip can significantly improve the switched energy/bit. We present an early experimental switched CMOS-vertical-cavity surface-emitting laser (VCSEL) system operating at Gigabit Ethernet line rates that achieves a switched interconnect energy of less than 19 pJ/bit for a fully nonblocking network with 16 ports and an aggregate capacity of 20 Gb/s/port. The CMOS-VCSEL switch achieves an optical bandwidth density of 37 Gb/s/mm2 even when operating at a modest line rate of 1.25 Gb/s and is capable of scaling to much higher peak bandwidth densities (~350 Gb/s/mm2) with 5-10 pJ/switched bit. We also review a silicon photonic system design that will lower link energies to 300 fJ/bit, while providing multiterabits per second per square millimeter bandwidth densities. This system will ultimately provide switched optical interconnect at less than a picojoule per switched bit and computer/router system energies of tens of picojoule per bit. We review progress made to date on the silicon photonic components and analyze an energy and bandwidth-density roadmap for future advances toward these goals.

Fast escape of photocreated carriers out of shallow quantum wells

We report that at room temperature the field‐induced escape of photogenerated carriers out of shallow GaAs/AlxGa1−xAs multiple quantum wells is as fast as for pure GaAs of the same thickness, if the value of x does not exceed 0.04. Our experimental findings can be explained by assuming that carriers are efficiently scattered into the unconfined barrier states by absorption of a LO phonon, as long as the effective barrier height is less than the LO‐phonon energy. The application of shallow quantum wells with x≤0.04 in self‐electro‐optic effect devices, providing not only strong excitonic electroabsorption but also fast sweep‐out times at small biases, should lead to shorter switching times.

Excitonic electroabsorption in extremely shallow quantum wells

We report the remarkable observation of strong room‐temperature excitonic features in the absorption spectra of GaAs‐AlxGa1−xAs quantum wells (QWs) for values of x as low as 0.02. This has important implications for high‐power modulators, since saturation intensities have been shown to be higher in QW modulators with low barriers. In addition, very shallow QWs have enhanced electroabsorption at small biases because of ease of ionization. In our p‐i(multi‐QW)‐n device with x=0.02, we obtain a transmission change from 29% to 47% for a voltage change from +1 to −3 V.

GaAs-AlGaAs multiquantum well reflection modulators grown on GaAs and silicon substrates

Measurements of GaAs-AlGaAs multiple-quantum-well (MQW) reflection modulators grown simultaneously on GaAs and silicon substrates are presented. Comparable electroabsorption is observed, with contrast ratios of about 4:1 for both modulators at 20 V. The absorption spectrum of the GaAs-on-Si quantum well shows a single exciton peak, which leads to certain improvements in modulator performance. This study is very encouraging for the growth of GaAs MQW modulators on silicon integrated circuit chips for off-chip communication.<<ETX>>

EXCITON SATURATION IN ELECTRICALLY BIASED QUANTUM WELLS

We have measured the heavy hole excitation saturation intensity in GaAs/AlGaAs quantum wells as a function of applied electric field and AlGaAs barrier design. We find that the saturation intensity increased with increasing applied field, and decreasing barrier thickness or height, because of increased carrier sweep‐out rates. Time‐resolved sweep‐out time and temperature‐dependent saturation intensity measurement point out the roles of both thermionic emission and tunneling in the field and barrier‐dependent carrier escape time. By reducing the barrier Al composition from 30 to 20%, we achieved an increase in the saturation intensity by a factor of ∼6.

GaAs 850 nm modulators solder-bonded to silicon

GaAs/AlGaAs p-i-n multiple-quantum-well modulators solder-bonded to a silicon substrate are reported. The GaAs substrate is then chemically removed to allow operation at 850 nm. The gold contact to the modulator is used as the reflector. A change in reflectivity from 26% to 52% is achieved for a 0 to 10 V bias swing. The device has a modulation saturation intensity of 80 kW/cm/sup 2/, demonstrating superb heat-sinking and ohmic contact. The hybrid was cycled from 30 degrees C to 100 degrees C over a 100 times, and it showed no degradation, exhibiting the practicality of the technique.<<ETX>>

Saturable Bragg reflector self-starting passive mode locking of a Cr 4+ :YAG laser pumped with a diode-pumped Nd:YVO 4 laser

We demonstrate self-starting passive mode locking of a Cr (4+):YAG laser, using an intracavity nonlinear mirror as a saturable absorber. The pump source is a diode-pumped Nd:YVO(4) laser. Output pulses are centered at 1541 nm, with 26-nm spectral bandwidth and 110-fs pulse width. Output powers of 70 mW are obtained with 8 W of pump power. This mode locking technique is compared with Kerr-lens mode locking.