K.-K. Law

Self‐electro‐optic device based on a superlattice asymmetric Fabry–Perot modulator with an on/off ratio ≳100:1

A self‐electro‐optic effect device based on a Fabry–Perot reflection modulator has been demonstrated for the first time. This modulator is a normally‐off high‐contrast asymmetric Fabry–Perot modulator using Wannier–Stark localization in a superlattice. Optical bistability has been achieved with a record‐high on/off ratio of 130:1 at the operating wavelength of 7620 A. The modulator with an appropriate reverse‐bias voltage supply was connected in series to a silicon photodiode which when illuminated with an appropriate light source acted as a current source load for the modulator.

Lateral carrier diffusion and surface recombination in InGaAs/AlGaAs quantum‐well ridge‐waveguide lasers

We measured the increase in threshold currents due to lateral carrier diffusion in InGaAs/AlGaAs quantum‐well ridge‐waveguide laser diodes. The ridge stripes were fabricated by using both in situ monitored pure Cl2 reactive ion etching and selective wet etching to completely eliminate the spreading current in the conductive upper cladding layer while keeping the ridge sidewalls straight. After comparing the threshold data with a theoretical model, the ambipolar diffusion coefficient is found to be 22 cm2/s in the population‐inverted InGaAs layer. This model is based on the calculated optical gain curve and the ambipolar carrier transport in the quantum‐well and waveguiding layers. The dependence of carrier lifetime on the local carrier concentration is included in the calculation. Moreover, from another set of devices with the portions of the active layer outside the ridge stripes etched away, the surface recombination velocity is found to be around 1–2×105 cm/s.

Normally‐off high‐contrast asymmetric Fabry–Perot reflection modulator using Wannier–Stark localization in a superlattice

A normally‐off surface normal Fabry–Perot reflection modulator with a contrast ratio of more than 60:1 with an operating voltage swing of 8 V has been demonstrated. The asymmetric Fabry–Perot structure consists of 100 periods of a 30 A GaAs/30 A Al0.3Ga0.7As superlattice sandwiched between a highly reflecting quarter‐wavelength stack on the substrate side, and a less reflecting front mirror (air‐semiconductor interface) on the top. The net Fabry–Perot reflection is turned on by reducing the cavity loss at resonance through the blue‐shifted electroabsorption effect of Wannier–Stark localization in the superlattice.

Low-voltage superlattice asymmetric Fabry-Perot reflection modulator

A normally off transverse superlattice asymmetric Fabry-Perot modulator that has a contrast ratio of more than 26:1 at the Fabry-Perot resonance, and a reflection change of 33% at a wavelength of approximately 20 degrees AA away from Fabry-Perot mode for an operating voltage swing of <3 V is discussed. The structure contains an active region of 52 periods of a 30 AA GaAs-30 AA Al/sub 0.3/Ga/sub 0.7/As superlattice, embedded between top and bottom quarter-wave grating mirrors. The modulation is achieved by reducing the cavity loss at the Fabry-Perot resonance through the field-induced effective absorption edge blue-shift in the superlattice.<<ETX>>

Superlattice surface-normal asymmetric Fabry-Perot reflection modulators: optical modulation and switching

The surface-normal asymmetric Fabry-Perot reflection modulators discussed are based on the electroabsorption stemming from the effective absorption edge blue shift caused by Wannier-Stark localization in superlattices. Due to the modulators normally off characteristics at the Fabry-Perot resonance, they exhibit negative differential photoconductivity. When the modulator is connected to a similar modulator or a simple photodiode, the negative differential photoconductance at resonance enables the modulator to operate as a self-electrooptic effect device (SEED) that shows clear bistable loops, and also has very large on-off ratios at its output. The operating principle of these modulators and SEEDs, and their sensitivity to variations of operating wavelengths, layer thickness and composition, and temperature are discussed. >

Effect of layer thickness variations on the performance of asymmetric Fabry–Perot reflection modulators

We present a study of the effect of the active cavity layer thickness variations on the operating characteristics of normally on low voltage high performance asymmetric Fabry–Perot modulators. For a modulator consisting of 25.5 periods of 100 A GaAs quantum wells confined by 45 A (GaAs/AlAs) short period superlattices with 5 pairs and 20.5 pairs of top and bottom quarter‐wave stacks, respectively, and assuming only layer thickness variation caused by Ga flux nonuniformity, the shift of the Fabry–Perot mode wavelength with respect to the fractional change of GaAs thickness inside the active cavity is ∼6 times that of the quantum well heavy hole exciton. This affects the relative distance between the wavelengths of the quantum well exciton and the Fabry–Perot resonance, and hence the performance of the modulators. Also, the tolerable percentage change of the Fabry–Perot mode wavelength should be less than 0.13% in order that such modulator arrays have at least 10:1 contrast ratios at a fixed optimum operati...

Electric‐field‐induced absorption changes in triangular quantum wells grown by pulsed‐beam molecular‐beam‐epitaxy technique

We present an observation of electric‐field‐induced excitonic quenching in GaAs/AlGaAs triangular quantum wells, grown by pulsed‐beam molecular‐beam‐epitaxy technique. We have measured photocurrent spectra for both symmetric and asymmetric triangular quantum wells as a function of electric field. In both cases, at the ground‐state exciton energy, we observed significant electroabsorption changes that were associated with the vanishing of sharp exciton resonances when the triangular quantum‐well structures were electrically switched to systems with no quantum confinement for one or both types of carriers.

Analysis and optimization of graded‐index separate‐confinement heterostructure waveguides for quantum well lasers

Waveguide structures for quantum well (QW) lasers are analyzed numerically by a straightforward 2×2 matrix approach. It is shown that this approach is capable of analyzing separate‐confinement heterostructure (SCH) waveguides, having any arbitrarily graded‐index (GRIN) profile in the waveguide layers and any number of QWs in the active layer, to any desired level of accuracy. Using this waveguide analysis, general GRIN‐SCH waveguide structures of QW lasers can be optimized for maximum confinement factors. It is estimated that the laser threshold current density can be reduced typically by 10% as a result of this waveguide optimization.

Temperature‐dependent threshold and modulation characteristics in InGaAs/GaAs quantum‐well ridge‐waveguide lasers

The observed temperature dependence of threshold currents in InGaAs/GaAs quantum‐well ridge‐waveguide lasers is modeled. This method incorporates both experimental data and theoretical gain calculations. The results show that lateral leakage currents must be included in the ridge‐waveguide laser models. Based on the same model, the temperature‐dependent modulation characteristics for InGaAs/GaAs quantum‐well ridge‐waveguide lasers are also investigated.

K-band operation of asymmetric Fabry-Perot modulators

The authors report preliminary high-frequency, small-signal optical measurements of asymmetric multiple-quantum well (MQW) Fabry-Perot electrooptic modulators which indicate that the electrical bandwidth for these devices is about 15 GHz at the onset of saturation, and as high as 21 GHz at low optical intensity-higher than any other measurements published to date. The modulators, 30 mu m*30 mu m in size, are integrated with on-chip microwave probe pads. The authors detail the fabrication process developed to achieve these high operating frequencies and predict from device models the maximum RC-limited operating frequencies for these devices.<<ETX>>