Nathan Bickel

Optical beam steering using InGaAsP multiple quantum wells

We report an efficient optical beam steering device based on InGaAsP multiple quantum wells. An area-selective zinc in-diffusion process is used to define highly localized p-n junctions through which electrical currents are injected into the quantum wells. The extent of the lateral spreading of the electrical carriers can be optimized by selecting the appropriate diffusion depth. Using a twin-parallel-stripe structure, an optical beam at a wavelength of 1.51 /spl mu/m was steered over a 17-/spl mu/m range using dc electrical currents of less than 13 mA.

Intermixing of InP-based multiple quantum wells for integrated optoelectronic devices

The intermixing characteristics of three widely used combinations of InP-based quantum wells (QW) are investigated using the impurity-free vacancy disordering (IFVD) technique. We demonstrate that the bandgap energy shift is highly dependent on the concentration gradient of the as-grown wells and barriers, as well as the thickness of the well, with thinner wells more susceptible to interdiffusion at the interface between the barrier and well. According to our results, the InGaAsP/InGaAsP and InGaAs/InP are well suited for applications requiring a wide range of bandgap values within the same wafer. In the case of the InGaAs/InGaAsP system, its use is limited due to the significant broadening of the photoluminescence spectrum that was observed. The effect of the top InGaAs layer over the InP cladding is also investigated, which leads to a simple way to obtain three different bandgaps in a single intermixing step.

MMI-based 2x2 photonic switch

We propose a robust, multi-mode interferometer-based, 2x2 photonic switch, which demonstrates high tolerance to typical fabrication errors and material non-uniformity. This tolerance margin is dependent upon the properties inherent to the MMI design and benefits from the high symmetry of the switch. The key design parameter of the device is to form a pair of well defined self-images from the injected light in the exact center of the switch. In allowing the index modulated regions to precisely overlap these positions, and by creating identical contact features there, any refractive index change induced in the material due to electrical isolation will be duplicated in both self-images. Since the phase relation will remain unchanged between the images, the off-state output will be unaltered. Similarly, offset and dimension errors are reflected symmetrically onto both self-images and, as a result, do not seriously impact the imaging. We investigate the characteristics of the switch under different scenarios using the finite difference beam propagation method. Crosstalk levels better than -20 dB are achievable over a wavelength range of 100 nm when utilizing this configuration. Polarization independence is maintained during device operation.

Angular-scattering characteristics of ferroelectric liquid-crystal electro-optical devices operating in the transient-scattering and the extended-scattering modes

The angular distribution of forward-scattered light in transient-scattering-mode (TSM) and extended-scattering-mode (ESM) ferroelectric liquid-crystal (FLC) devices was evaluated by use of circularly polarized incident light. For both modes the intensity and the distribution of forward-scattered light depended primarily on the FLC birefringence, spontaneous polarization, and the cell path length. In the FLC materials examined, the forward-scattering intensity under ESM drive conditions increased with longer FLC pitch lengths, whereas under TSM conditions stronger forward scattering was observed with increasing FLC spontaneous polarization. Although both TSM and ESM drive conditions displayed a similar angular distribution for forward-scattered light, the intensity of ESM scattering over a 0 degrees -6 degrees range was considerably smaller than that observed in earlier experiments with linearly polarized incident light.

Integrated beam-steered optical switch

We report an optical switch that is based on the beam steering of an optical waveguide formed by injection of electrons in a p-i-n slab waveguide structure. The structure consists of an undoped InGaAsP multiple quantum well (MQW) layer, with a total thickness of 0.28 μm that is sandwiched between n-doped InP cladding layers. Zinc is diffused into the top cladding layer through a silicon nitride mask to form the p-regions on top of which a pair of 10 um wide parallel titanium-zinc-gold contact stripes are deposited by evaporation and lift-off. The gap between the stripes is 20 μm wide and the device is cleaved to a length of 800 um. Electrical currents are injected through the electrodes and a laser beam is launched into the middle of the gap region. The injected electrons accumulate in the MQW layer and spread sideways by diffusion. The regions that are saturated with electrons experience a decrease in refractive index and surround a narrow high index region effectively forming a channel waveguide. By carefully controlling the current ratio through the two parallel stripes, the waveguide can be shifted, thereby steering the guided laser beam.

Controlled intermixing of multiple quantum wells for broadly tunable integrated lasers

A broadly tunable MQW laser utilizing a combined impurity-free vacancy disordering and beam steering techniques is proposed and investigated experimentally. The device consists of a beam-steering section and an optical amplifier section fabricated on a GaAs/AlGaAs MQW p-i-n hetrostructure substrate. The beam steering section forms a reconfigurable single mode waveguide that can be positioned laterally by applying electrical currents to two parallel contact stripes. The active core of the gain section contains a GaAs/AlGaAs MQW that is progressively disordered such that an optical beam steered through the selected region experience a peak in the gain spectrum that is determined by the degree of disordering of the MQWs. Furthermore the MQW in the beam-steering section is disordered to the largest extent to minimize optical beam attenuation. The MQW structure was intermixed using an impurity-free vacancy induced disordering technique. The MQW sample is encapsulated with a SiO2 film grown by plasma enhanced chemical vapor deposition (PECVD). The beam steering region is coated with a 400nm thick SiO2 film whereas in the gain section, the SiO2 film is selectively etched such that the thickness grades linearly ranging from 0 to 325nm. The disordering of the entire slab region is then induced by a single rapid thermal annealing step at 975°C for a 20s. Experimental results showed a controllable 10 to 60 nm wavelength blue shift of the peak of the photoluminescence spectrum corresponding to the change in SiO2 caps thickness and a lateral beam steering range up to 20 μm over the slab region.

2 x 2 quantum dot based switching device employing multimode interference effects

An integrated 2 x 2 multimode interference switching device was fabricated with InAs/In0.15Ga0.85As quantum dots as the active medium. The device, when probed with a 1.31 μm wavelength laser beam, showed similar responses for TE and TM polarization with initial power splitting ratios of 1:29 (TE) and 1:52 (TM) that were continuously adjustable to 49:1 (TE) and 38:1 (TM) when a change in current of 24 mA was applied through one of the electrodes. This is equivalent to achieving channel-to-channel crosstalk values of better than -15 dB for both polarizations. A 50:50 split ratio was reached at a current of 17 mA. We also present the preliminary results from an integrated variable power splitter that is based on a half-length multimode interference structure.

Etched quantum dots for all-optical and electro-optical switches

We present progress to date in the production of quantum dots etched from multiple quantum well structures for use in all-optical and electro-optical switches. Details of fabrication and comparisons to self-assembled quantum dot materials are described, and the direction of our continuing research is outlined.

Intermixing properties of InP-based MQW’s

The intermixing characteristics of three widely used combinations of InP based quantum wells (QW) are investigated using the impurity-free vacancy disordering (IFVD) technique. We demonstrate that bandgap tuning is highly dependent on the concentration gradient of the as-grown wells and barriers, as well as the thickness of the well, with thinner wells more susceptible to interdiffusion at the interface between the barrier and well. According to our results, the InGaAsP/InGaAsP and InGaAs/InP are well suited for applications requiring a wide range of bandgap values within the same wafer. In the case of the InGaAs/InGaAsP system, its use is limited due to the significant broadening of the photoluminescence spectrum that was observed. The effect of the top InGaAs layer over the InP cladding is also investigated, and leads to a simple way to obtain three different bandgaps in a single intermixing step.

Intermixing of InP‐Based Multiple Quantum Wells for Photonic Integrated Circuits

The intermixing characteristics of three widely used combinations of InP based quantum wells (QW) (InGaAsP/InGaAsP, InGaAs/InGaAsP, InP/InGaAs) are investigated using the impurity‐free vacancy disordering (IFVD) technique. We demonstrate that the shift of the bandgap energy is highly dependent on the concentration gradient of the as‐grown wells and barriers, as well as the thickness of the well, with thinner wells more susceptible to interdiffusion at the interface between the barrier and well. According to our results, the InGaAsP/InGaAsP and InGaAs/InP are well suited for applications requiring a wide range of bandgap values within the same wafer. The InGaAs/InP resulted in a blue shift of more than 200 nm, while the InGaAsP/InGaAsP provided approximately 100 nm. For the InGaAs/InGaAsP system, we suggest that although a decent bandgap shift of near 80 nm can be obtained, its use is limited due to the significant broadening of the photoluminescence spectrum that was observed. The effect of the top InGaAs...