Terry Hargett

Oxide-defined GaAs vertical-cavity surface-emitting lasers on Si substrates

By employing a reactive low-temperature wafer bonding technique, we have demonstrated oxide-defined 850 nm vertical-cavity surface-emitting lasers (VCSELs) on Si substrates. Devices reach a differential quantum efficiency of 53% and a light output power of 7.1 mW under room temperature and continuous-wave operation without a heat sink.

Fabrication and performance of two-dimensional matrix addressable arrays of integrated vertical-cavity lasers and resonant cavity photodetectors

Massively parallel interconnects and scannerless imaging are applications that would benefit from high-density two-dimensional arrays of lasers. Vertical-cavity surface-emitting lasers (VCSELs) are uniquely suited for these applications due to their small size and high efficiency. We have successfully fabricated 64 /spl times/ 64 element arrays containing alternating rows of selectively-oxidized 850 nm VCSELs and resonant-cavity photodetectors (RCPDs) monolithically integrated on semi-insulating GaAs substrates. In order to reduce the input and output connections to the array, we employ a matrix addressable architecture, where all the VCSELs (or RCPDs) in each row are connected by a common metal trace at the base of their mesas. The columns are connected by metal traces that bridge from mesa top to mesa top, connecting every other row (i.e., only VCSELs or only RCPDs). The pitch of devices in the array is 55 /spl mu/m, and the total resistance contributed by the long (up to 3.5 mm) row and column traces is below 50 /spl Omega/. The design, fabrication, and performance of these arrays are discussed.

Index Tuning for Precise Frequency Selection of Terahertz Quantum Cascade Lasers

We demonstrate precise shifts of terahertz quantum cascade laser frequencies by micron-scale changes to the duty cycle of a distributed-feedback grating, which alters the mode index, as an alternative to nanometer scale changes to the grating pitch. This method allows fabrication of lasers within a couple of gigahertz of a specific target frequency, enabling the temperature/bias to be independently optimized. Waveguides incorporating a central stripe were found to promote lasing on the fundamental lateral (TM00) mode, while an added oxide refill and metal cap made the lasers insensitive to wire bond locations for improved longitudinal mode control.

Consequences of spin-orbit coupling at the single hole level: Spin-flip tunneling and the anisotropic g factor

Hole transport experiments were performed on a gated double quantum dot device defined in a p-GaAs/AlGaAs heterostructure with a single hole occupancy in each dot. The charging diagram of the device was mapped out using charge detection confirming that the single hole limit is reached. In that limit, a detailed study of the two-hole spin system was performed using high bias magnetotransport spectroscopy. In contrast to electron systems, the hole spin was found not to be conserved during interdot resonant tunneling. This allows one to fully map out the two-hole energy spectrum as a function of the magnitude and the direction of the external magnetic field. The heavy-hole g factor was extracted and shown to be strongly anisotropic, with a value of 1.45 for a perpendicular field and close to zero for an in-plane field as required for hybridizing schemes between spin and photonic quantum platforms.

High-density interleaved VCSEL-RCPD arrays for optical information processing

Vertical-cavity surface-emitting lasers (VCSELs) are uniquely suited for massively parallel interconnects and scannerless imaging applications due to their small size, high efficiency and amiability to formation of high-density 2-dimensional arrays. We have successfully fabricated 4096 element arrays (64×64) containing alternating rows of selectively-oxidized 850 nm VCSELs and resonant-cavity photodetectors (RCPDs) on a 55 micron pitch monolithically integrated on semi-insulating GaAs substrates. We employ a matrix addressable architecture to reduce the input and output electrical connections to the array, where all the VCSELs (or RCPDs) in each row are connected by a common metal trace at the base of their mesas. The columns are connected by metal traces that bridge from mesa top to mesa top, connecting every other row (i.e., only VCSELs or only RCPDs). The design, fabrication and performance of these arrays is discussed.

Fabrication and performance of large (64x64) arrays of integrated VCSELs and detectors

Vertical-cavity surface-emitting lasers (VCSELs) are uniquely suited for applications requiring high-density 2-dimensional arrays of lasers, such as massively parallel interconnects or imaging applications. We have successfully fabricated 64x64 arrays containing alternating rows of selectively-oxidized 850 nm VCSELs and resonant-cavity photodetectors (RCPDs) on semi-insulating GaAs. In order to reduce the input/output pin count, we employed a matrix addressable architecture, where all the VCSELs (or RCPDs) in each row are connected by a common metal trace at the base of their mesas. The columns are connected by metal traces that bridge from mesa top to mesa top, connecting every other row (i.e., only VCSELs or only RCPDs). The pitch of devices in the array is 55 microns, and total resistance contributed by the long (up to 3.5 mm) row and column traces is below 50 ohms. The epitaxial design, fabrication and performance of these arrays is discussed.

Optical Bistability From Domain Formation in Terahertz Quantum Cascade Lasers

Novel transverse-mode behavior and bistability are observed in spectral and far-field measurements of terahertz-distributed feedback quantum cascade lasers. Prior to onset of negative differential resistance (NDR) at the maximum superlattice current, lasing is exclusive to a single (TM00) transverse mode. Precisely at NDR, lasing switches to a distinct (TM01) transverse mode. The spatial redistribution of optical power in the NDR regime is explained in terms of formation of bias domains localized to the lateral edges of the waveguide. As the bias voltage is reduced below NDR, hysteresis is observed as continued lasing on the TM01 mode. Photon-assisted current transport is proposed as the dominant mechanism for the bistability.

MilliKelvin HEMT Amplifiers for Low Noise High Bandwidth Measurement of Quantum Devices.

We demonstrate ultra-low power cryogenic high electron mobility transistor (HEMT) amplifiers for measurement of quantum devices. The low power consumption (few uWs) allows the amplifier to be located near the device, at the coldest cryostat stage (typically less than 100 mK). Such placement minimizes parasitic capacitance and reduces the impact of environmental noise (e.g. triboelectric noise in cabling), allowing for improvements in measurement gain, bandwidth and noise. We use custom high electron mobility transistors (HEMTs) in GaAs/A1GaAs heterostructures. These HEMTs are known to have excellent performance specifically at mK temperatures, with electron mobilities that can exceed 106 cm2/Vs, allowing for large gain with low power consumption. Low temperature measurements of custom HEMT amplifiers at T = 4 K show a current sensitivity of 50 pA at 1 MHz bandwidth for 5 mW power dissipation, which is an improvement upon performance of amplifiers using off-the-shelf HEMTs.

Fabrication and Characterization of a Single Hole Transistor in p-type GaAs/AlGaAs Heterostructures

Most spin qubit research to date has focused on manipulating single electron spins in quantum dots. However, hole spins are predicted to have some advantages over electron spins, such as reduced coupling to host semiconductor nuclear spins and the ability to control hole spins electrically using the large spin-orbit interaction. Building on recent advances in fabricating high-mobility 2D hole systems in GaAs/AlGaAs heterostructures at Sandia, we fabricate and characterize single hole transistors in GaAs. We demonstrate p-type double quantum dot devices with few-hole occupation, which could be used to study the physics of individual hole spins and control over coupling between hole spins, looking towards eventual applications in quantum computing. Intentionally left blank

High-mobility 2D hole systems for quantum computing applications.

One of the leading candidates for a solid-state quantum bit is the spin of a single electron confined in a semiconductor. Coherent control of individual electron spins has already been demonstrated in quantum dots in high-mobility 2D electron systems in GaAs/AlGaAs heterostructures. The major source of decoherence in such experiments is coupling between electron spins and nuclear spins in the host GaAs semiconductor. It has been proposed that hole spins in GaAs would be better suited for such experiments due to a lesser coupling between hole and nuclear spins. Building on recent successes in the growth of high-mobility 2D hole systems (2DHS) via carbon doping of (100) oriented GaAs/AlGaAs heterostructures, we have developed 2DHS at Sandia to enable experiments investigating the physics of hole spins in GaAs, looking towards eventual applications in the area of quantum computing.