Sabine Freisem

Lens-shaped all-epitaxial quantum dot microcavity

Data are presented on a new type of microcavity that uses epitaxial overgrowth to localize self-organized quantum dots in a microcavity optical mode. The all-epitaxial design eliminates free surfaces from the active material, eliminates quantum dots from the mirror and passive cavity regions, and provides a mechanically robust design with high thermal conductivity. The epitaxial overgrowth leads to a new type of lens-shaped microcavity, while the mesa confinement leads to lithographically defined placement of the quantum dots near the center of the optical mode.

Small-sized lithographic single-mode VCSELs with high-power conversion efficiency

Small, single mode VCSELs have been pursued almost since the inception of the device, but have been difficult to realize. Here we present data on lithographic and oxide-free VCSELs as small as 2 μm in diameter that produce single transverse mode powers of 8 mW and have high efficiency. The efficiencies reach 46% power conversion with greater than 73 % slope efficiency, with threshold current as low as 300 μA. Smaller VCSELs of 1 μm diameter produce 37 % power conversion efficiency with greater than 79 % slope efficiency, and single mode power over 5 mW. The keys to the high performance are the lithographic control and oxide elimination that reduce the electrical and thermal resistances.

Simultaneous deterministic control of distant qubits in two semiconductor quantum dots.

In optimal quantum control (OQC), a target quantum state of matter is achieved by tailoring the phase and amplitude of the control Hamiltonian through femtosecond pulse-shaping techniques and powerful adaptive feedback algorithms. Motivated by recent applications of OQC in quantum information science as an approach to optimizing quantum gates in atomic and molecular systems, here we report the experimental implementation of OQC in a solid-state system consisting of distinguishable semiconductor quantum dots. We demonstrate simultaneous high-fidelity π and 2π single qubit gates in two different quantum dots using a single engineered infrared femtosecond pulse. These experiments enhance the scalability of semiconductor-based quantum hardware and lay the foundation for applications of pulse shaping to optimize quantum gates in other solid-state systems.

Properties of Up Conversion Phosphors Necessary for Small Size Emissive Displays

Up converters combined with GaAs based semiconductor light sources are described as having potential for emissive displays with very large color gamut and very high resolution. To enable such displays the efficiency and temporal response of the up converting materials were studied and are reported. High efficiency is shown to be possible by proper preparation and utilization of the up converters. Up conversion displays can operate with refresh rates as high as 240 Hz. Operation as either a visible or near infrared display is possible.

Scaling properties of lithographic VCSELs

Data are presented demonstrating lithographic vertical-cavity surface-emitting lasers (VCSELs) and their scaling properties. Lithographic VCSELs have simultaneous mode- and current-confinement defined only by lithography and epitaxial crystal growth. The lithographic process of these devices allows getting uniform device size throughout a wafer and easy scaling to manufacture very small lasers. The semiconductors high thermal conductivity enables the small lithographic VCSEL to have lower thermal resistance than an oxide-aperture VCSEL, while the lithographic fabrication produces high VCSEL uniformity even at small size. Very dense packing is also possible. Devices of 3 μm to 20 μm diameters are fabricated and scaling properties are characterized. 3 μm lithographic VCSELs produce output power of 4.1 mW, with threshold current of 260 μA and slope efficiency of 0.76 W/A at emission wavelength of ~980 nm. These VCSELs also have single-mode single-polarization lasing without the use of a surface grating, and have >25 dB sidemode- suppression-ratio up to 1 mW of output power. Lifetime tests demonstrate that 3 μm VCSEL operates for hundreds of hours at high injection current level of 85 kA/cm2 with 3.7 mW output power without degradation. Scaling properties and low thermal resistance of the lithographic VCSELs can extend the VCSEL technology to manufacturable and reliable small size lasers and densely packed arrays with long device lifetime.

New VCSEL technology with scalability for single mode operation and densely integrated arrays

Data are presented demonstrating a new lithographic vertical-cavity surface-emitting laser (VCSEL) technology, which produces simultaneous mode- and current-confinement only by lithography and epitaxial crystal growth. The devices are grown by solid source molecular beam epitaxy, and have lithographically defined sizes that vary from 3 μm to 20 μm. The lithographic process allows the devices to have high uniformity throughout the wafer and scalability to very small size. The 3 μm device shows a threshold current of 310 μA, the slope efficiency of 0.81 W/A, and the maximum output power of more than 5 mW. The 3 μm device also shows single-mode single-polarization operation without the use of surface grating, and has over 25 dB side-mode-suppression-ratio up to 1 mW of output power. The devices have low thermal resistance due to the elimination of oxide aperture. High reliability is achieved by removal of internal strain caused by the oxide, stress test shows no degradation for the 3 μm device operating at very high injection current level of 142 kA/cm2 for 1000 hours, while at this dive level commercial VCSELs fail rapidly. The lithographic VCSEL technology can lead to manufacture of reliable small size laser diode, which will have application in large area 2-D arrays and low power sensors.

Physics of quantum dot lasers: Threshold temperature dependence, internal loss effects, and threshold current density

The physics of quantum dot lasers are studied theoretically and experimentally to study their threshold temperature dependence, and the relationship between internal loss and threshold current density.

Buried all-epitaxial microcavity for cavity-QED with quantum dots

Optical characterization of a novel type of semiconductor microcavity based on a fully-buried, all-epitaxial design reveals many properties essential for a manufacturable technology. We demonstrate detailed mode-imaging, lasing, as well as a sizeable Purcell effect.

Applications of femtosecond pulse engineering in the control of excitons in quantum dots

Pulse shaping techniques are used to demonstrate quantum control of exciton qubits in InAs quantum dots. Linearly chirped laser pulses are used to demonstrate adiabatic rapid passage in a single quantum dot on a subpicosecond timescale. The observed dependence of the exciton inversion efficiency on the sign of the pulse chirp identifies phonons as the dominant source of dephasing, which can be suppressed for positive chirp at low temperatures. The use of optimal quantum control theory to engineer a single optical pulse to implement simultaneous π and 2π single qubit gates in two uncoupled quantum dots is demonstrated. This work will support the use of pulse shaping in solid-state quantum hardware.

Optimal two-qubit quantum control in InAs quantum dots

Simultaneous control of exciton qubits in two distinguishable InAs semiconductor quantum dots with emission wavelengths near 1.3 microns is demonstrated through the development and application of general femtosecond pulse shaping protocols.