Homogeneous linewidth of quantum dots measured
Developed and applied a pump-probe, spectral-hole-burning technique to directly measure, with high spectral resolution, the exciton homogeneous absorption linewidth of self-assembled InGaAs quantum dots. The technique uses a local oscillator to separate detection of the collinear pump and probe beams in the waveguide geometry. Pump-power-dependent and temperature-dependent data were collected and modeled.
Contact: Dr. Kevin Silverman

Quantum dot coherence time measured
Used high resolution spectral hole burning (SHB) to perform the best measurements of InGaAs/GaAs quantum dot coherence time to date. The measured linewidth is nearly spontaneous-emission-lifetime limited. The SHB method used enabled a measurement of coherence time of 1.76 ns, which is a factor of two longer than other groups have reported, and makes quantum dots more attractive candidates as qubits in quantum logic operations.
Contact: Dr. Kevin Silverman

Semiconductor photon-number-resolving detector with high quantum efficiency
Demonstrated single photon detection using a quantum-dot-gated, field effect transistor (QDOGFET). The detector exhibits time-gated, single-shot, single-photon sensitivity; a linear response to average photon number; and an internal quantum efficiency of 70 % at 4 K through careful design to efficiently detect photons absorbed in the GaAs absorbing layer. The detector also demonstrated, for a mean photon number of 1.1, the ability to determine the number of detected photons with a probability of accuracy greater than 83 %.
Contact: Dr. Rich Mirin

Single photon source characterized with low-jitter superconducting detector
Demonstrated, with the Quantum Information and Terahertz Technology Project, the first use of superconducting single-photon detectors (SSPDs) for single-photon source characterization. The source was an optically pumped, microcavity-coupled, InGaAs quantum dot, emitting single photons at 902 nm. The two SSPDs replaced the silicon Avalanche Photodiodes (APDs) in a Hanbury-Brown Twiss interferometer measurement of the source second-order correlation function. The SSPD system electronics jitter was 68 ps, versus 550 ps for the APD units, enabling the source spontaneous emission lifetime to be measured with improved resolution.
Contact: Dr. Martin Stevens

Time-correlated infrared single-photon counting with superconducting detector
Demonstrated, with the Quantum Information and Terahertz Technology Project, the advantages of a NbN-based superconducting single photon-detector (SSPD) in a time-correlated, single-photon counting scheme to measure short spontaneous emission lifetimes of quantum wells in the infrared. The instrument response function (IRF) of the SSPD was shown to be Gaussian over nearly 5 decades of dynamic range. The FWHM of the IRF, which is a measure of the timing jitter, was 68 ps. These response characteristics are of great advantage over those of Si avalanche photodiodes (APDs) for detailed measurement of emission decay curves with high temporal resolution. In contrast, the IRFs of Si APDs exhibit long tails and multi-component responses. Quantum well emission out to 1650 nm in wavelength was measured with the SSPD, well beyond the photosensitive range of Si. APDs based on InGaAs or Ge can have reasonable detection efficiencies in the infrared, but are limited in available dynamic range.
Contact: Dr. Martin Stevens

Bistable mode-locked quantum dot laser
Wemonstrated, in collaboration with JILA, a mode-locked, InGaAs/GaAs quantum dot laser with an emission wavelength that is bistable with respect to applied bias on the saturable absorber region. The two stable lasing wavelengths for this device are about 1173 nm and 1166 nm, and the power contrast ratio is greater than 30 dB. The largest switchable wavelength range is 7.7 nm. The optical power and pulse width (6.5 ps) are almost identical in the two lasing modes under optimized conditions. The operation of this laser can be explained by the interplay of the spectral-hole burning and the quantum-confined Stark effect.
Contact: Dr. Kevin Silverman

Light extraction efficiency greatly enhanced
Demonstrated enhanced light extraction from circular Bragg grating-coupled semiconductor microcavities. A distributed Bragg reflector (DBR), behind the InGaAs quantum dot emitters and consisting of high-index-contrast GaAs/AlOx layers, increased the integrated photoluminescence by a factor of 2.7 over that of a device with no oxide DBR, indicating the role of the vertical cavity in enhancing extraction efficiency. To extract guided emission otherwise trapped in the plane of the device, circular Bragg gratings (CBGs) were formed on the periphery of the light-emitting mesa my means of a chlorine-assisted, ion-beam etch. Mode extraction was maximized by near-perfect momentum matching of the guided circular cavity modes to the resonant-extracted modes of the vertical cavity. The effect of the CBGs was an additional measured 7.5 times enhancement of light extraction, yielding a calculated absolute external efficiency of 41 % for the whole device. The modeling of modal dispersion diagrams and light extraction aided the design of the devices and interpretation of the data. The development of nanophotonic structures such as the DBR/CBG device offers potential advantages in efficiency, integration, and cost over present LED chip shaping and packaging for display and biomedical applications.
Contact: Dr. Rich Mirin