 |
 |
 |
 |
 |
 |
|
|
|
|
|
|
 |
 |
|
TECHNICAL
STRATEGY |
| Growth and characterization of semiconductor quantum dots |
| We
use molecular beam epitaxy (MBE) to grow self-assembled quantum dots. We have developed ultrafast pump-probe methods to characterize carrier lifetimes of quantum dots. We have used a novel time-resolved waveguide-coupling method to directly measure the dipole moment of quantum dots. Cavity ring-down spectroscopy provides an extremely sensitive direct measurement of absorption. Differential transmission spectroscopy yields information on carrier transfer and an estimate of the homogeneous linewidth of quantum dots. Spectral hole-burning techniques have been developed to directly measure the lineshape. Contact: Dr. Richard Mirin |
| Single photon sources |
| For practical implementation, single-photon sources should operate at high temperatures. Colloidal CdSe quantum dots, nitrogen vacancy centers, and single molecules have demonstrated room-temperature single-photon emission; however, the first two systems exhibit blinking, and some single molecules exhibit photobleaching, both of which degrade performance. Epitaxial InGaAs/GaAs quantum dots are attractive as single photon emitters due to their ease of fabrication and inclusion with monolithic microcavities, their short spontaneous emission lifetimes, and the possibility of electrical injection. The photon repetition rate of the single-quantum-dot, single-photon turnstile (SPT) can be controlled by the frequency of an applied AC voltage, allowing precisely defined emission of single photons. The ultimate goal is the embedding of an SPT inside a photonic crystal nanocavity, which will enable enhanced emission efficiency and directionality. We have implemented a Hanbury Brown-Twiss apparatus to measure the photon statistics from our SPT. A true single-photon device exhibits photon emission times that are nonclassical. Specifically, photon antibunching of an optically pumped SPT has been observed up to a temperature of 135K. Contact: Dr. Richard Mirin |
| Single-photon detectors |
| Photon detectors made with field-effect transistors (FETs) that use an optically addressable floating gate consisting of a layer of quantum dots have the potential to be fast, flexible, and efficient single-photon detectors. With compound semiconductors one can control the absorption spectrum via material choices. In addition, resonant-cavity designs, which raise the efficiency, are compatible with these structures. Contact: Dr. Richard Mirin |
| Semiconductor photonic crystals |
| Photonic crystal cavities and waveguides are expected to play important roles in advanced optoelectronic devices such as ultracompact photonic integrated circuits for communication and sensing. The fabrication of photonic crystals requires the precision growth and anisotropic etching of semiconductor epilayers. We are etching semiconductor nanocavities in layers grown by MBE. These nanocavities contain quantum dots, and the spontaneous emission from the quantum dots is influenced by the effects of the cavity. Careful control of the cavity structure and the position of the quantum dots within the cavity can lead to weak coupling (enhanced spontaneous emission, also known as the Purcell effect) or to strong coupling (Rabi oscillations). Semiconductor light-emitting diodes can efficiently generate light throughout the ultraviolet, visible, and near-infrared. However, because of the high index mismatch at the semiconductor-air interface, only a small fraction of light in a conventional LED is extracted normal to the surface, the rest being emitted into the substrate, or trapped and re-absorbed in the active layer. The development of nanophotonic structures such as photonic crystals to enhance light extraction could offer superior efficiency, integration, and cost over present efforts in LED chip shaping and packaging. We are using Bragg gratings to enhance the extraction efficiency of light-emitting devices. Contact: Dr. Richard Mirin |
| Mode-locked quantum dot lasers |
|
There are many applications of ultrashort/broadband optical pulses that are hindered by the cost, complexity and inefficiency of the large-frame mode-locked lasers used to produce such pulses. These include optical atomic clocks, terahertz generation for homeland security, and optical coherence tomography (OCT). The problems would be eliminated if high-performance mode-locked diode lasers could be developed. Efforts to develop mode-locked diode lasers ran up against a fundamental barrier that limited the brevity of the pulses — carrier dynamics in the gain region of semiconductor lasers that result in a complex phase profile of the generated pulse. Ultimately 200 fs pulses have proven to be the shortest achievable from a conventional mode-locked diode laser, i.e., one that uses a simple heterojunction or quantum well(s) for the gain region. This falls well short of the performance (~20 fs pulses) required for the most promising applications. We plan to overcome this fundamental barrier by utilizing self-organized quantum dots for the gain region. It is anticipated that the quantum dots will be immune to the deleterious effects of complex carrier dynamics and enable a major improvement of pulse durations. In addition, the inherent size distribution in self-assembled QD ensembles leads to a very large inhomogeneously broadened lineshape with the bandwidth necessary to support short-pulse mode locking. This project requires a close coupling between expertise in QD growth, semiconductor laser design and fabrication, and in ultrafast lasers and measurement. |
| Quantum optical metrology | Many precision measurements of length or refractive index are based on interferometry. However, the phase resolution of these measurements scales as the reciprocal of the square root of the amount of optical power used for the measurement. This is not a fundamental limitation, however, and the goal of this new project is to explore optical interferometry at the fundamental Heisenberg limit, in which phase resolution increases as the reciprocal of the number of photons. We are developing new sources of N entangled photons, such as NOON states and Schroedinger cat states, and plan on using these to perform interferometry. |