G. S. Solomon

Controlling the spontaneous emission rate of single quantum dots in a two-dimensional photonic crystal

We observe large spontaneous emission rate modification of individual InAs quantum dots (QDs) in a 2D photonic crystal with a modified, high-Q single-defect cavity. Compared to QDs in a bulk semiconductor, QDs that are resonant with the cavity show an emission rate increase of up to a factor of 8. In contrast, off-resonant QDs indicate up to fivefold rate quenching as the local density of optical states is diminished in the photonic crystal. In both cases, we demonstrate photon antibunching, showing that the structure represents an on-demand single photon source with a pulse duration from 210 ps to 8 ns. We explain the suppression of QD emission rate using finite difference time domain simulations and find good agreement with experiment.

Triggered single photons from a quantum dot

We demonstrate a new method for generating triggered single photons. After a laser pulse generates excitons inside a single quantum dot, electrostatic interactions between them and the resulting spectral shifts allow a single emitted photon to be isolated. Correlation measurements show a reduction of the two-photon probability to 0.12 times the value for Poisson light. Strong antibunching persists when the emission is saturated. The emitted photons are also polarized.

Secure communication: quantum cryptography with a photon turnstile.

Quantum cryptography generates unbreakable cryptographic codes by encoding information using single photons, which until now have relied on highly attenuated lasers as sources. But these sources can create pulses that contain more than one photon, making them vulnerable to eavesdropping by photon splitting. Here we present an experimental demonstration of quantum cryptography that uses a photon turnstile device, which is more reliable for delivering photons one at a time. This device allows completely secure communication in circumstances under which this would be impossible with an attenuated laser.

Substrate temperature and monolayer coverage effects on epitaxial ordering of InAs and InGaAs islands on GaAs

For InAs Stranski–Krastanov (SK) island growth on GaAs by molecular‐beam epitaxy, we show that the in‐plane island diameter varies exponentially with the growth temperature over the range of 390–540u2009°C. A transition region in SK growth between isolated island growth and island coalescing is investigated as functions of growth temperature and equivalent InAs layer‐by‐layer monolayer (ML) coverage in order to extend the isolated island regime for quantum confinement applications. InAs islands of 150 A in diameter have been grown. Growth of In0.5Ga0.5As islands indicates an increased 2D epitaxial region before island nucleation and a decreased island concentration compared to growth of InAs islands.

Effects of monolayer coverage, flux ratio, and growth rate on the island density of InAs islands on GaAs

We have studied the effects of monolayer coverage, V/III flux ratio and growth rate on the density of three‐dimensional growth induced isolated InAs islands grown on GaAs by molecular‐beam epitaxy. Within the isolated island growth regime, increasing the monolayer coverage increases the InAs island density with only a small increase in island size. Decreasing the V/III flux ratio or decreasing the growth rate increases the island density without changing the average in‐plane island diameter. We have observed island densities that are 80% of the ideal close‐packed island density. We propose a model explaining the island density increase with monolayer coverage; local variations in accumulated strain in the wetting layer vary the point at which local islanding is initiated. As more material is deposited more islands are nucleated and the island density increases. The island density increases with decreasing V/III flux ratio or growth rate by increasing the adatom surface diffusion in the underlying wetting ...

Optically pumped InAs quantum dot microdisk lasers

We have achieved lasing in InAs quantum dot embedded GaAs microdisks under optical pumping. Above the lasing threshold, a drastic increase of emission intensity is accompanied by a decrease of the spectral linewidth of the whispering gallery modes. The laser light is linearly polarized. The polarization direction is parallel to the disk plane. The wide gain spectrum of quantum dots allows simultaneous lasing in several whispering gallery modes of a microdisk.

Enhanced single-photon emission from a quantum dot in a micropost microcavity

We demonstrate a single-photon source based on a quantum dot in a micropost microcavity that exhibits a large Purcell factor together with a small multiphoton probability. For a quantum dot on resonance with the cavity, the spontaneous emission rate is increased by a factor of 5, while the probability to emit two or more photons in the same pulse is reduced to 2% compared to a Poisson-distributed source of the same intensity. In addition to the small multiphoton probability, such a strong Purcell effect is important in a single-photon source for improving the photon outcoupling efficiency and the single-photon generation rate, and for bringing the emitted photon pulses closer to the Fourier transform limit.

Atom-resolved scanning tunneling microscopy of vertically ordered InAs quantum dots

We present cross-sectional scanning tunneling microscopy (STM) images of strain-induced, self-organized InAs quantum dots grown on GaAs. Samples containing 5 and 10 sequentially grown dot layers are investigated, and dots from different layers are seen to align in vertical columns. Our STM images are in general agreement with previous structural imaging, such as cross-sectional transmission electron microscopy, except that dot crowns appear more truncated. Although the size of the dots in most columns is roughly constant, monotonic changes in diameter are observed in some cases. STM analysis also reveals many new atom-resolved details of electronic structure, including dissolution of the InAs wetting layer and the presence of indium between the dot columns, which we attribute to segregation and diffusion of indium out of the wetting layer during overgrowth.

Quantum degenerate exciton-polaritons in thermal equilibrium

We study the momentum distribution and relaxation dynamics of semiconductor microcavity polaritons by angle-resolved and time-resolved spectroscopy. Above a critical pump level, the thermalization time of polaritons at positive detunings becomes shorter than their lifetime, and the polaritons form a quantum degenerate Bose-Einstein distribution in thermal equilibrium with the lattice.

Submicrosecond correlations in photoluminescence from InAs quantum dots

Photon correlation measurements reveal memory effects in the optical emission of single InAs quantum dots with time scales from 10 to 800 ns. With above-band optical excitation, a long-time scale negative correlation (antibunching) is observed, while with quasiresonant excitation, a positive correlation (blinking) is observed. A simple model based on long-lived charged states is presented that approximately explains the observed behavior, providing insight into the excitation process. Such memory effects can limit the internal efficiency of light emitters based on single quantum dots, and could also be problematic for proposed quantum-computation schemes.