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We demonstrate progress in developing a new microwave-optical quantum transducer. We demonstrate focusing and stable surface acoustic wave cavities at 3.4 GHz and characterize the acoustic performance. This work is a contribution of the National Institute of Standards and Technology; not subject to copyright in the United States of America.

We present a novel semiconductor single-photon source based on tensile-strained (111)-oriented GaAs/InAlAs quantum dots (QDs) exhibiting ultrasmall exciton fine-structure splitting (FSS) of ≤ 8 µeV. Using low-temperature micro-photoluminescence spectroscopy, we identify the biexciton-exciton radiative cascade from individual QDs, which, combined with small FSS, indicates these self-assembled GaAs(111) QDs are excellent candidates for polarization-entangled photon-pair generation.

We report on the optical gain and loss of waveguides containing lithographically defined quantum dots. Lasing action has previously been demonstrated in a nominally identical structure. Measurements are made by monitoring the transmission of a resonant pulse while varying the injection current. We measure a maximum modal gain of 1.8 cm \(^{{-1}}\) at the peak of the ground state emission for a two-layer structure. The peak gain is insufficient for ground state lasing to be achieved in a structure with as-cleaved facets, but the gain per dot is comparable with that demonstrated in self-assembled quantum dots.

We report lasing wavelength bistability with respect to applied bias on the saturable absorbers in two-section mode-locked quantum dot lasers. We show data from three different devices exhibiting wavelength bistability. All lasers display wavelength bistability. Only one lasing wavelength is present at a time, with all other wavelengths totally quenched. The switchable ranges (the wavelength difference between two bistable lasing branches) are different for all three lasers and in one device can be manipulated by changing the current injection. All lasers show the remarkable property of switching only in integer multiples of about 8 nm. The bistable operation can be explained by the interplay of the cross-saturation and self-saturation properties in gain and absorber, and the quantum-confined Stark effect in absorber. The measured switching time between bistable wavelengths is 150 ps.

We report development of two absorption-based spectroscopic methods that have been adapted from atomic physics techniques to elucidate the basic physical properties of InGaAs/GaAs self-assembled quantum dots(SAQDs). Absorptive spectroscopic measurements allow the examination of the SAQDs optical transitions free from carrier relaxation effects. In addition, we employ these techniques to study SAQDs' optical transition with a level of spectral resolution not available using ultra-fast techniques. The first of the two approaches we discuss is cavity ring-down spectroscopy (CRDS), which permits an absolute measurement of absorption. We report on initial application of CRDS to SAQDs and present an assessment based on these measurements for single SAQD spectroscopy using this technique. The second method, spectral hole burning, is applied to SAQDs in a semiconductor ridge waveguide and permits the homogeneous linewidth of the SAQDs to be separated from the broad inhomogeneous spectrum.

In semiconductor quantum dots, the electron hyperfine interaction with the nuclear spin bath is the leading source of spin decoherence at cryogenic temperature. Using high-resolution two-color differential transmission spectroscopy, we demonstrate that such electron-nuclear coupling also imposes a lower limit for the positively charged exciton dephasing rate, \gamma, in an ensemble of InAs/GaAs quantum dots. We find that the dephasing rate is sensitive to the strength of the hyperfine interaction, which can be controlled through the application of an external magnetic field in the Faraday configuration. At zero applied field, strong electron-nuclear coupling induces additional dephasing beyond the radiative limit and \gamma = 230 MHz (0.95 \mu eV). Screening of the hyperfine interaction is achieved for an external field of ~1 T, resulting in \gamma = 172 MHz (0.71 \mu eV) limited only by spontaneous recombination. On the other hand, application of a Voigt magnetic field mixes the spin eigenstates, which increases the dephasing rate by up to 75%. These results are reproduced with a simple and intuitive model that captures the essential features of the electron hyperfine interaction and its influence on \gamma., Comment: 9 pages, 6 figures

We present a new technique for Multi-Dimensional Coherent spectroscopy of nano-structures. We measure the Four-Wave Mixing (FWM) amplitude and phase via photocurrent detection. The measurement is suitable for any nano-structures that can be electrically contacted.

We directly measure the optical gain and threshold current of lithographically-defined quantum dots. A peak groundstate gain of 1.8 cm−1 is determined. We also measure the recovery dynamics of the groundstate with the device biased above and below transparency using ultrafast differential transmission spectroscopy.

The propagation of optical pulses resonant with the ground-to-excited state transition of InGaAs quantum dots is investigated. An analysis of low intensity excitation yields a dipole moment of 8.8×10−29 to 10.9×10−29 C m, depending on the quantum dot growth conditions. We observe polarization of the dipole moment exclusively in the plane perpendicular to the growth direction.

Passively mode-locked semiconductor lasers with self-assembled quantum dot active regions can be operated in exotic output modes, stabilized by the complex gain and absorption dynamics inherent in these structures. One such device emits dark pulses—sharp dips on an otherwise stable continuous wave background—in an extended cavity design. We show that a dark pulse train is a solution to the master equation for mode-locked lasers and perform numerical modeling to test the stability of such a solution. A separate, monolithic design displays wavelength bistability and can be electrically switched between these two modes within just a few cavity round trips. This device can be made to switch between two stable wavelengths separated by just 7 nm up to over 40 nm with a contrast ratio of over 40 dB.

We describe an operating regime for passively mode-locked quantum dot diode laser where the output consists of a train of dark pulses, i.e., intensity dips on a continuous background. We show that a dark pulse train is a solution to the master equation for mode-locked lasers. Using simulations, we study stability of the dark pulses and show they are consistent with the experimental results.

We investigate the wavelength switching properties of a bistable two-section quantum-dot diode laser. The switching time is about 150 ps, which is approximately two round trips times of the laser.

Direct measurement of the absorption of an ensemble of InGaAs quantum dots using a heterodyne multibounce technique reveals that the saturation fluence decreases dramatically with decreasing temperature. The dependence is attributed to homogeneous linewidth narrowing.

We examine the temperature dependence of the ground state homogeneous linewidth in InGaAs/GaAs quantum dots. Measurements using a high-resolution spectral hole burning technique are performed on quantum dots in a semiconductor waveguide. This is a submission of the United States government and is not subject US copyright laws.

Coupling between quantum dots is investigated using differential transmission spectroscopy. Two-color pump-probe techniques are used to spectrally resolve the carrier dynamics, revealing carrier transfer between quantum dots at room temperature. The time constant for this process is shown to increase from at room temperature to at .

We employ two-color differential transmission spectroscopy to investigate coupling between InGaAs/GaAs QDs. Resonantly excited carriers escape and are captured by non-resonant dots with a time constant of 32 ps at room temperature

Summary from only given. We measure the dipole moment of InGaAs/GaAs self-assembled quantum dots by investigating the propagation of femtosecond light pulses interacting resonantly with the ground to excited state transition. The measurement involves coupling light into a waveguide containing quantum dots in the core and time resolving the output. The advantages are increased interaction length with the weakly absorbing quantum dots and observation of the decay of light pulses over successive cavity round trips. The large uncertainty in waveguide coupling efficiency that has hindered the determination of the absolute dipole moment in similar experiments' does not affect our results.

We measure the ultrafast optical response of lithographically defined quantum dot amplifiers at 40 K. Recovery of the gain mostly occurs in less than 1 picosecond, with some longer-term transients attributable to carrier heating. Recovery of the absorption proceeds on a much longer timescale, representative of relaxation between quantum dot levels and carrier recombination. We also measure transparency current-density in these devices.

We demonstrate InGaAs/GaAs quantum dot lasers with multimode lasing at room temperature immediately above threshold. The lasing modes are separated by about ten times the Fabry-Perot mode spacing, with several dark modes in between the lasing modes. Rate equation simulations indicate that this multimode behavior can be explained by a homogeneous broadening that is on the order of the mode spacing.

We measure the time-resolved photoluminescence characteristics of a novel type of lithographically patterned quantum dot fabricated by electron beam lithography, wet chemical etching, and overgrowth of the barrier layers by metalorganic chemical vapor deposition.We find that the quantum dot (QD) photoluminescence exhibits a bi-exponential decay that we explain in terms of the fast capture of carriers by defect states followed by a slower radiative relaxation process. We also perform a systematic investigation of the rise time and decay time as a function of the QD density, size, and temperature. These measurements indicate that the carrier capture process in this type of QD is limited by carrier drift within the GaAs barrier material.

We demonstrate photon antibunching from a single lithographically defined quantum dot fabricated by electron beam lithography, wet chemical etching, and overgrowth of the barrier layers by metalorganic chemical vapor deposition. Measurement of the second-order autocorrelation function indicates g(2)(0) = 0.395 ± 0.030, below the 0.5 limit necessary for classification as a single photon source.

We report the use of continuous wave spectral hole burning to perform high-resolution spectroscopy of the homogeneous linewidth of self-assembled InGaAs-GaAs quantum dots at low temperature. We use this technique to examine the power broadening behavior of the homogeneous InGaAs-GaAs quantum dot line. We find that at a temperature of 9.8 K and over the majority of the pump powers considered, the spectral hole signal is well fit by a single Lorentizian line shape. Analysis of the power broadening yields a full width at half maximum of 0.74μeV for the homogeneous linewidth and a corresponding coherence time T2 of 1.76 ns.

Self-assembled InGaAs quantum dots have been grown using alternating molecular beams of In, Ga, and As2. The size distribution changes from bimodal to monodisperse as the quantum dots grow larger. Room-temperature photoluminescence experiments on ensembles of these quantum dots show that the emitted intensity remains high as the center wavelength changes from about 1130 to 1345 nm. The linewidths are less than 30 meV for all samples studied, with the narrowest measured linewidth being 18 meV at a peak emission energy of 930.1 meV (1333 nm).

We demonstrate electronic control of the charged exciton optical coherence time in quantum dots embedded in a photonic ridge waveguide. Through electronic manipulation of the electron and hole wavefunctions, coherence is extended by ∼50%.