Single-photon sources with a high extraction efficiency are a prerequisite for applications in quantum communication and quantum computation schemes. One promising approach is the fabrication of a quantum dot containing membrane structure in combination with a solid immersion lens and a metal mirror. We have fabricated an 80 nm thin semiconductor membrane with incorporated InP quantum dots in an AlGaInP double hetero barrier via complete substrate removal. In addition, a gold layer was deposited on one side of the membrane acting as a mirror. The optical characterization shows in detail that the unique properties of the quantum dots are preserved in the membrane structure.

Summary form only given. Semiconductor disk lasers (SDLs), also known as vertical-external surface-emitting-lasers (VECSELs) are an ideal choice to reach high output powers. Further advantageous characteristics [1] of this laser type, as e.g. superior beam quality with a radial symmetric TEM00 beam profile, are provided by the external cavity. A further big advantage of VECSELs is given by the possibility of bandgap engineering. By proper adjustment of the semiconductor material and their composition, many different wavelength areas can be covered. The use of quantum dot (QD) layers instead of quantum wells (QWs) should further lead, according to theory [2], to broader gain spectra as well as to lower laser thresholds accompanied by decreased temperature sensitivity.We present a continuous-wave VECSEL system, based on a RPG structure with multiple InP QD layers (see Fig. 1a), emitting around 655 nm. All samples of this study were fabricated by metal-organic vapor-phase epitaxy. The seven single InP QD layers are embedded in a separate confinement heterostructure (SCH) which consist of tensile strained (Al0.1Ga0.9)0.52In0.48P barriers and (Al0.55Ga0.45)0.52In0.48P cladding layers. Below the active region an Al0.45GaAs / AlAs distributed Bragg reflector (DBR) consisting of 55 λ/4 pairs to generate a reflectivity of R>99.9 % is fabricated. The QD characteristics in ensemble and micro-photoluminescence investigations indicate that really the QDs are contributing to this emission. Auto-correlation measurements on a sample with a single QD layer, proves that the luminescence consists of emission of individual QDs. Measurements of the standard laser parameters reveal maximum output powers of 1.4 W at a low emission wavelength ~ 654 nm with a slope efficiency of ηdiff = 25.4% (Fig. 1b). Laser characteristics like high output power at the mentioned wavelenth and the possibility of inserting optical intra-cavity elements for wavelength selectio- , frequency doubling and tuning, given by the external cavity, make the here introduced VECSEL a very well suited laser source for medical applications like photodynamic therapy (in the red sprctral range) or for scientific and bio-technological applications as coherent light source (frequency doubled to ultraviolet spectral range) for micro-photoluminescence of nitride structures and for luminescence microscopy on biological samples.

In this letter, we report about mode characteristics of microcavity lasers with red-emitting InP quantum dots. The mode spectra and the quality factor of devices with different oxide aperture sizes are analyzed. The lateral mode confinement in the electrical devices is defined via oxide apertures. We found a good agreement between a simple analytical modeling of the mode structure and measurements, which allows to adjust the design of future devices. The quality factors show an analogous behavior as etched micropillars. The enhanced intensity of the higher order modes compared to the fundamental mode can be explained with the current density distribution within the device favoring higher order modes.

The work demonstrates frequency downconversion of single photons from a quantum dot and prove the preservation of the single photon character. The authors were able to translate the frequency of single photons of a semiconductor quantum dot from the red spectral range to the telecom O-band. It was proven that this conversion process leaves the quantum properties of the photons untouched. The high over-all efficiency of 32% for this conversion interface makes it well suited for an application in long-range quantum transmission links.

We demonstrate an optically pumped semiconductor disk laser (OP-SDL) using InP quantum dots (QDs) as active material fabricated by metal-organic vapor-phase epitaxy. The QDs are grown within [(Al0.1Ga0.9)0.52In0.48]0.5P0.5 (abbr. Al0.1GaInP) barriers in order to achieve an emission wavelength around 655 nm. We present optical investigations of the active region showing typical QD behavior like blue shift with increasing excitation power and single emission lines, which show anti-bunching in an intensity auto-correlation measurement. We report maximum output powers of the OP-SDL of 1.39 W at low emission wavelength of ∼654 nm with a slope efficiency of ηdiff=25.4 %.

The influence of the bias voltage on emission properties of a red emitting InP/GaInP quantum dot based single-photon source was investigated. Under pulsed electrical excitation, we can influence the band bending of the p-i-n diode with the applied bias voltage and thus the charge carrier escape by quantum tunneling. This leads to control over the non-radiative decay channel and allows carrier escape times as low as 40 ps, effectively reducing the time jitter of the photon emission. We realized high excitation repetition rates of up to 2 GHz while autocorrelation measurements with g(2)(0)-values of 0.27 attest dominant single-photon emission.

We demonstrate efficient (>30%) quantum frequency conversion of visible single photons (711 nm) emitted by a quantum dot to a telecom wavelength (1313 nm). Analysis of the first- and second-order coherence before and after wavelength conversion clearly proves that pivotal properties, such as the coherence time and photon antibunching, are fully conserved during the frequency translation process. Our findings underline the great potential of single photon sources on demand in combination with quantum frequency conversion as a promising technique that may pave the way for a number of new applications in quantum technology.

We report on in-lab free space quantum key distribution (QKD) experiments over 40 cm distance using highly efficient electrically driven quantum dot single-photon sources emitting in the red as well as near-infrared spectral range. In the case of infrared emitting devices, we achieve sifted key rates of 27.2 kbit s−1 (35.4 kbit s−1) at a quantum bit error rate (QBER) of 3.9% (3.8%) and a g(2)(0) value of 0.35 (0.49) at moderate (high) excitation. The red emitting diodes generate sifted keys at a rate of 95.0 kbit s−1 at a QBER of 4.1% and a g(2)(0) value of 0.49. This first successful proof of principle QKD experiment based on electrically operated semiconductor single-photon sources can be considered as a major step toward practical and efficient quantum cryptography scenarios.

For a single quantum dot under excitation with short electrical pulses the dependence of the photon anti-bunching on pulse width and excitation strength is studied in a theory-experiment collaboration.

Single vertically stacked pairs of InP quantum dots (QDs) with different interdot barrier width are investigated using microphotoluminescence, photoluminescence excitation spectroscopy and time-resolved techniques. The results indicate unidirectional carrier tunneling to be the main coupling mechanism between the two layers containing QDs with intentionally different size. The coupling is changing from electronhole tunneling to electron tunneling and finally vanishing tunneling with increasing barrier size.

We present excitation-pulse-width- and pump-power-dependent microelectroluminescence and photon statistics measurements on electrically driven single-photon devices based on InP/AlGaInP quantum dots (QDs). For an excitation regime far below QD saturation, the results show a characteristic decrease of the purity of the single-photon emission [${g}^{(2)}(0)$ value] with increasing excitation pulse width. For stronger excitation pulses close to QD saturation, strong antibunching is maintained for a much larger pulse width. In this case the ground-state exciton emission, which is used for the single-photon source, is inhibited during the pump pulse due to the presence of higher excited states. This prevents multiple-ground-state emission and reexcitation during long pump pulses and delays the single-photon emission to the end of the pulse, as predicted by theory and confirmed experimentally.

We report on the epitaxial growth of vertically stacked InP and In(Ga)As quantum dot (QD) layers to realize a triple dot quantum gate structure consisting of an asymmetric control double dot and a single target dot suitable for a CNOT gate structure. Structural analysis as well as studies on the optical properties are presented. For studies on control dot structures we analyze the growth of InP islands in a GaInP barrier on (100) GaAs substrates. By stacking InP QD layers with intentional asymmetric design in QD size of each layer and adjustment of the barrier width between the double dots, coupling and control of the coupling via barrier layer width‐design can be demonstrated. For defined gate action single dot spectra of aligned dots are indispensable. Therefore QD density reduction is studied. We study two possibilities to affect the QD density and control the dot site by manipulating the surface potential of InP island nucleation. (i) Growth of InP islands on top of a low density In(Ga)As QD seed layer (ii) Growth of InP islands on patterned (100) GaAs substrates. We present microsphere photolithography in combination with wet chemical etching as a fast and low‐cost method to produce regular hole arrays in a GaAs surface, which are suitable for controlled nucleation of self‐assembled InP islands.

The heteroepitaxy of III-V semiconductors on silicon is a promising approach for making silicon a photonic platform. Mismatches in material properties, however, present a major challenge, leading to high defect densities in the epitaxial layers and adversely affecting radiative recombination processes. However, nanostructures, such as quantum dots, have been found to grow defect-free even in a suboptimal environment. Here we present the first realization of indium phosphide quantum dots on exactly oriented Si(001), grown by metal-organic vapour-phase epitaxy. We report electrically driven single-photon emission in the red spectral region, meeting the wavelength range of silicon avalanche photodiodes' highest detection efficiency.

In our work pulsed electrical excitation was used to excite single InP/Ga 0.51 In 0.49 P quantum dots which were embedded into the intrinsic region of a p-i-n mesa structure. To enhance collection efficiency the active region is surrounded by two Bragg reflectors forming a low-Q cavity. The samples are excited using a pulse pattern generator together with a fast amplifier. We obtain triggered single-photon emission in the red spectral range (~ 650 nm) at an excitation repetition rate of up to 200 MHz.

We demonstrate electrical pumping of self-assembled InP/Ga(0.51)In(0.49)P quantum dots embedded in a p-i-n resonant-cavity-diode structure with emission in the red spectral region. A high aluminum containing Al(0.98)Ga(0.02)As layer allows wet thermal oxidation and implementation of a current restricting oxide aperture above the active region. The intended use of these InP-quantum dots in such a resonant-cavity-LED structure as a pulsed electrically driven single-photon emitter was confirmed by measuring the second order intensity correlation function g((2))(tau) with a Hanbury-Brown and Twiss type setup. The correlation measurements performed on a single quantum dot (approximate to 40K) show a clear antibunching behavior (g((2))(0) <0.24) up to 200 MHz as expected for a single-photon emitter. (C) 2010 Elsevier B.V. All rights reserved.

We used sub‐nanosecond electrical pulses to excite single InP/GaInP quantum dots to realize triggered single‐photon emission in the red spectral range. The electroluminescence of different quantum dots was investigated and the successful injection of short voltage pulses was verified by time—resolved and autocorrelation measurements.