C. A. Kessler

Quantum key distribution using quantum dot single-photon emitting diodes in the red and near infrared spectral range

We report on in-lab free space quantum key distribution (QKD) experiments over 40cm 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 27.2kbits 1 (35.4kbits 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

Electrically driven quantum dot single-photon source at 2 GHz excitation repetition rate with ultra-low emission time jitter

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.

High-power InP quantum dot based semiconductor disk laser exceeding 1.3 W

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 %.

Triggered single-photon emission from electrically excited quantum dots in the red spectral range

Pulsed electrical excitation was used to excite single InP/Ga0.51In0.49P quantum dots and obtain triggered single-photon emission in the red spectral range at an excitation repetition rate of up to 200 MHz. Increased repetition rates are prevented by the finite decay-time, and autocorrelation measurements look similar to what is expected for dc injection above 1 GHz. Finally, it is shown that negative voltage pulses can increase the decay-rate considerably such that 1 GHz excitation rates should be possible.

Low-density InP quantum dots embedded in Ga0.51In0.49P with high optical quality realized by a strain inducing layer

We present a method to reduce the intrinsically high InP quantum dot density embedded in a Ga0.51In0.49P barrier by introducing an InGaAs quantum dot seed layer. The additional strain reduces the total InP quantum dot density by around one order of magnitude from 2×1010 to 3×109 cm−2 but only ∼1% of the InP nanostructures seem to be optically active (107 cm−2). Therefore, microphotoluminescence measurements could be accomplished without masks. We found resolution-limited photoluminescence linewidths (ΔE<100 μeV), good signal-to-noise ratios (∼65), single-photon emission behavior [g(2)(τ=0)=0.3], and excitonic decay times of typically between 1 and 2 ns. Furthermore the structural quantum dot properties were investigated.

Influence of the oxide aperture radius on the mode spectra of (Al,Ga)As vertical microcavities with electrically excited InP quantum dots

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.

Fabrication and optical characterization of large scale membrane containing InP/AlGaInP quantum dots.

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.

InP quantum dot based semiconductor disk laser emitting at 655 nm

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 selection, 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.

High-frequency electrically driven quantum dot single-photon source

Compact and efficient single-photon sources are key components for several future applications, e.g in quantum cryptography, random number generators, and for a future standard of optical brightness. To date, commercial single-photon detectors provide highest sensitivity in the red spectral range and single-photon based technologies such as quantum communication benefit from their low detector dark count rates. Thus, the red spectral range is suited for free space communication or via polymer optical fibers in last mile networks.

High‐frequency Triggered Single—Photon Emission From Electrically Driven InP/(Al,Ga)InP Quantum Dots

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.