Wolfgang-Michael Schulz

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 pumped single-photon emission in the visible spectral range up to 80 K

We present an electrically pumped single-photon emitter in the visible spectral range, working up to 80 K, realized using a self-assembled single InP quantum dot. We confirm that the electroluminescense is emitted from a single quantum dot by performing second-order autocorrelation measurements and show that the deviation from perfect single-photon emission is entirely related to detector limitations and background signal. Emission from both neutral and charged exciton complexes was observed with their relative intensites depending on the injection current and temperature.

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.

Room-temperature lasing of electrically pumped red-emitting InP/(Al0.20Ga0.80)0.51In0.49P quantum dots embedded in a vertical microcavity

We demonstrate electrically pumped laser light emission in the visible (red) spectral range using self-assembled InP quantum dots embedded in a microcavity mesa realized by monolithically grown high-reflectivity AlGaAs distributed Bragg reflectors. We used common semiconductor laser processing steps to fabricate stand-alone index-guided vertical-cavity surface-emitting lasers with oxide apertures for optical wave-guiding and electrical current constriction. Ultra-low threshold of around 10 A/cm2 and room temperature lasing were demonstrated. Additionally, the temperature independence of the threshold current, which was predicted in theory for quantum dot lasers, is displayed.

Triggered single-photon emission in the red spectral range from optically excited InP/(Al,Ga)InP quantum dots embedded in micropillars up to 100 K

Systematic excitation power and temperature-dependent measurements on the emission lines of single self-assembled InP/(Al0.20Ga0.80)0.51In0.49P quantum dots embedded in micropillars have been performed. The quantum dots were excited optically via a pulsed laser and their luminescence was collected using a micro-photoluminescence setup. The exciton and biexciton intensity, linewidth, and spectral position was investigated in a temperature range from 4 K up to 130 K. Single-photon emission from the quantum dots is presented up to a temperature of 100 K, confirmed by photon-statistics measurements.

Low density MOVPE grown InGaAs QDs exhibiting ultra-narrow single exciton linewidths

Low density (approximately 10(7) cm(-2)), small sized InGaAs quantum dots were grown on a GaAs substrate by metal-organic vapor-phase epitaxy and a special annealing technique. The structural quantum dot properties and the influence of the annealing technique was investigated by atomic force microscope measurements. High-resolution micro-photoluminescence spectra reveal narrow photoluminescence lines, with linewidths down to 11 microeV and fine structure splittings of 25 microeV. High signal to noise ratios (approximately 140) and a nearly background free autocorrelation measurement indicate an excellent optical quality and single photon emission behavior. Furthermore, time resolved measurements reveal excitonic decay times typically in the range between 800 and 2300 ps and biexcitonic decay times around 300 ps.

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.

Polarization fine structure and enhanced single-photon emission of self-assembled lateral InGaAs quantum dot molecules embedded in a planar microcavity

Single lateral InGaAs quantum dot molecules have been embedded in a planar micro-cavity in order to increase the luminescence extraction efficiency. Using a combination of metal-organic vapor phase and molecular beam epitaxy samples could be produced that exhibit a 30 times enhanced single-photon emission rate. We also show that the single-photon emission is fully switchable between two different molecular excitonic recombination energies by applying a lateral electric field. Furthermore, the presence of a polarization fine-structure splitting of the molecular neutral excitonic states is reported which leads to two polarization-split classically correlated biexciton exciton cascades. The fine-structure splitting is found to be on the order of 10 micro-eV.

Low Threshold InP/AlGaInP Quantum Dot In-Plane Laser Emitting at 638 nm

Within this contribution, results for a laser structure consisting of InP quantum dots embedded in an (AlxGa1-x)0.51In0.49P matrix lattice matched to GaAs are presented. The structure was fabricated using metal–organic vapor-phase epitaxy, showing electrically pulsed laser operation at room temperature with a low threshold current density of 870 A/cm2 and a lasing wavelength of 638 nm for a 2000 µm long device with uncoated facets. Optical output powers of more than 55 mW per facet and lasing up to 313 K is demonstrated.

Optical properties of red emitting self-assembled InP/(Al0.20Ga0.80)0.51In0.49P quantum dot based micropillars

Using focused ion beam etching techniques, micropillar cavities were fabricated from a high reflective AlAs/AlGaAs distributed Bragg reflector planar cavity containing self-assembled InP quantum dots in (Al(0.20)Ga(0.80))(0.51)In(0.49)P barrier layers. The mode spectra of pillars with different diameters were investigated using micro-photoluminescence, showing excellent agreement with theory. Quality factors of the pillar cavities up to 3650 were observed. Furthermore, for a microcavity pillar with 1.26 mum diameter, single-photon emission is demonstrated by performing photon correlation measurements under pulsed excitation.