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The impact ionization characteristics of (Al x Ga1- x )0.52In0.48P have been studied comprehensively across the full composition range. Electron and hole impact ionization coefficients ( $\alpha $ and $\beta $ , respectively) have been extracted from avalanche multiplication and excess noise data for seven different compositions and compared to those of Al x Ga1- x As. While both $\alpha $ and $\beta $ initially decrease gradually with increasing bandgap, a sharp decrease in $\beta $ occurs in (Al x Ga1- x )0.52In0.48P when ${x} > 0.61$ , while $\alpha $ decreases only slightly. $\alpha $ and $\beta $ decrease minimally with further increases in ${x}$ and the breakdown voltage saturates. This behavior is broadly similar to that seen in Al x Ga1- x As, suggesting that it may be related to the details of the conduction band structure as it becomes increasingly indirect in both alloy systems.

An AlInP 3 × 3 pixel monolithic array was fabricated from a p + -i-n + structure wafer (6 μ m thick i layer) grown by metalorganic vapour phase epitaxy; each pixel (square mesa photodiode) had an area of 200 μ m × 200 μ m. The pixels were initially electrically characterized and were then subjected to illumination by X-ray and γ -ray photons of energies ≤ 88 keV while each of them was in turn coupled to a custom-made, low noise, charge-sensitive preamplifier. The photon counting X-/ γ - ray spectroscopic performance of the pixels was investigated by obtaining and analysing 55Fe X-ray, 241Am X-/ γ - ray, and 109Cd X-/ γ - ray spectra; the energy resolution (Full Width at Half Maximum), when the array and preamplifier were operated at the maximum investigated temperature (100 ∘ C), was 1.54 keV ± 0.08 keV at 5.9 keV, 1.58 keV ± 0.08 keV at 22.16 keV, and 1.57 keV ± 0.08 keV at 59.54 keV. This work sets the agenda for future development of an AlInP photon counting X-/ γ - ray spectroscopic imager for uncooled operation in high temperature environments.

Quantum light emitting diodes are important for many secure quantum networking applications. Here, we address some remaining practical challenges, and present progress in improving the electrical frequency, fiber compatibility and optical coherence of these devices.

A prototype In0.53Ga0.47As p+-i-n+ x-ray photodiode, fabricated from material grown by metalorganic vapour phase epitaxy, was investigated as a novel detector of x-rays. The detector was connected to a custom low-noise charge sensitive preamplifier and standard readout electronics to produce an x-ray spectrometer. The detector and preamplifier were operated at a temperature of 233 K (−40 °C). An energy resolution of 1.18 keV ± 0.06 keV Full Width at Half Maximum at 5.9 keV was achieved. This is the first time InGaAs (GaInAs) has been shown to be capable of spectroscopic photon counting x-ray detection.

Teleportation is a fundamental concept of quantum mechanics with an important application in extending the range of quantum communication channels via quantum relay nodes. To be compatible with real-world technology such as secure quantum key distribution over fiber networks, such a relay node should ideally operate at gigahertz clock rates and accept time-bin-encoded qubits in the low-loss telecom band around 1550 nm. Here, we show that In As-In P droplet-epitaxy quantum dots, with their sub-Poissonian emission near 1550 nm, are ideally suited for the realization of this technology. To create the necessary on-demand photon emission at gigahertz clock rates, we develop a flexible-pulsed optical-excitation scheme and demonstrate that the fast driving conditions are compatible with a low multiphoton emission rate. We show further that, even under these driving conditions, photon pairs obtained from the biexciton cascade show an entanglement fidelity close to 90%, comparable to the value obtained under continuous-wave excitation. Using asymmetric Mach-Zehnder interferometers and our photon source, we finally construct a time-bin qubit quantum relay able to receive and send time-bin-encoded photons and demonstrate mean teleportation fidelities of 0.82 ± 0.01, exceeding the classical limit by more than ten standard deviations.

Previously, an Al 0.52 In 0.48 P p + -i-n + spectroscopic photon counting X-ray photodiode with 2 μ m thick i layer ( 200 μ m diameter) was shown to suffer from energy-dependent incomplete charge collection noise (Lioliou et al., 2019). Subsequent measurements on a larger ( 400 μ m diameter) Al 0.52 In 0.48 P p + -i-n + photodiode (reported here) revealed the presence of even greater incomplete charge collection noise. Given these findings, an expectation would have been that thicker Al 0.52 In 0.48 P structures (which would be required for efficient absorption of all but the softest X-rays) would have a greater incomplete charge collection noise contribution, thus suggesting that thick Al 0.52 In 0.48 P photodiodes may be of limited practicality as high performance detectors for photon counting X-ray spectroscopy. However, two new Al 0.52 In 0.48 P p + -i-n + photodiodes (with 6 μ m i layers) were fabricated from material grown by the same technique (metalorganic vapour phase epitaxy) in the same reactor, and are now shown here to exhibit no signs of detectable incomplete charge collection noise under the illumination of X-ray photons of energy 4.95 keV to 21.17 keV. As such, now that greater experience has been built with Al 0.52 In 0.48 P, concerns about incomplete charge collection noise in X-ray detectors made from the material appear to have been unwarranted; the path towards thick Al 0.52 In 0.48 P X-ray detectors is now clear.

A technique for direct study of the distribution of the applied voltage within a quantum cascade laser (QCL) has been developed. The detailed profile of the potential in the laser claddings and laser core region has been obtained by gradient scanning Kelvin probe force microscopy (KPFM) across the cleaved facets for two mid-infrared quantum cascade laser structures. An InGaAs/InAlAs quantum cascade device with InP claddings demonstrates a linear potential distribution across the laser core region with constant voltage drop across the doped claddings. By contrast, a GaAs/AlGaAs device with AlInP claddings has very uneven potential distribution with more than half of the voltage falling across the claddings and interfaces around the laser core, greatly increasing the overall voltage value necessary to achieve the lasing threshold. Thus, KPFM can be used to highlight design and fabrication flaws of QCLs.

We show deployment of a tunable telecom entangled photon source based on semiconductor quantum dots in an installed fiber network. We further report about network integration of a pulsed device operating at GHz clock rates.

Quantum networks are essential for realising distributed quantum computation and quantum communication. Entangled photons are a key resource, with applications such as quantum key distribution, quantum relays, and quantum repeaters. All components integrated in a quantum network must be synchronised and therefore comply with a certain clock frequency. In quantum key distribution, the most mature technology, clock rates have reached and exceeded 1GHz. Here we show the first electrically pulsed sub-Poissonian entangled photon source compatible with existing fiber networks operating at this clock rate. The entangled LED is based on InAs/InP quantum dots emitting in the main telecom window, with a multi-photon probability of less than 10% per emission cycle and a maximum entanglement fidelity of 89%. We use this device to demonstrate GHz clocked distribution of entangled qubits over an installed fiber network between two points 4.6km apart.

InAs thermophotovoltaic (TPV) cells with external quantum efficiency at the peak wavelengths reaching 71% at low temperature and 55% at room temperature are reported, which are the highest values to date for InAs. The TPV exhibited 10% power conversion efficiency at 100 K cell temperature. The dark and light current-voltage characteristics were measured at different cell temperatures (100–340 K) in response to heat sources in the range 500–800 °C. The resulting dependences of the output voltage and current as well as the spectral response of the InAs TPV have been extensively characterized for waste heat recovery applications. The performance of these cells is strongly determined by the dark current which increases rapidly with increasing cell temperature originating from bandgap narrowing, which resulted in a reduction of open circuit voltage and output power.

A 2 × 2 square pixel In 0.5 Ga 0.5 P p + -i-n + mesa photodiode array was fabricated and investigated for its suitability in photon counting X-ray and γ -ray spectroscopy. Each pixel, which had an area of 200 μ m × 200 μ m and a 5 μ m thick i layer, was coupled to a low-noise charge-sensitive preamplifier and standard onwards readout electronics to form an X-ray and γ -ray photon counting spectrometer. The pixels were illuminated in turn with an 55Fe radioisotope X-ray source, an 241Am radioisotope X-ray and γ -ray source, and a 109Cd radioisotope X-ray and γ -ray source. The mean value (across all pixels) of the best energy resolution (Full Width at Half Maximum, FWHM) at 20 °C was 770 eV ± 30 eV at 5.9 keV, 840 eV ± 20 eV at 22.16 keV, and 870 eV ± 30 eV at 59.54 keV. The spectroscopic response of one of the pixels was then investigated at temperatures up to 100 °C; noise analysis was performed and the different noise contributions were identified. The FWHM at 100 °C was 1.29 keV ± 0.04 keV at 5.9 keV, 1.32 keV ± 0.06 keV at 22.16 keV, 1.34 keV ± 0.06 keV at 59.54 keV, and 1.43 keV ± 0.08 keV at 88.03 keV. The results indicate that the detector did not suffer from incomplete charge collection, and that the spectrometer had better energy resolution at 100 °C than any other multi-pixel radiation spectrometer so far reported.

The development of new x-ray and γ-ray spectrometers based on AlInP photodiodes with increased quantum detection efficiency and improved energy resolution is reported. The spectroscopic responses of two AlInP p+–i–n+ mesa photodiodes (10 μm i layer, the thickest so far reported) were investigated at photon energies from 4.95 to 88.03 keV; the detectors and preamplifier were operated at 30 °C. Energy resolutions (full width at half maximum) of 750 ± 40 eV and 850 ± 30 eV at 4.95 keV were achieved with the two detectors. The energy resolution deteriorated with increasing photon energy; this was in accordance with the increasing Fano noise with energy and suggested negligible incomplete charge collection noise across the photon energy range investigated. The measured voltage output of each spectrometer was found to be linear as a function of incident x-ray photon energy. The count rate (measured at 8.63 keV) was also found to linearly increase with incoming x-ray photon flux for the investigated spectrometers. These results, which were obtained using the thickest AlInP photodiodes produced so far, suggest that AlInP detectors are highly promising candidates for future uncooled x-ray and γ-ray spectrometers.

Quantum cascade lasers can be efficient infrared radiation sources and consist of several hundreds of very thin layers arranged in stacks that are repeated periodically. Both the thicknesses of the individual layers as well as the period lengths need to be monitored to high precision. Different transmission electron microscopy methods have been combined to analyse AlGaAs/GaAs quantum cascade laser structures in cross-section. We found a small parabolic variation of the growth rate during deposition, affecting the stack periodicity and a reduced aluminium content of the AlGaAs barriers, whereas their widths as well as those of the GaAs quantum wells agreed with the nominal values within one atomic layer. Growth on an offcut substrate led to facets and steps at the interfaces.

InP/GaInP quantum dot laser structures exhibiting broad optical gain spectra suitable for mode-locking have been demonstrated. Two-section narrow ridge passive mode-locked lasers were fabricated from this material. Mode-locking conditions have been investigated for devices with different cavity lengths, with maximum frequency of 15.21 GHz.

Two custom-made Al0.52In0.48P p+-i-n+ mesa photodiodes with different diameters (217 µm ± 15 µm and 409 µm ± 28 µm) and i layer thicknesses of 6 µm have been electrically characterised over the temperature range 0 °C to 100 °C. Each photodiode was then investigated as a high-temperature-tolerant photon counting X-ray detector by connecting it to a custom-made low-noise charge-sensitive preamplifier and illuminating it with an 55Fe radioisotope X-ray source (Mn Kα = 5.9 keV; Mn Kβ = 6.49 keV). At 100 °C, the best energy resolutions (full width at half maximum at 5.9 keV) achieved using the 217 µm ± 15 µm diameter photodiode and the 409 µm ± 28 µm diameter photodiode were 1.31 keV ± 0.04 keV and 1.64 keV ± 0.08 keV, respectively. Noise analysis of the system is presented. The dielectric dissipation factor of Al0.52In0.48P was estimated as a function of temperature, up to 100 °C. The results show the performance of the thickest Al0.52In0.48P X-ray detectors so far reported at high temperature. The work has relevance for the development of novel space science instrumentation for use in hot space environments and extreme terrestrial applications.

In this work, a 200 μm diameter InGaP (GaInP) p+-i-n+ mesa photodiode was studied across the temperature range 100 °C to 20 °C for the development of a temperature-tolerant electron spectrometer. The depletion layer thickness of the InGaP device was 5 μm. The performance of the InGaP detector was analysed under dark conditions and then under the illumination of a 183 MBq 63Ni radioisotope beta particle source. The InGaP photodiode was connected to a custom-made low-noise charge-sensitive preamplifier to realise a particle counting electron spectrometer. Beta spectra were collected at temperatures up to 100 °C with the InGaP device reverse biased at 5 V. The spectrum accumulated at 20 °C was compared with the spectrum predicted using Monte Carlo simulations; good agreement was found between the predicted and experimental spectra. The work is of importance for the development of electron spectrometers that can be used for planetary and space science missions to environments of high temperature or extreme radiation (e.g. Mercury, Jupiter’s moon Europa, near-Sun comets), as well as terrestrial applications.

Quantum light sources based on single-photon emitters are a core technology, which is expected to enable a multitude of emerging quantum-photonic applications with superior performance. As the development of sources is approaching a certain level of maturity, the important next step for quantum-network related applications is to distribute the generated photons in the most practical way. However, so far these remained within research facilities or were only transmitted over short specially installed fiber. In this regard, operation of the sources at standard telecom wavelengths such that existing classical communication infrastructure can be used, seems the most desirable solution.

Indium Arsenide (InAs) infrared photodiodes provide high quantum efficiency in the wavelength range of 1.0-3.0 μm. Planar diode configuration has been adopted to reduce surface leakage. In this work, new fabrication procedures for planar InAs avalanche photodiodes (APDs) are reported. Beryllium (Be) ions were implanted into InAs at a relatively low energy of 34 keV. Effects of duration of post implant annealing on the electrical characteristics of InAs APDs were investigated. It was found that a combination of post implant annealing at 500 °C for 15 min and a shallow surface etch produces planar APDs with good characteristics (room temperature dark current density of 0.52 A/cm 2 at -0.2 V and external quantum efficiency of 51% at 1520 nm at -0.3 V). These represent a 3 times reduction in dark current and 1.4 times increase in responsivity, compared to earlier Be-implanted planar InAs APDs. The APDs' avalanche gain characteristics remain similar to those from earlier reports, with a gain of 4 at a relatively low operating bias of 5 V. This suggests the potential of integrating InAs APDs with low voltage readout integrated circuits (ROIC) for development of infrared imaging arrays. The data reported in this paper is available from the ORDA digital repository (DOI: 10.15131/shef.data.6955037).

Four Al0.52In0.48P p+-i-n+ mesa photodiodes with 6 μm thick i layers and two different diameters (217 μm ± 15 μm and 409 μm ± 28 μm) were studied at room temperature (24 °C). Electrical characterisation measurements are reported along with measurements showing the performance of the devices as x-ray detectors. The devices exhibited leakage currents

Quantum communication networks are formed of secure links, where information can be transmitted with security guaranteed by the quantum nature of light. An essential building block of such a network is a source of single photons and entangled photon pairs, compatible with the low-loss fibre telecom window around 1550 nm. Previous work based on semiconductor quantum dots (QDs), colour centres in diamond and single atoms has been limited by emission wavelengths unsuitable for long distance applications. Efforts have been made to use standard gallium arsenide based QDs by extending their operating wavelength range, however, electrically driven quantum light emission from quantum dots in this telecommunication window has not yet been demonstrated. In this work, indium phosphide based QD devices have been developed to address this problem. The industry favoured growth method, metalorganic vapour phase epitaxy (MOVPE), has been used to create droplet QDs with low fine structure splitting (FSS). This growth scheme allows us to produce the first optoelectronic devices for single and entangled photon emission in the 1550 nm telecom window. We show single-photon emission with multi-photon events suppressed to 0.11±0.02. Furthermore, we obtain entangled light from the biexciton cascade with a maximum fidelity of 0.87± 0.04 which is sufficient for error correction protocols. We also show extended device operating temperature up to 93 K, allowing operation with liquid nitrogen or simple closed-cycle coolers. Our device can be directly integrated with existing long distance quantum communication, cryptography and quantum relay systems providing a new platform for developing quantum networks.

A practical way to link separate nodes in quantum networks is to send photons over the standard telecom fibre network. This requires sub-Poissonian photon sources in the telecom wavelength band around 1550 nm, where the photon coherence time has to be sufficient to enable the many interference-based technologies at the heart of quantum networks. Here, we show that droplet epitaxy InAs/InP quantum dots emitting in the telecom C-band can provide photons with coherence times exceeding 1 ns even under non-resonant excitation, more than a factor two longer than values reported for shorter wavelength quantum dots under similar conditions. We demonstrate that these coherence times enable near-optimal interference with a C-band laser qubit, with visibilities only limited by the quantum dot multiphoton emission. Using entangled photons, we further show teleportation of such qubits in six different bases with average fidelity reaching 88.3$\pm$4%. Beyond direct applications in long-distance quantum communication, the high degree of coherence in these quantum dots is promising for future spin based telecom quantum network applications., 8 pages, 3 figures

A practical way to link separate nodes in quantum networks is to send photons over the standard telecom fibre network. This requires sub-Poissonian photon sources in the telecom wavelength band around 1550 nm, where the photon coherence time has to be sufficient to enable the many interference-based technologies at the heart of quantum networks. Here, we show that droplet epitaxy InAs/InP quantum dots emitting in the telecom C-band can provide photons with coherence times exceeding 1 ns even under non-resonant excitation, more than a factor two longer than values reported for shorter wavelength quantum dots under similar conditions. We demonstrate that these coherence times enable near-optimal interference with a C-band laser qubit, with visibilities only limited by the quantum dot multiphoton emission. Using entangled photons, we further show teleportation of such qubits in six different bases with average fidelity reaching 88.3$\pm$4%. Beyond direct applications in long-distance quantum communication, the high degree of coherence in these quantum dots is promising for future spin based telecom quantum network applications.

The ability of two photons to interfere lies at the heart of many photonic quantum networking concepts and requires that the photons are indistinguishable with sufficient coherence times to resolve the interference signals. However, for solid-state quantum light sources, this can be challenging to achieve as they are in constant interaction with noise sources in their environment. Here, we investigate the noise sources that affect InAs/InP quantum dots emitting in the telecom C-band by comparing their behavior on a wetting layer for Stranski–Krastanov grown quantum dots with a nearly wetting layer-free environment achieved with the droplet epitaxy growth mode. We show that the droplet epitaxy growth mode is beneficial for a quiet environment, leading to 96% of exciton transitions having a coherence time longer than the typical detector resolution of 100 ps, even under non-resonant excitation. We also show that the decay profile indicates the presence of slow dephasing processes, which can be compensated for experimentally. We finally conduct Hong–Ou–Mandel interference measurements between subsequently emitted photons and find a corrected two-photon interference visibility of 98.6 ± 1.6% for droplet-epitaxy grown quantum dots. The understanding of the influence of their surroundings on the quantum optical properties of these emitters is important for their optimization and use in future quantum networking applications.

Multiplication and avalanche excess noise measurements have been undertaken on a series of AlInP homo-junction p-i-n and n-i-p diodes with $i$ region widths ranging from 0.04 to 0.89 $\mu \text{m}$ , using 442 and 460 nm wavelength light. Low dark currents of $^{\mathrm {-2}}$ at 95% of breakdown voltage were obtained in all the devices because of its wide bandgap and there was no tunneling dark current present even at high fields >1000 kV/cm. For a given multiplication factor, the excess noise decreased as the avalanche width decreased due to the dead-space effect. Using 460 nm wavelength light, measurements showed that a separate absorption multiplication avalanche photodiode with a nominal multiplication region width of 0.2 $\mu \text{m}$ had an effective $k$ (hole to electron ionization coefficient ratio) of $\sim 0.3$ .

A GaAs/Al0.8Ga0.2As separate absorption and multiplication (SAM) x-ray avalanche photodiode (APD) structure was grown by metalorganic vapor phase epitaxy. Mesa photodiodes of different diameters (200 μm and 400 μm) were fabricated from the structure. Two of the photodiodes (one of each diameter) were characterized at 20 °C for their electrical properties and response to x rays using an 55Fe radioisotope x-ray (Mn Kα = 5.9 keV; Mn Kβ = 6.49 keV) source. An energy resolution of 508 eV ± 5 eV full width at half maximum (FWHM) at 5.9 keV was achieved at an apparent avalanche gain, M, of 1.1. This is the best energy resolution so far reported for GaAs/AlxGa1−xAs x-ray SAM APDs. The noise components associated with the achievable spectroscopic energy resolutions are reported. Comparisons between the 200 μm and 400 μm diameter GaAs/AlxGa1−xAs SAM x-ray APDs and recently studied GaAs p+-i-n+ detectors were made, showing that the inclusion of the avalanche layer improves the achievable energy resolution; energy resolutions of 508 eV FWHM at 5.9 keV at M = 1.1 and 603 eV FWHM at 5.9 keV at M = 1.2 were achieved with the 200 μm and 400 μm diameter GaAs/AlxGa1−xAs SAM x-ray APDs, respectively; this is better than was previously reported for similar devices without avalanche layers: 690 eV FWHM at 5.9 keV and 730 eV FWHM at 5.9 keV for 200 μm and 400 μm diameter GaAs p+-i-n+ detectors, respectively [Lioliou et al., J. Appl. Phys. 122, 244506 (2017)].

Two custom-made In0.5Ga0.5P p+-i-n+ circular mesa spectroscopic X-ray photodiodes with different diameters (200 μm and 400 μm) and a 5 μm i layer have been characterized for their response to X-ray photons within the energy range 4.95 keV to 21.17 keV. The photodiodes, operating uncooled at 30 °C, were coupled, in turn, to the same custom-made charge-sensitive preamplifier. X-ray fluorescence spectra of high-purity calibration foils excited by a Mo target X-ray tube were accumulated. The energy resolution (Full Width at Half Maximum) increased from 0.79 keV ± 0.02 keV at 4.95 keV to 0.83 keV ± 0.02 keV at 21.17 keV, and from 1.12 keV ± 0.02 keV at 4.95 keV to 1.15 keV ± 0.02 keV at 21.17 keV, when using the 200 μm and 400 μm diameter devices, respectively. Energy resolution broadening with increasing energy was attributed to increasing Fano noise (negligible incomplete charge collection noise was suggested); for the first time the Fano factor for In0.5Ga0.5P was experimentally determined to be 0.13, suggesting a Fano limited energy resolution of 145 eV at 5.9 keV. The charge output of each system had a linear relationship with photon energy, across the investigated energy range. The count rate of both spectroscopic systems increased linearly with varying X-ray tube current up to ~105 photons s-1 cm-2 incident photon fluences. The development of In0.5Ga0.5P based spectrometers is particularly important for hard X-/γ-ray astronomy, due to the material’s large linear X-ray and γ-ray absorption coefficients and ability to operate uncooled at high temperatures.

In this paper for the first time, an InGaP photodiode was used in a high temperature tolerant X-ray spectrometer. The use of InGaP in X-ray spectrometers shows a significant advance within this field allowing operation up to 100 °C. Such results are particularly important since GaP and InP (the InGaP binary parent compounds) are not spectroscopic even at room temperature. The best energy resolution (smallest FWHM) at 5.9 keV for the InGaP spectrometer was 1.27 keV at 100 °C and 770 eV at 20 °C, when the detector was reverse biased at 5 V. The observed FWHM were higher than the expected statistically limited energy resolutions indicating that other sources of noise contributed to the FWHM broadening. The spectrometer’s Si preamplifier electronics was the limiting factor for the FWHM rather than the InGaP photodiode itself. The InGaP electron-hole pair creation energy (eInGaP) was experimentally measured across the temperature range 100 °C to 20 °C. eInGaP was 4.94 eV ± 0.06 eV at 20 °C.

Two 400 μm diameter Al0.52In0.48P p+-i-n+ mesa photodiodes (6 μm i layer) were fabricated from a wafer grown by metalorganic vapour phase epitaxy (MOVPE) and then studied at temperatures from 140 °C to -20 °C for the development of temperature tolerant 55Fe X-ray photovoltaic and 63Ni betavoltaic microbatteries. The Al0.52In0.48P epitaxial layers are the thickest so far reported for this emerging application. At each temperature, the performances of the Al0.52In0.48P detectors were analysed in dark conditions, as well as under the illumination of a 182 MBq 55Fe radioisotope X-ray source and a 185 MBq 63Ni radioisotope source. An open circuit voltage as high as 1.13 V was found for both the Al0.52In0.48P X-ray photovoltaic cells at -20 °C; whilst an open circuit voltage of 0.47 V was found for the best 63Ni betavoltaic cell, at the same temperature. Maximum output powers of 1.44 pW and 1.36 pW were obtained from the two X-ray photovoltaic cells at -20 °C; combining the output powers of these two Al0.52In0.48P X-ray photovoltaic cells, a total maximum output power as high as 2.8 pW could be obtained at 20 °C. Maximum output powers of 0.18 pW and 0.13 pW were instead extracted from the two betavoltaic cells at -20 °C, these could lead to a total maximum output power as high as 0.3 pW at 20 °C. Conversion efficiencies of 2.2% and 0.06% were found, respectively, for the best Al0.52In0.48P X-ray photovoltaic and betavoltaic cells at -20 °C. With respect to previously reported Al0.52In0.48P X-ray photovoltaic cells with thinner i layers, the 6 μm Al0.52In0.48P X-ray photovoltaic cells had higher short circuit current, open circuit voltage, maximum output power, and conversion efficiency. The 6 μm Al0.52In0.48P betavoltaic cells instead presented similar performances to previously analysed Al0.52In0.48P betavoltaic cells.

A semiconductor saturable absorber mirror (SESAM) passively mode-locked Alexandrite laser was demonstrated. Using an InP/InGaP quantum-dot saturable absorber mirror, pulse duration of 420 fs at 774 nm was obtained. The laser was pumped at 532 nm and generated 325 mW of average output power in mode-locked regime with a pump power of 7.12 W. To the best of our knowledge, this is the first report of a passively mode-locked Alexandrite laser using SESAM in general and quantum-dot SESAM in particular.

An Alexandrite laser passively mode-locked using an InP/InGaP quantum-dot semiconductor saturable absorber mirror (QD-SESAM) was demonstrated. The laser was pumped at 532 nm and generated pulses as short as 380 fs at 775 nm with an average output power of 295 mW. To the best of our knowledge, this is the first report on a passively mode-locked femtosecond Alexandrite laser using a SESAM in general and a QD-SESAM in particular.

We report an evaluation of an epitaxially grown uncooled InAs photodiode for the use in radiation thermometry. Radiation thermometry measurements was taken using the photodiode covered blackbody temperatures of 50 °C–300 °C. By determining the photocurrent and signal-to-noise ratio, the temperature error of the measurements was deduced. It was found that an uncooled InAs photodiode, with the peak and cutoff wavelengths of 3.35 and 3.55 $\mu \text{m}$ , respectively, measured a temperature of 50 °C, with an error of 0.17 °C. Many plastics have $C$ – $H$ molecular bond absorptions at 3.43 $\mu \text{m}$ and hence radiate thermally at this wavelength. Our results suggest that InAs photodiodes are well suited to measure the temperature of plastics above 50 °C. When tested with a narrow bandpass filter at 3.43 $\mu \text{m}$ and blackbody temperatures from 50 °C–300 °C, the InAs photodiode was also found to produce a higher output photocurrent, compared with a commercial PbSe detectors.

GaAs nanowires with elongated cross sections are formed using a catalyst-free growth technique. This is achieved by patterning elongated nanoscale openings within a silicon dioxide growth mask on a (111)B GaAs substrate. It is observed that MOVPE-grown vertical nanowires with cross section elongated in the [21̅1̅] and [1̅12] directions remain faithful to the geometry of the openings. An InGaAs quantum dot with weak radial confinement is realized within each nanowire by briefly introducing indium into the reactor during nanowire growth. Photoluminescence emission from an embedded nanowire quantum dot is strongly linearly polarized (typically90%) with the polarization direction coincident with the axis of elongation. Linearly polarized PL emission is a result of embedding the quantum dot in an anisotropic nanowire structure that supports a single strongly confined, linearly polarized optical mode. This research provides a route to the bottom-up growth of linearly polarized single photon sources of interest for quantum information applications.

Efficient sources of individual pairs of entangled photons are required for quantum networks to operate using fiber-optic infrastructure. Entangled light can be generated by quantum dots (QDs) with naturally small fine-structure splitting (FSS) between exciton eigenstates. Moreover, QDs can be engineered to emit at standard telecom wavelengths. To achieve sufficient signal intensity for applications, QDs have been incorporated into one-dimensional optical microcavities. However, combining these properties in a single device has so far proved elusive. Here, we introduce a growth strategy to realize QDs with small FSS in the conventional telecom band, and within an optical cavity. Our approach employs ‘‘droplet-epitaxy’’ of InAs quantum dots on (001) substrates. We show the scheme improves the symmetry of the dots by 72%. Furthermore, our technique is universal, and produces low FSS QDs by molecular beam epitaxy on GaAs emitting at ∼900 nm, and metal-organic vapor-phase epitaxy on InP emitting at ∼1550 nm, with mean FSS 4× smaller than for Stranski-Krastanow QDs.

InAs avalanche photodiodes (APDs) can be designed such that only electrons are allowed to initiate impact ionization, leading to the lowest possible excess noise factor. Optimization of wet chemical etching and surface passivation produced mesa APDs with bulk dominated dark current and responsivity that are comparable and higher, respectively, than a commercial InAs detector. Our InAs electron-APDs also show high stability with fluctuation of ~0.1% when operated at a gain of 11.2 over 60 s. These InAs APDs can detect very weak signal down to ~35 photons per pulse. Fabrication of planar InAs by Be implantation produced planar APDs with bulk dominated dark current. Annealing at 550 °C was necessary to remove implantation damage and to activate Be dopants. Due to minimal diffusion of Be, thick depletion of 8 μm was achieved. Since the avalanche gain increases exponentially with the thickness of avalanche region, our planar APD achieved high gain > 300 at 200 K. Our work suggest that both mesa and planar InAs APDs can exhibit high gain. When combined with a suitable preamplifier, single photon detection using InAs electron-APDs could be achieved.

Quantum dots based on InAs/InP hold the promise to deliver entangled photons with wavelength suitable for the standard telecom window around 1550 nm, which makes them predestined to be used in future quantum networks applications based on existing fiber optics infrastructure. A prerequisite for the generation of such entangled photons is a small fine structure splitting (FSS) in the quantum dot excitonic eigenstates, as well as the ability to integrate the dot into photonic structures to enhance and direct its emission. Using optical spectroscopy, we show that a growth strategy based on droplet epitaxy can simultaneously address both issues. Contrary to the standard Stranski-Krastanow technique, droplet epitaxy dots do not rely on material strains during growth, which results in a drastic improvement in dot symmetry. As a consequence, the average exciton FSS is reduced by more than a factor 4, which in fact makes all the difference between easily finding a dot with the required FSS and not finding one at all. Furthermore, we demonstrate that droplet epitaxy dots can be grown on the necessary surface (001) for high quality optical microcavities, which increases dot emission count rates by more than a factor of five. Together, these properties make droplet epitaxy quantum dots readily suitable for the generation of entangled photons at telecom wavelengths.

We demonstrate an ${\rm Al}_{0.52}{\rm In}_{0.48}{\rm P}$ homo-junction Separate Absorption Multiplication Avalanche Photodiode as a detector with narrow spectral response in the blue-green part of the optical spectrum. Due to its wide band-gap, this device has a dark current density of ${\rm cm}^{-2}$ at 99.9% of the breakdown voltage at room temperature. This device has a peak responsivity at 483 nm of 0.15 A/W when punched-through and is capable of an avalanche gain higher than 100.

We have studied twinning in GaAs nanowires grown via holes in a silica mask deposited on GaAs(111)B substrates by metal-organic chemical vapour epitaxy without catalysts. Twins perpendicular to the growth direction form in the nanowires, their {111}-type side facets leading to corrugation of their {110} side walls. The top facets are almost atomically smooth. Aberration corrected annular dark-field scanning transmission electron microscopy reveals all twins are rotational, commencing with layers of Ga and finishing with As atoms. Energy-loss spectroscopic profiling has shown no significant changes in the band-gap or bulk plasmon energy at those twin boundaries, and the observed reduction of the interface plasmon energy by ∼0.13 eV is close to the detection limit of the technique, reflecting the very low energetic electronic changes related to twin formation.

We investigate the nonlinear mechanical properties of GaAs nanowires with anisotropic cross-section. Fundamental and second order flexural modes are studied using laser interferometry with good agreement found between experiment and theory describing the nonlinear response under mechanical excitation. In particular, we demonstrate that the sign of the nonlinear coupling between orthogonal modes is dependent on the cross-section aspect ratio. The findings are of interest for applications such as amplitude to frequency conversion and vectorial force sensing.

A custom-made Al 0.52 In 0.48 P p + -i-n + circular mesa X-ray photodiode ( 200 μ m diameter; 2 μ m i layer thickness) has been investigated for its use in photon counting X-ray spectroscopy within the energy range 4.95 keV to 21.17 keV. This is the widest energy range so far used to characterize an AlInP X-ray detector. High-purity X-ray fluorescence calibration samples were excited by a Mo target X-ray tube to give characteristic energy photons with which to illuminate the detector. X-ray fluorescence spectra were accumulated with the Al 0.52 In 0.48 P X-ray detector coupled to a custom-made charge-sensitive preamplifier (both operated at a temperature of 30 °C ± 2 °C). The charge output linearly increased with X-ray photon energy. The energy resolution (Full Width at Half Maximum, FWHM) degraded from 0.99 keV ± 0.02 keV (80 e − rms) at 4.95 keV to 1.12 keV ± 0.02 keV (89 e − rms) at 21.17 keV. The increase of the FWHM with increased energy exceeded the increase expected as a consequence of Fano noise. Hence, the presence of energy dependent incomplete charge collection noise was suggested. The count rate linearly increased with increasing incident X-ray flux across the investigated flux range, up to ∼ 3 . 8 × 105 photons s−1 cm−2.

We study self-assembled InP quantum dot (QD) laser structures grown at two temperatures (690°C and 730 °C) each with three different quantities of deposited quantum dot material (2, 2.5, and 3 mono-layers). The absorption spectra of these structures show features associated with the QD distributions and the magnitude of the absorption increases for samples where more material is deposited and for lower growth temperature. The 690°C growth temperature structures exhibit nonradiative recombination and internal optical mode loss that increase with the quantity of material deposited; we suggest that the laser performance is limited by the presence of defects. The higher growth temperature samples have lower threshold current density and are limited by gain saturation. For these samples and for 2-mm long lasers with uncoated facets, the threshold current density is as low as 150 A cm-2, emitting in the wavelength range around 730 nm.

The potential for scale-up coupled with minimized system size is likely to be a major determining factor in the realization of applicable quantum information systems. Nanofabrication technology utilizing the III-V semiconductor system provides a path to scalable quantum bit (qubit) integration and a materials platform with combined electronic/photonic functionality. Here, we address the key requirement of qubit-site and emission energy control for scale-up by demonstrating uniform arrays of III-V nanowires, where each nanowire contains a single quantum dot. Optical studies of single nanowire quantum dots reveal narrow linewidth exciton and biexciton emission and clear state-filling at higher powers. Individual nanowire quantum dots are shown to emit nonclassically with clear evidence of photon antibunching. A model is developed to explain unexpectedly large excited state separations as revealed by photoluminescence emission spectra. From measurements of more than 40 nanowire quantum dots, we find emission energies with an ensemble broadening of 15 meV. The combination of deterministic site control and the narrow distribution in ensemble emission energy results in a system readily capable of scaling for multiqubit quantum information applications.

A prototype 200 μm diameter Al0.52In0.48P p+-i-n+ mesa photodiode (2 μm i-layer) was characterised at temperatures from 100 °C to −20 °C for the development of a temperature tolerant photon counting X-ray spectrometer. At each temperature, X-ray spectra were accumulated with the AlInP detector reverse biased at 0 V, 5 V, 10 V, and 15 V and using different shaping times. The detector was illuminated by an 55Fe radioisotope X-ray source. The best energy resolution, as quantified by the full width at half maximum (FWHM) at 5.9 keV, was observed at 15 V for all the temperatures studied; at 100 °C, a FWHM of 1.57 keV was achieved, and this value improved to 770 eV FWHM at −20 °C. System noise analysis was also carried out, and the different noise contributions were computed as functions of temperature. The results are the first demonstration of AlInP's suitability for photon counting X-ray spectroscopy at temperatures other than ≈20 °C.

Sensitive detection of mid-infrared light (2 to 5 μm wavelengths) is crucial to a wide range of applications. Many of the applications require high-sensitivity photodiodes, or even avalanche photodiodes (APDs), with the latter generally accepted as more desirable to provide higher sensitivity when the optical signal is very weak. Using the semiconductor InAs, whose bandgap is 0.35 eV at room temperature (corresponding to a cut-off wavelength of 3.5 μm), Sheffield has developed high-sensitivity APDs for mid-infrared detection for one such application, satellite-based greenhouse gases monitoring at 2.0 μm wavelength. With responsivity of 1.36 A/W at unity gain at 2.0 μm wavelength (84 % quantum efficiency), increasing to 13.6 A/W (avalanche gain of 10) at -10V, our InAs APDs meet most of the key requirements from the greenhouse gas monitoring application, when cooled to 180 K. In the past few years, efforts were also made to develop planar InAs APDs, which are expected to offer greater robustness and manufacturability than mesa APDs previously employed. Planar InAs photodiodes are reported with reasonable responsivity (0.45 A/W for 1550 nm wavelength) and planar InAs APDs exhibited avalanche gain as high as 330 at 200 K. These developments indicate that InAs photodiodes and APDs are maturing, gradually realising their potential indicated by early demonstrations which were first reported nearly a decade ago.

In this paper, the performance of an Al0.52In0.48P 63 Ni radioisotope cell is reported over the temperature range of −20 °C to 140 °C. A 400 μm diameter p+-i-n+ (2 μm i-layer) Al0.52In0.48P mesa photodiode was used as a conversion device in a novel betavoltaic cell. Dark current measurements on the Al0.52In0.48P detector showed that the saturation current increased increasing the temperature, while the ideality factor decreased. The effects of the temperature on the key cell parameters were studied in detail showing that the open circuit voltage, the maximum output power, and the internal conversion efficiency decreased when the temperature was increased. At −20 °C, an open circuit voltage and a maximum output power of 0.52 V and 0.28 pW, respectively, were measured.