Direct measurement of the hole-nuclear spin interaction in single quantum dots

We use photoluminescence spectroscopy of ''bright'' and ''dark'' exciton states in single InP/GaInP quantum dots to measure hyperfine interaction of the valence band hole with nuclear spins polarized along the sample growth axis. The ratio of the hyperfine constants for the hole (C) and electron (A) is found to be C/A~-0.11. In InP dots the contribution of spin 1/2 phosphorus nuclei to the hole-nuclear interaction is weak, which enables us to determine experimentally the value of C for spin 9/2 indium nuclei as C_In~-5 micro-eV. This high value of C is in good agreement with recent theoretical predictions and suggests that the hole-nuclear spin interaction has to be taken into account when considering spin qubits based on holes.Comment: to be submitted to Phys Rev Let

Energy response characterization of InGaP X-ray detectors

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

InGaP (GaInP) mesa p-i-n photodiodes for X-ray photon counting spectroscopy

In this paper, for the first time an InGaP (GaInP) photon counting X-ray photodiode has been developed and shown to be suitable for photon counting X-ray spectroscopy when coupled to a low-noise charge-sensitive preamplifier. The characterisation of two randomly selected 200 μm diameter and two randomly selected 400 μm diameter In0.5Ga0.5P p+-i-n+ mesa photodiodes is reported; the i-layer of the p+-i-n+ structure was 5 μm thick. At room temperature, and under illumination from an 55Fe radioisotope X-ray source, X-ray spectra were accumulated; the best spectrometer energy resolution (FWHM) achieved at 5.9 keV was 900 eV for the 200 μm In0.5Ga0.5P diameter devices at reverse biases above 5 V. System noise analysis was also carried out and the different noise contributions were computed

Temperature characterisation of spectroscopic InGaP X-ray photodiodes

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 (εInGaP) was experimentally measured across the temperature range 100 °C to 20 °C. εInGaP was 4.94 eV ± 0.06 eV at 20 °C

Overhauser effect in individual InP/GaInP dots

Sizable nuclear spin polarization is pumped in individual InP/GaInP dots in a wide range of external magnetic fields B_ext=0-5T by circularly polarized optical excitation. We observe nuclear polarization of up to ~40% at Bext=1.5T and corresponding to an Overhauser field of ~1.2T. We find a strong feedback of the nuclear spin on the spin pumping efficiency. This feedback, produced by the Overhauser field, leads to nuclear spin bi-stability at low magnetic fields of Bext=0.5-1.5T. We find that the exciton Zeeman energy increases markedly, when the Overhauser field cancels the external field. This counter-intuitive result is shown to arise from the opposite contribution of the electron and hole Zeeman splittings to the total exciton Zeeman energy

Pumping of nuclear spins by the optical solid effect in a quantum dot

We demonstrate that efficient optical pumping of nuclear spins in semiconductor quantum dots (QDs) can be achieved by resonant pumping of optically "forbidden" transitions. This process corresponds to one-to-one conversion of a photon absorbed by the dot into a polarized nuclear spin, which also has potential for initialization of hole spin in QDs. Pumping via the "forbidden" transition is a manifestation of the "optical solid effect", an optical analogue of the effect previously observed in electron spin resonance experiments in the solid state. We find that by employing this effect, nuclear polarization of 65% can be achieved, the highest reported so far in optical orientation studies in QDs. The efficiency of the spin pumping exceeds that employing the allowed transition, which saturates due to the low probability of electron-nuclear spin flip-flop.Comment: 5 pages, 3 figures, submitted to Phys. Rev. Let

Overhauser effect in individual InP/GaInP dots

Sizable nuclear spin polarization is pumped in individual InP/GaInP dots in a wide range of external magnetic fields B_ext=0-5T by circularly polarized optical excitation. We observe nuclear polarization of up to ~40% at Bext=1.5T and corresponding to an Overhauser field of ~1.2T. We find a strong feedback of the nuclear spin on the spin pumping efficiency. This feedback, produced by the Overhauser field, leads to nuclear spin bi-stability at low magnetic fields of Bext=0.5-1.5T. We find that the exciton Zeeman energy increases markedly, when the Overhauser field cancels the external field. This counter-intuitive result is shown to arise from the opposite contribution of the electron and hole Zeeman splittings to the total exciton Zeeman energy

AlInP photodiode x-ray detectors

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  <3 pA (corresponding to leakage current densities  <2 nA cm−2) at 100 V reverse bias (electric field strength of 167 kV cm−1). The photodiodes were coupled to a custom-made low-noise charge-sensitive preamplifier, the noise characteristics of the resultant spectrometers were investigated as functions of shaping times. The best energy resolutions (full width at half maximum of the 5.9 keV photopeak from an 55Fe radioisotope x-ray source) achieved with the 217 μm  ±  15 μm and 409 μm  ±  28 μm diameter photodiodes were 0.89 keV and 1.05 keV, respectively. The dielectric dissipation factor of Al0.52In0.48P was estimated to be (2.2  ±  1.1)  ×  10−3 at room temperature

AlInP X-ray photodiodes without incomplete charge collection noise

Previously, an Al0.52In0.48P p+-i-n+ spectroscopic photon counting X-ray photodiode with 2 μm thick i layer (200 μm diameter) was shown to suffer from energy-dependent incomplete charge collection noise [Lioliou et al., 2019, Nucl. Instr. and Meth. A Vol. 943, Art. No. 162467]. Subsequent measurements on a larger (400 μm diameter) Al0.52In0.48P 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 Al0.52In0.48P 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 Al0.52In0.48P photodiodes may be of limited practicality as high performance detectors for photon counting X-ray spectroscopy. However, two new Al0.52In0.48P p+ -i-n+ photodiodes (with 6 μm 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 Al0.52In0.48P, concerns about incomplete charge collection noise in X-ray detectors made from the material appear to have been unwarranted; the path towards thick Al0.52In0.48P X-ray detectors is now clear