We have studied the electron spin relaxation in semiconductor InAs/GaAs quantum dots by time-resolved optical spectroscopy. The average spin polarization of the electrons in an ensemble of p-doped quantum dots decays down to 1/3 of its initial value with a characteristic time T(Delta) approximately 500 ps, which is attributed to the hyperfine interaction with randomly oriented nuclear spins. We show that this efficient electron spin relaxation mechanism can be suppressed by an external magnetic field as small as 100 mT.

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Perovskite solar cells are analyzed by photo- and electroluminescence hyperspectral imaging. Significant spatial inhomogeneities in the quasi-Fermi level splitting are observed.

Photovoltaic generation has stepped up within the last decade from outsider status to one of the important contributors of the ongoing energy transition, with about 1.7% of world electricity provided by solar cells. Progress in materials and production processes has played an important part in this development. Yet, there are many challenges before photovoltaics could provide clean, abundant, and cheap energy. Here, we review this research direction, with a focus on the results obtained within a Japan–French cooperation program, NextPV, working on promising solar cell technologies. The cooperation was focused on efficient photovoltaic devices, such as multijunction, ultrathin, intermediate band, and hot-carrier solar cells, and on printable solar cell materials such as colloidal quantum dots.

We report on optical orientation experiments in undoped GaAsN epilayers and InGaAsN quantum wells (QW), showing that a strong electron spin polarisation can persist at room temperature. We demonstrate that the spin dynamics in these dilute nitride structures is governed by a spin-dependent recombination process of free conduction electrons on deep paramagnetic centres.

In common photovoltaic devices, the part of the incident energy above the absorption threshold quickly ends up as heat, which limits their maximum achievable efficiency far below the thermodynamic limit for solar energy conversion. Conversely, if the excess kinetic energy of the photogenerated carriers could be converted into additional free energy, it would be possible to approach the thermodynamic limit. This is the principle of hot carrier devices. Unfortunately, such a device operation in conditions relevant for utilisation has never been evidenced. Here we show that the quantitative thermodynamic study of the hot carrier population, with luminance measurements, allows us to discuss the hot carrier contribution to the solar cell performance. We demonstrate that voltage and current can be enhanced in a semiconductor heterostructure due to the presence of the hot carrier population in a single InGaAsP quantum well at room temperature. These experimental results substantiate the potential of increasing photovoltaic performances in the hot carrier regime.

We report on both experimental and theoretical study of conduction-electron spin polarization dynamics achieved by pulsed optical pumping at room temperature in GaAs1−xNx alloys with a small nitrogen content (x = 2.1, 2.7, 3.4%). It is found that the photoluminescence circular polarization determined by the mean spin of free electrons reaches 40-45% and this giant value persists within 2 ns. Simultaneously, the total free-electron spin decays rapidly with the characteristic time ≈ 150 ps. The results are explained by spin-dependent capture of free conduction electrons on deep paramagnetic centers resulting in dynamical polarization of bound electrons. We have developed a nonlinear theory of spin dynamics in the coupled system of spin-polarized free and localized carriers which describes the experimental dependencies, in particular, electron spin quantum beats observed in a transverse magnetic field.

We demonstrate electrical control of the single photon emission spectrum from chromium-based colour centres implanted in monolithic diamond. Under an external electric field the tunability range is typically three orders of magnitude larger than the radiative linewidth and at least one order of magnitude larger than the observed linewidth. The electric and magnetic field dependence of luminescence gives indications on the inherent symmetry and we propose Cr-X or X-Cr-Y type noncentrosymmetric atomic configurations as most probable candidates for these centres.Room temperature operation of diamond-based colour centres offers a unique platform to the field of quantum photonics [1] . The nitrogen-vacancy (NV) colour centre in diamond has received interest in diverse research fields ranging from spin-based quantum information [2][3][4][5] to nanoscale magnetometry [6][7][8][9] within the last decade. The impressive level of spin coherence [5] , the fast microwave spin manipulation [10] , and the recently demonstrated spin-photon interface [11] are essential to the proposed applications. However, the NV centre's broad photon spectrum is unfavourable for many of these experiments. Although emission from the zero-phonon line (ZPL) has been shown to be radiative lifetime broadened, it only accounts for about 4% of the total spectrum, owing to the dominance of phonon-assisted decay. This motivated the search for alternative centres, where potentially a similar degree of spin control could coexist with superior photonic properties. Consequently, silicon-vacancy (Si-V) [12,13] and nickel-based centres, such as NE8 [14] , were studied in the last few years for this purpose. Recently, chromium-based colour centres were reported among the brightest singlephoton emitters in the near infrared spectrum [15,16] . Here, we show that these centres have an impressive level of spectral tunability on the order of a few meVs, while