The concept of nuclear spin temperature is one of the cornerstones of the nuclear magnetism in solids. It has made possible realisation of the cryogenic cooling into the microKelvin range and observation of nuclear spin ordering in metals and insulators [1]. However, proving its validity for semiconductor nano- and microstructures is challenging due to the lack of techniques capable of precise sensing of weak nuclear magnetisation in a small volume. We develop a method of non-perturbative optical control over adiabatic remagneti- sation of the nuclear spin system in semiconductors and apply it to study nuclear spin thermodynamics in GaAs microcavities. The nuclear spin system is found to exactly follow the predictions of the spin-temperature theory, despite the quadrupole interaction that was earlier reported to disrupt nuclear spin thermalisation [2]. These ndings open a way to deep cooling of nuclear spins in semiconductor structures, with a prospect of realisation of nuclear spin-ordered states for high delity spin-photon interfaces. References [1] A. S. Oja, O. V. Lounasmaa, Rev. Mod. Phys 69, 1 (1997). [2] P. Maletinsky, M. Kroner, A. Imamoglu, Nat. Phys 5, 407 (2009).

Abstract-Biochemical detection applications and on-chip optical interconnects are examples of the useful applications considered for integrated photonic platforms dedicated to the UV and visible spectral range. This achievement requires the realization of efficient and compact microlaser sources that can be coupled to optical waveguides and are compatible with photonic circuitry. We develop a cost-effective practical approach relying on the controlled growth of thin AlN buffer layers on silicon substrates, followed by the growth of multiple quantum wells (MQW) GaN/AlN (for UV operation), or InGaN/GaN MQWs (for violet and blue operation) with a high radiative efficiency up to T=300K. This unique Nitride-on-Silicon platform could be released into a membrane by the selective under-etching of the silicon substrate.We present a series of microdisk lasers operating at room temperature under pulsed optical excitation over a broad spectral range extending from 275nm to 470nm. All microdisks present a Q factor above 1000 and reach 4000 in the best resonators. We are able to investigate the gain threshold of the different active layers. The microlasers operate under pulsed optical excitation, and the lasing threshold is reduced by a factor 10 from deep-UV GaN/AlN microdisks to the violet InGaN/GaN ones. This work allows us to expect a variety of integrated photonics applications, including for example multi-color integrated laser sources: we demonstrate here the versatility of this nitride-on-silicon platform, and the first realization on this platform of efficient active layers for a tunable lasing action over a 200nm broad UV to visible spectral range. [1,2].Acknowledgements: The authors acknowledge support from the projects GANEX (ANR-11-LABX-0014) and QUANONIC (ANR-13-BS10-0010). GANEX belongs to the public funded 'Investissements d'Avenir' program managed by the French ANR agency. This work was also partly supported by the RENATECH network.REFERENCES1.Sellés et al., APL 109, 231101 (2016)2.Sci. Rep. 6, 21650 (2016)

The polariton emission in a microcavity is usually measured from the surface, and little is known about the actual role of guided modes below the light cone in air, that can couple to the exciton transition, collect part of the emission and contribute to polariton losses. In this work we investigate a ZnO planar microcavity with mesas. The emission of the guided modes is observed at the mesa’s edge, together with the vertical emission. The microcavity operates as a polariton laser under pulsed optical pumping [1].The spectra collected under the excitation spot and at the mesa’s edge are very different, as shown on figure 1a. The vertical emission is dominated by the lower polariton branch (LPB) associated to the cavity mode whereas the edge emission forms a broad peak modulated by fringes which spacing decreases as the spot moves away from the edge. While these fringes could indicate Fabry-Perot oscillations of polaritons propagating in-plane, in analogy with radiative polaritons in GaAs microcavity wires [2], a careful analysis shows that they are the signature of beatings between 5 to 10 guided modes, without being necessary to invoke multiple reflections. These modes have a large attenuation length of 27 µm, intermediate between the propagation length of LPB polaritons (a few µm in a ZnO microcavity [3]) and the one of waveguide polaritons (100µm in a GaN microcavity [4]).Under pulsed optical pumping, the guided modes are strongly excited. They form leaky modes for the excitonic reservoir, as well as for the LPB polaritons themselves. Most interestingly, when the vertical microcavity displays polariton lasing, the polariton laser emission feeds the guided modes, which then cascade in energy (Fig. 1b,c).This work evidences therefore that the threshold of the polariton laser in conventional vertical microcavities is increased due to the existence of usually hidden guided modes, that can deplete the exciton reservoir and collect resonantly scattered light from the lasing LPB mode.

The broad-area vertical-cavity surface-emitting lasers (VCSELs) can be brought into a bistable regime by the use of a frequency-selective feedback (as well as saturable absorbers), leading to the formation of cavity solitons (CSs). Such CSs are generated under electrical injection below the lasing threshold of the VCSEL, and appear in the broad area laser as bright spots. They can also be arranged into self-organized arrays, switched on and off with an external laser holding beam, and brought to high-order states [1]. The self-localization can even be obtained in space and time, and self-pulsing spatial solitons have been demonstrated in a mode-locked VCSEL [2]. Here we explore the topological properties of the high-order states of the CS, and we report the spontaneous formation of anti-vortex CSs with a hyperbolic polarization structure. Beams with a spatially non-uniform distribution of polarization are interesting due to their intrinsic beauty, novel functionalities in quantum optics and newly enabled applications [3]. They possess a circular symmetric intensity structure, most often in form of a doughnut, combined with a spatially non-uniform polarization field containing polarization singularities and are referred to as `vector vortex beams'. The engineering of such vectorial vortex beams has been demonstrated in solid-state lasers with a mesa or a meta-surface shaping the spatial pattern [4], whereas they are here spontaneously obtained in the broad-area laser. Vortex beams, including the exotic anti-vortex reported here, have been predicted within the framework of spin-flip models of VCSELs [5], and result from the interplay between the natural anisotropies of the laser and the spin-dependent gain dynamics.

Three-dimensional solitons are highly sought after objects in nonlinear science. In nonlinear photonics they correspond to light bullets, but their experimental observation as long-term stable structures remains elusive in spite of impressive advances [1]. Broad-area vertical-cavity surface-emitting lasers (VCSELs) combine a large Fresnel number with a high nonlinearity and fast time scales. Stationary spatial solitons in 2D as well as mode-locking and temporal solitons without self-localization in space are known to exist. It is intriguing to explore a combination of these two aspects to achieve self-localization of light in three dimensions [2]. Here we report on the observation of mode-locked states of spatial solitons.The experimental system consists of a VCSEL with a 200 micrometer emission aperture emitting around 980 nm. It is coupled to a self-imaging external cavity closed by a volume Bragg grating (VBG) as a frequency-selective element. The round-trip time in the external cavity is about 0.7 ns. By varying the VCSEL current, the detuning between the VCSEL and the VBG resonances is changed and single-mode operation in the external cavity and multi-mode operation can be realized. The latter may correspond to irregular dynamics or fairly regular self-pulsing which represents mode-locked states (Fig. 1). These data are in a range where the self-pulsing spatial soliton is bistable, i.e. it coexists with the non-lasing state as expected for a self-localized state. We obtain self-pulsing at the fundamental and the harmonic round-trip frequency, as well as occasionally at even higher harmonics. The peaks in the RF-spectrum are extremely narrow (<; 100 kHz, close to the resolution limit) indicating a high phase-correlation between the participating modes. The optical spectrum displays three dominant modes. The pulse width (FWHM) is about 110 ps for the harmonic mode-locking and 270 ps for the fundamental mode-locking, roughly consistent with the expectation from spectral bandwidth. The results clearly establish mode-locking of spatial solitons on 3 modes creating temporal structures shorter than the external cavity. Numerical simulations of the model introduced in [4] are in good agreement with the experimental data [3].

Beams with a spatially non-uniform distribution of polarization attracted considerable recent interest due to their intrinsic beauty, novel functionalities in quantum optics and newly enabled applications [1]. Typical realizations investigated possess a circular symmetric intensity structure, most often in form of a doughnut, combined with a spatially non-uniform polarization field containing polarization singularities and are referred to as `vector vortex beams'. Typically, considerable engineering effort is necessary to create these non-trivial polarization configurations. In contrast, we report on the spontaneous formation of vector vortex beams in a broad-area vertical-cavity surface-emitting laser (VCSEL) with frequency-selective feedback [2]. In particular, an anti-vortex state with a hyperbolic polarization structure is observed. The relation to vectorial high-order solitons is discussed.

This special issue contains 9 original review papers, research papers and discussion papers on indirect excitons. An exciton is a Coulomb-correlated electron-hole pair. Frenkel excitons dominate optical properties of organic semiconductors, while Wannier-Mott excitons are responsible for the hydrogen-like absorption spectra of inorganic semiconductors at low temperatures. The interest to the physics of excitons has strongly increased in the new century. This interest is motivated by unique bosonic properties of excitons that lead to the phenomena of exciton-polariton lasing and stimulated scattering, build-up of the spontaneous coherence and polarisation in cold exciton gases. In addition to the rich fundamental physics, excitons offer the perspective of applications in opto-electronic devices such as exciton transistors, switches, optical integrated circuits, etc.The indirect excitons form an important research sub-field of the exciton physics. The reduction of electron-hole overlap in the real space leads to longer exciton radiative lifetimes, which is crucial for the studies of thermolyzed exciton gases. Large spatial dipole moments of indirect excitons are responsible for the strong exciton-exciton repulsion. This prevents formation of bi-excitons and excitonic molecules and facilitates formation of coherent collective excitonic states.The papers contained in this volume are devoted to different aspects of the modern physics of indirect excitons. In the review paper by Leonid Butov the intriguing perspective for implementation of excitonic device concepts is discussed. This review is nicely complemented by the work by Anton Nalitov et al. who propose using indirect excitons for realisation of bosonic cascade lasers emitting the coherent terahertz light. The review of Joe Wilkes and Egor Muljarov discusses the effect of electric and magnetic fields on indirect exciton and microcavity exciton-polaritons. Ursula Wurstbauer et al. present the experimental study of laterally trapped dipolar exciton ensembles at high magnetic fields in the Faraday configuration. Colin Hubert et al. study experimentally the dynamics of indirect excitons in GaAs/AlGaAs double quantum wells grown along the (111) direction. Tao Liu et al. study theoretically the spin dynamics of spatially trapped Bose-Einstein condensates of indirect excitons. Carsten Leding Vendelbjerg and Massimo Rontani present the theoretical research of the spin transfer torque induced onto a nano-magnet in hybrid magnetic nanostructures. Andrea Bertoni et al. address the time-dependent quantum scattering of a spatially indirect exciton by an external potential. Finally, Yotam Mazuz-Harpaz et al. analyse and discuss the recent experimental results on cold dipolar exciton gases in electrostatic traps.The nine groups that contributed to this special issue are the world-leaders in the rapidly growing research field of the physics of indirect excitons. The papers that composed this volume represent the state-of-the-art in this research domain. We hope that this volume may be used by young researchers and PhD students as well as the leading scientists as a reference book for further studies of indirect excitons.