The main interest of group-III-nitride nanophotonic circuits is the integration of active structures and laser sources. A photonic platform of group-III-nitride microdisk lasers integrated on silicon and emitting in the blue spectral range is demonstrated. The active microdisks are side-coupled to suspended bus waveguides, and the coupled emission is guided and outcoupled to free space using grating couplers. A small gap size of less than 100 nm between the disk and the waveguide is required in the blue spectral range for optimal evanescent coupling. To avoid reabsorption of the microdisk emission in the waveguide, the quantum wells are etched away from the waveguide. Under continuous-wave excitation, loaded quality factors greater than 2000 are observed for the whispering gallery modes for devices with small gaps and large waveguide bending angles. Under pulsed excitation conditions, lasing is evidenced for 3 μm diameter microdisks integrated in a full photonic circuit. We thus present a first demonstration of a III-nitride microlaser coupled to a nanophotonic circuit.

Cooling of nuclear spin system (NSS) in doped semiconductors via dynamic polarization by optical pumping is a powerful method for harnessing ubiquitous fluctuations of nuclear spin. The idea of NSS cooling is based on the hypothesis of spin temperature, which states that NSS reaches an internal thermal equilibrium long before it comes to equilibrium with the external bath (crystal lattice). Although thermodynamic framework has been successfully employed for the description of a variety of the experimental data, a rigorous check of this concept in semiconductors was impossible until recently, in particular at low magnetic field. The reason for that is the lack of experimental techniques allowing nonperturbative optical control over adiabatic transformation of the NSS. We have recently developed such methods, based on off-resonant Faraday rotation and spin noise spectroscopy [1,2]. Using these techniques, combined with photoluminescence spectroscopy, we established a comprehensive picture of the nuclear spin relaxation efficiency, its magnetic field, temperature, and carrier concentration dependence in both n- and p-doped GaAs, a model system in the field of nuclear spin physics in semiconductors [3-5]. We also analyzed the interplay between four relevant relaxation mechanisms: hyperfine interaction, quadrupole interaction, spin diffusion towards paramagnetic impurities, and Korringa mechanisms. Figure 1 illustrates these processes in a nuclei-electron coupled system. Understanding of field dependence of NSS dynamics allowed us to obtain a new insight into the NSS thermodynamics, and verify the spin temperature concept in GaAs bulk material and microcavities [6]. We have demonstrated that NSS exactly follows the predictions of the spin temperature theory, despite the quadrupole interaction that was earlier reported to disrupt nuclear spin thermalization in quantum dots [7]. These results open a way for the deep cooling of nuclear spins in semiconductor structures, with the prospect of realizing nuclear spin-ordered states for high-fidelity spin-photon interfaces. References[1] R. Giri et al, Physical Review Letters, 111, 087603 (2013)[2] I. I. Ryzhov et al, Applied Physics Letters, 106, 242405 (2015)[3] M. Kotur et al, Physical Review B, 94, 081201(R) (2016)[4] M. Vladimirova et al, Physical Review B, 95, 125312 (2017)[5] M. Kotur et al, Physical Review B, 97, 165206 (2018)[6] M. Vladimirova et al, Physical Review B, 97, 041301 (2018)[7] P. Maletinsky et al, Nature Physics, 5, 407 (2009)

Excitons in wide nitride Quantum Wells (QWs) are naturally indirect due to the strong internal electric field: within such excitons the electron and the hole are spatially separated, resulting in strong dipole moments and long radiative lifetimes. These properties offer the possibility to explore the collective exitonic phases with complex phase diagrams predicted theoretically in the temperature-density-dipole moment parameter space. Here we study 8 nm-wide GaN/(AlGa)N QWs presenting 1MV/cm built-in electric field in the growth direction, microsecond-long radiative lifetimes, 8 meV exciton emission linewidth and 100 µm transport lengths at cryogenic temperatures. To address the quasi-equilibrium properties of the strongly dipolar excitons (i.e. whose electric dipoles are twice as long as their Bohr radii d = 2aB), we confine them in the QW’s plane using electrostatic traps with a semi-transparent Schottly electrode. These traps consist in semitransparent electrodes deposited on the sample’s surface. We make micro-photoluminescence (µPL) measurements (less than 1µm- resolution) upon point-like optical excitation. We thus obtain the spectrally resolved information on the emission of the excitons as a function of power and density. Our experiments demonstrate an efficient trapping of excitons in the plane of the QW, with a trapping potential of an order of 50 meV. We demonstrate that it is possible to uniformly fill areas up to hundreds of microns in length with excitons and/or electron hole pairs in the QW’s plane. At T=4K, the emission from these areas exhibits a peculiar behavior. At low densities, such that there is no measurable screening of the built-in electric field, the emission from the trap is weak and spatially homogeneous. However, for weak built-in electric field screenings (as low as 10 meV, ~1011 cm-2 pair density), the PL brightens up and becomes spatially inhomogeneous – droplet like. We tentatively interpret this fragmentation as the coexistence of the gas and liquid phases of the excitons. As expected, the fragmented character of the emission disappears when either power and/or temperature is further increased, due to either exciton heating (gas phase), ionization or dissociation (Mott transition). Our results suggest that excitons in GaN QWs are promising candidates for the study of collective phenomena in cold dipolar bosonic systems. The advantages of the nitrides over arsenides include both higher critical temperatures for the onset of collective phenomena, and the ability to increase the dipolarity of systems based on their small exciton Bohr radius.

Density and spin dynamics of indirect excitons in asymmetric GaAs/AlGaAs coupled quantum wells is studied by time-resolved photoluminescence. Under electric bias applied in the growth direction the lifetime of indirect excitons formed by an electron in a wide and hole in a narrow quantum well reaches 12 ns, while the circular polarisation lifetime is about 5 ns. The structure is suited for further studies of dark indirect excitons by time-resolved pump-probe spectroscopy

spin fluctuations in atomic vapors and in semiconductors. As such it is quite attractive for probing non-perturbatively electrons or nuclear spins in atomic vapours or in semiconductors, particularly when the laser is detuned from optical resonances, an aspect which has triggered considerable interest to spin noise spectroscopy in recent years. The signal in spin noise spectroscopy can be considered as arising from intensity fluctuations due to the interference between the light scattered by Raman spin-flip excitations and the probe laser. This suggests that homodyne or heterodyne detection of spin noise by mixing of the Raman signal with a local oscillator is feasible, which we have demonstrated recently [1]. This opportunity is essential to spin noise spectroscopy because it allows us to increase its sensitivity and accessible frequency range.Homodyne and/or heterodyne detection of spin noise for scattered light directions different from that of the probe should give access to spatio-temporal spin correlations [2], and therefore could considerably extend the application range of spin noise spectroscopy. Here we demonstrate non-collinear homodyne detection of spin noise in n-doped CdTe epilayers. This allows us to exploit different scattering geometries based on spin-flip Raman selection rules. We are able to measure simultaneously different fluctuating spin components by homodyne mixing in different space directions, and also for different local oscillator polarizations. [1] S. Cronenberger and D. Scalbert, Review of Scientific Instruments 87, 093111 (2016).[2] G. G. Kozlov, I. I. Ryzhov, and V. S. Zapasskii, Phys. Rev. A 95, 043810 (2017).

In the recent years the spectroscopy of spin noise has won its spurs for spin dynamics related studies in semiconductors, largely because it exhibits several quite attractive features. Noticeably it enables almost perturbation-free detection of spontaneous electron spin fluctuations. In addition as the amplitude of spin noise grows when the size of the probed region is reduced, it is well adapted for spatially resolved studies. Also the spin noise spectrum is quite sensitive to internal effective fields, which allows to probe locally the existence of nuclear fields. Finally, the combination of spin noise spectroscopy and optical heterodyning has been demonstrated, which permits enhanced sensitivity and broadband detection, while keeping a high spectral resolution [1].In this poster we will present results obtained by heterodyne detection of spin noise in an n-doped CdTe epilayer with donor density n∼3×〖10〗^17cm-3. Thanks to the enhanced sensitivity gained by heterodyne detection we could detect the spin noise of electrons bound to neutral donors for probe powers as low as 5 µW. In these conditions we observe an extremely slow hopping rate W0 of electron spin between neighbouring donors, and extremely slow electron spin relaxation rate s (see Figure 1). The observed noise spectrum at zero field is characteristic of electron spin precession in the frozen nuclear field acting on the electrons bound to the donors and exhibits two components. A central lorentzian line corresponding to the electron spin component along the nuclear field, and two satellites gaussian lines corresponding to the spin precession in the nuclear field. We analyze our results in the framework of a theoretical model, which takes into account both the electron spin precession, and the hopping between donors, but slightly modified to take into account an eventual non-zero nuclear spin polarization [2]. In agreement with the theory we can see that the satellites merge with the central line as the hopping rate increases (at the highest probe power). Surprisingly W0 and s become quite small at the lowest probe power. We find that the electron spin relaxation time becomes longer than 1 µs in this regime.

Spin noise spectroscopy is a new very promising technique for non-perturbative measurement of spin dynamics in semiconductors.I will show how the homodyne and heterodyne detection of spin noise allows us to extend the possibilities of spin noise. These possibilities we be illustrated with our recent results on q-selective detection of spin noise, and on nuclear spin thermometry.