Canalization of developmental processes ensures the reproducibility and robustness of embryogenesis within each species. In its extreme form, found in ascidians, early embryonic cell lineages are invariant between embryos within and between species, despite rapid genomic divergence. To resolve this paradox, we used live light-sheet imaging to quantify individual cell behaviors in digitalized embryos and explore the forces that canalize their development. This quantitative approach revealed that individual cell geometries and cell contacts are strongly constrained, and that these constraints are tightly linked to the control of fate specification by local cell inductions. While in vertebrates ligand concentration usually controls cell inductions, we found that this role is fulfilled in ascidians by the area of contacts between signalingand responding cells. We propose that the duality between geometric and genetic control of inductions contributes to the counterintuitive inverse correlation between geometric and genetic variability during embryogenesis.

Hexagonal boron nitride is a model lamellar compound where weak, non-local van der Waals interactions ensure the vertical stacking of two-dimensional honeycomb lattices made of strongly bound boron and nitrogen atoms. We study the isotope engineering of lamellar compounds by synthesizing hexagonal boron nitride crystals with nearly pure boron isotopes ( 10 B and 11 B) compared to those with the natural distribution of boron (20 at% 10 B and 80 at% 11 B). On the one hand, as with standard semiconductors, both the phonon energy and electronic bandgap varied with the boron isotope mass, the latter due to the quantum effect of zero-point renormalization. On the other hand, temperature-dependent experiments focusing on the shear and breathing motions of adjacent layers revealed the specificity of isotope engineering in a layered material, with a modification of the van der Waals interactions upon isotope purification. The electron density distribution is more diffuse between adjacent layers in 10 BN than in 11 BN crystals. Our results open perspectives in understanding and controlling van der Waals bonding in layered materials.

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.