In the first part we will show a study of the Casimir torque between two metallic one-dimensional gratings rotated by an angle θ with respect to each other. We find that, for infinitely extended gratings, the Casimir energy is anomalouslydiscontinuous at θ = 0, due to a critical zero-order geometric transition between a 2D- and a 1D-periodicsystem. We will comment on the relevant implication of this finding.In the second part I will discuss the Radiative heat transfer (RHT) and radiative thermal energy (RTE) for two-dimensional (2D) nanoparticle ensembles in the framework of many-body radiative heat transfer theory. We consider nanoparticles made of different materials: metals (Ag), polar dielectrics (SiC), or insulator-metallic phase-change materials(VO2). We start by investigating the RHT between two parallel 2D finite-size square-lattice nanoparticleensembles, with particular attention to many-body interactions (MBI) effects. We fix the particle radius (a)as the smallest length scale, and we describe the electromagnetic scattering from particles within the dipoleapproximation. Depending on the minimal distance between the in-plane particles (the lattice spacing p forperiodic systems), on the separation d between the two lattice and on the thermal wavelength,we systematically analyze the different physical regimes characterizing the RHT. Four regimes are identified,rarefied regime, dense regime, non-MBI regime, and MBI regime, respectively.

The value and the nature of the bandgap of In4Se3 are still not well defined, with a large spread of the experimental data between 0.42 and 1.68 eV and an uncertain nature, predicted to be indirect by ab initio band structure calculations. Here we report on the optical transmission and photoluminescence (PL) performed in In4Se3 thin films grown by coevaporation on (0001)-oriented sapphire wafers. The quality of the polycrystalline layers allows the first detection of the excitonic-like transition in the optical absorption of this compound at low temperature. The PL detected under weak laser excitation shows a bound exciton emission at 0.75 eV. Strong laser irradiation reveals a quadratic dependence of the PL intensity on the optical excitation, which demonstrates a stimulated emission at 0.79 eV in relation with an exciton–exciton scattering process. On the basis of a reasonable estimate of the exciton energy, equal to 10 − 15 meV, we evaluate the direct bandgap of In4 Se3 to 0.82 ± 0.01 eV at low temperature.

After a rapid introduction to the basic physical concepts of radiative heat transfer and the presentation of the general theory valid for arbitrary objects, I will focus on the study of the radiative heat transfer between two identical metallic one-dimensional lamellar gratings. To this aim I will present and exploit a modification to the widely used Fourier modal method, known as adaptive spatial resolution, based on a stretch of the coordinate associated with the periodicity of the grating. I show that this technique dramatically improves the rate of convergence when calculating the heat flux, and that there is a remarkable amplification of the exchanged energy, ascribed to the appearance of spoof-plasmon modes. By comparing our results to recent studies, we find a consistent quantitative disagreement with some previously obtained results going up to 50%.

A radiative thermostat system senses its own temperature and automatically modulates heat transfer by turning on/off the cooling to maintain its temperature near a desired set point. Taking advantage of far- and near-field radiative thermal technologies, we propose an intelligent radiative thermostat induced by the combination of passive radiative cooling and near-field radiative thermal diode for thermal regulation at room temperature. The top passive radiative cooler in thermostat system with static thermal emissivity uses the cold outer space to passively cool itself all day, which can provide the bottom structure with the sub-ambient cold source. Meanwhile, using the phase-transition material vanadium dioxide, the bottom structure forms a near-field radiative thermal diode with the top cooler, which can significantly regulate the heat transfer between two terminals of the diode and then realize a stable temperature of the bottom structure. Besides, the backsided heat input of the thermostat has been taken into account according to real-world applications. Thermal performance of the proposed radiative thermostat design has been analyzed, showing that the coupling effect of static passive radiative cooling and dynamic internal heat transfer modulation can maintain an equilibrium temperature approximately locked within the phase transition region. Besides, after considering empirical indoor-to-outdoor heat flux, rendering its thermal performance closer to that of passive solar residential building walls, the calculation result proves that the radiative thermostat system can effectively modulate the temperature and stabilize it within a controllable range. Passive radiative thermostats driven by near-field radiative thermal diode can potentially enable intelligent temperature regulation technologies, for example, to moderate diurnal temperature in regions with extreme thermal swings.

We study the deep-ultraviolet emission in Bernal boron nitride as a function of temperature. The quasidegeneracy of indirect and direct excitons in Bernal boron nitride leads to their simultaneous recombination, allowing a comparison of phonon-assisted broadening in the two cases. Temperature-dependent measurements reveal that below 200 K, the efficiency of phonon-assisted broadening is one order of magnitude lower for the direct transition at 6.035 eV than for the phonon replicas of the indirect exciton. This striking effect results from the inhibition of quasielastic acoustic phonon scattering in the strong-coupling regime of the light-matter interaction, where the density of final states is reduced by the curvature of the polaritonic dispersion.

We will discuss recent results: (i) on the theory of the Casimir torque between two gratings rotated by an angle theta with respect to each other, and (ii) on the theory and experiment on the Casimir force between interpenetrating gratings. These findings pave the way to the design of contactless quantum vacuum torsional spring and sensors with possible relevance to micro and nanomechanical devices.