We study the near-field radiative heat transfer between two twisted finite-size polar dielectric nanoparticle gratings. Different from previ- ous studies of the same configuration, we do not rely on any approximated effective medium theory to describe the gratings. By the full many-body radiative heat transfer theory, we are able to investigate how the size, distance, and relative orientation between the gratings influence the radiative heat flux. By changing the twisting angle h, we show a significant oscillation of the thermal conductance G(\theta), due to the size effect for gratings of both square and circular shapes. The distance- and twisting-dependent coupling between the gratings accounts for a strong and characteristic modulation of radiative thermal conductance with implications for the energy management, sensing, and the micro-electromechanical system (MEMS) and nano-electromechanical system (NEMS) devices.

Radiative heat transfer (RHT) and radiative thermal energy (RTE) for two-dimensional (2D) nanoparticle en- sembles are investigated 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 nanoparticle ensembles, 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 dipole approximation. Depending on the minimal distance between the in-plane particles (the lattice spacing p for periodic systems), on the separation d between the two lattice and on the thermal wavelength λT = h ̄c/kBT, 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. When p ≪ λT , a multiple scattering of the electromagnetic field inside the systems gives rise to a MBI regime. MBI effects manifest themselves in different ways, depending on the separation d: (a) If d > λT , due to the pure intra-ensemble MBI inside each 2D ensemble, the total heat conductance is less affected, and the thermal conductance spectrum manifests a single peak which is nonetheless shifted with respect to the one typical of two isolated nanoparticles. (b) If d < λT , there is a strong simultaneous intra-ensemble and inter-ensemble MBI. In this regime there is a direct quantitative effect on the heat conductance, in addition to a qualitative effect on the thermal conductance spectrum, which now manifests a new second peak. As for the RTE, to correctly describe the radiation emitted by metallic nanoparticles, we derive an expression of the Poynting vector including also magnetic contribution, in addition to the electric one. By analyzing both periodic and nonperiodic ensembles, we show that the RTE emitted by a single 2D nanoparticle ensemble is sensitive to the particle distribution. As instance, we see that the RTE emitted by 2D concentric-ring-configuration ensemble has an inhibition feature near the center of the ensemble.

We investigate the possibility of spatially and spectrally controlling the thermal infrared emission by exploitation of the Yagi–Uda antenna design. Hybrid antennas composed of both SiC and Au rods are considered and the contributions of emission from all the elements, at a given equilibrium temperature, are taken into account. We show that the detrimental effect due to thermal emission from the not ideal parasitic elements drastically affect the performances of conventional thermal Au antennas in the 12 μm wavelength range. Nevertheless, our results show that the hybrid approach allows the development of efficient narrow-band and high directivity sources. The possibility of exploiting the Yagi-Uda design both in transmission and in reception modes, may open the way to the realization of miniaturized, efficient, robust and cheap sensor devices for mass-market applications.

By using the self-consistent Born approximation, we investigate disorder effect induced by the short-range impurities on the band gap in two-dimensional Dirac systems with the higher order terms in momentum. Starting from the Bernevig-Hughes-Zhang (BHZ) model, we calculate the density of states as a function of the disorder strength. We show that due to quadratic corrections to the Dirac Hamiltonian, the band gap is always affected by the disorder even if the system is gapless in the clean limit. Finally, we explore the disorder effects by using an advanced effective Hamiltonian describing the side maxima of the valence subband in HgTe quantum wells. We show that the band gap and disorder-induced topological phase transition in the real structures may differ significantly from those predicted within the BHZ model.

In this Letter, active control of near-field radiative heat transfer (NFRHT) between two isotropic materials is realized by a coating-twisting method. The two slabs are coated with graphene gratings, and then the NFRHT can be not only enhanced but also weakened, by tuning the twisted angle between the two gratings. The physical mechanism is attributed to the modes coupled by the graphene gratings and the isotropic material, which can vary with the twisted angle. The proposed method is also applicable for other kinds of anisotropic films and may provide a way to realize high-precision nanoscale thermal management, nimble thermal communications, and thermal switch.

Unidirectional surface plasmon polaritons (SPPs) at the interface between a gyrotropic medium and a simple medium are studied in a newly recognized frequency regime wherein the SPPs form narrow, beam-like patterns due to quasi-hyperbolic dispersion. The SPP beams are steerable by controlling parameters such as the cyclotron frequency (external magnetic bias) or the frequency of operation. The bulk band structure along different propagation directions is examined to ascertain a common bulk bandgap, valid for all propagation directions, which the SPPs cross. In addition, group velocity and Poynting vector for the SPPs are presented. The case of a finite-thickness gyrotropic slab is also considered, for which we present the Green function and examine the thickness and loss level required to maintain a unidirectional SPP.

We study 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 anomalously discontinuous at θ 1⁄4 0, due to a critical zero-order geometric transition between a 2D- and a 1D-periodic system. This transition is a peculiarity of the grating geometry and does not exist for intrinsically anisotropic materials. As a remarkable practical consequence, for finite-size gratings, the torque per area can reach extremely large values, increasing without bounds with the size of the system. We show that for finite gratings with only ten period repetitions, the maximum torque is already 60 times larger than the one predicted in the case of infinite gratings. These findings pave the way to the design of a contactless quantum vacuum torsional spring, with possible relevance to micro- and nanomechanical devices.