We use numerical simulations to demonstrate third-harmonic generation with near-unity nonlinear circular dichroism (CD) and high conversion efficiency $\left({10}^{-2}{\text{W}}^{-2}\right)$ in asymmetric Si-on-$\mathrm{Si}{\mathrm{O}}_2$ metasurfaces. The working principle relies on the spin-selective excitation of a quasibound state in the continuum, characterized by a very high $(>{10}^5)$ quality factor. By tuning multimode interference with the variation of the metasurface geometrical parameters, we show the possibility to control both linear CD and nonlinear CD. Our results pave the way for the development of all-dielectric metasurfaces for nonlinear chiro-optical devices with high conversion efficiency.

We demonstrate optically tunable control of second-harmonic generation in all-dielectric nanoantennas: by using a control beam that is absorbed by the nanoresonator, we thermo-optically change the refractive index of the radiating element to modulate the amplitude of the second-harmonic signal. For a moderate temperature increase of roughly 40 K, modulation of the efficiency up to 60% is demonstrated; this large tunability of the single meta-atom response paves the way to exciting avenues for reconfigurable homogeneous and heterogeneous metasurfaces.

The temperonic crystal, a periodic structure with a unit cell made of two slabs sustaining temperature wavelike oscillations on short timescales, is introduced. The complex-valued dispersion relation for the temperature scalar field is investigated for the case of a localized temperature pulse. The dispersion discloses frequency gaps, tunable upon varying the slabs’ thermal properties. Results are shown for the paradigmatic case of a graphene-based temperonic crystal. The temperonic crystal extends the concept of superlattices to the realm of temperature waves, allowing for coherent control of ultrafast temperature pulses in the hydrodynamic regime at above liquid nitrogen temperatures.

The optical constants of germanium antimony telluride (GST), measured by spectroscopic ellipsometry (SE), for the spectral range of 350-30,000 nm are presented. Thin films of GST with composition Ge₂Sb₂Te₅ are prepared by sputtering. As-deposited samples are amorphous, and when heated above the phase transition temperature near 150 °C, films undergo an amorphous to face-centered cubic crystalline phase transition. The optical constants and thicknesses of amorphous and crystalline GST films are determined from multi-angle SE measurements, applying a general oscillator model in both cases. Then, in order to evaluate the optical constants at intermediate states throughout the phase transition, GST films are heated in situ on a temperature stage, and single-angle SE measurements are carried out at discrete temperature steps in a range from 120–158 °C. It is shown that ellipsometric data for partially crystallized states can be fit by treating the GST as an effective medium consisting of its amorphous and crystalline states. Its optical constants, fractional crystallinity, and thickness can be determined at intermediate crystallization states throughout the phase transition. As a practical demonstration of the usefulness of this method, samples are held at fixed temperatures near the transition temperature, and SE is performed periodically. The fraction of crystallinity is determined as a function of time, and an activation energy for the amorphous to crystalline phase transition is determined.

The possibility of investigating the dynamics of solids on timescales faster than the thermalization of the internal degrees of freedom has disclosed novel non-equilibrium phenomena that have no counterpart at equilibrium. Transition metal oxides (TMOs) provide an interesting playground in which the correlations among the charges in the metal d -orbitals give rise to a wealth of intriguing electronic and thermodynamic properties involving the spin, charge, lattice and orbital orders. Furthermore, the physical properties of TMOs can be engineered at the atomic level, thus providing the platform to investigate the transport phenomena on timescales of the order of the intrinsic decoherence time of the charge excitations. Here, we review and discuss three paradigmatic examples of transient emerging properties that are expected to open new fields of research: (i) the creation of non-thermal magnetic states in spin–orbit Mott insulators; (ii) the possible exploitation of quantum paths for the transport and collection of charge excitations in heterostructures; (iii) the transient wave-like behavior of the temperature field in strongly anisotropic TMOs.