Andrea Toma

Spontaneous parametric down-conversion (SPDC) in subwavelength nanostructures is a promising source of quantum light, owing to its multifunctionality and ability to generate complex quantum states. Nevertheless, the mechanisms governing photon-pair generation in such systems remain only partially understood. In particular, experimental investigations of key emission properties in individual resonators, such as directionality and spectral distribution, are still lacking, and predictive theoretical frameworks have not yet been experimentally validated. Here, we report a measurement of the directional and spectral distributions of photon pairs generated via SPDC in a nanostructured resonator. Both distributions exhibit resonant behavior, which we describe using an extended quasi-normal-mode theory. This comparison is enabled by photon-pair count rates of up to 0.45 Hz/mW—to our knowledge, the highest reported for a nanostructured resonator. Our results provide a physical insight into nanoscale SPDC and represent an important step toward designing efficient miniaturized quantum light sources.

We demonstrate tunable THz generation by optical rectification in LiNbO3 rod metasurfaces supporting localized surface phonon polaritons (LSPhPs). By varying the rod length, the LSPhP resonance is tuned across the 4–5 THz range, producing a systematic red-shift of the generated THz spectral peak. THz time-domain spectroscopy and numerical calculations show that the emitted spectrum is governed by the geometry-dependent LSPhP response. Although the total far-field emission remains lower than that of the unpatterned film, numerical calculations show that the metasurfaces enhance the volume-averaged mean-square nonlinear current density and strongly localize the generated THz field near the rods. These results establish phonon-polaritonic metasurfaces as compact platforms for spectrally tailoring optical-to-THz conversion and enhancing localized THz fields.

We have investigated the effect of CdCl2 as an additive in the colloidal synthesis of amino-As-based InAs quantum dots (QDs). Our results demonstrate that this metal halide promotes the incorporation of Cd into the InAs QDs, leading to the formation of In(Cd)As alloyed QDs featuring a narrower bandgap and enhanced photoluminescence (PL) compared to their undoped InAs counterparts. By varying the relative amount of CdCl2, it was possible to synthesize, in a one-pot process, QDs with PL tunable from ∼1000 to ∼1430 nm with a quantum yield as high as 17% at ∼1430 nm, without requiring any postsynthesis shell growth. These In(Cd)As QDs feature longer exciton lifetimes compared to InAs QDs, due to suppressed ultrafast nonradiative trap pathways, and a fast biexciton lifetime that follows the expected linear scaling with nanocrystal volume. Density functional theory (DFT) calculations revealed that (i) In(Cd)As alloys, which are thermodynamically unfavorable in the bulk phase, are stabilized by finite-size effects; (ii) the bandgap narrowing induced by Cd incorporation arises primarily from a pronounced upward shift of the valence band maximum (VBM); and (iii) this upward shift of the VBM also buries the surface trap states below the valence band edge, thereby accounting for the observed high PL efficiency in In(Cd)As QDs. Overall, this work provides mechanistic insights into defect passivation in InAs QDs and establishes metal-halide-mediated growth as an effective strategy for infrared emitting InAs-based QDs.

We have studied halide exchange reactions in large colloidal nanocrystals of lead halide perovskites, with sizes (∼40–60 nm) that are intermediate between those of typical nanocrystals synthesized and studied by the community (5–10 nm) and those of microcrystals or thin films. This size regime still allows an investigation of the nanocrystals with techniques used for colloidal nanocrystals while enabling the observation of phenomena such as phase segregation, generation of planar defects, strain accumulation and relaxation, which are harder to identify in smaller nanocrystals, also given their high instability under the electron beam. We compare nanocubes with hollow nanocubes, and highlight how the presence of the hollow region can influence structural and morphological changes during the reactions. Notably, we report core–shell epitaxial CsPbI3/CsPbCl3 heterostructures by partial I– to Cl– exchange on CsPbI3 nanocrystals. Transient absorption spectroscopy provides evidence consistent with electronic coupling and exciton transfer between the Cl-rich shell and I-rich core in these heterostructures, evidenced by the accelerated bleach decay of the CsPbCl3 component and the delayed population of the CsPbI3 band edge.

The use of periodic arrays of nanostructures has become a powerful strategy for enhancing light–matter interactions. These systems support collective modes known as lattice resonances, which are characterized by strong and spectrally narrow optical responses. While all lattice resonances arise from the multiple, coherent scattering among the nanostructures of the array, the particular geometrical characteristics of the system can strongly affect the interaction strength, leading to very distinct optical responses. Despite the extensive research on this topic, a consistent framework for quantifying and comparing these differences remains lacking. In this work, we introduce the degree of collectivity of a lattice resonance, defined as a measure of the interaction strength between the nanostructures in the array, and relate it to the spectral detuning of the lattice resonance—an easily quantifiable metric. This allows us to establish a direct link between the geometrical characteristics of the array and the optical properties of the lattice resonances it supports. Through a combination of theoretical modeling and experimental validation, we investigate how the degree of collectivity affects the robustness of lattice resonances when the system deviates from perfect periodicity. Our results show that lattice resonances with higher degree of collectivity are more sensitive to common experimental imperfections, such as finite array size, positional disorder, and refractive index mismatch between the substrate and the superstrate. The degree of collectivity introduced in this study provides a valuable tool for predicting the performance and experimental feasibility of the lattice resonances supported by periodic arrays of nanostructures, thus paving the way for the development of advanced photonic devices.

Nonlinear optical phenomena are generally governed by geometry in matter systems, as they depend on the spatial arrangement of atoms within materials or molecules. Metasurfaces, through precisely designed geometries on a subwavelength scale, allow the optical response of a material to be tailored far beyond its natural properties. Therefore, metasurfaces are highly appealing for enabling the engineering of nonlinear optical interactions. Current studies of nonlinear metasurfaces predominantly focus on the phase control of the generated light. Nonetheless, investigating the tensorial nature of the nonlinearity of metasurfaces and its effect on the polarization of the generated light is critical to fully unlocking a range of applications, such as nonlinear vector beam generation and nonlinear polarization imaging. Here, we study the artificial optical nonlinearity of a dielectric metasurface originating from its meta-atom symmetry and describe the third-order nonlinear behavior by considering the polarization degree of freedom. We establish an effective nonlinear medium model that serves as a design toolbox for developing amorphous silicon-based geometric metasurfaces with customizable features for third-harmonic generation. We further extract quantitative values of the artificial nonlinear susceptibility tensor elements related to the investigated nonlinear process and geometry. The implemented functional devices demonstrate the versatility of dielectric metasurfaces in shaping the emitted light in terms of amplitude, phase, and polarization for the precise engineering of advanced nonlinear architectures targeting applications in nonlinear imaging and complex light generation.

Halide exchange in lead‐based halide perovskites has been studied extensively. While mixed Cl/Br or Br/I alloy compositions can be formed with no miscibility gaps, this is precluded for mixed Cl/I compositions, due to the large difference in Cl ⁻ and I ⁻ ionic radii. Here, perovskite‐chalcohalide CsPbCl ₃ –Pb ₄ S ₃ Cl ₂ nanocrystal heterostructures are exploited to study the Cl→I exchange and to isolate new types of intermediate structures. The epitaxial interface between the Pb ₄ S ₃ Cl ₂ chalcohalide and the CsPbCl ₃ perovskite significantly influences the intermediate stages of halide exchange in the perovskite domain, leading to coexisting CsPbCl ₃ and CsPbI ₃ domains, thereby delivering segmented CsPbI ₃ –CsPbCl ₃ –Pb ₄ S ₃ Cl ₂ , energetically favorable heterostructures, with partial I‐alloying of the CsPbCl ₃ domain and at the perovskite–chalcohalide interface. The I:CsPbCl ₃ domain between CsPbI ₃ and Pb ₄ S ₃ Cl ₂ enables a gradual lattice expansion across the heterostructure. This design accommodates interfacial strain, with a 5.6% mismatch at the CsPbCl ₃ –CsPbI ₃ interface and a 3.4% mismatch at the perovskite–chalcohalide interface. Full halide exchange leads to CsPbI ₃ –Pb ₄ S ₃ Cl ₂ heterostructures. Both in intermediate and fully exchanged heterostructures, the CsPbI ₃ domain is emissive. In the intermediate structures, the band alignment between the two perovskite domains is type‐I, with the carriers photogenerated in the CsPbCl ₃ domain quickly transferring to the CsPbI ₃ domain, where they can recombine radiatively.

2D materials, including transition metal dichalcogenides, are attractive for a variety of applications in electronics as well as photonics and have recently been envisioned as an appealing platform for phonon polaritonics. However, their direct characterization in the terahertz spectral region, of interest for retrieving, e.g., their phonon response, represents a major challenge, due to the limited sensitivity of typical terahertz spectroscopic tools and the weak interaction of such long‐wavelength radiation with sub‐nanometer systems. In this work, by exploiting an ad‐hoc engineered metallic surface enabling a ten‐thousand‐fold local absorption boost, enhanced terahertz spectroscopy of a monolayer transition metal dichalcogenide (tungsten diselenide) is performed and its dipole‐active phonon resonance features are extracted. In addition, these data are used to obtain the monolayer effective permittivity around its phonon resonance. Via the direct terahertz characterization of the phonon response of such 2D systems, this method opens the path to the rational design of phonon polariton devices exploiting monolayer transition metal dichalcogenides.

Bound states in the continuum (BICs) garnered significant interest for their potential to create new types of nanophotonic devices. Most prior demonstrations were based on arrays of dielectric resonators, which cannot be miniaturized beyond the diffraction limit, reducing the applicability of BICs for advanced functions. Here, we demonstrate BICs and quasi-BICs based on high-quality factor phonon-polariton resonances in isotopically pure h¹¹BN and how these states can be supported by periodic arrays of nanoresonators with sizes much smaller than the wavelength. We theoretically illustrate how BICs emerge from the band structure of the arrays and verify both numerically and experimentally the presence of these states and enhanced quality factors. Furthermore, we identify and characterize simultaneously quasi-BICs and bright states. Our method can be generalized to create a large number of optical states and to tune their coupling with the environment, paving the way to miniaturized nanophotonic devices with more advanced functions.