arXiv subjects

Giancarlo Soavi

Publications and source records attributed to Giancarlo Soavi.

At least 19 recordsLinked to original sources

arXiv2026

All-Optical Control of Interfacial Polarization in MoS$_2$/WSe$_2$ Heterobilayers

All-optical tuning of van der Waals heterostructures with coherent radiation offers a promising path toward ultrafast memory and optoelectronic devices. In the first-principles framework of real-time time-dependent density functional theory, we predict the induction of a persistent, long-lived out-of-plane polarization in MoS$_2$/WSe$_2$ heterobilayers, resonantly driven by intense ultrafast pulses. While weak fields preserve the intrinsic type-II band alignment, intermediate intensities trigger a four-fold enhancement of interlayer charge transfer. By analyzing the high-harmonic generation spectrum, we identify a transition from the perturbative to the strong-field regime inducing photoinduced interfacial polarity. We additionally show that lattice strain, ubiquitously present in heterobilayers, can be used as additional knob to adjust the resonant condition without compromising the permanent dipole induction. Our findings provide a theoretical blueprint for the all-optical manipulation of polar phases in low-dimensional heterostructures at the femtosecond scale.

cond-mat.mtrl-sci↗

arXiv2026

Ultrafast tuning of the focusing efficiency of a nonlinear atomically thin lens

Control over light focusing is paramount for optical and imaging systems. However, its implementation in active and integrated nanophotonic devices is hindered by the bulky and static nature of standard lenses. Here, we show that Fresnel Zone Plate Lenses (FZPL) based on a monolayer Transitional Metal Dichalcogenide are a viable solution to the challenges of miniaturization, integration, and active control of the focusing efficiency, featuring both ultrafast and large modulation depth. Using a monolayer $WSe_2$ FZPL, we demonstrate all-optical ultrafast ($\sim$ ps) modulation of the focusing efficiency close to $\sim$30\%, that we achieve by exploiting the nonlinear exciton-resonant enhancement and modulation of second harmonic generation.

physics.optics↗

arXiv2026

Twist-angle Control of Nonlinear Interference in a ZnO Nanowire/Monolayer WSe$_2$ Hybrid Structure

Nanoscale devices that integrate materials of different dimensionalities (0D, 1D, and 2D) hold great potential for advanced applications in photonics and optoelectronics. A fundamental requirement for the development of such devices is the engineering and control of light-matter interactions beyond the simple enhancement or quenching of linear and nonlinear optical emission. In this study, we demonstrate control over nanoscale light-matter interactions by achieving twist-angle tunability of the nonlinear optical response in a hybrid system composed of a ZnO nanowire and a monolayer of WSe$_2$. By varying the relative orientation between the ZnO polar axis and the WSe$_2$ crystal axes, we realize both constructive and destructive interference in second-harmonic generation, as well as full material selectivity in second harmonic polarization-dependent measurements. These outcomes arise from the distinct dimensionalities and symmetries of the hybrid constituents, underscoring the generality of our approach. Thus, our work presents an advanced framework for the design and control of nonlinear light-matter interactions in nanoscale hybrid devices, thereby paving the way for their future use in photonic and optoelectronic technologies.

physics.optics↗

arXiv2026

Highly Efficient Exciton Modulation in MoSe$_2$/PdSe$_2$ Heterostructures

Controlling exciton recombination in atomically thin semiconductors is central to their optoelectronic functionality, as the competition between radiative and non-radiative decay channels governs emission efficiency. Existing approaches, such as defect passivation, chemical doping, dielectric engineering, and strain tuning, primarily aim to suppress non-radiative losses. Here, we report a pronounced $\sim$6-fold enhancement of room-temperature A-exciton emission in a type-I MoSe$_2$/PdSe$_2$ van der Waals heterostructure, yielding a photoluminescence quantum yield of 6 %, compared to $\sim$1 % for as-exfoliated monolayer MoSe$_2$. This enhancement is accompanied by strong quenching of the B-exciton, consistent with interlayer electronic coupling that redistributes exciton populations toward the radiative A-exciton channel. Power- and temperature-dependent measurements reveal a suppression of exciton-exciton annihilation and a crossover to quenched emission at low temperature, indicating a redistribution of exciton relaxation pathways. Photoluminescence excitation spectroscopy further reveals a broadband enhancement spanning 450-725 nm, ruling out a resonance-specific mechanism. These results demonstrate that interlayer electronic coupling can be used as an efficient means to redirect exciton populations toward radiative channels, enhancing emission efficiency in two-dimensional semiconductors without chemical modification or strain.

cond-mat.mes-hall↗

arXiv2026

Nonlinear Circular Dichroism Reveals the Local Berry Curvature

Light-matter interactions are governed by conservation laws of energy and momentum. For harmonic generation in crystalline solids, energy conservation imposes that $m$ incoming photons with energy $\hbar ω_0$ are combined to form one photon at energy $m\hbar ω_0$. Linear momentum conservation governs phase matching, whereas angular momentum conservation connects the angular momentum carried by photons to the discrete rotational symmetry of the crystal lattice. As a consequence, circular harmonic generation exerts a torque on the lattice and, conversely, a macroscopic rotation of the crystal induces a nonlinear rotational Doppler shift. These cornerstone laws of nonlinear optics rely on macroscopic symmetry arguments, and therefore provide little insight into the microscopic origin of angular momentum transfer. Here we uncover a direct connection between angular momentum conservation in nonlinear optics and the electronic quantum geometry, by proving that the transferred angular momentum from light to the crystal is proportional to the local Berry curvature at one optical resonance. This relation is encoded in the nonlinear harmonic circular dichroism, which we measure experimentally in an atomically thin semiconductor. With this, we extend our understanding of nonlinear optics, and we establish a method for the all-optical control and read-out of the local Berry curvature.

cond-mat.mes-hall↗

arXiv2026

Valleytronics in 2D Materials Roadmap

Valleytronics exploits non-equivalent energy extrema in the electronic band structure of crystalline solids -- the valley degree of freedom -- to encode, manipulate, and read out information. The advent of 2D materials, first graphene and then transition-metal dichalcogenides, made valley control practical through optical, electrical, and magnetic routes. This foundation has enabled remarkable progress in recent years spanning established frontiers, such as valley exciton physics and valley Hall effects, as well as emerging directions including lightwave valleytronics, nanophotonic integration, flat-band valleytronics, and spin-valley qubits. In parallel, there are sustained efforts to scale up valleytronic materials and to predict new valleytronic platforms. This Roadmap brings together perspectives from leading experts to chart the key opportunities and challenges at the forefront of 2D material valleytronics. Each section captures a snapshot of progress in a key research area, identifies critical open challenges, and outlines pathways toward future valleytronics breakthroughs.

cond-mat.mes-hall↗

arXiv2026

Photoluminescence Quenching in WSe$_2$ via p-Doping Induced by Functionalized Rylene Dyes

Hybrid heterostructures combining transition metal dichalcogenides (TMDs) with light-harvesting dyes are promising materials for next-generation optoelectronics. Yet, controlling and understanding interfacial charge transfer mechanisms in these complex systems remains a major challenge. Here, we investigate the microscopic origin of photoluminescence (PL) quenching in $\text{WSe}_2$ functionalized with a novel, strongly electron-deficient perylene monoimide dye, $\text{CN}_4\text{PMI}$. Experimentally, the hybridization induces a $\sim$97\% PL quenching in $\text{WSe}_2$, confirming substantial static charge transfer and increased $p$-doping from the dye. To isolate the dominant electronic mechanism, we investigate from first principles various interface morphologies, including differing molecular orientations and layer thicknesses. Our density-functional theory results confirm that $\text{CN}_4\text{PMI}$ acts as a strong electron acceptor, inducing $p$-doping and forming a type-II level alignment with all considered configurations, giving rise to a small or vanishing band gap. Based on these findings, we attribute the observed PL suppression in $\text{WSe}_2$ to these strong electronic interactions with the dye. Our study provides a clear and validated strategy for tailoring the electronic structure of TMDs through targeted, electron-deficient organic functionalization.

cond-mat.mtrl-sci↗

arXiv2025

Intrinsically chiral exciton polaritons in an atomically-thin semiconductor

Photonic bound states in the continuum (BICs) have emerged as a versatile tool for enhancing light-matter interactions by strongly confining light fields. Chiral BICs are photonic resonances with a high degree of circular polarisation, which hold great promise for spin-selective applications in quantum optics and nanophotonics. Here, we demonstrate a novel application of a chiral BIC for inducing strong coupling between the circularly polarised photons and spin-polarised (valley) excitons (bound electron-hole pairs) in atomically-thin transition metal dichalcogenide crystals (TMDCs). By placing monolayer WS$_2$ onto the BIC-hosting metasurface, we observe the formation of intrinsically chiral, valley-selective exciton polaritons, evidenced by circularly polarised photoluminescence (PL) at two distinct energy levels. The PL intensity and degree of circular polarisation of polaritons exceed those of uncoupled excitons in our structure by an order of magnitude. Our microscopic model shows that this enhancement is due to folding of the Brillouin zone creating a direct emission path for high-momenta polaritonic states far outside the light cone, thereby providing a shortcut to thermalisation (energy relaxation) and suppressing depolarisation. Moreover, while the polarisation of the upper polariton is determined by the valley excitons, the lower polariton behaves like an intrinsic chiral emitter with its polarisation fixed by the BIC. Therefore, the spin alignment of the upper and lower polaritons ($\uparrow\downarrow$ and $\uparrow \uparrow$) can be controlled by $σ^+$ and $σ^-$ polarised optical excitation, respectively. Our work introduces a new type of chiral light-matter quasi-particles in atomically-thin semiconductors and provides an insight into their energy relaxation dynamics.

cond-mat.mes-hall↗

arXiv2025

Tunable Resonant Metasurfaces Empowered by Atomically Thin Semiconductors

Nanophotonics has recently gained new momentum with the emergence of a novel class of nanophotonic systems consisting of resonant dielectric nanostructures integrated with single or few layers of transition metal dichalcogenides (2D-TMDs). Thinned to the single layer phase, 2D-TMDs are unique solid-state systems with excitonic states able to persist at room temperature and demonstrate notable tunability of their energies in the optical range. Based on these properties, they offer important opportunities for hybrid nanophotonic systems where a nanophotonic structure serves to enhance the light-matter interaction in the 2D-TMDs, while the 2D-TMDs can provide various active functionalities, thereby dramatically enhancing the scope of nanophotonic structures. In this work, we combine 2D-TMD materials with resonant photonic nanostructures, namely, metasurfaces composed of high-index dielectric nanoparticles. The dependence of the excitonic states on charge carrier density in 2D-TMDs leads to an amplitude modulation of the corresponding optical transitions upon changes of the Fermi level, and thereby to changes of the coupling strength between the 2D-TMDs and resonant modes of the photonic nanostructure. We experimentally implement such a hybrid nanophotonic system and demonstrate voltage tuning of its reflectance as well as its different polarization-dependent behavior. Our results show that hybridization with 2D-TMDs can serve to render resonant photonic nanostructures tunable and time-variant $-$ important properties for practical applications in optical analog computers and neuromorphic circuits.

physics.optics↗

arXiv2025

Ultrafast transition from coherent to incoherent polariton nonlinearities in a hybrid 1L-WS2/plasmon structure

Exciton polaritons based on atomically thin semiconductors are essential building blocks of quantum optoelectronic devices. Their properties are governed by an ultrafast and oscillatory energy transfer between their excitonic and photonic constituents, resulting in the formation of polaritonic quasiparticles with pronounced nonlinearities induced by the excitonic component. In metallic nanoresonators, dissipation phenomena limit the polariton lifetime to a few ten femtoseconds, so short that the role of these polaritons for the nonlinearities of such hybrids is yet unexplored. Here, we use ultrafast two-dimensional electronic spectroscopy (2DES) to uncover coherent polariton dynamics in a hybrid monolayer (1L) WS2/plasmonic nanostructure. With respect to an uncoupled WS2 flake, we observe an over 20-fold, polarization-dependent enhancement of the optical nonlinearity and a rapid evolution of the 2DES spectra within ~70 fs. We relate these dynamics to a transition from coherent polaritons to incoherent excitations, unravel the microscopic optical nonlinearities, and show the potential of coherent polaritons for ultrafast all-optical switching.

cond-mat.mes-hall↗

arXiv2025

Photonics in Flatland: Challenges and Opportunities for Nanophotonics with 2D Semiconductors

Two-dimensional (2D) semiconductors are emerging as a versatile platform for nanophotonics, offering unprecedented tunability in optical properties through exciton resonance engineering, van der Waals heterostructuring, and external field control. These materials enable active optical modulation, single-photon emission, quantum photonics, and valleytronic functionalities, paving the way for next-generation optoelectronic and quantum photonic devices. However, key challenges remain in achieving large-area integration, maintaining excitonic coherence, and optimizing amplitude-phase modulation for efficient light manipulation. Advances in fabrication, strain engineering, and computational modelling will be crucial to overcoming these limitations. This perspective highlights recent progress in 2D semiconductor-based nanophotonics, emphasizing opportunities for scalable integration into photonics.

physics.optics↗

arXiv2025

The role of Berry curvature derivatives in the optical activity of time-invariant crystals

Quantum geometry and topology are fundamental concepts of modern condensed matter physics, underpinning phenomena ranging from the quantum Hall effect to protected surface states. The Berry curvature, a central element of this framework, is well established for its key role in electronic transport, whereas its impact on the optical properties of crystals remains comparatively unexplored. Here, we derive a relation between optical activity, defined by the gyration tensor, and the k-derivatives of the Berry curvature at optical resonances in the Brillouin zone. We systematically determine which of these derivatives are non-zero or constrained by symmetry across all time-reversal-invariant crystal classes. In particular, we analytically demonstrate that circular dichroism emerges in chiral crystal classes as a result of a non-zero Berry curvature k-derivative along the optical axis, and we interpret this finding based on the conservation of angular momentum in light-matter interactions. This work establishes a quantum-geometric framework for optical activity in solids and it opens new routes to probe quantum geometry via linear and nonlinear optics.

cond-mat.mes-hall↗

arXiv2025

Roadmap for Photonics with 2D Materials

Triggered by the development of exfoliation and the identification of a wide range of extraordinary physical properties in self-standing films consisting of one or few atomic layers, two-dimensional (2D) materials such as graphene, transition metal dichalcogenides (TMDs), and other van der Waals (vdW) crystals currently constitute a wide research field protruding in multiple directions in combination with layer stacking and twisting, nanofabrication, surface-science methods, and integration into nanostructured environments. Photonics encompasses a multidisciplinary collection of those directions, where 2D materials contribute with polaritons of unique characteristics such as strong spatial confinement, large optical-field enhancement, long lifetimes, high sensitivity to external stimuli (e.g., electric and magnetic fields, heating, and strain), a broad spectral range from the far infrared to the ultraviolet, and hybridization with spin and momentum textures of electronic band structures. The explosion of photonics with 2D materials as a vibrant research area is producing breakthroughs, including the discovery and design of new materials and metasurfaces with unprecedented properties as well as applications in integrated photonics, light emission, optical sensing, and exciting prospects for applications in quantum information, and nanoscale thermal transport. This Roadmap summarizes the state of the art in the field, identifies challenges and opportunities, and discusses future goals and how to meet them through a wide collection of topical sections prepared by leading practitioners.

cond-mat.mtrl-sci↗

arXiv2025

Ultrafast Coherent Bandgap Modulation Probed by Parametric Nonlinear Optics

Light-matter interactions in crystals are powerful tools that seamlessly allow both functionalities of sizeable bandgap modulation and non-invasive spectroscopy. While we often assume that the border between the two regimes of modulation and detection is sharp and well-defined, there are experiments where the boundaries fade. The study of these transition regions allows us to identify the real potentials and inherent limitations of the most commonly used optical spectroscopy techniques. Here, we measure and explain the co-existence between bandgap modulation and non-invasive spectroscopy in the case of resonant perturbative nonlinear optics in an atomically thin direct gap semiconductor. We report a clear deviation from the typical quadratic power scaling of second-harmonic generation near an exciton resonance, and we explain this unusual result based on all-optical modulation driven by the intensity-dependent optical Stark and Bloch-Siegert shifts in the $\pm$K valleys of the Brillouin zone. Our experimental results are corroborated by analytical and numerical analysis based on the semiconductor Bloch equations, from which we extract the resonant transition dipole moments and dephasing times of the used sample. These findings redefine the meaning of perturbative nonlinear optics by revealing how coherent light-matter interactions can modify the band structure of a crystal, even in the weak-field regime. Furthermore, our results strengthen the understanding of ultrafast all-optical control of electronic states in two-dimensional materials, with potential applications in valleytronics, Floquet engineering, and light-wave electronics.

physics.optics↗

arXiv2025

Direct measurement of broken time-reversal symmetry in centrosymmetric and non-centrosymmetric atomically thin crystals with nonlinear Kerr rotation

Time-reversal symmetry, together with space-inversion symmetry, is one of the defining properties of crystals, underlying phenomena such as magnetism, topology and non-trivial spin textures. Transition metal dichalcogenides (TMDs) provide an excellent tunable model system to study the interplay between time-reversal and space-inversion symmetry, since both can be engineered on demand by tuning the number of layers and via all-optical bandgap modulation. In this work, we modulate and study time-reversal symmetry using third harmonic Kerr rotation in mono- and bilayer TMDs. By illuminating the samples with elliptically polarized light, we achieve spin-selective bandgap modulation and consequent breaking of time-reversal symmetry. The reduced symmetry modifies the nonlinear susceptibility tensor, causing a rotation of the emitted third harmonic polarization. With this method, we are able to probe broken time-reversal symmetry in both non-centrosymmetric (monolayer) and centrosymmetric (bilayer) crystals. Furthermore, we discuss how the detected third harmonic rotation angle directly links to the spin-valley locking in monolayer TMDs and to the spin-valley-layer locking in bilayer TMDs. Thus, our results define a powerful approach to study broken time-reversal symmetry in crystals regardless of space-inversion symmetry, and shed light on the spin, valley and layer coupling of atomically thin semiconductors.

physics.optics↗

arXiv2024

High Performance Graphene Integrated Photonics Platform Enabled by Gold-assisted Transfer

Graphene is promising for nanoscale, efficient, ultra-fast photo- and opto-electronic devices because of its remarkable electrical and optical properties, such as fast electron relaxation and heat dissipation. Here, we realize high-performance graphene integrated photonics platform enabled by gold-assisted transfer. Thanks to our optimized transfer technique, we fabricate and demonstrate (1) a microscale thermo-optic modulator with a tuning efficiency of 0.037 nm/mW and high heating performance of 67.4 K$μm^{3}mW^{-1}$ on a small active area of 7.54 $μm^{2}$ and (2) a graphene electro-absorption modulator featuring an high modulation bandwidth up to 26.8 GHz and a high-speed data rate reaching 48 Gb/s, and (3) a graphene Mach-Zehnder interferometer modulator with a high normalized modulation efficiency of 0.027 dBV$^{-1}μm^{-1}$. Our graphene integrated photonics platform has far superior performances compared to state of the art in terms of efficiency, low process complexity, and compact device footage. Thus, our approach and results provide the background for the realization of high-performance integrated photonic circuits with CMOS compatibility.

physics.optics↗

arXiv2024

Unveiling the Role of Electron-Phonon Scattering in Dephasing High-Order Harmonics in Solids

High-order harmonic generation (HHG) in solids is profoundly influenced by the dephasing of the coherent electron-hole motion driven by an external laser field. The exact physical mechanisms underlying this dephasing, crucial for accurately understanding and modelling HHG spectra, have remained elusive and controversial, often regarded more as an empirical observation than a firmly established principle. In this work, we present comprehensive experimental findings on the wavelength-dependency of HHG in both single-atomic-layer and bulk semiconductors. These findings are further corroborated by rigorous numerical simulations, employing ab initio real-time, real-space time-dependent density functional theory and semiconductor Bloch equations. Our experimental observations necessitate the introduction of a novel concept: a momentum-dependent dephasing time in HHG. Through detailed analysis, we pinpoint momentum-dependent electron-phonon scattering as the predominant mechanism driving dephasing. This insight significantly advances the understanding of dephasing phenomena in solids, addressing a long-standing debate in the field. Furthermore, our findings pave the way for a novel, all-optical measurement technique to determine electron-phonon scattering rates and establish fundamental limits to the efficiency of HHG in condensed matter.

physics.optics↗

arXiv2023

Ultrafast Opto-Electronic and Thermal Tuning of Third-Harmonic Generation in a Graphene Field Effect Transistor

Graphene is a unique platform for tunable opto-electronic applications thanks to its linear band dispersion, which allows electrical control of resonant light-matter interactions. Tuning the nonlinear optical response of graphene is possible both electrically and in an all-optical fashion, but each approach involves a trade-off between speed and modulation depth. Here, we combine lattice temperature, electron doping, and all-optical tuning of third-harmonic generation in a hBN-encapsulated graphene opto-electronic device and demonstrate up to 85% modulation depth along with gate-tunable ultrafast dynamics. These results arise from the dynamic changes in the transient electronic temperature combined with Pauli blocking induced by the out-of-equilibrium chemical potential. Our work provides a detailed description of the transient nonlinear optical and electronic response of graphene, which is crucial for the design of nanoscale and ultrafast optical modulators, detectors and frequency converters.

cond-mat.mes-hall↗