arXiv subjects

Cristian Manzoni

Publications and source records attributed to Cristian Manzoni.

16 recordsLinked to original sources

arXiv2026

Topological Control of Polaritonic Flatbands in Anisotropic van der Waals Metasurfaces

Anisotropic van der Waals (vdW) materials exhibit direction-dependent optical and electronic properties, making them valuable for tailoring directional light-matter interactions. Rhenium disulfide (ReS$_2$) stands out for its strong in-plane anisotropy and its thickness-independent direct-bandgap excitons, which can hybridize with light to form exciton-polaritons. In parallel, metasurfaces, engineered arrays of nanoscale subwavelength resonators, can support ultra-sharp photonic modes in the form of quasi-bound states in the continuum (qBICs). Topological transformations of photonic modes can give rise to flatbands, i.e., dispersionless states with quenched kinetic energy and vanishing group velocity. Intrinsic material anisotropy offers an unexplored route to robust far-field flatband formation and control. Here, we demonstrate how structuring an intrinsically anisotropic excitonic material into a resonant metasurface fundamentally transforms its photonic topological features and light-matter coupling behavior, allowing us to drive and topologically control extended far-field flatband formation. To this end, we fabricate C$_4$-symmetric metasurfaces directly from bulk ReS$_2$. The intrinsic anisotropy lifts the initial double degeneracy of the qBIC mode and yields two distinctly polarized resonances. It also reshapes the topological landscape: the integer topological charge of the qBIC mode splits into momentum-separated half-integer singularities, thereby flattening the far-field photonic dispersion. The resulting topologically-controlled photonic flatbands are then tuned in resonance with the linearly polarized excitonic transitions of ReS$_2$, resulting in two distinct, directionally hybridized exciton-polariton flatband regimes. These findings establish anisotropic vdW metasurfaces as a new platform for topologically engineered flatbands and flatband-driven light-matter coupling.

physics.optics↗

arXiv2026

A Dual-Sideband Attosecond Interferometry Setup

We present the development and implementation of an experimental setup designed to investigate attosecond photoionization delays using a dual-sideband RABBITT (Reconstruction of Attosecond Beating By Interference of Two-Photon Transitions) technique. The setup utilizes an attosecond extreme ultraviolet source from high-harmonic generation driven by a carrier-envelope-phase-stabilized Ti:sapphire laser centered at 800 nm. The extreme ultraviolet radiation is synchronized with a 1200 nm infrared probe pulse generated via a non-collinear optical parametric amplifier. Active delay stabilization by means of a spectrally resolved interferometer signal achieves 45 as root-mean-square timing precision and enables the observation of sideband oscillations. Taking advantage of the dependence of the sideband signal on the carrier-envelope phase of the driving field, we report sideband-yield oscillations as a function of this parameter.

physics.optics↗

arXiv2025

Integration of 2D Materials in Radial van der Waals Heterostructure Metasurfaces

Two-dimensional semiconductors, such as monolayer transition metal dichalcogenides (TMDC), exhibit strong excitonic transitions at room temperature and offer a unique platform for exploring light-matter interactions in nanoscale photonic systems. In this work, we demonstrate a compact and polarization-invariant photonic metasurface, fabricated from hexagonal boron-nitride (hBN) and based on radial bound states in the continuum (BIC), which are formed by radially distributed pairs of structurally asymmetric resonators. The metasurface employs multiple symmetry-breaking perturbations to support high quality-(Q-)factor resonances within a footprint smaller than 8 x 8 $μm^2$ - one-sixth of the area of previous approaches. Compared to established hBN metasurface designs, the radial geometry furthermore achieves significantly higher Q-factors with a reduced footprint. By integrating the hBN photonic structure with a WS$_2$ monolayer, we observe enhanced photoluminescence when its resonance is spectrally aligned with the exciton resonance, accompanied by signatures of discrete momentum-space patterns that identify the orbital-angular-momentum-carrying ring eigenmodes. These features persist over a wide range of excitation powers and show minimal linewidth broadening, indicating robust and spatially modulated exciton-photon coupling. This work establishes a scalable approach for generating hybrid photonic-excitonic states with momentum-space structure, offering new opportunities for exciton localization, valley emission, spatially programmable light-matter interaction in two-dimensional material platforms and compact luminescent devices based on 2D material-integrated metasurfaces.

physics.optics↗

arXiv2025

Engineering High-Order Harmonic Generation through Gas Confinement at Sub-Millimeter Lengths

Attosecond light sources based on high-order harmonic generation (HHG) constitute to date the only table-top solution for producing coherent broadband radiation covering the spectral range from the extreme ultraviolet to the soft X-rays. The so-called emission cutoff can be extended towards higher photon energies by increasing the driving wavelength at the expense of conversion efficiency. An alternative route is to overdrive the process by using higher laser intensities, with the challenging requirement of interacting with higher plasma densities over short propagation distances. Here, we address this challenge by using a differentially pumped glass chip designed for optimal gas confinement over sub-mm lengths. By driving HHG with multicycle pulses at either 800 nm or 1500 nm, we demonstrate a cutoff extension by a factor of two compared to conventional phase matching approaches and surpassing the present record using multicycle fields. Our three-dimensional propagation simulations, in excellent agreement with the experiment, confirm that gas confinement is crucial since efficient phase matching of cutoff harmonics occurs only for short propagation lengths. Additionally, we show that the high photon energy component is not only temporally confined to the leading edge of the driving pulse, but also spatially confined in the near-field to an off-axis contribution due to reshaping of the driving field along propagation inside the medium. Our findings contribute to the fundamental understanding of HHG across different regimes.

physics.optics↗

arXiv2025

Ultrafast dynamics of coherent exciton-polaritons in van der Waals semiconductor metasurfaces

Enabling coherent light-matter interactions is a critical step toward next-generation quantum technologies. However, achieving this under ambient temperature conditions remains challenging due to rapid dephasing in optically excited systems. Optical metasurfaces based on quasi-bound states in the continuum have recently emerged as a powerful platform for reaching the strong light-matter coupling regime in flat, subwavelength thickness devices. Here, we investigate ultrafast exciton-polariton dynamics in self-hybridized WS$_2$ thin-film metasurfaces. Using hyperspectral momentum-resolved imaging, we reconstruct the highly anisotropic exciton-polariton dispersion, with a transition from positive to negative effective mass along orthogonal symmetry axes. Femtosecond pump-probe and multidimensional spectroscopy reveal detuning-dependent polariton dynamics with a coherence time up to ~110 fs, and allow direct observation of the coherent dynamics through ultrafast Rabi oscillations with ~45 fs period. We describe this behaviour with a three-eigenstate model that couples the photonic resonance with both bright and dark excitons, extending the conventional two-state picture of strong coupling. Our results establish van der Waals metasurfaces as a promising platform for next-generation polaritonic devices, enabling coherent quantum transfer of matter excitations at room temperature.

physics.optics↗

arXiv2024

Strong-field quantum control in the extreme ultraviolet using pulse shaping

Tailored light-matter interactions in the strong coupling regime enable the manipulation and control of quantum systems with up to unit efficiency, with applications ranging from quantum information to photochemistry. While strong light-matter interactions are readily induced at the valence electron level using long-wavelength radiation, comparable phenomena have been only recently observed with short wavelengths, accessing highly-excited multi-electron and inner-shell electron states. However, the quantum control of strong-field processes at short wavelengths has not been possible, so far, due to the lack of pulse shaping technologies in the extreme ultraviolet (XUV) and X-ray domain. Here, exploiting pulse shaping of the seeded free-electron laser (FEL) FERMI, we demonstrate the strong-field quantum control of ultrafast Rabi dynamics in helium atoms with high fidelity. Our approach unravels a strong dressing of the ionization continuum, otherwise elusive to experimental observables. The latter is exploited to achieve control of the total ionization rate, with prospective applications in many XUV and soft X-ray experiments. Leveraging recent advances in intense few-femtosecond to attosecond XUV to soft X-ray light sources, our results open an avenue to the efficient manipulation and selective control of core electron processes and electron correlation phenomena in real time.

physics.atom-ph↗

arXiv2024

Phase-cycling and double-quantum two-dimensional electronic spectroscopy using a common-path birefringent interferometer

Selecting distinct quantum pathways in two-dimensional electronic spectroscopy (2DES) can give particularly deep insights into coherent and incoherent interactions and quantum dynamics in various materials. This includes isolating rephasing and non-rephasing pathways for conventional single-quantum 2DES, but also the ability to record double- and zero-quantum spectra. Such advanced 2DES schemes usually require phase-cycling when performed in a partially or fully collinear geometry. A particularly simple and effective implementation of 2DES utilizes an in-line birefringent interferometer, the Translating-Wedge-based Identical pulses eNcoding System (TWINS), for the generation of an inherently phase-stable collinear excitation pulse pair. Here, we demonstrate how the TWINS can be adapted to allow for phase-cycling and experimental access to isolated quantum pathways. These new capabilities are demonstrated by recording rephasing, non-rephasing, zero-quantum and double-quantum 2DES on a molecular J-aggregate. This easy-to-implement extension opens up new experimental possibilities for TWINS-based 2DES in multidimensional all-optical and photoemission spectroscopy and microscopy.

physics.chem-ph↗

arXiv2023

Measurement principles for quantum spectroscopy of molecular materials with entangled photons

Nonlinear spectroscopy with quantum entangled photons is an emerging field of research that holds the promise to achieve a superior signal-to-noise ratio and effectively isolate many-body interactions. Photon sources used for this purpose however lack the frequency tunability and spectral bandwidth demanded by contemporary molecular materials. Here, we present design strategies for efficient spontaneous parametric downconversion to generate biphoton states with adequate spectral bandwidth and at visible wavelengths. Importantly, we demonstrate, by suitable design of the nonlinear optical interaction, the scope to engineer the degree of spectral correlations between the photons of the pair. We also present an experimental methodology to effectively characterize such spectral correlations. Importantly, we believe that such a characterization tool can be effectively adapted as a spectroscopy platform to optically probe system-bath interactions in materials.

physics.optics↗

arXiv2021

Roadmap on multimode light shaping

Our ability to generate new distributions of light has been remarkably enhanced in recent years. At the most fundamental level, these light patterns are obtained by ingeniously combining different electromagnetic modes. Interestingly, the modal superposition occurs in the spatial, temporal as well as spatio-temporal domain. This generalized concept of structured light is being applied across the entire spectrum of optics: generating classical and quantum states of light, harnessing linear and nonlinear light-matter interactions, and advancing applications in microscopy, spectroscopy, holography, communication, and synchronization. This Roadmap highlights the common roots of these different techniques and thus establishes links between research areas that complement each other seamlessly. We provide an overview of all these areas, their backgrounds, current research, and future developments. We highlight the power of multimodal light manipulation and want to inspire new eclectic approaches in this vibrant research community.

physics.optics↗

arXiv2021

Angle-tunable intersubband photoabsorption and enhanced photobleaching in twisted bilayer graphene

Van der Waals heterostructures obtained by artificially stacking two-dimensional crystals represent the frontier of material engineering, demonstrating properties superior to those of the starting materials. Fine control of the interlayer twist angle has opened new possibilities for tailoring the optoelectronic properties of these heterostructures. Twisted bilayer graphene with a strong interlayer coupling is a prototype of twisted heterostructure inheriting the intriguing electronic properties of graphene. Understanding the effects of the twist angle on its out-of-equilibrium optical properties is crucial for devising optoelectronic applications. With this aim, we here combine excitation-resolved hot photoluminescence with femtosecond transient absorption microscopy. The hot charge carrier distribution induced by photo-excitation results in peaked absorption bleaching and photo-induced absorption bands, both with pronounced twist angle dependence. Theoretical simulations of the electronic band structure and of the joint density of states enable to assign these bands to the blocking of interband transitions at the van Hove singularities and to photo-activated intersubband transitions. The tens of picoseconds relaxation dynamics of the observed bands is attributed to the angle-dependence of electron and phonon heat capacities of twisted bilayer graphene.

cond-mat.mes-hall↗

arXiv2019

Broadband optical parametric amplification by two-dimensional semiconductors

Optical parametric amplification is a second-order nonlinear process whereby an optical signal is amplified by a pump via the generation of an idler field. It is the key ingredient of tunable sources of radiation that play an important role in several photonic applications. This mechanism is inherently related to spontaneous parametric down-conversion that currently constitutes the building block for entangled photon pair generation, which has been exploited in modern quantum technologies ranging from computing to communications and cryptography. Here we demonstrate single-pass optical parametric amplification at the ultimate thickness limit; using semiconducting transition-metal dichalcogenides, we show that amplification can be attained over a propagation through a single atomic layer. Such a second-order nonlinear interaction at the 2D limit bypasses phase-matching requirements and achieves ultrabroad amplification bandwidths. The amplification process is independent on the in-plane polarization of the impinging signal and pump fields. First-principle calculations confirm the observed polarization invariance and linear relationship between idler and pump powers. Our results pave the way for the development of atom-sized tunable sources of radiation with applications in nanophotonics and quantum information technology.

physics.optics↗

arXiv2017

Photo-Induced Bandgap Renormalization Governs the Ultrafast Response of Single-Layer MoS2

Transition metal dichalcogenides (TMDs) are emerging as promising two-dimensional (2d) semiconductors for optoelectronic and flexible devices. However, a microscopic explanation of their photophysics -- of pivotal importance for the understanding and optimization of device operation -- is still lacking. Here we use femtosecond transient absorption spectroscopy, with pump pulse tunability and broadband probing, to monitor the relaxation dynamics of single-layer MoS2 over the entire visible range, upon photoexcitation of different excitonic transitions. We find that, irrespective of excitation photon energy, the transient absorption spectrum shows the simultaneous bleaching of all excitonic transitions and corresponding red-shifted photoinduced absorption bands. First-principle modeling of the ultrafast optical response reveals that a transient bandgap renormalization, caused by the presence of photo-excited carriers, is primarily responsible for the observed features. Our results demonstrate the strong impact of many-body effects in the transient optical response of TMDs even in the low-excitation-density regime.

cond-mat.mtrl-sci↗

arXiv2017

Non equilibrium optical properties in semiconductors from first--principles: a combined theoretical and experimental study of bulk silicon

The calculation of the equilibrium optical properties of bulk silicon by using the Bethe--Salpeter equation solved in the Kohn--Sham basis represents a cornerstone in the development of an ab--initio approach to the optical and electronic properties of materials. Nevertheless calculations of the {\em transient} optical spectrum using the same efficient and successful scheme are scarce. We report, here, a joint theoretical and experimental study of the transient reflectivity spectrum of bulk silicon. Femtosecond transient reflectivity is compared to a parameter--free calculation based on the non--equilibrium Bethe--Salpeter equation. By providing an accurate description of the experimental results we disclose the different phenomena that determine the transient optical response of a semiconductor. We give a parameter--free interpretation of concepts like bleaching, photo--induced absorption and stimulated emission, beyond the Fermi golden rule. We also introduce the concept of optical gap renormalization, as a generalization of the known mechanism of band gap renormalization. The present scheme successfully describes the case of bulk silicon, showing its universality and accuracy.

physics.optics↗

arXiv2014

Charge photogeneration in few-layer MoS2

The two-dimensional semiconductor MoS2 in its mono- and few-layer form is expected to have a significant exciton binding energy of several 100 meV, leading to the consensus that excitons are the primary photoexcited species. Nevertheless, even single layers show a strong photovoltaic effect and work as the active material in high sensitivity photodetectors, thus indicating efficient charge carrier photogeneration (CPG). Here we use continuous wave photomodulation spectroscopy to identify the optical signature of long-lived charge carriers and femtosecond pump-probe spectroscopy to follow the CPG dynamics. We find that intitial photoexcitation yields a branching between excitons and charge carriers, followed by excitation energy dependent hot exciton dissociation as an additional CPG mechanism. Based on these findings, we make simple suggestions for the design of more efficient MoS2 photovoltaic and photodetector devices.

cond-mat.mtrl-sci↗

arXiv2013

Carrier-envelope phase effects on the strong-field photoemission of electrons from metallic nanostructures

Sharp metallic nanotapers irradiated with few-cycle laser pulses are emerging as a source of highly confined coherent electron wavepackets with attosecond duration and strong directivity. The possibility to steer, control or switch such electron wavepackets by light is expected to pave the way towards direct visualization of nanoplasmonic field dynamics and real-time probing of electron motion in solid state nanostructures. Such pulses can be generated by strong-field induced tunneling and acceleration of electrons in the near-field of sharp gold tapers within one half-cycle of the driving laser field. Here, we show the effect of the carrier-envelope phase of the laser field on the generation and motion of strong-field emitted electrons from such tips. This is a step forward towards controlling the coherent electron motion in and around metallic nanostructures on ultrashort length and time scales.

physics.optics↗

arXiv2011

Nonlinear phononics: A new ultrafast route to lattice control

To date, two types of coupling between electromagnetic radiation and a crystal lattice have been identified experimentally. One is direct, for infrared (IR)-active vibrations that carry an electric dipole. The second is indirect, it occurs through intermediate excitation of the electronic system via electron-phonon coupling, as in stimulated Raman scattering. Nearly 40 years ago, proposals were made of a third path, referred to as ionic Raman scattering (IRS). It was posited that excitation of an IR-active phonon could serve as the intermediate state for a Raman scattering process relying on lattice anharmonicity as opposed to electron phonon interaction. In this paper, we report an experimental demonstration of ionic Raman scattering and show that this mechanism is relevant to optical control in solids. The key insight is that a rectified phonon field can exert a directional force onto the crystal, inducing an abrupt displacement of the atoms from the equilibrium positions that could not be achieved through excitation of an IR-active vibration alone, for which the force is oscillatory. IRS opens up a new direction for the coherent control of solids in their electronic ground state, different from approaches that rely on electronic excitations.

cond-mat.other↗