Efficient generation of entangled photons typically relies on spontaneous parametric down-conversion (SPDC) in phase-matched macroscopic nonlinear media. However, generating entanglement under phase-matching constraints requires additional bulk optics or interferometers. In contrast, ultrathin van der Waals semiconductors - such as transition metal dichalcogenides (TMDs) - exhibit strong enough optical nonlinearities for SPDC to be observed from subwavelength-thick media, thereby bypassing conventional phase-matching constraints. In this microscopic domain, the intrinsic crystal symmetry governs the nonlinear optical response, enabling the native generation of polarization-entangled photon pairs. However, generating these states efficiently has been fundamentally restricted by the material's coherence length (L_c), which limits the attainable conversion efficiency. Here, we investigate periodically-poled TMDs (PPTMDs) designed to scale up this interaction via quasi-phase matching. We demonstrate that mechanically flipping the sign of the nonlinearity at precise intervals of L_c introduces quasi-phase matching, that scales the pair-production rate while preserving the pristine, symmetry-generated polarization entanglement, with fidelities exceeding 99

The nonlinear Hall effect (NHE) is a recently discovered member of the Hall effect family in which the Hall voltage shows a nonlinear behavior when a transverse electric field is applied. While the NHE does not require broken time-reversal symmetry, such as that induced by a magnetic field, it requires broken inversion symmetry, which limits the range of suitable systems and potential applications. Here, we demonstrate an ultrafast NHE in centrosymmetric black phosphorus through dynamical symmetry breaking using femtosecond light pulses. We provide a detailed microscopic picture of excited carrier dynamics and induced fields using momentum-resolved photoemission spectroscopy combined with ab-initio calculations. The ultrafast NHE is observed exclusively for the light polarization aligned with the armchair high-symmetry direction and persists over 300 fs, which opens new possibilities for selective and ultrafast light-to-current conversions.

We investigate the third harmonic response of platinum ditelluride (PtTe_2), a Dirac semimetal belonging to the transition metal dichalcogenides class. Due to its topological properties, this material has drawn a lot of attention, particularly because it hosts type-II (super-critically tilted) Dirac fermions in the A-Γ- A high symmetry direction. Adopting a low-energy model fitted directly from density functional theory band structure simulations, we calculate analytically the nonlinear conductivity. We observe that third-order optical nonlinearities are efficiently modulated by the cones tilting, which produces a significant enhancement of the nonlinear susceptibility. Our results, besides shedding light on topological transitions of platinum ditelluride, are relevant for future nanophotonic devices exploiting the tunable nonlinear properties of type-II Dirac fermions.

Enantiomeric excess detection in a chiral molecular mixture is paramount because very often opposite enantiomers exhibit profound functional dissimilarities that play decisive roles in biochemical applications. Existing chiral sensing methods mostly rely on large operational sample volumes, hindering compatibility with integrated sensing schemes. Here, we propose a novel chiroptical sensing technique based on the inverse Faraday effect in a photonic micro-capillary filled with nl-volume chiral drug solution. We theoretically demonstrate that, upon excitation by intense laser light, an isotropic assembly of chiral drugs produces a static magnetisation, with amplitude and direction depending on the enantiomeric excess. In turn, by measuring the chirally-sensitive static magnetic field in the vicinity of the micro-tube one can retrieve the enantiomeric excess of the chiral drug solution. Our theoretical predictions unlock new opportunities for the development of innovative nanophotonic devices suitable for efficient chiroptical sensing with nl-volume sensitivity.

Self-diffraction is a four-wave mixing process proportional to the square modulus of third-order nonlinearity susceptibility χ(3), which is related to the material's electronic and thermal properties. In this study, we investigate the wavelength dependence of the self-diffracted signal generated by a femtosecond pulsed laser in a dye solution to directly evaluate the electronic third-order nonlinear susceptibility spectrum. By accounting for absorption effects and phase matching conditions, we determine the |χ(3)| for different concentrations. Experimental results complemented with theoretical predictions show that, in the low absorption and thin sample limits, the signal reproduces the |χ(3)| spectral profile. These findings demonstrate the feasibility of measuring nonlinear susceptibility spectra arising solely from the bound-electronic response across a wide spectral range and for various compounds.

Chiroptical sensing is central to gain fundamental insight into electronic, vibrational and rotational degrees of freedom of chiral molecules, and is a cornerstone for nanomedicine and drug discovery platforms. Current chiral sensing technologies to assess the enantiomeric imbalance of chiral pharmaceutical compounds are sensitive to ml volumes but are time-consuming and cannot be integrated on a chip, thus creating a major bottleneck for drug discovery and nanomedicine. Here, we propose a novel chiroptical sensing approach based on optical rectification in a photonic micro-cavity filled by a drug solution with nl volume. We theoretically demonstrate that, upon optical excitation by intense pulsed laser light, such a nonlinear effect produces a chirally-sensitive nV voltage burst at the electrically-gated micro-cavity boundaries, with sign depending solely on the drug enantiomeric imbalance. Our results shed light on the potential of optical rectification as a robust platform for innovative lab-on-a-chip devices enabling chiral sensing with nl sensitivity.

We propose a new laboratory strategy to generate and detect axion-like particles via third-harmonic generation induced by two non-collinear, polarised high-intensity laser beams of peak intensity of the order of 10^24 W/cm^2, where the third-harmonic signal is generated by the axion field. Starting from the axion-modified Maxwell equations, we analytically derive the axion-induced third-harmonic field, and show that by using state-of-the-art petawatt laser facilities, a detectable signal can be obtained over a broad range of axion masses and couplings. A key feature of the setup is that the axion-photon conversion rate can be resonantly enhanced by tuning the angle between the two beams through a mechanism that does not depend on the physical volume of the apparatus. The proposed configuration may therefore probe an unexplored region of axion parameter space and pave the way for next-generation high-power laser-based axion searches.

The ongoing evolution of hollow-core fibers (HCFs) continues to inspire the development of optofluidic platforms with enhanced sensitivity and minimal sample requirements. Here, we utilize the intrinsic advantages of anti-resonant reflection HCFs-such as low optical loss and broadband transmission-to realize a twisted single-ring HCF (SR-HCF) tailored for polarization-sensitive chiral detection. We optimize the fiber geometry to ensure single-mode operation by strongly attenuating higher-order modes ( >50 dB m(-1)) while maintaining low loss for the fundamental mode ( <0.1 dB m(-1)) and reducing the sample volume to only similar to 660 nl per 34 cm fiber length. By applying a constant twist along the fiber length, we minimize birefringence and ensure stable transmission of linear polarization states with polarization extinction ratios surpassing 38 dB. After injecting an aqueous solution of an optically active molecule, we measure its optical rotation at different wavelengths with millidegree-level sensitivity and remarkable robustness against misalignment. Measurements with different enantiomeric excess concentrations are in good agreement with independent liquid chromatography characterization.

Electromagnetically induced transparency and lasing without inversion are optical manifestations of quantum coherence control. Here we theoretically investigate these effects in a chiral molecule, coherently driven by an optical pump, finding that signal fields can undergo chiroptical absorption quenching or amplification within particular spectral windows at pump intensities as low as approximate to 1kW/cm2. Our calculations reveal that molecular chirality has the capacity to dramatically alter quantum coherence phenomena at peculiar wavelength ranges, modulating efficiently circular dichroism depending on the polarisation state of both pump and signal fields and the molecular orientation. We find that, for a given molecular orientation, the dissymmetry factor probed upon right/left-circular polarisation excitation by the signal can get maximised by pump polarisation tuning and the signal direction. By averaging over impinging signal orientations, we find that the orientation-averaged dissymmetry factor can diverge owing to electromagnetically induced transparency producing vanishing averaged absorption cross-sections.

We investigate the mid-infrared chiroptical response of Aluminum-doped Zinc Oxide (AZO)-based plasmonic nanostructures incorporating pharmaceutical chiral drug solutions. We systematically examine plasmon-enhanced vibrational circular dichroism (VCD) of the chiral drug solution to develop efficient chiroptical sensing techniques.

We theoretically model third-harmonic generation in near-zero heterogeneous nanostructures arising from collision-driven nonlinear electron dynamics. We explore the potential of harmonic generation process on a sodium-aluminum bilayer for the development of integrated XUV sources.

The capability of anisotropic media to exert an optical torque on electromagnetic fields is crucial for polarization control. Here we investigate twisted layered stacks of uniaxial anisotropic media to engineer chiral mirrors for the manipulation of circularly polarized light by adjusting the layer number, thicknesses, and orientation. To this aim, we undertake a comprehensive theoretical analysis devising a multilayered twisted anisotropic photonic crystal enabling highly efficient circular polarization manipulation functionalities, i.e., showing concurring close-to-unitary helicity-preserving reflectance of one circular polarization and transmittance of the opposite one. Owing to the angular-momentum manipulation concept enabled by our proposed devices, we envisage that such chiral mirrors are promising candidates for an alternative class of integrated circular polarizers, holding great potential for applications in quantum technologies, polarization-sensitive optical detectors, and electro-optical information processing.

The enhancement of the inherently weak optical activity of solvated molecules by superchiral fields, crucial for detecting their chirality, is a research frontier of photonics and the basis of novel chiroptical detection schemes. Here, we show that an effective medium consisting of randomly dispersed metal-based nanoparticles embedded within an optically active solvated drug (aqueous reparixin) can enhance vibrational optical rotation and circular dichroism thanks to superchirality produced by slow light in near-zero index conditions. We evaluate from first principles the effective bianisotropic response of the bulk chiral effective medium, showing that, by adjusting the nanoparticles filling fraction, vibrational optical activity is greatly enhanced by a factor ≃ 10^2-10^3 at the near-zero index resonance. Our results are relevant for the development of innovative devices capable of detecting the chirality of low-volume samples, with applications in quantum chemistry and nanomedicine.

The dynamical screening of the electron-phonon vertex is due to the retarded oscillations of the electronic charge following the phonon annihilation into an electron-hole pair. This retardation induces a frequency dependence of the electron-phonon interaction that is commonly neglected. In this work I propose a dynamical perturbative expansion that, while being diagrammatically consistent, defines a controllable and physically sound method to include dynamical screening effects in the electron-phonon vertex. The method is applied to the phonon self-energy of the homogeneous electron gas where I show how retardation effects are driven by the ratio between the plasma and the phonon frequencies. I finally propose a simple approach to estimate the importance of dynamical corrections. This method is applied to the paradigmatic case of MgB_2 to show the non–perturbative and large retardation effects that characterize this peculiar material.

Polarization control is achievable through the optical torque exerted by anisotropic media. We engineer miniaturized uniaxial anisotropic stacks as chiral mirrors reflecting over 99% of one circular polarization and less than 1% of the opposite.

Materials with a vanishing dielectric constant provide an ideal platform for achieving plasmon-enhanced light-matter interactions and are widely employed in various cutting-edge nonlinear photonics applications. In this study, we present the first experimental demonstration of extreme ultraviolet (XUV) plasmon-enhanced self-driven spectral modification using a submicrometric foil of aluminium. This is achieved through the excitation of widely tunable Ferrell-Berreman epsilon-near-zero resonances with extremely low absorption. Our angle-dependent measurements of spectral modulation enhancement, supported by theoretical analysis, reveal efficient spectral modification at peak intensities as low as 380 GW/cm2, which we attribute to ultrafast heating and saturation effects. These findings mark a breakthrough in the enhancement of typically weak nonlinearities in the XUV regime through nonlinear plasmonics, potentially paving the way for unprecedented tools for the manipulation and control of XUV radiation.

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.

The dynamics of nonlinear optical processes in epsilon-near-zero (ENZ) transparent conductive oxides (TCOs) are primarily governed by hot electron relaxation with a sub-picosecond response. However, there is currently a lack of comprehensive understanding of the ultrafast electron dynamics in nonlinear TCO ENZ materials. This study investigates the effects of laser peak power and ENZ mode excitation on hot electron relaxation in TCOs. Our experimental analysis theoretically supported by a hydrodynamic model reveals that increasing laser pulse intensity extends hot electron relaxation time by more than 200%, while ENZ mode excitation increases it by more than 40% in representative TCO ENZ materials. This research demonstrates the controllable modulation of ultrafast ENZ nonlinearity via pulse peak power and ENZ mode field enhancement. These findings provide substantial insights into the potential utilization of ENZ nonlinearity for the development of optical and quantum computing components, including ultrafast optical switches, dynamic pulse shapers, and modulators.