Quantum technologies are powered by platforms to generate complex non-classical states of matter or light to realize applications. We investigate the non-classical properties of high-harmonic generation in semiconductors, an emerging photonic platform. Measuring the click statistics of three double-digit orders, we evaluate witness operators to certify the non-classicality of the generated states. We show that higher-order harmonics driven by a coherent laser are squeezed and entangled. The properties of the emission are well retrieved with an entangled Gaussian state model, obtained by numerical state optimization to multiple observables. Additionally, we perform inter-order heralded measurements to engineer the quantum state of the emission. The heralded states have distinct properties, showing sub-Poissonian photon statistics. Further, we witness the generation of a quantum non-Gaussian state, a resource highly relevant for quantum information. With this, we establish high-harmonic generation as a platform for generating quantum optical resources.

High-order harmonic generation (HHG) in semiconductor thin films from ultrashort mid-infrared laser drivers holds the potential for the realization of integrated sources of extreme ultraviolet light. Here, we demonstrate solid-state HHG in zinc oxide thin films synthesized by the radiofrequency reactive magnetron sputtering process directly on the cleaved facets of optical fibers. Harmonics 3 to 13 of the radiation from a fiber-based laser system delivering 500 kW, 96 fs pulses at 3130 nm are produced in the thin film and guided along the fiber. A proper choice of the laser wavelength and fiber material allows for filtering out the mid-IR pump laser and achieving the HHG mode selection. The possibility to nanostructure the fiber exit by, e.g., focused ion beam milling paves the way to an increased control over the HHG spatial mode.

Incoherent Diffraction Imaging - IDI - is a diffraction-based imaging technique that has been recently proposed to exploit the partial coherence of incoherently scattered light to retrieve structural information from the scattering centers. Similar to the stellar intensity interferometry of Hanbury Brown and Twiss, the signal builds up on the second-order spatial correlations of the emitted light. The complex spatial distribution of the target is thereby encoded in the spatial intensity fluctuations of the scattered light. The first experimental realisations of this imaging technique have been realised using the fluorescence excited by an ultra-short X-ray pulse at Free Electron Laser (FEL) facilities. Here, we propose an alternative set-up based on a table-top Pseudo-Thermal Light Source. This set-up allows us to explore IDI under a wide range of physically relevant conditions as well as to benchmark numerical and analytical models currently used to determine the imaging capabilities of this technique.

The demand for precise, high-energy Extreme Ultraviolet (EUV) sources is growing across scientific and industrial fields. Studies have demonstrated that bulk crystals driven by mid-infrared laser pulses can generate high-order harmonics with enhanced intensities, extended cut-off energies, and improved damage thresholds [1]. While high-energy OPCPA and OPA systems have been successfully utilized to achieve these results, their complexity highlights the need for simpler, more compact solutions [1]–[3].

High-harmonic generation (HHG) and quantum optics are historically two distant fields of science. HHG as a central technique to generate attosecond pulses via atoms, molecules, or solids has been mostly treated as a classical source of light. Entanglement and squeezing of HHG light would open fascinating perspectives in quantum technology, with the emergence of a field of basic and applied research: attosecond quantum optics. Here, we measure entanglement in a tripartite solid-state HHG system through the violation of a Cauchy-Schwarz inequality. Displaced squeezing is experimentally observed and supported by numerical simulations with excellent agreement to the data. The Schmidt number is estimated, which indicates a low-dimensional multimode structure for each harmonic, an important property for most quantum technologies. At the applied level, the semiconductor HHG source is scalable, integrable, operates at room temperature with compact lasers, and thus represents a useful and exploitable resource in quantum photonics.

High harmonic generation is a resource of extremely broad frequency combs of ultrashort light pulses. The non-classical nature of this new quantum source has been recently evidenced in semiconductors by showing that high harmonic generation generates multimode squeezed states of light. Applications in quantum information science require the knowledge of the mode structure of the created states, defining how the quantum properties distribute over the spectral modes. To achieve that, an effective Schmidt decomposition of the reduced photonic state is performed on a tripartite harmonic set by simultaneously measuring the second- and third-order intensity correlation function. The Schmidt number is estimated which indicates an almost single-mode structure for each harmonic, a useful resource in quantum technology. By modelling our data with a displaced squeezed state, we retrieve the dependencies of the measured correlation as a function of the high harmonic driving laser intensity. The effective high-harmonic mode distribution is retrieved, and the strength of the contributing squeezing modes is estimated. Additionally, we demonstrate a significant violation of a Cauchy-Schwarz-type inequality for three biseparable partitions by multiple standard deviations. Our results confirm non-classicality of the high-harmonic generation process in semiconductors. The source operates at room temperature with compact lasers, and it could become a useful resource for future applications in quantum technologies.

We present findings on high harmonic generation (HHG) in solids utilizing a high-energy fiber laser system operating at 1550 nm. The driving laser source comprises an erbium-doped fiber chirped pulse amplifier (EDFA) combined with a post-compression stage employing a hollow-core photonic crystal fiber (HC-PCF) filled with noble gases. Nonlinear selfcompression in the HC-PCF enables the generation of ultrashort pulses with a duration of 50 fs and energy of 0.91 mu J at a repetition rate of 660 kHz. In a first step, harmonics up to H7 were observed when focusing the laser into small bandgap materials such as Zinc Oxide (ZnO). Subsequently, the system was enhanced to measure high harmonics in the extreme ultraviolet (XUV) range, with harmonics up to H25 observed using a large bandgap material, magnesium oxide (MgO). To the best of our knowledge, this represents the first solid-state HHG source driven by a high-energy few-cycle fiber laser in the telecom region.

High-order harmonics were generated from mono- and polycrystaline molybdenum disulfide (MoS2) monolayers with an infrared femtosecond pulse. We control the Orbital Angular Momentum (OAM) and spatial polarization distribution of the generation beam by using a liquid crystal Q-plate. We then measure the OAM and the full polarization map of the emitted harmonics. We observe that monocrystaline MoS2 behaves as a polarization converter, while polycrystaline MoS2 may be used as a phase mask.

Owing to their high mobility and immunity to topological deflection, skyrmions in antiferromagnetic (AFM) systems are gaining attention as a potential solution for next-generation magnetic data storage. Synthetic antiferromagnets (SAFs) offer a promising avenue to tune the properties of the individual magnetic layers, facilitating the conditions necessary for skyrmions to be used in practical devices. Despite recent advancements achieving fast skyrmion mobility, the nucleation of small and rigid circular skyrmions without an external field remains challenging in SAFs. Theoretical predictions suggest that optical vortex (OAM) beams can stabilize skyrmionic spin textures by transferring their spin and orbital angular momentum to the magnetic material. Here, this intriguing proposal is delved into and the creation of sub-50 nm compact skyrmions in SAFs using OAM beams is successfully demonstrated, eliminating the need for external magnetic fields. Additionally, the results underscore the importance of beam energy and the number of pulses, as both factors play critical roles in the stabilization of these AFM skyrmionic textures. This breakthrough is significant as it paves the way for stabilizing true zero-field skyrmions in AFM systems, where magnetization is minimally affected by external magnetic fields. This work will open a potential avenue for stabilizing small, compact skyrmions in antiferroic systems, facilitating their implementation in logic and memory devices. Skyrmions in antiferromagnets are attracting interest for next-gen magnetic data storage due to their high mobility and immunity to topological deflection. Despite advancements in skyrmion mobility, nucleating small, rigid skyrmions without an external field remains challenging in SAFs. Here it is demonstrated that optical vortex beams can stabilize skyrmionic textures, creating sub-50 nm skyrmions in SAFs without external fields. image

High-harmonic generation is a light up-conversion process occurring in a strong laser field, leading to coherent bursts of extreme ultrashort broadband radiation [Lewenstein et al., Phys. Rev. A 49 , 2117 (1994)]. As a new perspective, we propose that ultrafast strong-field electronic or photonic processes such as high-harmonic generation can potentially generate nonclassical states of light well before the decoherence of the system occurs [Gorlach et al., Nat. Commun. 11 , 4598 (2020); Stammer et al ., Phys. Rev. Lett. 128, 123603 (2022)]. This could address fundamental challenges in quantum technology such as scalability, decoherence, or the generation of massively entangled states [Lewenstein et al., Luca Argenti Michael Chini, 27 (2024)]. Here, we report experimental evidence of the nonclassical nature of the harmonic emission in several semiconductors excited by a femtosecond infrared laser. By investigating single- and double-beam intensity cross-correlation [Loudon, Rep. Prog. Phys. 43 , 913 (1980)], we measure characteristic nonclassical features in the single-photon statistics. We observe two-mode squeezing in the generated harmonic radiation, which depends on the laser intensity that governs the transition from super-Poissonian to Poissonian photon statistics. The measured violation of the Cauchy-Schwarz inequality realizes a direct test of multipartite entanglement in high-harmonic generation [Wasak, Phys. Rev. A 90 , 033616 (2014)]. This result is supported by the theory of multimodal detection and the Hamiltonian from which the effective squeezing modes of the harmonics can be derived [Gonoskov et al., Phys. Rev. B 109, 125110 (2024); Christ et al. New J. Phys. 13 , 033027 (2011)]. With this work, we show experimentally that high-harmonic generation is a new quantum bosonic platform that intrinsically produces nonclassical states of light with unique features such as multipartite broadband entanglement or multimode squeezing. The source operates at room temperature, using standard semiconductors and a standard commercial fiber laser, opening up new routes for the quantum industry, such as optical quantum computing, communication, and imaging.

High-harmonic generation (HHG) is a highly non-linear frequency up conversion process, mostly studied from a classical point of view. Recently, independent theoretical investigations about the quantum nature of HHG predicted several, nonclassical effects in the high-harmonic radiation [ 1, 2, 3]. In addition to the fundamental interest in understanding the physics behind HHG, a better understanding of the quantum nature of this process could potentially have a broad impact on the rapidly developing field of quantum technologies. It is in this context that present here our experimental photon statistics investigations showing the quantum nature of the HHG process.

Strong-field quantum electronics is emerging as a potential candidate in information processing but still coherence vs decoherence is a primary concern of the concept. Strong-field coherent processes in band gap materials have led during the last decade to the emergence of high harmonic generation in semiconductors, petahertz electronics, or strong-field quantum states. However, the coherent behavior of the sub-optical cycle-driven electrons has never been directly observed. We report here on the experimental evidence of coherent ultrashort emission of hot electrons from a nanostructured semiconductor. Our method uses sub-wavelength electric field enhancement to localize the electron emission within a nanometer-scale spot. We found similarities with the electron emission from metallic nanotips in the strong-field regime, a topic that has opened a vast domain of applications during the last decade. The electron spectra display both odd and even harmonic orders of the driving femtosecond laser frequency, a signature of the coherent nature of the electron emission and their attosecond timing. Our findings complete our knowledge of phenomena governing coherent strong-field processes in semiconductors and open perspectives for the generation of future quantum devices operating in the strong-field regime.

The use of ultrashort pulse lasers in medical treatments is increasing and is already an essential tool, particularly in the treatment of eyes, bones and skin. One of the main advantages of laser treatment is that it is fast and minimally invasive. Due to the interaction of ultrashort laser pulses with matter, X-rays can be generated during the laser ablation process. This is important not only for the safety of the patient, but also for the practitioner to ensure that the legally permissible dose is not exceeded. Although our results do not raise safety concerns for existing clinical applications, they might impact future developments at higher peak powers. In order to provide guidance to laser users in the medical field, this paper examines the X-ray emission spectra and dose of several biological materials and describes their dependence on the laser pulse energy.

We report on high harmonics generation in zinc oxide thin-films driven by an all-solid fiber-based architecture. The laser delivers 41 nJ 70 fs solitonic pulses at 1764 nm and drives harmonics generation up to H7.

We demonstrate a method to image an object using a self-probing approach based on semiconductor high-harmonic generation. On the one hand, ptychography enables high-resolution imaging from the coherent light diffracted by an object. On the other hand, high-harmonic generation from crystals is emerging as a new source of extreme-ultraviolet ultrafast coherent light. We combine these two techniques by performing ptychography measurements with nanopatterned crystals serving as the object as well as the generation medium of the harmonics. We demonstrate that this strong field in situ approach can provide structural information about an object. With the future developments of crystal high harmonics as a compact short-wavelength light source, our demonstration can be an innovative approach for nanoscale imaging of photonic and electronic devices in research and industry.

An interesting property of high harmonic generation in solids is its laser polarization dependent nature which in turn provides information about the crystal and band structure of the generation medium. Here we report on the linear polarization dependence of high-order harmonic generation from a gallium arsenide crystal. Interestingly, we observe a significant evolution of the anisotropic response of above bandgap harmonics as a function of the laser intensity. We attribute this change to fundamental microscopic effects of the emission process comprising a competition between intraband and interband dynamics. This intensity dependence of the anisotropic nature of the generation process offers the possibility to drive and control the electron current along preferred directions of the crystal, and could serve as a switching technique in an integrated all-solid-state petahertz optoelectronic device.

The surge of interest in nonperturbative high-harmonic generation in solids has been driven by the appeal of compact solid-state extreme ultraviolet sources and the prospect of untangling the material properties through high-harmonic generation response to strong fields. The traditional assumption is that the brighter, lower-order harmonics are purely perturbative in nature. However, the border between the perturbative and nonperturbative regimes often remains unclear. Here, we show that third-harmonic generation (THG) using 800-nm, 40-fs pulses displays a nonperturbative response in a wide-band-gap insulator MgO. Furthermore, we show that with the intro-duction of dopants, the nonperturbative THG reverts to the perturbative behavior. We attribute this to the blocking of intraband oscillations and the increased linear absorption pathways introduced by the dopant energy levels.

The occupied and unoccupied electronic states of MoS2 monolayer isolated flake were studied using laboratory based photoemission electron microscope (PEEM) nanoESCA equipped with He-I photon source. PEEM real-space imaging allowed selecting the high quality flake. Altogether, the data will allow accurately recovering the band structures of MoS2. The band structures will be used in future pump-probe experiments to explore the dynamics of electrons in the conduction band and photo-induced multitopological states using trefoil polarization.

On the basis of real-time ab initio calculations, we study the non-perturbative interaction of two-color laser pulses with MgO crystal in the strong field regime to generate isolated attosecond pulse from high-harmonic emissions from MgO crystal. In this regard, we examine the impact of incident pulse characteristics such as its shape, intensity, and ellipticity as well as the consequence of the crystal anisotropy on the emitted harmonics and their corresponding isolated attosecond pulses. Our calculations predict the creation of isolated attosecond pulses with a duration of ~ 300 attoseconds; in addition, using elliptical driving pulses, the generation of elliptical isolated attosecond pulses is shown. Our work prepares the path for all solid-state compact optical devices offering perspectives beyond traditional isolated attosecond pulse emitted from atoms.