High Harmonic Generation Process Research Articles

Highly Nonlinear Light-Nucleus Interaction.

The interaction between light and atomic nuclei is typically weak and confined within the linear and perturbative regime, which limits the achievable nuclear excitation probability and impedes potential applications such as nuclear optical clocks, nuclear lasers, and nuclear energy storage. In this Letter we show that the interaction between hydrogenlike thorium-229 ions (^{229}Th^{89+}) and contemporary intense lasers propels light-nucleus interaction into a highly nonlinear and nonperturbative regime. This interaction unlocks remarkably efficient nuclear excitation: over 10% of the ^{229}Th nuclei can be excited to the isomeric state by a single femtosecond laser pulse. Moreover, the laser-driven ^{229}Th^{89+} ions radiate multiple wavelengths of light that are high-order harmonics of the laser frequency, akin to the high harmonic generation process from laser-driven atoms but with distinct characteristics. These results pioneer a new frontier for exploring light-matter interaction, furnish a powerful method for efficient control over atomic nuclei, and pave a new way of nuclear coherent light emission.

Metrological robustness of high photon number optical cat states

Abstract In the domain of quantum metrology, cat states have demonstrated their utility despite their inherent fragility with respect to losses. Here, we introduce noise robust optical cat states which exhibit a metrological robustness for phase estimation in the regime of high photon numbers. These cat states are obtained from the intense laser driven process of high harmonic generation (HHG), and show a resilience against photon losses. Focusing on a realistic scenario including experimental imperfections we opt for the case in which we can maximize the lower bound of the quantum Fisher information (QFI) instead of analyzing the best case scenario. We show that the decrease of the QFI in the lossy case is suppressed for the HHG-cat state compared to the even and odd counterparts. In the regime of small losses of just a single photon, the HHG-cat state remains almost pure while the even/odd cat state counterparts rapidly decohere to the maximally mixed state. More importantly, this translates to a significantly enhanced robustness for the HHG-cat against photon loss, demonstrating that high photon number optical cat states can indeed be used for metrological applications even in the presence of losses.

Tabletop Tunable Chiral Photonic Emitter.

The increasing interest in chiral light stems from its spiral trajectory along the propagation direction, facilitating the interaction between different polarization states of light and matter. Despite tremendous achievements in chiral light-related research, the generation and control of chiral pulses have presented enduring challenges, especially at the terahertz and ultraviolet spectral ranges, due to the lack of suitable optical elements for effective pulse manipulation. Conventionally, chiral light can be obtained from intricate optical systems, by an external magnetic field, or by metamaterials, which necessitate sophisticated optical configurations. Here, leveraging the high harmonic generation process, we propose a versatile tunable chiral emitter, composed of only two planar Weyl semimetals slabs, addressing the challenges in both spectral ranges. Our results open the way to a compact tunable chiral emitter platform in both terahertz and ultra-violet frequency ranges. This advancement holds the potential to serve as the cornerstone for integrated chiral photonics.

Bloch-Wave Phase Matching of High Harmonic Generation in Solids.

There has been a longstanding doubt that the conversion efficiency of high harmonics in solids is much lower than expected at such a high level of electron density. To address this issue, we revisit the dynamical process of high harmonic generation (HHG) in solids in terms of wavelet interference. We find that the constructive interference among the wavelets has intrinsic consistency with the phase matching of coupled waves in nonlinear optics, which we call Bloch-wave phase matching. Our analysis indicates that most of the wavelets are out of phase and coherently canceled out during the solid HHG process, leading to only a small fraction of excited electrons effectively contributing to HHG. Consequently, the conversion from the excited electron to HHG is fairly low. Moreover, a Bloch-wave phase-matching scheme is proposed and a nearly 3 orders of magnitude enhancement of solid HHG can be achieved by engineering the crystal structures. Our Letter addresses a longstanding doubt and provides a novel idea and theoretical guidance for realizing efficient all-solid-state tabletop ultraviolet light sources.

Role of blue-shift length in macroscopic properties of high-harmonic generation

The production of brighter coherent XUV radiation by intense laser pulses through the process of high-harmonic generation (HHG) is one of the central challenges in contemporary nonlinear optics. We study the generation and spatial propagation of high harmonics analytically and via ab initio simulations. We focus on the length scales defining the growth of the harmonic signal with propagation distance and show that the well-known coherence length limits HHG only for relatively low driving intensities. For higher intensities, the photoionisation of the medium, naturally accompanying HHG, leads to essentially transient phase matching and laser frequency blue shift. By systematically taking both of these factors into account, we demonstrate that the behaviour of the harmonic signal at higher intensities is defined by another length scale—the blue-shift length. In this generation regime the XUV intensity at a given frequency first grows quadratically and then saturates passing the blue-shift length, but the total harmonic efficiency continues growing linearly due to the linear increase of the harmonic line bandwidth. The changeover to this generation regime takes place for all harmonic orders roughly simultaneously. The rate of the efficiency growth is maximal if the atomic dispersion is compensated by photoelectrons near the centre of the laser pulse. Our theory offers a robust way to choose the generation conditions that optimise the growth of the harmonic signal with propagation.

Entanglement and Squeezing of the Optical Field Modes in High Harmonic Generation.

Squeezed optical fields are a powerful resource for a variety of investigations in basic research and technology. However, the generation of intense squeezed light is challenging. Here, we show that intense squeezed light can be produced using strongly laser driven atoms and the so far unrelated process of high harmonic generation. We demonstrate that when the intensity of the driving field significantly depletes the ground state of the atoms, leading to dipole moment correlations, the quantum state of the driving field and the generated high harmonics are entangled and squeezed. Furthermore, we analyze how the resulting quadrature squeezing of the fundamental laser mode after the interaction can be controlled. The findings open the way for the generation of high intensity squeezed light states for a wide range of applications.

Theoretical analysis of perturbative high harmonic wave mixing from plasma surfaces

High harmonic generation modulated by a weakly perturbing laser field enables new wave mixing frequency components, thus allowing in-situ spatiotemporal measurements and wavefront control of attosecond optical pulses. However, perturbative high harmonic wave mixing from plasma surfaces has not been investigated extensively. In this study, we theoretically analyze the plasma high harmonic generation process in the relativistic regime modulated by a perturbing laser field with an arbitrary frequency. New wave mixing frequency components satisfying the conservation laws of photon energy and momentum are observed. The wave mixing component intensities adhere to a power law for the perturbating laser photon number as the perturbing laser intensity increases, thereby revealing perturbative behaviors in the nonperturbative, extremely nonlinear optical process of high harmonic generation. Detailed studies reveal the polarization selection rule and physical mechanism of high harmonic wave mixing. The modulation of the relativistic factor or mass enhancement of electrons on the plasma surface by the perturbing laser field is believed to result in high harmonic wave mixing in the relativistic regime.

Tracking Charge Migration with Frequency-Matched Strobo-Spectroscopy.

We present frequency-matched strobo-spectroscopy (FMSS) of charge migration (CM) in bromobutadiyne, simulated with time-dependent density functional theory. CM + FMSS is a pump-probe scheme that uses a frequency-matched high harmonic generation (HHG)-driving laser as an independent probe step, following the creation of a localized hole on the bromine atom that induces CM dynamics. We show that the delay-dependent harmonic yield tracks the phase of the CM dynamics through its sensitivity to the amount of electron density on the bromine end of the molecule. FMSS takes advantage of the intrinsic attosecond time resolution of the HHG process in which different harmonics are emitted at different times and thus probe different locations of the electron hole. Finally, we show that the CM-induced modulation of the HHG signal is dominated by the recombination step of the HHG process, with a negligible contribution from the ionization step.

Integrating artificial intelligence into the simulation of structured laser-driven high harmonic generation

High harmonic generation (HHG) stands as one of the most complex processes in strong-field physics, as it enables the conversion of laser light from the infrared to the extreme-ultraviolet or even the soft x-rays, enabling the synthesis and control of pulses lasting as short as tens of attoseconds. Accurately simulating this nonlinear and non-perturbative phenomena requires the coupling the dynamics of laser-driven electronic wavepackets, described by the three-dimensional time-dependent Schrödinger equation (3D-TDSE), with macroscopic Maxwell’s equations. Such calculations are extremely demanding due to the duality of microscopic and macroscopic nature of the process, thereby requiring the use of approximations. We develop a HHG method assisted by artificial intelligence that facilitates the simulation of macroscopic HHG within the framework of 3D-TDSE. This approach is particularly suited to simulate HHG driven by structured laser pulses. In particular, we demonstrate a self-interference effect in HHG driven by Hermite-Gauss beams. The theoretical and experimental agreement allows us to validate the AI-based model, and to identify a unique signature of the quantum nature of the HHG process.

High-order harmonic generation from ultrafast matter Talbot effect

High-order harmonic spectroscopy is a robust method for probing electron dynamics under the influence of a driving field, capturing phenomena as brief as attoseconds. It relies on the extreme non-linear process of high-harmonic generation (HHG), where intense laser pulses are directed at a material, causing it to emit high-energy photons in harmonics of the laser frequency. In this contribution we explore the possibility to generate high-order harmonics from low-dimensional crystalline solids driven under grazing incidence. We demonstrate that, in this unconventional geometry, the electron wavefunction is ejected from the solid and, subsequently, redirected to it to generate harmonics. Most appealingly, we show that the crystal’s periodicity imprinted in the electron’s wavefunction introduces a revival dynamics closely connected with the matter temporal Talbot effect. These Talbot oscillations are ultrafast (< femtosecond) and leave a distinct signature in the high-frequency harmonic spectrum, in the form of structures extending beyond the main spectral cutoff.

Investigation of High Harmonic Generation in Ar-Ne Gas Mixture

"In this study, we experimentally investigate the variation of the phase matching condition of the high harmonic generation (HHG) process with pure argon gas and an argon-neon gas mixture. Phase-matched HHG is generated around the absorption edge of argon gas and then neon gas is added to the original argon gas. The pressure-dependent intensity of the harmonics produced by pure argon gas and the gas mixture is examined. We show that as more neon gas is added to the mixture, the phase matching of the higher order harmonics is less favourable than that of the lower order harmonics. Finally, the total phase mismatch at various gas mixture pressures is discussed. Our experimental results are in agreement with the theoretical calculation."

Disorder-induced effects in high-harmonic generation process in fullerene molecules

The objective of this article is to investigate the profound nonlinear optical response exhibited by inversion symmetric fullerene molecules under the influence of different types of disorders described by the Anderson model. Our aim is to elucidate the localization effects on the spectra of high harmonic generation in such molecules. We show that the disorder-induced effects are imprinted onto molecules’ high-harmonic spectrum. Specifically, we observe a presence of strong even-order harmonic signals already for relatively small disorders. The odd-order harmonics intrinsic for disorder-free systems are generally robust to minor disorders. Both diagonal and off-diagonal disorders lift the degeneracy of states, opening up new channels for interband transitions, leading to the enhancement of the high-harmonic emission. The second harmonic signal has a special behavior depending on the disorder strength. Specifically in the case of diagonal disorder, the second harmonic intensity exhibits a quadratic scaling with the disorder strength, which enables the usage of the harmonic spectrum as a tool in measuring the type and the strength of a disorder.

Wavelength scaling of high harmonic yields and cutoff energies in solids driven by mid-infrared pulses.

The effect of driving wavelengths on high harmonic generation (HHG) have long been a fundamental research topic. However, despite of abundant efforts, the investigation of wavelength scaling of HHG in solids is still confined within the scope of theoretical predictions. In this work, we for the first time to the best of our knowledge, experimentally reveal wavelength scaling of HHG yields and cutoff energy in three typical solid media (namely pristine crystals GaSe, CdTe and polycrystalline ZnSe), driven in a broad mid-infrared (MIR) range from 4.0 to 8.7 µm. It is revealed that when the driving wavelength is shorter than 6.5-7.0 µm, HHG yields decrease monotonously with the MIR driving wavelengths, while they rise abruptly by 1-3 orders of magnitude driven at longer wavelength and exhibit a crest at 7.5 µm. In addition, the cutoff energies are found independent on driving wavelengths across the broad MIR pump spectral range. We propose that the interband mechanism dominates the HHG process when the driving wavelength is shorter than 6.5-7.0 µm, and as the driving wavelength increases, intraband contribution leads to an abrupt rise of the HHG yields, which is verified by the HHG polarization measurement driven at 3.0 and 7.0 µm. This work not only experimentally demonstrate the wavelength scaling of HHG in solids, but more importantly blazes the trail for optimizing the HHG performance by choosing a driving wavelength and provides experimental method to distinguish the interband and intraband dynamics.

Multiphoton excitation and high harmonic generation in rectangular graphene quantum dot

The multiphoton excitation and high harmonic generation (HHG) processes are considered using the microscopic quantum theory of nonlinear interaction of strong coherent electromagnetic (EM) radiation with rectangular graphene quantum dot (RGQD). The dynamic Hartree–Fock approximation is developed for the consideration of the quantum dot-laser field nonlinear interaction at the nonadiabatic multiphoton excitation regime. The many-body Coulomb interaction is described in the extended Hubbard approximation. By numerical results, we show the significance of the RGQD lateral size, shape, and EM wavefield orientation in RGQD of the zigzag edge compear to the armchair edge in the HHG process allowing for increasing the cutoff photon energy and the quantum yield of higher harmonics. The differences via edge on the elongated side of the RGQD have been explained by the investigation of the dipole momentum in both cases. Numerical results have shown that the HHG spectra have a strong anisotropy depending on the orientation of the laser wave, and the cutoff photon energy shifts toward blue with an increase in the transverse size of the RGQD.

Diagnosing the waveform of an ultrashort optical pulse by collinear interferometry

During the high-harmonic generation (HHG) process, information about field interaction with the medium is imprinted in the extreme ultraviolet (XUV) radiation. Here, we present a method for using HHG to diagnose the electric field of an optical pulse under a collinear geometry. When mixing a weak signal pulse with a strong driving pulse collinearly, the far field divergence of the XUV HHG is sensitive to the relative delay between the two pulses, which can be used as an ultrafast subcycle gate to reconstruct the electric field of the signal pulse. This collinear configuration is efficient, easy to operate and avoids artificial high order frequency components in reconstruction as compared to non-collinear schemes.

Realization of a Cost-Effective Deformable Grating for the Extreme-Ultraviolet

The application of a deformable grating to the realization of a time delay compensated monochromator is presented. The grating is realized as a replica on a thin glass substrate and the deformation is realized with a mechanical bender. The monochromator is conceived for the spectral selection of a single harmonic in the spectral range 20-80 nm in beamlines exploiting the high harmonic generation process. A possible optical design requiring only two deformable gratings, a mirror and an intermediate slit is presented.

Generation of isolated intense vortex laser with transverse angular momentum

A scheme is proposed to explore the generation of isolated intense vortex laser pulse with transverse angular momentum (AM), which implies that the total AM is non-collinear with the propagation direction. When two non-collinear vortex beams impinge on a solid thin target symmetrically on the same side, the generated harmonics containing the contributions of the two input pulses are emitted from the target at a predicted angle. The longitudinal AM of the harmonics can be predicted from the AM conservation regarding the photons involved in the high-harmonic generation process. The asymmetry of the energy flux in the vertical direction is confirmed as the transverse AM generation source. As an example, the related phenomenon of the fourth order harmonic has been well confirmed by theoretical analysis and three-dimensional particle-in-cell simulations.

High harmonic generation with many-body Coulomb interaction in rectangular graphene quantum dots of armchair edge

High harmonic generation (HHG) in graphene quantum dots of armchair edge by strong coherent electromagnetic radiation taking into account collective electron-electron scattering as well in such nanosystems is considered. The microscopic quantum theory describing the plane quantum dot-laser field nonlinear interaction within the dynamic Hartree–Fock approximation is developed. The many-body Coulomb interaction is considered in the extended Hubbard approximation. The obtained results indicate the significance of the nanostructure lateral size, shape, and bandgap width in HHG process that allows to increase in the quantum yield of high-order harmonics and the cutoff photon energy.

Probing phonon dynamics with multidimensional high harmonic carrier-envelope-phase spectroscopy

We explore pump-probe high harmonic generation (HHG) from monolayer hexagonal-boron-nitride, where a terahertz pump excites coherent optical phonons that are subsequently probed by an intense infrared pulse that drives HHG. We find, through state-of-the-art ab initio calculations, that the structure of the emission spectrum is attenuated by the presence of coherent phonons and no longer comprises discrete harmonic orders, but rather a continuous emission in the plateau region. The HHG yield strongly oscillates as a function of the pump-probe delay, corresponding to ultrafast changes in the lattice such as specific bond compression or stretching dynamics. We further show that in the regime where the excited phonon period and the pulse duration are of the same order of magnitude, the HHG process becomes sensitive to the carrier-envelope phase (CEP) of the driving field, even though the pulse duration is so long that no such sensitivity is observed in the absence of coherent phonons. The degree of CEP sensitivity versus pump-probe delay is shown to be a highly selective measure for instantaneous structural changes in the lattice, providing an approach for ultrafast multidimensional HHG spectroscopy. Remarkably, the obtained temporal resolution for phonon dynamics is ∼1 femtosecond, which is much shorter than the probe pulse duration because of the inherent subcycle contrast mechanism. Our work paves the way toward routes of probing phonons and ultrafast material structural changes with subcycle temporal resolution and provides a mechanism for controlling the HHG spectrum.

Saddle-point exciton signature on high-order harmonic generation in two-dimensional hexagonal nanostructures

The disclosure of basic nonlinear optical properties of graphene-like nanostructures with correlated electron-hole nonlinear dynamics over a wide range of frequencies and pump field intensities is of great importance for both graphene fundamental physics and for the expected novel applications of two-dimensional (2D) hexagonal nanostructures in extreme nonlinear optics. In the current paper, the nonlinear interaction of 2D hexagonal nanostructures with the bichromatic infrared driving field taking into account the many-body Coulomb interaction is investigated. Numerical investigation in the scope of the Bloch equations within the Houston basis that take into account $e\text{--}e$ and $e\text{--}h$ interactions in the Hartree-Fock approximation reveals significant excitonic effects in the high-harmonic generation process in 2D hexagonal nanostructures such as graphene and silicene. It is shown that, due to the correlated electron-hole nonlinear dynamics around the van Hove singularity, spectral caustics in the high-harmonic generation spectrum are induced near the saddle-point excitonic resonances.