scispace - formally typeset
Search or ask a question

Showing papers on "High harmonic generation published in 2019"


Journal ArticleDOI
TL;DR: In this article, a review of recent efforts at understanding and characterizing high-order harmonics from solid-state materials is presented, which can be used to probe driven ultrafast dynamics and its prospects for novel, compact short-wavelength light sources.
Abstract: High-harmonic generation in atomic gases has been studied for decades, and has formed the basis of attosecond science. Observation of high-order harmonics from bulk crystals was, however, reported much more recently, in 2010. This Review surveys the subsequent efforts aimed at understanding the microscopic mechanism of solid-state harmonics in terms of what it can tell us about the electronic structure of the source materials, how it can be used to probe driven ultrafast dynamics and its prospects for novel, compact short-wavelength light sources. Although most of this work has focused on bulk materials as the source, recent experiments have investigated high-harmonic generation from engineered structures, which could form flexible platforms for attosecond photonics. This Review surveys recent efforts at understanding and characterizing generation of high harmonics from solid-state materials.

357 citations


Journal ArticleDOI
TL;DR: Recently, a new class of materials with a vanishing permittivity, known as epsilon-near-zero (ENZ) materials, has been reported to exhibit unprecedented ultrafast nonlinear efficiencies within sub-wavelength propagation lengths as discussed by the authors.
Abstract: Efficient nonlinear optical interactions are essential for many applications in modern photonics. However, they typically require intense laser sources and long interaction lengths, requirements that often render nonlinear optics incompatible with new nanophotonic architectures in integrated optics and metasurface devices. Obtaining materials with stronger nonlinear properties is a crucial step towards applications that require lower powers and smaller footprints. Recently, a new class of materials with a vanishing permittivity, known as epsilon-near-zero (ENZ) materials, has been reported to exhibit unprecedented ultrafast nonlinear efficiencies within sub-wavelength propagation lengths. In this Review, we survey the work that has been performed on ENZ materials and the related near-zero-index materials, focusing on the observation of various nonlinear phenomena (such as intensity-dependent refraction, four-wave mixing and harmonic generation), the identification of unique field-enhancement mechanisms and the study of non-equilibrium dynamics. Degenerately doped semiconductors (such as tin-doped indium oxide and aluminium-doped zinc oxide) are particularly promising candidates for ENZ-enhanced nonlinear optical applications. We conclude by pointing towards possible future research directions, such as the search for ENZ materials with low optical losses and the elucidation of the mechanisms underlying nonlinear enhancements. Materials with vanishingly small dielectric permittivity, known as epsilon-near-zero materials, enable strong ultrafast optical nonlinear responses within a sub-wavelength propagation length. This Review surveys the various observations of nonlinear phenomena in this class of materials.

304 citations


Journal ArticleDOI
TL;DR: A general group theory based formalism for harmonic generation from dilute and dense media, yielding new symmetries and selection rules that are not explained by currently known conservation laws.
Abstract: Symmetry is one of the most generic and useful concepts in science, often leading to conservation laws and selection rules. Here we formulate a general group theory for dynamical symmetries (DSs) in time-periodic Floquet systems, and derive their correspondence to observable selection rules. We apply the theory to harmonic generation, deriving closed-form tables linking DSs of the driving laser and medium (gas, liquid, or solid) in (2+1)D and (3+1)D geometries to the allowed and forbidden harmonic orders and their polarizations. We identify symmetries, including time-reversal-based, reflection-based, and elliptical-based DSs, which lead to selection rules that are not explained by currently known conservation laws. We expect the theory to be useful for ultrafast high harmonic symmetry-breaking spectroscopy, as well as in various other systems such as Floquet topological insulators. It is commonly assumed that a complete theory for selection rules in optical nonlinear harmonic generation was developed previously. Here, the authors present more general group theory based formalism for harmonic generation from dilute and dense media, yielding new symmetries and selection rules.

144 citations


Journal ArticleDOI
TL;DR: A unique broadband natural quasi-phase-matching (QPM) mechanism underlying an observation of highly efficient second- and third-order harmonic generation at multiple wavelengths in an x-cut lithium niobate (LN) microdisk resonator is revealed.
Abstract: We reveal a unique broadband natural quasi-phase-matching (QPM) mechanism underlying an observation of highly efficient second- and third-order harmonic generation at multiple wavelengths in an $x$-cut lithium niobate (LN) microdisk resonator. For light waves in the transverse-electric mode propagating along the circumference of the microdisk, the effective nonlinear optical coefficients naturally oscillate periodically to change both the sign and magnitude, facilitating QPM without the necessity of domain engineering in the micrometer-scale LN disk. The second-harmonic and cascaded third-harmonic waves are simultaneously generated with normalized conversion efficiencies as high as $9.9%/\mathrm{mW}$ and $1.05%/{\mathrm{mW}}^{2}$, respectively, thanks to the utilization of the highest nonlinear coefficient ${d}_{33}$ of LN. The high efficiency achieved with the microdisk of a diameter of $\ensuremath{\sim}30\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ is beneficial for realizing high-density integration of nonlinear photonic devices such as wavelength convertors and entangled photon sources.

142 citations


Journal ArticleDOI
TL;DR: This work uncovers a form of simultaneous spin and orbital angular momentum conservation and shows, theoretically and experimentally, that this phenomenon allows for unprecedented control over the divergence and polarization of extreme-ultraviolet vortex beams.
Abstract: Optical interactions are governed by both spin and angular momentum conservation laws, which serve as a tool for controlling light-matter interactions or elucidating electron dynamics and structure of complex systems. Here, we uncover a form of simultaneous spin and orbital angular momentum conservation and show, theoretically and experimentally, that this phenomenon allows for unprecedented control over the divergence and polarization of extreme-ultraviolet vortex beams. High harmonics with spin and orbital angular momenta are produced, opening a novel regime of angular momentum conservation that allows for manipulation of the polarization of attosecond pulses-from linear to circular-and for the generation of circularly polarized vortices with tailored orbital angular momentum, including harmonic vortices with the same topological charge as the driving laser beam. Our work paves the way to ultrafast studies of chiral systems using high-harmonic beams with designer spin and orbital angular momentum.

114 citations


Journal ArticleDOI
TL;DR: In this article, the echo-enabled harmonic generation (EEHG) configuration was used to generate stable, intense, nearly fully coherent pulses at wavelengths as short as 5.9 nm.
Abstract: X-ray free-electron lasers (FELs), which amplify light emitted by a relativistic electron beam, are extending nonlinear optical techniques to shorter wavelengths, adding element specificity by exciting and probing electronic transitions from core levels. These techniques would benefit tremendously from having a stable FEL source, generating spectrally pure and wavelength-tunable pulses. We show that such requirements can be met by operating the FEL in the so-called echo-enabled harmonic generation (EEHG) configuration. Here, two external conventional lasers are used to precisely tailor the longitudinal phase space of the electron beam before emission of X-rays. We demonstrate high-gain EEHG lasing producing stable, intense, nearly fully coherent pulses at wavelengths as short as 5.9 nm (~211 eV) at the FERMI FEL user facility. Low sensitivity to electron-beam imperfections and observation of stable, narrow-band, coherent emission down to 2.6 nm (~474 eV) make the technique a prime candidate for generating laser-like pulses in the X-ray spectral region, opening the door to multidimensional coherent spectroscopies at short wavelengths. Echo-enabled harmonic generation in a free-electron laser enables 45th harmonic pulses from a 264 nm wavelength seed, yielding 5.9 nm wavelength coherent output.

100 citations


Journal ArticleDOI
TL;DR: It is shown how luminal metamaterials generalize the parametric oscillator concept, realize giant broadband nonreciprocity, achieve efficient one-way amplification, pulse compression, and harmonic generation, and propose a realistic implementation in double-layer graphene.
Abstract: Time has emerged as a new degree of freedom for metamaterials, promising new pathways in wave control. However, electromagnetism suffers from limitations in the modulation speed of material parameters. Here we argue that these limitations can be circumvented by introducing a traveling-wave modulation, with the same phase velocity of the waves. We show how luminal metamaterials generalize the parametric oscillator concept, realize giant broadband nonreciprocity, achieve efficient one-way amplification, pulse compression, and harmonic generation, and propose a realistic implementation in double-layer graphene.

99 citations


Journal ArticleDOI
13 Sep 2019
TL;DR: In this paper, the concept of bound states in the continuum with engineering of epsilon-near-zero substrates was combined with the engineering of Epsilon near zero substrates to boost multi-frequency and multi-step cascaded nonlinear processes at the nanoscale.
Abstract: This paper combines the concept of bound states in the continuum with engineering of epsilon-near-zero substrates to boost multi-frequency and multi-step cascaded nonlinear processes at the nanoscale. High-order nonlinear processes such as four-wave mixing, third- and fifth-harmonic generation in all-dielectric subwavelength resonators are shown to be enhanced in comparison with the state-of-the-art results.

99 citations


Journal ArticleDOI
04 Apr 2019
TL;DR: In this paper, the authors discuss the current state of the art in the area of all-dielectric nonlinear nanostructures and metasurfaces, including the role of Mie modes, Fano resonances, and anapole moments for harmonic generation, wave mixing, and ultrafast optical switching.
Abstract: Free from phase-matching constraints, plasmonic metasurfaces have contributed significantly to the control of optical nonlinearity and enhancement of nonlinear generation efficiency by engineering subwavelength meta-atoms. However, high dissipative losses and inevitable thermal heating limit their applicability in nonlinear nanophotonics. All-dielectric metasurfaces, supporting both electric and magnetic Mie-type resonances in their nanostructures, have appeared as a promising alternative to nonlinear plasmonics. High-index dielectric nanostructures, allowing additional magnetic resonances, can induce magnetic nonlinear effects, which, along with electric nonlinearities, increase the nonlinear conversion efficiency. In addition, low dissipative losses and high damage thresholds provide an extra degree of freedom for operating at high pump intensities, resulting in a considerable enhancement of the nonlinear processes. We discuss the current state of the art in the intensely developing area of all-dielectric nonlinear nanostructures and metasurfaces, including the role of Mie modes, Fano resonances, and anapole moments for harmonic generation, wave mixing, and ultrafast optical switching. Furthermore, we review the recent progress in the nonlinear phase and wavefront control using all-dielectric metasurfaces. We discuss techniques to realize all-dielectric metasurfaces for multifunctional applications and generation of second-order nonlinear processes from complementary metal–oxide–semiconductor-compatible materials.

98 citations


Journal ArticleDOI
TL;DR: In this article, the authors report high-harmonic emission up to the 9th order directly from a low-loss, solid-state ENZ medium: indium-doped cadmium oxide, with an excitation intensity at the GW cm-2 level.
Abstract: High-harmonic generation (HHG) from a compact, solid-state medium is highly desirable for applications such as coherent attosecond pulse generation and extreme ultra-violet (EUV) spectroscopy, yet the typically weak conversion of pump light to HHG can largely hinder its applications. Here, we use a material operating in its epsilon-near-zero (ENZ) region, where the real part of its permittivity vanishes, to greatly boost the efficiency of the HHG process at the microscopic level. In experiments, we report high-harmonic emission up to the 9th order directly from a low-loss, solid-state ENZ medium: indium-doped cadmium oxide, with an excitation intensity at the GW cm-2 level. Furthermore, the observed HHG signal exhibits a pronounced spectral red-shift as well as linewidth broadening, resulting from the photo-induced electron heating and the consequent time-dependent resonant frequency of the ENZ film. Our results provide a novel nanophotonic platform for strong field physics, reveal new degrees of freedom for spectral and temporal control of HHG, and open up possibilities of compact solid-state attosecond light sources.

92 citations


Journal ArticleDOI
TL;DR: Th thin-film-based ultraefficient periodically-poled lithium niobate nonlinear waveguides are presented, leveraging actively-monitored ferroelectric domain reversal engineering and nanophotonic confinement to address growing demands in integrated-photonic frequency conversion, frequency metrology, atomic physics, and quantum optics.
Abstract: Chip-scale implementations of second-order nonlinear optics benefit from increased optical confinement that can lead to nonlinear interaction strengths that are orders of magnitude higher than bulk free-space configurations. Here, we present thin-film-based ultraefficient periodically-poled lithium niobate nonlinear waveguides, leveraging actively-monitored ferroelectric domain reversal engineering and nanophotonic confinement. The devices exhibit up to 4600 %W−1cm−2 conversion efficiency for second-harmonic generation, pumped around 1540 nm. In addition, we measure broadband sum-frequency generation across multiple telecom bands, from 1460 to 1620 nm. As an immediate application of the devices, we use pulses of picojoule-level energy to demonstrate second-harmonic generation with over 10% conversion in a 0.6-mm-long waveguide. Our ultracompact and highly efficient devices address growing demands in integrated-photonic frequency conversion, frequency metrology, atomic physics, and quantum optics, while offering a coherent link between the telecom and visible bands.

Journal ArticleDOI
TL;DR: In this paper, the authors revisited the mechanism of high-harmonic generation from solids by comparing HHG in laser fields with different ellipticities but a constant maximum amplitude, and they showed that the cutoff of HHG is strongly extended in a circularly polarized field.
Abstract: We revisit the mechanism of high-harmonic generation (HHG) from solids by comparing HHG in laser fields with different ellipticities but a constant maximum amplitude. It is shown that the cutoff of HHG is strongly extended in a circularly polarized field. Moreover, the harmonic yield with large ellipticity is comparable to or even higher than that in the linearly polarized field. To understand the underlying physics, we develop a reciprocal-space-trajectory method, which explains HHG in solids by a trajectory ensemble from different ionization times and different initial states in the reciprocal space. We show that the cutoff extension is related to an additional preacceleration step prior to ionization, which has been overlooked in solids. By analyzing the trajectories and the time-frequency spectrogram, we show that the HHG in solids cannot be interpreted in terms of the classical recollision picture alone. Instead, the radiation should be described by the electron-hole interband polarization, which leads to the unusual ellipticity dependence. We propose a new four-step model to understand the mechanism of HHG in solids.

Journal ArticleDOI
TL;DR: Third-order harmonic generation and 60-fold enhanced three-photon luminescence are demonstrated, enabling optical encoding applications of perovskite metasurface in high-resolution nonlinear color nanoprinting and optical encoding.
Abstract: Lead halide perovskites have emerged as promising materials for photovoltaic and optoelectronic devices. However, their exceptional nonlinear properties have not been fully exploited in nanophotonics yet. Herein we fabricate methyl ammonium lead tri-bromide perovskite metasurfaces and explore their internal nonlinear processes. While both of third-order harmonic generation and three-photon luminescence are generated, the latter one is less affected by the material loss and has been significantly enhanced by a factor of 60. The corresponding simulation reveals that the improvement is caused by the resonant enhancement of incident laser. Interestingly, such kind of resonance-enhanced three-photon luminescence holds true for metasurfaces with a small period number of 4, enabling promising applications of perovskite metasurface in high-resolution nonlinear color nanoprinting and optical encoding. The encoded information ‘NANO’ is visible only when the incident laser is on-resonance. The off-resonance pumping and the single-photon excitation just produce a uniform dark or photoluminescence background. Lead halide perovskites attract high interest as semiconductor materials but their exceptional nonlinear properties have not been fully exploited. Here Fan et al. demonstrate third-order harmonic generation and 60-fold enhanced three-photon luminescence, enabling optical encoding applications.

Journal ArticleDOI
TL;DR: It is demonstrated that the third harmonic generation of deep UV light in an indium tin oxide (ITO) film can be substantially enhanced by a metasurface consisting of metallic toroidal meta-atoms covered with an alumina layer for protection against laser-induced damage.
Abstract: The harmonic generation of light with plasmonic and all-dielectric nanostructures has gained much recent interest. This approach is especially promising for short wavelength (i.e., ultraviolet (UV)) generation, where conventional nonlinear crystals reach their limits both in transparency and in their ability to achieve phase-matching between the input and output fields. Here, we demonstrate that the third harmonic generation of deep UV light in an indium tin oxide (ITO) film can be substantially enhanced by a metasurface consisting of metallic toroidal meta-atoms covered with an alumina layer for protection against laser-induced damage. This approach combines the benefits of the large nonlinear susceptibility of ITO with the unique field enhancement properties of a toroidal metasurface. This ITO-meta-atom combination produces a third harmonic signal at a wavelength of 262 nm that is nominally five times larger than that of an ITO film patterned with a conventional hotspot-enhanced plasmonic dimer array. This result demonstrates the potential for toroidal meta-atoms as the active engineered element in a new generation of enhanced nonlinear optical materials and devices.

Journal ArticleDOI
08 Jan 2019
TL;DR: In this article, the interaction of intense lasers with solid materials offers an alternative way to achieve high-order harmonic generation (HHG), since the underlying mechanisms of the harmonic emission remain uncerta...
Abstract: Interaction of intense lasers with solid materials offers an alternative way to achieve high-order harmonic generation (HHG). Since the underlying mechanisms of the harmonic emission remain uncerta...

Journal ArticleDOI
TL;DR: In this paper, the authors demonstrated nonperturbative high harmonics up to 18th order in monolayer transition metal dichalcogenides and found that the enhancement in the even-order high harmonic which is attributed to the resonance to the band nesting energy.
Abstract: High-harmonic generation in solids is a unique tool to investigate the electron dynamics in strong light fields. The systematic study in monolayer materials is required to deepen the insight into the fundamental mechanism of high-harmonic generation. Here we demonstrated nonperturbative high harmonics up to 18th order in monolayer transition metal dichalcogenides. We found the enhancement in the even-order high harmonics which is attributed to the resonance to the band nesting energy. The symmetry analysis shows that the valley polarization and anisotropic band structure lead to polarization of the high-harmonic radiation. The calculation based on the three-step model in solids revealed that the electron–hole polarization driven to the band nesting region should contribute to the high harmonic radiation, where the electrons and holes generated at neighboring lattice sites are taken into account. Our findings open the way for attosecond science with monolayer materials having widely tunable electronic structures. Monolayer materials have tunable electronic structures that may be useful for optical applications. Here the authors show even and odd high-harmonic generation up to 15th order from a variety of monolayers with different band properties and emphasize the role of nonlinear interband polarization.

Journal ArticleDOI
TL;DR: In this paper, the authors show that a topological edge mode at the first harmonic can produce strong propagating higher-harmonic signals, acting as a nonlocal cross-phase nonlinearity.
Abstract: Nonlinear transmission lines (NLTLs) are nonlinear electronic circuits used for parametric amplification and pulse generation, and it is known that left-handed NLTLs support enhanced harmonic generation while suppressing shock wave formation. We show experimentally that in a left-handed NLTL analogue of the Su-Schrieffer-Heeger (SSH) lattice, harmonic generation is greatly increased by the presence of a topological edge state. Previous studies of nonlinear SSH circuits focused on solitonic behaviours at the fundamental harmonic. Here, we show that a topological edge mode at the first harmonic can produce strong propagating higher-harmonic signals, acting as a nonlocal cross-phase nonlinearity. We find maximum third-harmonic signal intensities five times that of a comparable conventional left-handed NLTL, and a 250-fold intensity contrast between topologically nontrivial and trivial configurations. This work advances the fundamental understanding of nonlinear topological states, and may have applications for compact electronic frequency generators.

Journal ArticleDOI
TL;DR: A gas phase extreme ultraviolet (XUV) femtosecond light source, an XUV monochromator, and a time-of-flight electron analyzer are combined to develop XUV-based time-resolved ARPES which can access the first Brillouin zone of all materials with narrow energy resolution.
Abstract: High harmonic generation of ultrafast laser pulses can be used to perform angle-resolved photoemission spectroscopy (ARPES) to map the electronic band structure of materials with femtosecond time resolution. However, currently it is difficult to reach high momenta with narrow energy resolution. Here, we combine a gas phase extreme ultraviolet (XUV) femtosecond light source, an XUV monochromator, and a time-of-flight electron analyzer to develop XUV-based time-resolved ARPES. Our technique can produce tunable photon energy between 24–33 eV with an unprecedented energy resolution of 30 meV and time resolution of 200 fs. This technique enables time-, energy- and momentum-resolved investigation of the nonequilibrium dynamics of electrons in materials with a full access to their first Brillouin zone. We evaluate the performance of this setup through exemplary measurements on various quantum materials, including WTe2, WSe2, TiSe2, and Bi2Sr2CaCu2O8+δ. Currently, it is difficult to reach high momenta with narrow energy resolution via laser-based angle-resolved photoemission spectroscopy (ARPES). Here, Sie et al. develop a time-resolved XUV based ARPES setup which can access the first Brillouin zone of all materials with narrow energy resolution.

Journal ArticleDOI
TL;DR: In this paper, a low-loss, indium-doped cadmium oxide thin film was used for high-harmonic generation from a thin film by leveraging the epsilon-near-zero (ENZ) effect, whereby the real part of the material permittivity in certain spectral ranges vanishes, as well as the associated large resonant enhancement of the driving laser field.
Abstract: High-harmonic generation (HHG) is a signature optical phenomenon of strongly driven, nonlinear optical systems. Specifically, the understanding of the HHG process in rare gases has played a key role in the development of attosecond science1. Recently, HHG has also been reported in solids, providing novel opportunities such as controlling strong-field and attosecond processes in dense optical media down to the nanoscale2. Here, we report HHG from a low-loss, indium-doped cadmium oxide thin film by leveraging the epsilon-near-zero (ENZ) effect3–8, whereby the real part of the material’s permittivity in certain spectral ranges vanishes, as well as the associated large resonant enhancement of the driving laser field. We find that ENZ-assisted harmonics exhibit a pronounced spectral redshift as well as linewidth broadening, resulting from the photo induced electron heating and the consequent time-dependent ENZ wavelength of the material. Our results provide a new platform to study strong-field and ultrafast electron dynamics in ENZ materials, reveal new degrees of freedom for spectral and temporal control of HHG, and open up the possibilities of compact solid-state attosecond light sources. High harmonics are generated from a thin film by leveraging the epsilon-near-zero effect. These kinds of harmonic are found to exhibit a pronounced spectral redshift as well as linewidth broadening caused by the time-dependency of this effect.

Journal ArticleDOI
TL;DR: The measurement time reduction and the photon energy scalability render this technology viable for next-generation, high-repetition-rate, multidimensional attosecond metrology, and a large count rate improvement over state-of-the-art attose Cond setups under identical space charge conditions.
Abstract: Laser-dressed photoelectron spectroscopy, employing extreme-ultraviolet attosecond pulses obtained by femtosecond-laser-driven high-order harmonic generation, grants access to atomic-scale electron dynamics. Limited by space charge effects determining the admissible number of photoelectrons ejected during each laser pulse, multidimensional (i.e. spatially or angle-resolved) attosecond photoelectron spectroscopy of solids and nanostructures requires high-photon-energy, broadband high harmonic sources operating at high repetition rates. Here, we present a high-conversion-efficiency, 18.4-MHz-repetition-rate cavity-enhanced high harmonic source emitting 5 × 105 photons per pulse in the 25-to-60-eV range, releasing 1 × 1010 photoelectrons per second from a 10-µm-diameter spot on tungsten, at space charge distortions of only a few tens of meV. Broadband, time-of-flight photoelectron detection with nearly 100% temporal duty cycle evidences a count rate improvement between two and three orders of magnitude over state-of-the-art attosecond photoelectron spectroscopy experiments under identical space charge conditions. The measurement time reduction and the photon energy scalability render this technology viable for next-generation, high-repetition-rate, multidimensional attosecond metrology. Space charge effects can distort the results of photoelectron spectroscopic measurements, and usually limit the allowable photon flux in an experiment. Here, the authors present an 18.4 MHz repetition rate high harmonic source in the 25–60 eV range, with a large count rate improvement over state-of-the-art attosecond setups under identical space charge conditions.

Journal ArticleDOI
TL;DR: In this article, the authors calculate high-harmonic spectra of SSH chains that are coupled to an external laser field of a frequency much smaller than the band gap and find huge differences between the harmonic yield for the two topological phases, similar to recent results obtained with more demanding time dependent density functional calculations.
Abstract: Su-Schrieffer-Heeger (SSH) chains are the simplest model systems that display topological edge states. We calculate high-harmonic spectra of SSH chains that are coupled to an external laser field of a frequency much smaller than the band gap. We find huge differences between the harmonic yield for the two topological phases, similar to recent results obtained with more demanding time-dependent density functional calculations [D. Bauer and K. K. Hansen, Phys. Rev. Lett. 120, 177401 (2018)]. This shows that the tight-binding SSH model captures the essential topological aspects of the laser-chain interaction (while higher harmonics involving higher bands or screening in the metal phase are absent). We study the robustness of the topological difference with respect to disorder, a continuous phase transition in position space, and on-site potentials. Further, we address the question of whether the edges need to be illuminated by the laser for the huge difference in the harmonic spectra to be present.

Journal ArticleDOI
TL;DR: A new all-optical technique to reconstruct the momentum-dependent transition dipole moment using the harmonic spectrum from MgO crystal driven by an ultrashort mid-infrared laser pulse and paves a way to image the two-dimensional transition dipoles moment of crystals with the inversion symmetry.
Abstract: Band structure and transition dipole moment play important roles in high-order harmonic generation from solid materials. In this work we provide a new all-optical technique to reconstruct the momentum-dependent transition dipole moment using the harmonic spectrum from MgO crystal driven by an ultrashort mid-infrared laser pulse. Under the influence of the ultrashort laser pulse, the emitted photon energy and the crystal momentum form a one-to-one match, in the same way between the intensity of the harmonic above the minimum bandgap and the square of the amplitude of the transition dipole moment, resulting in a realization of directly probing the transition dipole moment. Our all-optical method paves a way to image the two-dimensional transition dipole moment of crystals with the inversion symmetry.

Journal ArticleDOI
TL;DR: This work identifies a practical source for TR-ARPES that achieves a flux of over 1011 photons/s delivered to the sample, and operates over a range of 8-40 eV with a repetition rate of 60 MHz, and addresses the challenge of achieving a high energy resolution while producing high photon energies and a high photon flux.
Abstract: With its direct correspondence to electronic structure, angle-resolved photoemission spectroscopy (ARPES) is a ubiquitous tool for the study of solids. When extended to the temporal domain, time-resolved (TR)-ARPES offers the potential to move beyond equilibrium properties, exploring both the unoccupied electronic structure as well as its dynamical response under ultrafast perturbation. Historically, ultrafast extreme ultraviolet sources employing high-order harmonic generation (HHG) have required compromises that make it challenging to achieve a high energy resolution—which is highly desirable for many TR-ARPES studies—while producing high photon energies and a high photon flux. We address this challenge by performing HHG inside a femtosecond enhancement cavity, realizing a practical source for TR-ARPES that achieves a flux of over 1011 photons/s delivered to the sample, operates over a range of 8–40 eV with a repetition rate of 60 MHz. This source enables TR-ARPES studies with a temporal and energy resolution of 190 fs and 22 meV, respectively. To characterize the system, we perform ARPES measurements of polycrystalline Au and MoTe2, as well as TR-ARPES studies on graphite.

Journal ArticleDOI
TL;DR: This work demonstrates an effective strategy for the compact generation of VUV light that could enable expanded access to this useful region of the electromagnetic spectrum.
Abstract: Dielectric metasurfaces have recently been shown to provide an excellent platform for the harmonic generation of light due to their low optical absorption and to the strong electromagnetic field en...

Journal ArticleDOI
TL;DR: In this article, a femtosecond enhancement cavity (fsEC) was used for high-order harmonic generation (HHG) for angle-resolved photoemission spectroscopy (ARPES).
Abstract: With its direct correspondence to electronic structure, angle-resolved photoemission spectroscopy (ARPES) is a ubiquitous tool for the study of solids. When extended to the temporal domain, time-resolved (TR)-ARPES offers the potential to move beyond equilibrium properties, exploring both the unoccupied electronic structure as well as its dynamical response under ultrafast perturbation. Historically, ultrafast extreme ultraviolet (XUV) sources employing high-order harmonic generation (HHG) have required compromises that make it challenging to achieve a high energy resolution - which is highly desirable for many TR-ARPES studies - while producing high photon energies and a high photon flux. We address this challenge by performing HHG inside a femtosecond enhancement cavity (fsEC), realizing a practical source for TR-ARPES that achieves a flux of over 10$^{11}$ photons/s delivered to the sample, operates over a range of 8-40 eV with a repetition rate of 60 MHz. This source enables TR-ARPES studies with a temporal and energy resolution of 190 fs and 22 meV, respectively. To characterize the system, we perform ARPES measurements of polycrystalline Au and MoTe$_2$, as well as TR-ARPES studies on graphite.

Journal ArticleDOI
TL;DR: A non-perturbative behavior of HHG in terahertz regime from 3D Dirac semimetal, Cd3As2, at room temperature, and reveal the underlying nonlinear kinetics.
Abstract: Harmonic generation is a general characteristic of driven nonlinear systems, and serves as an efficient tool for investigating the fundamental principles that govern the ultrafast nonlinear dynamics. In atomic gases, high-harmonic radiation is produced via a three-step process of ionization, acceleration, and recollision by strong-field infrared laser. This mechanism has been intensively investigated in the extreme ultraviolet and soft X-ray regions, forming the basis of attosecond research. In solid-state materials, which are characterized by crystalline symmetry and strong interactions, yielding of harmonics has just recently been reported. The observed high-harmonic generation was interpreted with fundamentally different mechanisms, such as interband tunneling combined with dynamical Bloch oscillations, intraband thermodynamics and nonlinear dynamics, and many-body electronic interactions. Here, in a distinctly different context of three-dimensional Dirac semimetal, we report on experimental observation of high-harmonic generation up to the seventh order driven by strong-field terahertz pulses. The observed non-perturbative high-harmonic generation is interpreted as a generic feature of terahertz-field driven nonlinear intraband kinetics of Dirac fermions. We anticipate that our results will trigger great interest in detection, manipulation, and coherent control of the nonlinear response in the vast family of three-dimensional Dirac and Weyl materials.

Journal ArticleDOI
TL;DR: Even and odd high-harmonic generation up to 15th order from a variety of monolayers with different band properties are shown and the role of nonlinear interband polarization is emphasized.
Abstract: We demonstrated nonperturbative high harmonics induced by intense mid-infrared light up to 18th order that well exceed the material bandgap in monolayer transition metal dichalcogenides. The intensities of the even-order high-harmonic radiation did not monotonically decrease as the harmonic order increased. By comparing the high harmonic spectra with the optical absorption spectra, we found that the enhancement in the even-order high harmonics could be attributed to the resonance to the band nesting energy. The symmetry analysis shows that the valley polarization and anisotropic band structure lead to polarization of the high-harmonic radiation under excitation with the polarization along the zigzag direction. We also examined the possible recombination pathways of electrons and holes by calculating their dynamics in real and momentum spaces based on three-step model in solids. It revealed that, by considering the electrons and holes generated at neighboring lattice sites, the electron-hole polarization driven to the band nesting region should contribute to the high harmonic radiation. Our findings open the way for attosecond science with monolayer materials having widely tunable electronic structures.

Journal ArticleDOI
TL;DR: The effects of wavelength, pulse power, intensity, propagation length, and crystallinity on supercontinuum and high harmonic generation are investigated experimentally using ultrafast mid-infrared pulses, finding harmonic conversion efficiency scales linearly in propagation length.
Abstract: Polycrystalline ZnSe is an exciting source of broadband supercontinuum and high-harmonic generation via random quasi phase matching, exhibiting broad transparency in the mid-infrared (0.5−20 μm). In this work, the effects of wavelength, pulse power, intensity, propagation length, and crystallinity on supercontinuum and high harmonic generation are investigated experimentally using ultrafast mid-infrared pulses. Observed harmonic conversion efficiency scales linearly in propagation length, reaching as high as 36%. For the first time to our knowledge, n2 is measured for mid-infrared wavelengths in ZnSe: n2(λ=3.9 μm)=(1.2±0.3)×10−14 cm2/W. Measured n2 is applied to simulations modeling high-harmonic generation in polycrystalline ZnSe as an effective medium.

Journal ArticleDOI
TL;DR: In this article, a donor-doped band gap material can enhance the overall high-order harmonic generation (HHG) efficiency by several orders of magnitude, compared with undoped and acceptordoped materials.
Abstract: We find that a donor-doped band-gap material can enhance the overall high-order harmonic generation (HHG) efficiency by several orders of magnitude, compared with undoped and acceptor-doped materials. This significant enhancement, predicted by time-dependent density functional theory simulations, originates from the highest occupied impurity state which has an isolated energy located within the band gap. The impurity-state HHG is rationalized by a three-step model, taking into account that the impurity-state electron tunnels into the conduction band and then moves according to its band structure until recombination. In addition to the improvement of the HHG efficiency, the donor-type doping results in a harmonic cutoff different from that in the undoped and acceptor-doped cases, explained by semiclassical analysis for the impurity-state HHG.

Journal ArticleDOI
TL;DR: In this article, the authors proposed the use of the maximally localized Wannier functions that provide a framework to map ab initio calculations to an effective tight-binding Hamiltonian with great accuracy.
Abstract: In this work, the nonlinear optical response, and in particular, the high harmonic generation of semiconductors, is addressed by using the Wannier gauge. One of the main problems in the time evolution of the Semiconductor Bloch equations resides in the fact that the dipole couplings between different bands can diverge and have a random phase along the reciprocal space, and this leads to numerical instability. To address this problem, we propose the use of the maximally localized Wannier functions that provide a framework to map ab initio calculations to an effective tight-binding Hamiltonian with great accuracy. We show that working in the Wannier gauge, the basis set in which the Bloch functions are constructed directly from the Wannier functions, the dipole couplings become smooth along the reciprocal space, thus avoiding the problem of random phases. High harmonic generation is computed for a two-dimensional monolayer of hexagonal boron nitride as a numerical demonstration.