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Nanostructured optical surfaces allow exquisite control over linear and nonlinear light interactions, where the surface actively creates new frequencies up to high-order harmonics of an intense infrared driving laser field. The function and performance of these surfaces depend sensitively on the distribution of the high-harmonic fields in and between the nanostructured elements, as the high-harmonic wavelength becomes comparable to the nanoscale features of the surface. Imaging the nonlinear response at the active surface with nanometer resolution would greatly benefit the optimization of the metasurface's function. Here we demonstrate an approach to lensless imaging of extreme ultraviolet high harmonics that resolves the amplitude and phase of nonlinear polarization at the active nanostructured surface of an MgO crystal. Imaging the near-field distribution of high harmonics is the gateway to optimized functional high-harmonic metasurfaces and the integration of high harmonics on a chip.

Introduction Vitamin D plays a vital role in the proper functioning of various systems of the human body. Conventionally, it is known that its deficiency can impair, among other factors, calcium homeostasis, bone metabolism, and immune system regulation. Besides having anti-inflammatory and antiangiogenic properties, 25(OH)D also acts in maintaining the integrity of the human retina, favoring the idea of it being a protective factor for certain ocular diseases. Objective Therefore, the purpose of this review was to collect data from the literature which support the importance of vitamin D for ocular health. Methods Through the Scientific Electronic Library Online and the National Library of Medicine databases, a compendium was created on the subject. Results In total, 31 articles in English published between 2016 and 2020 were found. These articles were reviewed and categorized by diseases in order to study each disorder thoroughly. It is noticed that 25(OH) D is, in fact, a potential protective factor against such diseases, including diseases that affect both the surface of the cornea and the ocular tissues that have vitamin D receptors. Conclusion Vitamin D is a protective hormone in the eye, and its deficiency is related to a variety of ocular diseases. Therefore, the supplementation of 25(OH)D can be considered as a complementary treatment for patients with these diseases, since it brings relevant benefits.

Lead telluride (PbTe), a narrow bandgap semiconductor commonly used in infrared detectors, exhibits anomalous vibrational and structural properties, making it appealing for thermoelectrics. Despite...

In this paper, the unit log-logistic distribution was proposed. This distribution is obtained through by transformation of a random variable with log-logistic distribution. The unit log-logistic has closed forms for the cumulative distribution function and quantile function. Subsequently, the unit log-logistic regression model, with parametrization in the median was defined. So, in the presence of outliers, this model is more robust than the models with parametrization in the mean. The maximum likelihood method was used to estimate the parameters. The validity of the estimators of this model is shown through Monte Carlo simulations. Application to real data showed that the new model has a better fit than the popular beta regression model.

A new lifetime distribution has been defined. This distribution is obtained from a transformation of a random variable with beta distribution and is called here the kagebushin-beta distribution. Some mathematical properties such as mode, quantile function, ordinary and incomplete moments, mean deviations over the mean and median and the entropies of Rényi and Shannon are demonstrated. The maximum likelihood method is used to obtain parameter estimates. Monte Carlo simulations are carried out to verify the accuracy of the maximum likelihood estimators. Applications to real data showed that the kagebushin-beta model can be better than the Weibull, gamma and exponentiated exponential distributions.

We investigate coupled electron-lattice dynamics in the topological insulator Bi2Te3 with time-resolved photoemission and time-resolved x-ray diffraction. It is well established that coherent phonons can be launched by optical excitation, but selection rules generally restrict these modes to zone-center wavevectors and Raman-active branches. We find that the topological surface state couples to additional modes, including a continuum of surface-projected bulk modes from both Raman- and infrared-branches, with possible contributions from surface-localized modes when they exist. Our calculations show that this surface vibrational spectrum occurs naturally as a consequence of the translational and inversion symmetries broken at the surface, without requiring the splitting-off of surface-localized phonon modes. The generality of this result suggests that coherent phonon spectra are useful by providing unique fingerprints for identifying surface states in more controversial materials. These effects may also expand the phase space for tailoring surface state wavefunctions via ultrafast optical excitation.

We report a coherent layer-by-layer build-up of high-order harmonic generation (HHG) in artificially stacked transition metal dichalcogenides (TMDC) crystals in their various stacking configurations. In the experiments, millimeter-sized single crystalline monolayers are synthesized using the gold foil-exfoliation method, followed by artificially stacking on a transparent substrate. High-order harmonics up to the 19th order are generated by the interaction with a mid-infrared (MIR) driving laser. We find that the generation is sensitive to both the number of layers and their relative orientation. For AAAA stacking configuration, both odd- and even-orders exhibit a quadratic increase in intensity as a function of the number of layers, which is a signature of constructive interference of high-harmonic emission from successive layers. Particularly, we observe some deviations from this scaling at photon energies above the bandgap, which is explained by self-absorption effects. For AB and ABAB stacking, even-order harmonics remain below the detection level, consistent with the presence of inversion symmetry. Our study confirms our capability of producing nonperturbative high-order harmonics from stacked layered materials subjected to intense MIR fields without damaging samples. Our results have implications for optimizing solid-state HHG sources at the nanoscale and developing high-harmonics as an ultrafast probe of artificially stacked layered materials. Because the HHG process is a strong-field driven process, it has the potential to probe high-momentum and energy states in the bandstructure combined with atomic-scale sensitivity in real space, making it an attractive probe of novel material structures such as the Moiré pattern.

Interactions of quantum materials with strong-laser fields can induce exotic nonequilibrium electronic states. Monolayer transition-metal dichalcogenides, a new class of direct-gap semiconductors with prominent quantum confinement, offer exceptional opportunities toward Floquet engineering of quasiparticle electron-hole states, or excitons. Strong-field driving has a potential to achieve enhanced control of electronic band structure, thus a possibility to open a new realm of exciton light-matter interactions. However, experimental implementation of strongly-driven excitons has so far remained out of reach. Here, we use mid-infrared laser pulses below the optical bandgap to excite monolayer tungsten disulfide up to a field strength of 0.3 V/nm, and demonstrate strong-field light dressing of excitons in the excess of a hundred millielectronvolt. Our high-sensitivity transient absorption spectroscopy further reveals formation of a virtual absorption feature below the 1s-exciton resonance, which is assigned to a light-dressed sideband from the dark 2p-exciton state. Quantum-mechanical simulations substantiate the experimental results and enable us to retrieve real-space movies of the exciton dynamics. This study advances our understanding of the exciton dynamics in the strong-field regime, and showcases the possibility of harnessing ultrafast, strong-field phenomena in device applications of two-dimensional materials.

Este artigo de revisão sumariza conhecimentos sobre a sintomatologia, patogenia, fatores epidemiológicos para algumas espécies da fauna silvestre acometidas pela virose, lista espécies silvestres vulneráveis à doença do vírus Ebola (EVD) e espécies de animais domésticos não vulveráveis, evidencia o uso de modelos animais para estudos objetivando a defesa de humanos e de animais contra essa doença e destaca a ameaça à saúde dos animais silvestres representada pela EVD.

The basolateral complex of the amygdala (BLA) is capable of modulating memory and is thought to do so via projections to regions such as the hippocampus. The present study used optogenetic stimulation of glutamatergic projection neurons in the BLA as rats learned object-context associations during a well-studied hippocampus-dependent memory task. Relative to a control condition, optogenetic BLA stimulation resulted in the accelerated acquisition of when stimulation was delivered following correct choices but not when it was delivered during the intertrial interval. These results extend prior examples of amygdala-mediated memory enhancement to a canonical example of hippocampus-dependent memory and provide an opportunity for future dissection of amygdalar modulation of object-context associative memory. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

This study demonstrates how zirconia additive transforms to zirconium hydride and substantially lowers the dehydrogenation temperature of magnesium hydride. We pre- pared MgH2þxZrO2 (x 1⁄4 0.125 and 0.5) powder samples reacted for 15 min, 1 h, 5 h, 10 h, 15 h, 20 h and 25 h, and monitored the phase changes at each stage of the reaction. Differential scanning calorimetry (DSC) study provides the first crucial evidence regarding the chemical transformation of zirconia. Subsequently, detailed additional sample testing by X-ray diffraction (XRD), energy dispersive x-ray spectroscopy and confocal Raman microscopy provide strong supports that low temperature dehydrogenation of magnesium hydride is a result of formation of an active in situ product (zirconium hydride). This observation is validated by the negative Gibbs free energy values obtained for the formation of zirconium hydride over a broad working temperature range of 0e600 ºC. Scanning electron microscopy (SEM) results prove the high dispersion of tiny nanoparticles all across the surface after the chemical interaction between MgH2 and ZrO2 and atomic force microscopy (AFM) study further proves that objects with grain sizes of ~10 nm are abundant throughout the scanned surfaces. These observations reiterate that better metal oxide additives interact with MgH2 and results to the evolution of highly active in situ nanocatalysts. published

The generalized gamma-generated family adds one shape parameter to a baseline distribution. We define the gamma-Chen and derive some of its mathematical properties. Its hazard rate may have increasing, decreasing, bathtub and unimodal shapes due to the extra parameter, which portrays a positive point of the proposed model. We perform Monte Carlo simulations to prove that the asymptotic properties of the maximum likelihood estimators hold. We show empirically that the new distribution is better than ten others known distributions using engineering-related data sets.

Since the new millennium coherent extreme ultra-violet and soft x-ray radiation has revolutionized the understanding of dynamical physical, chemical and biological systems at the electron’s natural timescale. Unfortunately, coherent laser-based upconversion of infrared photons to vacuum-ultraviolet and soft x-ray high-order harmonics in gaseous, liquid and solid targets is notoriously inefficient. In dense nonlinear media, the limiting factor is strong re-absorption of the generated high-energy photons. Here we overcome this limitation by generating high-order harmonics from a periodic array of thin one-dimensional crystalline silicon ridge waveguides. Adding vacuum gaps between the ridges avoids the high absorption loss of the bulk and results in a ~ 100-fold increase of the extraction depth. As the grating period is varied, each high harmonic shows a different and marked modulation, indicating their waveguiding in the vacuum slots with reduced absorption. Looking ahead, our results enable bright on-chip coherent short-wavelength sources and may extend the usable spectral range of traditional nonlinear crystals to their absorption windows. Potential applications include on-chip chemically-sensitive spectro-nanoscopy. Control of nonlinear optical processes at the nanoscale is vital for the generation of on-chip short-wavelength sources, yet strong re-absorption of this radiation limits its efficiency in solids. Here, high harmonics are generated in an array of 1D silicon ridge waveguides, mitigating bulk re-absorption.

We report high-harmonic generation from the artificially stacked monolayer tungsten disulfide. The harmonic yield scales quadratically with the number of layers, limited by reabsorption by subsequent layers for above bandgap energies.

In this work we use helium ion beam nanolithography to structure the surface of an MgO crystal and control the emission of extreme-ultraviolet high harmonics. We demonstrate their focusing to below 300 nm in waist diameter.

Resumo: Introdução: A iniciação científica (IC) é o processo de treinamento e estudo sobre a metodologia científica ou a execução de pesquisa com orientação. Tal prática é prevista pelas Diretrizes Curriculares Nacionais de 2014, verificada pelo Sistema Nacional de Avaliação da Educação Superior e apoiada pelo Sistema de Acreditação de Escolas Médicas. A IC tem o potencial de produzir e permitir o avanço do conhecimento, bem como favorecer uma prática de Medicina Baseada em Evidências. Objetivo: Este estudo teve como objetivo delimitar o panorama acerca dos programas de pesquisa para graduandos de Medicina no Brasil, buscando os requisitos mínimos para caracterizar uma IC e as potencialidades e os desafios na realidade brasileira. Método: Trata-se de um estudo de revisão sistemática, conforme critérios PRISMA, realizado no período de maio a junho de 2021 nas bases de dados SciELO, PubMed, BVS, Google Acadêmico e BDTD. Efetuou-se análise qualitativa dos artigos selecionados por meio de avaliação de possíveis vieses e organização em seis categorias temáticas: 1. políticas institucionais, 2. engajamento e interesse de discente, 3. desafios à prática de pesquisa, 4. orientadores, 5. efeitos e produtos da IC, e 6. formação científica. Resultado: Foram incluídos 12 estudos na análise final. As escolas investigadas, em sua maioria, não tinham programas de IC estruturados e possuíam a maior parte de seus estudantes interessados por pesquisa científica ou em realizar trabalhos científicos. Os principais desafios à prática da pesquisa por estudantes foram a falta de estímulo institucional, de financiamento e de infraestrutura adequada. E, como efeitos da IC, grande parte dos estudantes apresentou um trabalho em congressos, mas dificilmente publicaram em periódicos. Conclusão: A IC é um programa com atividades que envolvem a aplicação do método científico com grande relevância para estimular a condução de um projeto de pesquisa com carácter ético, técnico e científico. A potencialidade político-social mais interessante da participação na IC é a desmistificação da pós-graduação e da carreira acadêmica, e o desafio mais presente na realidade brasileira é a falta de estímulo institucional.

The collision of 10-13 GeV level electrons with tightly focused 20 TW laser pulses at FACET II will enable experimental studies in the strong-field QED regime beyond the Schwinger limit. Experimental parameters, diagnostics and perspectives are described.

Resumo: Introdução: A iniciação científica (IC) é o processo de treinamento e estudo sobre a metodologia científica ou a execução de pesquisa com orientação. Tal prática é prevista pelas Diretrizes Curriculares Nacionais de 2014, verificada pelo Sistema Nacional de Avaliação da Educação Superior e apoiada pelo Sistema de Acreditação de Escolas Médicas. A IC tem o potencial de produzir e permitir o avanço do conhecimento, bem como favorecer uma prática de Medicina Baseada em Evidências. Objetivo: Este estudo teve como objetivo delimitar o panorama acerca dos programas de pesquisa para graduandos de Medicina no Brasil, buscando os requisitos mínimos para caracterizar uma IC e as potencialidades e os desafios na realidade brasileira. Método: Trata-se de um estudo de revisão sistemática, conforme critérios PRISMA, realizado no período de maio a junho de 2021 nas bases de dados SciELO, PubMed, BVS, Google Acadêmico e BDTD. Efetuou-se análise qualitativa dos artigos selecionados por meio de avaliação de possíveis vieses e organização em seis categorias temáticas: 1. políticas institucionais, 2. engajamento e interesse de discente, 3. desafios à prática de pesquisa, 4. orientadores, 5. efeitos e produtos da IC, e 6. formação científica. Resultado: Foram incluídos 12 estudos na análise final. As escolas investigadas, em sua maioria, não tinham programas de IC estruturados e possuíam a maior parte de seus estudantes interessados por pesquisa científica ou em realizar trabalhos científicos. Os principais desafios à prática da pesquisa por estudantes foram a falta de estímulo institucional, de financiamento e de infraestrutura adequada. E, como efeitos da IC, grande parte dos estudantes apresentou um trabalho em congressos, mas dificilmente publicaram em periódicos. Conclusão: A IC é um programa com atividades que envolvem a aplicação do método científico com grande relevância para estimular a condução de um projeto de pesquisa com carácter ético, técnico e científico. A potencialidade político-social mais interessante da participação na IC é a desmistificação da pós-graduação e da carreira acadêmica, e o desafio mais presente na realidade brasileira é a falta de estímulo institucional.

A grande barreira à utilização generalizada de veículos leves alimentados a hidrogénio é neste momento a capacidade de armazenamento a bordo. Para tal tem vindo a ser desenvolvidos diversos trabalhos de forma a explorar novas opções. Nomeadamente o armazenamento em sólidos. É nesse âmbito que se desenvolveu este trabalho com o objetivo de projetar e construir um equipamento capaz de quantificar uma série de propriedades relativas ao armazenamento de hidrogénio em sólidos. A técnica de medição implementada diverge ligeiramente da maioria dos equipamentos, no entanto existe literatura suficiente que permitiu à priori que essa técnica fosse considerada a melhor opção. O equipamento foi testado e validado com sucesso comparando com versões anteriores e comparando com dados da literatura. The big barrier to the widespread of hydrogen powered light vehicles is at the moment the on board storage capacity. In order to address this problem there have been a lot of studies to explore new options. It is in such view that this work was developed with the objective of design and assembling a device capable of quantifying a series of properties relevant to hydrogen storage in solids. The measurement technique implemented differs slightly from most equipment, however there is enough literature that allowed this technique to be considered the best option. The equipment has been successfully tested and validated against previous versions and against literature data. Mestrado em Engenharia Mecânica

The advent of chirped-pulse amplification in the 1980s and femtosecond Ti:sapphire lasers in the 1990s enabled transformative advances in intense laser–matter interaction physics. Whereas most of experiments have been conducted in the limited near-infrared range of 0.8–1 μm, theories predict that many physical phenomena such as high harmonic generation in gases favor long laser wavelengths in terms of extending the high-energy cutoff. Significant progress has been made in developing few-cycle, carrier-envelope phase-stabilized, high-peak-power lasers in the 1.6–2 μm range that has laid the foundation for attosecond X ray sources in the water window. Even longer wavelength lasers are becoming available that are suitable to study light filamentation, high harmonic generation, and laser–plasma interaction in the relativistic regime. Long-wavelength lasers are suitable for sub-bandgap strong-field excitation of a wide range of solid materials, including semiconductors. In the strong-field limit, bulk crystals also produce high-order harmonics. In this review, we first introduce several important wavelength scaling laws in strong-field physics, then describe recent breakthroughs in short- (1.4–3 μm), mid- (3–8 μm), and long-wave (8–15 μm) infrared laser technology, and finally provide examples of strong-field applications of these novel lasers. Some of the broadband ultrafast infrared lasers will have profound effects on medicine, environmental protection, and national defense, because their wavelengths cover the water absorption band, the molecular fingerprint region, as well as the atmospheric infrared transparent window.

Objetiva analisar as transmissões de preços entre o mercado da pera no Brasil. Especificamente, os mercados escolhidos foram Porto Alegre-RS, São Paulo-SP e Recife-PE, sendo as séries de periodicidade mensal no período de junho de 2009 a dezembro de 2015. O teste de causalidade de Granger mostrou que o mercado de São Paulo causa os demais, porém o inverso não é verdadeiro. O teste de cointegração de Johansen mostrou que a matriz de interesse tem posto completo (?=?), o que indica que as colunas da matriz são linearmente independentes e, assim, não é necessário estimar um VEC, e sim, um VAR em nível. A elasticidade de transmissão de preços conforme o modelo VAR estimado mostra que o aumento de preço em São Paulo é repassado em maior magnitude para Porto Alegre do que para o Recife. O aumento de 10% do preço da pera no período ??1, ceteris paribus, fará com que Porto Alegre e Recife aumentem em 3,5% e 1,7%, respectivamente, no período ?. A decomposição de variância confirma o que o teste de causalidade de Granger mostrou, ou seja, que o mercado de São Paulo é o mercado central e é este que determina os preços, enquanto os outros são tomadores de preços. Made available in DSpace on 2022-10-13T17:08:49Z (GMT). No. of bitstreams: 1 Mercado-de-pera-no-Brasil-2021.pdf: 823426 bytes, checksum: 2dc30aa245a2a93a61ff8d7a51d323d6 (MD5) Previous issue date: 2021

Short wavelength high-harmonic sources are undergoing intense development for applications in spectroscopy and microscopy. Despite recent progress in peak and average power, spatial control over coherent extreme ultraviolet (XUV) beams remains a formidable challenge due to the lack of suitable optical elements for beam shaping and control. Here we demonstrate a robust and precise approach that structures XUV high-order harmonics in space as they are emitted from a nanostructured MgO crystal. Our demonstration paves the way for bridging the numerous applications of shaped light beams from the visible to the short wavelengths, with potential uses for applications in microscopy and nanoscale machining.

We report the observation of a novel nonlinear optical response from the prototypical three-dimensional topological insulator Bi$_2$Se$_3$ through the process of high-order harmonic generation. We find that the generation efficiency increases as the laser polarization is changed from linear to elliptical, and it becomes maximum for circular polarization. With the aid of a microscopic theory and a detailed analysis of the measured spectra, we reveal that such anomalous enhancement encodes the characteristic topology of the band structure that originates from the interplay of strong spin-orbit coupling and time-reversal symmetry protection. Our study reveals a new platform for chiral strong-field physics and presents a novel, contact-free, all-optical approach for the spectroscopy of topological insulators. The implications are in ultrafast probing of topological phase transitions, light-field driven dissipationless electronics, and quantum computation., 21 pages, 5 figures

Resumo: Este artigo estuda a eficiência técnica e a produtividade total dos fatores (PTF) da agricultura dos países da América Latina e do Caribe. Para tanto, utilizaram-se a abordagem de fronteira estocástica de produção e o índice de produtividade de Malmquist. Os dados de valor da produção e a quantidade de insumos agrícolas abrangeram os anos de 1991 a 2012. Os resultados mostraram que todos os países apresentaram ineficiência técnica de produção agrícola ao longo do período. As variáveis que explicaram significativamente a ineficiência da produção agrícola foram o crédito e o consumo de energia na agricultura, o que representou uma inovação nessa temática. O índice de produtividade de Malmquist mostrou que não houve ganhos de produtividade total dos fatores. Abstract: This article studies the technical efficiency and the total productivity of agricultural factors in the countries of Latin America and of Caribbean. For this, it was applied the stochastic production frontier approach and the Malmquist productivity index. The data used were the value of production and the quantity of agricultural inputs from the years 1991 to 2012. The results showed that all countries presented technical inefficiency of agricultural production over the period. The variables that significantly explained the inefficiency of agricultural production were credit and energy consumption in agriculture, which represented an innovation in this area. The Malmquist productivity index showed that there were no gains in total factor productivity.

Considering the growth of the Gross Domestic Product (GDP) of the Northeast in the last years, this study aims to analyze if such growth occurred in a way to reduce the income disparities between the municipalities of this region. For this, the hypothesis of -convergence, absolute and conditional, as well as -convergence was tested. The results showed that for the region as a whole, the municipalities have the two types of convergence, in addition to the convergence. The conditional -convergence proved to be poor, since the convergence velocity was greater than in absolute -convergence, and consequently a shorter half-life, being a reduction of two and a half years. This shows that the variables used in conditional -convergence (Firjan health and education), contribute to reduce the income disparities between the Northeastern municipalities. Tendo em vista o crescimento do Produto Interno Bruto (PIB) da região Nordeste nos últimos anos, este trabalho tem como objetivo analisar se tal crescimento ocorreu de uma maneira a diminuir as disparidades de rendas entre os municípios desta região. Para tanto foi testado a hipótese de -convergência, absoluta e condicional, e também a -convergência. Os resultados mostraram que para a região como toda, os municípios apresentam os dois tipos de beta convergência, além da sigma convergência. A - convergência condicional mostrou-se mais satisfatória, já que, a velocidade de convergência foi maior do que na -convergência absoluta, e, consequentemente um tempo de meia vida menor, sendo uma redução de dois anos e meio. O que mostra que as variáveis utilizadas na -convergência condicional (Firjan saúde e educação), contribuem para diminuir as disparidades de renda entre os municípios nordestinos.

We report on the use of multilayer Laue lenses to focus the intense X-ray Free Electron Laser (XFEL) beam at the European XFEL to a spot size of a few tens of nanometers. We present the procedure to align and characterize these lenses and discuss challenges working with the pulse trains from this unique x-ray source.

We demonstrate an experimental approach to determining the excited-state interatomic forces using femtosecond x-ray pulses from an x-ray free-electron laser. We determine experimentally the excited-state interatomic forces that connect photoexcited carriers to the nonequilibrium lattice dynamics in the prototypical Peierls-distorted material, bismuth. The forces are obtained by a constrained least-squares fit of a pairwise interatomic force model to the excited-state phonon dispersion relation as measured by the time- and momentum-resolved x-ray diffuse scattering. We find that photoexcited carriers weaken predominantly the nearest-neighbor forces, which drives the measured softening of the transverse acoustic modes throughout the Brillouin zone as well as the zone-center ${A}_{1g}$ optical mode. This demonstrates a bond-selective approach to measuring electron-phonon coupling relevant to a broad range of photoinduced phase transitions and transient light-driven states in quantum materials.

Under strong electromagnetic excitation, electron–hole (e-h) pairs may be generated in solids, which are subsequently driven to high energy and high momentum, producing high harmonics (HH) of the driving field. The HH efficiency depends on the degree of coherence between the driven electron and hole created by the laser field. Here, we disrupt this e-h coherence in an atomically thin semiconductor by photodoping via incoherent e-h pairs. We observe a strong, systematic harmonic order-dependent intensity reduction. This trend is explained by an exponential decay of the inter-band polarization, proportional to the sub-cycle excursion time of the e-h pair. Our study provides a platform to probe the importance of many-body effects, such as excitation density-dependent decoherence time for strongly driven electrons without the need of ultrashort laser pulses.

Understanding the temporal profiles of high harmonics is essential for their applications in attosecond science1,2. Microscopically, the dipole phase plays an important role in determining the high-harmonic emission phase2–4. In gas-phase high-harmonic generation, the tunnel-ionized electron spends much of its travel time in the continuum—far from the parent ion, where it accumulates the dipole phase5,6. Therefore, the atomic dipole phase is largely independent of the target atom3. In solid-state high-harmonic generation7–11, since the driven electron experiences a periodic potential during the entire travel time, the dipole phase may depend on the electronic structures of source materials9,12–14. Here, we employ an interferometric method to characterize high harmonics from magnesium oxide and quartz crystals. We measure material-dependent intensity-induced high-harmonic phase delays that we attribute to the intensity-induced changes in the dipole phase originating from the interband polarization10,15,16. The material-dependent dipole phase can provide a robust platform for high-harmonic spectroscopy of solids. An interferometric homodyne method is employed to measure material-dependent intensity-induced phase shifts of extreme-ultraviolet high harmonics emerging from bulk magnesium oxide and quartz crystals, providing a robust platform for high-harmonic spectroscopy of solids.

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.

Snapshots of a phase transition Time-resolved x-ray scattering can be used to investigate the dynamics of materials during the switch from one structural phase to another. So far, methods provide an ensemble average and may miss crucial aspects of the detailed mechanisms at play. Wall et al. used a total-scattering technique to probe the dynamics of the ultrafast insulator-to-metal transition of vanadium dioxide (VO 2 ) (see the Perspective by Cavalleri). Femtosecond x-ray pulses provide access to the time- and momentum-resolved dynamics of the structural transition. Their results show that the photoinduced transition is of the order-disorder type, driven by an ultrafast change in the lattice potential that suddenly unlocks the vanadium atoms and yields large-amplitude uncorrelated motions, rather than occurring through a coherent displacive mechanism. Science , this issue p. 572 ; see also p. 525

We study ultrafast x-ray diffraction on the charge density wave (CDW) of ${\mathrm{SmTe}}_{3}$ using an x-ray free-electron laser. The high momentum and time resolution afforded by the x-ray laser enabled capturing fine wave-vector and time-dependent features of the CDW that originate from fast (in time) and sharp (in real space) variations of the CDW lattice distortion, which we attribute to an inversion of the order parameter. These domain inversions occur near the surface and are caused by the short penetration depth of the near-infrared pump with the wavelength centered at 800 nm, resulting in CDW domain walls perpendicular to the sample surface. These domain walls break the CDW long-range order on the scale of the x-ray probe depth, controlled experimentally by the x-ray incidence angle and suppress the diffraction intensity of the CDW for times much longer than the $\ensuremath{\sim}1\text{\ensuremath{-}}\mathrm{ps}$ recovery of the electronic gap observed in time and angle-resolved photoemission spectroscopy. We model the spatial and temporal dependences of the order parameter using a simple Ginzburg-Landau model with all the parameters obtained from the published literature. We find reasonable agreement between the calculated and the measured diffraction across the momentum, time, fluence, and incidence angle dependence without adjusting any parameters. We reconstruct the spatial and temporal dependences of the lattice order parameter and find that at long times, depending on the pump fluence, multiple domain walls remain at distances of a few nanometers from the surface.

We present a systematic study of the crystal-orientation dependence of high-harmonic generation in monolayer transition-metal dichalcogenides, WS 2 and MoSe 2 , subjected to intense linearly polarized midinfrared laser fields. The measured spectra consist of both odd- and even-order harmonics, with a high-energy cutoff extending beyond the 15th order for a laser-field strength around ~1 V/nm. In WS 2 , we find that the polarization direction of the odd-order harmonics smoothly follows that of the laser field irrespective of the crystal orientation, whereas the direction of the even-order harmonics is fixed by the crystal mirror planes. Furthermore, the polarization of the even-order harmonics shows a flip in the course of crystal rotation when the laser field lies between two of the crystal mirror planes. By numerically solving the semiconductor Bloch equations for a gapped-graphene model, we qualitatively reproduce these experimental features and find the polarization flipping to be associated with a significant contribution from interband polarization. In contrast, high-harmonic signals from MoSe 2 exhibit deviations from the laser-field following of odd-order harmonics and crystal-mirror-plane following of even-order harmonics. We attribute these differences to the competing roles of the intraband and interband contributions, including the deflection of the electron-hole trajectories by nonparabolic crystal bands.

We report the influence of photo-carrier doping on high-harmonic generation in monolayer MoS2. The observed decrease in efficiency suggests a strong role for interband transitions in mid-IR driven HHG in this prototypical 2D semiconductor.