Solid-state high-order harmonic generation: emerging frontiers in ultrafast and quantum light science

Photon sciences and technologies establish the building blocks for myriad scientific and engineering frontiers in life and energy sciences. Because of their overarching functionality, the developmental roadmap and opportunities underpinned by photonics are often semiotically mediated by the delineation of subject areas of application. In this perspective article, we map current and emerging linkages between three intersecting areas of research stewarded by advanced photonics technologies, namely light by design, outlined as (a) quantum and structured photonics, (b) light–matter interactions in accelerators and accelerator-based light sources, and (c) ultrafast sciences and quantum molecular dynamics. In each section, we will concentrate on state-of-the-art achievements and present prospective applications in life sciences, biochemistry, quantum optics and information sciences, and environmental and chemical engineering, all founded on a broad range of photon sources and methodologies. We hope that this interconnected mapping of challenges and opportunities seeds new concepts, theory, and experiments in the advancement of ultrafast photon sciences and light–matter interactions. Through this mapping, we hope to inspire a critically interdisciplinary approach to the science and applications of light by design.

Isolated attosecond pulses make it possible to study and control the ultrafast electron processes in atoms and molecules. High order harmonic generation (HHG) is the most promising way to generate such pulses, benefiting from the broad plateau structure of the typical HHG spectrum. In previous HHG studies on the polarization gating scheme, atomic ionization caused by the laser cycles before the polarization gate not only places a limit on the pulse width and intensity of the driving laser, but also affects the phase matching of harmonics generated in a polarization gate. According to these, in this paper we propose a new double optical gating scheme, in which the polarization of the laser pulse changes from linear to elliptical and back to linear again. Thus, only the linearly polarized field in the leading of the pulse contributes to high harmonic generation. By using a strong field approximation theory, we first simulate high order harmonic and attosecond pulse generation from helium atom irradiated by a double optical gating pulse based on the orthogonal polarization field. Here the orthogonal polarization field consists of two linearly polarized pulses with a certain time delay, orthogonal polarization directions and equal amplitudes. And for the double optical gating pulse, the second harmonic of the driving field is added to an orthogonal polarization field with an appropriate phase and energy. It is found that the high harmonic spectrum with higher efficiency and supercontinuum plateau is obtained by reasonably adjusting the parameters of the combined pulse. After inverse Fourier transform, an isolated 143-as pulse with higher intensity can be realized by superposing supercontinuum harmonics from the 50th to the 150th order. Compared with the double optical gating scheme proposed by Chang et al. (Zhao K, Zhang Q, Chini M, Wu Y, Wang X, Chang Z 2012 <i>Opt. Lett.</i> <b>37</b> 3891), our scheme not only overcomes the limit on the pulse duration and intensity of the incident pulse laser, but also avoids the harmonic phase mismatching in the process of the propagation due to unwanted ionization of the gas target caused by the laser cycles before the polarization gate.

The generation of high-order harmonics based on the interaction between ultrafast intense laser and matter provides a platform for studying the light-matter interaction in the non-perturbative region. It is also the main route to generating desktop extreme ultraviolet light source and attosecond pulse. The non-perturbative solid high-order harmonic involves the core content of ultrafast strong field physics, condensed matter physics, materials science, information science and other fields. Since it was first experimentally observed in 2011, it has rapidly become the research frontier of strong field physics and attosecond science. This review summarizes the research progress and important applications of solid high-order harmonics from the perspective of an experimentalist. Firstly, distinct characteristics are shown for solid high-order harmonic by comparing the dependence of harmonic yield and cut-off energy on driving laser parameters with gas high-order harmonic. Then, the progress of manipulation and application are highlighted for solid high-order harmonic, including the precise control of harmonic yield, polarization, space-time distribution through the design of target structure or laser field, as well as the application of solid high-order harmonic spectroscopy in the fields of material structure characterization and ultrafast electron dynamics. Finally, the future is prospected for the study of solid high-order harmonics.

This paper reviews our recent studies on ultrafast, strong-ˆeld interactions with atoms, molecules and solid surfaces. The major in- terest in atomic and molecular processes is concerned with nonperturbative nonlinear processes induced with intense ultrashort laser pulses, especially, high-order harmonic generation from nonadiabatically aligned molecules and its dynamics measurements. Ex- perimental and theoretical results demonstrate characteristic properties of the harmonic generation from coherently rotating mole- cules, depending on molecular structures. Concerning the interaction with solid surfaces, the periodic nanostructure formation observed in femtosecond laser ablation of thinlm surfaces has been the subject for the purpose of potential applications of ultrashort laser pulses to nanoprocessing. It is found that the nano-scale ablation is initiated with laser-induced near-ˆeld on solid surfaces, and the nano-periodicity is attributed to the excitation of surface plasmon polaritons. The results obtained show that the intense ultrashort- pulse laser is a promising tool for investigating new and/or latent dynamics and functions of matter.

High-order harmonic generation (HHG) in atoms and molecules allows the study of the static and dynamic properties of these systems. We present the results of HHG studies in the plasmas produced using femtosecond and picosecond laser pulses on the surfaces of lanthanides and their oxides (La, Yb, Pr6O11, and Tb4O7). The plasmas induced by femtosecond pulses have proven to be a more efficient medium for HHG than the plasmas produced by picosecond pulses in the case of two-color pump HHG. We analyze the advantages of laser ablation using femtosecond pulses for the extension of the cutoff energy of generated harmonic in lanthanide plasma. We have shown that Yb plasma is the efficient medium for the harmonic generation up to the 73rd order, which is one of the largest orders generated in laser-produced plasmas.

Since 1998, the interaction of precision spectroscopy and ultrafast laser science has led to several notable accomplishments. Femtosecond laser optical frequency 'combs' (evenly spaced spectral lines) have revolutionized the measurement of optical frequencies and enabled optical atomic clocks. The same comb techniques have been used to control the waveform of ultrafast laser pulses, which permitted the generation of single attosecond pulses, and have been used in a recently demonstrated 'oscilloscope' for light waves. Here we demonstrate intra-cavity high harmonic generation in the extreme ultraviolet, which promises to lead to another joint frontier of precision spectroscopy and ultrafast science. We have generated coherent extreme ultraviolet radiation at a repetition frequency of more than 100 MHz, a 1,000-fold improvement over previous experiments. At such a repetition rate, the mode spacing of the frequency comb, which is expected to survive the high harmonic generation process, is large enough for high resolution spectroscopy. Additionally, there may be many other applications of such a quasi-continuous compact and coherent extreme ultraviolet source, including extreme ultraviolet holography, microscopy, nanolithography and X-ray atomic clocks.

The landscape of ultrafast structured light pulses has significantly advanced thanks to the ability of high-order harmonic generation (HHG) to translate the spatial properties of infrared laser beams to the extreme-ultraviolet (EUV) spectral range. In particular, the up-conversion of orbital angular momentum (OAM) has enabled the generation of high-order harmonics whose OAM scales linearly with the harmonic order and the topological charge of the driving field. Having a well-defined OAM, each harmonic is emitted as an EUV femtosecond vortex pulse. However, the order-dependent OAM across the harmonic comb precludes the synthesis of attosecond vortex pulses. Here we demonstrate a method for generating attosecond vortex pulse trains, i.e., a succession of attosecond pulses with a helical wavefront, resulting from the coherent superposition of a comb of EUV high-order harmonics with the same OAM. By driving HHG with a polarization tilt-angle fork grating, two spatially separated circularly polarized high-order harmonic beams with order-independent OAM are created. Our work opens the route towards attosecond-resolved light-matter interactions with two extra degrees of freedom, spin and OAM, which are particularly interesting for probing chiral systems and magnetic materials.

Grating eliminated no-nonsense observation of ultrafast incident laser light e-fields is an experimental technique for the full characterization of ultrashort laser pulses. It requires the nonlinear conversion of broadband ultrashort optical pulses into their second harmonic generation (SHG) that is relatively tightly focused on a thick SHG crystal. In this paper, the laser focusing problem is experimentally presented and demonstrated in the measurements of 780 nm chirped pulses emitted from a Ti:sapphire oscillator. In this experimental study, it has been shown how the focusing effect can cause distortions in the trace of the chirped pulses. It is demonstrated that the measurement of broadband pulses requires that the beam have a wide angular divergence. Therefore, a tight focusing is essential. A long focal length leads to a smaller range of angles incident on the crystal causing an insufficient divergence angle. Finally, the ability of this device in the characterization of double and complicated chirped pulses is described. The broadening of complex chirped pulses after passing through a thick BK7 glass is measured and the group velocity dispersion of the BK7 glass is calculated as about 47.3 fs2 mm−1. Also the train of double chirped pulses created using a Michelson interferometer is measured and the spectral fringes marked by phase jumps are observed. The experimental results of the retrieved phases and intensities are in good agreement with independently measured data.

The measurement and characterization of ultrashort laser pulses remains an arduous task. The most commonly used pulse-measurement method is known as frequency-resolved optical gating (FROG), and another version with great experimental simplification and low-priced setup is known as grating-eliminated no-nonsense observation of ultrafast incident laser light E fields (GRENOUILLE). Nevertheless, there is interest in elaborating other, more accessible or simpler and cheaper, setups with equal or better assets. We explored modification of the GRENOUILLE method in which we replaced the original Fresnel biprism with a beam splitter and two mirrors and used a cheap webcam to measure the pulse traces. We have evaluated our system, and we propose a method to correct border effects caused by the beam intensity's profile based on the characterization of three pulse classes: Fourier-transform limited, double, and chirped. We compare the recovered electric field with further spectral and second-order correlation data of the corresponding pulses.

Solid-state high-order harmonic generation (HHG) presents a promising approach for achieving controllable broadband coherent light sources and dynamically detecting materials. In this study, we demonstrate the all-optical control of HHG in a strongly correlated system, vanadium dioxide (VO2), through photo-carrier doping. It has been discovered that HHG can be efficiently modified using a pump laser, achieving modulation depths approaching 100% (extinction ratio ≥40 dB) on femtosecond timescales. Quantitative analysis reveals that the driving forces behind pump-dependent HHG are attributed to two distinct many-body dynamics: the scattering-induced dephasing and the insulator-to-metal transition (IMT) caused by photo-induced electron shielding. These two dynamics play a crucial role in defining the intensity and transient response of the HHG. Furthermore, we demonstrate that it is possible to quantitatively extract the metallic phase fraction from time-resolved HHG (tr-HHG) signals throughout the IMT. This study highlights the benefits of utilizing many-body dynamics for controlling HHG and underscores the necessity for further theoretical research on HHG in strongly correlated systems.

Directly monitoring atomic motion during a molecular transformation with atomic-scale spatio-temporal resolution is a frontier of ultrafast optical science and physical chemistry. Here we provide the foundation for a new imaging method, fixed-angle broadband laser-induced electron scattering, based on structural retrieval by direct one-dimensional Fourier transform of a photoelectron energy distribution observed along the polarization direction of an intense ultrafast light pulse. The approach exploits the scattering of a broadband wave packet created by strong-field tunnel ionization to self-interrogate the molecular structure with picometre spatial resolution and bond specificity. With its inherent femtosecond resolution, combining our technique with molecular alignment can, in principle, provide the basis for time-resolved tomography for multi-dimensional transient structural determination.

We report the application of femtosecond and picosecond laser pulses for plasma formation on metal targets (Al, Cu, In, Sn) for non-resonant and resonant high-order harmonic generation using laser pulses with 1 kHz repetition rate. The resonance enhancement of single harmonics in the picosecond and femtosecond pulse induced In and Sn plasmas has been studied. These studies reveal comparable harmonic yields from both types of plasmas, which indicate that the pump fluence is of central importance for the optimal plasma formation for efficient harmonic generation.

The "International Topical Conference on Plasma Physics: New Frontiers in Nonlinear Sciences" was held at the University of Algarve (UA), Faro (Portugal), during the period 6–10 September 1999. The conference was organized by P K Shukla, R Bingham, J T Mendonça and L Stenflo with the help of an international advisory board and a program committee that included scientists from all over the world. The conference enters into a series of previous biennial activities that we have held at the Abdus Salam International Centre for Theoretical Physics, Trieste, since 1989.The purpose of the Faro meeting was to provide an informal forum for scientists who have dealt with various aspects ofnonlinear processes in space, astrophysical and laboratory plasmas, as well as in fluids and nonlinear optics. The selectedtopics, which were interdisciplinary, are expected to have a great deal of impact on the development of nonlinear sciencesin the coming millennium.The response of the conference was almost overwhelming. It was attended by approximately 120 delegates from Europe,USA, Japan, and developing countries. Many participants were young researchers from both the industrial and developingcountries, as the organizers tried to keep a good balance in inviting senior and younger generations of nonlinear scientiststo our Faro conference.The scientific program included five review talks (45 minutes) and thirty-five invited topical lectures (30 minutes). Inaddition, there were about eighty poster papers in three sessions. The latter gave opportunities to younger physicistsfor displaying the results of their recent work and to obtain comments from the other participants. During the five daysat the UA, we focused on fundamental aspects of: (i) the nonlinear physics in various branches of sciences (plasmas, fluids,and optics), (ii) nonlinearities in space and astrophysics, (iii) coherent processes in non-ideal systems (colloidal and dusty as well as pure electron plasmas), (iv) nonlinear charged particle beams and laser/neutrino-plasma interactions, and(v) nonlinearities in controlled laboratory devices. The focus was on nonlinear phenomena involving wave–wave and wave–particle interactions in ideal and non-ideal complex plasmas, intense short laser pulse interactions with plasmas and atomic clusters, the Kerr nonlinearity in optics, generation of harmonics as well as modulated wave packets in fluids, anomalous transport processes and complexities, etc. The nonlinear Schrödinger-like models for the propagation of particle and laser beams and optical pulses are still common in nonlinear dispersive media. The formation of coherent nonlinear structures (voids, envelope solitons, shocks, various types of solitary vortices, and vortex crystals) was exclusively demonstrated in data from low-temperature laboratory and space plasmas. For example, observations from the auroral ionosphere and microgravity dusty plasma experiments reveal the simultaneous presence of solitary vortex structures of various scale sizes as well as large scale density holes (voids). During the Faro meeting, some talks also concerned the role of self-organized criticality in nature as well as techniques for controlling chaos. Computer modeling of numerous kinetic instabilities and phase space vortex structures revived renewed interest at the conference. Furthermore, charged particle acceleration involving collective processes in ionospheric, astrophysical, and laboratory plasmas remained frontiers of the nonlinear sciences. Novel fields identified during the conference were dusty and neutrino plasma physics, which take advantage of knowledge from condensed matter and particle physics, making plasma physics truly cross-disciplinaryand very fascinating with wide ranging applications. Most of the contributions from the Faro meeting appear in this TopicalIssue of Physica Scripta, which will be distributed to all the participants. It is expected that the papers of the present proceedings, which systematically describe the advancement of a particular subject matter, shall be useful for understanding the many complex nonlinear phenomena that are occurring in science and technology.The organizers are grateful to Professor Adriano Pimpão, the President of the UA, for his generous support and warm hospitality in Faro. The excellent work of the scientific secretary Dr Rui Guerra is also acknowledged. The Editors want to express sincere gratitude to their colleagues and co-organizers Profs Tito Mendonça and Bob Bingham for their constant and wholehearted support in our endeavours. Thanks are also due to the Universidade do Algarve for providing the venue, and for administrative support, the Universidade Técnica de Lisbon for secretarial and other support, and theFCT-Fundação para a Ciência e a Tecnologia for a limited financial support to our conference at the UA, Faro.Finally, the organizers cordial thanks are extended to the speakers and the attendees for their contributions which resulted in the success of the Faro conference. Specifically, we appreciate the speakers for delivering excellent talks, supplying well prepared manuscripts for publication, and enhancing the nonlinear science activity at the UA, where we hope to create a new center of excellence for carrying out high quality research and for training young researchers in the fascinating new areas of nonlinear sciences which may have a tremendous impact on the development of new technologies andnew materials in the twenty-first century.

This study investigates the use of ultrafast lasers for postprocessing fused deposition modeling 3D-printed parts, focusing on improving surface roughness and analyzing its corresponding effects on tensile strength and fatigue life. We explore the adoption of high repetition rate ultrafast laser light and raster scanning techniques to address the limitations associated with as-deposited surface roughness in 3D-printed objects. By employing a design of experiment framework using Taguchi’s orthogonal arrays, we analyze the effects of various laser parameters on the surface finish and mechanical integrity of printed polylactic acid parts. Our study indicates significant enhancements: a 90% reduction in surface roughness, a 20% increase in ultimate tensile strength, and a 165% increase in high-cycle fatigue life, showcasing the considerable benefits of ultrafast laser processing. We demonstrate that low-thermal-impact surface processing can substantially elevate the quality and durability of 3D-printed materials. The analysis points to the importance of controlling certain factors during the laser postprocessing phase, as they impact surface conditions and broader material properties. This work positions ultrafast laser processing as a viable technique to bridge the gap between additive manufacturing and traditional fabrication methods, particularly in the context of improving the surface quality and structural performance of 3D-printed thermoplastics. The outcomes could significantly benefit industries where additive manufacturing is prevalent by expanding the practical applications of 3D-printed components.

Imaging using sources in the XUV and X-ray spectral range combines high resolution with longer penetration depth (compared to electron/ion microscopy) and found applications in many areas of science and technology. Coherent diffractive imaging (CDI) techniques, in addition, lift the performance limitation of conventional XUV/X-ray microscopes imposed by image forming optics and enable diffraction limited resolutions. Until recently, CDI techniques were mainly confined to large scale facilities e.g. synchrotrons and X-ray free electron lasers due to unavailability of suitable table-top XUV/X-ray sources. Table-top sources based on high-order harmonic generation (HHG) nowadays offer high and coherent photon flux which widened the accessibility of CDI techniques. So far, table-top CDI systems were not able to resolve sub-100 nm features using performance metrics that can qualify these systems for real world applications. In this work, CDI experiments with the highest resolutions in different modalities using a high flux fiber laser driven HHG source are presented. In conventional CDI, a record-high resolution of 13 nm is demonstrated together with the possibility of high speed acquisition with sub-30 nm resolution. In a holographic implementation of CDI, features with a half-distance of 23 nm are resolved which are the smallest features to ever be resolved with a table-top XUV/X-ray imaging system. Ptychographic imaging of extended samples is also performed using a reliable Rayleigh-like resolution metric and resolving of features as small as 2.5 wavelengths is demonstrated. These systems can find applications in material and biological sciences, study of ultrafast dynamics, imaging of semiconductor structures and EUV lithographic mask inspection.