In this paper, I describe an overview of the special issue on the new frontier for high-order harmonic generation (HHG), which consists of 6 review papers. These papers will review the authors’ recent research works from the view point of the new trend of HHG research such as HHG in solids, circularly- polarized HHG, and resonant HHG from laser ablation media.

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Solid-state materials have recently emerged as a new stage of strong-field physics and attosecond science. The mechanism of high-harmonic generation (HHG) from solids is being scrutinized. Here we theoretically discuss the momentum-space pictures of HHG from graphene and crystalline dielectrics and semiconductors. Within massless Dirac fermion and tight-binding models, we rigorously derive intraband displacement and interband transition, which form the basis for understanding solid-state HHG. Then based on the results of simulations that solve the time-dependent Schrödinger equation for a one-dimensional model crystal, we introduce a simple momentum-space three-step model that incorporates intraband displacement, interband tunneling, and recombination with a valence band hole. We also present a time-dependent density-matrix method that simulates HHG from actual three-dimensional materials whose results are compared with experiments to increase the understanding of measurement results.

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Carrier-envelope phase-stable, femtosecond intense light sources in the mid-IR region are developed to produce high harmonics in solids. Using these light sources, high harmonics are produced in GaSe. Polarization analysis along with a first-principle simulation suggests that the intraband current is the primary source for lower-order odd harmonics, which show unusual 30-degree periodicity. High harmonic generation with circularly polarized electric field is also investigated. The selection rule for circularly polarized high harmonics is experimentally confirmed for the first time.

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Non-perturbative or strong-field optical phenomena are potentially useful for attosecond pulse generation in the extreme ultraviolet region, table-top accelerators, lighwave-electronics, etc. Nanoscale engineering of the optical field allows us to downsize the driving lightsource and may provide better control over electron trajectories and new functionalities. In this article, we discuss field enhancement properties of metal nanoantennas in the femtosecond time domain and introduce our recent results on plasmonically- enhanced strong-field photoemission and high harmonic generation. We also discuss the prospects of strong-field science enabled by nanoscale engineering.

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High harmonic generation is sensitive to the electronic structure of the highest valence electron. Here, circularly polarized high harmonic generation from chiral molecules by the laser field synthesized by two-color counter rotating circularly polarized lasers is investigated to elucidate the relation between the chirality and high harmonic generation. The intensity ratio between (3m + 1)-th and (3m + 2)-th orders is found to depend on the chirality. High harmonic generation can be a new probe into the chirality of bio-molecules.

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We review a complete solution to generate polarization-controllable, table-top, isolated attosecond pulses [Nature Photonics, 12, 349 (2018)] – that is realized by adjusting the ellipticity of counter-rotating few-cycle polarized fundamentals for non-collinear high harmonic generation. The polarization state of the attosecond pulses was fully analyzed with an EUV polarimeter composed of two rotatable sets of triple-reflection polarizers, which unambiguously determines the ellipticity and helicity of attosecond pulses. Furthermore, the isolation property, the pulse contrast and coherence time of EUV pulses was characterized by a Fourier-transform field autocorrelation. The proposed polarization control scheme is simple and robust, and enables the real-time measurement of the basic process associated with energy and angular momentum transfer between the electron/spin system and symmetry-dependent characteristics in molecules and materials with unprecedented temporal resolution.

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High-order harmonics generated from low-temperature, low-density laser ablation media is a new and promising method for generating intense coherent soft X-ray and extreme ultraviolet pulses. In this review, we focus on recent advances in resonant harmonics from such ablation media, including studies on their physical mechanisms as well as future applications.

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We performed 2D Particle-In-Cell plasma simulations to study the interaction of an ultra-thin film plasma mirror (PM) with a high-intensity laser for two cases; 1) interaction with a main-pulse and 2) interaction with a back-reflected pulse from the interaction target. PM’s reflectivity decreased from 79% to 0.1% after expansion of the plasma mirror when the back-reflected pulse reentered the mirror. The result indicates that a harmful back-reflected light from the main target can be removed by using an ultra-thin film PM.

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