There are intense research activities in the development of ultrafast mid-infrared pulse generation technology and its application to attosecond pulse generation at shorter wavelengths and non-perturbative optical physics. This paper describes the overview of the special issue on “forefront of ultrafast mid-infrared pulse generation technology and its application to material science.” The topics of this issue covers intense mid-infrared pulse generation by optical parametric processes, higher-order harmonic generation in gases and solids, and manipulation of electrons in vacuum and quantum materials.

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We report optical parametric amplification (OPA) of phase-stable multi-terahertz pulses which can be tuned from 16.9 to 44.8 THz (6.7 ~ 17.8 μm). The 255-fs laser output of the Yb:KGW regenerative amplifier is compressed to 11-fs pulses using a multi-plate broadening scheme, which generates multi-terahertz pulses with a spectrum extending to approximately 50 THz by intra-pulse difference frequency generation (DFG) in GaSe. The multi-terahertz pulses are further amplified using a two-stage OPA in GaSe. The temporal dynamics and photocarrier effects during OPA are characterized in the time domain. Owing to the intra-pulse DFG, the long-term phase drift of the multi-terahertz pulses after two-stage OPA is as small as 16 mrad during a 6-h operation without any active feedback. Our scheme using the intra-pulse DFG and post-amplification proposes a new route to intense multi-terahertz light sources with extreme phase stability.

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We have experimentally demonstrated the generation of sub-half-cycle phase-stable mid-infrared (MIR) pulses through two-color filamentation in nitrogen. Intense femtosecond pulses from a Ti:sapphire amplifier and the second harmonic pulses are focused into nitrogen at atmospheric pressure, and sub-halfcycle pulses with the carrier wavelength of 10.2 μm are generated through four-wave mixing processes. The carrier-envelope phase (CEP) of the MIR pulse is passively stabilized and controlled by the attosecond time delay between the two-color input pulses. The duration of the MIR pulse is 13.7 fs, which corresponds to 0.402 cycles. The standard deviation of the CEP instability is 0.0397π (124 mrad). The absolute value of the CEP of the generated sub-half-cycle pulse is consistent with an one dimensional fourwave difference frequency generation model.

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High-energy infrared (IR) femtosecond laser sources are important for applications in ultrafast and strong-field laser science; thus, considerable effort has recently been made to develop such laser systems. In this paper, we review our recent work on developing TW-class high power mid-infrared (MIR) few-cycle lasers in the 1 ~ 4 μm region using a dual-chirped optical parametric amplification (DC-OPA) method. By employing a Ti:sapphire laser with joule energy to pump the DC-OPA system, MIR femtosecond pulses with 100-mJ-class energy are demonstrated. High conversion efficiency, high energy scalability, and wide wavelength tunability in DC-OPA are confirmed experimentally. By precisely optimizing the chirps of the seed and pump pulses in the DC-OPA system, gain narrowing of amplified pulses can be suppressed.

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Extremely nonlinear optical phenomena that appear when high-intensity light and solid matter interact with each other are reviewed from the viewpoint of the Floquet state. In particular, we show the recent experimental results on the generation of high-order harmonics generation and high-order sidebands generation in atomically thin materials.

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The emerging field of all-optical electron-beam control enables the temporal shaping of free-electron beams at the level of optical cycles. Here we review our two recent experiments where we temporally compressed sub-relativistic electron beams with mid-infrared laser pulses. In the first experiment, we produced a train of few-femtosecond electron pulses with sub-picosecond multi-cycle mid-infrared pulses. In the second, we modulated the electron beams with single-cycle mid-infrared fields. The modulation with a minus-sine-like waveform produced an isolated attosecond peak. These results provide novel opportunities in ultrafast electron microscopy, laser-driven electron accelerators, and electron-matter collisions.

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Charge acceleration during the application of an intense light field to solids attracts much attention as elementary processes in high-harmonic generation and photoelectron emission. For manipulating such attosecond dynamics of charge, carrier-envelope-phase (CEP: relative phase between carrier oscillation of light field and its envelope function) control has been employed in insulators, nanometal and graphene. In superconducting materials, charge motion is expected to be controlled by taking advantage of the strongly coherent nature of quasi-particles. Here, in a layered organic superconductor, a non-linear petahertz current driven by a single-cycle 6 femtosecond near infrared field shows up as second harmonic generation (SHG), which is in contrast to the common belief that even harmonics are forbidden in the centrosymmetric system. The SHG represents a CEP sensitive nature and an enhancement near the superconducting temperature. The result and its quantum many-body analysis indicate that a polarized current is induced by non-linear acceleration of charge, which is amplified by superconducting fluctuations.

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We demonstrated salt damage assessment in concrete structures using remote laser-induced breakdown spectroscopy (LIBS). A LIBS signal ratio of Na at 589.6 nm to Ca at 616.6 nm allows us to evaluate the surface salt concentration of highway viaducts at a relatively remote distance of 5 m. Our results suggest that a remote LIBS system will be useful for salt damage assessment in difficult to access areas.

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