The primary goal of this review is to present a clear chemical perspective of borates in order to stimulate and facilitate the discovery of new borate-based optical materials. These materials, whic...

Borates: A Rich Source for Optical Materials.

Advances in attosecond science have led to a wealth of important discoveries in atomic, molecular, and solid-state physics and are progressively directing their footsteps toward problems of chemical interest. Relevant technical achievements in the generation and application of extreme-ultraviolet subfemtosecond pulses, the introduction of experimental techniques able to follow in time the electron dynamics in quantum systems, and the development of sophisticated theoretical methods for the interpretation of the outcomes of such experiments have raised a continuous growing interest in attosecond phenomena, as demonstrated by the vast literature on the subject. In this review, after introducing the physical mechanisms at the basis of attosecond pulse generation and attosecond technology and describing the theoretical tools that complement experimental research in this field, we will concentrate on the application of attosecond methods to the investigation of ultrafast processes in molecules, with emphasis i...

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Attosecond Electron Dynamics in Molecules.

Attosecond metrology has so far largely remained limited to titanium:sapphire lasers combined with an active stabilization of the carrier-envelope phase (CEP). These sources limit the achievable photon energy to ∼100 eV which is too low to access X-ray absorption edges of most second- and third-row elements which are central to chemistry, biology and material science. Therefore, intense efforts are underway to extend attosecond metrology to the soft-X-ray (SXR) domain using mid-infrared (mid-IR) drivers. Here, we introduce and experimentally demonstrate a method that solves the long-standing problem of the complete temporal characterization of ultra-broadband (≫10 eV) attosecond pulses. We generalize the recently proposed Volkov-transform generalized projection algorithm (VTGPA) to the case of multiple overlapping photoelectron spectra and demonstrate its application to isolated attosecond pulses. This new approach overcomes all key limitations of previous attosecond-pulse reconstruction methods, in particular the central-momentum approximation (CMA), and it incorporates the physical, complex-valued and energy-dependent photoionization matrix elements. These properties make our approach general and particularly suitable for attosecond supercontinua of arbitrary bandwidth. We apply this method to attosecond SXR pulses generated from a two-cycle mid-IR driver, covering a bandwidth of ∼100 eV and reaching photon energies up to 180 eV. We extract an SXR pulse duration of (43±1) as from our streaking measurements, defining a new world record. Our results prove that the popular and broadly available scheme of post-compressing the output of white-light-seeded optical parametric amplifiers is adequate to produce high-contrast isolated attosecond pulses covering the L-edges of silicon, phosphorous and sulfur. Our new reconstruction method and experimental results open the path to the production and characterization of attosecond pulses lasting less than one atomic unit of time (24 as) and covering X-ray absorption edges of most light elements.

Streaking of 43-attosecond soft-X-ray pulses generated by a passively CEP-stable mid-infrared driver.

The motion of electrons in the microcosm occurs on a time scale set by the atomic unit of time—24 attoseconds. Attosecond pulses at photon energies corresponding to the fundamental absorption edges of matter, which lie in the soft X-ray regime above 200 eV, permit the probing of electronic excitation, chemical state, and atomic structure. Here we demonstrate a soft X-ray pulse duration of 53 as and single pulse streaking reaching the carbon K-absorption edge (284 eV) by utilizing intense two-cycle driving pulses near 1.8-μm center wavelength. Such pulses permit studies of electron dynamics in live biological samples and next-generation electronic materials such as diamond. Isolated attosecond pulses are produced using high harmonic generation and sources of these pulses often suffer from low photon flux in soft X-ray regime. Here the authors demonstrate efficient generation and characterization of 53 as pulses with photon energy near the water window.

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53-attosecond X-ray pulses reach the carbon K-edge

Time-resolved x-ray absorption spectroscopy (TR-XAS) has so far practically been limited to large-scale facilities, to subpicosecond temporal resolution, and to the condensed phase. We report the realization of TR-XAS with a temporal resolution in the low femtosecond range by developing a tabletop high-harmonic source reaching up to 350 electron volts, thus partially covering the spectral region of 280 to 530 electron volts, where water is transmissive. We used this source to follow previously unexamined light-induced chemical reactions in the lowest electronic states of isolated CF 4 + and SF 6 + molecules in the gas phase. By probing element-specific core-to-valence transitions at the carbon K-edge or the sulfur L-edges, we characterized their reaction paths and observed the effect of symmetry breaking through the splitting of absorption bands and Rydberg-valence mixing induced by the geometry changes.

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Time-resolved x-ray absorption spectroscopy with a water window high-harmonic source

The higher-harmonic generation of laser pulses is used to achieve short-wavelength attosecond pulses for ultrashort experiments, but has been limited in the achievable energy. Here, Ishii et al. develop a scheme to break this barrier and to achieve photon energies higher than the carbon K edge of about 284 eV.

Carrier-envelope phase-dependent high harmonic generation in the water window using few-cycle infrared pulses

We report on the generation of 9.0 fs, 550 μJ, carrier-envelope phase (CEP)-stabilized optical pulses around 1.6 μm at 1 kHz. Few-cycle IR pulses are obtained from a BiB3O6 optical parametric chirped-pulse amplifier. The amplification of nearly octave-spanning ultrabroad pulses without spectral broadening results in good stability in output energy (0.85% rms) and CEP (160 mrad rms). We observed high harmonics in the water window from a neon cell that corresponds to a laser intensity of 4.1×1014  W/cm2.

Sub-two-cycle, carrier-envelope phase-stable, intense optical pulses at 1.6 μm from a BiB3O6 optical parametric chirped-pulse amplifier.

We propose a method for achieving molecular orientation by two-step excitation with intense femtosecond laser and terahertz (THz) pulses. First, the femtosecond laser pulse induces off-resonant impulsive Raman excitation to create rotational wave packets. Next, a delayed intense THz pulse effectively induces resonant dipole transition between neighboring rotational states. By controlling the intensities of both the pulses and the time delay, we can create rotational wave packets consisting of states with different parities in order to achieve a high degree of molecular orientation under a field-free condition. We numerically demonstrate that the highest degree of orientation of $\ensuremath{\langle}\mathrm{cos}\ensuremath{\theta}\ensuremath{\rangle}g0.8$ in HBr molecules is feasible under experimentally available conditions.

High degree of molecular orientation by a combination of THz and femtosecond laser pulses

We report the direct observation of the orientation of jet-cooled molecules induced by a THz pulse. The intense THz field ($\ensuremath{\sim}$30 kV/cm) creates a superposition between the rotational ground state ($J=0$) and the excited state ($J=1$) of HBr molecules. The rotational dynamics is probed by Coulomb explosion imaging of the fragment ions produced by a near-infrared femtosecond laser pulse. Comparing with the simulations based on a time-dependent Schr\"odinger equation, the rotational wave packets are completely reconstructed, which reveal that $\ensuremath{\sim}$$7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ of the ground-state population is transferred to the $J=1$ state at an initial temperature of 8 K, achieving a degree of orientation of $\ensuremath{\langle}cos\ensuremath{\theta}\ensuremath{\rangle}\ensuremath{\sim}0.017$.

Orientation of jet-cooled polar molecules with an intense single-cycle THz pulse

We demonstrate the octave-spanning optical parametric amplification (OPA) of infrared pulses in the range from 1100 to 2200 nm using a BiB3O6 (BIBO) crystal and 800 nm pump pulses. Difference frequency generation is used to produce carrier-envelope-phase(CEP)-stabilized octave-spanning seed pulses. BIBO-based degenerate OPA amplifies the seed pulses to the 10 µJ level, while preserving their bandwidth and phases. We use an f-to-2f interferometer without external spectral broadening to measure the CEP of the output pulses from OPA.

https://iopscience.iop.org/article/10.1143/APEX.4.022701/pdf

Carrier-Envelope-Phase-Preserving, Octave-Spanning Optical Parametric Amplification in the Infrared Based on BiB3O6 Pumped by 800 nm Femtosecond Laser Pulses

Kenta Kitano

Papers

Carrier-envelope phase-dependent high harmonic generation in the water window using few-cycle infrared pulses

Sub-two-cycle, carrier-envelope phase-stable, intense optical pulses at 1.6 μm from a BiB3O6 optical parametric chirped-pulse amplifier.

High degree of molecular orientation by a combination of THz and femtosecond laser pulses

Orientation of jet-cooled polar molecules with an intense single-cycle THz pulse

Carrier-Envelope-Phase-Preserving, Octave-Spanning Optical Parametric Amplification in the Infrared Based on BiB3O6 Pumped by 800 nm Femtosecond Laser Pulses