Kenta Kitano

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

High harmonic generation (HHG) using waveform-controlled, few-cycle pulses from Ti:sapphire lasers has opened emerging researches in strong-field and attosecond physics. However, the maximum photon energy of attosecond pulses via HHG remains limited to the extreme ultraviolet region. Long-wavelength light sources with carrier-envelope phase stabilization are promising to extend the photon energy of attosecond pulses into the soft X-ray region. Here we demonstrate carrier-envelope phase-dependent HHG in the water window using sub-two-cycle optical pulses at 1,600 nm. Experimental and simulated results indicate the confinement of soft X-ray emission in a single recombination event with a bandwidth of 75 eV around the carbon K edge. Control of high harmonics by the waveform of few-cycle infrared pulses is a key milestone to generate soft X-ray attosecond pulses. We measure a dependence of half-cycle bursts on the gas pressure, which indicates subcycle deformation of the waveform of the infrared drive pulses in the HHG process.

Sub-two-cycle, carrier-envelope phase-stable, intense optical pulses at 1.6 μm from a BiB 3 O 6 optical parametric chirped-pulse amplifier

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 BiB(3)O(6) 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 4.1×10(14) W/cm(2).

Quantum unidirectional rotation directly imaged with molecules.

High resolution imaging fully characterizes quantum-mechanical signatures of direction-controlled molecular rotational dynamics. A gas-phase molecular ensemble coherently excited to have an oriented rotational angular momentum has recently emerged as an appropriate microscopic system to illustrate quantum mechanical behavior directly linked to classical rotational motion, which has a definite direction. To realize an intuitive visualization of such a unidirectional molecular rotation, we report high-resolution direct imaging of direction-controlled rotational wave packets in nitrogen molecules. The rotational direction was regulated by a pair of time-delayed, polarization-skewed laser pulses, introducing the dynamic chirality to the system. The subsequent spatiotemporal propagation was tracked by a newly developed Coulomb explosion imaging setup. From the observed molecular movie, time-dependent detailed nodal structures, instantaneous alignment, angular dispersion, and fractional revivals of the wave packet are fully characterized while the ensemble keeps rotating in one direction. The present approach, providing an accurate view on unidirectional rotation in quantum regime, will guide more sophisticated molecular manipulations by utilizing its capability in capturing highly structured spatiotemporal evolution of molecular wave packets.

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

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.

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

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 >0.8 in HBr molecules is feasible under experimentally available conditions.

High-energy half-cycle cutoffs in high harmonic and rescattered electron spectra using waveform-controlled few-cycle infrared pulses

We developed a few-cycle waveform-controlled light source for infrared pulses at that is based on optical chirped-pulse amplification in (BIBO) crystals pumped by Ti:sapphire lasers. Using this source, we observe soft x-ray high harmonics that extend up to a photon energy of , as well as high-energy photoelectrons up to . The spectra of the high harmonics and photoelectrons have clear signatures of half-cycle cutoffs that can be used to extract electronic and molecular dynamics on an attosecond time scale.

Generation of Isolated Soft X-Ray Pulses Around the Carbon K -Edge Using CEP-Stabilized Few-Cycle IR Pulses

We generate a 75-eV-wide continuum in the water window via the HHG process using CEP-stabilized, few-cycle IR pulses. A pressure dependence of harmonic spectra indicates sub-cycle deformation of the IR pulses in the process.

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

Most of strong-field experiments such as high harmonic generation (HHG), attosecond pulse generation, and their applications rely on femtosecond Ti:sapphire lasers around 800 nm. Currently the photon energy range in HHG is limited to 200 eV or less for spectroscopic applications. To overcome this barrier, an IR source is promising because the ponderomotive potential is proportional to the square of a lasers wavelength, therefore, we expect the extension of an electron kinetic energy toward ~keV [1]. However an infrared (IR) light source must meets all requisites for attosecond pulse generation in soft X-rays such as few-cycle pulse duration, carrier-envelope phase (CEP) stabilization, mJ output energy, and high repetition rate. Although many IR systems have been constructed so far, CEP-sensitive strong field phenomena have not been exploited so far because of difficulty in meeting all the requisites. We have constructed a novel optical parametric chirped pulse amplifier (OPCPA) by use of BiB3O6 crystals and Ti:sapphire lasers. The OPCPA produces 9.0-fs, 550-μJ, CEP-stabilized optical pulses around 1.6 μm at 1 kHz [2]. In this presentation, using this light source, we report on the generation of an attosecond continuum in the water window and their CEP dependences, which can be understood by half-cycle cutoffs [3].

Carrier-envelope phase-dependent high harmonic generation in the water window by few-cycle IR pulses

We report on the generation of carrier-envelope phase-dependent high harmonics in the water window using few-cycle IR pulses. The results suggest the production of a 50-eV-wide isolated attosecond continuum at the carbon K edge.

Few-Cycle, Phase-Locked, Octave IR OPA Using BIBO and 800-nm Pump

Few-cycle, high-energy, infrared (IR) lasers have been recognized as promising drivers to generate coherent radiation and attosecond pulses beyond the softX-ray, reaching the water window. Based on a high harmonic generation (HHG) process driven by IR pulses, preliminary researches have demonstrated the drastic extension of a cutoff energy to a soft X-ray regime compared to Ti:sapphire-based lasers at 800 nm [1–3]. Although few IR drivers aiming at soft X-ray attosecond pulse generation have been constructed [4–7], these prototypes still require further improvement for potential applications in strong field physics and ultrafast spectroscopy.