Katsumi Midorikawa

Classical Trajectory Methods for Simulation of Laser-Atom and Laser-Molecule Interaction

The classical trajectory Monte Carlo method applied to the simulation of many-electron atoms and molecules in intense laser fields is reviewed. Two ways to solve the problem of constructing a stable, many-body ground state in classical mechanics are presented: (i) Fermionic molecular mechanics and (ii) interaction via soft-core Coulomb potentials. Five examples of the application of classical trajectory methods to the simulation of laser-driven atomic and molecular systems are introduced, ranging from the non-sequential double ionization of He and inner-shell electron ejection in C to the ionization and dissociation dynamics of H(_3) (^+) and proton recollision in ({}^{15})NH.

High-Pulse-Energy Yb:YAG Thin Disk Mode-Locked Oscillator for Intra-Cavity High Harmonic Generation

We have generated 1-mJ pulses with a pulse duration of 520 fs at a repetition rate of 2.85 MHz in a mode-locked oscillator cavity. These pulses can be used for generating high-harmonics in the cavity.

Focusing Intense High-Order Harmonics to a Micron Spot Size

The focusability of intense coherent radiation generated by high-order harmonic conversion is investigated. A variety of focusing mirrors are tested and used in the focusing experiments. We demonstrate the focusing of an extremeultraviolet/soft X-ray beam to a 1 µm spot size with a peak intensity as high as 1 × 1014 W/cm2, which is sufficient to stimulate nonlinear optical phenomena in the short wavelength region.

Generation of TW-scale mid-IR femtosecond pulses using a dual-chirped optical parametric amplification

By using a dual-chirped optical parametric amplification (DC-OPA) scheme, we demonstrate high-energy infrared (IR) pulses with TW-scale peak power in the wavelength region of 1 ∼ 3.5 μm. In the central wavelengths between 1.2 ∼ 1.8 μm, we obtained over 100 mJ pulse energy. DC-OPA, which is an energy scalable method for an OPA process, can allow us to use a joule-class Ti:sapphire laser system for pumping an OPA. To the best of our knowledge, our DC-OPA laser is the first 100-mJ-class IR source with pulse duration of shorter than 100 fs. Additionally, to show the ability of our DC-OPA laser system, we demonstrate an energy-scaling strategy of high-order harmonic driven by a loosely focused 1.5 μm pulse. Using a 4-cm Ne gas cell with a 3.5-m laser focusing length, soft x-ray harmonics beyond 200 eV are clearly enhanced by a phase-matching effect.

Classical Trajectory Models for Laser-Atom and Laser-Molecule Interactions

When an atom or a small molecule in the gas phase is exposed to an intense laser field, a complex response is generally induced. In the present context, “intense” signifies a peak-field intensity in the range between 1014 and 1016 W/cm2, assuming a wavelength of typically 800 nm. Such a laser pulse triggers a violent rearrangement of valence electrons in the atom or molecule.

Nanoaquarium: Manipulation of bio-cells and worms in electrofluidics fabricated by hybrid femtosecond laser processing

We present our recent progress on a new type of nanoaquarium based on electrofluidic devices fabricated by hybrid femtosecond (fs) laser processing. The hybrid fs laser processing involves two steps of (1) fs laser direct writing followed by thermal treatment, successive chemical wet etching and additional annealing for fabrication of three-dimensional (3D) microfluidic structures inside photosensitive glass, and then (2) water-assisted fs laser direct-write ablation followed by electroless metal plating for flexible deposition of patterned metal films on any desired locations in fabricated microfluidic structures. To show the applications of the nanoaquarium, fabricated electrofluidics are used to electrically manipulate the movement of microorganisms and worms in the microscale spaces. Flexible patterning and arrangement of electrodes to produce controllable AC electric fields in the closed microfluidic channels allows us to three-dimensionally manipulate the motions of Euglena cells due to electro-orientation. Meanwhile, 3D glass microfluidic channels monolithically integrated with vertical electrodes between which a DC voltage is applied enable us to flexibly control the movement of the nematode worm C. elegans in a closed channel based on electrotaxis.We present our recent progress on a new type of nanoaquarium based on electrofluidic devices fabricated by hybrid femtosecond (fs) laser processing. The hybrid fs laser processing involves two steps of (1) fs laser direct writing followed by thermal treatment, successive chemical wet etching and additional annealing for fabrication of three-dimensional (3D) microfluidic structures inside photosensitive glass, and then (2) water-assisted fs laser direct-write ablation followed by electroless metal plating for flexible deposition of patterned metal films on any desired locations in fabricated microfluidic structures. To show the applications of the nanoaquarium, fabricated electrofluidics are used to electrically manipulate the movement of microorganisms and worms in the microscale spaces. Flexible patterning and arrangement of electrodes to produce controllable AC electric fields in the closed microfluidic channels allows us to three-dimensionally manipulate the motions of Euglena cells due to electro-orie...

Observation of Vibrational Wave-Packet Dynamics in \mathrm{D}_{2}^{+} Using High-Order Harmonic Pulses

High-order harmonic generation is a unique phenomenon to provide coherent ultrashort light source ranging to the extreme ultraviolet region. In recent years, ultrashort high-harmonic pulses have been used to study ultrafast dynamics in atoms and molecules. We present the pump-probe experimental results on the observation of the vibrational wave-packet dynamics in (mathrm{D}_{2}^{+}) utilizing the high-order harmonic laser pulses of a sub-15 fs Ti:sapphire laser pulse. We launch the wave-packet of (mathrm{D}_{2}^{+}) by using a sub-10 fs extreme ultraviolet high-harmonic pump pulse, and observe its evolution by using the split harmonic probe pulse, whose wavelength ranges from near infrared to vacuum ultraviolet. This pump-probe scheme can provide us with a powerful experimental tool for investigating a variety of wave packets evolving with a time scale of ∼20 fs.

Electrical manipulation of biological samples in glass-based electrofluidics fabricated by 3D femtosecond laser processing

Electrical manipulation of biological samples using glass-based electrofluidics fabricated by femtosecond laser, in which the microfluidic structures are integrated with microelectric components, is presented. Electro-orientation of movement of living cells with asymmetric shapes such as Euglena gracilis of aquatic microorganisms in microfluidic channels is demonstrated using the fabricated electrofluidics. By integrating the properly designed microelectrodes into microfluidic channels, the orientation direction of Euglena cells can be well controlled.

Nonlinear Fourier-transform spectroscopy revealing wave-packet dynamics of D+2 with multicolor harmonic field

We report on a series of studies concerning D2 molecules irradiated by high-harmonic pulses generated from intense femtosecond laser pulses. The kinetic energy (KE) spectrum of dissociated D+ ion fragments with a scanning delay between two replica harmonic pulses exhibits specific characteristics that are completely different from a conventional interferometric autocorrelation signal. We have successfully determined and separated three distinct ionization/dissociation processes by analyzing KE-resolved interferometric fringes by Fourier transform. We call this method for analyzing the KE spectrum of ion fragments nonlinear Fourier-transform spectroscopy (NFTS). NFTS provides us molecular information in stationary states because it is intrinsically a frequency domain analysis. Nevertheless, we have resolved the real-time evolution of the vibrational wavepacket of D+ ions with a period of 22 fs by shortening the pulse duration of the fundamental laser pulse to 12 fs and extending the scanning delay range of two harmonic pulses to 150 fs. The probing process of the wavepacket can be described as a model with one-photon absorption of a multicolor harmonic field. We discuss a possible method of reconstructing the phase and magnitude of the wavepacket from the measured delay-KE spectrogram.

High-performance laser processing using manipulated ultrafast laser pulses

We employ manipulated ultrafast laser pulses to realize microprocessing with high-performance. Efficient microwelding of glass substrates by irradiation by a double-pulse train of ultrafast laser pulses is demonstrated. The bonding strength of two photostructurable glass substrates welded by double-pulse irradiation was evaluated to be 22.9 MPa, which is approximately 22% greater than that of a sample prepared by conventional irradiation by a single pulse train. Additionally, the fabrication of hollow microfluidic channels with a circular cross-sectional shape embedded in fused silica is realized by spatiotemporally focusing the ultrafast laser beam. We show both theoretically and experimentally that the spatiotemporal focusing of ultrafast laser beam allows for the creation of a three-dimensionally symmetric spherical peak intensity distribution at the focal spot.