Kenichi L. Ishikawa

Quantum path interference in the wavelength dependence of high-harmonic generation

We investigate the fundamental-wavelength dependence of high-harmonic generation yield. Superimposed on a smooth power-law dependence, we find surprisingly strong and rapid fluctuations on a fine wavelength scale, due to quantum-path interferences.

Observation of the complex propagation of a femtosecond laser pulse in a dispersive transparent bulk material

Pulse shapes in a dispersive transparent material modulated by group-velocity dispersion, self-phase modulation, and self-focusing induced by a femtosecond laser light were observed directly with femtosecond time-resolved optical polarigraphy probing the induced instantaneous birefringence. The first observation of the state of femtosecond laser pulses about the interaction region inside the transparent bulk material indicated that the pulse propagation was accomplished with a multiple conelike structure that was hypothesized from a numerical simulation with an extended nonlinear Schrodinger equation.

Intracycle and Intercycle Interferences in Above-Threshold Ionization: the Time Grating

Within a semiclassical description of above-threshold ionization (ATI) we identify the interplay between intracycle and intercycle interferences. The former is imprinted as a modulation envelope on the discrete multiphoton peaks formed by the latter. This allows one to unravel the complex interference pattern observed for the full solution of the time-dependent Schroedinger equation (TDSE) in terms of diffraction at a grating in the time domain. These modulations can be clearly seen in the dependence of the ATI spectra on the laser wavelength. Shifts in energy modulation result from the effect of the long Coulomb tail of the atomic potential.

Gene expression analysis in human malignant melanoma cell lines exposed to carbon beams.

Purpose: To elucidate the molecular changes in response to carbon beams (C-ions) in melanoma. Materials and methods: We examined expression profiles of 6 melanoma cell lines exposed to C-ions or X-rays with 2 Gy using single-color microarrays. Results: Twenty-two genes, including nuclear factor of kappa light polypeptide gene enhancer in B-cells inhibitor, alpha (NFKBIA), responded to C-ions in all six cell lines, based on analysis of variance (ANOVA) filtering (p < 0.001). We found 173 genes that responded in common to C-ions in four cell lines. We identified many down-regulated genes including the cell cycle – related genes that were more responsive to C-ions than X-rays. In contrast, most of the up-regulated genes including the tumor protein p53 (p53) target genes responded to both C-ions and X-rays. C-ions induced G2/M arrest significantly more than X-rays at 30 h (p < 0.05). Conclusion: Our findings suggest that down-regulation of gene expression plays a key role in the response to C-ions. Regulation of cell cycle – related genes and induction of prolonged G2/M arrest may be responsible for the extra sensitivity to C-ions, whereas p53-related genes may have similar roles in the sensitivities to both C-ions and X-rays.

Direct amplification of terawatt sub-10-fs pulses in a CPA system of Ti:sapphire laser

We have developed a chirped pulse amplification system of Ti:sapphire laser generating a 9.9 fs pulse with a pulse energy of 11 mJ at a repetition rate of 10 Hz. Spectral narrowing during amplification is successfully compensated by using specially designed partial mirrors and broadband high-damage-threshold mirrors. This is the first demonstration, to the best of our knowledge, of the direct amplification of terawatt sub-10-fs pulses in a chirped pulse amplification system of Ti:sapphire laser.

The Proangiogenic Factor Ephrin-A1 Is Up-Regulated in Radioresistant Murine Tumor by Irradiation

While the pre-treatment status of cancer is generally correlated with outcome, little is known about microenvironmental change caused by anti-cancer treatment and how it may affect outcome. For example, treatment may lead to induction of gene expression that promotes resistance to therapy. In the present study, we attempted to find a gene that was both induced by irradiation and associated with radioresistance in tumors. Using single-color oligo-microarrays, we analyzed the gene expression profiles of two murine squamous cell carcinomas, NR-S1, which is highly radioresistant, and SCCVII, which is radiosensitive, after irradiation with 137-Cs gamma rays or carbon ions. Candidate genes were those differentially regulated between NR-S1 and SCCVII after any kind of irradiation. Four genes, Efna1 (Ephrin-A1), Sprr1a (small proline-rich protein 1A), Srgap3 (SLIT-ROBO Rho GTPase activating protein 3) and Xrra1 [RIKEN 2 days neonate thymus thymic cells (NOD) cDNA clone E430023D08 3′], were selected as candidate genes associated with radiotherapy-induced radioresistance. We focused on Efna1, which encodes a ligand for the Eph receptor tyrosine kinase known to be involved in the vascular endothelial growth factor (VEGF) pathway. We used immunohistochemical methods to detect expression of Ephrin-A1, VEGF, and the microvascular marker CD31 in radioresistant NR-S1 tumor cells. Ephrin-A1 was detected in the cytoplasm of NR-S1 tumor cells after irradiation, but not in SCCVII tumor cells. Irradiation of NR-S1 tumor cells also led to significant increases in microvascular density, and up-regulation of VEGF expression. Our results suggest that radiotherapy-induced changes in gene expression related with angiogenesis might also modulate microenvironment and influence responsiveness of tumors.

Resolving vibrational wave-packet dynamics of D2+ using multicolor probe pulses

We demonstrate the generation and real-time observation of the vibrational wave packet of D(2)(+) by using a sub-10-fs extreme UV high-harmonic pump pulse and a three-color probe laser pulse whose wavelength ranges from near-IR to vacuum UV. This multicolor 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.

Time-dependent multiconfiguration self-consistent-field method based on the occupation-restricted multiple-active-space model for multielectron dynamics in intense laser fields

The time-dependent multiconfiguration self-consistent-field method based on the occupation-restricted multiple active space model is proposed (TD-ORMAS) for multielectron dynamics in intense laser fields. Extending the previously proposed time-dependent complete-active-space self-consistent-field method [TD-CASSCF; Phys. Rev. A, {\bf 88}, 023402 (2013)], which divides the occupied orbitals into core and active orbitals, the TD-ORMAS method {\it further} subdivides the active orbitals into an arbitrary number of subgroups, and poses the {\it occupation restriction} by giving the minimum and maximum number of electrons distributed in each subgroup. This enables highly flexible construction of the configuration interaction (CI) space, allowing a large-active-space simulation of dynamics, e.g., the core excitation or ionization. The equations of motion both for CI coefficients and spatial orbitals are derived based on the time-dependent variational principle, and an efficient algorithm is proposed to solve for the orbital time derivatives. In-depth descriptions of the computational implementation are given in a readily programmable manner. The numerical application to the one-dimensional lithium hydride cluster models demonstrates that the high flexibility of the TD-ORMAS framework allows for the cost-effective simulations of multielectron dynamics, by exploiting systematic series of approximations to the TD-CASSCF method.

Time-dependent complete-active-space self-consistent-field method for atoms: Application to high-order harmonic generation

We present a numerical implementation of the time-dependent complete-active-space self-consistent-field (TD-CASSCF) method [Phys. Rev. A 88, 023402 (2013)] for atoms driven by a strong linearly polarized laser pulse. The present implementation treats the problem in its full dimensionality and introduces a gauge-invariant frozen-core approximation, an efficient evaluation of the Coulomb mean field scaling linearly with the number of basis functions, and a split-operator method specifically designed for stable propagation of stiff spatial derivative operators. We apply this method to high-harmonic generation in helium, beryllium, and neon and explore the role of electron correlations.

A Review on Ab Initio Approaches for Multielectron Dynamics

In parallel with the evolution of femtosecond and attosecond laser as well as free-electron laser technology, a variety of theoretical methods have been developed to describe the behavior of atoms, molecules, clusters, and solids under the action of those laser pulses. Here, we review major ab initio wave-function-based numerical approaches to simulate multielectron dynamics in atoms and molecules driven by intense long-wavelength and/or ultrashort short-wavelength laser pulses. Direct solution of the time-dependent Schrödinger equation, though its applicability is limited to He, H2, and Li, can provide an exact description and has been greatly contributing to the understanding of dynamical electron-electron correlation. Multiconfiguration self-consistent-field (MCSCF) approach offers a flexible framework from which a variety of methods can be derived to treat both atoms and molecules, with possibility to systematically control the accuracy. The equations of motion of configuration interaction coefficients and molecular orbitals for general MCSCF ansatz have recently been derived. Time-dependent extension of the R-matrix theory, originally developed for electron-atom collision, can realistically and accurately describe laser-driven complex multielectron atoms.