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Katsumi Midorikawa

Publications and source records attributed to Katsumi Midorikawa.

18 recordsLinked to original sources
PubMed2006

Interferometric autocorrelation of an attosecond pulse train in the single-cycle regime.

We report on the direct observation of the phase locking of the attosecond pulse train (APT) via interferometric autocorrelation in the extreme ultraviolet region. APT is formed with Fourier synthesis of high-order harmonic fields of a femtosecond laser pulse. Time-of-flight mass spectra of N+, resulting from the Coulomb explosion of N2 absorbing two photons of APT, efficiently yield correlated signals of APT. The measured autocorrelation trace exhibits that the duration of the pulse should be only 1.3 periods of the extreme ultraviolet carrier frequency. A few interference fringes within the short pulse duration clearly show two types of symmetry, which ensure the phase locking between pulses in APT.

Journal Article↗
PubMed2006

Optical pulse compression of ultrashort laser pulses in an argon-filled planar waveguide.

We investigate the possibility of optical pulse compression of high energy ultrashort laser pulses in an argon-filled planar waveguide, based on two level coupled mode theory and the full 3D nonlinear Schrödinger equation. We derive general expressions for controlling the spatial beam profile and the extent of the spectral broadening. The analysis and simulations suggest that the proposed method should be appropriate for optical pulse compression of ultrashort laser pulses with energies as high as 600 mJ.

Journal Article↗
PubMed2006

Development of high-throughput, high-damage-threshold beam separator for 13 nm high-order harmonics.

We demonstrate a high-throughput, high-damage-threshold beam separator for wavelengths shorter than 30 nm, which uses a 10 nm thick niobium nitrogen film prepared on a Si substrate, set at the Brewster angle relative to the pump wavelength. The film was deposited by rf reactive magnetron sputtering on a Si substrate. The beam separator has an attenuation ratio of 0.01 and a damage-threshold intensity of at least 0.8 TW/cm2 for a 26 fs pump pulse. The measured reflectivity of the beam separator exceeded 70% in a wavelength range of 13-18 nm. This broadband beam separator may be used to eliminate energetic laser pulses from longitudinally pumped x ray lasers as well as from high-order harmonics.

Journal Article↗
PubMed2006

Conclusive evidence of an attosecond pulse train observed with the mode-resolved autocorrelation technique.

We report on the direct observation of an attosecond pulse train with a mode-resolved autocorrelation technique. The chirp among the three harmonic fields is specified by analyzing two-photon above-threshold ionization spectra of electrons, resulting in a pulse duration that should be shorter than 450 as, which is, to our knowledge, the first determination of the chirp in the attosecond pulse train with an autocorrelation technique. These results will open the way to full characterization of an attosecond pulse train with its envelope.

Journal Article↗
PubMed2006

Focusing multiple high-order harmonics in the extreme-ultraviolet and soft-x-ray regions by a platinum-coated ellipsoidal mirror.

The focusability of multiple high-order harmonics in the extreme-ultraviolet and soft-x-ray regions is described, together with the design and performance of the ellipsoidal mirror used for this purpose. The mirror focuses intense coherent light in the spectral-region from 25 to 40 nm into a 2.4 microm spot size with a focused peak intensity of 6 x 10(13) W/cm2. The focal images indicate that a good beam profile is obtained with a near-Gaussian distribution and a beam quality factor (M2 value) as low as 2.4.

Journal Article↗
PubMed2006

Femtosecond laser microprocessing with three-dimensionally isotropic spatial resolution using crossed-beam irradiation.

We describe the use of a crossed-beam irradiation system in three-dimensional femtosecond laser microprocessing to obtain three-dimensionally isotropic spatial resolution. In the crossed-beam geometry, two orthogonal objective lenses are arranged to share a common focal point. The synthesized focal spot produces an isotropic illumination volume. We demonstrate that microfluidic channels with substantially circular cross-sectional shapes can be directly fabricated inside glass by using the crossed-beam irradiation system.

Journal Article↗
PubMed2005

Production of doubly charged helium ions by two-photon absorption of an intense sub-10-fs soft x-ray pulse at 42 eV photon energy.

We report on the observation of doubly charged helium ions produced by a nonlinear interaction between a helium atom and photons with a photon energy of 42 eV which are generated with the 27th harmonic of a femtosecond pulse from a Ti:sapphire laser. The number of ions is proportional to the square of the intensity of the 27th harmonic pulse, and thus two-photon double ionization should be dominantly induced as compared with other nonlinear processes accompanying sequential ionization via a singly charged ion. This phenomenon is utilized to measure the pulse duration of the 27th harmonic pulse by using an autocorrelation technique, for the first time to our knowledge, and as a result a duration of 8 fs is found.

Journal Article↗
PubMed2004

Microfluidic laser embedded in glass by three-dimensional femtosecond laser microprocessing.

Microfluidic dye lasers three-dimensionally embedded in glass have been fabricated for what is believed to be the first time by integrating micro-optical and microfluidic components by use of a femtosecond laser. By pumping the microfluidic laser, in which the microfluidic chamber was filled with the laser dye Rhodamine 6G dissolved in ethanol, with a frequency-doubled Nd:yttrium aluminum garnet laser, lasing action was confirmed by analysis of the emission spectra at different pump powers. In addition, by arranging two microfluidic chambers serially in the glass, we built a microfluidic twin laser that produces an array of two simultaneous laser emissions with one pump laser.

Journal Article↗
PubMed2004

Focusing coherent soft-x-ray radiation to a micrometer spot size with an intensity of 10(14) W/cm2.

We investigate the focusability of intense coherent soft-x-ray radiation generated by high-order harmonic conversion. The 27th-harmonic wave at 29.6 nm is focused by an off-axis parabolic mirror with a SiC/Mg multilayer coating. Focal-spot images are observed from the visible fluorescence induced by the soft-x-ray photons on a Ce:YAG scintillator. We demonstrate focusing of the soft-x-ray beam to a 1-microm spot size with a peak intensity of 1 x 10(14) W/cm2, which is to our knowledge the highest ever reported in the soft-x-ray region.

Journal Article↗
PubMed2004

Two-color two-photon 4Pi fluorescence microscopy.

In 4Pi fluorescence microscopy the point-spread function is composed of a strong central lobe accompanied by interference sidelobes that produce artifacts in the image. We propose to combine two-color two-photon fluorescence microscopy and 4Pi fluorescence microscopy to overcome this sidelobe problem. Simulation results show that a single sharp fluorescence spot can be produced by use of two excitation wavelengths of 400 and 800 nm and detected at 350-nm emission wavelength.

Equipment Design↗
PubMed2004

High-throughput, high-damage-threshold broadband beam splitter for high-order harmonics in the extreme-ultraviolet region.

We demonstrate a high-throughput and high-damage-threshold beam splitter for high-order harmonics in the soft-x-ray region that uses Si and (or) SiC plates set at Brewster's angle with respect to the pump wavelength. The beam splitters are guaranteed to have a damage threshold of at least 0.8 TW/cm2 (average power density, 0.25 W/cm2) and an attenuation rate of 10(-4)-10(-5) for a 30-fs pump pulse. The measured reflection efficiency at the 27th harmonic (29.6 nm) was 0.56 for Si and 0.45 for SiC. These beam splitters are useful not only for high harmonics but also for longitudinally pumped x-ray lasers.

Journal Article↗
PubMed2003

Attenuation of photobleaching in two-photon excitation fluorescence from green fluorescent protein with shaped excitation pulses.

The two-photon excitation fluorescence (TPEF) process of an enhanced green fluorescent protein (EGFP) for fluorescence signals was adaptively controlled by the phase-modulation of femtosecond pulses. After the iteration of pulse shaping, a twofold increase in the ratio of the fluorescence signal to the laser peak power was achieved. Compared with conventional pulses optimized for peak power, phase-optimized laser pulses reduced the bleaching rate of EGFP by a factor of 4 while maintaining the same intensity of the fluorescence signal. Our method will provide a powerful solution to various problems confronting researchers, such as the photobleaching of dyes in two-photon excitation microscopy.

Fluorescent Dyes↗
PubMed2003

Efficient sum-frequency generation of continuous-wave single-frequency coherent light at 252 nm with dual wavelength enhancement.

Highly efficient frequency conversions were conducted to obtain deep-ultraviolet single-mode coherent light by use of two-stage external cavities. A power of 154 mW at approximately 252 nm was obtained with a conversion efficiency of more than 8% by doubly resonant sum-frequency mixing of 373-nm light from the first-stage conversion and 780-nm light from a single-mode Ti:sapphire laser. The output performance of the deep-ultraviolet light source is sufficient for use in the laser cooling of neutral silicon atoms.

Journal Article↗
PubMed2003

Three-dimensional micro-optical components embedded in photosensitive glass by a femtosecond laser.

We show that three-dimensional micro-optical components can be embedded in a photosensitive glass by a femtosecond (fs) laser. After exposure to the tightly focused fs laser beam, latent images are written inside the sample. Modified regions are developed by a postbaking process and then preferentially etched away in a 10%-dilute solution of hydrofluoric acid. After this process, hollow internal structures are formed that act as a mirror and a beam splitter. Furthermore, we find that postannealing smoothes the surfaces of the fabricated hollow structures, resulting in great improvement of their optical properties.

Journal Article↗
PubMed2003

Control of the cross-sectional shape of a hollow microchannel embedded in photostructurable glass by use of a femtosecond laser.

Theoretical and experimental investigations have been made of the three-dimensional microchannel fabrication of photostructurable glass by use of a femtosecond (fs) laser. Generally, a microchannel fabricated inside glass by the scanning focal spot of a fs laser perpendicular to the direction of laser propagation assumes an elliptical shape with a cross section of large aspect ratio. We demonstrate that one can greatly reduce the aspect ratio merely by inserting a slit, which is oriented parallel to the laser's scanning direction, before the focusing lens. Computer simulations show that a more symmetrical pattern is obtained in the vicinity of the focal point with the help of such a slit, owing essentially to a diffraction effect.

Journal Article↗
PubMed2002

High-power regime of femtosecond-laser pulse propagation in silica: multiple-cone formation.

We present a numerical study of the (2+1)-dimensional propagation dynamics of femtosecond-laser pulses in silica. In particular, considered are pulses, whose power is tens to hundreds of times higher than the threshold for self-focusing. We solve the axially symmetric, extended, nonlinear Schrödinger equation for the laser electric field, including group velocity dispersion, Kerr nonlinearity, plasma formation and defocusing, self-steepening, and space-time focusing. Our simulation results reveal that the high-power pulses are split spatially, as well as temporally, several times into multiple cones during its propagation. This new structure is formed as a result of the interplay of strong Kerr self-focusing and plasma defocusing. The number of cones and their angle with respect to the propagation axis increase with incident pulse energy. The uncertainty, which may be contained in the evaluation of plasma response and band-to-band transition rate, and the pulse disturbance by modulation instability are also analyzed. Although these influence the details of the pulse propagation, they do not affect the essence of our results: the multiple-cone formation.

Journal Article↗
PubMed2002

Generation and characterization of ultrafast white-light continuum in condensed media.

The generation of white-light continuum by femtosecond laser pulses in transparent condensed media is investigated comprehensively with 262-, 393-, and 785-nm pump wavelengths. We find that the ratio of the medium's bandgap energy to the photon energy of the incident wavelength determines the amount of anti-Stokes broadening, independently of the pump wavelength and the medium's bandgap. It is also shown that, although the amount of anti-Stokes broadening is greater for the longer pump wavelength, the shorter pump wavelength is more advantageous for generating shorter-wavelength continua in the UV region. In addition, a self-induced change in polarization of the white-light continuum that is generated is observed in an isotropic material with a cubic crystal structure, such as CaF2 and LiF. After the investigation of polarization, the frequency chirp of the continuum is characterized by the Kerr-gate method with 70-fs temporal and 10-nm wavelength resolution.

Journal Article↗