[show abstract][hide abstract] ABSTRACT: 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.
[show abstract][hide abstract] ABSTRACT: Large-scale transmission gratings were produced for a stretcher and a compressor in the Yb-fiber chirped-pulse amplification system. A 23-W, 200-fs laser system with a 10-MHz repetition rate was demonstrated. Focused intensity as high as 10<sup>14</sup> W/cm<sup>2</sup> was achieved, which is high enough for multi-photon processes such as high-order harmonics generation and multi-photon ionization of neutral atoms. High-order harmonics up to 7th order were observed using Xe gas as a nonlinear medium.
[show abstract][hide abstract] ABSTRACT: We report the generation of coherent deep ultraviolet light at 199 nm with a maximum average power of 32 mW by frequency quadrupling a picosecond pulse train from a Ti:sapphire laser. Two frequency-doubling stages were taking place in resonant cavities that can enhance conversion efficiency of second harmonic generations (SHGs). To reduce the extra-loss of the resonant cavity, a Brewster-input KBe2BO3F2 (KBBF) prism coupling device (PCD) was designed and employed in the second stage SHG for the first time.
[show abstract][hide abstract] ABSTRACT: Time- and angle-resolved photoemission spectroscopy is performed on the
doped Bi2Se3^topological insulator. We observe
unusual variation in the efficiency of photoemission from
femto-to-picosecond non-equilibrium particularly when two-dimensional
electron gas (2DEG) states are developed on surface, while the surface
confinement potential is virtually unchanged. The results indicate that
a surface sheet polarization, which is induced nonlinearly by both the
photon field and inversion-symmetry-breaking field, grows in magnitude
as the 2DEG states become pronounced and opens a so-called surface
photoemission channel, divA, that can be varied transiently. Matrix
element effects investigated by linearly-polarized angle-resolved
photoemission also supports the presence of divA. The asymmetric charge
distribution developed around vacuum-surface interface is considered as
a key to understand and control Rashba splitting of the 2DEG states.
[show abstract][hide abstract] ABSTRACT: We have studied the electronic structure of optimally doped
FeTe0.6Se0.4 (Tc = 14.5 K), using
laser-excited angle-resolved photoemission spectroscopy (laser ARPES).
We observe sharp superconducting coherence peaks in the hole band
slightly shifted from the γ point at T = 2.5 K. In contrast to
earlier ARPES studies but consistent with thermodynamic results, the
momentum dependence shows a (4) modulation of the SC-gap anisotropy. In
addition, we found an electron band at the γ point, lying just
above EF. This electron band also shows a sharp
superconducting coherence peak with gap formation below Tc.
The hole and electron bands show significantly different values of
superconducting gap δ and Fermi energy ɛF ,
while the associated Bogoliubov quasiparticle dispersions get merged.
The results suggest composite superconductivity in an iron-based
superconductor, consisting of strong-coupling Bose-Einstein condensation
(BEC) in the electron band while the hole band superconductivity lies
closer to the weak-coupling Bardeen-Cooper-Schrieffer (BCS) limit.
[show abstract][hide abstract] ABSTRACT: Ultrashort pulses with a 25-μJ output energy were generated at 200 nm by dual broadband frequency doubling with a thin KBe(2)BO(3)F(2) (KBBF) crystal at 1 kHz as the fourth harmonic of a high power Ti:sapphire laser. The spectrum was broadened to a spectral width of 2.25 nm. The pulse duration of 56 fs was measured by single-shot autocorrelation with two-photon fluorescence from self-trapped excitons in a CaF(2) crystal.
[show abstract][hide abstract] ABSTRACT: We developed 18-W, femtosecond Yb-fiber laser system at 10-MHz repetition rate with large-scale transmission gratings for chirped-pulse amplification. Focused intensity was high enough for highly nonlinear processes such as ionization of atoms.
[show abstract][hide abstract] ABSTRACT: We present a quasi-cw laser in vacuum ultraviolet region at megahertz
repetition rate. The narrowband pulses generated from an ytterbium-fiber
laser system at 33 MHz repetition rate at the central wavelength of 1074
nm is frequency-converted by successive stages of LBO crystals and KBBF
crystals. The generated radiation at 153 nm has the shortest wavelength
achieved through phase-matched frequency conversion processes in
nonlinear optical crystals to our knowledge.
[show abstract][hide abstract] ABSTRACT: Multimillijoule, few-cycle, carrier-envelope-phase (CEP)-locked, near-IR pulses at 750 nm from an optical parametric chirped-pulse amplifier are applied to the generation of CEP-dependent, soft x-ray high harmonics around the boron K-edge at 188 eV. The dependence on the CEP manifests the phase coherence of high harmonics preserved in the highest-photon energy ever reported. Multimillijoule optical pulses also allow the extension of the cutoff energy up to 325 eV, exceeding the carbon K-edge of the water window. However, in this spectral range, the CEP dependence of harmonic spectra is not observed, suggesting the degradation of temporal coherence due to the heavy ionization of helium atoms.
[show abstract][hide abstract] ABSTRACT: We present a quasi-cw laser in a vacuum ultraviolet region at megahertz repetition rate. The narrowband pulses generated from an ytterbium-fiber laser system at 33 MHz repetition rate at the central wavelength of 1074 nm are frequency-converted by successive stages of LiB(3)O(5) crystals and KBe(2)BO(3)F(2) crystals. The generated radiation at 153 nm has the shortest wavelength achieved through phase-matched frequency conversion processes in nonlinear optical crystals to our knowledge.
[show abstract][hide abstract] ABSTRACT: We report on octave-spanning OPA of IR pulses from 1100 to 2200 nm using BIBO and 800-nm pump. We confirmed that 10-μJ OPA output pulses were CEP-stabilized using an f-to-2f interferometer without spectral broadening.
[show abstract][hide abstract] ABSTRACT: 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.
[show abstract][hide abstract] ABSTRACT: By frequency quadrupling a picosecond pulse train from a Ti:sapphire laser with a wavelength of 798 nm and a repetition rate of 82 MHz, we generated a coherent deep ultraviolet light below 200 nm. Two frequency-doubling stages take place in resonant cavities that can increase conversion efficiency. This allows an overall efficiency of 4.3% and produces an output power of up to 60 mW. In the second stage, in which a KBe2BO3F2 (KBBF) crystal was employed as a nonlinear medium, conversion efficiency is increased by about three times compared with that of the single-pass configuration. Taking into account reflective loss, conversion efficiency is improved five times.
Japanese Journal of Applied Physics 01/2011; 50. · 1.07 Impact Factor
[show abstract][hide abstract] ABSTRACT: Coherent quasi-cw radiation at 153 nm is generated at 33 MHz repetition rate by successive frequency conversion of the output from Yb-fiber-based laser system using two LBO crystals and two KBBF crystals. OCIS codes: (140.7240) UV, EUV, and X-ray lasers; (190.2620) Harmonic generation and mixing.
[show abstract][hide abstract] ABSTRACT: A novel transmission grating with a large scale (170 mm long, 40 mm high, and 1 mm thick) has been developed. The diffraction efficiency is over 93% and is almost flat from 750 to 850 nm. We developed a 1-kHz, 10-mJ, 0.5-TW Ti:sapphire laser system by using transmission gratings in a compressor. The throughput of the compressor was about 70%. The large-scale grating is capable of increasing the peak power up to multi-terawatts at 1 kHz.
[show abstract][hide abstract] ABSTRACT: Compensation of the intracavity dispersion in the mode-locked oscillator is known to be one of the most important factors for ultrashort pulse generation. However, recent investigations of a Yb-doped fiber mode-locked oscillator revealed that precise third-order dispersion (TOD) compensation is not always necessary for ultrashort pulse generation, owing to the strong nonlinearity that compensates residual TOD without reducing its spectral bandwidth. The origin of the nonlinear TOD compensation has remained unclear. To investigate the process in detail, we studied the pulse evolution inside a 30 fs Yb-doped fiber mode-locked oscillator both experimentally and numerically, and we found that the nonlinear phase shift with a temporally asymmetric pulse shape introduces an appropriate amount of TOD that exactly cancels the residual cavity dispersion.
[show abstract][hide abstract] ABSTRACT: We demonstrated a 1.3-GHz, Yb:KYW Kerr-lens mode-locked oscillator and a chirped-pulse amplifier system by using a double-clad Yb-doped fiber. The pulse duration of 180 fs was obtained with the average power of 20 W.
[show abstract][hide abstract] ABSTRACT: We have investigated the pulse evolution in a 30-fs Yb-doped mode-locked oscillator experimentally. We found that nonlinear-phase shift plays an important role for the passive third-order dispersion compensation.
[show abstract][hide abstract] ABSTRACT: We demonstrate the carrier-envelope phase (CEP) stabilization of the 5-fs, multi-mJ parametric chirped-pulse amplification system. The slow CEP drift induced in the parametric chirped-pulse amplifier was precompensated in the phase-control loop of the seed oscillator, and arbitrary CEP control of the whole system was realized over a few hours. We discuss several possible origins that induce CEP noise of the amplified pulses.
Japanese Journal of Applied Physics 01/2010; 49(3). · 1.07 Impact Factor
[show abstract][hide abstract] ABSTRACT: We have obtained an average output power as much as 1.2 W at 200 nm by using a 2.71-mm thick KBe2BO3F2 crystal optically contacted by CaF2 and SiO2 prisms on the both sides. Watt-level average-power was generated tunably in the deep ultraviolet region from 185 nm to 200 nm. The average power is the highest, to our knowledge, ever obtained by nonlinear crystals in the wavelength region below 200 nm.