Yoshihito Tanaka

We have developed a control system to synchronize intense picosecond laser pulses to the hard x-ray synchrotron radiation (SR) pulses of SPring-8. A regeneratively amplified mode-locked Ti:sapphire laser is synchronized to 40 ps SR pulses by locking the laser to the radio frequency of the ring. The synchronization of the pulses is monitored by detecting both beams simultaneously on a gold photocathode of a streak camera. This method enabled us to make a precise measurement of the time interval between the beams, even if the trigger of the streak camera drifts. Synchronization between the laser and the SR pulses has been achieved with a precision of +/- 2 ps for some hours. The stable timing control ensures the possibility of making two-photon excitation and pump-probe experiments with time resolution of a few tens of ps (limited by the pulse duration of the SR). We have used this system to show that closing undulator gaps in the storage ring shifts the arrival time of the SR pulses, in accord with expectations for the increased power loss. (C) 2000 American Institute of Physics. [S0034-6748(00)02803-3].

The spectral profiles of sum-frequency signal from CH vibrational modes of octadecanethiol (ODT) self-assembled on gold have been studied for several optical configurations of incident beams. The observed spectra, generally of the shape of dispersion type, have been interpreted by the interference between the resonant contribution from the CH stretching modes of adsorbed molecules and the nonresonant contribution from the gold substrate. We have shown for the first time that the contribution from the zzz component of the resonant nonlinear susceptibility chi(zzz)((R)) is dominant in the observed resonant signals, whereas all of the ijk components contribute to the nonresonant signal. The transition frequencies and the relative amplitude of resonant signals are also determined for the CH3 vibrational modes of ODT on gold.

Amplified spontaneous emission (ASE) from single rotational levels of Rydberg states, 4p sigma M (2) Sigma(+), 4p pi K (2) Pi, 5s sigma S (2) Sigma(+), 4d sigma O (2) Sigma(+), 4d pi O' (2) Pi, 5p sigma R (2) Sigma(+), and 5p pi Q (2) Pi of NO molecules has been investigated through optical-optical double resonance excitation via the A (2) Sigma(+) state. Several new inter-Rydberg transitions were identified in the midinfrared region. The rotational structure in dispersed ASE spectra was resolved completely, which provided detailed information on cascading radiative relaxation processes among Rydberg states. (C) 1998 American Institute of Physics. [S0021-9606(98)02603-8].

We use a novel picosecond source (a seeded optical parametric amplifier tunable near 3 mu m) to pump a type-II ZnGeP2 traveling-wave optical parametric generator. With a rather simplified two-pass arrangement, the tunability of the driving Ti:sapphire regenerative amplifier is extended continuously up to 11 mu m. As a result of the short pulse pumping and the pulse compression accompanying the parametric amplification, nearly bandwidth-limited pulses could be generated for the first time with this crystal. We report microjoule output energies with 20% quantum efficiency and unprecedentedly low (<100 MW/cm(2) for a crystal length of only 1 cm) parametric gain thresholds.

We report on a new scheme for seeded optical parametric amplification using the phase matching retracing behavior in a 7% MgO:LiNbO3 crystal to generate picosecond widely tunable pulses in the mid-infrared range. A regeneratively amplified 2 ps Ti:sapphire laser and a tunable 3 ns narrow-band optical parametric oscillator serve as pumping and seeding sources; respectively. By setting the crystal at a phase matching angle very close to the retracing point of the crystal for a pumping wavelength of 778 nm, a wide tunability from 3.35 to 3.85 mu m of 2.3 ps pulses at a very intense constant pulse energy of 60 mu J has been demonstrated by varying only the seeding wavelength from the nanosecond optical parametric oscillator. The constant energy range could be blue- or redshifted as a function of the pumping wavelength.

When the laser frequency was tuned through the C-2 Pi (upsilon = 0) <-- X(2) Pi (upsilon '' = 0) two-photon transition of NO, near-IR as well as UV radiations were ejected along the laser propagation with a narrow solid angle. It was shown that the UV wave corresponds to the A(2) Sigma(+) (upsilon' = 0) --> X(2) Pi (upsilon '' = 0) transition. Temporal profiles and polarization properties of output waves indicate that the parametric four wave mixing (PFWM) process is mainly responsible for the simultaneous generation of near-IR and UV radiations. Excitation spectra corresponding to the C-2 Pi (upsilon = 0) <-- X(2) Pi (upsilon '' = 0) band exhibited an excellent S/N ratio, implying that the PFWM technique might be utilized as a novel spectroscopic method which can interrogate the two-photon resonant levels.

Phase matching angles for sum and difference frequency mixing in a 7% MgO:LiNbO3 crystal of the radiation of a Ti:Sapphire laser and the radiation of 2.2 to 3.4 mu m from a Ramam shifter are used to determine the refractive indices of the crystal, A dispersion equation accurate in the midinfrared spectral range as well as in the visible and near infrared range is firstly given, Based on this result the phase matching characteristic of a mid-infrared optical parametric generator using 7% MgO:LiNbO3 crystal is described.

Time-of-flight spectra of He and Ar beams produced with a pulsed beam source of short open time (almost-equal-to 50-mu-s) are investigated as a function of the distance from the nozzle both experimentally and theoretically. The beam is found to be under non-equilibrium conditions even at a distance of longer than 1 m. The observed decrease of the contribution from slow molecules is explained on the basis of the Boltzmann equation with the effect of collisions within the beam.

Rotational transitions in NH3 induced by the dipole-quadrupole collisional interaction are studied by use of the supersonic molecular beam technique and the double-resonance spectroscopy. The measured state-to-state cross section agrees well with the cross section calculated nonempirically based on the Anderson-Tsao-Curnutte theory, which depends only on the dipole matrix element between the initial and final states of NH3. It is deduced that the state-to-all states cross section is independent of the molecular quantum states.

The energy distribution among rotational levels of NH3 molecules in a supersonic beam is measured at a position distant from the nozzle by the highly sensitive infrared laser Stark spectroscopic method. A nonequilibrium energy distribution is observed within the rotational degree of freedom. In particular, population inversion is observed on some rotational levels. Computer simulation based on a novel rate-equation model, in which the dipole-dipole interaction, detailed balance, and hard-sphere collisions are assumed, reproduces the observed results well. This model is able to characterize the internal energy distribution of a supersonic beam under various experimental conditions by means of a scaling parameter.