Takao Fuji

Visible pulse compression to 4 fs by optical parametric amplification and programmable dispersion control.

Angular dispersion of pump frequencies is shown to be an efficient mechanism for bandwidth enhancement in a noncollinear optical parametric amplifier. We demonstrate the generation of a continuous, simultaneously phase-matched 250-THz parametrically amplified spectrum. The resultant visible-near-IR signal-wave pulses were compressed to a 4-fs duration by a micromachined flexible mirror. Feedback for an iterative computer-controlled dispersion compensation algorithm is based on pulse characterization by second-harmonic generation frequency-resolved optical gating.

Self-referencing of the carrier-envelope slip in a 6-fs visible parametric amplifier.

We demonstrate a scheme for parametric amplification that allows us to measure the drift of the carrier-envelope phase of the output signal pulses. The method is based on the unique double phase-matching properties of a noncollinearly pumped BBO crystal, making possible the detection of the interference between the signal and the frequency-doubled idler. Additionally, the suggested device greatly simplifies the single-shot measurement of the phase evolution in Ti:sapphire laser amplifiers by dispensing with harmonic synthesis from the spectral edges of an octave-wide supercontinuum.

SHG FROG and XFROG methods for phase/intensity characterization of pulses propagated through an absorptive optical medium

Abstract Second-harmonic generation frequency-resolved optical gating and cross-correlation frequency-resolved optical gating were used as a characterization method of the phase and amplitude of pulses propagated through a 0.5-mm-thick plate of Nd 3+ -doped glass. Both experimental results of field intensity and phase were in fair agreement with the calculated result.

Dynamical observation of Duschinsky rotation by sub-5-fs real-time spectroscopy

Abstract Excitation-induced modulation of vibrational frequency and amplitude in dye molecules was studied using sub-5-fs pulses. The frequency and amplitude modulations of the ring-breathing mode were clearly observed, indicating that the mode is coupled to another vibrational mode. The common modulated frequency and the depth of phase modulations were determined from the instantaneous frequency and amplitude. The two-dimensional (2D) potential surfaces were determined using a simple model of two-mode coupling. The anharmonicity appearing in the potential surfaces rep-resents the Duschinsky effect.

First observation of Auger-process induced anharmonic oscillation in molecular systems under high-density excitation

Abstract Unusually intense overtones of three vibrational modes were found just after high-density excitation of phthalocyanine microcrystal film and they decay approximately with a power-law ( t 2 ) due to the Auger-type bi-excitonic quenching which induces anharmonic vibration. The decay is associated with vibrational relaxation in the lowest excited singlet state (exciton) after the internal conversion from a higher excited state (excited exciton). The observation of the high-frequency-overtone has become possible by feeding the higher vibrational levels by the Auger process, since small Franck-Condon factor do not allow population of the anharmonic vibrational levels by direct excitation.

Toward a terawatt few-optical-cycle driver laser for attosecond spectroscopy

We discuss routes towards developing an ultra-high peak power phase-stable source of few-cycle laser pulses suitable for driving a wide range of strong-field applications. Experiments with a phase-stable 0.1-TW 5-fs system based on a Ti:sapphire amplifier and the progress in construction of a 1-TW few-cycle optical parametric amplifier are presented.

Vibrational phase characterization in femtosecond-pumped molecules by path-length modulation

The ultrafast dynamics of a cyanine dye is studied by the pump-probe spectroscopy using 20 fs pulses establishing a high sensitivity on the molecular vibration. The simulation reproduces the observed phase dependence on the mode frequencies.

Generation of ultra broadband spectra from a mirror-dispersion controlled Ti:sapphire oscillator

Recently a lot of applications of femtosecond Tksapphire lasers are demonstrated in various regioiis of science and technology. In the most applications, broadness and flatness of the spectrum of the laser pulse are very iniportant parameter for improving the potential of the equipnients. The broadest spectrum directly from the Ti:sappbire laser consisted by chirped mirrors, prisms, and a glass for additional self-phase modulation is over 1 octave‘. However, the spectrum is consisted by several sharp peaks. Therefore, a lot of satellite pulses exist around the main pulse. This feature is not very nice for most applications. In this contribution, we report that generation of ultra broadband and smooth spectrum with a Tksapphire laser in which prism pairs are not included. This oscillator is just modified by putting chirped concave mirrors instead of ordinary high reHective concave mirrors and putting wedges for controlling the dispersion in the cavity very precisely into a mirror-dispersion controlled oscillator supplied by FemtoLasers GmbHZ. The advantage of prismless laser is that the cavity can be designed more compact. The laser is pninped by a 532-nin laser (Coherent Verdi). The thickness of the crystal is 2.5 mm. The spectrum generated by the oscillator is shown in Fig. 1. The bandwidth of the spectrum is 2G5 nni in full width of half maximuin (FWHM). The fine structure in long wavelength region is caused by vapor absorption and/or the group delay dispersion structure of chirped mirrors in the cavity. The spectrum stays for at least 10 hours. This period is caused by accumulation of dust in the optics. Average power is 250 mW for a pump power of 3.65 W and repetition rate is 64 MHz. The transmission of the output coupler is 10%. This broad and smooth spectrum is probably caused by strong Kerr effect in the Ti:sapphire crystal. This effect is successfully occurred by strong focusing on the crystal with small radius of curvature concave chirped mirror ( T = 50 mm). The output pulses are compressed by 6 reflections of chirped mirrors, and the temporal characteristics of the pulses are measured using interferometric autocorrelator (FemtoLasers). The dots in Fig. 2 shows the measured interferometric autocorrelation trace (IAC). The solid line shows the reconstructed IAC trace by a phase retrieval algorithm. The FWHM of the intensity envelope is estimated as 6.5 fs. Assuming a Hat phase, the FWHM of the pulses would be 5.0 fs. The reason why we cannot compress the pulse is due to nonlinear frequency dependence of the spectral phase. This nonlinearity would be caused by strong phase modulation in the Ti:sapphire crystal. Nevertheless, we can say that the sDectrum is coherent. This laser is useful for various applications, spatially optical coherence tomography

Measurement and self-stabilization of carrier-envelope phase drift by use of an optical parametric amplifier

Summary form only given. We show that severe technical problems associated with the practical implementation of the active carrier-envelope phase (CEP) control at kHz repetition rates can be readily obviated by employing unique properties of parametric amplification. We prove that the self-stabilization of the CEP is achieved in an OPA seeded with a white light that is derived from the pump pulse. The mechanism enabling this regime relies on a combination of three principal effects: 1) The pulse-to-pulse fluctuation of CEP (although not the CEP value itself), is inherited by the white-light continuum pulse from the input laser pulse. This fact is widely utilized in the CEP drift measurement based on the generation of octave-spanning spectra and creating a spectral overlap with the second-harmonic (SH) radiation. 2) In a similar way, in a seeded optical parametric amplifier, the CEP fluctuation is passed from the seed wave onto the amplified signal. This is a characteristic feature of white-light seeded OPAs only, since the CEP value of the signal wave is random in an OPA seeded by superfluorescence. 3) The phases of all three pulses taking part in a parametric interaction are linked, which can be described by a simplified phase equation. The phase properties of different white-light seeded OPAs are summarized. We believe that this work provides a new insight into the experimental capabilities of various OPA systems and opens attractive possibilities of their use in the applications of nonlinear optics with demands for sub-fringe resolution.

Photon echo signature of vibrational superposition states created by femtosecond excitation of molecules

A pair of coherent femtosecond pulse excitations applied to a molecule with strong electron-phonon coupling creates a coherent superposition of a low-momentum and a high-momentum wavepacket in the vibrational states of both the excited state and the ground state of the coherent transition. As the excited state is accelerated further, interference between the high-momentum ground state contribution and the low-momentum excited state contribution causes a photon echo. This photon echo is a direct consequence of quantum interference between separate vibrational trajectories and can therefore provide experimental evidence of the non-classical properties of molecular vibrations.