Satoshi Ashihara

In the liquid phase, water molecules form a disordered fluctuating network of intermolecular hydrogen bonds. Using both inter- and intramolecular vibrations as structural probes in ultrafast infrared spectroscopy, we demonstrate a two-stage structural response of this network to energy disposal: vibrational energy from individually excited water molecules is transferred to intermolecular modes, resulting in a sub-100 fs nuclear rearrangement that leaves the local hydrogen bonds weakened but unbr

We describe efficient soliton compression of femtosecond pulses by use of cascade quadratic nonlinearity and normal dispersion in quadratic media. Pulse compression by a factor of ∼3 is achieved in ∼30-mm-long beta-barium borate at a wavelength of 800 nm. We investigate the dependence of compression performance on phase mismatch, input intensity, and propagation length. The compressed pulses are fully characterized by use of the frequency-resolved optical gating method.

We propose a novel scheme of achromatic phase matching for second-harmonic generation (SHG) in tilted quasi-phase matching gratings. The spectral angular dispersion is introduced in fundamental waves such that each frequency component satisfies the two-dimensional quasi-phase matched condition. This is equivalent to simultaneous quasi-phase- and group-velocity-matched SHG for ultrashort pulses. Equations to describe achromatic conditions are derived and applied specifically to periodically poled

Efficient pulse compression was achieved by the use of cascaded quadratic nonlinearities in 10-mm-long periodically poled MgO-doped lithium niobate. An off-diagonal component of the nonlinear coefficient was utilized for group-velocity matched cascaded interactions at the fundamental wavelength of 1560 nm. Simultaneously compressed fundamental and second-harmonic pulses of ∼35 fs duration were obtained from the 110-fs-pump pulses.

Vibrational strong coupling (VSC) between a vacuum field and molecules in a cavity offers promising applications in cavity-modified chemical reactions and ultrasensitive vibrational spectroscopy. At present, in order to realize VSC, bulky microcavities with large mode volume are utilized, which limits their potential applications at the nanoscale. Here, we report on the experimental realization of strong coupling between molecular vibrations and infrared photons confined within a deeply subwavel

We achieved efficient spectral broadening for mid-IR pulses of few-microjoule energy. The spectral bandwidth of the femtosecond pulses at the center wavelength of 5000 nm increased from 540 nm to 2060 nm (from 220 to 910 cm(-1) in frequency) by nonlinear propagation in a gallium arsenide single crystal. The spectral broadening was accompanied by nonlinear absorption loss of 25%. The demonstrated scheme should be available at any operation wavelength within the material transparency range and pro

A nonlinear shift in the dispersion relation of a surface phonon-polariton (SPhP) is observed with grating-coupled pump–probe reflection spectroscopy. Upon excitation of an SPhP on a 4H-SiC surface, an instantaneous frequency shift of the SPhP mode at a constant wavevector is observed. This pump-induced frequency shift is equivalent to a nonlinear dispersion shift and to a Kerr-like nonlinear phase shift. The effective nonlinear index is evaluated to be orders of magnitude larger than the typica

Femtosecond pulses with the bandwidth in excess of 600 nm in 3–4 µm spectral range were generated by an optical parametric amplifier based on periodically-poled MgO:LiNbO3. We utilized broadband quasi-phase-matching condition, which appears around the idler wavelength of 3400 nm when pumped at 800 nm. The generated mid-infrared pulses were characterized by the frequency-resolved optical gating method in the second-harmonic generation geometry. After the second-order spectral phase was compensate

Large nonlinear phase shifts were generated with femtosecond pulses at 1560 nm through cascaded quadratic interactions in periodically poled MgO-doped LiNbO3. The off-diagonal component of the nonlinear coefficient was utilized for simultaneous quasi phase matching and group-velocity matching. The effective nonlinear refractive index was varied from -2.9 x 10(-14) to +3.3 x 10(-14) cm2/W by tuning the phase-mismatch conditions.