For the study of attosecond physics and petahertz electronics, it is necessary to measure precisely the optical waveform of short pulses, including the carrier-envelope phase. A promising approach is to use the optical field emission from metal nanostructure, where the electron tunneling from metal surface is driven by plasmonic near fields. However, there have been problems of low current levels and laser-induced damages of metal nanostructures. Here, we develop an all-solid-state optical-field detector based on metal-insulator hybrid nanostructures, which works in the nanojoule range. The photoelectric efficiency is substantially increased because of the lowered energy barrier for photoemission and the higher near-field enhancements originating from the metal-insulator-metal plasmon. Laser-induced damage resistance is also improved by encapsulating the metal nanoantennas with dielectric materials.
We demonstrate that metal-carbonyl compounds in liquid n-hexane are dissociated and adsorbed on gold surfaces upon vibrational excitation. We illuminate gold nanoantennas with temporally-shaped mid-infrared pulses to produce intense plasmonic near-fields. The produced near-fields induce vibrational ladder climbing and the resultant dissociation of metal-carbonyl compounds. A new band, cumulatively increase with irradiation, is attributed to the molecular species which are dissociated and adsorbed on gold surfaces. This demonstration proves that the plasmonic near-fields of midinfrared pulses are useful for mode-selective reaction control at electronic ground states and for possible subsequent manipulation of molecules like trapping and alignment.
Plasmonic enhancements of optical near-fields with metal nanostructures offer extensive potential for amplifying lightmatter
interactions. We analytically formulate the enhancement of linear and nonlinear optical responses of molecular
vibrations through resonant nanoantennas, based on a coupled-dipole model. We apply the formulae to evaluation of
signal enhancement factors in the antenna-enhanced vibrational spectroscopy.
Coherent spectral expansion of the mid-infrared femtosecond pulses is beneficial for monitoring and controlling
molecular vibrational dynamics. We investigate the spectral broadening of mid-infrared pulses due to nonlinear optical
effects in semiconductor materials. The mid-infrared pulses of 100 fs duration and 180 cm-1 bandwidth at the center
wavelength of about 5 micron are focused onto the semiconductor materials. With only few-micro-joule pulse energy,
the spectral broadening by a factor of more than 3 is observed for Si, Ge, and GaAs. The output spectral component
extends from 1500 cm-1 to 3000 cm-1. The intensity and the phase profiles of the self-phase modulated pulses are
characterized by the modified auto-interferometric autocorrelation method and its phase-retrieval algorithm, indicating
the spectral phase to be compensated for pulse compression.
Formation and relaxation dynamics of electron polarons in lithium niobate crystals is investigated by measuring transient
absorption induced by blue femtosecond pulses. Anisotropy in absorption change distinguishes between small free
polarons and small bound polarons, revealing that the dynamics is influenced by MgO-doping and stoichiometry control.
In crystals doped with MgO at concentrations above threshold, small free polarons are generated within 100 fs and decay
at tens of nanosecond. In the presence of antisite defects, sequential formation of polaronic states occurs: electrons
initially trapped as small free polarons become trapped as small bound polarons at picosecond time scale. The results are
relevant for nonlinear optical applications of pulsed or high-power lasers.
We experimentally demonstrate that adiabatic compression of femtosecond pulse can be achieved by employing the
management of quadratic cascading nonlinearity in quasi-phase-matching gratings. Cascading nonlinearity is not a
simple analogy with third-order optical nonlinearity in term of the engineering properties of the magnitude and focusing
(or defocusing) nonlinearity. Femtosecond pulse compression is investigated based on type-I (e: o + o) collinear QPM
geometry of aperiodically poled MgO-doped LiNbO3 (MgO: LN). Group-velocity-matching condition is chosen to
generate quadratic femtosecond soliton consisting of fundamental (FF) and second harmonic (SH) pulses. Adiabatic-like
compression process is observed in the length of 50 mm linearly chirped QPM. Cascading nonlinearity is local managed,
instead of dispersion management used in fiber adiabatic soliton compression. Quadratic soliton including FF and SH
pulses are obtained from the compression of 95 fs FF pulse in the initial experiments. Dependence on the phase
mismatch and group velocity mismatch, cascading nonlinearity has a flexible property and presents a new challenge for
exploring femtosecond pulse shaping and control. The demonstrated pulse compression and control based on cascading
nonlinearity is useful for generation of shorter pulses with clean temporal profiles, efficient femtosecond second
harmonic generation and group-velocity control.
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