Generation of few-cycle pulses and control of the spatiotemporal phase
Mono-
or few-cycle ultrashort pulses can be generated by broadening the
spectrum by self-phase modulation followed by dispersion
compensation. In particular, the use of gas-filled hollow fibers
is effective in broadening the spectrum and obtaining excellent beam
quality. Ultrashort pulses are attracting much attention as a
light source for generating attosecond x-ray pulses. However,
when the incident energy into a hollow fiber becomes more intense,
spectral broadening by self-phase modulation can be impeded due to
self-focusing and multi-photon ionization of the beam. In order
to overcome this problem, we have developed a pressure-gradient hollow
fiber technique. As a result, few-cycle pulses with peak powers
as high as 1 TW were obtained. For further intensification,
multi-photon ionization occurring in the hollow fiber needs to be
avoided. As a solution, we have proposed the use of chirped
pulses to generate intense few-cycle pulses, and our research is now in
progress.
Generation of coherent soft-x-ray pulses by high-order harmonic conversion
Coherent soft-x-ray and extreme-ultraviolet pulses can be generated by high-order harmonic conversion of a femtosecond laser. Since high-order harmonics exhibit excellent special coherence and focusability, it is expected that these be utilized as short wave coherent pulse sources for stimulating ultrafast nonlinear optical phenomena. In addition, the 2.3 -4.4 nm range, so called the‘water window’since water molecules are transparent at these wavelengths, is extremely effective for in vivo biomolecule imaging. In order to generate such high-order harmonics with high-conversion efficiencies, it is important to achieve the phase matching between the fundamental wave and the high-harmonics. In our laboratory, research has been carried out in generating coherent soft x-ray pulses utilizing various phase-matching technologies, in addition to developing intense few-cycle pulse sources as the fundamental wave.
Multi-photon bioimaging and nonlinear spectroscopy of fluorescent proteins
Imaging technologies with which the molecular dynamics and interactions inside living cells can be observed are essential for research and development in the life sciences. Fluorescent imaging that utilizes fluorescent proteins plays a pivotal role in leading-edge technologies. In recent years, photo-functional fluorescent proteins that enable photo activated switching of the fluorescence or photo-conversion of the fluorescent wavelength have been realized. Thus, a variety of fluorescent imaging technologies have been developed. However, photobleaching of fluorescent proteins, which occurs with continuous irradiation, causes irreversible decay of the fluorescence intensity. Photobleaching of fluorescent molecules is an outstanding crucial issue in two-photon excitation fluorescent imaging.