It is essential to understand details of reaction mechanism for the efficient development of new reaction schemes and functional materials. Recent development of ultrashort laser pulse allowed us to observe the transient states in photochemical reactions. However, almost all chemical reactions in the synthesis of medical products and functional materials are based on the thermal reaction, and observation of transient states in the thermal reaction has been considered impossible. The photoreaction occurs in the electronic excited state after photo irradiation, while thermal reaction occurs through activation of molecular vibrations in the electronic ground state. We assumed that it is possible to observe the thermal reaction by inducing molecular vibrations through Raman process using laser pulses with the photon energy being lower than the electronic transition levels. Previously, we have observed transient states in photochemical reaction. (Real-time spectroscopy of transition states in bacteriorhodopsin during retinal isomerization, T. Kobayashi, T. Saito, and H. Ohtani Nature、414、531-534 (2001)). In the present study, we observed for the first time the transient states in thermal reaction of chloroform with oxygen on the basis of the method described above. The details were published in Chem. Phys. Lett.
Carrier-envelope phase in ultrashort pulse is one of the most important factors in the nonlinear spectroscopy, especially for generation of soft X ray. Furthermore, stabilization of the carrier-envelope phase is essential for an optical frequency standard, and researchers in German and USA received the Novel prize for it. Our group has proposed and demonstrated new methods to improve the phase sensitivity by orders of magnitude by means of carrier-envelope phase-controlled quantum interference technique （“Carrier-envelope phase-controlled quantum interference in optical poling,” S. Adachi and T. Kobayashi, Phys. Rev. Lett. 94, 153903（2005）. In the previous study, we employed dye molecules dispersed in polymer for the quantum interference measurement. However, the dye density was low and thus the signal was not sufficient. In the present study, we employed a new polymer to gain high density film by attaching the dye to the side-chain of the polymer. This material significantly improved the signal intensity and shortened measurement time below tithe. Furthermore, the pulse duration of the second harmonics of the idler with stabilized carrier-envelope phase from the NOPA was successfully compressed with a wedge quartz plate and a deformable mirror used to compensate the low- and high-order group dispersion, respectively. The pulse was characterized by using a grating, a cylindrical collimating mirror and a telescope, and the pulse duration was found to be 2.4 fs, which is close to the Fourier transform limit of 2.2 fs.
Initial deformation process of bacteriorhodopsin is different from that assumed over thirty years. Ultrafast dynamics of bacteriorhodopsin was clarified experimentally and theoretically. We performed real time measurement of the amplitude and phase change of molecular vibrations in the isomerization process at 128 wavelengths, and found that retinal conformation changes within 30 fs after photo excitation around the C=N bound in the Schiff base. Furthermore, time-resolved measurement was performed to study the photo-dissociation process of the oxygen from the oxyhemoglobin. It appeared that the oxygen molecule dissociates soon after the photo-excitation with the exponential decay time of 45 fs. It also appeared that the frequency of the vibrational modes between the iron ion and the oxygen gradually decreases due to the dissociation. We observed for the first time the dissociation process by the real time measurement.
(4-1) Multicolor ultrashort pulse generated by cascade four-wave mixing
The understanding of physiological and pathological phenomena is a matter of urgency to meet the needs in the aging society. Vital phenomena arise from the complex interactions among many proteins in a single as well as a number cell. Thus, it is important to observe the collective phenomena of cells to study the signaling and its interactions among the related cells. The collective processes among proteins, signal molecules and cells arise from their well-ordered communication system. Fluorescence imaging by using a laser is the most general method employed for this study. Spatial distribution of molecules can be easily visualized by labeling the target low-molecular compounds or biological molecules with fluorescence dyes. In this study, we developed multicolor ultrashort pulses suited for the fluorescence imaging using five fluorescence proteins as a light source for the nonlinear laser microscopy. Up to fifty-color beams are available by this system. In contrast, two or three color beams were used before. Furthermore, the special coherences of the beams are close to the theoretical limit. It is expected that we can observed five fluorescence proteins simultaneously by using the multicolor beams and the fundamental IR beam as a light source for two photon excitation.
The principle of method is not based on spontaneous emission which has been widely used for the fluorescence imaging before, but stimulated emission; The Stokes beam of the multicolor beams is used for two photon excitation of fluorescence proteins or quantum wells, while the remaining anti-Stokes beams are used to induce stimulated emission from the excited state. Since photon energy absorbed by the molecule is then derived through stimulated emission, phototoxic effects and photo-degradation due to the relaxation to long-lasting triples states through level crossing are reduced. Therefore, this method is bio-friendly.
In contrast, super continuum light, which is widely used, have many detects for the microscopy such as the unsuitable spectral profile, poor intensity stability of 20% (rms), and low light intensity. Optical parametric amplifier, an alternative wavelength-tunable light source, is also not suitable because only one color is available and the system is expensive and difficult in the daily use. The multicolor beams employed in this study are wavelength tunable by simply rotating a nonlinear media. Furthermore, spectral widths and the number of color are also tunable, and the intensity stability is 0.95% (rms), so that this new light source is suitable for the multicolor imaging.
(4-2) Development of high-performance ultra-fast pulse laser by degenerate four-wave mixing
High-intensity lasers are presently studied at a lot of research institution in various countries. Researches on interactions between the high-intensity laser and material are particularly expected to induce development of novel fields in material science. The high-intensity lasers are required to be clean in the time domain for the application to material science. In fact, when the pulses are not clean, the satellite pulses of the high-intensity lasers cause degradation of the materials through nonlinear interaction between the satellite pulses and the materials because of significantly high intensity of the satellites. In such case, the observations reflect the interactions between the pulses and the degraded materials.
The high-intensity lasers are expected in medical fields for the application to the ion acceleration. The lasers are required to be clean for the application; otherwise it is impossible to generate high-energy ion beams. Therefore, clean high-intensity pulse lasers with no satellite pulses in time domain are required. The highest enhancement ratio of contrast in the world has been 105 with the traditional pulse-cleaning technology, and the highest contrast has been 10 – 14. In the traditional technology for the highest enhancement ratio of contrast, birefringence is utilized, and the contrast is restricted by the extinction ratio of birefringence prisms. The highest value of the extinction ratio of existing prisms is 10 – 5, and it has determined the upper limit of the enhancement ratio of contrast. It is almost impossible to improve the value.
We have been succeeded to develop novel technology for the enhancement with degenerate four-wave mixing. This technology is a completely different approach from that noted above. This method is not restricted by the extinction ratio of the prisms, and the highest enhancement ratio of contrast in the world, 105 has been readily obtained with the method. The current restriction of the value is caused by scattering by the nonlinear medium. The reduction of the scattering, however, is possible with current technology, and the enhancement of 108 is possible with existing materials. The method has a potential to output the constant ratio of 10 – 13.
Broad-band ultraviolet pulses have been often generated by the four-wave mixing (2ω+ 2ω - ω = 3ω) between near infrared pulse (800 nm; ω) and near ultraviolet pulse (400 nm; 2ω) in a hollow fiber, while the band width is not sufficiently broad. It is also significantly difficult to generate ultra-fast pulses because of impracticability for complete compensation of the group velocity dispersion caused by large refractive indices in the ultraviolet region.
We have generated broad-band near infrared pulses with self-phase modulation in a hollow fiber, and controlled the phase with the group velocity dispersion in a transparent medium. We have also controlled the phase of near ultraviolet pulses with a prism pair. We have indirectly controlled the phase of deep ultraviolet light generated by the four-wave mixing with the phase controls noted above. Sub-ten femto second deep ultraviolet pulses have been generated by the facile and precise control the phase of the broad-band deep ultraviolet light. We have also generated near ultraviolet 8 fs ultra-short pulses with the spectral expansion by the self-phase modulation in the hollow fiber and the precise compensation of group velocity dispersion by chirp mirrors or a deformable mirror.
In the consequence of the developments, sub-10 fs pulses have been generated in the ultraviolet region of 260 – 290 nm and 360 – 440 nm. These light sources are suited for spectroscopy because of a low ratio (less than 5%) of energy of satellite pulses.
The above study is being deepened as a project of “Study of the imaging and photo-activation of cellular systems, and mechanism of photo-damage and uncaging of caged molecules” supported by the JST: CREST, a project of “Development of pre- and post-pulse free ultra-high intensity laser pulse and study of attosecond intra-molecule dynamics” with the Max-Planck-Institute supported by the Humboldt financial group, and the project of “Study of ultra-fast chemical reaction” with the Max-Born-Institute.