Efficient nonlinear conversion requires that interacting optical waves maintain a consistent phase relationship when traveling in a medium despite its dispersion. Birefringent phase-matching, which is often used to compensate for the dispersion, is not applicable to optically isotropic nonlinear materials. Here, we present a one-dimensional photonic crystal structure that allows the propagation of optical surface waves, both at the fundamental and third-harmonic frequencies, as an efficient medium for phase-matched third-harmonic generation. A unique advantage of this structure is that the effective refractive indices for the surface waves are similar to the refractive index of air at both frequencies. This allows phase-matching between the first and third harmonics, and a visible collinear beam of the third harmonic is produced at the prism-coupled output. Moreover, these optical surface waves propagate over long distances even if a lossy nonlinear nanofilm is deposited onto the photonic crystal surface. We provide experimental results for third-harmonic generation at a wavelength of 410 nm for a bare dielectric Ta2O5/SiO2 multilayer structure and for the same structure coated with a 15-nm GaAs film.
The scenario of the formation of light bullets in the presence of anomalous group velocity dispersion is presented within the same general scenario for condensed matter and humid air. The temporal and spectral parameters of light bullets during filamentation in fused silica and humid air are obtained. A light bullet (LB) is a short-lived formation in a femtosecond filament with a high spatiotemporal light field localization. The sequence formation of the quasi-periodical LB is obtained numerically and is confirmed experimentally by autocorrelation measurements of the LB’s duration. The estimation of the LB duration reaches few-cycle value. It is established that the generation of each LB is accompanied by the ejection of a supercontinuum (SC) in the visible spectrum and an isolated anti-Stokes wing is formed in the visible area of the SC as a result of destructive interference of broadband spectral components. It was found that the energy of a visible SC increases discretely according to the number of LBs in the filament. We demonstrated that the model of ionization in solid dielectric which is used in numerical simulation fundamentally affects the obtained scenario of LB formation. The possibility of the formation of LBs under the filamentation of middle-IR pulses in the atmosphere was shown with numerical simulation.
The formation of light bullets during femtosecond laser pulse filamentation in the presence of anomalous group velocity dispersion has been recorded for the first time. The minimum experimentally detected width of the light bullet autocorrelation function is 27 fs, which corresponds to a duration of about 13.5 fs. The duration of the light bullet at a wavelength of 1800 nm is about two periods of the light field oscillation. The numerically calculated width of the autocorrelation function for such a light bullet is 23 fs, which is in good agreement with the experimental value.
Femtosecond measurements of kinetics and spectra of absorbance changes (AA) were carried out with modified reaction centers (RCs) from Rhodobacter sphaeroides (R-26) from which nonactive bacteriochlorophyll BM (located in the M protein subunit) was removed. The band of BM at 800 nm in native RCs is shifted in femtosecond measurements and obscures the AA of active bacteriochlorophyll BL (L subunit). The spectrum of AA in modified RCs at 6 ps delay includes the bleachings of the bands of P (primary electron donor) at 870 nm, of BL at 805 nm and of HL (bacteriopheophytin located in the L subunit) at 755 mn showing the reduction of -0.5 mol BL and -0.5 mol HL per mol P+. These data confirm an earlier suggestion that BL participates as an electron acceptor in the light-induced primary charge separation and agree with recent X-ray analysis of Rhodopseudomonas viridis and R. sphaeroides RCs which shows a location of BL between P and HL.
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