Search Results (2)

Recent discoveries in nonlinear optical properties of nanoparticle colloids make actual the challenge to lower the energy threshold of phase conjugation and move it into the domain of shorter pulse widths. A novel effect of the stimulated Rayleigh-Mie scattering (SRMS) in two-photon absorbing nanocolloids is considered as a promising answer to this challenge. We report the results of experimental and theoretical study of the two-photon-assisted SRMS in Ag and ZnO nanocolloids in the nanosecond-to-picosecond pulse width domain. For 12 ns 0.527 μm laser pulses, the four-wave mixing SRMS scheme provides lasing and amplification of backscattered anti-Stokes signal in Ag nanocolloids in toluene at the threshold 0.2 mJ and the spectral shifts up to 150 MHz. For 100 ps 0.532 μm pulses, we observed for the first time efficient (over 50% in signal-to-pump ratio of pulse energies) SRMS backscattering of the anti-Stokes signal in Ag nanocolloids in toluene and predominantly Stokes signal in ZnO nanocolloids in water, with the spectral shifts up to 0.25 cm⁻¹. We develop the first order-in-perturbation model of the four-wave mixing two-photon absorption-assisted SRMS process which shows that at nanosecond pulses, amplification is predominantly due to the thermal-induced coherent oscillations of polarization while the slow temperature wave acts also as a dynamic spatial grating which provides a self-induced optical cavity inside the interaction region. At a picosecond pulse width, according to our model, the spectral overlap between pump and signal pulses results in formation of only the dynamic spatial temperature grating, and we succeeded at recovering the linear growth of the spectral shift with the pump power near the threshold. Full article

We develop theoretical background for the new nonlinear optical phenomenon of narrowly directed monochromatic IR conical emission which has been recently observed when 248-nm UV filaments propagate in xenon (V. D. Zvorykin, et al., Laser Phys. Lett. 13, 125404 (2016)). We treat it as coherent stimulated four-wave mixing process in which two pump KrF laser photons are converted into the coupled pair of resonance IR(828 nm) and VUV (147 nm) photons through $5p^5{(}^2{P}_{3/2})6p{[1/2]}_0\to 5p^5{(}^2{P}_{3/2})6s{[3/2]}_1^{\mathrm{o}}$ and $5p^5{(}^2{P}_{3/2})6s{[3/2]}_1^{\mathrm{o}}\to$${}^1{S}_0$ transitions. We explore the coherent interaction regime which proceeds at a time scale shorter than transverse relaxation time $T_2$. The momentum and energy conservation laws determine the characteristic angle of conical emission. We find that the threshold of this coherent process is determined by the KrF laser pump pulse area. Full article