Overcoming refractive index limit of mesoscale light focusing by means of specular-reflection photonic nanojet.

The physical origin of subwavelength photonic nanojet in specular-reflection mode (s-PNJ) is theoretically considered. This specific type of photonic nanojet (PNJ) emerges upon double focusing of a plane optical wave by a transparent dielectric microparticle located near a flat mirror. For the first time, to the best of our knowledge, we report a unique property of s-PNJ for increasing its focal length and intensity using circular-shaped microparticles with a refractive index ratio exceeding the known limiting value that fundamentally distinguishes s-PNJ from regular PNJ behavior. This may drastically increase the trapping potential of PNJ-based optical tweezers.

Engineering polyshell spherical microcapsules for optimal localized light absorption.

Optical radiation absorption in a polylayer spherical microparticle simulating an inorganic/organic polyshell absorbing microcapsule is considered. With the aim of the finite-difference time-domain technique, the spatial distribution of the absorbed light power in microcapsules of various sizes and internal structure is numerically calculated. For the purpose of light absorption enhancement, we have engineered the optimal structure of a capsule consisting of a strong-refracting transparent outer coating and an absorbing layer which covers a liquid core. The proposed microcapsule prototype provides for a manifold increase in the absorbed light power density in comparison with the usual single-layer absorbing capsule. We show that for light-wavelengths-scaled microcapsules it is optimal to use a material with the refractive index larger than two as an outer shell, for example, titanium dioxide (TiO2). The highest values of the absorbed power density can be obtained in microcapsules with absorbing shell thickness of approximately a tenth of a laser wavelength.

Dynamics of CO(2) laser pulse filamentation in air influenced by spectrally selective molecular absorption.

The theoretical aspects of self-focusing and filamentation of high-power pulsed CO(2) laser radiation with carrier wavelength 10.6 µm in air are considered. The spectrally selective molecular absorption of realistic atmospheric air is included in the theoretical model. In the conditions of strong pulse self-phase modulation and pulse spectral broadening, the supercontinual radiation spectrum is substantially influenced by the selective atmospheric absorption that destabilizes the filamentation process and results in considerable shortening of the filamentation length.

Angular diagram of broadband emission of millimeter-sized water droplets exposed to gigawatt femtosecond laser pulses.

We report on the experiments on the interaction of gigawatt femtosecond laser pulses with suspended millimeter-sized water droplets. The transparent droplets experienced laser-induced breakdown and explosive boiling up and emitted a broadband radiation. This radiation covers the spectral range from 450 to 1100 nm and consists of the spectrum of laser pulse scattered and transformed by the droplet due to self-phase modulation and plasma emission produced in water during photoionization. The droplet emission spectrum showed remarkable broadening at all viewing angles and is maximal in the direction of the laser exit from the droplet. The enlargement of the droplet results in additional spectral spreading of the emitted radiation. The depth and amount of laser pulse spectral self-transformations upon propagation through the water droplet are simulated by means of numerical calculations.

Broadband emission spectrum dynamics of large water droplets exposed to intense ultrashort laser radiation.

We report on experiments on the interaction of a gigawatt femtosecond laser pulse train with hanging isolated millimeter-sized water droplets. A transparent droplet experienced explosive boiling-up and emitted light in the visible spectrum as a result of laser-induced plasma formed inside the droplet volume. The droplet emission spectra showed remarkable broadening, depending on the laser power. The role of pulse self-phase modulation in measured spectral broadening when the pulse propagates through the droplet is discussed.

Whispering-gallery mode excitation in a microdroplet illuminated by a train of chirped ultrashort laser pulses.

The peculiarities of resonant optical field excitation inside a water microdroplet under illumination by a spatially bounded Gaussian beam with a temporal regime of a single chirped ultrashort laser pulse and a chirped pulse train are considered. It is established that the coupling efficiency of incident radiation to a selected high-Q whispering-gallery mode significantly depends on the interpulse interval in the train and chirping parameter of pumped laser radiation. The influence of the geometry of particle illumination by a laser beam and of the number of pulses in the train on the whispering-gallery mode buildup and its peak intensity is investigated.