Phase-control over nonlinear interactions in plasmonic nanostructures

Metasurfaces, and in particular those containing plasmonic-based metallic elements,
constitute an attractive set of materials with a potential for replacing standard bulky optical elements. In recent years, increasing attention has been focused on their nonlinear optical properties, particularly in the context of second and third harmonic generation and beam steering by phase gratings. In our lab, we harness the full phase-control enabled
by subwavelength plasmonic elements to demonstrate a unique metasurface-phase-matching which is required for efficient nonlinear processes. We discuss the difference between scattering by a grating and by subwavelength phase-gradient elements. We show that for such interfaces a new, anomalous phase matching condition prevails, which is the nonlinear analog of the generalized Snell’s law. The subwavelength phase control of optical nonlinearities paves the way for the design of ultrathin, flat nonlinear optical elements. We demonstrate nonlinear metasurface lenses which act both as generators and as manipulators of the frequency-converted signal.

Rational Design of Plasmonic Metasurfaces

Optimizing the shape of nanostructures and nano-antennas for specific optical properties has evolved to be a very fruitful activity.  With modern fabrication tools a large variety of possibilities is available for shaping both nanoparticles and nanocavities; in particular nanocavities in thin metal films have emerged as attractive candidates for new metamaterials and strong linear and nonlinear optical systems. In our lab, we rationally design metallic nanocavities to boost their Four Wave Mixing response by resonating the optical plasmonic resonances with the incoming and generated beams. The linear and nonlinear optical responses as well as the propagation of the electric fields inside the cavities are derived from the solution of Maxwell’s equations by using the 3D finite-differences time domain method.  The observed conversion-efficiency of near infra-red to visible light equals or surpasses that of BBO of equivalent thickness. Implications to further optimization for efficient and broadband ultrathin nonlinear optical materials are discussed..

Orientation and Alignment Echoes

Strong femtosecond pulses are known to align and orient molecules by applying torques and forces on freely rotating molecules. The controlled application of several pulses (two or more) can selectively excite individual species form amongst several which are exposed to the radiation. In collaboration with the group of Professor Ilya Averbukh in our department, we studied this subject extensively. [See a review by Fleischer et al. Israel Journal of Chemistry, 52, 414-437 (2012)]. Presently, with the Averbukh group and in collaboration with our French colleagues, we are focusing on the fascinating phenomena of Echoes.  Recently we presented one of the simplest classical systems featuring the echo phenomenon—a collection of randomly oriented free rotors with dispersed rotational velocities.  Following excitation by a pair of time delayed impulsive kicks, the mean orientation or alignment of the ensemble exhibits multiple echoes and fractional echoes. We elucidate the mechanism of the echo formation by the kick-induced filamentation of phase space, and provide the first experimental demonstration of classical alignment echoes in a thermal gas of CO2 molecules excited by a pair of femtosecond laser pulses.