B.F. Soares

Light-induced switching between structural forms with different optical properties in a single gallium nanoparticulate.

In a single gallium nanoparticulate, self-assembled (from an atomic beam) in a nano-aperture at the tip of a tapered optical fiber, we have observed evidence for a sequence of reversible light-induced transformations between five different structural phases (gamma - epsilon - delta - beta - liquid), stimulated by optical excitation at nanowatt power levels.

Polymorphic nanoparticles as all-optical memory elements

A nanoparticle undergoing light-induced transformations between structural phases with different optical properties is an inheritably bistable structure and this bistability can be used to create a resonator-free optical memory element, operating at very low power levels. We experimentally demonstrate this memory functionality using a film of gallium nanoparticles, and we present a method for differentially accessing the logic state of the memory using a modulated optical probe beam.

Controlling light with light via structural transformations in metallic nanoparticles

Reversible light-induced structural transitions in metallic nanoparticles, taking the form of surface-driven dynamic phase coexistences, provide an optical nonlinearity suitable for controlling light with light in nanophotonic and plasmonic switching devices. Using low-power diode lasers (giving excitation powers of a few tens of nanowatts per particle) operating at telecom wavelengths, we studied this nonlinearity in gallium nanoparticles grown from atomic beam at the tip of silica optical fiber. The nonlinear response characteristics indicate the occurrence of both solid-solid and solid-liquid transitions and are consistent with an effective medium model for the optical properties of closely packed nanoparticle films. The megahertz dynamics of light-by-light control in the particle film provide an insight into the kinetics of structural transformations in nanoparticles. Transitions between two solid phases of gallium in the nanoparticles, occurring on application and withdrawal of optical excitation, take considerably less than a microsecond. The transient characteristics of the nonlinearity associated with the solid-liquid transition are dominated by the recrystallization time, which is typically of the order of a microsecond.

Resetting single nanoparticle structural phase with nanosecond pulses

The authors demonstrate that transitions from high temperature phases to low temperature phases in a gallium nanoparticle can be stimulated by nanosecond optical pulses, and explain how this phenomenon could be used in a single nanoparticle all-optical memory.

Nanophotonics under a scanning electron microscope: Studying resonator-less all-optical switching and memory functionality in gallium nanoparticles

Light- and electron beam-induced structural transformations in nanoparticles of polymorphic metals provide a paradigm for achieving resonator-less all-optical switching and optical memory functionality with picojoule energy requirements.

Nanowatt photonics of structural transformations in a single nanoparticle

Metallic nanoparticles have the potential to play a key role in future, highly-integrated photonic devices, not only as elements of waveguiding structures; (in plasmonic wave-guiding chains, or as scattering centres in band-gap structures), but also as active all-optical switching elements operating at very low power levels. We show that this functionality can be achieved by reversibly controlling the phase composition, and thereby the optical properties, of a nanoparticle with optical excitation at nanowatt power levels. For the first time, phase transition phenomena and associated optical effects have been studied in a single gallium nanoparticle, i.e. without the inhomogeneous broadening associated with size and shape distributions in nanoparticle films. We have been able to dynamically control coexistences between various combinations of galliums crystalline and disordered phases in very narrow temperature intervals and to detect substantial changes in optical properties at very low power levels. The nanoparticle was grown on the aperture at the tip of a gold-coated, tapered silica optical fibre. Its optical characteristics were then studied, at temperatures between 80 and 300 K, using low power infrared diode lasers. Power levels as low as 5 nW were found to be sufficient to substantially alter the particles reflectivity and transmission. The light-induced changes are fully reversible, with relaxation times in the microsecond range, and are critically enhanced near the transition temperatures between gallium’s metastable crystalline phases and the liquid. Such behaviour indicates that structural transformations, including solid-solid transitions between different crystalline forms, occur continuously, through the dynamic coexistence of phases.