Controlling the coexistence of structural phases and the optical properties of gallium nanoparticles with optical excitation

Abstract

We have observed reversible structural transformations, induced by optical excitation at 1.55 µm, between the β, γ and liquid phases of gallium in self-assembled gallium nanoparticles, with a narrow size distribution around 50 nm, on the tip of an optical fiber. Only a few tens of nanowatts of optical excitation per particle are required to control the transformations, which take the form of a dynamic phase coexistence and are accompanied by substantial changes in the optical properties of the nanoparticle film. The time needed to achieve phase equilibrium is in the microsecond range, and increases critically at the transition temperature.We report that the coexistence of different crystalline and disordered phases in nanoparticles can be controlled by optical excitation in a controllable, continuous and reversible fashion. This has been observed in nanoparticles of polymorphic elemental gallium. Shifts in the balance between phases are accompanied substantial changes in the optical properties of the nanoparticle film. The light-induced transformations display phenomenological features typical of 1st-order Ehrenfest phase transitions (such as a reflectivity hysteresis) but simultaneously display characteristics of 2nd-order transitions (such as critical dependencies on temperature of the 'susceptibility' and relaxation time of the stimulated response). Our study is motivated by a desire to understand the exciting physics of phase equilibria in nanoparticles [1,2,3] and in particular in metallic nanoparticles, which have the potential to play a key role in future highly integrated photonic devices as the active elements of waveguiding [4] and switching [5] structures.We studied light-induced structural transformations in gallium nanoparticles by monitoring the optical reflectivity of nanoparticle films. The particles, typically 50 nm in diameter with a relatively narrow size distribution (±14 nm), were prepared on the tips of silica optical fibers, using the recently developed light-assisted self-assembly technique [6,7]. This process produced a nanoparticle film on the fiber's core (9 µm in diameter) comprising ∼2.10 4 nanoparticles. Phase transitions in the nanoparticles ware stimulated by a 1.4 mW , 1.55 µm diode laser launched into the fiber. Its output was modulated with 50% duty cycle at 6.2 kHz providing an average excitation power of about 22 nW per nanoparticle. Another 0.4 mW cw diode laser operating at 1.31 µm and phasesensitive detection apparatus were used to monitor the reflectivity of the film. The detection system had an overall bandwidth of 300 kHz.The nanoparticle film's reflectivity shows a very wide (>100 K) hysteresis, with two distinct steps, at T ′ 0 ∼ 233 K and T 0 ∼ 253 K, in the rising temperature part of the cycle (Fig. 1a). The presence of these steps indicates that there are two structural phase transitions and therefore that either the nanoparticles undergo a transition from one solid phase to another and then from that phase to the liquid, or that two solid phases coexist at low tem...

Gallium Polymorphs: Phase‐Dependent Plasmonics

“…It is found that there are significant differences in the dielectric function of the various Ga polymorphs, especially in the spectral region below 2.5 eV, with interband transitions in α‐ and β‐Ga and Drude metallic behavior for γ‐Ga and δ‐Ga. Until present, although the different phases have been identified in confined systems (nanostructures), the lack of information on the dielectric dispersion of the different Ga‐phases has hampered the accurate electromagnetic modeling of these Ga nanostructures, pushing experimentalists to approximate the optical constants as linear combinations of the liquid and solid (α‐Ga) phases . Furthermore, we demonstrate that optical constants attributed to α‐Ga in the literature, obtained by supercooling l‐Ga, actually corresponds to the mixture of α‐ with other Ga phases.…”

“…Synthesized solid Ga nanoparticles have been reported to present the coexistence of different phases whose structural transformation can be driven by low power (approximately nW) optical excitation . In order to model the plasmonic response of these NPs, and due to the lack of information on the dielectric dispersion profiles of the different Ga phases, the optical properties of the Ga polymorphs were approximated through the linear combination of the dielectric function of l‐ and α‐Ga . Therefore, we have simulated the plasmonic response of NPs with different geometries made of the different Ga phases.…”

“…These characteristics are responsible for the unique plasmonic properties of Ga NPs, recently demonstrated to span the ultraviolet (UV), visible, and near infrared (NIR) spectral regions . Already, Ga nanostructures have been exploited for a variety of applications, including chemical sensing (using UV surface‐enhanced Raman spectroscopy), molecular sensing, delivery of cancer therapy drugs (using transformable liquid‐metal nanospheres composed of a liquid‐phase gallium core and a thiolated polymeric solid shell), phase‐change memories, reversible light‐induced switching, phase‐change nonlinear systems, and “active plasmonics.” Many of these applications exploit changes in optical reflectivity produced by phase transformations between the various structural forms of Ga, changes stimulated by very low power (approximately nW) optical excitation . The nature of the Ga–Ga bond is of particular interest because the way structural transformations affect optical properties may yield fascinating new mechanisms for photonic functionality, including multifunctional reconfigurable frequency‐ and polarization‐selective surfaces …”

Microwave antennas and heterogeneous substrates using nanomaterial fabrication techniques

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Resetting single nanoparticle structural phase with nanosecond pulses

“…3a). [7,11] and are related to the nature of the phase transition process in nanoparticles. In bulk solids, phase transitions are characterized by a discontinuous (irreversible) rearrangement of the crystalline lattice at a specific temperature.…”