Davon W. Ferrara

Modulation of the gold particle–plasmon resonance by the metal–semiconductor transition of vanadium dioxide

We report experimental observations of relative blue-shifts in the particle?plasmon resonance of gold nanoparticles (Au NPs) covered with a vanadium dioxide (VO2) film as the VO2 material undergoes a semiconductor-to-metal transition at approximately 67??C. Although the extinction spectra of the Au NPs exhibit significant red-shifts in the presence of the surrounding VO2 film as compared to the same particles in air, the key result of this work is the dynamically controlled blue-shift of the Au-NP dipole resonance upon thermal switching of the VO2 overlayer from the semiconducting to the metallic state. We also report on the size and polarization dependence of the extinction spectra for both states, and present Mie theory calculations that confirm in a semi-quantitative way the observed trends in the VO2-induced modulation of the Au-NP plasmon resonance, and their origin in the VO2 dielectric function.

Plasmonic Probe of the Semiconductor to Metal Phase Transition in Vanadium Dioxide

An array of 180 nm diameter gold nanoparticles (NPs) embedded in a thin vanadium dioxide film was used as a nanoscale probe of the thermochromic semiconductor-to-metal transition (SMT) in the VO2. The observed 30% reduction in plasmon dephasing time resulted from the interaction between the localized surface plasmon resonance of the NPs with the 1.4 eV electronic transitions in VO2. The NPs act as nanoantennas probing the SMT; homogeneous broadening of the gold plasmon resonance is observed at the temperatures where electron correlations are strongest in VO2.

Plasmon-enhanced low-intensity laser switching of gold::vanadium dioxide nanocomposites

Transient absorption of gold nanoparticle (NP) arrays covered by a 60 nm thick film of VO2 was measured using a mechanically shuttered 785 nm pump laser and a 1550 nm cw probe. Even though the Au NPs constitute only 4% by volume of the nanocomposite, they increase the effective absorption coefficient by a factor of 1.5 and reduce the threshold laser power required to induce the semiconductor-to-metal transition (SMT) by as much as 37%. It is argued that the NPs function as thermal initiators for the SMT and as “nanoradiators” to increase the scattering and absorption of light into interband transitions of the VO2.

Spectral modulation of single plasmonic nanostructures

We discuss in this paper the feasibility of dynamically modulating both resonance wavelength and spectral width of single nanostructures exhibiting plasmonic effects by cycling through a metal-insulator transition (MIT) in vanadium dioxide (VO2). Using full-field 3D finite-difference time domain (FDTD) simulation method with nonuniform mesh techniques, we study the effects of this modulation by varying the lateral dimensions of these nanostructures from 40 nm to 120 nm radially and changing its configuration as well, that is VO2 nanodisk on gold one and vice-versa. As an initial step towards fabricating those single composite nanostructures showing the greatest modulating effect, we start by making single NPs of VO2 and single gold NPs embedded between two 60 nm layers of VO2. The samples are fabricated on 130 μm thin glass substrates by electron-beam lithography, pulsed laser deposition of VO2 and electron-beam evaporation of gold. Using confocal extinction spectroscopy, we hereafter provide for the first time experimental observations of spectral tuning in these lithographically prepared single nanostructures. However, we discussed the variability in spectra obtained. Indeed, as the gold NP size decreases, it becomes comparable to the domain sizes of the embedding VO2 and this prevent the correct acquisition of the flat field. Hence the study of the tunability of gold particle plasmon resonance is imparted. However, we conclude that this study will be feasible for truly hybridized NP, that is gold nanodisk stacked on VO2 nanodisk and vice-versa. As hinted by our simulation studies and preliminary experimental results, these hybridized composite NPs could potentially be used in the dynamic spectral tuning of plasmonic waveguides.

Far-field coupling in arrays of gold and gold::vanadium dioxide nanodimers

Previous observations on arrays of single nanoparticles (NPs) have shown that particle separation and grating constant determine the peak extinction wavelength of the local surface plasmon resonance (LSPR). Recently, it has been predicted that the LSPR peak extinction wavelength in arrays of nanodimers (NDs) exhibit enhanced sensitivity to changes in the local dielectric function compared to single NPs. In order to test this prediction, arrays of NPs, NDs and heterodimers comprising three different NP sizes were fabricated by electron-beam lithography with various grating constants, particle diameters, and interparticle separations. Another set of arrays were also fabricated and coated with approximately 60-nm of vanadium dioxide, which undergoes an insulator to metal phase transition at a critical temperature near 68.C. By tuning the temperature of the samples through the strong-correlation region around the critical temperature, we varied the effective dielectric constant surrounding the NP arrays over a significant range. Linear extinction measurements on the arrays were made at temperatures above and below the critical temperature, with linear polarizers placed in the incident beam in order to distinguish between LSPR modes. Measurements show a clear dependence of LSPR sensitivity to interparticle separation as well as the dielectric function of the surrounding medium. Finally, finite-difference time-domain (FDTD) simulations were carried out for comparison with the experimental results.

Linear and nonlinear plasmonic effects modulated by a metalinsulator transition

Localized and propagating plasmonic effects in noble-metal nanostructures are receiving worldwide attention as potential enabling principles for optical and electro-optic devices. A critical need in plasmonic device design is a general technique for active modulation of the plasmon response. In this paper, we describe the use of a reversible, solid-solid phase transition in VO2 to modulate plasmonic response. Case studies are drawn from experiments on nano structured hole and particle arrays in which VO2 acts as a modulator by altering the local dielectric constant.

Plasmonic effects on the laser-induced metal-insulator transition of vanadium dioxide

Vanadium dioxide (VO2) is a strongly-correlated electron material with a well-known semiconducting to metallic phase transition that can be induced thermally, optically, or electrically. When switched to the high-temperature (T > 68°C) metallic phase, the greatest contrast in the optical properties occurs at wavelengths in the near-to-mid-infrared and beyond. In the visible to near-infrared, however, upon switching for wavelengths between ~500-1000 nm, VO2 transmits more light in the metallic phase. In this paper, we report studies of the effect of near-IR irradiation (785 nm) on lithographically prepared arrays of gold nanoparticles (NPs) covered with a thin film of VO2 and find that the presence of the NPs substantially lowers the laser threshold for low-power induction of the phase transition. Hybrid Au::VO2 structures were created by coating lithographically prepared arrays of gold nanoparticles (NPs) (diameters 140 and 200 nm, array spacing 450 nm) with 60 nm thick films of VO2 by pulsed laser deposition. Due to resonant absorption of the Au particle-plasmon resonance (PPR) at 785 nm, a temperature-dependent shift in the PPR can be generated by switching the VO2 from one phase to another. We have measured the switching behavior of VO2 and Au::VO2 structures using shuttered CW laser irradiation in order to study both optical and thermal mechanisms of the phase transition. Transient absorption measurements using a shuttered 785 nm pump laser corresponding to the PPR resonance of the Au NPs and 1550 nm CW probe show that the presence of the Au NPs lowers the threshold laser power required to induce the phase transition.

Modulating linear and nonlinear nanoplasmonic effects using the metal-insulator transition in vanadium dioxide

Linear and nonlinear nanoplasmonic effects - such as resonant absorption, second-harmonic generation and extraordinary optical transmission- are modulated in metallic nanostructures when the local dielectric environment is altered by the ultrafast metal-insulator transition in vanadium dioxide.

Effects of Surface Asymmetry on Femtosecond Second-Harmonic Generation from Metal Nanoparticle Arrays

We describe experiments aimed at distinguishing possible mechanisms of second-harmonic generation (SHG) in lithographically prepared arrays of metal nanoparticles. It is well-known that even-order harmonics cannot be generated by electric dipole-dipole interactions in centrosymmetric systems. The experiment employs two basic sample geometries. In our first geometry, as in our previous work, the NPs are left exposed to air, producing an asymmetric local dielectric environment with ITO on one side and air on the other. In the second geometry, we propose coating the arrays with the same material as they are created on, thus producing a centrosymmetric environment in which any SHG observed can not be due to asymmetry in the medium, but to nonlocal or retardation mechanisms in the particles. The arrays are fabricated using focused ion-beam lithography and vapor deposition of the metal, followed by standard lift-off protocols. This procedure yields typical NP dimensions between 60 nm and 200 nm in diameter, and between 15 nm and 30 nm in height, as characterized by scanning electron and atomic-force microscopy. By tuning the NP resonances to the excitation wavelength the SHG signal can be substantially enhanced. Surface melting effects are minimized by the use of ultra-short (50-fs) pulses which give high intensity while allowing us to work at relatively low fluence.

Time-resolved Second-harmonic Generation from Gold Nanoparticle Arrays

We have studied the effects of planar inversion symmetry and particle-coupling of gold nanoparticle (NP) arrays by angle dependent second-harmonic generation (SHG). Time- and angle- resolved measurements were made using a mode-locked Ti:sapphire 800 nm laser onto gold NP arrays with plasmon resonance tuned to match the laser wavelength in order to produce maximum SHG signal. Finite-difference time domain simulations are used to model the near-field distributions for the various geometries and compared to experiment. The arrays were fabricated by focused ion-beam lithography and metal vapor deposition followed by standard lift-off protocols, producing NPs approximately 20nm high with various in-plane dimensions and interparticle gaps. Above a threshold fluence of ~ 7.3 × 10-5 mJ/cm2 we find that the SHG scales with the third power of intensity, rather than the second, and atomic-force microscopy shows that the NPs have undergone a reshaping process leading to more nearly spherical shapes.