J. Y. Suh

Semiconductor to metal phase transition in the nucleation and growth of VO2 nanoparticles and thin films

The optical and morphological characteristics of vanadium dioxide nanoparticles and thin films during their nucleation and growth phases have been studied by correlating the temperature and sharpness of the transition with the processing parameters. Thermal annealing results in grain growth and improved crystallinity. Normally, larger crystallites show smaller hysteresis, as there is a greater probability of finding a nucleating defect in the larger volume. But at the same time, this improved crystal perfection, which accompanies the thermal annealing and grain growth, tends to a larger hysteresis, as there are fewer nucleating defects within the volume. We show that the width and shape of the hysteresis cycle are thus determined by the competing effects of crystallinity and grain size.

Modulated optical transmission of subwavelength hole arrays in metal-VO2 films

We demonstrate the modulation of the transmission of near-infrared light through a periodic array of subwavelength apertures in Ag–VO2 and Au–VO2 double-layer films using the semiconductor-to-metal phase transition in VO2. The transmitted intensity ratio increases by a factor of 8 as the VO2 goes from the semiconductor to the metal phase. We attribute this modulation to the switchable dielectric-permittivity contrast between the air-filled holes in the array and the surrounding VO2 material, a conjecture that is semiquantitatively confirmed by simulation.

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.

X-ray diffraction studies of the growth of vanadium dioxide nanoparticles

We have characterized for the first time in situ the growth of vanadium dioxide nanoparticulate films prepared by pulsed-laser deposition, using a five-circle x-ray diffractometer, in order to provide structural information as the films are grown. A vanadium metal target was ablated in the presence of a pulsed N2O reactive gas source, and films were grown on Si(001) and Al2O3(0001) substrates. Optical measurements confirmed that the films deposited in this way exhibit the well-known VO2 metal–insulator transition at approximately 70 °C. The VO2 films grown at room temperature on silicon substrates are amorphous and extremely smooth. These become considerably rougher upon thermal annealing, as the VO2 phase crystallizes out in the form of hemispherical islands. These films also contain traces of a V2O5 nanoparticle phase in the first few monolayers, although the degree of VO2 crystallinity in the nanoparticles is quite high. In contrast, the VO2 nanoparticles grown on the sapphire substrates are in the for...

Using a Semiconductor-to-Metal Transition to Control Optical Transmission through Subwavelength Hole Arrays

We describe a simple configuration in which the extraordinary optical transmission effect through subwavelength hole arrays in noble-metal films can be switched by the semiconductor-to-metal transition in an underlying thin film of vanadium dioxide. In these experiments, the transition is brought about by thermal heating of the bilayer film. The surprising reverse hysteretic behavior of the transmission through the subwavelength holes in the vanadium oxide suggest that this modulation is accomplished by a dielectric-matching condition rather than plasmon coupling through the bilayer film. The results of this switching, including the wavelength dependence, are qualitatively reproduced by a transfer matrix model. The prospects for effecting a similar modulation on a much faster time scale by using ultrafast laser pulses to trigger the semiconductor-to-metal transition are also discussed.

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.

Fabricating active plasmonic structures by combining laser-, electron- and ion-beam techniques

The burgeoning interest in sub-wavelength metal optics is fueled by applications ranging from cancer therapy and proteomics to opto-electronics and novel photovoltaic materials. The optical properties of nanoscale metallic structures are determined not only by the electromagnetics of localized and propagating surface plasmons, but also by the details of metal nanoparticle size, shape and spatial arrangement, and dielectric environment. This paper presents several ways in which interesting plasmonic structures and applications are made possible by nanoscale materials synthesis and structuring techniques based on pulsed laser and electron-beam deposition, electron-beam lithography and focused ion-beam nanomachining. Examples include (1) using a focused ion beam to create arrays of subwavelength holes in which the extraordinary optical transmission effect can be switched; (2) fabricating gold::VO2 composite nanoparticles to measure the size dependence of the metal-insulator transition in vanadium dioxide using surface-enhanced Raman scattering; and (3) the construction of heterostructures in which excitons can be coupled to surface plasmons.The burgeoning interest in sub-wavelength metal optics is fueled by applications ranging from cancer therapy and proteomics to opto-electronics and novel photovoltaic materials. The optical properties of nanoscale metallic structures are determined not only by the electromagnetics of localized and propagating surface plasmons, but also by the details of metal nanoparticle size, shape and spatial arrangement, and dielectric environment. This paper presents several ways in which interesting plasmonic structures and applications are made possible by nanoscale materials synthesis and structuring techniques based on pulsed laser and electron-beam deposition, electron-beam lithography and focused ion-beam nanomachining. Examples include (1) using a focused ion beam to create arrays of subwavelength holes in which the extraordinary optical transmission effect can be switched; (2) fabricating gold::VO2 composite nanoparticles to measure the size dependence of the metal-insulator transition in vanadium dioxide usi...

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.

Nanoscale nonlinear optics coupling plasmonics to the metal-insulator transition in vanadium dioxide

Nanostructured vanadium dioxide and oxide-metal composites exhibit unusual nonlinear optical and plasmonic effects, including size- and shape-dependent optical switching and extraordinary optical transmission controlled by either thermal or ultrafast laser-initiated metal-insulator transition in vanadium dioxide.

Metal-Insulator Phase Transition in VO 2 : A Look from the Far Infrared Side

Vanadium dioxide (VO 2 ) displays a well-known metal-insulator (MI) transition at a temperature of 68°C. In this study we use terahertz time-domain spectroscopy (THz-TDS) to investigate the optical properties of VO 2 thin films in the vicinity of the MI transition temperature in the frequency range 0.1 – 1.5 THz. We observe the interesting effect that the phase of the transmitted THz field through the conducting VO 2 film is delayed in comparison to the phase of the same THz signal transmitted through the insulating VO 2 film. This is in contrast to the expected behavior of a homogeneous, conducting film. This observation shows that even at temperatures significantly above the transition temperature, the formation of a homogeneous, conducting film is incomplete. We demonstrate that effective-medium theory (EMT) in combination with a Drude model accounting for the conductivity of metallic domains formed in the VO 2 film accounts for all our observations. We show that the Maxwell-Garnett EMT is consistent with our observations, whereas the Bruggeman EMT fails to account for our observations.