Current induced polycrystalline-to-crystalline transformation in vanadium dioxide nanowires.

Vanadium dioxide (VO2) exhibits a reversible insulator-metal phase transition that is of significant interest in energy-efficient nanoelectronic and nanophotonic devices. In these applications, crystalline materials are usually preferred for their superior electrical transport characteristics as well as spatial homogeneity and low surface roughness over the device area for reduced scattering. Here, we show applied electrical currents can induce a permanent reconfiguration of polycrystalline VO2 nanowires into crystalline nanowires, resulting in a dramatically reduced hysteresis across the phase transition and reduced resistivity. Low currents below 3 mA were sufficient to cause the local temperature in the VO2 to reach about 1780 K to activate the irreversible polycrystalline-to-crystalline transformation. The crystallinity was confirmed by electron microscopy and diffraction analyses. This simple yet localized post-processing of insulator-metal phase transition materials may enable new methods of studying and fabricating nanoscale structures and devices formed from these materials.

Electrically controllable extraordinary optical transmission in gold gratings on vanadium dioxide.

Highly tunable optical transmission through one-dimensional gold gratings patterned on top of a film of the phase transition material, vanadium dioxide (VO2), is demonstrated. Dense electrical integration is enabled by grating features that also function as electrical contacts to the VO2. Extraordinary optical transmission is observed in the VO2 insulator phase, and the optical transmission is extinguished by up to about 6 dB in a 170 nm thick VO2 film. Measurements of gratings with varying duty cycles demonstrate the dependence of the optical transmission and tuning on the device geometry.

Wavelength-size hybrid Si-VO(2) waveguide electroabsorption optical switches and photodetectors.

Ultra-compact waveguide electroabsorption optical switches and photodetectors with micron- and sub-micron lengths and compatible with silicon (Si) waveguides are demonstrated using the insulator-metal phase transition of vanadium dioxide (VO(2)). A 1 µm long hybrid Si-VO(2) device is shown to achieve a high extinction ratio of 12 dB and a competitive insertion loss of 5 dB over a broad bandwidth of 100 nm near λ = 1550 nm. The device, operated as a photodetector, can measure optical powers less than 1 µW with a responsivity in excess of 10 A/W. With volumes that are about 100 to 1000 times smaller than today's active Si photonic components, the hybrid Si-VO(2) devices show the feasibility of integrating transition metal oxides on Si photonic platforms for nanoscale electro-optic elements.

Design of electrically driven hybrid vanadium dioxide (VO2) plasmonic switches.

We present two types of designs for plasmonic switches based on hybridization between single interface surface plasmon polaritons and modes of a thin film of transition metal oxide material, vanadium dioxide (VO(2)). The design includes integrated, localized heaters that activate the VO(2) transition. The device operation is investigated and optimized by electromagnetic, electrical, and thermal simulations. The large change in the VO(2) refractive index in the infrared wavelength range enables highly compact and efficient plasmonic switches. The proposed designs achieve extinction ratios of 23-32 dB using only a 5 µm active region, a switching voltage of about 60 mV, and a switching power of about 9 mW.

Generalized exact dynamic localization in curved coupled optical waveguide arrays.

We observed exact dynamic localization in its general case in strongly coupled curved waveguide arrays with periodic, nonsquare-wave curvatures. We achieved a six times change in the dynamic localization bandwidth by changing a design parameter in our deviated-square-wave curvature. By analyzing the relation between the bandwidth and curvature profile, we offer a general method to control the localization bandwidth.

Surface plasmon polariton induced optical amplitude and phase modulation in sub-wavelength apertures.

We report on the amplitude and phase modulation of picosecond optical pulses, near λ = 800 nm, transmitted through sub-wavelength rectangular apertures in thin gold films with thicknesses of λ/10 at per-pulse energies of <0.3 nJ or 9 pJ per aperture. Due to the excitation and strong confinement of surface plasmon polaritons in the apertures, the leading edge of a pulse causes a rapid heating of the electrons and lattice to modulate its falling edge. By comparing cross-correlation frequency resolved optical gating measurements with simulations, the thermal effects responsible for the induced pulse dynamics are identified.

Quasi-BLOCH oscillations in curved coupled optical waveguides.

We report the observation of quasi-Bloch oscillations, a recently proposed, new type of dynamic localization in the spatial evolution of light in a curved coupled optical waveguide array. By spatially resolving the optical intensity at various propagation distances, we show the delocalization and final relocalization of the beam in the waveguide array. Through comparisons with other structures, we show that this dynamic localization is robust beyond the nearest-neighbor tight-binding approximation and exhibits a wavelength dependence different from conventional dynamic localization.

A palladium-catalyzed alkylation/direct arylation synthesis of nitrogen-containing heterocycles.

A norbornene-mediated palladium-catalyzed sequence is described in which an alkyl-aryl bond and an aryl-heteroaryl bond are formed in one reaction vessel. The aryl-heteroaryl bond-forming step occurs via a direct arylation reaction. A number of six-, seven-, and eight-membered ring-annulated indoles, pyrroles, pyrazoles, and azaindoles were synthesized from the corresponding bromoalkyl azole and an aryl iodide.