Optical antenna structures have revolutionized the field of nano-optics by confining light to deep subwavelength dimensions for spectroscopy and sensing. In this work, we fabricated coaxial optical antennae with sub-10-nanometer critical dimensions using helium ion lithography (HIL). Wavelength dependent transmission measurements were used to determine the wavelength-dependent optical response. The quality factor of 11 achieved with our HIL fabricated structures matched the theoretically predicted quality factor for the idealized flawless gold resonators calculated by finite-difference time-domain (FDTD). For comparison, coaxial antennae with 30 nm critical dimensions were fabricated using both HIL and the more common Ga focus ion beam lithography (Ga-FIB). The quality factor of the Ga-FIB resonators was 60% of the ideal HIL results for the same design geometry due to limitations in the Ga-FIB fabrication process.

We experimentally investigate surface-plasmon assisted photoemission to enhance the efficiency of metallic photocathodes for high-brightness electron sources. A nanohole array-based copper surface was designed to exhibit a plasmonic response at 800 nm, fabricated using the focused ion beam milling technique, optically characterized and tested as a photocathode in a high power radio frequency photoinjector. Because of the larger absorption and localization of the optical field intensity, the charge yield observed under ultrashort laser pulse illumination is increased by more than 100 times compared to a flat surface. We also present the first beam characterization results (intrinsic emittance and bunch length) from a nanostructured photocathode.

In this Letter, we report on the efficient generation of electrons from metals using multiphoton photoemission by use of nanostructured plasmonic surfaces to trap, localize, and enhance optical fields. The plasmonic surface increases absorption over normal metals by more than an order of magnitude, and due to the localization of fields, this results in over 6 orders of magnitude increase in effective nonlinear quantum yield. We demonstrate that the achieved quantum yield is high enough for use in rf photoinjectors operating as electron sources for MHz repetition rate x-ray free electron lasers.

We describe a relatively simple, reliable, and reproducible preparation technique, the precursor oxidation process, for making VO 2 films with strong semiconductor-to-metal phase transition. Sputter-deposited metal precursor V films were oxidized in situ in the deposition chamber for 2.5-7 h at 370-415°C in 0.2 Torr O 2 to form 22-220 nm VO 2. The strength ͓resistivity ratio, RR= S / M ͔ and sharpness ͑hysteresis width ⌬T C ͒ of T-dependent semiconductor-to-metal hysteretic phase transition in VO 2 were our most immediate and relevant quality indicators. In 200-nm-range films, the process was optimized to yield RR= ͑1-2͒ ϫ 10 3 , ⌬T C ϳ 11°C and absolute resistivity in a semiconducting phase S = 0.4± 0.2 ⍀ m, close to resistivity in bulk single crystals of VO 2. Films were characterized by scanning electron microscopy, atomic force microscopy, grazing-incidence x-ray diffraction, and Raman spectroscopy, and found to be polycrystalline single-phase VO 2. We also measured optical reflectivity R T ͑͒ from 200 to 1100 nm, and R ͑T͒ from 20 to 100°C. R T ͑͒ measured in thin-film interference structures allowed us to calculate the index of refraction in the two phases, which agrees well with the published data and, together with structural measurements, confirms that our films are essentially pure VO 2. The limited study of these films in terms of stability, aging, lithographic processing, and thermal cycling shows that they can be used in applications.

High quality GaN epilayers have been grown on oxygen and zinc surfaces of ZnO (0001) substrates by reactive molecular beam epitaxy and the effect of the intermediate buffer layer on the structural and optical properties of the GaN films has been investigated. The optical and structural characterization of the GaN epilayers and ZnO substrates were performed using photoluminescence, reflectivity, x-ray double diffraction, atomic force microscopy, and transmission electron microscopy. The optical results indicated that GaN was grown with compressive strain due to the difference in thermal expansion coefficient between GaN and ZnO. The surface roughness has been reduced by using an intermediate low temperature GaN buffer layer. The low temperature photoluminescence spectra of GaN/ZnO epilayers did not reveal any sign of the well-known midgap yellow signal. Linear polarized reflectivity and photoluminescence indicated that GaN epilayer planes were not misoriented with respect to the ZnO substrate planes: this result was confirmed by x-ray double diffraction measurements.