Vanadium dioxide (VO2) is one of the most extensively studied materials in the strongly correlated electron family capable of sustaining an insulator-to-metal transition. Here we present our studies of high-quality thin films of epitaxially grown VO2 on c-Al2O3(0001) and TiO2(001) via reactive DC pulsed magnetron sputtering. We present the structural transition probed via Reflection High Energy Electron Diffraction (RHEED) for the first time and we correlate the surface microstructure measurements with simulations before, during, and after the thermally induced transition. We also study the photoelectric conversion of VO2 on TiO2 (001) and c-Al2O3(0001) under 405 nm light and demonstrate up to a 2000% increase in quantum efficiency as the power of the light is varied for VO2 on TiO2(001).

Bulk Niobium (Nb) SRF (superconducting radio frequency) cavities are currently the preferred method for acceleration of charged particles at accelerator facilities around the world. However, bulk Nb cavities have poor thermal conductance and impose material and design restrictions on other components of a particle accelerator. Since the SRF phenomena occurs at surfaces within a shallow depth of ~ 1 mm, a proposed solution to this problem has been to deposit a superconducting Nb thin film on the interior of a cavity made of a suitable alternative material such as copper or aluminum. While this approach has been attempted in the past using DC magnetron sputtering (DCMS), such cavities have never performed at the bulk Nb level. However, new energetic condensation techniques for film deposition offer the opportunity to create suitably thick Nb films with improved density, microstructure and adhesion compared to traditional DCMS. One such technique that has been developed somewhat recently is “High Power Impulse Magnetron Sputtering” (HiPIMS). Here we report early results from various thin film coatings carried out on a 1.5GHz Nb cavity and small coupon samples coated at Jefferson Lab using HiPIMS.

Vanadium Dioxide is a strongly correlated material that can exhibit photoelectric properties via substrate-film hole transfer. We study these photoelectric properties in epitaxially grown VO2 thin films under near UV-light on various substrates, namely TìO2(001), and TiO2:Nb in development of new, fast UV photodetectors.

While vanadium dioxide (VO2) is one of the most extensively studied highly correlated materials, there are intriguing similarities and differences worth exploring in another highly correlated oxide, niobium dioxide (NbO2). Both materials exhibit a thermally-induced first-order insulator-metal transition at a material-dependent critical temperature, which is considerably higher in NbO2 than in VO2 - approximately 1080 K and 340 K in bulk, respectively. This transition, evidenced by up to 6 orders of magnitude change in DC and optical conductivities, can also be induced in VO2 via photo-doping on a sub-picosecond timescale. Here, we present the first ultrafast pump-probe studies on the optically-induced transition of NbO2 thin films and the comparison with similar VO2 films. It is observed that NbO2 films transition faster and exhibit significantly faster recovery time than VO2 films of similar thickness and microstructure, showcasing that NbO2 is a promising material for next generation high-speed optoelectronic devices. (C) 2016 Optical Society of America

Superconducting thin films have a wide range of dc and RF applications, from detectors to superconducting radio frequency. Amongst the most used materials, niobium (Nb) has the highest critical temperature (T-C) and highest lower critical field (H-C1) of the elemental superconductors and can be deposited on a variety of substrates, making Nb thin films very appealing for such applications. Here, we present temperature-dependent dc studies on the critical temperature and critical fields of Nb thin films grown on copper and r-plane sapphire surfaces. Additionally, we correlate the dc superconducting properties of these films with their microstructure, which allows for the possibility of tailoring future films for a specific application.

The current technology used in linear particle accelerators is based on superconducting radio frequency (SRF) cavities fabricated from bulk niobium(Nb), which have smaller surface resistance and therefore dissipate less energy than traditional nonsuperconducting copper cavities. Using bulk Nb for the cavities has several advantages, which are discussed elsewhere; however, such SRF cavities have a material-dependent accelerating gradient limit. In order to overcome this fundamental limit, a multilayered coating has been proposed using layers of insulating and superconducting material applied to the interior surface of the cavity. The key to this multilayered model is to use superconducting thin films to exploit the potential field enhancement when these films are thinner than their London penetration depth. Such field enhancement has been demonstrated in MgB2thin films; here, the authors consider films of another type-II superconductor, niobium nitride (NbN). The authors present their work correlating stoichiometry and superconducting properties in NbN thin films and discuss the thickness dependence of their superconducting properties, which is important for their potential use in the proposed multilayer structure. While there are some previous studies on the relationship between stoichiometry and critical temperature TC, the authors are the first to report on the correlation between stoichiometry and the lower critical field HC1.

Current superconducting radio frequency technology, used in various particle accelerator facilities across the world, is reliant upon bulk niobium superconducting cavities. Due to technological advancements in the processing of bulk Nb cavities, the facilities have reached accelerating fields very close to a material-dependent limit, which is close to 50 MV/m for bulk Nb. One possible solution to improve upon this fundamental limitation was proposed a few years ago by Gurevich [Appl. Phys. Lett. 88, 012511 (2006)], consisting of the deposition of alternating thin layers of superconducting and insulating materials on the interior surface of the cavities. The use of type-II superconductors with Tc > TcNb and Hc > HcNb, (e.g., Nb3Sn, NbN, or NbTiN) could potentially greatly reduce the surface resistance (Rs) and enhance the accelerating field, if the onset of vortex penetration is increased above HcNb, thus enabling higher field gradients. Although Nb3Sn may prove superior, it is not clear that it can be grown as a suitable thin film for the proposed multilayer approach, since very high temperature is typically required for its growth, hindering achieving smooth interfaces and/or surfaces. On the other hand, since NbTiN has a smaller lower critical field (Hc1) and higher critical temperature (Tc) than Nb and increased conductivity compared to NbN, it is a promising candidate material for this new scheme. Here, the authors present experimental results correlating film microstructure with superconducting properties on NbTiN thin film coupon samples while also comparing films grown with targets of different stoichiometry. It is worth mentioning that the authors have achieved thin films with bulk-like lattice parameter and transition temperature while also achieving Hc1 values larger than bulk for films thinner than their London penetration depths.

Correlated experimental and simulation studies on the modulation of Surface Plasmon Polaritons (SPP) in Au/VO2 bilayers are presented. The modification of the SPP wave vector by the thermally-induced insulator-to-metal phase transition (IMT) in VO2 was investigated by measuring the optical reflectivity of the sample. Reflectivity changes are observed for VO2 when transitioning between the insulating and metallic states, enabling modulation of the SPP in the Au layer by the thermally induced IMT in the VO2 layer. Since the IMT can also be optically induced using ultrafast laser pulses, we postulate the viability of SPP ultrafast modulation for sensing or control.

We have been interested in the plasmonic properties of alternative conducting materials to metals, such as conducting oxides, and we have recently expanded our studies to include highly correlated oxides, such as vanadium dioxide (VO2) thin films. VO2 exhibits a metal-insulator transition (MIT) just above ambient temperature at ∼ 340K. Interestingly, this transition can be induced thermally, optically or applying electric fields. Across the MIT, the optical properties are completely modified over a broad frequency range. We will present our recent optical investigations on the photon induced transition studies on such films, as well as the surface plasmon resonance (SPR) modulation in nanopatterned Au gratings by the thermally induced MIT in VO2 thin films, addressing possibilities of ultrafast SPR modulation with VO2.