Zhaozhu Li

Magnetic Shielding Larger Than the Lower Critical Field of Niobium in Multilayers

The current technology in superconducting radio frequency linear accelerators is based on the use of bulk niobium cavities. However, optimization of bulk technology is approaching the accelerating gradient limit set by the thermodynamic critical field of niobium, HC = 200 mT . In order to surpass niobiums ultimately achievable accelerating gradient, it has been proposed to use multilayer coatings to shield bulk niobium from higher fields. These multilayer coatings involve alternating superconducting and insulating layers. The superconductor used in this multilayer structure must have a higher HC than that of niobium. NbN is one such superconductor that has potential application in these multilayer coatings. Recently, it has been shown that NbN can sufficiently shield an underlying niobium layer. However, this reported shielding has never been shown to be above the lower critical field of niobium. In this work, we present NbN multilayers that for the first time are shown to be capable of shielding an underlying niobium layer beyond the lower critical field of bulk niobium.

Surface plasmon resonance modulation in nanopatterned Au gratings by the insulator-metal transition in vanadium dioxide films.

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.

NbN thin films for superconducting radio frequency cavities

NbN thin films have the potential to be incorporated into radio frequency cavities in a multilayer coating to overcome the fundamental field gradient limit of 50 MV m−1 for the bulk niobium based technology that is currently implemented in particle accelerators. In addition to having a larger critical field value than bulk niobium, NbN films develop smoother surfaces which are optimal for cavity performance and lead to fewer losses. Here, we present a study on the correlation of film deposition parameters, surface morphology, microstructure, transport properties and superconducting properties of NbN thin films. We have achieved films with bulk-like lattice parameters and superconducting transition temperatures. These NbN films have a lower surface roughness than similarly grown niobium films of comparable thickness. The potential application of NbN thin films in accelerator cavities is discussed.

Tailored Fano resonance and localized electromagnetic field enhancement in Ag gratings

Metallic gratings can support Fano resonances when illuminated with EM radiation, and their characteristic reflectivity versus incident angle lineshape can be greatly affected by the surrounding dielectric environment and the grating geometry. By using conformal oblique incidence thin film deposition onto an optical grating substrate, it is possible to increase the grating amplitude due to shadowing effects, thereby enabling tailoring of the damping processes and electromagnetic field couplings of the Fano resonances, hence optimizing the associated localized electric field intensity. To investigate these effects we compare the optical reflectivity under resonance excitation in samples prepared by oblique angle deposition (OAD) and under normal deposition (ND) onto the same patterned surfaces. We observe that by applying OAD method, the sample exhibits a deeper and narrower reflectivity dip at resonance than that obtained under ND. This can be explained in terms of a lower damping of Fano resonance on obliquely deposited sample and leads to a stronger localized electric field. This approach opens a fabrication path for applications where tailoring the electromagnetic field induced by Fano resonance can improve the figure of merit of specific device characteristics, e.g. quantum efficiency (QE) in grating-based metallic photocathodes.

Temperature and Microstructural Effects on the Superconducting Properties of Niobium Thin Films

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 (TC) and highest lower critical field (HC1) 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.

Enhancing Silver Photocathode Performance by Exciting Surface Plasmon Resonance on Ag corrugated thin films

The silver photocathodes have been limited into application due to its low quantum efficiency. We explore the enhancement of silver photocathode performance by exciting surface plasmon resonance on grating Ag thin film to overcome high reflectivity.

Photocathode materials able to sustain high currents

We will present preliminary work on photocathode materials able to sustain high currents, pertinent to the technology of accelerators and associated systems and essential to develop strategies and technologies for next generation nuclear physics accelerator capabilities. To this end, metallic photocathodes offer several clear advantages over semiconductor photocathodes because they are robust against degradation due to surface contamination and against damage resulting from conditioning or heating and can withstand high electric surface fields present at the cathode in RF accelerators. Other advantages include their very short response time (less than picoseconds) and their very long lifetime (years or longer), which is much longer than of other types of photocathodes (hours to months). However, the main problem with metallic photocathodes is the rather low quantum efficiency (QE), even for UV radiation. A possibility to improve the QE of metallic photocathodes is to exploit surface Plasmon resonance using adequate geometries for the intended application, as well as possible cap layers able to lower the metal work function. In this way, metal photocathodes designed to support surface Plasmons could produce high electron yields by enhancing their QE.