Enhancing the QE of Niobium by Exploiting Plasmonics

Enhancing the QE of Niobium by Exploiting Plasmonics and the Superconducting Proximity Effect J. F. Zasadzinski, S. Asalzadeh, N. Samuelson, M. Warren, L. Spentzouris, J. Power* Physics Department, Illinois Institute of Technology, Chicago, IL USA *Argonne Wakefield Accelerator, HEP Division, ANL, Argonne IL USA Introduction The development of superconducting photocathodes is presented which explores ultra-thin film coatings to enhance the quantum efficiency (QE) of superconducting Nb above its bulk value of < 10 -6. Deposition of a 10 nm layer of Mg (bulk work function = 3. 66 e. V) onto Nb foils after UHV anneal increases QE by a factor of 10 -20. Tunneling measurements on similar foils reveals the superconducting gap close to bulk Nb. Deposition of ultra thin islands of In (4 nm thick) on top of Nb/Mg/(Mg oxide) leads to overall enhancements of QE by up to 400 times. We attribute this latter enhancement to plasmonic effects where the stored EM fields in the In islands couple to surface electrons. Such cathodes are robust in air. Test of Nb/Mg Cathode in AWA Gun – Phase I • • • Nb plug cathode Mechanical polish UHV anneal 600 C In situ Mg deposition 10 nm or 100 nm QE Measurement System AWA Test Gun 1. 3 GHz up to 100 MV/m UV LED Wavelengths Photon Power 245 nm 1 m. W 260 nm 30 m. W 285 nm 40 m. W Dark Current Test up to 60 MV/m Figure 5 Fig. 4 AFM Topography of Nb/Mg . Proximity Effect in the Arnold Limit High Field enhacement, b, likely due to surface roughness (field emiiters) Clean Specular N/S Interface Andreev Reflection Relevant Fig. 3 Dark Charge vs E field for various cathodes N QE Results All Samples Exposed to Air No significant damage to Mg layer up to 60 MV/m S Quasiparticle excitations above the induced gap in the normal metal inhibited up to bound state E 0 ~ DS due to Andreev Reflection. Nb/Mg Test Phase II (in development) ARGONNE CATHODE TEST-STAND (ACT) Tunneling Spectroscopy on Nb/Mg • • Recrystallized Nb foils UHV anneal > 2000 C Nb single crystal 261 nm, ps laser Cabot Industries polishing 25 nm RMS UHV anneal 600 C In-situ Mg deposition High Gradient Cathode Testing in NCRF gun • NCRF gun characterization of photocathodes and field emission cathodes • High gradient testing • Charge, QE measurement and mapping • thermal emittance measurement and mapping Cathode Development • In situ QE measurements • Kelvin probe work function (CPD) measurements D. M. Burnell* and E. L. Wolf, “Proximity-Effect Tunneling Study of Mg” Journal of Low Temperature. P hysics, Vo. L 50° , Nos. 1/2, 1984 Fig. 7. QE measurements for pure Nb and various thin Coatings. Plasmon Resonance: Indium Islands Optical Transmittance of PVD In on Sapphire Surface Plasmon Mode Tunable with Nominal Indium Thickness A B Fig. 2 Gap region I-V for various Mg thicknesses Mg is Superconducting! Displays the Nb Gap Parameter D = 1. 55 me. V for thin layers. Same RF losses. Figure 5 Work Function and QE of Nb/Mg Reflectance: In Islands on Nb/Mg/Mg-oxide Reproducible Plasmon Mode in Deep UV Conclusions • Mg film can be induced in superconducting state with gap parameter similar to Nb • Anticipated minimal effect on RF impedance • QE of Nb/Mg is enhanced by factor of 10 -20 after air exposure. • Vapor deposition of In (4 nm thick) onto air- exposed Nb/Mg leads to QE -4 up to 4 X 10 Future Work • Robust in air • Proof of principal for plasmonic • Mg film on single-crystal Nb (25 nm RMS polis enhancement • AWA test gun, high field QE, thermal emittanc • E-beam lithography, patterned plasmonics Kelvin Probe Measurements nanodisks Contact Information John Zasadzinski Mg thin films on Nb approach bulk Work Function of Mg (3. 66 e. V). QE increased by 10 -20 over bulk Nb (see Fig. 7) www. meteconferences. org Address Figure 6 Optical reflectance peak is plasmon resonance Email: zasadzinski@iit. edu