Hydrogen sensors based on single Pd nanowires show promising results in speed, sensitivity, and ultralow power consumption. The utilization of single Pd nanowires, however, face challenges in nanofabrication, manipulation, and achieving ultrasmall transverse dimensions. We report on hydrogen sensors that take advantage of single palladium nanowires in high speed and sensitivity and that can be fabricated conveniently. The sensors are based on networks of ultrasmall (<10 nm) palladium nanowires deposited onto commercially available filtration membranes. We investigated the sensitivities and response times of these sensors as a function of the thickness of the nanowires and also compared them with a continuous reference film. The superior performance of the ultrasmall Pd nanowire network based sensors demonstrates the novelty of our fabrication approach, which can be directly applied to palladium alloy and other hydrogen sensing materials.

We report on the first successful operation of a field-emitter-array cathode in a conventional Lband radio-frequency electron source. The cathode consisted of an array of ∼ 10 6 diamond diamond tips on pyramids. Maximum current on the order of 15 mA were reached and the cathode did not show appreciable signs of fatigue after weeks of operation. The measured Fowler-Nordheim characteristics, transverse beam density, and current stability are discussed. Numerical simulations of the beam dynamics are also presented.PACS numbers: 41.75.Fr Over the past years, field-emission (FE) electron sources have been the subject of intense investigations due to several advantages they offer over photoemission and thermionic sources. The main advantages of FE sources stem from their ability to produce very lowemittance bunched beams, their capability to generate high-average current beams, and the absence of requirement for an auxiliary laser system. A single-tip FE cathode emits electrons from a very small transverse area and can therefore produce beams with extremely small, near quantum-degenerate, transverse emittances [1,2]. When arranged as large arrays, field-emission-array (FEA) cathodes can provide substantial average currents [3] to the detriment of emittance which then scales linearly with the FEA macroscopic radius [4,5].Pulsed field-emission occurs when a FE cathode experiences a time-dependent field, e.g., when located in a resonant radiofrequency (RF) cavity. Taking the example of a cylindrical-symmetric resonant pillbox cavity operating on the TM 010 mode with axial electric field E z (r = 0, z, t) = E 0 cos(2πf t), where f and E 0 are respectively the field frequency and peak amplitude, field-emitted bunches have a root-mean-square (rms) duration σ t ≃ ω −1 [β e E 0 /B(φ)] 1/2 where ω ≡ 2πf . The latter pulse duration is obtained by taking the current density to follow the Fowler-Nordheim's (F-N) law [6] j(t) = A(φ)β 2 e E(t) 2 exp[−B(φ)/(β e E(t))] where A(φ) and B(φ) are functions of the work function φ of the cathode material and β e is a field-enhancement factor [7,8]. Nominally, the bunch rms duration is a significant fraction of the RF field period typically resulting in beams with large energy spread. This limitation can however be circumvented by exposing the FE cathode to superimposed electromagnetic fields operating at harmonic frequencies with properly tuned relative phases and amplitudes. A practical implementation of this technique consists in a RF gun supporting two harmonic modes with axial electric fields [9].In this letter we report on the first operation of a diamond FEA (DFEA) cathode in a conventional L-band RF gun nominally operated with a Cesium Telluride (Cs 2 Te) photocathode. The DFEA is composed of ungated diamond pyramids which have proven to be rugged. Depending on the size and pitch of the pyramids, tests under DC voltages have showed field emission to begin at macroscopic fields E 0 ≃ 5 MV/m, and peak currents per tip as high as 15 µA has been obtained [10].The geometry of the DFE...

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We explore nonlinear photoemission in cesium telluride (Cs2Te) photocathodes where an ultrashort (∼ 100 fs full width at half max) 800-nm infrared laser is used as the drive-laser in lieu of the typical ∼ 266-nm ultraviolet laser. An important figure of merit for photocathodes, the quantum efficiency, we define here for nonlinear photoemission processes in order to compare with linear photoemission. The charge against drive-laser (infrared) energy is studied for different laser energy and intensity values and cross-compared with previously performed similar studies on copper [P. Musumeci et al., Phys. Rev. Lett., 104, 084801 (2010)], a metallic photocathode. We particularly observe two-photon photoemission in Cs2Te using the infrared laser in contrast to the anticipated three-photon process as observed for metallic photocathodes.Photocathodes are widely used to generate bright electron bunches with durations comparable to the emissiontriggering laser [1]. Semiconductor photocathodes are uniquely attractive due to their high quantum efficiencies (QEs) defined as the number of electrons emitted per unit photon of the drive-laser. Cesium telluride (Cs 2 Te) cathodes have substantial QEs (up to 20 %) [2,3], long lifetimes, and picosecond response times [1]. The photoemission photon energy requirement of Cs 2 Te dwells around ∼ 4 eV corresponding to an excitation-laser wavelength in the ultraviolet (UV) region. The photoemission band extends from UV to higher wavelengths [4] but any reasonable amount of charge extraction has so far been accomplished using UV laser pulses. Since most of the high-gain lasers use solid state media lasing in the infrared (IR), the typical production of a UV pulse relies on frequency up-conversion from IR to the UV. For laser systems based on the titanium sapphire (Ti:Sapph) medium (wavelength ∼ 800 nm), the UV pulses needed for photoemission are obtained from frequency tripling of the IR pulses using a two-stage process consisting of a second harmonic generation (SHG) stage followed by a sum frequency generation (SFG) stage. In order to preserve the short pulse duration during the up-conversion process, both stages generally use thin barium borate (BBO) crystals which limits the IR-to-UV conversion efficiency to typically < 10%.Operation of UV drive-lasers is technically challenging for high-average-current photoinjectors, while finding cathodes at the longer wavelengths available from solidstate lasers has proven challenging to date [5]. It was pointed out that nonlinear photoemission from metallic photocathodes could be advantageous [6,8] prompting us to explore possible multi-photon photoemission from Cs 2 Te using a Ti:Sapph laser.If an IR laser were to be used for photoemission from Cs 2 Te, then a nonlinear photoemission processspecifically three-photon photoemission (simultaneous absorption of three photons)-would have to take place if we assume the charge emission is strictly from photoemission and the photoemission threshold is unaltered; this is because, in the current context,...

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