Femtosecond ultrabright electron sources with spatially structured emission are an enabling technology for free-electron lasers, compact coherent X-ray sources, electron diffractive imaging, and attosecond science. In this work, we report the design, modeling, fabrication, and experimental characterization of a novel ultrafast optical field emission cathode comprised of a large (>100,000 tips), dense (4.6 million tips·cm(-2)), and highly uniform (<1 nm tip radius deviation) array of nanosharp high-aspect-ratio silicon columns. Such field emitters offer an attractive alternative to UV photocathodes while providing a direct means of structuring the emitted electron beam. Detailed measurements and simulations show pC electron bunches can be generated in the multiphoton and tunneling regime within a single optical cycle, enabling significant advances in electron diffractive imaging and coherent X-ray sources on a subfemtosecond time scale, not possible before. At high charge emission yields, a slow rollover in charge is explained as a combination of the onset of tunneling emission and the formation of a virtual cathode.

This paper provides a detailed study on surface acoustic wave (SAW) induced acoustic streaming and pumping, focusing on the effects of the wave mode and surface modification. SAW devices with wavelengths of 32 and 64 µm were fabricated on 128° Y-cut lithium niobate substrates with aluminium interdigitated transducers. A higher order harmonic mode wave appears in addition to the fundamental Rayleigh wave for samples with metallization ratios less than 0.6. Both waves have demonstrated the ability to induce acoustic streaming and to pump liquid. A high streaming velocity, and hence a high mixing efficiency and a higher acoustic force, can be obtained using the fundamental Rayleigh wave as the high harmonic waves have large propagation losses. A linear relationship between the streaming velocity and RF signal voltage has been obtained, and effective mixing can be achieved. An acoustic wave has also been utilized to manipulate and pump droplets with sizes up to 5 µl, and a moving speed of ∼1.4 cm s−1 has been obtained on an octadecyltrichlorosilane-treated SAW device using a signal voltage of 40 V.

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Two near-ultraviolet (UV) sensors based on solution-grown zinc oxide (ZnO) nanowires (NWs) which are only sensitive to photo-excitation at or below 400 nm wavelength have been fabricated and characterized. Both devices keep all processing steps, including nanowire growth, under 100 °C for compatibility with a wide variety of substrates. The first device type uses a single optical lithography step process to allow simultaneous in situ horizontal NW growth from solution and creation of symmetric ohmic contacts to the nanowires. The second device type uses a two-mask optical lithography process to create asymmetric ohmic and Schottky contacts. For the symmetric ohmic contacts, at a voltage bias of 1 V across the device, we observed a 29-fold increase in current in comparison to dark current when the NWs were photo-excited by a 400 nm light-emitting diode (LED) at 0.15 mW cm(-2) with a relaxation time constant (τ) ranging from 50 to 555 s. For the asymmetric ohmic and Schottky contacts under 400 nm excitation, τ is measured between 0.5 and 1.4 s over varying time internals, which is ~2 orders of magnitude faster than the devices using symmetric ohmic contacts.

Abstract. We demonstrate the design, fabrication, characterization, and operation of highdensity arrays of Au nanorod electron emitters, fabricated by high-resolution electron beam lithography, and excited by ultrafast femtosecond near-infrared radiation. Electron emission characteristic of multiphoton absorption has been observed at low laser fluence, as indicated by the power-law scaling of emission current with applied optical power. The onset of spacecharge-limited current and strong optical field emission has been investigated so as to determine the mechanism of electron emission at high incident laser fluence. Laser-induced structural damage has been observed at applied optical fields above 5 GVm -1 , and energy spectra of emitted electrons have been measured using an electron time-of-flight spectrometer.