Michael E. Swanwick

Nanostructured Ultrafast Silicon-Tip Optical Field-Emitter Arrays

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.

Near-ultraviolet zinc oxide nanowire sensor using low temperature hydrothermal growth

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.

A portable x-ray source with a nanostructured Pt-coated silicon field emission cathode for absorption imaging of low-Z materials

We report the design, fabrication, and characterization of a portable x-ray generator for imaging of low-atomic number materials such as biological soft tissue. The system uses a self-aligned, gated, Pt-coated silicon field emitter cathode with two arrays of 62 500 nano-sharp tips arranged in a square grid with 10 μm emitter pitch, and a natural convection-cooled reflection anode composed of a Cu bar coated with a thin Mo film. Characterization of the field emitter array demonstrated continuous emission of 1 mA electron current (16 mA cm − 2) with >95% current transmission at a 150 V gate-emitter bias voltage for over 20 h with no degradation. The emission of the x-ray source was characterized across a range of anode bias voltages to maximize the fraction of photons from the characteristic K-shell peaks of the Mo film to produce a quasi-monochromatic photon beam, which enables capturing high-contrast images of low-atomic number materials. The x-ray source operating at the optimum anode bias voltage, i.e. 35 kV, was used to image ex vivo and nonorganic samples in x-ray fluoroscopic mode while varying the tube current; the images resolve feature sizes as small as ~160 µm.

High-density Au nanorod optical field-emitter arrays

We demonstrate the design, fabrication, characterization, and operation of high-density 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 space-charge-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 GV m(-1), and energy spectra of emitted electrons have been measured using an electron time-of-flight spectrometer.

Nanostructured silicon photo-cathodes for x-ray generation

We report the fabrication and characterization of ultrafast laser triggered nanostructured silicon photo-cathodes for x-ray generation via inverse Compton scattering. A highly uniform array of ~2200 silicon pillars with 5 μm array pitch, where each pillar is capped by a nanosharp tip, shows stable current emission using 35 fs, 800 nm laser pulses. The cathodes can emit at 3.6 nA average current over 8-million 1.2 pC electron bunches when excited with 9.5 μJ laser pulses with no degradation of the emission characteristic of the cathode, showing that silicon-based photon-triggered cathodes processed with standard CMOS processes and operated at high vacuum can function for extended periods without performance degradation.

Multiplexing and scaling-down of nanostructured photon-triggered silicon field emitter arrays for maximum total electron yield

Femtosecond ultrabright cathodes with spatially structured emission are a critical technology for applications such as free-electron lasers, tabletop coherent x-ray sources, and ultrafast imaging. In this work, the optimization of the total electron yield of ultrafast photon-triggered field emission cathodes composed of arrays of nanosharp, high-aspect-ratio, single-crystal silicon pillars is explored through the variation of the emitter pitch and height. Arrays of 6 nm tip radius silicon emitters with emitter densities between 1.2 and 73.9 million tips cm(-2) (hexagonally packed arrays with emitter pitch between 1.25 and 10 μm) and emitter height between 2.0 and 8.5 μm were characterized using 35 fs 800 nm laser pulses. Three-photon electron emission for low-energy (<0.3 μJ) light pulses and strong-field emission for high-energy (>1 μJ) light pulses was observed, in agreement with the literature. Of the devices tested, the arrays with emitter pitch equal to 2.5 μm produced the highest total electron yield; arrays with larger emitter pitch suffer area sub-utilization, and in devices with smaller emitter pitch the larger emitter density does not compensate the smaller per-emitter current due to the electric field shadowing that results from the proximity of the adjacent tips. Experimental data and simulations suggest that 2 μm tall emitters achieve practical optimal performance as shorter emitters have visibly smaller field factors due to the proximity of the emitter tip to the substrate, and taller emitters show marginal improvement in the electron yield at the expense of greater fabrication difficulty.

Space Charge Effects in Strong-Field Emission From a Nanostructured Si Cathode

Ultrafast photoemission from a nanostructured Si-cathode array comprised of nano-sharp tips (~5 nm radius of curvature) was studied. Total current yield and electron spectra indicate a transition from multiphoton to strong-field emission followed by virtual cathode formation.

High-density optically actuated Au nanorod electron emitter arrays

In this work we investigate the use of Au nanorods as optically actuated electron emitter arrays. We have fabricated high-density arrays of 10 nm diameter Au nanorods via electron beam lithography and studied the effects of, emitter array density on charge yield, and laser intensity on emitter morphology, respectively. Additionally, we have numerically simulated the magnitude of the local electric field at the surface of emitter arrays with various layouts and geometries. This investigation thus provides us with a better understanding of optically actuated electron emission phenomena within high-density electron emitter arrays.

Pitch scaling of ultrafast, optically-triggered silicon field emitter arrays

Ultrafast, optically-triggered field emitter arrays are an exciting new area of research with potential application in ultrafast imaging and in coherent x-ray sources based on inverse Compton scattering. The fabrication and preliminary experimental results of pitch scaling on emitter arrays using n-type silicon wafers and scalable CMOS processing is studied to better understand the effect of tip to tip field interaction, space charge and fabrication limitations.

Current limitation in large-area self-aligned gated field emission arrays

The emission current of Pt-coated self-aligned gated tip arrays deviates from FN behavior at current levels above 100 nA/tip. The space charge force on emitting electrons has been calculated for the worst-case scenario in which the electrons are emitted from a single point and travel to the anode along the same trajectory. It is suggested that the current limitation is most likely due to limitation in supply of electrons rather than the space charge effect.