Onset of charge interaction in strong-field photoemission from nanometric needle tips

Strong-field photoemission from nanostructures and the associated temporally modulated currents play a key role in the development of ultrafast vacuum optoelectronics. Optical light fields could push their operation bandwidth into the petahertz domain. A critical aspect of their functionality in the context of applications is the impact of charge interaction effects. Here, we investigated the photoemission and photocurrents from nanometric tungsten needle tips exposed to carrier-envelope phase (CEP)-controlled few-cycle laser fields. We report a characteristic rapid increase in the intensity-rescaled cutoff energies of emitted electrons beyond a certain intensity value. By comparison with simulations, we identify this feature as the onset of charge-interaction dominated photoemission dynamics. Our results are anticipated to be relevant also for the strong-field photoemission from other nanostructures, including photoemission from plasmonic nanobowtie antennas used in CEP-detection and for PHz-scale devices.

A Universal Method to Weld Individual One-Dimensional Nanostructures with a Tungsten Needle Based on Synergy of the Electron Beam and Electrical Current

One-dimensional (1D) nanostructures are extensively used in the design of novel electronic devices, sensors, and energy devices. One of the major challenges faced by the electronics industry is the problem of contact between the 1D nanostructure and electrode, which can limit or even jeopardize device operations. Herein, a universal method that can realize good Ohmic and mechanical contact between an individual 1D nanostructure and a tungsten needle at sub-micron or micron scale is investigated and presented in a scanning electron microscope (SEM) chamber with the synergy of an electron beam and electrical current flowing through the welded joint. The linear I‒V curves of five types of individual 1D nanostructures, characterized by in-situ electrical measurements, demonstrate that most of them demonstrate good Ohmic contact with the tungsten needle, and the results of in-situ tensile measurements demonstrate that the welded joints possess excellent mechanical performance. By simulation analysis using the finite element method, it is proved that the local heating effect, which is mainly produced by the electrical current flowing through the welded joints during the welding process, is the key factor in achieving good Ohmic contact.

Development of Minimally Invasive Microneedle Made of Tungsten – Sharpening Through Electrochemical Etching and Hole Processing for Drawing up Liquid Using Excimer Laser –

A tungsten needle was fabricated by electrochemically etching a thin wire with a diameter of 100 µm, with the goal of using it in minimally invasive medical treatments. The sharpness and smoothness of the tip were effective for easy insertion because they provided a large stress concentration and small amount of friction, respectively. An experiment involving the insertion of the fabricated needle into artificial skinmade of silicone rubber was carried out. The resistance force during the insertion was greatly reduced because of the small size of the needle, which was comparable to a mosquito’s proboscis. Despite the ultra-thin shape, the microneedle neither buckled nor broke because of the high hardness of the tungsten material. A hole was fabricated in the tungsten needle using excimer laser processing and electrochemical etching. Water and blood sampling were successfully achieved using this needle.

Welding of Fine Metal Particles by Using a Tungsten Needle and Measurement of the Strength of the Joints

Gold particles several tens of micrometers in size were welded onto a gold substrate. High voltages of 4 kV or more were applied to a tungsten needle in contact with the particle on the substrate at a very low contact pressure. The particle was welded to the substrate in an instant with sparks. The needle was then retracted to 20 !m above the particle and a high voltage of about 2kV was applied to the needle. An electric discharge between the needle and the substrate enveloped the particle, and the joint was strengthened. The joint strength was measured, and the fractured surface was examined by scanning electron microscopy and scanning laser microscopy. The mechanism of joint strengthening is discussed. It is clear that an inert gas flow during the electric discharge is necessary to strengthen the joints.