Manuel Brugat

Modulation of the current in a field emitter caused by a continuous wave or pulsed laser: Simulations and experimental results

Numerical simulations suggest that a laser would increase field emission current due to a resonant interaction in which tunneling electrons exchange quanta with the laser, and this has been confirmed experimentally. Floquet methods are used to determine the steady-state response to a cw laser, and the transient response to a laser pulse is computed with a product formulation that does not require absorbing boundary conditions or wave packets. Resonances in the steady-state response typically result in a single broad peak when the full distribution of energies in a metal are considered. The transient solutions show delays equal to the semiclassical tunneling time, which is defined as the time for traversing the inverted barrier. A 1 MHz tunneling current of 0.4 nA was measured when a tungsten field emitter with a dc current of 5 μA was illuminated with a laser diode amplitude modulated at 1 MHz. The laser increases the dc tunneling current by 270 nA, which is attributed to tip heating, but the rf current h...

Prototypes using metal, carbon fiber and composite field emission sources modulated by a laser beam.

Field emission of electrons from a variety of metallic, carbon fiber and composite metal-insulator micropoint cathodes was employed in this study. Tungsten, carbon fiber and ZrC tips, were studied using a field emission microscope. These cathodes were characterized and the current-voltage (I-V) characteristics were determined. A variety of surface treatment procedures were carried out to increase the stability of emission. These electron sources were mounted in sealed prototype field emission tubes, while others were tested under medium, high and UHV conditions. The emission current switch-on phenomenon was found with all non-metallic cathodes. The emitters were then subjected to a square wave-modulated, maximally focused laser diode beam (lambda = 658 nm, 30mW). The beam impedance (approximately 1 Gohms) and the anode capacitance (approximately 10 pF) act as a low-pass filter.

Measurement of field emission current variations caused by an amplitude modulated laser

Abstract A laser diode ( λ =658 nm, 30 mW) was focused on a tungsten field emitter tip in vacuum. The time dependent emitted current was measured while the laser diode was TTL-amplitude modulated at frequencies from 10 Hz to 50 kHz. It was found that below 100 Hz, the rise and fall times of the field emitted current ranged from 2 to 3 ms, but beginning at 300 Hz, the rise and fall times become shorter as the frequency is increased. The incremental change in the total emitted current is greater at 5 kHz than at any of the other frequencies. Since the only parameter changed was the modulation frequency, we conclude that these effects are not consistent with the assumptions from photofield studies that (1) the increase in the total current is mainly thermal and therefore slow, (2) the current increase is small, and (3) the effect is negligible at the red end of the spectrum. From the carried out experiments, we conclude that the current increase is not caused by tip heating, but rather, they show the significance of the circuit parameters of the field emitter tube itself.

Design of field emitter devices for microwave and terahertz applications

Simulations show that photomixing in resonant laser-assisted field emission can cause the emitted current to oscillate at frequencies from dc to over 100 THz, so this technique shows promise for new ultrawideband devices. However, it is necessary to find a means to couple these signals from the apex of the tip to an external load while avoiding the effects of dispersion of the bunched electron beam and the low-pass filter action caused by the series impedance of the electron beam and the shunting capacitance of the electrodes. We have designed a new type of device in which the signal would propagate as a surface wave on the emitting tip itself. The procedures for design are defined, and graphs are given to determine the dimensions. Examples of designs are presented, which include devices to generate signals from 10 GHz to 10 THz, from 100 GHz to 100 THz, and a device that would be simpler to construct for use from 10 MHz to 10 GHz.

Measurements of the self-sustained enhancement of field emission by carbon fiber microemitters.

Two types of self-sustained enhancement in field emission by carbon fibers are described. In the first, the field is increased until the emission current switches from zero to between 1 and 10 microA. Next the field is reduced, but not so far that the current would drop. Then the current remains for several hours to several days, with transient increases from the 10 microA to between 14 and 22 microA. It is believed that the transients are caused by the activation of new microtips on the fiber surface. These effects were noted when the carbon fiber tip was mounted in a closed glass vacuum bulb pumped by barium getters, and also in a vacuum system using the combination of a molecular drag pump and ion pumps. The second type of enhancement occurs under ultrahigh vacuum conditions, during in situ thermal treatment of the carbon fiber tip while the emission current is about 2.5 microA. A specially built cathode assembly enables heating the tip to approximately 725 degrees C. After continuous heating at 570 degrees C for 20 to 35 h, the current suddenly increases to between 13 and 25 microA. This enhancement is reversible if the emitted current is kept at the newly increased value for at least 30 min. The current-voltage characteristics at several temperatures were recorded and analyzed. Similar field-forming phenomena were previously observed with Molybdenum and ZnO-W tips.

Simulations of photon-assisted tunneling using the Fokker-Planck equation to model the scattering of electrons within the emitting metal tip.

A method to simulate photon-assisted tunneling is developed, and applied to model laser-assisted field emission from metals. Our simulations show that most of the exchange of quanta between the electrons and the radiation occurs within the emitting metal tip. In typical experiments (lambda = 670 nm with tungsten metal) the depth of penetration for the radiation is four times the mean free path for electrons at the Fermi level, so it is necessary to allow for scattering. We use a Floquet expansion with the time-dependent Schrödinger equation to allow for the exchange of quanta between the electrons and the radiation field. Multiparticle effects are modeled with the density functional theory within the local density approximation for the Kohn-Sham exchange and correlation, and the Fokker-Planck formulation is used to determine the effects of scattering on the energy distribution of the electrons.