Optical beam modulation is accomplished with the aid of a semiconductive nanomembrane, such as a silicon nanomembrane. A photocathode modulates a beam of charged particles that flow between the carbon nanotube emitter and the anode. A light source, or other source of electromagnetic radiation, supplies electromagnetic radiation that modulates the beam of charged particles. The beam of charged particles may be electrons, ions, or other charged particles. The electromagnetic radiation penetrates a silicon dioxide layer to reach the nanomembrane and varies the amount of available charge carriers within the nanomembrane, thereby changing the resistance of the nanomembrane. As the resistance of the nanomembrane changes, the amount of current flowing through the beam may also change.

TECHNICAL FIELD

This disclosure relates to modulating a beam of charged particles with electromagnetic radiation.

BACKGROUND INFORMATION

In a variety of electronic systems, it is useful to modulate a beam of charged particles, such as electrons or ions. Electron beams are employed in heating systems, imaging systems, display systems, and high-frequency (e.g., radio frequency) signal processing. Examples of systems employing ion beams include neutron generators, which may be used to detect nuclear materials, explosives, landmines, drugs, or other contraband, and which may have industrial applications, such as qualifying coal streams, cement, or other commodity items. In these systems, as well as others, the flow of charged particles may be modulated, e.g., turned on, turned off, increased, decreased, or cycled at some frequency.

In particular, it may be useful to modulate the beam of charged particles with an electromagnetic radiation source, e.g. a light source, such as a laser. Electromagnetic radiation may convey signals with a relatively high frequency, and in some instances, these signals may be transmitted between electrically isolated components.

DETAILED DESCRIPTION

As explained below, optical beam modulation may be accomplished with the aid of a semiconductive nanomembrane, such as a silicon nanomembrane. A silicon nanomembrane (“SiNM”) is a kind of semiconductor with a band gap around 1 eV, which is similar to bulk silicon. It is, however, different from bulk silicon in that its conductivity significantly varies with thickness. As illustrated in the graph inFIG. 1, the sheet resistance of a SiNM on isolated SiO2(e.g., on a silicon-on-insulator substrate “SOI substrate”) increases sharply as the thickness of the silicon nanomembrane is reduced. This effect may be due to carrier depletion. When the membrane thickness is less than 100 nm, the sheet resistance can reach as high as 107to 1011Ω/unit of square area. Such a high resistance is about equivalent to, or larger than, the typical working impedance of carbon nanotube (CNT) field emission devices (e.g., in devices operating near a kV at sub mA or mA regime, or around 107Ω impedance).

Silicon nanomembranes are electrically responsive to electromagnetic radiation. As a semiconductor with a relatively narrow band gap, a SiNM's resistance is adjustable by visible light illumination or infrared (IR) light illumination. And, its ultra-thin thickness, absence of defects, and single crystalline characteristics are believed to provide a relatively fast photo-response and relatively high sensitivity to light.

By exploiting these properties, semiconductive nanomembranes can be used to modulate a beam of charged particles with electromagnetic radiation, e.g., in a photocathode. Examples of such embodiments are described below: an off-chip CNT/SiNM photocathode, an on-chip CNT/SiNM photocathode, and a photocathode formed on a glass substrate. In some embodiments, these devices may generate high-frequency modulated electron beams that are optically controlled. Note that the present invention is not limited to these specific embodiments.

FIG. 2illustrates a system10having a photocathode12. The system10may be part of an imaging system, such as a radar system, a medical imaging system (e.g., an x-ray system), a terrestrial or satellite-based communications system, a heating system (e.g., a microwave oven), an electron accelerator, a particle accelerator, a neutron generator, or any system utilizing an electron beam source. For instance, the photocathode12may be an electron beam source in a traveling wave tube, a klystron, a magnetron, or other microwave amplifier, microwave device, or x-ray device.

The illustrated photocathode12includes a nanomembrane14, an electrode16, a silicon dioxide layer18, a carbon nanotube emitter20, and a substrate22, and it may be in electrical communication with an anode24, a current source26, and a voltage source28. The nanomembrane14may be a semiconductive material having a thickness less than about 200 nm, or more preferably about 150 nm, or more preferably about 100 nm, or more preferably about 50 nm. The nanomembrane14may include or consist essentially of silicon, e.g., single-crystal silicon, or other semiconductive materials. The electrode16may include a conductive material, such as aluminum or an aluminum alloy, and may include various liner materials. The silicon dioxide layer18may be deposited or grown, e.g., as a native oxide. The carbon nanotube emitter20may include carbon nanotubes deposited or grown on the nanomembrane14. The substrate22may include a dielectric material, such as silicon oxide, formed on a silicon wafer or other substrate material, and the photocathode12may be formed on the dielectric material.

In operation, the photocathode12modulates a beam of charged particles30that flow between the carbon nanotube emitter20and the anode24, as illustrated byFIG. 3. A light source, or other source of electromagnetic radiation32, supplies electromagnetic radiation that modulates the beam of charged particles30. The beam of charged particles30may be electrons, ions, or other charged particles. The source of electromagnetic radiation32may be a laser, a light-emitting diode, ambient light, or other source. Electromagnetic radiation from the electromagnetic radiation source32penetrates the silicon dioxide layer18to reach the nanomembrane14and varies the amount of available charge carriers within the nanomembrane14, thereby changing the resistance of the nanomembrane14. As the resistance of the nanomembrane14changes, the amount of current flowing through the beam30may also change. Thus, the beam of charged particles30may be controlled by the source of electromagnetic radiation32.

The photocathode12illustrated byFIGS. 2 and 3may be characterized as an off-chip type photocathode, as the beam of charged particles30travels to an anode24that is separate from the substrate22.

FIG. 4illustrates an embodiment of a process34for making an of type photocathode, such as described above. The process34may begin with obtaining a nanomembrane substrate, as illustrated by block36. Obtaining a nanomembrane substrate may include purchasing a nanomembrane substrate or manufacturing a nanomembrane substrate, such as a silicon-on-insulator substrate having an appropriate silicon thickness. Next, the nanomembrane substrate may be chemically cleaned, as illustrated by block38, and an aluminum electrode may be formed on a selected area of the nanomembrane substrate, as illustrated by block40. Forming an aluminum electrode may include depositing, e.g., with physical vapor deposition, a layer of aluminum on the nanomembrane substrate, and patterning the resulting aluminum film with lithography (e.g., photolithography) and etching. A silicon dioxide layer may be formed on the nanomembrane substrate, as illustrated by block42, by depositing and patterning silicon dioxide or by growing a native oxide layer in exposed areas. Next, carbon nanotubes may be deposited or gown on a third selected area of the nanomembrane substrate, as illustrated by block44. To test the photocathode produced by these steps, the nanomembrane substrate may be illuminated, and a resulting current may be measured, as illustrated by block46.

FIG. 5illustrates an embodiment of an on-chip photocathode48. In this embodiment, an anode50is formed on a substrate22. The anode50may be formed in an exposed region52of the substrate22in which a nanomembrane14has been thinned or removed. In operation, a beam of charged particles30travels across the substrate22, between the carbon nanotube emitter20and the anode50.

The on-chip photocathode48may be formed with a process54illustrated inFIG. 6. The process54may begin with obtaining a nanomembrane substrate, as illustrated by block56, and removing the nanomembrane from a selected area, as illustrated by block58. The nanomembrane may be removed from the selected area by patterning the substrate with photolithography and etching the nanomembrane from the selected area to leave silicon dioxide exposed. For instance, the nanomembrane may be etched with a chemical etch. Next, the nanomembrane substrate may be chemically cleaned, as illustrated by block60, and aluminum electrodes may be formed both in the above-mentioned selected area and in another selected area, as illustrated by block62. In some embodiments, this step may form both the anode and the electrode that connects to the carbon nanotube emitter. A layer of silicon dioxide may be formed or gown on a third selected area of the nanomembrane substrate, as illustrated by block64, and carbon nanotubes may be formed (e.g., deposited or grown) on a fourth selected area of the nanomembrane substrate, as illustrated by block66. Finally, the nanomembrane substrate may be tested by illuminating the nanomembrane substrate and measuring a resulting current, as illustrated by block68.

FIG. 7illustrates an embodiment of a photocathode70that may be formed on a glass substrate72(or an equivalent substrate transparent to the utilized electromagnetic radiation from the source73). An electromagnetic radiation source73may be communicatively coupled to the photocathode70through the glass substrate72. For instance, an optical fiber may be bound to the back surface of the glass substrate72, and light may be transmitted through the glass substrate72to the nanomembrane14. The remainder of the photocathode70operates similarly as the photocathode48.

The photocathode70may be formed with a process74illustrated inFIG. 8. The process74may include obtaining a nanomembrane substrate, as illustrated by block76, and transferring the nanomembrane to a glass-substrate, as illustrated by block78. Transferring the nanomembrane may include lifting the nanomembrane from the nanomembrane substrate, e.g., by cleaving the nanomembrane. Next, the nanomembrane and glass substrate may be annealed to enhance bonding between the nanomembrane and the glass substrate, as illustrated by block80. The resulting bonded substrate may then be chemically cleaned, as illustrated by block82, and an aluminum electrode may be formed on a selected area of the bonded substrate, as illustrated by block84. Next, a silicon dioxide layer may be formed in another selected area of the bonded substrate, as illustrated by block86, and carbon nanotubes may be formed on a third selected area of the bonded substrate, as illustrated by block88. Finally, the photocathode yielded by the process74may be tested by illuminating the bonded substrate and measuring a resulting current, as illustrated by block90.

In some embodiments, the previously described photocathodes may include electrodes configured to further enhance the response and the sensitivity of the photocathodes. For example, the electrodes in one or more of the previously described embodiments may have a comb-like shape or other shape designed to increase responsiveness or sensitivity. It should also be noted that while the previously described embodiments show the beam of charged particles flowing toward the voltage source, in other embodiments, the polarity of the voltage source may be reversed, and the previously described devices may be used to form optically modulated ion beams. Such ion beams made be used in a variety of systems, such as a high-frequency ionizer or a neutron generator.

In other embodiments, the anode (24inFIGS. 2 and 3;50inFIGS. 5 and 7) may be a screen, grid, or a perforated conducting electrode that allows part of the electron or ion beam to pass through and be acted on by electric fields imposed by other electrodes, such as focusing electrodes or high voltage targets, as in the case of x-ray sources or neutron sources.