Abstract:
A nanoscale electron shuttle with two elastically mounted conductors positioned within a gap between conductors produces asymmetrical electron conduction between the conductors when the conductors receive an AC signal to provide for rectification, detection and/or power harvesting.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with United States government support awarded by the following agencies:
         NAVY N66001-07-1-2046   USAF/AFOSR FA9550-08-0337       

     The United States government has certain rights in this invention. 
    
    
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     N/A 
     BACKGROUND OF THE INVENTION 
     The present invention relates to devices for converting free-space electromagnetic radiation to electrical power and in particular to a rectification element employing an electron shuttle useful for such a device. 
     “Rectennas” are antennas that may receive radio signals and rectify them to generate electrical power for wireless power transfer. An example rectenna system was used in 1964 to power a tethered helicopter holding the rectenna and receiving a beam of microwave radiation from a ground-based microwave transmitter. 
     Potential applications for rectennas include both large-scale power transfer applications such as the communication of power between satellite and earth based stations as well as smaller scale applications such as powering RFID tags, biomedical implants, or the like. The use of rectennas is not limited to radio signals but has been proposed for electromagnetic signals at light frequencies as an alternative to standard photocells. 
     A limitation in the use of rectennas, particularly for low power density radiation, comes from the rectifying element necessary to convert an electromagnetic signal to useful power. A free-space electromagnetic signal will, in general, be an alternating current (AC) signal with an average current (and voltage) of zero (zero bias). In order to obtain useful continuous electrical power, the AC signal normally must be converted by rectification to a signal with a non-zero average (DC signal). 
     Standard junction semiconductors, such as pn diodes, may be used for rectification but are relatively inefficient and have high forward bias voltages resulting in lost power in the junction during the rectification process. Such high forward bias values can also make it impractical to extract power from low power density signals where these voltages are not readily obtained at the antenna output. For light frequency electromagnetic signals, the junction capacitance of a standard junction diode can prevent the required high-speed operation. 
     SUMMARY OF INVENTION 
     The present invention provides a rectifier using an electron shuttle that operates by transferring electrons between two terminals in vibratory mode which may be asymmetrical under certain operating conditions to rectify current. The potentially high-speed operation of this rectifier and low energy loss may permit improved rectenna design. 
     In one embodiment, the present invention provides a power collector for electromagnetic radiation having an antenna structure tuned into at least one wavelength of a free-space electromagnetic signal and a rectification unit communicating with the antenna structure. The rectification unit includes a first and second electrical conductor having corresponding first and second ends approaching each other across a gap and at least two elastically mounted conducting elements positioned within the gap, each to permit shuttling of electrons between each other and at least one of the first and second electrical conductors with vibration of the two elastically mounted conducting elements. The conducting elements operate in a coupled mode to provide a non-zero, average current flow between the first and second electrical conductor when excited by an electrical signal of the free-space electromagnetic signal. 
     It is thus a feature of at least one embodiment of the invention to provide a new rectenna design having substantially improved performance particularly for low power density signals. 
     The elastically mounted conducting elements may have a static separation from one of the first and second ends of less than 100 nanometers. The height of the pillars may be less than 1000 nm and a diameter of the pillars maybe less than 100 nm. 
     It is thus a feature of at least one embodiment of the invention to provide a nanoscale device suitable for efficient high-frequency operation. 
     The first and second electrical conductors may be metallization layers on a planar substrate and the elastically mounted conducting elements may be metallization layers on the top of pillars extending upward from the substrate from a depression between the first and second electrical conductors. The substrate may be a silicon-on-oxide substrate and the pillars may terminate in the oxide layer for electrical isolation. 
     It is thus a feature of at least one embodiment of the invention to provide a simple method of producing the necessary electrically isolated elements using standard integrated circuit techniques and materials. 
     The arrangement of the elastically mounted conducting elements with respect to the first and second electrical conductors and/or the shape of at least one of the elastically mounted conducting elements and the first and second electrical conductors may include a predetermined asymmetry to promote a predetermined direction of spontaneous symmetry breaking. 
     It is thus a feature of at least one embodiment of the invention to produce predictable spontaneous symmetry breaking necessary for a practical rectifier. 
     The first and second electrical conductors may be brachiated to have multiple first and second ends each with corresponding elastically mounted conducting elements, the conducting elements operating in a coupled mode to provide parallel current flow between the first and second electrical conductors. 
     Alternatively or in addition, the power collector may further include a third and fourth electrical conductor having corresponding first and second ends approaching each other across a gap and at least two elastically mounted conducting elements positioned within the gap to permit shuttling of electrons between each other and at least one of the third and fourth electrical conductors with vibration of the two elastically mounted conducting elements so that the conducting elements operate in a coupled mode to provide a net average current flow between the third and fourth electrical conductor when excited by an AC waveform applied across the first and second electrical conductor having an average value of zero. The second conductive element may be connected to the first conduct development to provide for serial current flow from the first conductive element to the fourth conductive element. 
     It is thus a feature of at least one embodiment of the invention to provide a rectification system having an arbitrary current capacity or voltage breakdown by the parallel and/or serial connection of many devices. 
     The rectification unit may provide rectification in a first polarity at a first set of frequencies and may further include a frequency filter selectively passing the first set of frequencies from the antenna to the rectification unit. 
     It is thus a feature of at least one embodiment of the invention to preprocess the electromagnetic signal to promote operation at a given polarity and/or efficiency. 
     The frequency filter may be implemented at least in part by antenna geometry. 
     It is thus a feature of at least one embodiment of the invention to provide a simple and flexible way of eliminating inefficient modes of operation, for example, of frequencies which cause reverse current flow. 
     These particular features and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a perspective exploded view of a rectenna of the present invention showing an array of antennas each having an associated rectification unit comprised of at least two vibratory pillars separated across a gap; 
         FIG. 2  is a cross-sectional view along line  2 - 2  of  FIG. 1  showing the suspension of conducting elements on top of the pillars as metallization layers; 
         FIG. 3  is a graph showing DC current obtained across the gap of the rectification circuit at different frequencies; 
         FIG. 4  is an expanded portion of the graph of  FIG. 3  at approximately 589 MHz showing on and off resonance points having greater and lesser DC current flow; 
         FIG. 5  is an IV-diagram comparing current flow at the on and off resonance points of  FIG. 4  showing rectification at the on resonance; 
         FIG. 6  is a simplified diagram of the rectification unit of the present invention showing the arraying of multiple units in series and parallel connections; and 
         FIG. 7  is a block diagram of the electrical connection of multiple antennas of a rectenna using the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIG. 1 , antenna array  10  of the present invention may provide for multiple antenna elements  12  designed to receive electromagnetic radiation  14 . The multiple antenna elements  12  maybe electrically interconnected in series or in parallel to provide for desired power voltage and current as will be described below. 
     Each antenna element  12  may, for example, be a dipole providing for a pair of arms  15 , here shown in a spiral configuration, for broadband frequency sensitivity. The arms  15  may connect to a rectification element  16  for extracting power from the electromagnetic radiation  14  received by the antenna element  12 . The rectification element  16  may be an individual rectifier or a full wave bridge of a type understood in the art comprised of one or more rectifiers  17 . 
     Referring also to  FIG. 2 , each rectifier  17  may include a first and second conductor  18  and  20  opposed across a gap  22  containing a first and second elastically mounted conducting element  24   a  and  24   b  therebetween. The rectifier  17  may be constructed on a substrate  26 , for example, a silicon on insulator (SOI) wafer having a first upper silicon layer  28  approximately 190 nm in thickness separated by a thin silicon dioxide insulator  30  of approximately 350 nm thickness from a lower silicon handle  32  of arbitrary thickness. The first and second conductor  18  and  20  may be metallization layers on top of the upper silicon layer  28 . 
     A depression  34  in the form of a channel may be etched between proximate ends of the conductors  18  and  20  excluding the material of two pillars  36   a  and  36   b  extending upward from the depression  34  and aligned along an axis  40  extending between the first and second conductors  18  and  20 . The upper ends of the pillars  36   a  and  36   b  may be metalized to create two elastically mounted conducting elements  24   a  and  24   b , the elasticity provided by flexure of the pillars  36   a  and  36   b.    
     The pillars  36  may be approximately 250 nm tall with a diameter of approximately 65 nm. A spacing  38  between the pillars may be 17 nm and less than the gaps  41  between either pillar  36   a  or  36   b  and the closest conductor  18  or  20 . This spacing provides increased electrostatic communication between the pillars  36   a  and  36   b  providing the necessary coupling for spontaneous symmetry breaking as will be described. The gaps  41  are approximately equal making the structure essentially symmetric along the axis  40  extending from conductor  18  to conductor  20  and through each of elastically mounted conducting elements  24   a  and  24   b . Pillar diameter as used herein refers to the diameter of a cylinder that would closely contain the pillar with the pillar axis aligned with the cylinder axis and does not require that the pillars be perfect cylinders. 
     An alternating current electrical signal  46  from one or more antenna elements  12  maybe applied across conductors  18  and  20  to promote a vibratory oscillation  42  of the pillars  36   a  and  36   b  under the influence of the variable electrostatic field between the conductors  18  and  20 . This vibratory oscillation  42  may have a component aligned with axis  40  but will generally occur in three dimensions to provide for complex vibratory modes. 
     During in the vibratory oscillations  42 , elastically mounted conductive elements  24   a  and  24   b  may exchange charges between conductive element  24   a  and conductor  18  and between conductive element  24   b  and conductor  20  by electron tunneling. The general operation and construction of such charge transfer devices is described, for example, in: “Nanopillar Arrays On Semiconductor Membranes As Electron Amplifiers”, H. Qin, H. S. Kim, and R. H. Blick, Nanotechnology 19, 095504 (2008); “Field Emission from a Single Nanomechanical Pillar”, Hyun-Seok Kim, Hua Qin, Lloyd M. Smith, Michael Westphall, and Robert H. Blick, Nanotechnology 18, 065201 (2007); “Effects of Low Attenuation in a Nanomechanical Electron Shuttle”, D. V. Scheible, Ch. Weiss, and R. H. Buick, Journal of Applied Physics 96, 1757 (2004); “A Quantum Electro Mechanical Device The Electro-Mechanical Single Electron Pillar”, Robert H. Blick and D. V. Scheible, Superlattices and Microstructures 33, 397 (2004); “Silicon Nano-Pillars for Mechanical Single Electron Transport”, D. V. Scheible and R. H. Buick, Applied Physics Letters 84, 4632 (2004); “Nanomechanical Resonator Shuttling Single Electrons at Radio Frequencies”, A. Erbe, Ch. Weiss, W. Zwerger, and R. H. Blick, Physical Review Letters 87, 096106 (2001); “Coulomb blockade in Silicon Nanostructures”, A. Tilke, F. Simmel, R. H. Buick, H. Lorenz, and, J. P. Kotthaus, Progress in Quantum Electronics 25, 97 (2001), all hereby incorporated by reference. 
     Referring now to  FIG. 3 , at different frequencies of the signal  46  (having an average or DC voltage of zero per a free-space electromagnetic signal), a net average current I DS  will flow between conductor  18  and  20 . While the inventors do not wish to be bound by a particular theory, this rectification is believed to be caused by spontaneous symmetry breaking theoretically predicted by Ahn, K. H., Park H. C., Wiersig J, Hong J. as described in the paper: “Current Rectification By Spontaneous Symmetry Breaking In Coupled Nanomechanical Shuttles”, Phys. Rev. Lett. 2006 Nov. 24; 97(21): 216804. Epub 2006 Nov. 22, hereby incorporated by reference. This spontaneous symmetry breaking results in an asymmetrical current flow despite the symmetrical structure of the rectifier  17 . In the graph of  FIG. 3 , a number of resonance peaks are shown labeled with fractions p/q based on a deduced fundamental mode at 504 MHz where p/q equals one. It should be noted that the upwardly extending peaks represent the first polarity of current rectification while the downwardly extending peaks represent the opposite direction of current rectification. Referring momentarily to  FIG. 1 , the antenna elements  12  may be tuned to preferentially receive only the frequencies of the upward (or downwardly) extending peaks to ensure maximum power harvesting capabilities. Alternatively, a filter may be placed between the antenna and the rectification element  16  to accomplish a similar purpose. 
     Referring now to  FIG. 4 , a detail of one peak  50  of  FIG. 3  is shown for two operating frequencies: on-resonance frequency  52  and off-resonance frequency  54 .  FIG. 5  shows the current-voltage characteristics at these frequencies of approximately 590 MHz and 630 MHz, respectively. Of significance, the IV-curve  56  for the off-resonance frequency  54  passes closely through zero current and zero voltage in the manner of a conventional resistor whereas the curve  58  for the on-resonance frequency  52  shows a current of approximately 30 pico amps at zero voltage. The voltage indicated in the IV-curve is the average voltage or DC offset of the signal  46 . Accordingly at resonance, a rectification of the signal  46  occurs. 
     Referring now to  FIG. 6 , the rectification element  16  of the present invention may be assembled in series chains of rectifiers  17  as indicated by rectifier  17   a  and  17   b  where the first conductor  18  is positioned across a first set of elastically mounted conductive elements  24  from a second conductor  20  which is joined to a third conductor  60  positioned across a second set of elastically mounted conductive elements  24  from a fourth conductor  62  so that current flows in series from conductor  18  to  62 . This configuration decreases the amount of voltage across each element  24  thus allowing higher voltage capacity of the rectification element  16 . 
     Alternatively or in addition, rectifier  17   a  may be placed in parallel with rectifier  17   c  and  17   d  so the current may pass in parallel through each of these rectifying elements increasing the total current handling capacity of the rectification element  16 . 
     Referring now to  FIG. 7 , the antenna array  10  may receive electromagnetic radiation  14  at multiple antenna elements  12  that may, for example, be connected in series as shown to provide increased voltage to a voltage conditioner  72  or in parallel (not shown) to provide increased current to the voltage conditioner  72 , the latter which may include filter elements such as capacitors and the like and/or DC to DC converters for providing power to a load  74 . In this way the invention may scavenge or collect the energy from electromagnetic radiation  70  to be used to provide power to a device. 
     In alternative embodiments more than two elastically mounted conductive elements  24  may be placed in the gap between the conductors  18  and  20 . 
     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.