Abstract:
A rectifying nanoscale structure is disclosed, which, upon exposure to incident light, is induced to propagate electrons in an anisotropic fashion, with exceptionally low losses. A rectifying nanoscale structure which exploits the phenomena of plasmons to modify its light absorption and rectification properties is also disclosed.

Description:
FIELD OF THE INVENTION 
       [0001]    Embodiments of this invention relate to the methods, materials, and devices for the conversion of radiation into electricity 
       BACKGROUND 
       [0002]    A rectenna is a device that converts solar energy into electrical energy. Essentially, a rectenna comprises an optical antenna that absorbs incident solar radiation coupled to a diode that rectifies an AC voltage produced in the antenna by the incident solar radiation. 
         [0003]      FIG. 1  shows a schematic drawing of a rectenna  100 , which includes several components electrically coupled together. The rectenna  100  comprises optical antennas  102  that are integral with or connected with structures  104  that support a rectifying element  106 . Bias/control circuitry  108  is located at an end of the structures  104 . In use polarized light  110  is oriented so that E-field  112  is maximally coupled with the optical antennas  102  so that alternating currents are produced in the optical antennas  102  and the structures  104 . Under action of the rectifying element  106 , the currents are converted into a half-wave rectified output voltage. Bias/control circuitry  108  acts to apply a voltage bias to the rectifying element  106  so that its performance is maximized. The output voltage is conditioned by bias/control circuitry  108  to present minimum voltage ripple so that it is useful as a power source. 
         [0004]    Rectennas operate on incoming radiation in the microwave frequency range which can run into the high GHz range. In the case of incoming radiation higher than the microwave frequency range, e.g. radiation in the 5-500 THz range or greater, the efficiency at which a rectenna converts incoming radiation into an output voltage is reduced. This loss in efficiency may be attributed to skin and conductor resistance effects, parasitic capacitance of the rectifying element and the coupling junctions, as well as impedance mismatch between components of the rectenna. 
         [0005]    The above-described rectenna requires the integration of several components which include the optical antennas and the rectifying element, each of which may comprise several sub-components. The resulting geometry and interfaces between the components all contribute to the aforementioned loss in efficiency. 
       SUMMARY OF THE INVENTION 
       [0006]    According to a first aspect of the invention, there is described a rectifying nanoscale structure which, upon exposure to incident light, is induced to propagate electrons in an anisotropic fashion, with exceptionally low losses. 
         [0007]    According to a second aspect of the invention, there is described a rectifying nanoscale structure which exploits the phenomena of plasmons to modify its light absorption and rectification properties. 
         [0008]    According to a third aspect of the invention, there is described a rectifying nanoscale structure which exploits the phenomena of interference to enhance its efficiency. 
         [0009]    Other aspects of the invention will be apparent from the detailed description below: 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  shows a schematic drawing of rectenna known to the inventors; 
           [0011]      FIG. 2  shows a material for solar flux conversion, in accordance with one embodiment of the invention; 
           [0012]      FIG. 3  shows a cross section through several emitter-collector pairs, in accordance with one embodiment of the inventions; 
           [0013]      FIG. 4   a  shows a side view of a material for solar flux conversion, in accordance with another embodiment of the invention; 
           [0014]      FIG. 4   b  shows the material of  FIG. 4   a  in plan view; 
           [0015]      FIG. 5   a  shows a plan view of a device for solar flux conversion, in accordance with one embodiment of the invention; 
           [0016]      FIG. 5   b  shows the device of  5   a  in packaged form; 
           [0017]      FIG. 6  shows a solar flux conversion system, in accordance with one embodiment of the invention. 
           [0018]      FIGS. 7   a  and  7   b  show embodiments of an edge emitter designed to produce plasmons, in accordance with one embodiment of the invention; and 
           [0019]      FIGS. 8   a  and  8   b  show embodiments of an edge emitter coupled with an optical structure designed to improve efficiency of flux conversion, in accordance with one embodiment of the invention, 
           [0020]      FIGS. 9   a  and  9   b  show embodiments of a collector/edge emitter array, in accordance with another embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the invention. It will be apparent, however, to one skilled in the art that the invention can be practiced without these specific details. 
         [0022]    Reference in this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Moreover, various features are described which may be exhibited by some embodiments and not by others. Similarly, various requirements are described which may be requirements for some embodiments but not other embodiments. 
         [0023]    Embodiments of the present invention disclose a flux conversion method whereby radiation incident on a first structure results in an electron emission from the first structure towards a second structure by the process of field emission. Under the influence of the incident radiation, electrons in the first structure oscillate forming an alternating current or, more accurately, the composite of an almost infinite number of alternating currents. The geometry of the first and the second structures is such that electron field emission occurs at the first structure and not at second structure which serves to attract the electrons from the first structure. Advantageously, the first and the second structures together define a rectifier to rectify the alternating currents, without the need for a separate rectifier. Devices and materials based the flux conversion method are also disclosed. 
         [0024]    Referring now to  FIG. 2 , a material  200 , in accordance with one embodiment of the invention, for solar flux conversion is illustrated. Incident light  202  is shown propagating toward a first structure in the form of edge emitters  204 . The edge emitters  204  are positioned laterally apart and perpendicular to a second structure in the form of a collector structure  206 . The edge emitters  204  are shaped and dimensioned to emit electrons responsive to the light  202  incident thereon. In one embodiment, the edge emitters  204  may comprise one or more emitter films. The emitter films may be less than several hundred nanometers thick. As will be seen, the edge emitters  204  are mounted on supports  208 . The edge emitters  204  and the collector structure  206  are supported by a substrate  210 . The supports  208  may be insulating. Alternatively, the supports  208  may be conducting if the substrate  210  is insulating. The supports  208  may also be opaque or transparent depending on the needs of the application. The dimensions of the supports  208  may be dependent on the characteristics of the support material of which it is comprised, and the degree to which the supports  208  and the proximity of the edge emitters  204  to the substrate  210  contribute to parasitic capacitances. In one embodiment, the supports  208  have a height less than 10 microns, and a width some fraction of the width of the edge emitters  204 . The edge emitters  204  are biased by a voltage source  212 , which is connected to the edge emitters  204  through current limiting resistors  214 . Current limiting resistors  214  may be replaced by current limiting diodes or other semiconductor elements. In one embodiment, the incident light  202  may be polarized in which case currents  216  are induced. The currents  216  are oriented in a direction perpendicular to collectors  206 . These currents are aligned with the electric field applied by the voltage source  212 . With the proper bias voltage and appropriate dimensions for the edge emitters  204  and collectors  206 , the currents  216  induced by the incident radiation  202  will result in electron emission in the direction indicated by arrow  218  towards the collectors  206 . This will create a net direct current (DC) whose magnitude will be directly related to the intensity of the incident radiation  202 . In one embodiment, the material  200  may have a number of the above-described edge emitters  204  and collectors  206  connected to a bias voltage source to convert incident radiation into a DC current. 
         [0025]    In the material  200 , the geometry of each edge emitter  204  has to be such that when coupled with the incident radiation  202  the induced electric currents  216  are sufficiently strong result in electron emission from the edge emitter by the process of field emission. In the embodiment  200 , each edge emitter  204  has a body that defines as active area  204   a  that is operatively exposed to the incident radiation  202  and a thickness that is measured in a direction transverse to the active area  204   a.  In contrast to the edge emitters  204 , the collector  206  is shaped and dimensioned such that any induced electric currents in the collector  204  responsive to coupling with the incident radiation  202  is too weak to result in electron emission. As will be seen, collector  206  has a collection surface  206   a  on which electrons from the edge emitters  204  impinge, and a thickness measured in a direction transverse to the collection surface  206   a.  If the aspect ratio of each edge emitter  204  is defined as the ratio of the width of the active area  204   a  to the thickness of the edge emitter  204 , and the aspect ratio of the collector  206  is defined as the ratio of the thickness of the collector  206  to the width of the collection surface  206   a,  then the aspect ratio of the edge emitters  204  is higher than that aspect ratio of the collectors  206 . 
         [0026]    One skilled in the art would recognize that each edge emitter  204  is actually an antenna that is operatively coupled with incident radiation to produce free electrons, and that the combination of the edge emitter  204  with the collector  206  functions as a rectifier to rectify the alternating currents defined by the flow of free electrons within the edge emitter  204 . Thus, the material  200  is a form of rectenna. 
         [0027]    This material  200  exhibits low loss because the functions of antenna and rectifying element are structurally integrated and simple. Further, the antenna/edge emitter is a single component. Thus, parasitic capacitances are significantly reduced because there is no intervening structure. As described, the rectifying element in the form of a vacuum diode is realized in the combination of the edge emitter and the collector. The edge emitter and the collector are coupled via electrons which propagate ballistically from emitter to collector, minimizing conduction losses, and additional parasitic capacitance. 
         [0028]    In the material  200 , the geometry of each edge emitter  204  reduces the work function of the surface electrons such that incoming photons can more easily remove electrons from the edge emitter  204 . 
         [0029]    Referring now to  FIG. 3 , there is shown a cross-section through several emitter-collector pairs fabricated on a substrate  300 , in accordance with one embodiment. As can be seen edge emitters  302  are paired with several different collector structures  304 . The different collector shapes may be more easily fabricated depending on the particular manufacturing process in use. Under the influence of a similarly applied bias, carrier flow is induced along the direction indicated by arrows  306 . 
         [0030]      FIGS. 4   a  and  4   b  show an embodiment of a material  400  that may be used to convert radiation into electrical energy in accordance with the above-described conversion method. The material is shown in side view in  FIG. 4   a  and in plan view in  FIG. 4   b.  As will be seen, the material  400  comprises circular or disc-shaped edge emitters  402  fabricated on a substrate  404 . The edge emitter discs  402  are encircled by collector cylinders  406 . One advantage of the material  400  is it is less polarization dependent. That is to say that all of the currents induced by light which is randomly polarized can result in a net electron flow towards a collector  406  that encircles the disc  402 . The electron flow is indicated by arrows  408 . One skilled in the art will recognize that many other geometries are possible for emitter-collector pairs. 
         [0031]    Referring to  FIG. 5 , a conversion device in the form of a strip array  500  comprising an array of edge emitters  502  connected in parallel to a bias/controller  504 .  FIG. 5   a  shows the strip array in plan view, whereas  FIG. 5   b  shows the array  500  mounted in a package. For attracting electrons emitted from the edge emitters  502 , the device  500  comprises a plurality of collectors  506 . Each edge emitter  502  as a lateral dimension  508  which may be bounded approximately on the upper end by the transverse spatial coherence of the incident radiation. One possible mechanism for bounding the lower end relies on classical antenna theory. 
         [0032]    The transverse spatial coherence length for direct sunlight is given by L=0.16Rλ/ρ, where R is the distance to the sun (1.5×10 11  m) and ρ is the sun&#39;s radius (7×10 8  m). For optical wavelengths this is greater than 10 microns, so the lateral dimension  508  is less than 10 microns for the incident light electric field across an emitter  502  to be in phase. Optimum efficiency for a half-wave dipole antenna requires the antenna dimension in the wave oscillation direction to be integer multiples of ½ the wavelength of the incident light. With the shortest wavelength contained in solar radiation being about 200 nanometers, this corresponds to an emitter  502  dimension of greater than 100 nanometers. 
         [0033]    The separation  510  between edge emitters  502  and collectors  506  may be constrained by a combination of required diode behavior, manufacturing capabilities, and the required bias voltage, in one embodiment. In one embodiment, the separation  510  may range from 0.05 microns to 1 microns. 
         [0034]    Bias/controller  504  serves to provide a bias voltage between the edge emitters and collectors to increase the conversion efficiency of the embodiment  500 . A variety of factors determine the bias voltage, including the aforementioned dimensions, properties of the materials comprising the structures, and the strength of the solar flux. In one embodiment the bias voltage may be adaptively changed based on changes in the solar flux strength due to the time of day and weather conditions. 
         [0035]    Referring now to  FIG. 5   b,  the array  500  is shown in a vacuum enclosure/package defined by layers  512  where one of the layers must be transparent. Depending on the separation  510  between the emitter  502  and collectors  506 , in some embodiments it may not be necessary to package the device in a vacuum. If the separation  510  is sufficiently small then carrier transport may occur without any significant effects due to scattering. For distances of 1 micron or less, electron transport at atmospheric pressures is essentially collisionless. 
         [0036]    Referring now to  FIG. 6 , a solar flux conversion system  600 , in accordance with one embodiment of the invention is shown. The system  600  comprises an array  602  of flux conversion devices as described above. Each of the devices in the array  602  may be connected in a serial and/or parallel via a bus  604  to a bias/controller element  606 . The bias/controller element  606  performs bias functions and optimizes conversion efficiency. The bias/controller element  606  also controls the charging of an energy storage unit  608 , as well as the inversion (conversion from DC to AC) and distribution of energy to an electrical grid  610 . The system  600  can act as a self-contained energy generation node that is part of a larger energy generation and distribution network. Each node may be capable of satisfying some or all the needs of a local user or host, and then intelligently supplying excess power to an existing electrical distribution grid. 
         [0037]    Referring now to  FIG. 7   a,  there is shown a schematic drawing of an edge emitter  700 , in accordance with one embodiment of the invention. The edge emitter is similar to the edge emitter  204  described above, but includes an active area  702  that has a morphology. In other words, the active area  702  has surface features or periodic structures  704  which introduce variations in height along the axis illustrated by arrow  706 . These features may reside on the top surface., the bottom surface, or both. These features may be of any geometry or arrangement. In general, however, their dimensions vary from the sub-micron to sub-nanometer scale. 
         [0038]      FIG. 7   b  shows an embodiment  710  of an edge emitter in accordance with one embodiment of the invention. The embodiment  710  is very similar to the embodiment  700  except that the active area  702  includes discontinuities. In the embodiment shown the discontinuities are in the form of apertures  712 . The apertures  712  may be of arbitrary geometry and arrangement and may be in the sub-micron to sub-nanometer size range 
         [0039]    The surface features of edge emitters  700  and  710  exploit the phenomena of plasmons to enhance overall performance. The theory of plasmons is described in “A Hybridization Model for the Plasmon Response of Complex Nanostructures,”, N., Halas E. Prodan et. al., Journal of Science, Oct. 17, 2003. Plasmons can be described as electron density waves which propagate on a metal surface. The specific nature of the plasmon is related to the geometry of the surface which accommodates it. The phenomenon is of use because plasmons can produce and alter the nature of the electric fields generated when light is incident on a metallic structure, as well as the manner in which light is absorbed and/or converted into electric fields. The incorporation of sub-micron and sub-nanometer structures into the edge emitter disclosed herein provides an additional mechanism by which the electron emission and light conversion and absorption properties of edge emitters may be manipulated. 
         [0040]    In some embodiments the edge emitter may be coupled with an optical structure to increase an angle of incidence at which radiation strikes the active area of the edge emitter.  FIG. 8   a  shows an edge emitter  800  which is similar to any of the above described edge emitters to which is coupled an optical structure in the form of a prism  802 . The prism  802  may be coupled to the active area of the edge emitter  800  via at least one coupling film  804 .  FIG. 8   b  shows an embodiment in which the edge emitter  800  is coupled to an optical structure in the form of a hemispherical lens  806  via at least one coupling film  808 . 
         [0041]    The optical structures  804  and  806  serve to take light  810  which is incident normally, and increase the angle of incidence with which it strikes the active area to which it is coupled. Light which is incident on a surface at higher angles of incidence has enhanced ability to induce surface plasmons. Consequently the efficiency of this effect can be increased by proper design of the optical structure. These structures may take on arbitrary shapes depending on the expected average angle of incidence on the structure as a whole during operation. The structure is generally made from a material which is transparent at the wavelengths of interest. In addition to optical structure designs based on refraction, designs exploiting the phenomena of diffraction and interference are also useful and may present design advantages. 
         [0042]    The coupling films  804  and  808  are made of materials that are also optically transparent to allow radiation to pass thorough to the edge emitter on which it resides. The thickness and refractive properties of these films are chosen to enhance coupling of the light into the surface. In general this requires that at least one of the films be of a material with a refractive index less than that of the optical structure. One of the films may also be air. 
         [0043]    Referring now to  FIG. 9   a,  a rectenna  900  is shown, in accordance with one embodiment of the invention. The rectenna  900  is designed to ensure greater conversion of energy from incident light. The rectenna  900  comprises a transparent collector-emitter structure indicated generally by reference numeral  902 . The collector-emitter structure  902  includes an emitter and a collector as described above. The collector-emitter structure  902  is laterally spaced from a reflecting substrate  904  by a gap  906 . Radiation  908  reaches the emitter of the collector-emitter structure  902  and causes an alternating current to flow in a body of the emitter. This alternating current is rectified in a process in which electrons are emitted from the emitter body and made to flow towards the collector of the collector-emitter structure  902 . The reflecting substrate  904  produces a standing wave upon reflection due to interference between the radiation  908  and a reflected wave  910 . In one embodiment, the peak of the standing wave may be positioned to coincide with the location of the collector-emitter structure  902  by fixing the gap  906  appropriately. The gap  906  may be fixed at less than one micron for visible light. If the peak of the standing wave is coincident with the collector-emitter structure  902  then an increased field strength results which can enhance the emission of electrons from the emitter body.  FIG. 9   b  illustrates a standing wave peak  912  produced by reflection by the substrate  904 . As will be seen, peak  912  is coincident or aligned with collector-emitter structure  902 . 
         [0044]    In the embodiments of the flux conversion device described above, radiation in the form of solar energy is converted into electrical energy. However, one skilled in the art would appreciate that the techniques and devices disclosed herein are suitable for the conversion of energy from other regions of the electromagnetic spectrum into electrical energy. 
         [0045]    While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative and not restrictive of the broad invention and that this invention is not limited to the specific constructions and arrangements shown and described, since various other modifications may occur to those ordinarily skilled in the art upon studying this disclosure. In an area of technology such as this, where growth is fast and further advancements are not easily foreseen, the disclosed embodiments may be readily modifiable in arrangement and detail as facilitated by enabling technological advancements without departing from the principals of the present disclosure or the scope of the accompanying claims.