Abstract:
The Smith Purcell effect, in which a beam of electrons passes close to a conducting grating and induces electromagnetic radiation from the grating surface, can be used as a source of THz radiation. A grating composed of negative index metamaterial (NIM) enhances the output of the Smith Purcell source. Of particular interest is the use of a NIM grating in a Smith-Purcell source to provide a tunable coherent CW source of terahertz (THz) radiation.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates to electromagnetic radiation sources, and more specifically to radiation sources based on the Smith-Purcell effect with particular application in the THz regions of the spectrum.  
         [0003]     2. Description of the Related Art  
         [0004]     THz-frequency radiation, in the frequency region from 0.3 THz to 10 THz, has been relatively unexploited compared to the adjacent radio frequency (RF) and IR spectral bands that lie on either side of it. This is largely because of transmission difficulties due to absorption by atmospheric water vapor but also due to a lack of practical radiation sources and detectors. In recent years there has been a significant growth of interest in applications of this previously underutilized portion of the electromagnetic spectrum in the areas of active short range imaging systems, space-based communications, imaging, and spectroscopy.  
         [0005]     Prior work by Urata et al. of Dartmouth College (Urata, J., et al., “Superradiant Smith Purcell Emission”, PRL 80#3, January 1998), which is hereby incorporated by reference, has established that the classic Smith Purcell effect can be used as a source of radiation in the far infrared (FIR) to terahertz (THz) spectral regions. As illustrated in  FIG. 1 , the beam  10  from an electron microscope is focused over a properly sized metal diffraction grating  12  thereby producing electromagnetic radiation  14 . The wavelength of the radiation is determined from the expression  
       λ   =       l        n          ⁢     (       1   β     -     sin   ⁢           ⁢   θ       )           
 
 where l is the grating period, β≡v/c is the electron velocity v relative to the speed of light c, θ is the angle of emission measured from a direction normal to the surface of the grating, and n is the spectral order. Urata et al. conclude “extensions of this technique promise a new tunable coherent CW source for the difficult to access . . . 30-1000 micron range of the spectrum”. As shown in  FIG. 2 , limited tunability of the emission spectrum  16  can be achieved by varying the voltage, hence velocity of the electron beam. Vermont Photonics (in collaboration with a Dartmouth group) has recently developed a source of THz radiation using this technique. 
 
         [0006]     Subsequent work by Yamaguti et al. (Yamaguti et al., “Photonic crystals versus diffraction gratings in Smith-Purcell radiation”, Physical Review B 66, 195202 (2002)) demonstrated that the use of a two-dimensional photonic crystal in place of the one-dimensional diffraction grating created resonant structures of enhanced intensity whose peak heights were an order of magnitude larger than the emission intensity of the one dimensional Bragg diffraction grating. Yamaguti et al. conclude “the results of calculations thus show a possible advantage of photonic crystals over diffraction gratings as converter of the evanescent field from a running charge into propagating waves.” 
         [0007]     Although Smith-Purcell sources hold great promise for THz sources, further advancements are needed to increase their intensity and efficiency to a point where commercialization is practicable.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention provides a high intensity Smith-Purcell radiation source with particular applicability in the THz regions of the spectrum.  
         [0009]     This is accomplished with a periodic array of interface discontinuities formed between first and second media having index of refraction of opposite sign in a specified bandwidth. A charged particle beam is directed across the periodic array thereby producing enhanced Smith-Purcell electromagnetic radiation in the specified bandwidth. The reflection and transmission coefficients for the evanescent waves generated at the interface between a positive and a negative-index medium have far greater magnitude than those generated at the surface between two media of the same index sign as is the case in known Smith-Purcell radiation sources. In a perturbative sense, the grating modulation leads to a coupling between the incident beam and all diffractive beams, mediated by the evanescent components of the surface waves. As a result, the intensity of the radiation emitted by the Smith-Purcell radiation source is much enhanced.  
         [0010]     In one embodiment, a 1, 2 or 3-dimensional grating (planar, wedge or other) is formed from a Negative-Index Metamaterial (NIM). The positive-index media may be a vacuum environment (n=1) or even material (n&gt;1) selectively placed on the grating at the interface discontinuities to further enhance radiation. The NIM has a resonant structure that may be defined by a periodic array of split-ring resonators and rod structures, a patterned photonic crystal or other man made structures that exhibit a negative index of refraction over the specified bandwidth. The resonant structure may also be configured in 1, 2 or 3 dimensions. To achieve the enhanced intensity, the resonant wavelength the grating must lie within the resonant bandwidth of the NIM material where the index is negative.  
         [0011]     In another embodiment, a THz radiation source directs an electron beam across the surface of a grating formed from a NIM. The bandwidth of the resonant structure in the NIM is matched to the resonance of the grating in a specified THz bandwidth.  
         [0012]     These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings, in which: 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1 , as described above, is a diagram illustrating the Smith-Purcell effect;  
         [0014]      FIG. 2 , as described above, is a diagram illustrating tunability of Smith-Purcell radiation;  
         [0015]      FIG. 3  is a diagram of a Smith-Purcell radiation source using Negative-Index Metamaterial (NIM) in accordance with the present invention;  
         [0016]      FIG. 4  is a diagram of a known refraction experiment with a NIM grating;  
         [0017]      FIG. 5  is a plot illustrating the overlapping resonances of the periodic array and NIM;  
         [0018]      FIG. 6  is a diagram of an alternate embodiment of the Smith-Purcell radiation source; and  
         [0019]      FIGS. 7 and 8  are diagrams of one and two-dimensional resonant structures for a NIM grating. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]     The present invention provides a high intensity Smith-Purcell radiation source with particular applicability in the THz regions of the spectrum, specifically 0.3×10 2  Hz to 10×10 12  Hz.  
         [0021]     As shown in  FIG. 3 , a Smith-Purcell radiation source  20  includes a source  22  that directs a charged particle beam  24  across a periodic array of interface discontinuities  26  formed between first and second media  28  and  30 , respectively, having index of refraction of opposite sign in a specified bandwidth. The interaction of the charged particle beam  24  with electrons in at least one of the media at the interface discontinuities thereby produces enhanced Smith-Purcell electromagnetic radiation  32  in the specified bandwidth. The reflection and transmission coefficients for the evanescent waves generated at the interface between the positive and a negative-index medium have far greater magnitude than those generated at the surface between two media of the same index sign as is the case in known Smith-Purcell radiation sources. In a perturbative sense, the interface discontinuities lead to a coupling between the incident beam and all diffractive beams, mediated by the evanescent components. As a result, the radiation  32  is at least several times greater than the radiation  33  emitted from a conventional grating formed from a material having a positive index of refraction. Limited tunability of the emission spectrum can be achieved by varying the voltage, hence velocity of the charged particle beam  24 .  
         [0022]     In this particular embodiment, the periodic array of interface discontinuities  26  and the first media  28  are a grating  34  formed of a Negative-Index Metamaterial (NIM). This particular grating is a one-dimensional wedge. Alternately, a planar-grating or two and three-dimensional gratings formed of a NIM can be used. Multi-dimensional gratings may take the form of sandwiched gratings, annular cylinders, spheres etc. The second media  30  is a vacuum environment in which the refractive index is, by definition, equal to plus one.  
         [0023]     A metamaterial is a composite or structured material that exhibits properties not found in naturally occurring materials or compounds. Negative-index metamaterials have specific electromagnetic properties, namely that the magnetic permeability μ and electric permittivity ε are negative over a specified bandwidth, and thus the index of refraction n is negative over the specified bandwidth. The NIM  28  has a resonant structure  36  that may, for example be defined by a periodic array of split-ring resonators  38  and rod structures  40 , a patterned photonic crystal or other man made structures that exhibit a negative index of refraction over the specified bandwidth. The resonant structure stores energy so that more energy is coupled out of the grating. The theory of negative-index metamaterials was first described in V. G. Veselago, Soviet Physics USPEKHI, 10, 509 (1968) and later reintroduced by J. B. Pendry, Phys. Rev. Lett. 85, 3966 (2000), which are hereby incorporated by reference.  
         [0024]     Enhanced diffraction from a grating on the surface of a Negative-Index Metamaterial was just recently discovered by D. R. Smith et al. (Physical Review Letters, Vol. 93, Number 13, Sep. 24, 2004). Smith et al. passed an electromagnetic wave  41  through a wedge  42  formed of NIM in order to study the effects of NIM on the refracted beam  43  as illustrated in  FIG. 4 . The finite unit-cell size of the NIM led to an unavoidable stepping of the surface to achieve the desired angle for the refractive surface. The surface stepping constitutes a grating  44 , which would be expected to generate a first-order diffracted beam  45  in addition to the zeroth order refracted beam  43 . What they found was that the coupling to the first  45  and higher order diffracted beams  46  is much larger than would be the case for a positive index material.  
         [0025]     By combining the enhanced coupling of the diffraction beam in a NIM grating with the Smith-Purcell effect, we have constructed a high-intensity radiation source with particular applicability as a source of THz radiation. To achieve the enhanced electromagnetic radiation  32 , the resonant wavelength  48  of the grating must lie within the resonant bandwidth  49  of the NIM material where the refractive index is negative (n(ω)&lt;1 for ω 1 &lt;ω&lt;ω 2 ) as shown in  FIG. 5 . The resonant wavelength  48  of the grating is determined by the size and spacing of the individual surface step elements and the observer angle with respect to the surface. The resonant bandwidth  49  of the NIM material is determined by dimensions of the metamaterial building blocks or unit cells. Outside this bandwidth the NIM has a positive-index of refraction and thus does not enhance the radiation at those wavelengths. Because of the lack of availability of high-power cost-effective THz sources, the THz bandwidths are of particular importance.  
         [0026]     As shown in  FIG. 6 , Smith-Purcell radiation  50  from a grating formed of a NIM may be further enhanced by selectively placing positive-index material  52  (n&gt;1) at the interface discontinuities. The effect of having material where n&gt;1 instead of a vacuum where n=1 at the interface is to provide increased coupling between the evanescent components and the radiated wave.  
         [0027]     An exemplary embodiment of a Smith-Purcell THz radiation source  60  including a one-dimensional wedge grating  62  formed from a NIM  64  having a resonant structure defined by a periodic array  66  of metal posts  68  and split-ring resonators  70  is depicted in  FIG. 7 . The split-ring resonators  70  are patterned on an insulating substrate  74  and mounted with the metal posts  68  on a substrate  76  in electrical isolation from each other. J. B. Pendry, A. J. Holden, W. J. Stewart, I. Youngs,  Phys. Rev. Lett.  76, 4773 (1996) suggested that an array of conducting wires could respond to the electrical component of incident radiation, displaying a negative permittivity. J. B. Pendry, A. J. Holden, D. J. Robbins, W. J. Stewart, “Magnetism from conductors and enhanced non-linear phenomena,”  IEEE Trans. MTT,  47, 2075 (1999) suggested that an array of ring resonators could respond to the magnetic component of incident radiation, displaying an effective permeability. By arranging the split-ring resonators  70  into a strongly interacting array, a bandwidth of negative permeability is created. Combining the two as shown in  FIG. 7  creates a NIM with a negative refractive n(ω)&lt;1 for ω 1 &lt;ω&lt;ω 2  index in a specific bandwidth. This particular resonant structure for a NIM is well known in the literature and has been tested to demonstrate, among other properties, the reversal of Snell&#39;s law, reversed Doppler shift and reversed Cerenkov radiation.  
         [0028]     A source directs a charged particle beam  78 , suitably electrons, across the grating  64  thereby stimulating resonances in the electric and magnetic elements of the negative index metamaterial. In a manner similar to surface plasmons in a traditional conducting PIM grating in the Smith Purcell effect, these resonances couple to electromagnetic fields outside the grating and result in propagation of electromagnetic waves away from the surface. The coupling of the NIM element resonant fields to the external medium is much stronger that that produced by surface plasmons in the PIM grating, resulting in an enhanced radiated power compared to a standard Smith Purcell source.  
         [0029]     An exemplary embodiment of a Smith-Purcell THz radiation source  80  including a one-dimensional flat grating  82  formed from a NIM  84  having a two-dimensional resonant structure defined by a first periodic array  86  of metal posts  88  and split-ring resonators  90  formed on opposite sides of an insulator  91  and a second periodic array  92  of metal posts  94  and split-ring resonators  96  formed on opposite sides of an insulator  97  is depicted in  FIG. 8 . The two-dimensional resonant structure is very similar to the one-dimensional structure illustrated in  FIG. 7  except that it includes a second periodic array  92  perpendicular to the first. Furthermore, a three-dimensional resonant structure may be created by including a third periodic array that is perpendicular to the first two (a horizontal plane in  FIG. 8 ). Multi-dimensional resonant structures may be more efficient at coupling energy from the input beam  98  into the sourced electromagnetic radiation  100  because they contain more resonant elements per unit volume, although their orientation is not optimal. Furthermore, in the multi-dimensional resonant structures the charged particle beam can be directed across the grating from multiple directions. This permits radiation in several orthogonal directions from the same sample.  
         [0030]     While several illustrative embodiments of the invention have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. Such variations and alternate embodiments are contemplated, and can be made without departing from the spirit and scope of the invention as defined in the appended claims.