Abstract:
Methods and apparatus for gathering image information from nanostructures includes a composite waveguide of conductive nanoparticles in a dielectric medium. The waveguide is irradiated with preferably coherent blue light to form a slow surface wave. The evanescent wave that is the “tail” of the surface wave exists outside the waveguide contiguous to its surface. The nanostructures are located to encounter the evanescent wave. The slowing of the wave that occurs in the waveguide reduces the wave&#39;s speed and wavelength sufficiently such that nanostructures can be imaged. Upon encountering the evanescent wave, the nanostructures radiate. This radiation causes a backward scattering from the structures and a forward perturbation of the wavefront of the surface wave. From the scattering and perturbation information about the physical characteristics of the nanostructures sufficient to form an image is derived.

Description:
RELATED APPLICATIONS 
     This application is related to and claims priority from U.S. Provisional Application Ser. No. 60/280,644 entitled Coherent Evanescent Wave Imaging of Rudolfo Diaz, Ampere Tseng, Karl Booksh, Jose Menendez, Michael Wagner and Sethuraman Panchanathan, filed Mar. 30, 2001. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to methods and apparatuses for observing the characteristics of nanostructures and more particularly methods and apparatus for producing images of nanostructures by causing the nanostructures to encounter a slow, electromagnetic, evanescent wave formed by a slowed electromagnetic surface wave in a waveguide irradiated with electromagnetic energy having a free-space wavelength substantially greater than that of the resultant slow wave in the medium of the wave guide. 
     BACKGROUND OF THE INVENTION 
     There has been considerable interest in understanding and developing nanostructures, but this has been hampered by limitations on observation that these structures impose by nature of their size. 
     Whereas in the past, various microscopy techniques have been sufficient to detect defects of a surface at the nanostructure level, or the mere presence of a nanostructure, these have been inadequate to provide characteristics of the nanostructure such as would permit development of images or details of physical features, such as dimensions, shape and other characteristics. 
     One daunting difficulty in characterizing nanostructures based on observation has been the wavelength of light. At 488 nm., even blue light has a wavelength in free space (or air) that is too long for use in imaging objects where those objects have dimensions that may range from just a few nm. to several times the wavelength of the light. Ultraviolet light degrades too readily under most conditions to afford a reasonable alternative. In microscopy it is known to locate a specimen in a drop of liquid and to bring the object lens of the microscope into contact with the liquid to take advantage of the reduction in the speed of light that occurs within the liquid and the commensurate reduction in wavelength of the light impingent on the specimen. This technique is not suitable for imaging nanostructures. First, the reduction in wavelength is not sufficient to permit imaging of specimens or structures having dimensions of just a few nm. Second, nanostructures within a liquid are likely to have their observable characteristics and their observable motion altered or distorted. This may occur by the liquid pressure, by dissolving, by chemical reaction or by other interactions of the liquid and the nanostructure. To substantially reduce wavelength, blue light at 488 nm. would need to be used in a medium such as diamond. This would slow the speed of the light by a factor of two, consequently reducing the wavelength by half. Again, this is both impractical and insufficient. The nanostructures being examined cannot be situated in a diamond medium, and halving the 488 nm. wavelength is still not a sufficient reduction. For effective imaging of nanostructures a reduction of the 488 nm. wavelength of blue light by a factor of ten would be desirable. 
     In  Plasmonics - A Route to Nanoscale Optical Devices , Advanced Materials, 13, No. 19, Oct. 13, 2001 (Wiley-VCH Verlag GmbH), Maier et al. teach using gold nanoparticles with diameters between 30 and 50 nm., spaced “a few tens of nanometers apart,” as building blocks for “plasmon waveguides.” This publication is incorporated herein by reference. The speed of wave propagation at the center of the operating band along the series of spaced nanoscale spheres of Maier et al. is 1/10 the free-space speed of the electromagnetic radiation employed. Consequently, the wavelength is 1/10 that of the free-space wavelength. Maier et al. do not suggest using the speed reduction for light wavelength reduction enabling examination of nanostructures. Rather, Maier et al. suggest optical wave guides fashioned into optical path “Ls,” “Ts” and switches for use in optical circuitry, so slowing the speed of light is not an objective. And, in fact, Maier et al. mention they are seeking the fastest such propagation velocity, whereas we desire the slowest. Maier et al. do not suggest suspension of particles in a dielectric medium. 
     BRIEF DESCRIPTIONS OF THE INVENTION 
     In accordance with this invention there is provided methods and apparatuses for determining characteristics of one or more nanostructures using electromagnetic radiation. A slow moving electromagnetic wave is used to illuminate the nanostructure. In accordance with one aspect of the invention nanostructures are positioned to encounter an evanescent electromagnetic wave that is a characteristic of a slow electromagnetic surface wave. Effects of electromagnetic interactions between the evanescent wave and the one or more nanostructures are observed. These effects may be an electromagnetic scattering effect or a perturbation of the wavefront of the evanescent electromagnetic wave that encounters the nanostructure. 
     The evanescent electromagnetic wave can be produced by irradiating a wave guide having a boundary surface between a first, internal medium and a second, external medium such that a slow electromagnetic surface wave is produced in the wave guide at the surface with a characteristic evanescent electromagnetic wave in the second medium contiguous to the surface. The nanostructure or nanostructures are located in the second medium proximate the waveguide surface at which the surface wave is produced. 
     To produce the evanescent wave, the waveguide is irradiated by a electromagnetic energy having a wavelength in the medium of the wave guide that may be shorter than its wavelength in free space by a factor of ten or more. In a preferred embodiment the electromagnetic energy is blue light having a wavelength in free space of substantially 488 nm. The electromagnetic energy is directed through the medium of the waveguide in such a way as to create the surface wave. The coupling from the incident wave to the surface wave is typically accomplished via discontinuities on the surface such as a line grating or, as illustrated in  FIG. 1 , the end of the waveguide itself. In the preferred embodiment, this coupling is accomplished by a laser illuminating a conductive sphere or particle that is optically coupled to the waveguide. This creates a plasmon in the sphere or particle serving as a point source of coherent light for the waveguide. 
     The evanescent wave or “tail” portion of the surface wave that is produced in the second medium, outside of the waveguide and contiguous to its surface decays exponentially with distance from the surface. The electromagnetic energy directed through the medium of the wave guide is preferably coherent. This enables detection of effects of the interaction of the evanescent wave and the nanostructures that may include diffraction, scattering backward in a direction that is the reverse of the direction of wave propagation, and perturbation of the wavefront in the direction of wave propagation, a degradation or diminishing of the coherent wave that is caused by passage over the nanostructures. This latter effect may be thought of as the inverse of the backward scattering echo that occurs. 
     In one preferred embodiment the evanescent wave portion of a coherent collimated surface wave is split into a reference wave and a nanostructure-illuminating wave, the nanostructures are illuminated by the nanostructure-illuminating wave, and the split waves are brought together. Additive reinforcement and subtractive interference that occurs when the two waves combine give image information regarding the nanostructures. As used herein, the term “electromagnetic” and “electromagnetic energy” wave and the like are not limited to electric waves or waves in just the radio frequency or microwave ranges, but include as well light waves. 
     “Nanostructures” means structures having dimensions measured in nanometers that may range from just a few nanometers to less than a thousand nanometers, or put another way from a dimension very significantly less than the wavelength in free space of electromagnetic energy used to observe the nanostructure up to about 100 multiples of the wavelength in free space of that electromagnetic energy. A nanostructure could be a bacterium or a “nanomachine.” As used herein, referring to the nanoparticles, “cross-sectional dimension” means the particles&#39; greatest cross-sectional dimension. 
     The above and further objects and advantages of the invention will be better understood from the following detailed description of at least one preferred embodiment of the invention, taken in consideration with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a fractional diagrammatic illustration of a nanostructure imaging arrangement showing a wave guide with a surface wave, an evanescent wave and nanostructures being imaged. 
         FIG. 2  is a fragmentary, diagrammatic planar view of nanostructures illuminated by coherent light and illustrates diagrammatically the derivation of structure information from backward scattering and forward degradation of the wave front of the coherent light. 
         FIG. 3  is a fragmentary, diagrammatic planar view like FIG.  2  and illustrates diagrammatically a further technique for the derivation of structural information by varying the direction of illumination of the nanostructures with coherent light. 
         FIG. 4  is a plot of permittivity versus frequency for gold. 
         FIG. 5  is a plot like FIG.  4  and shows the change in the plot using a waveguide of gold particles dispersed in silicon dioxide according to one aspect of the invention. 
         FIG. 6  is a diagrammatic illustration of a waveguide utilizing a single row of spaced gold nanoparticles as described by Maier, et al. 
         FIG. 7  is a diagrammatic enlarged, fragmentary planar view of a waveguide in accordance with this invention. 
         FIG. 8  is an enlarged, diagrammatic, side elevation view of a transducer for imaging nanostructures in accordance with the invention and shows the nanostructures located on a surface below a waveguide of the kind illustrated in FIG.  7 . 
         FIG. 9  is a bottom plan view of the waveguide under-surface of the transducer of FIG.  8  and shows wave directing provisions of the waveguide and a detector for outputting information relating to nanostructures. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Turning to  FIG. 1 , a cross section of a portion of an instrument for detecting physical characteristics of nanostructures is diagrammatically illustrated for purpose of explanation. A surface wave is produced at the surface of a waveguide. The surface wave is an electromagnetic slow wave that does not travel in free space but is bound to a surface. The wave exists at the boundary between a medium (the waveguide) and the free space, air or other medium outside the waveguide boundary. Such a wave has associated with it an exponentially decaying tail located outward of the waveguide in the air or free space bounded by the surface of the guide. The free space portion of the surface wave thus created in the space proximate the waveguide&#39;s surface is termed an “evanescent wave.” The wave does not propagate away from the boundary, but it is detectable. It does not radiate, but an object placed in the wave will radiate. As graphically illustrated in  FIG. 1 , a waveguide  20  is irradiated with light  25  to create such a surface wave. At a coupling discontinuity  21 , light is coupled to the waveguide. In the wave guide, which is of a light transmissive medium with a higher index of refraction than air, the light is directed to the medium-air interface  28  at the coupling discontinuity so as to generate the surface wave through the near fields of the discontinuity&#39;s electromagnetic scattering. 
     A number of techniques are used to produce an image of structure using the phenomenon of the evanescent wave. A metal substrate  30  has nanostructures  32 ,  33 ,  34  and  35  projecting from its upper surface  38 . The waveguide  20  and substrate  30  should be relatively moveable toward and away from one another. Here, movement of the waveguide as indicated toward and away from the substrate is assumed as indicated by the unnumbered arrow on the guide. At  25 ′ the light propagated along the surface is the surface wave, graphically illustrated at  27 . The associated evanescent wave is graphically illustrated at  29 , diminishing exponentially in the direction away from and normal to the surface  28 . As it is moved toward the metal surface  38  the structure  35  interacts with the evanescent wave  29  and radiates as indicated at  40 . In other words, the wave  29  excites a current or plasmon in the structure  35  which accordingly radiates. That radiation can be detected. It causes scattering backward from the direction of propagation of the surface wave and its attendant tail (to the right as illustrated in  FIG. 1 ) and a decay or degradation in the surface wave  27  going forward (to the left in FIG.  1 ). At the separation distance of the substrate  30  and the waveguide  20  shown in  FIG. 1 , only the tallest nanostructure  35  is detected. The structures  32 ,  33  and  34  are shorter and are not irradiated by the evanescent wave. This, then, is information concerning the relative heights of the nanostructures  32 ,  33 ,  34  and  35 . Moving the guide  20  and substrate  30  closer, the nanostructure  33  will next be detected. If the evanescent wave has planar wave fronts  25  progressing across structures  32 ,  33 ,  34  and  35 , for example, as shown in  FIG. 3 , the structures will cause scattering of the evanescent wave. That scattering, backward from the direction of wave propagation, is the echo referred to above. In the wave going forward, a perturbation of the evanescent wave is the inverse of the scattering echo. This is the effect notes in the forward direction of wave propagation. In other words, as the evanescent wave proceeds, the planar wave front of the slow, surface wave  27  will now be altered by the radiation from the structure  40  caused by its encounter with the evanescent wave. 
     By making the surface wave  27  coherent, the irradiant “scattering” from the nanostructure is made coherent. As diagrammatically illustrated in  FIG. 2 , it then becomes possible by moving the location of observation to detect interference from out-of-phase light reflected from the nanostructures at, say, location A and reinforcement of in-phase reflected light from the nanostructures at, say, location B. The effect is analogous to a hologram in which the observer&#39;s location affects the observable image through interference and reinforcement of coherent light. At observation location C the wavefronts indicated by  45  are seen to be perturbed or degraded by their interaction with the nanostructures  32 ,  33 ,  34  and  35  as indicated schematically at  47 . The information observed at C is the inverse of the backwave scattering or echo. The technique by which the echoes received at A, B, and a multiplicity of other similarly disposed locations, can be inverted to obtain an image of the scattering objects is known as ISAR (Inverse Synthetic Aperture Radar). 
     In  FIG. 3  a similar technique to that employed in  FIG. 2  at A and B is used to image the structure  32 ,  33 ,  34  and  35  in the forward direction (to the right in FIG.  3 ). The observer&#39;s position is varied from C to D as shown. The direction of propagation of the surface and evanescent wave is changed from 42 to 49. By observing the disturbance of the wave fronts at various locations as the field traverses the structures at varying angles, an image can be created from the composite information gathered. Techniques similar to those used in tomography may be used here. It will be appreciated that numerous such observations are contemplated, not with the naked eye, but with instrumentation to gather and computer software and hardware suitable to store and compile the information thus garnered. 
     As noted above, the wavelength of the light is an important factor. It is desirable to slow the speed of light to 1/10 th  or less of its speed in free space. That has the effect of reducing the wavelength by a factor of 10 since λ 0 =C 0 /F 0 . However ordinary dielectrics cannot be used to effect such a reduction in wavelength. Even if the permittivity (∈) of the medium is increased by a factor of 10 the light velocity is only related to the square root of ∈ and so will not vary nearly as much. A velocity decrease to only approximately ⅓ of the original velocity is thus achieved. Consequently dispersive structures have been formulated to “fool” the wave.  FIG. 4  plots ∈ in gold (Au) versus frequency. ∈ crosses the zero axis at a point f 1 . Where ∈ is less than zero, as indicated in  FIG. 4 , very slow guided waves result. Maier et al. show that a propagated light wave along a chain of gold nanobeads or particles as shown in  FIG. 6  can theoretically reduce the velocity of wave propagation by a factor of 10. In the article cited above, Maier et al. were not interested in reducing speed, but in increasing speed for the purpose of improving light circuitry. High resolution nano imaging was not discussed. A preferred embodiment of this invention uses a material made up of gold globes or particles coated with silicon dioxide SiO 2  that bind the gold particles in a matrix. 
     At  FIG. 6  a field produced in a waveguide like the theoretical line of gold particles described in the above article of Maier et al. is shown graphically. A strongest portion  55  of the field  52  is within the guide which here is from one gold particle  50  to the next. However an evanescent field  58  with the configuration shown from particle to particle is the characteristic exponentially diminishing field that occurs in such a structure. Changing the spacing between the particles and altering the permittivity of the interstitial medium permits tuning of the guide to vary the field. In accordance with this invention, then, a composite guide  60  is provided, as shown in FIG.  7 . The exemplary waveguide  60  has an array of gold particles  62  that have cross-sectional dimensions of approximately 10 nm. and are spaced apart approximately 37 nm. in a SiO 2  binder  65 . A waveguide thus formed has an apparent permittivity altered as illustrated the full line plotted in FIG.  5 . 
     One preferred embodiment of an imaging transducer  70  is shown in  FIGS. 8 and 9 . A laser  72  is incident on a conductive bead or particle  74  to produce a surface plasmon point source as is common practice in a technique known as surface enhanced Raman scattering. The emerging light  76  is collimated at  78  by a Luneberg lens  79 . The collimated coherent beam  78  is split by a beam splitter  81  to produce a reference wave  82  directed to a first Bragg mirror  84  which may be, for example, a projection from the surface  28  of the waveguide  20  that is two gold particles in height. The portion of the coherent collimated beam  78  passing through the beam splitter  81  interacts with the structure being imaged in an interaction region  86  and impinges on a second Bragg mirror  88 . The two Bragg mirrors direct their reflected waves to a novel detector array  90 . The detector array  90  is a linear series of gold nanoparticles each coated with a different fluorescent material chosen to fluoresce when illuminated so as to produce different wavelengths of emitted light. Impinging light across the detector thus produces a color coded signature or profile. Where the two combined coherent waves interfere the fluorescent particle remains dark. Where they reinforce light of a particular wavelength is emitted. The variation in light along the length of the detector can be plotted based on the colors of emissions and this provides information on the features of the structures under observation. Alternatively the pattern of light reinforcements and interferences may be projected and enlarged to derive this information. 
     Surface and target nanostructures for the creation and characterization of the proposed imaging system can be manufactured using a combination of nano-deposition and nano-machining processes. An ultrahigh vacuum evaporation system used for thin-film deposition can be adopted for forming the nano-scale substrate of multi-material layered structures. The multi-layer substrates are then further shaped or sculpted by a nano-machining process to their final configurations. Equipment capable of producing beam spot sizes from 50 to 500 nm. with current densities up to 5 A/cm2 can be used to accomplish the nano-machining. Combinations of ion species, e.g., PdAsB, AuSiBe, or Ga, can be obtained from liquid metal ion sources. 
     The Bragg mirror is a known device. It consists of a series of alternating transparent obstacles with varying index of refraction, spaced one-half wavelength apart. Each obstacle by itself has negligible effect on an incident wave, but the combined scattering from 20 or more layers can approach 100% reflection. In our preferred embodiment, it has spheres or particles located half a wave length apart so as to interact with a desired wave length and reflect 100% of light at that wavelength. The beam splitter  81  may be a line of spheres or particles that are two spheres high (to simulate a pellicle) or it may be another integral part of the topography of the surface  28  of the waveguide  20 . In  FIG. 8  the “tail”  29  of the coherent, slow, surface wave  27  is shown as are the nanostructures  32 ,  33 ,  34  and  35 . Scattering echo waves and perturbations are indicated at  91 . The coherent, nanostructure-illuminating wave is output at  93  and directed at  95  to the detector  90  of  FIG. 9  where it interacts with the coherent, slow surface wave that is the reference wave  82  (FIG.  9 ). 
     Techniques currently used for imaging structures with waves such as, for example, Tomography, Nomarski microscopy, and Synthetic Aperture Radar, can be used to reconstruct the entire image field. Fusion of images obtained by different techniques and at different wavelengths will yield a rich multi-spectral characterization of the subject nanostructures. 
     The foregoing descriptions of at least one preferred embodiment are exemplary and not intended to limit the claimed invention. Obvious modifications that do not depart from the spirit and scope of the invention as claimed will be apparent to those skilled in the art.