Abstract:
A system and method for generating and using broadband surface plasmons in a metal film for characterization of analyte on or near the metal film. The surface plasmons interact with the analyte and generate leakage radiation which has spectral features which can be used to inspect, identify and characterize the analyte. The broadband plasmon excitation enables high-bandwidth photonic applications.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS  
       [0001]     This application claims priority to United States Provisional Patent Application No. 60/666,901 filed on Mar. 31, 2005, and is incorporated herein by reference.  
         [0002]     The United States Government has certain rights in this invention pursuant to Contract No. W-31-109-ENG-38 between the U.S. Department of Energy and The University of Chicago operating Argonne National Laboratories. 
     
    
     FIELD OF THE INVENTION  
       [0003]     This invention relates to a method for optically exciting and detecting surface plasmons in thin metal films, schematically illustrated in  FIG. 1 . More specifically this invention relates to a method for the excitation and detection of surface plasmons having a broad wavelength spectrum. Still more specifically this invention relates to a method for the excitation of surface plasmons which have utility for propagating plasmons in metal films, with applications in photonics. This invention also relates to the spectroscopy and sensing of adsorbates on metal films using the broadband plasmon feature. In addition the invention relates to use of surface plasmons for optical communications in microscale and nanoscale circuitry.  
       BACKGROUND OF THE INVENTION  
       [0004]     A plasmon is the quantization of plasma oscillations, which are density waves of the charge carriers in a conducting medium such as a metal, semiconductor, or plasma. Surface plasmons exist in various geometries, such as nanoparticles or two dimensional films. Thin film plasmons propagate in the micron range depending on the wavelength and type of material. Such plasmons are non-radiative in air and are sensitive to dielectric environment. Recent technological advances that allow metals to be structured and characterized on the nanometer scale triggered new interest in the application of surface plasmons (SPs). The control of SP properties is of interest to a wide spectrum of scientists, ranging from physicists, chemists and materials scientists to biologists. For instance, SPs are being explored for their potential in small-scale optical circuitry, high-resolution optical microscopy and bio-detection.  
         [0005]     Surface plasmons are known solutions of Maxwell&#39;s equations applied along an interface between a medium with a negative permittivity, i.e. a metal, and a dielectric. These solutions are traveling waves that are generally bound to the interface and are exponentially decaying in both media. The optical excitation of surface plasmons on flat metal interfaces is challenged by the phase matching condition between the plasmons and the exciting radiation. The surface plasmon dispersion ω(k) is located outside the light cone ω=ck and hence no SPS can be excited with freely propagating radiation. The excitation of surface plasmons can only occur if the photon momentum—or the wave vector—can be artificially increased. Various experimental techniques have been developed to accomplish this task, such as (i) increasing the index of refraction of the incident medium (total internal reflection (TIR) conditions) or (ii) engineering the surface of the film (grating coupler). While these approaches provide very efficient coupling between the incident photons and the SP waves, the interaction area is usually comparable or greater than the SP propagation distances.  
         [0006]     It was recognized very early that in an asymmetric structure, i.e. a thin metal film (permittivity ε m ) surrounded by two dielectric media (permittivities ε 1 , and ε 2 , with ε 1 &gt;ε 2 ), has four modes that are solutions of the dispersion relations. Two of these solutions exist at each of the interfaces ε m /ε i , i=1, 2) and are characterized by their fields decaying exponentially into the media. The two other modes are radiative leaky waves originating from the finite thickness of the film. As a non- radiative mode travels along an interface, the wave amplitude decays exponentially in the metal and is coupled into leakage radiation (“LR”) by the opposite interface. The far-field observation of this leakage radiation (LR) gives a direct measurement of the non-radiative surface plasmon propagation at the opposite interface. The intensity of the radiation, at a given lateral position in the film, is proportional to that of the SP—at the same position.  
         [0007]     Surface plasmons are thus well-known phenomena and commercial surface plasmon-based sensors are currently used in biological research and in industrial applications. Use of the surface plasmons allows manipulation of light in devices smaller than the wavelength and can be extremely localized. They also exhibit ultrafast dynamics for use in rapidly changing circumstances or for rapid data output. For example, the detection principle of a commercially available plasmon sensor relies on the surface plasmon resonance resulting from energy and momentum being transformed from incident photons into surface plasmons. This process is sensitive to the refractive index of the medium on the opposite side of the film from the reflected light. Heretofore, the light source used for optically exciting surface plasmons was a monochromatic laser directed at an angle through a prism to a metal (gold or silver) coated surface. The sensor operated by determining the variation in the angle of incidence for maximum plasmon absorption. The presence of an adsorbate material on the surface of the metal was detected by measuring the change in the angle of incidence of the monochromatic beam. Alternatively, the angle of incidence was fixed and the wavelength varied to extract the same information. These methods are, however, very time consuming and technically more difficult if one wishes to extract spectral information, e.g. multiple wavelengths, on the adsorbates material.  
         [0008]     Further, in the vast majority of other surface plasmon studies, they are optically excited in the so-called Kretschmann attenuated total internal reflection (“ATR”) configuration, where the momentum mismatch between free-propagating photons and SPs is taken from a material with a refractive index larger than air, e.g. a glass substrate. In the case of an ATR geometry and for smooth metal films, the leakage radiation (“LR”) interferes destructively with the incoming excitation light at the reflection spot and cannot be detected if the excitation area is larger or comparable to the lateral decay length of the surface plasmon. However, if surface plasmons are locally excited by electrons or near-field techniques, LR can be observed. This is, however, technologically ambitious and difficult because near-field optics is based on scanning probe microscopy.  
       SUMMARY OF THE INVENTION  
       [0009]     The invention is directed to a new system and method for the excitation and detection of a large spectrum of surface plasmons. With the increasing trend towards the miniaturization of photonic circuits, the confined nature of surface plasmons and their long propagation length make them suitable for integration in metallic planar circuitry designs. Rudimentary surface plasmon (“SP”) optical manipulations in structured thin films include propagation, interference, scattering, waveguiding, splitting and mirror-like reflection. Due to the evanescent nature of the SP field traveling at the metal/air interface, near-field and fluorescence techniques were applied to image the surface plasmon intensity distributions and have been essential in the characterization of SP devices.  
         [0010]     The preferred technique utilizes an incident white-light continuum beam as a excitation source and an index-matched immersion objective lens having a wide aperture in contact with the substrate being probed. The objective can be part of a conventional inverted optical microscope focused on the metal/glass interface. Importantly, the focusing of the white light continuum through the microscope objective produces a wide range of incident angles of excitation, so as to simultaneously launch a continuum of plasmons. This method and system enables an unusually large bandwidth or frequency of plasmons to be excited rather than the very narrow bandwidth of previous methods. Detection of the surface plasmon continuum is achieved by monitoring the leakage radiation including a continuum of wavelengths, including the visible and infra-red spectral region which has originated from the propagation of the surface plasmons. The radiation is recorded by a conventional CCD camera placed in an image plane. The instantaneous detection of the wide radiation bandwidth permits a new form of spectroscopy of adsorbates on the surface of the metal film. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1A  illustrates a system for excitation and detection of surface plasmons;  FIG. 1B  illustrates plasmon frequency versus reciprocal space vector k x ; and  FIG. 1C  illustrates a detail of surface plasmon creation from incident white light and coupling out of the material of leakage radiation;  
         [0012]      FIG. 2A  illustrates sensing of particles in a two-dimensional array;  FIG. 2B  illustrates sensing a line of particles and  FIG. 2C  illustrates sensing larger particles along a line;  
         [0013]      FIG. 3  illustrates dark field spectroscopy using surface plasmons to sense isolated nanoparticles;  
         [0014]      FIG. 4  illustrates blue-shift of surface plasmon resonance as a function of particle size;  
         [0015]      FIGS. 5A  illustrates the effect of increased damping of surface plasmons on resonance peak position as a function of particle spacing; and  FIG. 5B  the effect on resonance peak width;  
         [0016]      FIG. 6A  illustrates the effect of blue-shift of surface plasmon leakage radiation on resonance peak position for different particle sizes and  FIG. 6B  the effect on resonance width;  
         [0017]      FIG. 7A  illustrates red-shift of leakage radiation for smaller range particle spacing and  FIG. 7B  the effect for larger range particle separation;  
         [0018]      FIG. 8A  shows p-type light polarization effect on leakage radiation and  FIG. 8B  shows the effect for s-type polarization;  
         [0019]      FIG. 9A  illustrates broad band excitation producing a plasmon rainbow leakage radiation spectrum with changing colors of light and  FIG. 9B  shows for use of only s-type polarized light;  
         [0020]      FIG. 10A  illustrates the plasmon rainbow radiation with sections at different spacing from the origin with  FIG. 10B  showing intensity changes for a number of the spacing sections of  FIG. 10A ;  FIG. 10C  illustrates exponential decay of leakage radiation intensity for different propagation distance and for different radiation wavelengths; and  FIG. 10D  illustrates plasmon decay length versus radiation wavelength;  
         [0021]      FIG. 11  shows an example sensor with data for various surface coatings;  
         [0022]      FIG. 12A  illustrates absorption versus wavelength for radiation leakage; and  FIG. 12B  illustrates decay length of the plasmon versus leakage radiation for a reference Ag film and a J-aggregate coated film;  
         [0023]      FIG. 13A  illustrates a plasmon rainbow jet for a device microstructure;  FIG. 13B  shows a bridge section for the device of  FIG. 13A ;  FIG. 13C  shows another bridge structure detail for the device of  FIG. 13A ; and  FIG. 13D  shows transmission percent versus bridge width for different leakage radiation wavelengths. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]     A plasmon sensor system  10  constructed in accordance with a predefined form of the invention as illustrated in  FIGS. 1A-1C . The plasmon sensor system  10  both creates surface plasmons  11  (see  FIG. 1C ) in a material and also detects those surface plasmons  11  in the form of an emitted continuum of photon wavelengths (leakage radiation) throughout the visible and infra-red region. Most preferably, the visible light spectrum is used to sense the surface plasmons  11  and their character which is representative of surface and near surface constituents  15  (see  FIG. 11 ) of an underlying metal film or other metallic conductor. Heretofore, the dispersion relating the surface plasmon energy to the momentum has prevented the excitation of the surface plasmons  11  with a large spectral content due to the difficulty of attaining the necessary spread of momenta while keeping a low background signal for the attenuated total internal reflection (“ATR”). The broad-band surface plasmons  11  can be useful to investigate wavelength-sensitive planar photonic devices or to spectrally study adsorbates (or the constituents  15 ) on materials in which the surface plasmons  11  exist, such as, metal films (such as Ag or Au). This methodology has particular advantages for use in nanotechnology applications (See, for example,  FIGS. 2A-2C ,  3 ,  4 ,  5 A,  5 B,  6 A,  6 B,  7 A and  7 B).  
         [0025]     As shown in  FIGS. 1A-1C , an incident white-light beam  13  is applied to a material to generate the desired surface plasmons  11 . The surface plasmon propagation is visualized by recording the real-space distribution of leakage radiation  22  emitted by the surface plasmon continuum as it travels along an asymmetric air  12 /silver  14 /glass structure  16 . The surface plasmon  11  can be detected by scattering at defects and leakage radiation emitted back in the substrate (the silver  14 ).  
         [0026]     A spatial variation of the spectral components of the surface plasmon  11  produces a rainbow-like jet in the collected images for the resonance conditions of  FIG. 1B  (see  FIGS. 9A, 9B ,  10 A and  10 B). These illustrate broad band excitation wherein the color spread reflects the surface plasmon dispersion and the energy dependant velocities (d{acute over (ω)}/dk).  FIGS. 10A-10D  illustrates surface plasmon decay lengths.  FIG. 10A  shows the appearance of the surface plasmon  11  spread as a function of wavelength from a detected region of a specimen.  FIG. 10B  illustrates the intensity variation as a function of wavelength for a series of cross sectional lines shown in  FIG. 10A .  FIG. 10C  shows the exponential decay behavior for a variety of wavelengths of emitted leakage radiation.  FIG. 10D  shows decay length as a function of wavelength of the leakage radiation.  
         [0027]     The leakage radiation  22  (See  FIG. 1A ) emitted in the glass substrate  16  by the surface plasmons  11  traveling at the air  12 /metal  14 /interface  18  are characterized by a well-defined emission angle θ sp  for every wavelength. The value θ sp  is greater than the critical angle in the glass. Therefore, elements in optical contact with the substrate (the silver  14 ) are necessary to avoid total internal reflection of the leakage radiation  22  (“LR”) within the substrate structure. This is achieved by an index-matched immersion objective  20  in contact with the substrate (the silver  14 ). The objective is part of a conventional inverted optical microscope (not shown) focused on the metal  14 /glass  16 /interface  18 . The leakage radiation  22  is focused by objective lens  20  and then recorded by a radiation sensor capable of detecting and analyzing visible and IR light, such as, CCD camera  24  placed in the image plane. We used the same objective  20  as part of the system  10  to excite the surface plasmons  11  in a variant of the Kretschmann configuration. There are several key advantages for using the objective  20  with the immersion oil  26  to excite the surface plasmons  11  over the standard prism. First, in order to visualize the leakage radiation  22  (LR), the surface plasmons excitation area must be smaller than the SP propagation length, which is achieved by focusing the illumination beam  13 . As a result of the focusing, a broad distribution of rays or wavevectors are impinging on the glass  16 /silver  14 /interface  18 . For a given wavelength, an associated wavevector will be responsible for surface plasmon excitation, while the others will be reflected or transmitted through the silver  14 . But, if the illumination light beam  13  is composed of a white-light continuum, virtually all wavelengths will be able to couple into the surface plasmons  11 .  
         [0028]     The oil immersion objective  20  we used has a most preferred numerical aperture (N.A.) of 1.4, meaning that the angular spread ranges between 0° to 68°. The SP excitation angles for wavelengths throughout the visible are confined within a few degrees around 45°. Therefore, if the full N. A. of the objective  20  is used, only a small fraction of the light  13  will be converted into the surface plasmons  11 ; and the overwhelming remaining part will be reflected or transmitted through the silver film  14 . Instead of completely filling the back-aperture of the objective  20 , a small beam of the collimated white-light beam  13  was adjusted within the back-aperture of the objective  20  as depicted in  FIG. 1A . The angle of the reflected light beam  30  emerging from the objective  20 , and the angular spread of the light beam  30  was controlled by adjusting the incident beam  13  with respect to optical axis  32 .  
         [0029]     The incident white-light beam  13  continuum was produced by the output of a Coherent MIRA regeneratively amplified Ti:Sapphire laser system (not shown). The beam  13  is created through well-known methods, in particular by focusing the 800 nm pulses into a small piece of sapphire (50 fs/pulse at 250 kHz). The white light beam  13  produced in this manner is generally easier to manipulate, collimate, and focus than other typical white light sources. The polarization of the beam  13  was controlled by a conventional multi-wavelength waveplate (not shown). The asymmetric plasmonic films were produced by thermally evaporating about 45±5 nm thick silver films on cleaned ones of the glass cover slips  16 .  
         [0030]     The resulting plasmon sensor system  10  is a highly sensitive device which can analyze and detect extremely small quantities of adsorbates on a metallic conducting material. Various features of surface plasmon sensors can be exploited to determine the presence and amount of adsorbates and even near surface constituents which are different than the matrix of the material being studied. An example of plasmonics is shown in  FIGS. 2A-2C  which demonstrates spatial sensitivity to particle size and interparticle spacing. In  FIG. 3  is a “dark-field spectroscopy” image where the light  13  imparts a beam mask  15 ; and then the nanoparticles shown are detected by the surface plasmon  11  scattering from the nanoparticles  17  in layer  19  with no forward illumination light transmitted.  
         [0031]     Further work is illustrated in  FIG. 4 , which shows a blue-shift in the plasmon resonance value for decreasing particle size and also the narrowing of the resonance for decreasing particle size (reduced plasmon damping). In  FIGS. 5A and 5B  is shown the characteristic increased damping for small spacing in terms of resonance peak location and resonance peak width. In  FIGS. 6A and 6B  is shown the influence of interparticle spacing for a two-dimensional array. A far field effect is shown in the form of a grating effect and blue shift for spacings greater than 200 nm. A near field coupling is shown for a red shift of the plasmon for spacings less than 200 nm. An increased plasmon damping occurs in the near field. In  FIGS. 7A and 7B  is shown the influence of interparticle spacing for a one-dimensional array.  
         [0032]     One embodiment of the invention of  FIGS. 1A-1C  is shown in  FIGS. 8A and 8B , where polarization sensitivity can be used to advantage with “p” type polarization for the light  13  which results in the illuminated signature for plasmon excitation while  FIG. 8B  shows the signature for “s” type polarized light  13 . The spectra shown are for the silver  14  layer of 50 nm thickness, the light  13  is 532 nm; and the NA of the objective  20  is 1.4 for a magnification of 60X.  
         [0033]      FIG. 11  illustrates the surface plasmon decay length for several example substrates and coated layers. Note the systematic decline in decay length as the reference silver  14  is coated with 1-Nonanethiol and 1-Dodecanethiol.  
         [0034]      FIGS. 12A and 12B  illustrate another example of absorption spectroscopy for the system  10  wherein conventional J-aggregates are present on the silver  14 .  
         [0035]      FIGS. 13A-13D  illustrate surface plasmon transmission characteristics for various submicron structures, such as are typically present in electronics arts and the like.  FIG. 13A  shows the overall leakage radiation as a “rainbow-jet” dispersion.  FIG. 13B  shows a bridge structure in the electronic device,  FIG. 13C  shows another bridge structure and  FIG. 13D  shows percent transmission as a function of bridge width for leakage radiation wavelengths of 650 nm, 750 nm and 850 nm.  
         [0036]     It should be understood that various changes and modifications referred to in the embodiment described herein would be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present invention. For instances, the femtosecond laser system used to produce the white-light continuum can be replaced by a simple gas bulb (halogen, etc.), or light emitting diode (LED), or emissive element (tungsten or carbon for example), etc. Similarly, the broad spectrum of wave vectors produced by the objective lens can also be produced by a defect on the film (engineered or natural) that is sub-wavelength in dimensions, or by the proximity of a near-field probe (with or without aperture). Similarly, a solid immersion lens or other high numerical aperture optic can readily replace the oil immersion objective used here. Similarly, the research grade CCD can be replaced by simpler devices, such as a digital camera, diode, or integrated hand-held or on-chip spectrograph. The inverted microsocope is used only for versatility and exploring a range of initial optical configurations during research. Now optimized, it can be eliminated in a commercial system. Changes to the detection of the broadband leakage radiation can also be readily envisioned by those skilled in the art, e.g. by avoiding leakage radiation collection by the objective lens.