Abstract:
Tunable cross-coupling evanescent mode optical devices and methods of making the same are described. In one aspect, a tunable optical device includes a first layer, a second layer, a metal layer disposed between the first and second layers, and an electrode. The first layer is supportable of electromagnetic field modes over a range of wavelengths that includes a target wavelength range. The second layer is disposed between the metal layer and the electrode and has an index of refraction that is adjustable over a range of values. The metal layer is disposed between the first and second layers and has at least one corrugated metal film region with a corrugation periodicity enabling cross-coupling of evanescent modes of equal wavelength within the target wavelength range and localized on opposite sides of the metal layer with different respective wavenumbers. The cross-coupling evanescent modes have a cross-coupling wavelength determined at least in part by the corrugation periodicity and the index of refraction of the first and second layers and is substantially unaffected by the electrode.

Description:
BACKGROUND  
         [0001]    Surface plasmon resonance occurs when radiant energy is “coupled” (or transferred) to electrons in a metal. The wavelength of light at which coupling occurs depends on the characteristics of the metal that is illuminated and the optical properties of the surrounding environment. When there is a match or resonance between the energy and wavenumber of the light photons and the electrons at the metal surface, a transfer of energy occurs. The coupling of light into a metal surface produces a plasmon (i.e., a group of excited electrons which behave like a single electrical entity). The plasmon, in turn, generates an electro-magnetic field that typically extends on the order of about 100 nanometers (nm) above and below the metal surface and oscillates with optical frequencies.  
           [0002]    U.S. Pat. No. 5,841,143 has proposed a waveguide integrated fluorescence sensor that includes a corrugated dielectric-metal-dielectric thin film stack that is fabricated on a p-n junction. Fluorescent light of a single wavelength is selectively transmitted through the corrugated thin film stack and converted into an electronic signal by the p-n junction. Wavelength filtering is enabled by evanescent mode (or evanescent wave) cross-coupling across the metal film. Such cross-coupling includes cross-coupling between surface plasmons across the metal film and cross-coupling between surface plasmons and waveguide modes across the metal film.  
         SUMMARY  
         [0003]    The invention features tunable cross-coupling evanescent mode optical devices and methods of making the same. The invention enables the wavelength of light transmitted through a corrugated metal layer to be readily changed based on selective adjustment of the index of refraction of a variable index layer in the vicinity of the corrugated metal layer.  
           [0004]    In one aspect of the invention, a tunable optical device includes a first layer, a second layer, a metal layer disposed between the first and second layers, and an electrode. The first layer is supportable of electromagnetic field modes over a range of wavelengths that includes a target wavelength range. The second layer is disposed between the metal layer and the electrode and has an index of refraction that is adjustable over a range of values. The metal layer is disposed between the first and second layers and has at least one corrugated metal film region with a corrugation periodicity enabling cross-coupling of evanescent modes of equal wavelength within the target wavelength range and localized on opposite sides of the metal layer with different respective wavenumbers. The cross-coupling evanescent modes have a cross-coupling wavelength determined at least in part by the corrugation periodicity and the index of refraction of the first and second layers and is substantially unaffected by the electrode.  
           [0005]    In another aspect, the invention features a method of making the above-described tunable optical device.  
           [0006]    Other features and advantages of the invention will become apparent from the following description, including the drawings and the claims. 
       
    
    
     DESCRIPTION OF DRAWINGS  
       [0007]    [0007]FIG. 1 is a block diagram of a tunable optical device that has a photosensitive region and an overlying thin film stack that includes a corrugated metal layer disposed between a waveguide layer and a layer with an adjustable index of refraction.  
         [0008]    [0008]FIG. 2 is an exemplary graph of surface plasmon energy plotted as a function of wavenumber for the two metal interfaces of the tunable optical device of FIG. 1.  
         [0009]    [0009]FIG. 3 is a diagrammatic view of an implementation of the tunable optical device of FIG. 1 that is operable to detect fluorescence that is emitted from fluorescent material disposed near the waveguide layer.  
         [0010]    [0010]FIG. 4 is a block diagram of a tunable optical device that has a thin film stack that includes a corrugated metal layer disposed between a waveguide layer and a layer with an adjustable index of refraction. 
     
    
     DETAILED DESCRIPTION  
       [0011]    In the following description, like reference numbers are used to identify like elements. Furthermore, the drawings are intended to illustrate major features of exemplary embodiments in a diagrammatic manner. The drawings are not intended to depict every feature of actual embodiments nor relative dimensions of the depicted elements, and are not drawn to scale.  
         [0012]    Referring to FIG. 1, in some embodiments, a tunable optical device  10  includes a waveguide layer  12 , a corrugated metal layer  14 , a layer  16  with an adjustable index of refraction, an optional photosensitive region  18 , and an electrode  19 . In some embodiments, tunable optical device  10  is formed by known thin film fabrication techniques.  
         [0013]    Waveguide layer  12  is able to support electromagnetic field modes over a range of wavelengths that includes a target wavelength range. Waveguide layer  12  may be formed of any dielectric material, including silicon dioxide, silicon nitride, and lithium fluoride. In some embodiments, waveguide layer  12  has a thickness of about 200-500 nm.  
         [0014]    Corrugated metal layer  14  is a continuous thin film layer that has a thickness between about 20 nm and 100 nm. Corrugated metal layer  14  may be formed of any thin metal film, including thin films of silver and gold. The corrugation periodicity (i.e., the spatial distance between adjacent corrugation peaks or adjacent corrugation valleys) of metal layer  14  is selected to enable cross-coupling of evanescent modes (or evanescent waves) of equal wavelength within the target wavelength range and localized on opposite sides of the metal layer  14  with different respective wavenumbers. As used herein, the term evanescent mode cross-coupling refers to cross-coupling between surface plasmons and cross-coupling between surface plasmons and waveguide modes. In one implementation, metal layer  14  has corrugations that are characterized by a peak-to-valley depth of about 50 nm and a corrugation periodicity of about 1.1 micrometer (μm). In this implementation, the cross-coupling wavelength range extends from about 850 nm to about 400 nm when the waveguide layer  12  has a refractive index of about 1.5 and the refractive index of a liquid crystal layer  16  is varied from about 1.5 to about 1.7. The cross-coupling wavelength range in this implementation may be extended by increasing the grating periodicity. In some embodiments, metal layer  14  is localized in separate discrete regions of tunable optical device  10 . In some embodiments, tunable optical device  10  includes multiple separate metal film regions each of which is characterized by a different respective corrugation periodicity.  
         [0015]    In some implementations, the corrugations of metal layer  16  are characterized by a sinusoidal surface relief. In these implementations, the corrugation may be achieved by patterning one of the underlying layers (e.g., variable index layer  16  or photo-sensitive region  18 ). For example, a photoresist layer may be spun on a planar surface of variable index layer  16  or photosensitive region  18 . The photoresist is exposed to two interfering laser beams of the same wavelength. This causes a sinusoidal variation in the photoresist exposure. Upon development, the photoresist layer will have a sinusoidal surface relief with dimensions and periodicity corresponding to the desired corrugation peak-to-valley depth and the desired corrugation periodicity. The surface relief pattern may be transmitted into the variable index layer  16  or the photosensitive region  18  by etching (e.g., ion beam milling or dry chemical etching). If the sinusoidal surface relief is etched into the photosensitive region  18 , the variable index layer  16  is formed on the etched photosensitive region  18  and the metal layer is formed on the variable index layer  16 . The variable index layer  16  is sufficiently thin that the sinusoidal spatial variations are transferred from photosensitive region  18  to the overlying metal layer  14 . If the sinusoidal surface relief is etched into the variable index layer  16 , a metal film is simply formed on the variable index layer  16  to form the corrugated metal layer  14 . Waveguide layer  12  is formed over the corrugated metal layer  14 .  
         [0016]    Variable index layer  16  may be formed of any dielectric or electro-optic material that has an index of refraction that may be varied controllably over a range of refraction index values that includes refraction index values that are different from the refraction index of waveguide layer  12 . In some implementations, variable index layer  16  is formed of an electro-optic material. Exemplary electro-optic materials that may be used to form variable index layer  16  include: lithium niobate; lithium tantalite; potassium dihydrogen phosphate; potassium dideuterium phosphate; aluminum dihydrogen phosphate; aluminum dideuterium phosphate; barium sodium niobate; and liquid crystal. In these implementations, a refraction index adjuster  20  (e.g., a voltage source) is operable to apply a voltage between corrugated metal layer  14  and electrode  19 . The applied voltage is sufficient to create across variable index layer  16  an electric field of sufficient strength to controllably adjust the refractive index of variable index layer  16 . In some implementations, variable index layer  16  has a thickness between about 50 nm and 100 nm.  
         [0017]    The optional photosensitive region  18  may be formed of any material that responds to electromagnetic fields within the target wavelength range with a detectable photo-response. In some implementations, photosensitive region  18  includes a conventional semiconductor p-n (or n-p) junction. In these implementations, a photo-response detector  22  (e.g., an electronic circuit) is operable to measure electrical responses of photo-sensitive region  18  to evanescent mode fields that are transmitted across corrugated metal layer  14  (e.g., measure an electrical current generated in the photosensitive region  18  or measure a change in voltage or resistance across the photosensitive region  18 ).  
         [0018]    Electrode  19  may be formed of any electrically conducting material (e.g., a metal or indium-tin-oxide). Electrode  19  is configured so that it does not affect evanescent mode cross-coupling across metal layer  14 . For example, in the illustrated embodiment, photosensitive region  18  prevents electrode  19  from influencing evanescent mode cross-coupling across metal layer  14 . In some embodiments, electrode  19  is disposed between variable index layer  16  and photosensitive region  18 . In these embodiments, electrode  19  is formed of a material (e.g., indium-tin-oxide for visible and ultraviolet wavelengths) that is substantially transparent to light within the target wavelength range.  
         [0019]    In operation, incident radiation  24  couples into waveguide layer  12  by exciting surface plasmon modes supported at the metal/waveguide layer interface  28  or by exciting waveguide modes in the waveguide layer  12 . Both surface plasmons and waveguide modes are evanescent modes whose electromagnetic fields  30  decay rapidly with increasing distance away from the metal layer/waveguide layer interface  32  and the center of the waveguide layer  26 , respectively. Evanescent modes can also be supported at the variable index layer/metal layer interface  32  (surface plasmons) and in the variable index layer  16  (waveguide modes). If the media on opposite sides of the metal film have different values of refractive index, evanescent modes with the same wavelength and different wavenumbers, k z , are supported on opposite sides of the corrugated metal layer. The presence of the corrugation allows wavenumber matching and, as a result, evanescent modes on one side of the corrugated metal layer can cross couple with evanescent modes with equal wavelength on the opposite side of the corrugated metal layer. In particular, surface plasmons with the same wavelength on opposite sides of the corrugated metal layer can cross couple. Also, surface plasmons and TM waveguide modes of equal wavelength and localized to opposite sides of the corrugated metal layer can cross couple. TE waveguide modes have an orthogonal polarization with respect to surface plasmons and, therefore, cannot interact with surface plasmons. As a consequence of the corrugation-induced cross coupling, radiative energy at the desired wavelength is transmitted across the otherwise opaque corrugated metal layer. The corrugation periodicity of the corrugated metal layer  14  and the indices of refraction of layers  12 ,  16  determine the wavelength at which the surface plasmons cross-couple across corrugated metal layer  14 .  
         [0020]    Referring to FIG. 2, in one illustrative example, at the wavelength X, two surface plasmon states at opposite sides of the corrugated metal layer  14  couple when the wavenumbers (k z ) of the surface plasmon states that are parallel to the metal interfaces are matched via the corrugated metal layer  14 . In particular, the presence of the corrugation contributes a wavenumber contribution, Δk z , which is equal to +/−2 πn/Λ, where Λ is the corrugation periodicity and n is an integer having a value of 1 or more. Since the surface plasmon dispersion curves  36 ,  38  for surfaces  28 ,  32  of metal layer  14  are diverging, there is one and only one wavelength, λ, where wavenumber matching is accomplished. In the illustrated embodiment, if an electric field is applied across the variable index layer  16 , the refractive index of this layer changes. For the purpose of this discussion, without loss of generality, it is assumed that the refractive index increases; although in some implementations, the refraction index of layer  16  may decrease in response to an applied electric field. As the refraction index increases, the surface plasmon dispersion curve  38  “rotates” (as shown by arrow  40 ) towards larger k z  values. The cross-coupling wavelength is shifted to a longer wavelength λ′ (i.e., lower energy; as shown) because the corrugation period has not changed. Thus, the radiation wavelength that is allowed to pass through the metal film and, hence, be detected varies as a function of the voltage applied across the variable index layer.  
         [0021]    As explained in detail below, in some embodiments, the tunable optical device  10  may be implemented as an optical sensor that may be tuned to detect desired wavelengths of light by adjusting the voltage applied between the corrugated metal layer  14  and electrode  19 . In other embodiments, the optional photosensitive region  18  may be omitted and the resulting tunable optical device may be used as a tunable optical wavelength filter.  
         [0022]    As shown in FIG. 3, in some embodiments, the tunable optical device of FIG. 1 may be implemented as a fluorescence sensor  41 . In these embodiments, a fluorescent material  42  is placed adjacent to the waveguide layer  12  of the fluorescence sensor  41 . The fluorescent material  42  may be in a gaseous, liquid, or solid state. Alternatively, a film of fluorescent material  42  may be deposited on a surface of waveguide layer  12 . In operation, excitation radiation  44  is applied to the fluorescent material  42 . The excitation radiation  44  may be delivered from either an external source or from light confined to the waveguide layer  12  via a waveguide mode. The excitation radiation  44  includes wavelengths that excite atoms or molecules of interest in the fluorescent material  12 . The waveguide layer  12  supports the propagation of waveguide modes that generate a strong electromagnetic field in the vicinity of the fluorescent material  42 , enhancing the intensity of fluorescent light that is generated by the fluorescent material  42 .  
         [0023]    Fluorescent light flows away from the fluorescent material  42  and into waveguide layer  12 . The wavelength content of the resulting evanescent modes is the same as that of the fluorescent emission spectra of the fluorescing atoms or molecules in the fluorescent material  12 . Surface plasmons at interface  46  between the waveguide layer  12  and corrugated metal layer  14  are excited at all fluorescent emission wavelengths. TM waveguide modes in the waveguide layer  12  also are excited at all fluorescent wavelengths. Based on the refractive index to which variable index layer  16  is adjusted, surface plasmons or TM waveguide modes (or both) of a desired cross-coupling wavelength are supported at both metal interfaces  46 ,  48  and layers  12 ,  16  respectively. Thus, only fluorescent light at the desired wavelength is transmitted through the corrugated metal layer  14  to the photosensitive region  18 .  
         [0024]    Referring to FIG. 4, in some embodiments, a tunable optical wavelength filter  50  may be implemented by forming the multi-layer stack  12 - 16  on an electrode  52  that is substantially transparent to radiation within the target wavelength range. In operation, only radiation  54  that has a wavelength corresponding to the tuned evanescent mode cross-coupling wavelength is able to pass though tunable optical wavelength filter  50 .  
         [0025]    Other embodiments are within the scope of the claims.