Abstract:
A manufacture for supporting propagation of terahertz waves includes a stack of layers made of a conformal protective polymer coating material; and an array of cells patterned on each of said layers, each cell including a metallic structure.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application No. 61/429,318, filed on Jan. 3, 2011, the contents of which are incorporated herein. 
     
    
     FIELD OF DISCLOSURE 
       [0002]    This disclosure relates to electromagnetic structures that control wave propagation, and in particular, to metamaterials for supporting such propagation in the terahertz, far-infrared and millimeter-wave range. 
       BACKGROUND 
       [0003]    Terahertz radiation is useful for a variety of applications. For example, because of its ability to penetrate most clothing, terahertz radiation provides a way to detect concealed weapons. Because of its ability to detect differences in water content and density of tissue, terahertz radiation can be used to reliably distinguish between normal cells and cancerous cells. 
         [0004]    Electromagnetic metamaterials for supporting propagation of a particular wavelength consist of composites having metallic structures consisting of large number of unit cells each having dimensions an order smaller than the wavelength to be propagated. The joint interaction of these metallic structures in their surrounding medium results in a wave propagation medium that can have selected values of permittivity and/or permeability. Different values of permittivity/permeability can provide a diverse array of electromagnetic response such as filtering, focusing, negative reflection or refraction, lenses, cloaking and radiation. 
       SUMMARY 
       [0005]    In one aspect, the invention features a manufacture for supporting and altering propagation of terahertz and far-infrared electromagnetic waves. Such a manufacture includes a stack of layers made of a conformal protective polymer coating material; and an array of metamaterial unit cells patterned on each of the layers. Each such metamaterial unit cell includes a metallic structure 
         [0006]    Among the embodiments of the manufacture are those in which the stack of layers includes a parylene layer, those in which the stack of layers includes a parylene-C layer, those in which the stack of layers includes a parylene-N layer, those in which the stack of layers includes a parylene-D layer, those in which the layers are biocompatible, and those in which the stack of layers includes a poly para xylene layer. Also included are those embodiments in which the layers are made of any combination of the foregoing materials 
         [0007]    In some embodiments, the metallic structures have a maximum lineal dimension that can range anywhere from nanometers to meters. A maximum lineal dimension in the range from 100 nm to 10 mm is suitable for terahertz and far-infrared region of electromagnetic spectrum. 
         [0008]    In another aspect, the invention features a method of making a metamaterial for propagation of terahertz waves. Such a method includes vapor coating, onto a platform, a plurality of layers of conformal protective coating material; patterning metallic planar structures on each of the layers; and removing the plurality of layers from the platform. 
         [0009]    In some practices, the conformal coating material is a parylene. 
         [0010]    In another aspect, the invention features an apparatus for sub epithelial implantation for detection of skin cancer using terahertz radiation. Such an apparatus includes a detector having a metamaterial, the metamaterial including layers of a biocompatible conformal protective coating polymer, each of which has, formed thereon, an array of metallic structures. 
         [0011]    In some embodiments, the metallic structures have a maximum lineal dimension of between 0.1 mm and 1 mm. In other embodiments, the coating includes parylene. 
         [0012]    These and other features of the invention will be apparent from the following detailed description and the accompanying figures, in which: 
     
    
     
       DESCRIPTION OF THE FIGURES 
         [0013]      FIG. 1  shows a multi-layer parylene-based meta-material; 
           [0014]      FIG. 2  shows one layer of a parylene-based metamaterial of  FIG. 1 ; and 
           [0015]      FIG. 3  shows exemplary metallic structures for the cells of the metamaterial shown in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Referring to  FIG. 1 , a metamaterial  10  consists of a plurality of thin film layers  12 , each of which is between about 10 nm and 1 mm thick. A typical layer  12 , as shown in  FIG. 2 , has, patterned thereon, an array  14  of metamaterial unit cells  16 . Each cell  16  includes a metallic sub-wavelength structure  18 . In many embodiments, the thin film thickness is approximately 100 nm. 
         [0017]    In the particular cell  16  shown, the metallic sub-wavelength structure  18  is a planar split-ring resonator. However, other metallic sub-wavelength structures can be used. For example, instead of a split-ring resonator as shown, the cell  16  can have a split-ring structure with single and/or multiple loops, or a fishnet structure, or an arrangement of thin wires. In some embodiments, the metamaterial unit cell  16  can include magneto dielectric spheres. Examples of different metallic structures in a 100 μm×100 μm cell are shown in  FIG. 3 . 
         [0018]    Each layer  12  is made of a conformal protective polymer coating material. A suitable polymer is a poly para xylene parylene, and in particular, parylene-C, parylene-N, and parylene-D. 
         [0019]    In one method of fabrication, a silicon layer is used as a platform upon which the parylene layer  12  is fabricated and from which it is peeled off after fabrication. 
         [0020]    After dehydration baking at 150 C, a ten micron layer  12  of parylene-C is deposited onto the platform using a parylene deposition unit. A suitable deposition unit is sold under the name of LABCOPTER 2 PARYLENE DEPOSITION UNIT made by Specialty Coating Systems in Indianapolis, Ind. 
         [0021]    The deposition unit vaporizes a dimer charge at 175 C and 1 Ton, and then decomposes it into its monomer (paraxylylene) at 690 C and 0.5 Ton. It then deposits the monomer onto the platform at 25 C and 0.1 Ton to form the parylene-C layer  12 . 
         [0022]    Once the layer  12  is in place, the next step is to create the array  14  of unit cells  16 . This is carried out using a conventional photo resist, such as AZ nLOF 2020 using conventional photolithographic methods. A layer of titanium and/or gold, or any suitable conductor, is then sputtered onto the parylene layer to form the metallic sub-wavelength structures  18 . The thickness of the conductor ranges from 10 nm to 200 nm. The platform, now supporting one meta-material layer  12 , is then placed in an acetone bath and peeled off. 
         [0023]    To manufacture a laminated structure as shown in  FIG. 1 , one patterns a layer  12  and then carries out chemical vapor deposition on the patterned layer  12  to form a second layer, which can then be patterned in the same way as the first layer. This procedure repeats until the requisite number of layers is reached. 
         [0024]    Because of its biocompatibility, a metamaterial  10  made of parylene thin films is particularly suitable for medical applications. Because of their ability to interact with terahertz radiation, and because of the use of terahertz radiation in detecting skin cancer, diagnostic detectors that rely on parylene-based metamaterials can safely be implanted in a human. 
         [0025]    Various properties of metamaterials as described herein are described in more detail in an article entitled “Metamaterials on parylene thin film substrates: Design, fabrication and characterization at terahertz frequency” by X. Liu, et al., and published in Applied Physics Letters 96-011906, the contents of which are herein incorporated by reference.