Abstract:
An apparatus and a method for steering optical frequency beams using nanowire. A method includes providing one or more nanowire waveguide arrays, generating an optical frequency beam, wherein the optical frequency beam is incident on the one or more nanowire waveguide arrays, controlling the one or more nanowire waveguide arrays to produce a phase delay in the optical frequency beam as it traverses the nanowire waveguide array, wherein the phase delay causes the optical frequency beam to deflect upon exiting the one or more nanowire waveguide arrays, and steering the optical frequency beam exiting the one or more nanowire waveguide arrays by increasing or decreasing the phase delay, wherein the angle of deflection of the exiting optical frequency beam is determined by the amount of phase delay.

Description:
BACKGROUND 
     Recent advances in nanoscale technology have resulted in techniques for growing nanoscale wires (“nanowires”) from a variety of materials including Gallium Nitride (GaN), Silicon (Si), Silicon Germanium (SiGe), Zinc Oxide (ZiO), Lead Zirconate Titanate (PZT), Cadmium Sulfide (CdS), Indium Phosphide (InP) and others. These nanowires have demonstrated many remarkable properties. Of particular interest is the ability to act as waveguides for optical frequency radiation. 
     Nanowires can be grown with diameters on the order of a few hundred nanometers. This allows them to act as waveguides for UV, visible and near IR light. One of the most common techniques used for the formation of nanowires, nanotubes, or nanorods is template-based synthesis in which the desired materials are grown within the pores of a porous membrane such as track-etched polycarbonate or anodic alumina. This method is widely used to form metal and polymer nanorods, semiconducting and oxide nanowires and composite structures. After the material is grown the template is removed to leave the desired nanowires. 
     SUMMARY 
     An advantage of the embodiments described herein is that they overcome the disadvantages of the prior art. 
     These advantages and others are achieved by a method for steering optical frequency beams using nanowire. A method includes providing one or more nanowire waveguide arrays, generating an optical frequency beam, wherein the optical frequency beam is incident on the one or more nanowire waveguide arrays, controlling the one or more nanowire waveguide arrays to produce a phase delay in the incident optical frequency beam, wherein the phase delay causes the optical frequency beam to deflect upon exiting the one or more nanowire waveguide arrays, and steering the optical frequency beam exiting the one or more nanowire waveguide arrays by increasing or decreasing the phase delay, wherein the angle of deflection of the exiting optical frequency beam is determined by the amount of phase delay. 
     These advantages and others are also achieved by method of making a nanowire waveguide array used for steering an optical frequency beam. The method includes forming a substrate, growing a layer of amorphous material on substrate, etching away low density regions in layer of amorphous material, creating inter-columnar spaces in amorphous material, filling in inter-columnar spaces with nanowire material, whereby nanowires are grown in the inter-columnar spaces, etching away remaining amorphous material, and optionally removing substrate. 
     These advantages and others are also achieved by a system for steering optical frequency beams using nanowire. The system includes two nanowire waveguide arrays configured in a Risley prism-like arrangement, coaxially aligned with each other, a light source that generates an optical frequency beam incident on a first of the two nanowire waveguide arrays and exits a second of the two nanowire waveguide arrays, and a mechanism for rotating the two nanowire waveguide arrays, wherein rotating the two nanowire waveguide arrays increases or decreases an angle of deflection of the exiting optical frequency beam. 
     These advantages and others are also achieved by a system for electrically steering optical frequency beams using nanowire. The system includes a nanowire waveguide array fabricated from an electro-optic material, in which the nanowire waveguide array includes one or more electrodes, a light source that generates an optical frequency beam incident on the nanowire waveguide array, and wherein an electric field is applied to the nanowire waveguide array, causing a phase delay in the optical frequency beam so that optical frequency beam exits the nanowire waveguide array with an angle of deflection, in which increasing the intensity of the electric field increases the angle of deflection. 
     These advantages and others are also achieved by a system for magnetically steering optical frequency beams using nanowire. The system includes a nanowire waveguide array fabricated from a magneto-optic material, a quarter-wave plate aligned parallel to nanowire waveguide array, a light source that generates an optical frequency beam incident on the quarter-wave plate, wherein quarter-wave plate produces circularly polarized light beam that is incident on nanowire waveguide array, and a mechanism for applying a magnetic field to the nanowire waveguide array, wherein applied magnetic field causes a phase delay in the circularly polarized light beam so that circularly polarized light beam exits the nanowire waveguide array with an angle of deflection, in which increasing the magnetic field gradient increases the angle of deflection and the circularly polarized light beam is deflected in the direction of the magnetic field gradient. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The detailed description will refer to the following drawings, wherein like numerals refer to like elements, and wherein: 
         FIGS. 1A-1B  are a schematic diagram illustrating an embodiment of a system for steering a beam using nanowire; 
         FIGS. 2A-2E  are schematic diagrams graphically illustrating various steps of an embodiment of a method of forming a nanowire array; 
         FIG. 3  is photographs illustrating levels of microstructure in amorphous germanium and amorphous silicon; 
         FIG. 4  is a schematic diagram illustrating a nanowire array made using an embodiment of a method of forming a nanowire array; 
         FIG. 5A  is a schematic diagram illustrating an embodiment of a system for mechanically steering an optical beam using a nanowire array; 
         FIG. 5B  is a flowchart illustrating an embodiment of a method for mechanically steering an optical beam using a nanowire array; 
         FIG. 6A  is a schematic diagram illustrating an embodiment of a system for electronically steering an optical beam using a nanowire array; 
         FIG. 6B  is a flowchart illustrating an embodiment of a method for electronically steering an optical beam using a nanowire array; 
         FIG. 7A  is a schematic diagram illustrating an embodiment of a system for magnetically steering an optical beam using a nanowire array; and 
         FIG. 7B  is a flowchart illustrating an embodiment of a method for magnetically steering an optical beam using a nanowire array. 
     
    
    
     DETAILED DESCRIPTION 
     Described herein are systems and methods for steering optical frequency beams (optical beams) using nanowires and a method of making such systems. Embodiments include methods of steering an optical beam using an array of nanowire waveguides. A first embodiment uses a nanowire array that has been doped to form a structure similar to a diffraction grating. The structure is then rotated mechanically to steer the beam. A second embodiment uses nanowires made from an electro-optic material such as GaN, allowing the beam to be steered electronically. A third embodiment uses the Faraday effect to steer the beam magnetically. 
     Because of their small size, nanowire waveguides can form an array with spacing less than the wavelength of the light being transmitted. The surface of an array of waveguides is essentially an antenna array. If the light traversing the nanowires is phase coherent, the light will emerge from the array to form a beam. 
     With reference now to  FIGS. 1A-1B , shown is a schematic illustrating system  10  for steering a beam using nanowire array  12 . Applying a phase delay that is linearly increasing across the face of the array will cause the beam to steer to the side. Individual nanowires (or nanotubes or nanorods)  14  in array  12  act as waveguides for light (optical frequency radiation)  16 . When nanowire array  12  is illuminated with a phase coherent source such as a laser (from left-side), light  16  propagates down individual nanowires  14  and is reemitted. The end of each nanowire waveguide  14  acts like an independent light source. For points well away from the face of the array, the light from each individual source can add constructively or destructively. If the light from each source is emitted with the same phase, the sources will only add constructively for points perpendicular to the face of the nanowire array. Light will thus be emitted as a beam  18  perpendicular to the face of the array, as shown in  FIG. 1A . If however each waveguide applies a different phase delay to the light propagating down the waveguide and the phase delays are arranged such that the phase shift increases linearly across the face of the array, the light sources will only add coherently at points which lie along a line at an angle to the face of the array. That is, the beam  18 ′ emitted by the array will be deflected (steered) away from the perpendicular, as shown in  FIG. 1B . The amount of deflection is directly proportional to the gradient of the phase shift across the face of the array. Thus increasing the gradient of the phase shift will increase the angle of deflection. 
     According to the embodiments described herein, the linearly increasing phase delay can be achieved by changing the index of refraction of the material such as by doping or through the electro-optic effect. Likewise, according to the embodiments described herein, the linearly increasing phase delay can also be achieved for circularly polarized light using the Faraday (magneto-optic) effect. 
     Embodiments described herein use GaN nanowires to make the waveguide array. GaN is known to form nanowires that can act as optical waveguides. In addition, GaN has been reported to exhibit both electro-optic and Faraday effects. However, other materials, such as Barium Titanate (BaTiO3), Strontium-Barium Niobate (Sr x Ba (1-x) Nb 2 O 6 ) SBN, PZT, PLZT, SiGe, KH 2 PO 4  (KDP), LiNbO 3 , etc., may also be used. In embodiments described herein, the material should be capable of (a) forming nanowires of a substance that is transparent at the operating frequency that can act as optical waveguides and (b) 1) being doped to change the index of refraction, 2) exhibiting the electro-optic effect, and/or 3) exhibiting the Faraday effect. 
     Manufacture of Nanowire Arrays 
     As discussed above, one of the most common techniques used for the formation of nanowires, nanotubes, or nanorods is template-based synthesis in which the desired materials are grown within the pores of a porous membrane such as track-etched polycarbonate or anodic alumina. With reference to  FIGS. 2A-2E , described herein is a method of forming a nanowire array. Method is based on the fact that thin films, particularly films grown by physical vapor deposition, have three levels (macro, micro and nano) of columnar microstructure with low-density regions filling the inter-columnar space. The macro columns are a few microns in diameter, the micro columns a few thousand angstroms, and the nano columns are a few nanometers. With reference to  FIG. 3 , three levels of microstructure in an amorphous germanium (a-Ge) film are shown. The a-Ge film was etched in H 2 O 2  to remove the low-density regions and clearly reveal the different columnar structure levels. A similar observation was made with amorphous silicon (a-Si) films, also shown in  FIG. 3 . 
     With reference now to  FIG. 2A , substrate  22  is formed. Substrate  22  may be formed with bottom electrodes  24  for certain electronic steering embodiments. Substrate  22  may be formed from nearly any material, examples including silicon, sapphire, GaAs, quartz, and others. A layer or film  26  of a-Si or a-Ge is grown on substrate  22 , as shown in  FIG. 2B . Low density regions in film  26  are identified and etched away (or otherwise removed), as shown in  FIG. 2C . This creates the inter-columnar spaces or regions  28  described above. Inter-columnar regions  28  are then filled with the desired material (e.g., GaN, ZnSe, ZnTe, KTN, LiNbO 3 , LiTaO 3 , KNbO 3 , or ZnGeP2, CdGeP2, or a rare earth iron garnate in which the rare earth metal may be T, Y, Gd, Tb, Dy, or Yb) for growing nanowires  30 , and nanowires  30  are grown, as shown in  FIG. 2D . Then, top electrodes  32  are optionally deposited and the remaining a-Si or a-Ge is etched away, leaving only nanowire waveguide array  34 , as shown in  FIG. 2E . An advantage of this technique is that the inter-columnar spacing can be changed by varying the deposition conditions. Further, certain deposition conditions allow the columns to grow tilted permitting the nanowires to be grown at an angle if necessary. 
     Once nanowires  30  have been grown, the space between and around nanowires  30  can be filled to form a rigid structure. A variety of different materials exist which may be used for this purpose. The materials used should have a proper index of refraction to keep the optical radiation confined to the waveguide, not cause excessive optical losses, and be rigid enough to securely hold the nanowire waveguide array  34  in place during further processing. In certain embodiments, spaces between nanowires  30  may be filled with electrodes in order to provide for electrical steering of beam (see below). Once the space between nanowires  30  is filled, substrate  22  may be ground off and the top and bottom surfaces of nanowire waveguide array  34  polished. Optionally, the substrate need not be removed if it is made from a transparent material with the proper optical properties. This process may yield a nanowire waveguide array  34  similar to that depicted in  FIG. 4 . 
     Mechanically Steered Array 
     With reference now to  FIGS. 5A-5B , shown are system  40  and method  50  for mechanically steering an optical beam using a nanowire array. The simplest way to use a nanowire array to steer an optical beam is mechanical steering. In the embodiment shown, two nanowire waveguide arrays  42  are configured similar to a Risley prism. Individual nanowires  44  in arrays  42  may be doped to change their index of refraction. Linearly increasing the dopant concentration across nanowire waveguide array  42  will cause a linear increase in the index of refraction of nanowires  44 . This will result in a linear increase in the phase delay experienced by the light propagating through each waveguide  42 . Since the light emitted from the surface of nanowire waveguide array  42  will have a linearly increasing phase delay, the resulting beam will steer in the direction opposite the dopant gradient. 
     Nanowire waveguide arrays  42  are mechanically rotated independently to steer beam  46 . System  40  provides for maximum beam deflection when nanowire waveguide arrays  42  are aligned coaxially with a small gap between them, as shown in  FIG. 5A . Nanowire waveguide arrays  42  may be rotated mechanically, using known mechanisms such as a stepper motor, to achieve steering. When the doping gradients are aligned (i.e., dopant gradients are parallel), phase delay in beam  46  is greatest and beam  46  experiences maximum deflection (greatest angle of deflection). When the dopant gradients are anti-parallel, the phase delay is minimized and beam  46  will be un-deflected. System  40  may be configured with additional nanowire waveguide arrays  42  to provide additional steering options and flexibility. For example, a third nanowire waveguide array  42  may enable the angle of deflection to be further increased. 
     With reference to  FIG. 5B , method  50  includes providing two or more nanowire waveguide arrays  42  (block  52 ). Nanowire waveguide arrays  42  may be manufactured as described above with reference to  FIGS. 2A-2E . Nanowire waveguide arrays  42  are preferably doped with increasing dopant concentration across each nanowire waveguide array  42 . Nanowire waveguide arrays  42  are configured similar to a Risley-prism configuration (block  54 ). A mechanism(s) for rotating nanowire waveguide arrays  42  is provided (block  56 ). An optical beam source is provided, with light beam  46  incident on first nanowire waveguide array  42  (block  58 ). Optical beam source may be, for example, a laser, a laser diode, or an incoherent monochromatic source. Nanowire waveguide arrays  42  are rotated to increase or decrease the phase delay and, hence, the resulting angle of deflection of beam  46  at output from second (or last) nanowire waveguide array  42  (block  60 ). One nanowire waveguide array  42  may be rotated while the other remains stationary or both nanowire waveguide arrays  42  may be rotated. 
     Electrically Steered Array 
     With reference now to  FIGS. 6A-6B , shown are system  70  and method  80  for electrically steering an optical beam using a nanowire array. Electrical steering is generally much faster than mechanical steering, but electrically steered nanowire waveguide array  72  is generally more difficult to fabricate. The index of refraction (and thus phase delay) of nanowire waveguide array  72  made from electro-optic material can be modulated with an applied electric field. Generating the electric field using electrodes  74  in channels between nanowires  76  in nanowire waveguide array  72  permits a large beam deflection with a small applied voltage. Alternatively, electrodes could be deposited on top or bottom surface of nanotube array (not shown). 
     With reference to  FIG. 6A , nanowires  76  are constructed from an electro-optic material (e.g., GaN, ZnSe, ZnTe, KTN, LiNbO 3 , LiTaO 3 , KNbO 3 ). When an electric field is placed across the nanowire waveguide array  72 , the index of refraction of nanowires  76  increases, slowing optical beam (not shown) and increasing the phase delay. Increasing the phase delay increases the angle of deflection of the optical beam. Beam can be steered by varying the intensity of the electric field across the face of nanowire waveguide array  72 . 
     Electrodes  74  used to produce the electric field may be placed either on the top and bottom surfaces of nanowire waveguide array  72  (as described above with regards to  FIGS. 2A-2E ) or in channels between the individual nanowires  76 , as shown in  FIG. 6A . Placing electrodes  74  on the top and bottom surfaces is easier to fabricate, but the large separation between electrodes  74  limits the size of the electric field and thus the maximum beam deflection. Depositing electrodes  74  in the channels between nanowires  76  is more difficult to fabricate, but allow nanowire waveguide array  72  to achieve large deflection angles with low voltages. 
     With reference to  FIG. 6B , method  80  includes providing an electrically steered nanowire waveguide array  72  (block  82 ). Nanowire waveguide array  72  may be manufactured as described above with reference to  FIGS. 2A-2E . Nanowire waveguide arrays  72  may include top and bottom electrodes  74  or electrodes  74  deposited in the channels between nanowires  76 . An optical beam source is provided, with light beam incident on nanowire waveguide array  72  (block  84 ). An electric field is applied to the nanowire waveguide array  72  (block  86 ), increasing the index of refraction of nanowires  76  and the phase delay in beam. An electric field may be applied to nanowire waveguide array  72  through a power source connected to electrodes (e.g., via wires or other known means). Intensity of applied electric field may be varied to increase or decrease the index of refraction and, hence, the phase delay and resulting angle of deflection of beam, thereby steering the beam (block  88 ). 
     Magnetically Steered Array 
     With reference now to  FIGS. 7A-7B , shown are system  100  and method  120  for magnetically steering an optical beam using a nanowire array. System  100  and method  120  use the Faraday effect to steer the beam magnetically. The Faraday effect is a rotation of the plane of polarization of light passing through a magneto-optic medium in the presence of a magnetic field. For certain materials, the plane of polarization of a beam of light passing through the material is rotated when a magnetic field is applied parallel to the beam. The degree of rotation is proportional to the applied field. 
       FIG. 7A  shows a schematic of magneto-optic nanowire waveguide array  102  using the Faraday effect for beam steering. Linearly polarized beam  104  (e.g., laser beam  104 ) passes through quarter wave plate  106 , producing circularly polarized light beam  108 . Quarter wave plate  108  are commercially available optics components. Circularly polarized light beam  108  then passes through nanowire waveguide array  102 . Nanowire waveguide array  102  is fabricated from a magneto-optic material. If magnetic field  110  is applied to nanowire waveguide array  102 , magnetic field  110  will rotate the plane of polarization of polarized light beam  108 . For circularly polarized beam  108 , rotating the plane of polarization is equivalent to a phase shift. The phase shift produces an effective phase delay in circularly polarized beam  108 , deflecting circularly polarized beam  108 . The greater the phase shift, and hence phase delay, the greater the angle of deflection of circularly polarized beam  108 . 
     Magnetic field  110  may be generated, e.g., using a pair of counter-wound coils  112 . Power supply(ies) (not shown) may be connected to counter-wound coils  112  to provide necessary power and control to for counter-wound coils  112  to produce and control magnetic field  110 . Counter-wound coils  112  produce magnetic field  110  with a gradient in magnetic field  110  across the face of nanowire waveguide array  102 . The resulting gradient in the phase (the phase shift or delay) of polarized light beam  108  emitted from nanowire waveguide array  102  will cause polarized light beam  108  to steer in the direction of the magnetic field gradient. The greater the magnetic field gradient across nanowire waveguide array  102 , the greater gradient in the phase of polarized light beam  108  and, hence, the greater degree of deflection of polarized light beam  108  emitted from nanowire waveguide array  102 . 
     With reference to  FIG. 7B , method  120  includes providing magnetically steered nanowire waveguide array  102  (block  122 ). Nanowire waveguide array  102  is fabricated from magneto-optic material and may be manufactured as described above with reference to  FIGS. 2A-2E . Quarter wave plate  106  is provided and configured parallel to nanowire waveguide array  102  with an appropriate space between, as illustrated in  FIG. 7A  (block  124 ). A linearly polarized optical beam source, such as a laser, is provided, producing linearly polarized light beam  104  incident on quarter wave plate  106  (block  126 ). Circularly polarized light beam  108  incident on nanowire waveguide array  102  is emitted from quarter wave plate  106  (block  128 ). Magnetic field source(s), such as counter-wound coils  112 , is provided (block  130 ). Magnetic field  110  is applied to nanowire waveguide array  102  by magnetic field source (block  132 ), increasing the phase shift or delay of polarized light beam  108  emitted by nanowire waveguide array  102 . Magnetic field  110  gradient may be varied to increase or decrease the phase shift or delay, thereby increasing or decreasing angle of deflection of polarized light beam  108  emitted by nanowire waveguide array  102 , thereby steering polarized light beam  108  in direction of magnetic field gradient (block  134 ). 
     The description above with regards to  FIGS. 1A and 1B  explains how phase delay causes the output beam to steer. The same mechanism is involved in mechanically, electrically and magnetically steered arrays. The difference between these embodiments is in how the phase delay is generated. 
     The terms and descriptions used herein are set forth by way of illustration only and are not meant as limitations. Those skilled in the art will recognize that many variations are possible within the spirit and scope of the invention as defined in the following claims, and their equivalents, in which all terms are to be understood in their broadest possible sense unless otherwise indicated.