Abstract:
A method includes: providing an element having mutually exclusive first and second portions with an initial index of refraction; and applying energy to the first portion in a manner causing the index of refraction thereof to change by at least 0.05 in relation to the index of refraction of the second portion. According to one specific approach, the applied energy is laser energy.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates in general to techniques for forming optical structures and, more particularly, to techniques for modifying the index of refraction of one portion of a material relative to another portion thereof. 
       BACKGROUND 
       [0002]    Optical systems often use a waveguide to carry optical energy from one location to another. One technique for making a waveguide is to take a piece of glass or crystalline silicon, and focus ultra-fast laser pulses in an interior region. The laser energy induces a small change in the density of the material in the interior region, thereby changing the index of refraction of the interior region relative to the index of refraction of the remainder of the material. Although this pre-existing technique is interesting from an academic perspective, it has not been fully satisfactory in terms of practical application. For example, the maximum change to the index of refraction of glass or crystalline silicon is typically limited to about 0.01, or even less. A change this small limits the practicality of this pre-existing technique. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]    A better understanding of the present invention will be realized from the detailed description that follows, taken in conjunction with the accompanying drawing, in which: 
           [0004]      FIG. 1  is a diagrammatic fragmentary perspective view of an apparatus that embodies aspects of the invention. 
           [0005]      FIG. 2  is a graph showing the indexes of refraction exhibited by each of crystalline silicon and amorphous silicon within a selected range of wavelengths. 
           [0006]      FIG. 3  is a diagrammatic fragmentary perspective view of an apparatus that is an alternative embodiment of the apparatus of  FIG. 1 , and that embodies aspects of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0007]      FIG. 1  is a diagrammatic fragmentary perspective view of an apparatus  10  that embodies aspects of the invention. The apparatus  10  includes a substrate  16  with an upper surface, and a coating or layer  17  of amorphous silicon provided on the upper surface of the substrate. The substrate  16  can be made of any material that is amenable to being coated with the amorphous silicon layer  17 . 
         [0008]    Infrared optical systems often use infrared radiation with a wavelength in the range of approximately 3 to 5 microns. Where amorphous silicon is to be used for infrared radiation in this range, the amorphous silicon can be produced with any desired index of refraction within a relatively wide range, from approximately 3.4 to approximately 3.8. Various techniques for depositing a layer of amorphous silicon are well known in the art. For example, several such techniques are disclosed in Palik,  Handbook of Optical Constants of Solids , Academic Press, San Diego, Calif., 1998, pages 571-572. In  FIG. 1 , the amorphous silicon layer  17  is formed using a known technique, with an index of refraction that is about 3.8 and that is relatively uniform throughout the entire layer  17 . 
         [0009]    A laser  26  of a known type is provided. In the disclosed embodiment, the laser  26  is a commercially-available, chirped, pulse-amplified Ti:sapphire laser, shifted to a wavelength of 2400 nm. The laser produces a beam  27  in the form of a series of pulses, each pulse being about 70 fs long, and having an energy of about 600 μJ. A positioning mechanism  28  is provided, and can effect relative movement of the laser  26  in three dimensions with respect to the substrate  16  and the layer  17 . The beam  27  of the laser  26  is focused at a point or region  37  located within the amorphous silicon layer  17 , between the upper and lower surfaces of the layer  17 . The 2400 nm wavelength of the laser  26  was selected because it can pass into the bulk of the silicon without being highly absorbed. In contrast, some other wavelengths of laser radiation would be highly absorbed by the silicon material. 
         [0010]      FIG. 2  is a graph showing the indexes of refraction exhibited by each of crystalline silicon and amorphous silicon within a selected range of wavelengths. It will be noted that, for wavelengths in the range of approximately 2500 nm to 6000 nm, crystalline silicon exhibits a relatively constant index of refraction of approximately 3.4, and amorphous silicon exhibits a relatively constant index of refraction of approximately 3.8. In  FIG. 1 , the laser energy focused at the point or region  37  causes a change in the physical structure of the amorphous silicon there. In particular, silicon atoms have a tendency to favor a crystalline structure, and the energy from the laser will tend to cause silicon material at the point or region  37  to shift from a purely amorphous state toward a crystalline state. This may produce a partially crystalline structure. For example, the laser radiation may cause many different portions of the silicon material to each have a crystalline structure, and to have random orientations with respect to each other. As silicon material at the point or region  37  shifts from an amorphous state toward a greater degree of crystallinity, the index of refraction changes relative to the rest of the amorphous silicon layer  17 . The index of refraction can change by as much 0.4 or more. 
         [0011]    While the laser  26  is producing the beam  27 , the positioning mechanism  28  effects movement of the substrate  16  and the layer  17  relative to the laser  26 , for example in a manner so that the point or region  37  where the laser beam is focused passes through all points within an elongate cylindrical portion  41  of the layer  17 . This portion  41  is spaced below the top surface of the layer  17 , and is spaced above the bottom surface of the layer  17 . When this relative movement is completed and the laser  26  is turned off, the portion  41  of the layer  17  will have an index of refraction that is somewhat below the index of refraction of the remaining portion  42  of the layer  17 . For example, the portion  41  may have an index of refraction of approximately 3.4, and the portion  42  may have an index of refraction of approximately 3.8. This difference in indexes of refraction permits the portion  41  to effectively function as the core of a waveguide, and permits the portion  42  to effectively function as the cladding of the waveguide. Thus, a conventional radiation source shown diagrammatically at  51  can supply a beam  52  of infrared radiation to one end of the portion  51 , and this radiation can then propagate through the portion  41  to a conventional radiation detector shown diagrammatically at  54 . 
         [0012]    As explained above, the portions  41  and  42  of amorphous silicon layer  17  can each have the same initial index of refraction of about 3.8, and then laser energy can be applied to the portion  41  so as to reduce its index of refraction to about 3.4. Thus, the change in the index of refraction in the portion  41  is about 0.4, and this is about 40 times larger than the largest change in index of refraction achieved with pre-existing techniques. This significant improvement is due in part to the use of a different material in which the application of laser energy produces a change in crystalline structure, as well as a relatively large change in density. 
         [0013]      FIG. 3  is a diagrammatic fragmentary perspective view of an apparatus  110  that is an alternative embodiment of the apparatus  10  of  FIG. 1 , and that embodies aspects of the invention. The apparatus  110  of  FIG. 3  is generally equivalent to the apparatus  10  of  FIG. 1 , except for differences discussed below. In  FIG. 3 , the amorphous silicon layer  17  contains an optical circuit that is indicated diagrammatically at  112 . The optical circuit  112  can include various optical components, such as a radiation source  113 , and a radiation detector  114 . In a manner similar to that discussed above in association with  FIG. 1 , the laser  26  in  FIG. 3  can be used to change the index of refraction of a portion  41  of the layer  17  that extends between the source  113  and the detector  114 . The portion  41  can then serve as a waveguide. The laser  26  could also potentially be used to form portions of optical components, such as the source  113  or the detector  114 . 
         [0014]    In the disclosed embodiments, the layer  17  is made from amorphous silicon. However, the layer  17  could alternatively be made from any other suitable material that exhibits a significant change in its index of refraction in response to the application of energy such as laser radiation. For example, the layer  17  could be made from semi-amorphous silicon, aluminum oxide (Al 2 O 3 ), yttrium oxide (Y 2 O 3 ), titanium oxide (TiO 2 ), or indium-tin oxide (ITO). (ITO is a mixture of indium oxide (In 2 O 3 ) and tin oxide (SnO 2 ), typically about 90% indium oxide and 10% tin oxide by weight). As still another alternative, the layer  17  could be made from a material that is made by Merck KGaA and that is commercially available as Substance H4, for example through EMD Chemicals Inc. of Gibbstown, N.J. Substance H4 is believed to include a mixture of titanium oxide (TiO 2 ) and lanthanum oxide (La 2 O 3 ). 
         [0015]    In the disclosed embodiments, laser energy is used to modify the index of refraction of an interior portion of a material relative to an outer portion thereof. Alternatively, however, it would be possible to modify the index of refraction of the outer portion relative to that of the interior portion. 
         [0016]    A further consideration is that, in the embodiments discussed above, laser energy is used to modify a portion of a material by decreasing the index of refraction of that portion. Alternatively, however, it would be possible to use laser energy to increase the index of refraction of a portion of a material, for example by applying laser energy in a manner that tends to decrease rather than increase the degree of crystallinity. 
         [0017]    Still another consideration is that, in the embodiments discussed above, laser energy is used to change the index of refraction of a portion of a material. Alternatively, however, it would be possible to utilize any other suitable technique that can modify the index of refraction of one portion of a material relative to another portion thereof. 
         [0018]    Although selected embodiments have been illustrated and described in detail, it should be understood that a variety of substitutions and alterations are possible without departing from the spirit and scope of the present invention, as defined by the claims that follow.