Abstract:
An omnidirectional optical reflector structure (41, 52, 61) is made by forming a plurality of holes in a solid body (11) so as to result in a face-centered cubic lattice. Conventiently, the desired structure can be made by three successive steps of microfabrication, e.g., in the presence of a mask layer (12). Preferred reflector structures can be used, e.g., as external mirrors as filters, and as cavity materials of communications lasers with improved signal-to-noise ratio.

Description:
TECHNICAL FIELD 
     The invention is concerned with devices including a structure for reflecting electromagnetic radiation. 
     BACKGROUND OF THE INVENTION 
     Optical reflector structures, e.g., mirrors, gratings, and layered structures are of obvious importance in the field of optics in general, and such structures are being developed further for optical communications and information processing applications. In particular, for example, reflector structures are used in lasers, e.g., fiber-optics and integrated-optics communications lasers. Preferably, such lasers are designed for maximized stimulated emission and minimized spontaneous emission, and reflector structures are sought which contribute to these goals. 
     It has been recognized that spontaneous emission is diminished in omnidirectional reflector structures which form a three-dimensionally periodic, face-centered cubic lattice. As disclosed by E. Yablonovitch, &#34;Inhibited Spontaneous Emission in Solid-State Physics and Electronics&#34;, Physical Review Letters 58 (1987), pp. 2059-2062, such a structure can be made by successive layer deposition on a corrugated surface having a checkerboard pattern, the resulting structure being composed of cube-shaped elements or &#34;atoms&#34;. Alternatively, as disclosed by E. Yablonovitch et al., &#34;Photonic Band Structure: The Face-Centered-Cubic Case&#34;, Physical Review Letters 63 (1989), pp. 1950-1953, such a structure may be composed of a face-centered-cubic lattice of spheres. 
     While, conceptually, face-centered cubic lattices of cubes or spheres may appear as the simplest, they have been found difficult to implement, and their performance has not met expectations. The invention described below is motivated by the desire for a face-centered-cubic structure which is more readily manufacturable and which is highly effective as an omnidirectional reflector. 
     SUMMARY OF THE INVENTION 
     For electromagnetic radiation, e.g., visible, ultraviolet, or infrared light, a reflector structure is made by forming a plurality of holes, passages, or channels in a solid body. Preferred passages intersect by threes and impart face-centered cubic structure to the solid body. 
     Preferred reflector structures can be used as device elements, e.g., as external mirrors or to provide feedback in an optical cavity. When such a structure is used to make an optical communications laser, a transmitter with improved signal-to-noise ratio can be realized. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic perspective representation of a material body processed in accordance with a preferred method in accordance with the invention; 
     FIG. 2 is a perspective representation of a real-space unit cell (Wigner-Seitz primitive cell) of a preferred structure in accordance with the invention; 
     FIG. 3 is a perspective geometric representation of face-centered cubic lattice sites; 
     FIG. 4 is a schematic cross section of a preferred device embodiment of the invention; 
     FIG. 5 is a schematic cross section of a preferred further device embodiment of the invention; and 
     FIG. 6 is a schematic perspective representation of a laser structure, representing a preferred further device embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows body 11 and, on a nominally planar surface of body 11, a patterned mask layer 12 which leaves uncovered a regularly triangular array of surface sites 13. For example, as shown, such sites may be circular. The material of mask layer 12 may be chosen to protect underlying surface portions of body 11 during microfabrication, e.g., by reactive-ion etching. Shown associated with one site 13 are lines 14, 15, and 16, each forming the same preferred polar angle with a line 17 perpendicular to mask layer 12. Nominally, such polar angle is the trigonometric inverse-tangent of 2 -1/2  ; thus, such angle is approximately equal to 35.26 degrees. Furthermore, azimuthal angles between lines 14 and 15, 15 and 16, and 16 and 14 are 120 degrees nominally. Also identified in FIG. 1 are sites 31-36 as further described below with reference to FIG. 3. 
     Preferred processing in accordance with an aspect of the invention involves forming, at each site 13, three intersecting holes, channels, or passages essentially straight into or through body 11, such three holes being formed in the directions defined by lines 14, 15, and 16. On a sufficiently large scale, holes can be formed by mechanical drilling or machining; more typically, in microfabrication, lithographic or direct-writing methods may be used, e.g., reactive ion etching or ion milling. In such fabrication, nominal preferred directions may be realized to within a fraction of a degree. 
     In accordance with an aspect of the invention, a structure made as described can be identified as a face-centered-cubic lattice, consisting of a close-packed plurality of unit cells or &#34;atoms&#34; as depicted in FIG. 2. As shown, a unit cell takes the form of a regular rhombic dodecahedron having pair-wise adjoining facets 21, 22, and 23 with holes 24, 25, and 26 perpendicular to respective facets 21, 22, and 23. Holes 24, 25, and 26 intersect at the center of the unit cell, and they exit perpendicularly through three unseen bottom facets of the unit cell. 
     For the sake of relating a preferred structure to an alternative representation, FIG. 3 shows selected lattice sites 30-39 of a face-centered cubic structure. Sites 31-36 are readily seen to lie in a plane and to form a regular triangular array as shown also in FIG. 1; sites 30, 38, and 39 lie in a parallel plane below. Unit cells in accordance with FIG. 2 are centered on such sites. 
     When suitably dimensioned, a preferred structure has a &#34;forbidden band&#34; of wavelengths at which it can serve as an omnidirectional reflector; also, within the structure, electromagnetic radiation at such wavelengths is inhibited. For example, for a material having a refractive index of 3.6, a triangular pattern with hole sites spaced apart 0.3062&#34; (0.7777 centimeter), and a hole diameter of 13/64&#34; (0.5159 centimeter), the forbidden frequency band extends from 13 to 16 gigahertz approximately, corresponding to wavelengths from 1.9 to 2.3 centimeters approximately. 
     Among parameters available for influencing the optical properties of a preferred structure are the refractive index of the material of body 11, the shape, spacing, and size of the holes, and the refractive index of the medium occupying the holes. For example, for maximized fractional width of the forbidden band, optimal hole size depends on the refractive index of the material of body 11. 
     The material of body 11 may be homogeneous or inhomogeneous; e.g., as illustrated by the laser device of FIG. 6 below, body 11 may include an &#34;island layer&#34; whose composition differs from the surrounding material. Also, body 11 may be compositionally layered or graded. Preferred materials are chosen to be essentially transparent or nonabsorbing for a wavelength in the forbidden band; suitable materials include semiconductor materials, e.g., silicon, gallium arsenide, and indium phosphide, and dielectric materials, e.g., diamond and titanium dioxide. In the interest of a large refractive-index difference relative to the structural material, holes may be left unfilled; alternatively, the average refractive index of the structure can be influenced, e.g., by filling the holes with a suitable resin. 
     With circular shape of sites 13, etched holes will have elliptical cross section. For round holes, suitably shaped elliptical sites can be used, separate masks being required for processing in each of the three directions. Holes may also have straight-edged shapes, e.g., hexagonal or six-sided star shape. Typical hole size is such as to result in removal of at least 10 percent of the volume of an initial body. On the other hand, hole size is limited by considerations of structural integrity of the processed structure, so that, preferably, volume removed constitutes not more than approximately 90 percent of initial volume. 
     FIG. 4 shows a preferred structure 41 on support body 42, e.g., for use as an omnidirectional external mirror. Support body 42 may serve as mounting and positioning means. 
     FIG. 5 shows a preferred structure 52 partially supported by support body 51 which may have been shaped by etching before or after formation of structure 52 as described above with reference to FIG. 1. The resulting assembly can be used as an optical filter: incident radiation 53 is transmitted (54) except for radiation 55 having wavelengths in the forbidden band of structure 52. 
     FIG. 6 shows optical gain medium 62 embedded in a preferred reflector structure 61--inclusion of gain medium 62 resulting in an allowed mode of electromagnetic radiation within an otherwise forbidden band of reflector structure 61. When suitably pumped, this assembly can operate as a laser and emit radiation 63 into optical communications fiber 64. Radiation 63 may be directly (electrically) modulated as in an optical communications transmitter; in accordance with an aspect of the invention, a resulting transmitter has an improved signal-to-noise ratio.