Abstract:
An optical device includes a microdisk optical resonator element. The microdisk resonator element is formed on a substrate and has upper and lower portions respectively distal and proximal the substrate. An arcuate semiconductor contact region partially surrounds the microdisk resonator element. A first modulator electrode is centrally formed on the upper portion of the microdisk resonator element, and a second modulator electrode is formed on the arcuate contact region. A laminar semiconductor region smaller in thickness than the microdisk resonator element separates the arcuate contact region from the microdisk resonator element and is formed on the substrate so as to electrically connect the arcuate contact region to the lower portion of the microdisk resonator element.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application Ser. No. 61/622,596 entitled HIGH-SPEED PHOTONIC MODULATOR DESIGNS and filed on Apr. 11, 2012, which application is incorporated by reference in its entirety. 
    
    
     STATEMENT OF GOVERNMENT INTEREST 
     This invention was developed under Contract DE-AC04-94AL85000 between Sandia Corporation and the U.S. Department of Energy. The U.S. Government has certain rights in this invention. 
    
    
     TECHNICAL FIELD 
     The present invention is directed generally to high-speed photonic modulator designs, and, more particularly, to resonant modulators such as those in disk and ring geometries. 
     BACKGROUND OF THE INVENTION 
     One type of optical modulator that has promise for high-speed applications such as exascale computing and next generation optical communications is the resonant optical modulator. A resonant optical modulator includes an optical resonator that is typically a waveguiding ring or disk of silicon, although other geometries and other materials are not excluded. An input and output optical beam is coupled to the resonator by directing the beam through a waveguide, which may e.g. be a rectilinear planar waveguide, situated within an evanescent coupling distance of the resonator. 
     Within a characteristic wavelength band, such a modulator is relatively transmissive when light coupled into the modulator excites a resonant mode of the resonator, and is less transmissive when it goes out of resonance with the coupled light. The resonance may be controlled by, e.g., thermal or electronic modification of the optical velocity within the resonator. 
     One of the advantages of resonant modulators relative to competing technologies is that they occupy a relatively small volume, and as a consequence are conservative as to wafer real estate and as to power demands. Because the design of such modulators is a relatively new field, there remain opportunities for further improving their performance. 
     SUMMARY OF THE INVENTION 
     An exemplary embodiment of the present invention is a photonic modulator including a disk resonator and modulator electrodes. In contrast to more conventional designs in which both modulator electrodes are defined in a central portion of the device, one of the modulator electrodes is moved to the periphery of the inventive device. By this means, the series resistance between the modulator electrodes can be reduced, thereby increasing the operating radiofrequency bandwidth of the device. 
     Accordingly, the present invention in one aspect is an optical device including a microdisk optical resonator element. (By “microdisk resonator element” is meant an element that operates, at least in part but necessarily in entirety, as a microdisk optical resonator.) The microdisk resonator element is formed on a substrate and has upper and lower portions respectively distal and proximal the substrate. An arcuate, i.e. arc-shaped but not covering an entire 360 degrees, semiconductor contact region partially surrounds the microdisk resonator element. A first modulator electrode is centrally formed on the upper portion of the microdisk resonator element, and a second modulator electrode is formed on the arcuate contact region. A laminar semiconductor region smaller in thickness than the microdisk resonator element separates the arcuate contact region from the microdisk resonator element and is formed on the substrate so as to electrically connect the arcuate contact region to the lower portion of the microdisk resonator element. 
     In embodiments, the microdisk resonator element is subdivided into a portion that supports a microdisk resonant mode and a portion that supports a micro-ridge resonant mode. The resonator element further includes a taper region that adiabatically converts between said resonant modes. 
     In other embodiments, the laminar region is of substantially less thickness than a resonant wavelength of light, such that optical mode confinement by the microdisk resonator element is not substantially modified by the presence of the laminar region. Such embodiments do not necessarily include an adiabatic taper, but they offer the advantage of possibly including a heater integrated into the resonator structure. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  provides a partially schematic perspective view of a first exemplary embodiment of the present invention, which we refer to as a “top hat resonator”. 
         FIGS. 1B and 1C  provide an electric field profiles, obtained by numerical simulation, of the resonant mode within respectively an exemplary microdisk resonator and a corresponding micro-ridge resonator. These profiles are provided as a pedagogical aide for an enhanced understanding of the respective modes between which an adiabatic transition is effectuated by the top hat resonator of, e.g.,  FIG. 1A . 
         FIG. 2  provides a schematic top-down plan view of a second exemplary embodiment of the present invention, which we refer to as a “D-ring resonator. 
         FIG. 3  is a copy of  FIG. 2 , in which for ease of reference certain reference numerals have been omitted and others have been added. Such reference numerals as are common between  FIGS. 2 and 3  call out common elements. 
         FIG. 4  is a flowchart of an exemplary fabrication sequence for the device of  FIGS. 2 and 3 . 
     
    
    
     DETAILED DESCRIPTION 
     A first exemplary embodiment, which we refer to as a “top hat resonator”, is shown in  FIG. 1A . The top hat resonator includes a microdisk resonator that is adiabatically transformed into a ridge resonator. One feature of this exemplary design is that it can effectuate a significant reduction in the series resistance of the resonator, relative to older designs in which an n-type modulator contact and a p-type contact are both centrally located within the disk. In the current state of the art of microdisk modulators, series resistance ultimately limits the electrical bandwidth of the resonator. 
     As seen in the figure, an exemplary top hat resonator is implemented in silicon on an SOI substrate. The resonator structure  105  comprises portion  110 , which is formed on substrate oxide layer  100  and conformed as a portion of a conventional microdisk resonator. Exemplary design details of a silicon microdisk resonator may be gained, for example, from the description of the second exemplary embodiment herein, which is directed to a D-ring resonator. The resonator structure further comprises portion  120 , which is formed on silicon pedestal  130  and, although completing the full circular arc of the resonator structure, is conformed to function optically as a ridge resonator. It will be seen that portion  110  stands in relief above oxide layer  100  which underlies, among other things, pedestal layer  130 , and that portion  120  stands in relief above pedestal layer  130 . 
     Edge  140  of the pedestal, together with a symmetrically placed edge on the other side of the resonator structure (not visible in the drawing), is seen as tangential and continuous with the bounding part of microdisk portion  110 , and as diverging therefrom. The portions  150  of the structure near the tangent points are transition regions in which the confined and resonant optical mode is adiabatically transformed between the profile characteristic of a microdisk resonator and the profile characteristic of a micro-ridge resonator. Specific design parameters for effectuating an adiabatic transition may be found without undue experimentation by exploring the design space using readily available simulation tools. By way of example, the angle between edge  140  and its corresponding opposite edge will typically lie in the range 30-90 degrees. The thickness of the pedestal is preferably less than one-half the total thickness of the disk. 
     With further reference to the figure, it will be seen that central and peripheral doped silicon contact  160 ,  170  are respectively formed on a central portion of resonator structure  105  and as a spatially separated arcuate structure on pedestal  130 . Contact  160  exemplarily makes contact with the p-doped portion of a vertical pn junction within the resonator structure, and is p+-doped for that purpose. In that event, contact  170  makes contact with the n-doped portion of the same junction via the pedestal, and is n+-doped for that purpose. It should be noted, however, that in alternate implementations, the respective polarities may be reversed, and that a horizontal junction arrangement may be substituted for the vertical junction. 
     The resonator of  FIG. 1A  is readily used in conjunction with an optical waveguide, such as a ridge waveguide, to which it is coupled by evanescent coupling. As is well known in the art, such a coupled arrangement can be used, e.g., for purposes of optical switching, modulation, or filtering. Although such a coupled waveguide is not shown in  FIG. 1A , the conformation and placement of appropriate such elements will be readily apparent to those skilled in the art. 
       FIG. 1B  provides an electric field profile, obtained by numerical simulation, of the resonant mode within an exemplary microdisk resonator of conventional design.  FIG. 1C  provides an electric field profile, similarly obtained, of a corresponding micro-ridge resonator. As explained above, the inventive top-hat resonator effectuates an adiabatic transition between mode structures of these respective types. 
     A second exemplary embodiment, which we refer to as a “D-ring resonator”, is shown in top-down plan view in  FIG. 2 . The illustrated embodiment includes mode-guiding disk  200 , which is exemplarily a silicon disk 250-nm thick and 4.2 μm in diameter formed on an SOI substrate. The upper half of disk  200  is doped p-type and the lower half is doped n-type so that a vertical junction is defined. It should be noted, however, that alternate embodiments may instead employ a horizontal pn junction. 
     Optical resonance in disk  200  is typically excited by evanescent coupling to a guided wave in a nearby optical waveguide, not shown in the figure. Defined in disk  200  is cutout  210 , which those skilled in the art will recognize as a mode-pinning device that may be effective for limiting the resonator to single-mode operation. 
     Concentric with disk  200  and partially surrounding it is arcuate n+-doped silicon spacer  220 , which is exemplarily 50 nm thick and 1 μm wide. Spacer  220  establishes electrical continuity between the n-doped portion of disk  200  and n+-doped silicon contact  230 , which is a further arcuate body concentric with and partially surrounding disk  200 . Spacer  220  is intended to effectuate the electrical contact while exerting minimal influence on the confinement of the resonant electromagnetic field by disk  220 . Some variability in the design of spacer  220  is allowable, as the same purposes may be satisfied using, e.g., widths greater than 1 μm. 
     The use of spacer  220  is advantageous because it permits the n-type contact, i.e. contact  230 , to be situated peripherally. Although peripheral contacts are known in the context of ring-type optical resonators, we are unaware of any previous use thereof for disk-type resonators. Because this arrangement enables the device to be contacted from the outside, bandwidth performance typical of ring designs is achievable, even reaching bandwidths as great as 20 GHz or more. 
     Because contact  230  is substantially optically isolated from disk  200 , its height is not critical and may therefore be determined by factors such as ease of fabrication and ease of making electrical contact rather than by optical constraints. 
     The angular length of spacer  220  and contact  230  may vary over a substantial range. There are scaling effects that relate the amount of arc to the doping density of disk  200  needed to achieve a given shift λ shift  in the optical resonant wavelength under bias. The relevant design space can of course be explored through numerical modeling using available modeling tools. More generally, we note that the full width at half-maximum (FWHM) λ FWHM  of the resonant peak is closely determined by the amount of arc in the disk. 
     Moreover, the shift in the resonant wavelength varies with the FWHM in proportion to the cube root of the doping density N d : 
                 Δλ   shift       Δλ   FWHM       ∝       1       α   dop     +     α   other         ·       N   d     1   /   3       .             
In the preceding equation, α dop  represents the optical loss due to dopants, and α other  represents the optical loss due to other factors. From the equation it can be seen that reducing the FWHM (by reducing the arc) will result in less dopants being needed to get a similar amount of resonance shift, Δλ shift . As the FWHM and dopant levels change, the disk may have to be moved in relation to the bus waveguide in order to optimize the coupling.
 
     With further reference to the figure, it will be seen that p-type contact  240  is defined as a concentric region situated in the interior of disk  200 . The top of contact  240  is at the same level as the top of disk  200 . Contact  240  may be formed as a shallow surface region, or it may extend partway or even entirely down to the SOI substrate. It is significant that the conformation of contact  240  can result in less electrical resistance than a comparable ring design, because the corresponding contact in a typical ring design is conformed as a lowered pedestal, which will typically exhibit higher series resistance. 
     For ease of reference, the drawing of  FIG. 2  has been reproduced in  FIG. 3 , where some elements also called out in  FIG. 2  are called out by like reference numerals, while other elements are newly called out by new reference numerals. In some embodiments, it will be advantageous to include a heater for adjusting the optical properties of the device. In fact, a heater is readily integrated into the center portion of disk  200 . This is best seen in  FIG. 3 , where annular p-doped region  250  constitutes the relatively resistive region to be used for ohmic heating. Heater region  250  will typically occupy a shallow surface layer, but this is not critical, and implementations are feasible in which region  250  extends to a substantial depth. 
     Advantageously, the p-type modulation contact  240  can also serve as the ground contact for the heater. Accordingly, as seen in the figure, the circular array of metal, e.g. tungsten, contacts  260  are provided to make the ground connection for the pn junction and for the heater. Metal, e.g. tungsten, contact  270  provides the other external connection for the heater. The circular array of metal, e.g. tungsten, contacts  280  provides the external connection for n-type contact  230 . 
     As will be understood, our exemplary implementation has a central p-type contact and a peripheral n-type contact. It should be further understood that such a choice of polarities is merely illustrative and although it may be typical, it is not limiting, because interchanging the polarities can also result in an operative device. 
     It should also be understood that the dimensions provided for our exemplary implementation are merely illustrative and are subject to variation depending on design choices such as material compositions, fabrication techniques, and operating wavelengths. In particular it should be noted that although our example is implemented in silicon, other semiconductor materials can be used in this context, including III-V materials and II-VI materials. 
     In an exemplary fabrication sequence for the device of  FIGS. 2 and 3  as summarized in  FIG. 4 , a 250-nm thick layer of silicon is deposited  401  on 3 μm of buried oxide on an SOI substrate. The silicon layer is lithographically patterned and etched  402  to define the features that will become disk  200  and contact  230 . A further lithographic patterning and etching step  403  then defines the feature that will become spacer  220 . The dopants are then implanted  404  to define the electronic characteristics of each of the respective features of the device. Then oxide is deposited  405  and tungsten contacts are formed  406  according to conventional practices.