Abstract:
In one aspect, a photonic device includes a substrate layer comprising magnesium fluoride and an optical guiding layer disposed on the substrate layer. The optical guide layer includes silicon dioxide. The substrate layer and the optical guide layer are transparent at an ultraviolet and visible wavelength range. In another aspect, a method includes oxidizing silicon to form a silicon dioxide layer, bonding the silicon dioxide layer to magnesium fluoride, removing the silicon and performing lithography and etching of the silicon dioxide to form a photonic device.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority to Provisional Application Ser. No. 62/338,650 filed on May 19, 2016 and entitled “INTEGRATED PHOTONIC MATERIAL PLATFORM FOR THE UV VISIBLE WAVELENGTH,” which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    Integrated photonics has the advantage of miniaturization and large scale manufacturing of photonic functionalities for variety of applications. A range of applications are in the ultraviolet and visible (UV-vis) wavelength range. Examples of applications include integrated spectrometers, Raman spectroscopy, chemical/biological sensing, strong and nonlinear light-matter interaction at short wavelengths. Many current optical sensing platforms in the UV-vis range exploit table-top and bulky optical devices. As a result, such sensing platforms are not handheld and are mostly used in the labs. Many of the mature and existing integrated photonic platforms operate at infrared or near-infrared for application mostly in data interconnect and communications. Examples include Silicon Photonics and Indium Phosphide Photonics that are used for applications at 1550 nm, but cannot operate at UV-vis wavelengths. There are integrated photonic materials such as silicon nitride that can operate in the visible range; however, when going to shorter wavelength and in the UV range, these integrated photonic materials suffer from strong optical absorption. 
       SUMMARY 
       [0003]    Described herein is a new photonic material platform where the optical guiding layer is made of silicon dioxide and the underneath substrate layer is magnesium fluoride. Both materials have extremely high optical qualities over the entire UV and visible range. The refractive index difference between silicon dioxide and magnesium fluoride is large enough to provide optical waveguiding condition in the UV-vis range, and yet small enough (˜0.08-0.1) to avoid extra-small waveguide dimensions at short wavelengths. Single-mode waveguides with sub-micron or micron scale dimensions can be designed and these dimensions are well within the capabilities of lithography and microfabrication technology. Silicon dioxide is a very mature material in microelectronics and photonics and many of existing technologies can be borrowed to implement such silicon dioxide-on-magnesium fluoride photonic devices. 
         [0004]    Also, described herein are techniques to fabricate silicon dioxide-on-magnesium fluoride wafers which are used to make photonic devices on this platform. 
         [0005]    In one aspect, a photonic device includes a substrate layer comprising magnesium fluoride and an optical guiding layer disposed on the substrate layer. The optical guide layer includes silicon dioxide. The substrate layer and the optical guide layer are transparent at an ultraviolet and visible wavelength range. 
         [0006]    In another aspect, a method includes oxidizing silicon to form a silicon dioxide layer, bonding the silicon dioxide layer to magnesium fluoride, removing the silicon and performing lithography and etching of the silicon dioxide to form a photonic device. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a cross-sectional diagram of an example of a photonic waveguiding platform. 
           [0008]      FIG. 2A  is a diagram of an example of a photonic device that includes a ring resonator and a waveguide. 
           [0009]      FIG. 2B  is a diagram of an example of a simulation of the cross-sectional optical mode profile (electric field distribution) for a silicon dioxide-on-magnesium fluoride waveguide. 
           [0010]      FIG. 2C  is a diagram of an example of a cross-sectional optical mode profile for a silicon dioxide-on-magnesium fluoride ring resonator. 
           [0011]      FIGS. 3A to 3E  are cross-sectional diagrams to fabricate a silicon dioxide-on-magnesium fluoride photonic device. 
           [0012]      FIG. 4  is a flow diagram of an example of a process to fabricate a silicon dioxide-on-magnesium fluoride photonic device. 
           [0013]      FIG. 5  is a cross-sectional diagram of another particular example of the photonic platform of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    Described herein are methods to implement an integrated photonic material platform and devices functional in the ultraviolet (UV) and visible wavelength range (e.g., wavelengths as short as 200 nm to wavelengths as long as 800 nm). In one example, a photonic device may include at least one of a waveguide or a resonator. In other examples, the photonic device may include at least one of a directional coupler, a beam splitter, a Mach-Zehnder interferometer, a grating device, and so forth. 
         [0015]    Referring to  FIG. 1 , a photonic platform  100  operating in the UV-visible wavelength range includes an optical guiding layer  106  and a substrate layer  110 . The optical guiding layer material  106  is made of silicon dioxide that has a refractive index larger than the underneath substrate  110  which is crystalline magnesium fluoride. The difference between the refractive index of the optical guiding layer  106  and the refractive index of the substrate layer  110  is within ˜0.08-0.1. The optical guiding layer  106  and the substrate layer  110  are transparent with negligible or small optical absorption at the UV-visible wavelength range (e.g. less than &lt;0.1 dB/m at a wavelength of 350 nm). The optical guiding layer  106  and the substrate layer  110  are compatible with respect to each other to allow fabrication. In one example, overcladding material (e.g., surrounding at least a portion of the platform  100 ) can be air or a material like water that has a refractive index less than that of silicon dioxide and is transparent in UV and visible with very small optical absorption. 
         [0016]    Referring to  FIG. 2A , an example of a photonic device is a photonic device  112  using the photonic platform  100 . The photonic device  112  includes a photonic ring resonator  106 ′ (i.e., an optical guiding layer  106 ) that includes silicon dioxide, on the substrate layer  110  that includes magnesium fluoride. The photonic device  112  also includes a silicon dioxide waveguide  120  on the substrate layer  110 . In this configuration, the ring resonator  106 ′ is side-coupled to the waveguide  120  in order to excite the ring resonator  106 ′. In one particular example, as shown in  FIG. 2B , the silicon dioxide waveguide  120  has a cross sectional dimension of 800 nm×350 nm at a wavelength of 350 nm. In one particular example, as shown in  FIG. 2C  the ring resonator  106 ′ has a radius of 35 microns and a resonance at about a 350-nm wavelength. 
         [0017]    Referring to  FIG. 3A to 3E , a photonic device (e.g., a photonic device  300  ( FIG. 3E )) may be fabricated to include silicon dioxide and magnesium fluoride. Silicon  202  (e.g., in wafer form) is oxidized to form the silicon dioxide  206  ( FIG. 3A ). Magnesium fluoride  210  (e.g., in wafer form) is bonded to the silicon dioxide  206  ( FIG. 3B ). In one particular example, the magnesium fluoride  210  may also include a thin layer of deposited silicon dioxide that is then bonded to the silicon dioxide  206  on the silicon  202 . The silicon  202  is removed ( FIG. 3C ) and the silicon dioxide-on-magnesium fluoride wafer is formed. Using lithography and etching techniques, for example, which are conventional in microfabrication technology, the silicon dioxide layer  206  is patterned and etched ( FIG. 3D ) to form the photonics devices in this platform. In one example, lithography and etching may be used to form at least one of a waveguide, a ring resonator, a disk resonator, a directional coupler, a Mach-Zehnder interferometer, a multiplexor, a demultiplexor, an array waveguide grating device, a beam splitter or a grating and periodic device. 
         [0018]    Polydimethylsiloxane (PDMS) material  302  is added on portions of the magnesium fluoride  210  to form a fluidic channel  330  ( FIG. 3E ) that carries fluid such as water or air, for example. In one example, the photonic device  300  may be used in aqueous environments for chemical or biological sensing applications and water monitoring. In other examples, the photonic device  300  can be used to enhance the Raman sensing of chemical/biological material in aqueous environments. In another example, more complicated photonic devices such as spectrometers or optical spectrum analyzer operating at the UV or visible wavelength can be implemented on this platform with a very compact and chip-scale size. 
         [0019]    In one example, a metal microheater may be integrated with the photonic device to tune the optical properties using a thermo-optic effect. 
         [0020]    Referring to  FIG. 4 , an example of a process to form a photonic device on silicon dioxide-on-magnesium fluoride platform is a process  400 . Process  400  oxidizes silicon to form silicon dioxide ( 402 ). In one example, the silicon dioxide  206  is formed using thermal oxidation on silicon  202  (e.g., silicon wafer) (see, for example,  FIG. 3A ). 
         [0021]    Process  400  bonds the silicon dioxide to a magnesium fluoride. In one example, silicon dioxide  206  is bonded to magnesium fluoride  210  (see, for example,  FIG. 3B ). In one particular example, the magnesium fluoride  210  includes a layer of deposited silicon dioxide (e.g., 10-50 nm) (not shown) that was deposited using atomic layer deposition or plasma enhanced chemical vapor deposition, for example and the silicon dioxide  206  is bonded to the silicon dioxide on the magnesium fluoride. 
         [0022]    Process  400  removes the silicon ( 412 ). For example, the silicon may be removed using plasma etching or wet etching using KOH chemical, or a combination of plasma and wet etching. 
         [0023]    Process  400  performs lithography and etch ( 418 ). In one example, the lithography and etching process shapes the silicon dioxide to form a ring resonator. 
         [0024]    Process  400  forms a fluidic channel. In one example, the PDMS material  302  is deposited on at least a portion of the magnesium fluoride  210  and over the silicon dioxide to form the fluidic channel  330  (see, for example,  FIG. 3E ). 
         [0025]    Referring to  FIG. 5 , another particular example of a photonic platform  100  is a photonic platform  500 . The photonic platform  500  includes aluminum gallium nitride (AlGaN)  506  as a light guiding layer and aluminum nitride (AlN)  510  as the adjacent material. In one example, the operational wavelength of the photonic platform  500  is greater than 260 nm. 
         [0026]    The processes described herein are not limited to the specific examples described. For example, the process  400  is not limited to the specific processing order of  FIG. 4 . Rather, any of the processing blocks of  FIG. 4  may be re-ordered, combined or removed, performed in parallel or in serial, as necessary, to achieve the results set forth above. 
         [0027]    The processes described herein are not limited to the specific embodiments described. Elements of different embodiments described herein may be combined to form other embodiments not specifically set forth above. Other embodiments not specifically described herein are also within the scope of the following claims.