Xiaomin Zhang

Ultra-high Quality factor microcavities

 

Silica optical microcavities with quality (Q) factors above 100 million have applications throughout science and engineering. is directly related to type of silicon substrate used in the fabrication process. Dopants diffuse from the silicon into the oxide, at a rate which is dependent on the growth conditions.1These dopants can modify the refractive index, which in turn will affect the Q of the cavity.

  • toroid

    • Figure 1. left: A scanning electron micrograph of the fabricated silica microtoroid resonator. Middle: A fine scan (the forward scan direction) of the fundamental transverse mode of the microtoroid fabricated from the film with boron concentration of 1.63×1014cm-3 in silicon at 848.8nm with a dual-Lorentz fit (red). The resonance shows splitting and the quality factor of the left and right peak is 3.39×108 and 2.67×108 separately. Inset: Optical micrograph of a microtoroid coupled to tapered optical fiber during testing. Right: Measured quality factor as a function of refractive index or boron dopant concentration at three wavelengths: 630, 850 and 980nm. The Q factor decreases as the refractive index decreases and the wavelength increases.

    FEM Simulations

     
  • toroid

    • Figure 2. from left to right: the fundamental TE mode of a silica 2um thick microdisk with a diameter of 50 um at 980nm; a silica microtoroid with major(minor) dimater of 40(8) um at 850nm; a silica microsphere with diameter of 100um at 980nm.

    Hybrid devices

     

    Ultra-high-quality (UHQ) factor optical cavities have numerous applications throughout engineering and science. Incorporating active elements into these UHQ cavities to create dynamic devices would extend their applicability; however, it is inherently difficult to develop an active UHQ device. Ultra-thin films formed from optically active polymers provide one route to overcome this limitation. In the present work, hybrid devices composed of UHQ planar optical cavities with ultra-thin films are fabricated on a silicon wafer. Using finite element method simulations, the optical field overlap between the cavity and the polymer film is modeled and experimentally verified using two polymers: poly(methyl methacrylate) and polystyrene. These hybrid devices have demonstrated material-limited Q factors above 107.
  • toroid

    • Figure 3. Left: (a) Scanning electron micrograph of a toroidal microresonator.FEM simulation results for the optical field intensity distribution: (b) silica microtoroid, (c) hybrid microtoroidwith a 100 nm thick PS film, and (d) 200 nm thick PS film. Note that the optical field shifts from the silica toward the polymer film as the thickness of the polymer film increases. The major (minor) diameter is 40(8) um at 850 nm. Right: (Color online) Experimental and theoretical quality factor Q as a function of polymer thickness. PMMA at (a) 850 and (b) 980 nm. PS at (c) 850 and (d) at 980 nm. The results were fit to an equation of the form y=axb, which is included as a solid (dashed) curve for the theoretical (experimental) Q results. The black dotted line indicates the highest Q demonstrated with a silica toroidal resonant cavity to date, setting an upper bound on Q.

     
     
    The University of Southern California does not screen or control the content on this website and thus does not guarantee the accuracy, integrity, or quality of such content. All content on this website is provided by and is the sole responsibility of the person from which such content originated, and such content does not necessarily reflect the opinions of the University administration or the Board of Trustees