Research ArticlePHYSICS

Experimental realization of deep-subwavelength confinement in dielectric optical resonators

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Science Advances  24 Aug 2018:
Vol. 4, no. 8, eaat2355
DOI: 10.1126/sciadv.aat2355
  • Fig. 1 Comparison of light concentration in different photonic crystal unit cells.

    (A and B) Traditional circular unit cell of a photonic crystal and its electric energy profile at the dielectric mode band edge. (C and D) Slotted photonic crystal unit cell and its electric energy profile at the dielectric band edge. (E and F) Bowtie photonic crystal unit cell and its electric energy profile at the air band edge. The tip of the v-groove is modeled to extend down to the middle of the silicon slab. (G) 3D profile of the mode in the bowtie unit cell showing the electric energy distribution. All profiles are taken at the middle of the silicon slab. All color maps are scaled according to the minimum and maximum electrical energy values of each individual unit cell. The maximum electric field amplitude in each unit cell scales as follows: traditional circular unit cell = 1 (normalized), slotted unit cell = 10, bowtie unit cell = 80.

  • Fig. 2 Design of silicon photonic crystal using a bowtie-shaped unit cell.

    (A) The cavity is formed with a center unit cell of 150 nm radius and mirror unit cells of 187 nm radii on both sides of the cavity. The radius is gradually tapered from the center to the mirror segments. The photonic crystal lattice spacing is a = 450 nm, and the width of the waveguide is 700 nm. The structure is designed with a 220-nm silicon device layer and a 2-μm-thick buried oxide layer. (B) Optical band structures of the cavity unit cell (red curve) and mirror unit cell (blue curve). (C) Top view (xy plane) and (D) cross-sectional view (yz plane) schematics and associated air band edge electrical energy in the center unit cell. (E) Log plot of the photonic crystal cavity electric energy distribution at the resonance wavelength in the xy plane at z = 0 (v-groove tip). (F) Log plot of the photonic crystal cavity electric energy distribution at the resonance wavelength in the xz plane at y = 0 (bowtie tip). Figure S2 (A and B) shows the same mode profiles using a linear scale.

  • Fig. 3 Transmission of fabricated bowtie photonic crystal.

    (A) SEM image of the bowtie photonic crystal. (B) Zoomed-in image of a single unit cell in the red box in (A). (C) Tilted SEM image of an undercut bowtie photonic crystal revealing the out-of-plane profile. (D) Measured transmission spectrum. The fundamental mode has Q ~ 100,000 at λ = 1578.85 nm. The second-order and third-order peaks are located at 1562.20 and 1546.96 nm, with Q factors of 21,800 and 5156, respectively. a.u., arbitrary units.

  • Fig. 4 Analysis of spatial confinement via NSOM measurements.

    (A) Schematic of bowtie photonic crystal cavity with overlay of simulated electric energy 15 nm above the silicon surface, where the NSOM measures the scattered field. (B) AFM measurement and (C) corresponding electric energy distribution, as measured by NSOM. The inset in (B) shows a higher-resolution SEM image of one of the bowtie unit cells from the measured cavity. (D and E) Simulated and NSOM-measured near-field profile along the y direction and x direction, respectively, along with superimposed AFM line scan.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/8/eaat2355/DC1

    Fig. S1. Design of photonic crystal cavity in an FDTD simulation.

    Fig. S2. Resonance mode profile.

    Fig. S3. Broadband transmission spectrum of bowtie photonic crystal cavity shown in Fig. 3D.

    Fig. S4. SEM image and transmission of the bowtie photonic crystal cavity characterized by NSOM.

    Fig. S5. Position-dependent electric energy distribution in the central cavity unit cell of the silicon bowtie photonic crystal.

    Table S1. Calculated mode volume (Vm) and measured quality factor (Q) of different photonic crystal (PhC) cavities including the bowtie photonic crystal cavity presented in this work.

    Table S2. NSOM-measured mode sizes of plasmonic structures in comparison to dielectric bowtie photonic crystal reported in this work.

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. Design of photonic crystal cavity in an FDTD simulation.
    • Fig. S2. Resonance mode profile.
    • Fig. S3. Broadband transmission spectrum of bowtie photonic crystal cavity shown in Fig. 3D.
    • Fig. S4. SEM image and transmission of the bowtie photonic crystal cavity characterized by NSOM.
    • Fig. S5. Position-dependent electric energy distribution in the central cavity unit cell of the silicon bowtie photonic crystal.
    • Table S1. Calculated mode volume (Vm) and measured quality factor (Q) of different photonic crystal (PhC) cavities including the bowtie photonic crystal cavity presented in this work.
    • Table S2. NSOM-measured mode sizes of plasmonic structures in comparison to dielectric bowtie photonic crystal reported in this work.

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