Bingqing Zhu

Hyperuniform Disordered Network Polarizers

A TM-pass integrated waveguide polarizer based on hyperuniform disordered wall network is proposed and experimentally demonstrated for the first time. Its working principle is based on strong interaction between the waveguide modes and hyperuniform disordered solids (HUDS) with large photonic bandgaps, which creates distinct transmission for the TE and TM modes. In our fabricated waveguide polarizers with a length of 8.0 μm, we have achieved a measured bandwidth of 98 nm with extinction ratio larger than 30 dB, and an average insertion loss for the passing TM mode only 1.72 dB. The operational bandwidth is the largest demonstrated to date. The fabrication tolerance of the HUDS polarizers is also numerically investigated. Considering these superior features, our HUDS polarizers serve as a promising candidate for manipulating photonic polarization states on an integrated platform.

Fully suspended slot waveguides for high refractive index sensitivity

A fully suspended mid-infrared (FSMIR) slot waveguide is proposed and experimentally demonstrated on a silicon-on-insulator (SOI) platform for the first time. The slotted waveguide core is mechanically supported by lateral subwavelength grating claddings. The fabricated waveguides possess low propagation loss, which is measured to be 7.9  dB/cm at the wavelength of 2.25 μm. With the underlying buried oxide (BOX) removed, the FSMIR slot waveguide has a broad spectral range of transparency that is limited only by the absorption of silicon. Numerical simulation shows that its sensitivity, defined as the ratio between the change of the effective index and the ambient refractive index, can reach 1.123, which is 9.7% higher than the maximal sensitivity of conventional SOI slot waveguides on BOX.

Enhancement of self-phase modulation induced spectral broadening in silicon suspended membrane waveguides

We experimentally observed a possibly enhanced self-phase modulation (SPM) in silicon suspended membrane waveguides (SMWs) by measuring the spectral broadening of optical pulses. The nonlinear coefficient n 2 and the two-photon absorption coefficient β 2 of silicon SMWs were measured to be (4.6 ± 0.9) × 10−18 m2 W−1 and 0.46 cm GW−1 at 1555 nm wavelength. We also proposed a method of using SPM-induced spectral broadening to obtain the coupling loss of a single grating coupler and experimentally compared the spectra of two grating couplers in silicon SMWs and in silicon-on-insulator waveguides.

Ultraviolet optomechanical crystal cavities with ultrasmall modal mass and high optomechanical coupling rate

Optomechanical crystal (OMC) cavities which exploit the simultaneous photonic and phononic bandgaps in periodic nanostructures have been utilized to colocalize, couple, and transduce optical and mechanical resonances for nonlinear interactions and precision measurements. The development of near-infrared OMC cavities has difficulty in maintaining a high optomechanical coupling rate when scaling to smaller mechanical modal mass because of the reduction of the spatial overlap between the optical and mechanical modes. Here, we explore OMC nanobeam cavities in gallium nitride operating at the ultraviolet wavelengths to overcome this problem. With a novel optimization strategy, we have successfully designed an OMC cavity, with a size of 3.83 × 0.17 × 0.13 μm3 and the mechanical modal mass of 22.83 fg, which possesses an optical mode resonating at the wavelength of 393.03 nm and the fundamental mechanical mode vibrating at 14.97 GHz. The radiation-limited optical Q factor, mechanical Q factor, and optomechanical coupling rate are 2.26 × 107, 1.30 × 104, and 1.26 MHz, respectively. Our design and optimization approach can also serve as the general guidelines for future development of OMC cavities with improved device performance.

Integrated near-infrared photodetector based on colloidal HgTe quantum dot loaded plasmonic waveguide

We propose and demonstrate an ultracompact near-infrared (NIR) photodetector based on the hybrid integration of a surface plasmon polariton (SPP) optical guiding structure and the colloidal mercury telluride (HgTe) quantum dots (QDs). The photodetector achieves a responsivity of 10 mA/W at low input power. We also show that short ligand exchange process can dramatically increase the conductivity and responsivity of the photodetector.

Nanophotonic structures for waveguide couplers and polarizers

We review our recent work on nanophotonic waveguide devices. We shall discuss structures which efficiently couple light into a plasmonic slot waveguide, and the use of a hyperuniform disordered photonic bandgap structure to make a highly compact waveguide polarizers. Nanoscale photonic integration was successfully used to demonstrate the monolithic integration of 850 photonic components on a CMOS microprocessor with over 70 million transistors. This large disparity in the number of photonic and electronic devices on the same chip arises because of the limitation in how small conventional silicon photonic devices can be made. In this talk we shall describe our recent work in two different approaches to enable the integration of smaller photonic devices. One approach for smaller photonic devices is to exploit the guiding of light at metal-dielectric interfaces from surface plasmon-polariton (SPP) modes. Plasmonic devices not only offer the advantage of smaller size, but in the case of optical modulators they can also provide higher energy efficiency and high speed operation. One important challenge for plasmonic devices on silicon chips is how to couple light efficiently into the plasmonic device. We shall present some of the different designs of nanophotonic couplers to couple light efficiently from a conventional silicon waveguide to a plasmonic slot waveguide. Another approach to make smaller photonic devices is to exploit the photonic bandgap of nanophotonic structures for highly compact resonators, filters or polarizers. Unlike convention photonic crystals which require high precision in the periodicity of the photonic lattice, photonic bandgaps from hyperuniform disordered structures are more tolerant to fabrication errors. We shall describe a waveguide polarizer with over 30 dB extinction ratio over a 98nm optical bandwidth that occupies a short length of 8 μm (including the waveguide tapers) based on a hyperuniform disordered photonic bandgap structure [4].

Photoresponse of graphene-on-silicon nitride microring resonator

We report the design and fabrication of a graphene-on-silicon nitride microring resonator to achieve the optimized optical absorption. The bolometric effect in the graphene photodetector results in a responsivity of 1.31 mA/W at 1.55 μm.

Thermo-optic tunable silicon grating coupler

We demonstrate a thermally tunable silicon grating coupler. The experimental result of ~10nm shift in center wavelength at about 410mW electrical power was in reasonable agreement with the expected ~16nm tuning range.