Fully suspended slot waveguides for high refractive index sensitivity

Wide-range, ultra-compact, and high-sensitivity ring resonator biochemical sensor with CMOS-compatible hybrid plasmonic waveguide

A ring resonator-based biochemistry sensor with a wide range, ultra-compact footprint, and high sensitivity is proposed, which utilizes a suspended slot hybrid plasmonic (SSHP) waveguide. The waveguide consists of a suspended Si nanowire separated from a Cu metal surface by a nanoscale air gap. The hybridization of fundamental mode of a Si channel waveguide with the surface plasmon polariton (SPP) mode of Cu-Si interface achieves a strong light confinement, high waveguide sensitivity (Sw), and low optical loss, showing a great potential in integrated optical sensor. The sensitivity, the detection limit and the detection range of the SSHP waveguide-based biochemistry sensor with a miniaturized radius of 1 µm are numerically demonstrated as 458.1 nm/RIU, 3.7 × 10^-5 RIU and 0.225 RIU, respectively. These superior performances as well as the fully CMOS compatibility enable the integrated optical sensing applications.

Advancing the sensitivity of integrated epoxy-based Bragg grating refractometry by high-index nanolayers

In this Letter, we report on significantly improved surrounding RI sensitivity of epoxy polymer waveguide Bragg grating sensors. Uniform Bragg gratings were generated inside flat rectangular epoxy waveguides near the cutoff regime using standard phase mask excimer laser writing. Thickness controlled nanolayers of high-index titanium dioxide were deposited homogeneously on the waveguide sensor's surface area by repeated reactive sputter processing. Maximum Bragg wavelength shifts as high as 74.22 nm, as well as maximum sensitivities around 523 nm/RI unit corresponding to a minimum RI resolution of 1.9⋅10^-6, could be obtained by employing a ∼75nm thick titanium dioxide coating.

Hyperuniform disordered waveguides and devices for near infrared silicon photonics

We introduce a hyperuniform-disordered platform for the realization of near-infrared photonic devices on a silicon-on-insulator platform, demonstrating the functionality of these structures in a flexible silicon photonics integrated circuit platform unconstrained by crystalline symmetries. The designs proposed advantageously leverage the large, complete, and isotropic photonic band gaps provided by hyperuniform disordered structures. An integrated design for a compact, sub-volt, sub-fJ/bit, hyperuniform-clad, electrically controlled resonant optical modulator suitable for fabrication in the silicon photonics ecosystem is presented along with simulation results. We also report results for passive device elements, including waveguides and resonators, which are seamlessly integrated with conventional silicon-on-insulator strip waveguides and vertical couplers. We show that the hyperuniform-disordered platform enables improved compactness, enhanced energy efficiency, and better temperature stability compared to the silicon photonics devices based on rib and strip waveguides.

Dual-wavelength-band subwavelength grating coupler operating in the near infrared and extended shortwave infrared

We show that dual-wavelength-band (DWB) subwavelength grating couplers (SWGCs) can be designed for simultaneous coupling of the near-infrared and extended shortwave-infrared (SWIR) fundamental transverse electric polarized light at the same diffraction angle. Numerical simulations predict coupling efficiencies (CEs) larger than 34% for the DWB SWGCs operating in the S/C band (1.48/1.55 μm) and extended SWIR band (1.8-2.8 μm) with a widely and continuously tailorable peak wavelength separation between 250 and 1250 nm. The fabricated DWB SWGCs with peak wavelengths of (1.56, 2.255) μm and (1.487, 2.331) μm respectively obtain CEs of (20.2, 25.8)% and (20.6, 26.9)%, 1-dB bandwidths of 38 nm and 54 nm at a diffraction angle of 2°.

A High-Efficiency Multispectral Filter Based on Plasmonic Hybridization between Two Cascaded Ultrathin Nanogratings

Overcoming the disadvantages of low transmission and broad peak bandwidth of previously reported plasmonic color filters, a high-efficiency multispectral plasmonic color filter is theoretically proposed with two cascaded ultrathin metallic nanogratings separated by two heterogeneous dielectric layers, and its optical properties are theoretically investigated using the finite-difference time-domain method. The transmission spectrum presents three near-unity peak bands accompanied with three near-null dip bands adjacent around them. Both transmission efficiencies of above 90% and ultranarrow peak bandwidth of 20 nm are achieved in the visible regime. The peak band positions can be flexibly tailored by varying the structural parameters. The filter selects the visible color with high signal noise ratio at the peak bands. The outstanding spectral properties of this filter indicate significant improvement for the high-accuracy color filtering and multispectral imaging applications. The simulated near-field electromagnetic distributions suggest that the excitation of the hybrid antisymmetric surface plasmon polariton (SPP) leaky mode and metal-insulator-metal waveguide modes are responsible for the peak transmission bands, while the formation of the hybrid SPP bound modes confined on the bottom nanograting makes the dip transmission bands, all of which are the consequence of the plasmonic hybridization between the two neighboring metallic nanogratings.

Suspended low-loss germanium waveguides for the longwave infrared

Germanium is a material of high interest for mid-infrared (MIR) integrated photonics due to its complementary metal-oxide-semiconductor (CMOS) compatibility and its wide transparency window covering the 2-15 μm spectral region exceeding the 4 and 8 μm limit of the silicon-on-insulator platform and Si material, respectively. In this Letter, we report suspended germanium waveguides operating at a wavelength of 7.67 μm with a propagation loss of 2.6±0.3  dB/cm. To the best of our knowledge, this is the first demonstration of low-loss suspended germanium waveguides at such a long wavelength. Suspension of the waveguide is achieved by defining holes alongside the core providing access to the buried oxide layer and the underlying Si layer so that they can be wet etched using hydrofluoric acid and tetramethylammonium hydroxide, respectively. Our MIR waveguides create a new path toward long wavelength sensing in the fingerprint region.

Efficient perfectly vertical grating coupler for multi-core fibers fabricated with 193 nm DUV lithography

We propose a novel high-efficiency, low-reflection, and fabrication-tolerant perfectly vertical grating coupler (PVGC) with a minimum feature size >200  nm to allow for fabrication using 193 nm deep-ultraviolet lithography. The structural parameters of PVGC were optimized by a genetic optimization algorithm. Simulations predicted the coupling efficiency to be -2.0  dB (63.0%) and the back reflections to be less than -20  dB in the wavelength range of 1532-1576 nm. The design was fabricated in a multi-project wafer run for silicon photonics, and a coupling efficiency of -2.7  dB (53.7%) with a 1 dB bandwidth of 33 nm is experimentally demonstrated. The measured back reflection is less than -16  dB over the C-band. The PVGC occupies a compact footprint of 30  μm×24  μm and can be interfaced with the multi-core fibers for future space-division-multiplexing networks.

Subwavelength integrated photonics

In the late nineteenth century, Heinrich Hertz demonstrated that the electromagnetic properties of materials are intimately related to their structure at the subwavelength scale by using wire grids with centimetre spacing to manipulate metre-long radio waves. More recently, the availability of nanometre-scale fabrication techniques has inspired scientists to investigate subwavelength-structured metamaterials with engineered optical properties at much shorter wavelengths, in the infrared and visible regions of the spectrum. Here we review how optical metamaterials are expected to enhance the performance of the next generation of integrated photonic devices, and explore some of the challenges encountered in the transition from concept demonstration to viable technology.

Tailorable dual-wavelength-band coupling in a transverse-electric-mode focusing subwavelength grating coupler

A transverse-electric-mode focusing subwavelength grating coupler (FSWGC) is proposed and demonstrated for dual-wavelength-band (DWB) coupling from a single-mode fiber into a suspended-membrane waveguide for the first time, to the best of our knowledge. Location and separation of the two coupling peaks can be flexibly tailored based on a proposed design methodology. As a proof of concept, two DWB FSWGCs working at (1486.0, 1594.5) nm and (1481.5, 1661.5) nm are experimentally demonstrated with coupling efficiencies of (18.3%, 20.1%) and (14.5%, 17.5%), 3-dB bandwidths of (55.0, 30.5) nm and (44.0, >39.5) nm, respectively. A DWB FSWGC working at (1480, 1830) nm with a wavelength separation of 350 nm and coupling efficiencies of (38.0%, 33.2%) is also numerically predicted.

Suspended silicon waveguides for long-wave infrared wavelengths

In this Letter, we report suspended silicon waveguides operating at a wavelength of 7.67 μm with a propagation loss of 3.1±0.3  dB/cm. To our knowledge, this is the first demonstration of low-loss silicon waveguides at such a long wavelength, with loss comparable to other platforms that use more exotic materials. The suspended Si waveguide core is supported by a sub-wavelength grating that provides lateral optical confinement while also allowing access to the buried oxide layer so that it can be wet etched using hydrofluoric acid. We also demonstrate low-loss waveguide bends and s-bends.

High sensitivity visible light refractive index sensor based on high order mode Si3N4 photonic crystal nanobeam cavity

We design and demonstrate a suspended high sensitivity silicon nitride (Si₃N₄) photonic crystal (PhC) nanobeam cavity sensor. By utilizing the higher order mode, the optical field distribution in the analytes increases dramatically and the light matter interaction between the optical mode and the analytes has been enhanced. A high sensitivity of 321 nm/refractive index unit (nm/RIU) has been experimentally achieved at the wavelength ~700 nm which is the highest value reported so far for a resonator based sensor at such a short wavelength.

Loss reduction of silicon-on-insulator waveguides for deep mid-infrared applications

We report that propagation loss of optical waveguides based on a silicon-on-insulator (SOI) material platform can be greatly reduced. Our simulations show that the loss, including SiO₂ absorption and substrate leakage, but no scattering loss, is 0.024 and 0.53 dB/cm in the deep mid-infrared at 4.8 and 7.1 μm wavelengths, where the material absorption in SiO₂ is 100 and 1000 dB/cm, respectively. The loss becomes negligible, compared to scattering loss in Si waveguides. This is enabled by using the TE₁₀ mode in a pedestal waveguide. We also show that the TE₁₀ mode can be excited in the proposed waveguide by the fundamental mode with a coupling efficiency of >94%. Low propagation loss, high coupling efficiency, and fabrication-friendly design would make it promising for practical use of SOI devices in the deep mid-infrared.