Bridging the gap between nanophotonic waveguide circuits and single mode optical fibers using diffractive grating structures

High-Efficiency, Wide Working Bandwidth Antenna Based on SOI Platform for Optical Phased Array

A novel structure of a subwavelength surface optical antenna for optical phased array is proposed in this paper. An asymmetric vertical grating structure is applied to achieve high emission efficiency (73% at 1550 nm). Optical antennas with large fabrication tolerances can also maintain a wide working bandwidth of 1 dB between 1350 and 1850 nm. The far-field scanning characteristics of 16-channel optical phased array are investigated in this study by employing the proposed antenna. The results show that the background suppression without considering side lobes caused by the antenna arrangement is −24.5 dB when the phase difference is 0 and when the scan range is as large as ±14.8° × 73.6°.

Improving the performance of optical antenna for optical phased arrays through high-contrast grating structure on SOI substrate

A novel optical antenna for optical phased arrays is proposed and simulated. A high-contrast grating structure is used to achieve extremely efficient emission. The emission efficiency is as high as 93.94% at 1.55 μm, which exceeds 50% in a range of wavelength from 1.48 μm to 1.62 μm. The antenna can achieve a perfect grating lobe suppression with background suppression of 28.4 dB when the phase difference between adjacent waveguides is 0. A 16-wire optical phased array can easily achieve a scan range of ± 22.8° × 20.2° with a beam width of 2.4° × 2.5°, by employing the optical antenna proposed.

Very high efficiency optical coupler for silicon nanophotonic waveguide and single mode optical fiber

Integrated optical circuits are poised to open up an array of novel applications. A vibrant field of research has emerged around the monolithic integration of optical components onto the silicon substrates. Typically, single mode optical fibers deliver the external light to the chip, and submicron single-mode waveguides then guide the light on-chip for further processing. For such technology to be viable, it is critically important to be able to efficiently couple light into and out of the chip platform, and between the different components, with low losses. Due to the large volume mismatch between a fiber and silicon waveguide (on the order of 600), it has been extremely challenging to obtain high coupling efficient with large tolerance. To date, demonstrated coupling has been relatively lossy and effective coupling requires impractical alignment of optical components. Here, we propose the use of a high contrast metastructure (HCM) that overcomes these issues, and effectively couples the off-chip, out-of-plane light waves into on-chip, in-plane waveguides. By harnessing the resonance properties of the metastructure, we show that it is possible to spatially confine the incoming free-space light into subwavelength dimensions with a near-unity (up to 98%) efficiency. The underlying coupling mechanism is analyzed and designs for practical on-chip coupler and reflector systems are presented. Furthermore, we explore the two-dimensional HCM as an ultra-compact wavelength multiplexer with superior efficiency (90%).

A realistic fabrication and design concept for quantum gates based on single emitters integrated in plasmonic-dielectric waveguide structures

Tremendous enhancement of light-matter interaction in plasmonic-dielectric hybrid devices allows for non-linearities at the level of single emitters and few photons, such as single photon transistors. However, constructing integrated components for such devices is technologically extremely challenging. We tackle this task by lithographically fabricating an on-chip plasmonic waveguide-structure connected to far-field in- and out-coupling ports via low-loss dielectric waveguides. We precisely describe our lithographic approach and characterize the fabricated integrated chip. We find excellent agreement with rigorous numerical simulations. Based on these findings we perform a numerical optimization and calculate concrete numbers for a plasmonic single-photon transistor.

Spatial resolution effect of light coupling structures

The coupling of light between free space and thin film semiconductors is an essential requirement of modern optoelectronic technology. For monochromatic and single mode devices, high performance grating couplers have been developed that are well understood. For broadband and multimode devices, however, more complex structures, here referred to as “coupling surfaces”, are required, which are often difficult to realise technologically. We identify general design rules based on the Fourier properties of the coupling surface and show how they can be used to determine the spatial resolution required for the coupler’s fabrication. To our knowledge, this question has not been previously addressed, but it is important for the understanding of diffractive nanostructures and their technological realisation. We exemplify our insights with solar cells and UV photodetectors, where high-performance nanostructures that can be realised cost-effectively are essential.

Miniaturized Bragg-grating couplers for SiN-photonic crystal slabs

We report on an experimental and theoretical investigation of an integrated Bragg-like grating coupler for near-vertical scattering of light from photonic crystal waveguides with an ultra-small footprint of a few lattice constants only. Using frequency-resolved measurements, we find the directional properties of the scattered radiation and prove that the coupler shows a good performance over the complete photonic bandgap. The results compare well to analytical considerations regarding 1d-scattering phenomena as well as to FDTD simulations.

Near-field interference for the unidirectional excitation of electromagnetic guided modes

Wave interference is a fundamental manifestation of the superposition principle with numerous applications. Although in conventional optics, interference occurs between waves undergoing different phase advances during propagation, we show that the vectorial structure of the near field of an emitter is essential for controlling its radiation as it interferes with itself on interaction with a mediating object. We demonstrate that the near-field interference of a circularly polarized dipole results in the unidirectional excitation of guided electromagnetic modes in the near field, with no preferred far-field radiation direction. By mimicking the dipole with a single illuminated slit in a gold film, we measured unidirectional surface-plasmon excitation in a spatially symmetric structure. The surface wave direction is switchable with the polarization.

High-performance 90° hybrid based on a silicon-on-insulator multimode interference coupler

We propose a multimode interference coupler (MMI) design for high-index-contrast technologies based on a shallowly etched multimode region, which is, for the first time to our knowledge, directly coupled to deeply etched input and output waveguides. This reduces the phase errors associated with the high-index contrast, while still allowing for a very compact layout. Using this structure, we fabricate a 2 × 4 MMI operating as a 90° hybrid, with a footprint of only 0.65 mm × 0.53 mm, including all the structures necessary to couple light to a fiber array. We experimentally demonstrate a common mode rejection ratio better than -20 dBe and phase errors better than ±5° in a ~50 nm bandwidth.

Potential characterization of free-space-wave drop demultiplexer using cavity-resonator-integrated grating input/output coupler

A prototype free-space-wave drop demultiplexer consisting of a cavity-resonator-integrated grating input/output coupler (CRIGIC) and a different-guided-mode-coupling distributed Bragg reflector (DGM-DBR) was designed for constructing a high-density wavelength-division-multiplexing intra-board chip-to-chip optical interconnection. The CRIGIC consists of one grating coupler and two DBRs, and can vertically couple a guided wave and a free-space wave with high efficiency. A two-channel drop demultiplexer operating at around 850-nm wavelength with 5-nm channel spacing in wavelength was fabricated in a thin-film SiO2-based waveguide. The device performance was predicted theoretically, characterized experimentally, and discussed.