DOI: 10.1002/adom.201801433 Scientific American selects plasmonic sensing as the top 10 emerging technologies of 2018.[15] Almost every single new plasmonic or photonic structure would be explored to test its sensing ability.[16–29] These works tend to report the sensing performance of their own structure. Some declare that their sensitivity breaks the world record. However, there is still a missing literature on what the world record really is, the gap between the experiments and the theoretical limit, as well as the differences between metal-based plasmonic sensors and dielectric-based photonic sensors. To push plasmonic and photonic sensors into industrial applications, an optical sensing technology map is absolutely necessary. This review aims to cover a wide range of most representative plasmonic and photonic sensors, and place them into a single map. The sensor performances of different structures will be distinctly illustrated. Future researchers could plot the sensing ability of their new sensors into this technology map and gauge their performances in this field. In this review, we focus on optical refractive index (RI) sensors with no fluorescent labeling required. We will utilize two parameters to characterize and compare the performance of optical RI sensors: sensitivity to RI change (denoted by symbol SRI) and figure of merit (in short, FoM). For simplicity, we restrict our discussions to bulk RI change, where the change in RI occurs within the whole sample. There is another case where the RI variation occurs only within a very small volume close to the sensor surface. This surface RI sensitivity is proportional to the bulk RI sensitivity, the ratio of the thickness of the layer within which the surface RI variation occurs, and the penetration depth of the optical mode.[6] The bulk RI sensitivity defines the ratio of the change in sensor output (e.g., resonance angle, intensity, or resonant wavelength) to the bulk RI variations. Here, we limit our discussions to the spectral interrogations and the bulk RI sensitivity SRI is given by[3,5–7,30]

Optical Refractive Index Sensors with Plasmonic and Photonic Structures: Promising and Inconvenient Truth

https://onlinelibrary.wiley.com/doi/pdf/10.1002/admt.201901138

Silicon-Based Integrated Label-Free Optofluidic Biosensors: Latest Advances and Roadmap

An ultra-compact broadband 2 × 2 3 dB power splitter is proposed and demonstrated by utilizing a subwavelength- grating (SWG)-assisted asymmetric directional coupler. In the present design, a subwavelength grating is inserted into the straight coupling region of an asymmetric directional coupler, so that the refractive index and the dispersion properties of this structure are engineered for achieving an ultra-broad bandwidth and an ultra-compact footprint. For the present power splitter with a straight coupling region as short as 5.25 μm, the bandwidth is as large as 300 nm (1400∼1700 nm) for achieving an imbalance of <0.4 dB and an excess loss of <0.33 dB for the fundamental transverse electrical (TE) mode. The fabricated splitter works well with an imbalance less than 0.5 dB and with an excess loss less than 1 dB in a broad wavelength-band from 1450 to 1650 nm.

Ultra-Compact Broadband 2 × 2 3 dB Power Splitter Using a Subwavelength-Grating-Assisted Asymmetric Directional Coupler

/pdf/a-review-of-silicon-subwavelength-gratings-building-break-54chaj4osh.pdf

A review of silicon subwavelength gratings: building break-through devices with anisotropic metamaterials

We demonstrate a compact high-performance adiabatic 3-dB coupler for the silicon-on-insulator platform. The refractive index of the gap region between two coupling waveguides is effectively increased using subwavelength grating, which leads to high-performance operation and a compact design footprint, with a mode-evolution length of only 25 µm and an entire device length of 65 µm. The designed adiabatic 3-dB coupler has been fabricated using electron beam lithography and the feature size used in our design is CMOS compatible. The fabricated device is characterized in the wavelength range from 1500 nm to 1600 nm, with a measured power splitting ratio better than 3 ± 0.27 dB and an average insertion loss of 0.20 dB.

Compact high-performance adiabatic 3-dB coupler enabled by subwavelength grating slot in the silicon-on-insulator platform

It has been shown that the wavelength-dependent performance of a directional coupler (DC) in silicon-on-insulator (SOI) platform can be greatly engineered by suitable design optimizations. Semianalytical coupled mode theory is used to optimize a nearly wavelength-independent design of a DC in an SOI substrate with a device layer thickness of 220 nm, operating in TE-polarization ( $\lambda \sim$ 1550 nm). The transmission characteristics of fabricated DCs are found to be indeed wavelength independent over a bandwidth of ${\text{100 nm}}$ (1525 nm $\leq \lambda \leq$ 1625 nm), consistent with the theoretical predictions. The average excess loss of such directional couplers is evaluated as $\sim\text{0.8}$ dB and there are scopes for its further reduction. These DCs are then used further to demonstrate integrated optical building blocks like power splitters (2 $\times$ 2, 1 $\times$ 4), Mach–Zehnder interferometers (2 $\times$ 2), and all-pass microring resonators. Their performances are also found to be uniform within the wavelength range mentioned and, thus, making them suitable for integrated silicon photonics for broadband applications.

Wavelength-Independent Directional Couplers for Integrated Silicon Photonics

It is shown that the dispersion in transmission characteristics of a ring resonator designed with silicon-on-insulator waveguides in all-pass configuration can be enhanced significantly by increasing interaction length of the directional coupler. This in turn helps to single-out highly extinct resonance(s) at and around the critically coupled wavelength. Such a device is found to be useful for a wide range of refractive index sensing for the cladding materials/analytes ( ${\text{1.0}} ). As a proof of concept, the sensor devices were fabricated and characterization results are shown to be consistent with theoretical prediction. The fabricated devices have been also used successfully to determine unknown refractive index of a given analyte (Newport F-IMF-150) with an error limit of $\delta n \sim {\text{1.67}} \times {\text{10}}^{-2}$ RIU. Analyzing experimental results, it is shown that the limit of detection can be further reduced ( $\ll {\text{10}^{{-3}}}$ RIU), if the perimeter of the ring is increased without compromising the round-trip waveguide loss. The superiority of such a sensor device lies in its simpler design rule, easier operation, wider range, and nearly accurate detection mechanism.

Dispersion Enhanced Critically Coupled Ring Resonator for Wide Range Refractive Index Sensing

In this paper, we investigate the mode sensitivity (S
 mode
) of subwavelength grating slot (SWGS) waveguides. S
 mode
 is an important parameter in various waveguide-based photonic circuits such as sensors, modulators, and thermally-controlled devices. It is a measure of the sensitivity of the waveguide effective index towards the refractive index perturbations in the cladding medium. The SWGS waveguide exhibits high mode sensitivity, as it combines sensitivity enhancement features of both slot and subwavelength grating waveguides. Finite-difference time-domain simulations are performed for the analysis, design, and optimization of the hybrid structure. The SWGS waveguide is incorporated into a Mach-Zehnder interferometer and fabricated on a silicon-on-insulator platform for the experimental estimation of S
 mode
. The measured S
 mode
 value of 79% is consistent with the theoretical prediction of 83%.

/pdf/mode-sensitivity-analysis-of-subwavelength-grating-slot-53ri7bloze.pdf

Mode Sensitivity Analysis of Subwavelength Grating Slot Waveguides

A beam shaping approach has been implemented to realize high-performance waveguide crossings based on cosine tapers. Devices with a compact footprint of 4.7µm×4.7µm were fabricated on the GLOBALFOUNDRIES 45 nm monolithic silicon photonics platform (45 CLO technology). Fabricated devices are found to be nearly wavelength independent (±0.035dB for 1260nm≤λ≤1360nm) with low insertion loss (∼0.2dB) and crosstalk (-35dB). The measured response of the devices is consistent with the three-dimensional finite-difference time-domain simulation results. The design stability is validated by measuring the device insertion loss on eight chips, which is found to be 0.197±0.017dB at the designed center wavelength of 1310 nm.

Beam shaping for ultra-compact waveguide crossings on monolithic silicon photonics platform

We demonstrate a compact ( $367 \times 67 \, \mu {\text{m}}^2$ ) four-channel wavelength division multiplexer using an arrayed waveguide grating based on multimode interference couplers, and built on a monolithic silicon photonics platform. The design is particularly attractive due to the thin device layer. A semi-numerical approach was used for device design and simulation. Optical measurements were found to be consistent with simulation results. The device channel spacing, 3-dB bandwidth and crosstalk were measured to be 97 GHz, 87 GHz and 9.6 dB, respectively, for the designed wavelength of operation near 1310 nm.

/pdf/compact-mmi-based-awgs-in-a-scalable-monolithic-silicon-32s8b7egk8.pdf

Sujith Chandran

Papers

Wavelength-Independent Directional Couplers for Integrated Silicon Photonics

Dispersion Enhanced Critically Coupled Ring Resonator for Wide Range Refractive Index Sensing

Mode Sensitivity Analysis of Subwavelength Grating Slot Waveguides

Beam shaping for ultra-compact waveguide crossings on monolithic silicon photonics platform

Compact MMI-Based AWGs in a Scalable Monolithic Silicon Photonics Platform