Sensitive optical biosensors for unlabeled targets: a review.

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

Gate control of positive and negative carriers in graphene is used to guide current in a manner analogous to the guiding of light by an optical fibre.

Gate-controlled guiding of electrons in graphene.

We present an idea to generate an arbitrary space-variant vector beam with structured polarization and phase distributions. The vector beams are synthesized from the left- and right-hand polarized light, each carrying different phase distributions. Both the phase and the state of polarization of vector beams can be tailored independently and dynamically by a spatial light modulator.

Generation of vector beam with space-variant distribution of both polarization and phase.

In optics, the two main mechanisms for enhancing the Goos-H\"anchen (GH) shift of a reflected light beam have a common shortcoming: The maximum shift is located exactly at the reflectance dip, which makes the reflected beam hard to detect. Here the authors tune the excitation of guided modes in a compound grating-waveguide structure, to realize quasibound states in the continuum (quasi-BICs) with ultrahigh $Q$-factors. Assisted by these quasi-BICs, the GH shift at the reflectance peak can be greatly enhanced. This giant GH shift with high reflectance can be used for $e.g.$ ultrasensitive sensors, wavelength-division (de)multiplexers, optical switches, and polarization beam splitters.

Giant Enhancement of the Goos-Hänchen Shift Assisted by Quasibound States in the Continuum

By analogy of optical waveguides, we investigate the guided modes in graphene waveguides, which is made of symmetric quantum well. The unique properties of the graphene waveguide are discussed based on the two different dispersion relations, which correspond to classical motion and Klein tunneling, respectively. It is shown that the third-order mode is absent in the classical motion, while the fundamental mode is absent in the Klein tunneling case. We hope these phenomena can lead to the potential applications in graphene-based quantum devices.

Guided modes in graphene waveguides

A graphene-coated U-shaped fiber surface plasmon resonance sensor with high refractive index sensitivity is proposed. With the combination of graphene coating and U-shape design, the proposed sensor exhibits a significantly enhanced sensitivity as compared with the conventional Au-based straight fiber surface plasmon resonance sensor. The refractive index sensitivity of 15.52 μm/RIU (refractive index unit) is achieved by coating monolayer graphene on U-shaped fiber with a bent radius of 1.1 cm. A sensitivity enhancement factor of about 5-fold is obtained when compared with the conventional straight fiber surface plasmon resonance sensor with and without monolayer graphene coating. The effect of Au thickness, number of graphene layers, and the bent radius on the sensor performance was investigated. The high sensitivity of the proposed graphene-coated U-shaped fiber surface plasmon resonance sensor makes it a promising candidate for biosensing and chemical sensing.

Highly Sensitive Surface Plasmon Resonance Sensor Based on Graphene-Coated U-shaped Fiber

The transmission probability of electrons tunneling through a graphene superlattice with periodic potential patterns is investigated using the transfer matrix method. It is found that the transmission probability as a function of incidence energy has more than one gap. The number, width and position of transmission gaps can be modulated by changing the period number, the incidence angle, the height and width of the potential. These characteristics of the transmission gaps in graphene superlattices may facilitate the development of many graphene-based electronics such as a multichannel electron wave filter.

Transmission gaps in graphene superlattices with periodic potential patterns

Bound-state spectra for supersymmetric quantum mechanics

We have investigated theoretically the resonant tunneling phenomenon of Dirac electrons through graphene superlattices with periodic potentials of square barriers. It is found that there are two resonance conditions for the graphene superlattices. Some of the resonance transmission peaks present N – 1-fold resonance splitting for $ N{\text{-barriers}} $, which is the analogy of the resonance splitting in semiconductor superlattices. The resonance splitting effect depends on the incidence angle rather than the height and width of the potential. However, there is no explicit splitting rule for the conductance and shot noise, which is different from the magnetic case. Furthermore, the resonant splitting rule of the transmission is not sensitive to the shape of the potential barrier. These properties in graphene superlattices may lead to potential applications in graphene-based electron devices.

Resonant peak splitting in graphene superlattices with one-dimensional periodic potentials

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