Wide-angle ultrasensitive biosensors based on edge states in heterostructures containing hyperbolic metamaterials

Label-free optical biosensing: going beyond the limits

Label-free optical biosensing holds great promise for a variety of applications in biomedical diagnostics, environmental and food safety, and security. It is already used as a key tool in the investigation of biomolecular binding events and reaction constants in real time and offers further potential additional functionalities and low-cost designs. However, the sensitivity of this technology does not match the routinely used but expensive and slow labelling methods. Therefore, label-free optical biosensing remains predominantly a research tool. Here we discuss how one can go beyond the limits of detection provided by standard optical biosensing platforms and achieve a sensitivity of label-free biosensing that is superior to labelling methods. To this end we review newly emerging optical implementations that overcome current sensitivity barriers by employing novel structural architectures, artificial materials (metamaterials and hetero-metastructures) and using phase of light as a sensing parameter. Furthermore, we elucidate the mechanism of plasmonic phase biosensing and review hyper-sensitive transducers, which can achieve detection limits at the single molecule level (less than 1 fg mm-2) and make it possible to detect analytes at several orders of magnitude lower concentrations than so far reported in literature. We finally discuss newly emerging layouts based on dielectric nanomaterials, bound states in continuum, and exceptional points.

Ultra-sensitive refractive index sensing enabled by a dramatic ellipsometric phase change at the band edge in a one-dimensional photonic crystal

Surface plasmon polaritons (SPPs) and Bloch surface waves (BSWs) have been widely utilized to design sensitive refractive index sensors. However, SPP- and BSW-based refractive index sensors require additional coupling component (prism) or coupling structure (grating or fiber), which increases the difficulty to observe ultra-sensitive refractive index sensing in experiments. Herein, we realize dramatic ellipsometric phase change at the band edges in an all-dielectric one-dimensional photonic crystal for oblique incidence. By virtue of the dramatic ellipsometric phase change at the long-wavelength band edge, we design an ultra-sensitive refractive index sensor at near-infrared wavelengths. The minimal resolution of the designed sensor reaches 9.28×10^-8 RIU. Compared with SPP- and BSW-based refractive index sensors, the designed ultra-sensitive refractive index sensor does not require any additional coupling component or coupling structure. Such ultra-sensitive refractive index sensor would possess applications in monitoring temperature, humidity, pressure, and concentration of biological analytes.

Photonic Bandgaps of One-Dimensional Photonic Crystals Containing Anisotropic Chiral Metamaterials

Conventional photonic bandgaps (PBGs) for linear polarization waves strongly depend on the incident angle. Usually, PBGs will shift toward short wavelengths (i.e., blue-shifted gaps) as the incident angle increases, which limits their applications. In some practices, the manipulation of PBGs for circular polarization waves is also important. Here, the manipulation of PBGs for circular polarization waves is theoretically investigated. We propose one-dimensional photonic crystals (1DPCs) containing anisotropic chiral metamaterials which exhibit hyperbolic dispersion for left circular polarization (LCP) wave and elliptical dispersion for right circular polarization (RCP) wave. Based on the phase variation compensation effect between anisotropic chiral metamaterials and dielectrics, we can design arbitrary PBGs including zero-shifted and red-shifted PBGs for LCP wave. However, the PBGs remain blue-shifted for RCP wave. Therefore, we can design a high-efficiency wide-angle polarization selector based on the chiral PBGs. Our work extends the manipulation of PBGs for circular polarization waves, which has a broad range of potential applications, including omnidirectional reflection, splitting wave and enhancing photonic spin Hall effect.

Omnidirectional nonreciprocal absorber realized by the magneto-optical hypercrystal

Photonic bandgap design is one of the most basic ways to effectively control the interaction between light and matter. However, the traditional photonic bandgap is always dispersive (blueshift with the increase of the incident angle), which is disadvantageous to the construction of wide-angle optical devices. Hypercrystal, the photonic crystal with layered hyperbolic metamaterials (HMMs), can strongly modify the bandgap properties based on the anomalous wavevector dispersion of the HMM. Here, based on phase variation competition between HMM and isotropic dielectric layers, we propose for the first time to design nonreciprocal and flexible photonic bandgaps in one-dimensional photonic crystals containing magneto-optical HMMs. Especially the zero-shift cavity mode and the blueshift cavity mode are designed for the forward and backward propagations, respectively. Our results show maximum absorption about 0.99 (0.25) in an angle range of 20-75 degrees for the forward (backward) incident light at the wavelength of 367 nm. The nonreciprocal omnidirectional cavity mode not only facilitates the design of perfect unidirectional optical absorbers working in a wide-angle range, but also possesses significant applications for all-angle reflectors and filters.

Near-infrared ITO-based photonic hypercrystals with large angle-insensitive bandgaps

The angle-sensitive photonic bandgap (PBG) is one of the typical features of one-dimensional photonic crystals. Based on the phase-variation compensation effect between the dielectric and hyperbolic metamaterials (HMMs), angle-insensitive PBGs can be realized in photonic hypercrystals. However, since hypercrystals are usually constructed using metal components, these angle-insensitive PBGs are mostly limited to narrow bandwidths in visible range. Here, we replace metal with indium tin oxide (ITO) to construct HMMs in the near-infrared range. In these ITO-based HMMs, we experimentally demonstrate the negative refraction of light in transverse magnetic polarization. With this HMM component, we realize a photonic hypercrystal with an angle-insensitive PBG in the wavelength range of 1.15-2.02 µm. These ITO-based hypercrystals with large angle-insensitive PBGs can find applications in near-infrared reflectors or filters.

Photosensitivity and reflectivity of the active layer in a Tamm-plasmon-polariton-based organic solar cell

We report on a model of an organic solar cell in which a photosensitive layer doped with plasmon nanoparticles acts as not only an absorbing element but also a mirror involved in the formation of the Tamm plasmon polariton. It is shown that such solar cells can be fabricated without metal contacts, thus avoiding undesired losses in the system. Methods for an additional increase in the integral absorption by applying metal or dielectric mirrors to the lower boundary of the photonic crystal are proposed. It has been found that the integral absorption in the active layer can be increased by 15% compared to classical optimized planar solar cells.

Critical coupling vortex with grating-induced high Q-factor optical Tamm states

We investigate optical Tamm states supported by a dielectric grating placed on top of a distributed Bragg reflector. It is found that under certain conditions the Tamm state may become a bound state in the continuum. The bound state, in its turn, induces the effect of critical coupling with the reflectance amplitude reaching an exact zero. We demonstrate that the critical coupling point is located in the core of a vortex of the reflection amplitude gradient in the space of the wavelength and angle of incidence. The emergence of the vortex is explained by the coupled mode theory.

Effective optical nihility media realized by one-dimensional photonic crystals containing hyperbolic metamaterials

Owing to the omnidirectional perfect transmission and omnidirectional zero phase accumulation properties, S-type optical nihility media (ONM) have been utilized to design hyperlenses, optical waveguides, field concentrators and field rotators. Under the multiple interference mechanism, for conventional all-dielectric one-dimensional photonic crystals (1DPCs), all the transmittance peaks within the passband will shift towards short wavelengths (blueshift) with the increase in incident angle. Therefore, effective ONM cannot be realized in all-dielectric 1DPCs because the perfect transmission and zero phase accumulation conditions at the wavelength of the transmittance peak can only be satisfied at a specific incident angle. However, in a 1DPC composed of alternating dielectric and hyperbolic metamaterial (HMM) layers, one can realize a stopband of which one band edge is redshifted. At the same time, a transmittance peak in the passband is blueshifted. Therefore, between the redshift band edge and the blueshift transmittance peak, one can obtain an angle-independent transmittance peak. The HMM layer is mimicked by a dielectric/doped semiconductor multilayer. At the wavelength of the angle-independent transmittance peak, perfect transmission and zero phase accumulation conditions can be satisfied at any incident angle. Our work provides a route, under the current experimental conditions, to realize an effective S-type ONM by a simple one-dimensional structure in the near-infrared range.

Broadband omnidirectional near-infrared reflector based on an angle-insensitive photonic band gap

Owing to the Bragg interference mechanism, band gap in all-dielectric one-dimensional photonic crystal (1DPC) is strongly sensitive to the incident angle, which limits the width of omnidirectional band gap. Based on the angle-insensitive Bragg band gap in 1DPC composed of alternating dielectric and hyperbolic metamaterial layers, we design a broadband omnidirectional near-infrared mirror with the bandwidth of 943 nm (52.3% relative bandwidth) without using the conventional cascade method. In addition, we show that the angle-insensitive property of the band gap is robust against the layer thickness disturbance. This multilayer-based broadband omnidirectional reflector will possess potential applications for omnidirectional filters within broad stopbands and photonic crystal fibers with broad operating bandwidths.