Hybrid plasmonic slot waveguide with a metallic grating for on-chip biosensing applications

Ultrasensitive refractometric sensing via centimeter-scale metasurfaces with spatially gradient geometry generated by elastomer mechanical stretching

Plasmonic metasurfaces with gradient geometry are emerging two-dimensional optical elements with unique capabilities, such as manipulating light by imparting local, space-variant phase changes to an incident electromagnetic wave, eliminating the chromatic aberration. However, the costly and time-consuming fabrication process and the requirement of sophisticated optical characterization instruments restrict practical applications of plasmonic metasurfaces. Herein, we present a novel nanofabrication method to generate centimeter-scale metasurface with spatially gradient geometry over the whole metasurface by directional stretching of a trapezoid elastic carrier patterned with regular metallic nanostructures. This strategy eases the requirement of time-consuming and expensive lithographic techniques in conventional methods. The spatially gradient plasmonic metasurface exhibit variable transmittances under monochromatic and polar light illumination, resulting in grayscale patterns with different transmittance intensity distributions. An ultrahigh imaging-based sensitivity of 1495 pixel/refractive index unit (RIU) and a detection limit of 0.00068 RIU can be achieved based on the spatially gradient plasmonic metasurface, which is superior to the performance of the regular metasurface before stretching. This novel strategy is expected to be promising for fabrication of gradient metasurfaces to be employed in many fields of nanophotonics.

Overview of Optical Biosensors for Early Cancer Detection: Fundamentals, Applications and Future Perspectives

Conventional cancer detection and treatment methodologies are based on surgical, chemical and radiational processes, which are expensive, time consuming and painful. Therefore, great interest has been directed toward developing sensitive, inexpensive and rapid techniques for early cancer detection. Optical biosensors have advantages in terms of high sensitivity and being label free with a compact size. In this review paper, the state of the art of optical biosensors for early cancer detection is presented in detail. The basic idea, sensitivity analysis, advantages and limitations of the optical biosensors are discussed. This includes optical biosensors based on plasmonic waveguides, photonic crystal fibers, slot waveguides and metamaterials. Further, the traditional optical methods, such as the colorimetric technique, optical coherence tomography, surface-enhanced Raman spectroscopy and reflectometric interference spectroscopy, are addressed.

Kerr nonlinear medium assisted double-face absorbers for differential manipulation via an all-optical operation

Recently, light absorbers have attracted great attentions due to their promising in applications in functional optoelectronic devices. Herein, we theoretically propose and numerically demonstrate a new absorber platform, which consists of a 280-nm-thick photonic nonlinear waveguide film covering on the metal grating structure. Strong reflection inhibition and absorption enhancement is achieved in both the forward and backward directions, which indicates potential novel performances since the previous reports only achieved absorption in one side due to the using of opaque metal film substrate or the reflective mirror. The anti-reflection bands or the absorption peaks at the shorter and longer wavelength ranges are related to the excitation of the propagating surface plasmon resonance by the slit-assisted grating and the cavity mode by the slit in the metal film. Strong differential manipulation is realized for the double-face absorbers via the all-optical operation. Moreover, the operation wavelengths for the double-face light absorber can be modified strongly via using an asymmetric dielectric medium for the coating films. These new findings pave approaches for subtractive lightwave modulation technology, selective filtering, multiplex sensing and detection, etc.

Uniformity improvement of two-dimensional surface relief grating waveguide display using particle swarm optimization

Augmented reality head-mounted displays (AR-HMDs) based on diffractive waveguides have been a challenging and rewarding research topic focusing on near-eye displays. The size of the exit pupil and uniformity of the image illuminance are two important factors that affect the display performance of the diffractive waveguide. In this paper, a novel method for optimizing high uniformity of two-dimensional (2D) diffractive waveguide is proposed. A straight-line 2D surface relief grating (SRG) waveguide with divided grating regions is designed. An illuminance uniformity evaluation model of the energy propagation process is established, and non-sequential ray tracing is utilized to optimize the diffraction efficiency of multi-regions grating to achieve illuminance uniformity distribution. Then, the uniformity distribution of the diffraction efficiency in different fields of view (FOVs) is realized by combining the particle swarm optimization (PSO) algorithm and rigorous couple wave analysis (RCWA) to optimize the grating structural parameters, which further ensures the uniformity of the exit pupil illuminance and angular illuminance. The waveguide with exit pupil expansion (EPE) has exit pupil size of 16 mm × 14 mm at an eye relief (ERF) of 20 mm, exit pupil illuminance uniformity of 91%, and angular uniformity illuminance of 64%.

Design of 4.7 μm High-Efficiency Hybrid Dielectric Reflection Gratings

Traditional reflective diffraction gratings working at 4.7 μm are fabricated by metal coatings. Due to the absorption of the metal itself, the diffraction efficiency (DE) could not reach over 95%. In this paper, we propose a 3 μm period multilayer grating design using hybrid multilayer dielectrics. With a layer of 0.353 μm Si and a layer of 0.905 μm SiO₂ forming the rectangular grating, the maximum of larger than 99.99% and the overall first-order DE reached 97.88%. The usable spectrum width is larger than 0.2 μm, more than four times larger than that of the pure Si rectangular grating. This high DE multilayer grating is an ideal element for high-power laser systems with the spectrum beam combining method.

Efficient 4.95 µm-8.5 µm dual-band grating coupler with crosstalk suppression capability

In many integrated optics systems, grating couplers are a key component of interfacing the external light source with in-plane photonic devices. Grating couplers with dual-band capability are often desired for expanding the operation spectrum of photonic systems. Here, we propose and theoretically investigate, for the first time, a 4.95 µm-8.5 µm dual-band grating coupler on a Ge-on-SOI platform. In addition to conventional structures, Bragg gratings are introduced to two wavelength division directions for crosstalk suppression. With this design, the simulated coupling efficiencies have respectively reached 59.93% and 46.38% for the 4.95 µm and 8.5 µm bands. This mid-infrared dual-band grating coupler may be useful for defense and environmental monitoring applications.

Optical properties of one-dimensional plasma photonic crystals with inhomogeneous plasma density distribution functions

This paper investigates the optical properties of the two plasma photonic crystal structures. The first structure contains periodic thin layers of plasma with heterogeneous densities, and in the second one, the heterogeneous plasma layer has been applied as the defect layer of a one-dimensional photonic crystal. Herein, the plasma density distribution function is considered a continuous function of plasma critical density as follows: n=n_crf(r→), where n_cr=m_eε₀ω²/e² represents the critical density of the plasma and f(r→) indicates the plasma density distribution function. The heterogeneous plasma layer is assumed to be composed of several homogeneous sublayers with constant density. The transfer-matrix method is applied in calculations, and the total transfer matrix is obtained by multiplying the transfer matrices of each sublayer. Properties and behavior of the photonic bandgap and the defect mode for five different plasma density distribution functions and applied external constant magnetic field are comparatively investigated. Results show that, in the first structure, increasing the incident angle from 0° to 30° causes a bandgap shift to higher frequencies. So, the incident angle is a parameter that can control the bandgap. In the defective structure, the intensity of the defect modes is almost the same in all five distributions, but their locations in the spectrum are different. The effect of the lateral position of the maximum density and external constant magnetic field on the defect mode position is also investigated. The capability of controlling the photonic bandgaps and defect modes of the plasma photonic crystals creates wide applications in promising tunable optical devices, such as optical filters.

Vector bending sensor based on an edge-core cladding-type fiber Bragg grating

A two-dimensional vector bending sensor that is both compact and simple is proposed and demonstrated, based on an edge-core cladding-type fiber Bragg grating (ECLFBG) inscribed in an edge-core. The ECLFBG is written parallel to the edge-core using a femtosecond laser point-by-point technique. The reflection spectrum of this ECLFBG varies significantly depending on the magnitude and direction of the fiber's bend. Combining the trend and sensitivity of the wavelength shift and reflection intensity variations of the ECLFBG, the bending magnitude and direction can be measured simultaneously.