Yun-yun Chen

Waveguide-coupled surface phonon resonance sensors with super-resolution in the mid-infrared region

A waveguide-coupled surface phonon resonance (SPhR) sensor with super-resolution based on Fano resonance (FR) by using a multilayer system within the Kretschmann configuration in the mid-infrared wavelength region is proposed. Due to the coherent interference of the waveguide and the surface phonon polariton modes, the calculated reflectivity spectrum possesses sharp asymmetric FR dips. An ultra-small linewidth is formed because of the Fano coupling, and the physical features contribute to a highly efficient nano-sensor for refractive index sensing. The bulk and surface sensitivity by intensities are greatly enhanced relative to those of conventional SPhR sensors.

Four-step spatial phase-shifting shearing interferometry from moiré configuration by triple gratings

A spatial phase-shifting shearing interferometry is presented in this paper. The whole optical configuration is simple and consists of three Ronchi gratings. Four phase-shifted shearing interferograms can be obtained simultaneously. The explicit intensity distributions of shearing interferograms are given and a corresponding four-step spatial phase-shifting algorithm is proposed to extract phase information from the new interferometry. This spatial phase-shifting configuration is applied to extract phase projection of a propane flame and a mathematical error analysis is presented.

Spatial phase-shifting characteristic of double grating interferometer

Double grating interferometer is usually used to achieve phase information from distorted wave front by its temporal phase-shifting characteristic. In this paper, the spatial phase-shifting characteristic of double grating interferometer is presented. The explicit intensity distributions of interferograms produced by double gratings are derived with the scalar diffraction theory, and the stable phase shift is found between plus-first, zero and minus-first order interferograms. Results indicate that the phase shift only depends on the grating period and the distance between two gratings if no phase object exists. If phase object exists, it varies on the interferograms. But the phase shifts are equal at any special point of interferograms. In particular, the triple grating interferometer is presented to generate at least four phase shift interferograms simultaneously with the similar method.

Influence of pressure distribution on flow field temperature reconstruction

This research proposes an issue that has previously been omitted in flow field temperature reconstruction by optical computerized tomography (OCT). To prove that it is not reasonable to always assume an isobaric process occurs when OCT is adopted to obtain the temperature distributions of flow fields, a propane-air flame and an argon arc plasma are chosen as two practical examples for experiment. In addition, the measurement of the refractive index is achieved by moiré deflection tomography. The results indicate that the influence of pressure distribution on temperature reconstruction is a universal phenomenon for various flow fields. Hence, the condition that can be introduced to estimate when an isobaric process can no longer be assumed is presented. In addition, an equation is offered to describe the temperature reconstruction imprecision that is caused by using the supposed pressure instead of the practical pressure.

Angularly dense comb-like enhanced absorption of graphene monolayer with attenuated-total-reflection configuration

A multiline absorber based on the excitation of guided-mode resonance of one-dimensional photonic crystals (1D-PhCs), including a surface graphene monolayer under the attenuated-total-reflection configuration, is proposed and demonstrated. By carefully designing the structure parameters of the 1D-PhCs, the guided mode can be modulated by the periodic distribution of the refractive index. Our results reveal that the critical coupling of the guided resonance in periodical PhCs to graphene produces the perfect absorption. The number of absorption peaks within the photonic band corresponds to the number of unit cells. An ultrahigh Q-factor value of 4.75×106 is obtained at resonance with unity absorption, which could serve as a promising replacement of metallic thin film as a sensor probe for future biosensing applications.

Polarization-insensitive and wide-incident-angle optical absorber with periodically patterned graphene-dielectric arrays

A polarization-insensitive and angle-independent graphene absorber (GA) with periodically patterned grating is demonstrated. A periodic nanocavity composed of multilayer subwavelength grating and metal substrate supports a strongly localized mode inside the cavity, where the mode helps to absorb more electromagnetic waves. The proposed GA exhibits polarization-insensitive behavior and maintains the high absorption above 90% within a wide range of incident angle (more than 80°). We attribute the high absorption to the excitation of the cavity mode resonance and magnetic resonance for the transverse electric and transverse magnetic polarizations, respectively. The proposed GA has potential applications in the design of various devices, such as optical modulators or tunable absorption filters because of its remarkable angle-insensitive absorption performance.

Multiband enhanced absorption of monolayer graphene with attenuated total reflectance configuration and sensing application

An enhanced absorption of monolayer graphene is obtained in a multilayer film-based attenuated total reflectance configuration in the visible wavelength range. The enhanced absorption under transverse magnetic and electric conditions is associated with the excitation of the waveguide mode in the thin-film layer, which is verified by the numerical calculation of field profiles. The obtained results manifest that the model induces a high field enhancement at the graphene–dielectric interface with the resonant angle, which implies potential sensing applications. The magnitude of the figure of merit is found to be three times higher than that of a conventional surface plasmon sensor.

High-temperature flow field's electron number density measurement by two-wavelength moiré tomography.

In this Letter, a direct method is proposed to measure the electron number density distribution for high-temperature complex flow fields. The experimental system of two-wavelength moiré tomography is established, while four key issues are solved and well clarified. The argon arc plasma is adopted as an example for experiment, while 532 and 808 nm are chosen as the two probe wavelengths. The results indicate that the electron number densitys distribution of the measured argon arc plasma can be directly obtained by two-wavelength moiré tomography, which can avoid the imprecision of the indirect methods. This Letter can provide some reference for various high-temperature and high-density gradient flow field optical measurement and diagnosis.

Introducing nanoresonators into a metal–dielectric–metal waveguide array to allow beam manipulation

Stub and circular ring-shaped plasmonic resonators are introduced into a metal?dielectric?metal (MDM) waveguide array to allow light transmission control. Light focusing and splitting effects are verified by the finite difference time domain method; the simulation results reveal that the resonators can be used for modulating the superposition phase of the interference between the surface plasmon wave (SPW) from the end of the resonator and the passing SPW in the waveguide array. Furthermore, a structure utilizing a stub cavity with nonlinear material to control the phase of the transmitted SPW is proposed; the deflection angle of the light can be controlled by means of the intensity of the incident light. The proposed MDM waveguide array with plasmonic resonators, with its compact size, ease of integration, and high output, certainly has potential for application in nanophotonic circuits.

Ionization effect on arc plasma’s optical diagnosis by the measurement of the refractive index

The effect of arc plasma ionization on its temperature diagnosis by the measurement of the refractive index is discussed. The refractive index of arc plasma in two conditions is compared: 1) only the first ionization is considered and 2) both the first and second ionizations are considered. In order to facilitate plasma temperature reconstruction, two corresponding refractive index models are deduced. For the sake of making this study universal, both the monatomic and dual-atomic molecule arc plasmas are chosen as typical examples for theoretical deduction and analysis. A condition, which can be adopted to estimate whether the second ionization should be considered in temperature reconstruction, is proposed. Finally, an argon arc plasma is chosen as an example for experiment, and the experimental results match well with the theoretical analysis. This study is crucial to arc plasmas optical diagnosis, which is based on the measurement of the refractive index.