A novel plasmonic refractive index sensor based on gold/silicon complementary grating structure

A novel complementary grating structure is proposed for plasmonic refractive index sensing due to its strong resonance at near-infrared wavelength.The reflection spectra and the electric field distributions are obtained via the finite-difference time-domain method.Numerical simulation results show that multiple surface plasmon resonance modes can be excited in this novel structure.Subsequently,one of the resonance modes shows appreciable potential in refractive index sensing due to its wide range of action with the environment of the analyte.After optimizing the grating geometric variables of the structure,the designed structure shows the stable sensing performance with a high refractive index sensitivity of 1642 nm per refractive index unit(nm/RIU)and the figure of merit of 409 RIU^(-1).The promising simulation results indicate that such a sensor has a broad application prospect in biochemistry.

A theoretical study of a plasmonic sensor comprising a gold nano-disk array on gold film with a SiO_2 spacer

A plasmonic refractive index(RI) sensor with high RI sensitivity based on a gold composite structure is proposed.This composite structure is constructed from a perfect gold nano-disk square array on a gold film, with a SiO_2 spacer. The reflection spectra of the composite structure, with analyte RI in the range of 1.30 to 1.40, are theoretically studied using the finite-difference time-domain method. The incident light beam is partly coupled to the localized surface plasmons(LSP) of the single nano-disks and partly transferred to the propagating surface plasmons(PSP) by grating coupling. The reflectivity is nearly zero at the valley of the reflection spectrum because of the strong coupling between LSP and PSP. Also, the full width at half maximum(FWHM) of one of the surface plasmon polaritons(SPPs) modes is very narrow, which is helpful for RI sensing. An RI sensitivity as high as 853 nm/RIU is obtained. The influence of the structure parameters on the RI sensitivity and the sensor figure of merit(FOM) are investigated in detail. We find that the sensor maintains high RI sensitivity over a large range of periods and nano-disk diameters. Results of the theoretical simulation of the composite structure as a plasmonic sensor are promising. Thus, this composite structure could be extensively applied in the fields of biology and chemistry.

Efficient manipulation of terahertz waves by multi-bit coding metasurfaces and further applications of such metasurfaces

Benefiting from the unprecedented superiority of coding metasurfaces at manipulating electromagnetic waves in the microwave band,in this paper,we use the Pancharatnam-Berry(PB)phase concept to propose a high-efficiency reflectivetype coding metasurface that can arbitrarily manipulate the scattering pattern of terahertz waves and implement many novel functionalities.By optimizing the coding sequences,we demonstrate that the designed 1-,2-,and 3-bit coding metasurfaces with specific coding sequences have the strong ability to control reflected terahertz waves.The two proposed1-bit coding metasurfaces demonstrate that the reflected terahertz beam can be redirected and arbitrarily controlled.For normally incident x-and y-polarized waves,a 10 d B radar cross-section(RCS)reduction can be achieved from 2.1 THz to5.2 THz using the designed 2-bit coding metasurface.Moreover,two kinds of orbital angular momentum(OAM)vortex beams with different moduli are generated by a coding metasurface using different coding sequences.Our research provides a new degree of freedom for the sophisticated manipulation of terahertz waves,and contributes to the development of metasurfaces towards practical applications.

Plasmonic sensor with self-reference capability based on functional layer film composed of Au/Si gratings

We propose a simple one-dimensional grating coupling system that can excite multiple surface plasmon resonances for refractive index(RI)sensing with self-reference characteristics in the near-infrared band.Using theoretical analysis and the finite-difference time-domain method,the plasmonic mechanism of the structure is discussed in detail.The results show that the excited resonances are independent of each other and have different fields of action.The mode involving extensive interaction with the analyte environment achieves a high sensitivity of 1236 nm/RIU,and the figure of merit(FOM)can reach 145 RIU-1.Importantly,the mode that is insensitive to the analyte environment exhibits good self-reference characteristics.Moreover,we discuss the case of exchanging the substrate material with the analyte environment.Promising simulation results show that this RI sensor can be widely deployed in unstable and complicated environments.

Theoretical investigation of hierarchical sub-wavelength photonic structures fabricated using high-order waveguide-mode interference lithograph

This paper presents the theoretical investigation of hierarchical sub-wavelength photonic structures with various periods and numbers of layers, which were fabricated using a high-order waveguide-mode interference field. A 442-nm laser was used to excite high-order waveguide modes in an asymmetric metal-cladding dielectric waveguide structure. The dispersion curve of the waveguide modes was theoretically analyzed, and the distribution of the interference field of high-order waveguide modes was numerically simulated using the finite-element method. The various dependences of the characteristics of hierarchical sub-wavelength photonic structures on the thickness and refractive index of the photoresist and the waveguide mode were investigated in detail. These hierarchical sub-wavelength photonic structures have various periods and numbers of layers and can be fabricated by a simple and low-cost method.

Photonic spin Hall effect and terahertz gas sensor via InSb-supported long-range surface plasmon resonance

We propose a simple one-dimensional grating coupling system that can excite multiple surface plasmon resonances for refractive index(RI)sensing with self-reference characteristics in the near-infrared band.Using theoretical analysis and the finite-difference time-domain method,the plasmonic mechanism of the structure is discussed in detail.The results show that the excited resonances are independent of each other and have different fields of action.The mode involving extensive interaction with the analyte environment achieves a high sensitivity of 1236 nm/RIU,and the figure of merit(FOM)can reach 145 RIU-1.Importantly,the mode that is insensitive to the analyte environment exhibits good self-reference characteristics.Moreover,we discuss the case of exchanging the substrate material with the analyte environment.Promising simulation results show that this RI sensor can be widely deployed in unstable and complicated environments.

Independently tunable dual resonant dip refractive index sensor based on metal–insulator–metal waveguide with Q-shaped resonant cavity

A plasmonic resonator system consisting of a metal–insulator–metal waveguide and a Q-shaped resonant cavity is proposed in this paper. The transmission properties of surface plasmon polaritons in this structure are investigated by using the finite difference in time domain(FDTD) method, and the simulation results contain two resonant dips. The physical mechanism is studied by the multimode interference coupled mode theory(MICMT), and the theoretical results are in highly consistent with the simulation results. Furthermore, the parameters of the Q-shaped cavity can be controlled to adjust the two dips, respectively. The refractive index sensor proposed in this paper, with a sensitivity of 1578 nm/RIU and figure of merit(FOM) of 175, performs better than most of the similar structures. Therefore, the results of the study are instructive for the design and application of high sensitivity nanoscale refractive index sensors.

Ultra wide sensing range plasmonic refractive index sensor based on nano-array with rhombus particles

We propose a two-dimensional metal grating with rhombus particles on a gold film structure for refractive index sensing due to its perfect absorption at near-infrared wavelength.Via two-dimensional metal grating coupling,the incident light energy is effectively transformed into the surface plasmons which propagate along the upper surface of the gold film and interact with the surrounding environment in a wide range.The plasmonic resonance mechanism of the structure is discussed in detail by theoretical analysis and finite-difference time-domain method.After optimizing the geometrical parameters,the designed structure shows the sensing performance with a refractive index sensitivity of 1006 nm/RIU.More importantly,this plasmonic refractive index sensor achieves an ultra wide refractive index sensing range from 1.0 to 2.4 with a stable sensing performance.The promising simulation results of the structure show that the sensor has a broad application prospect in the field of biology and chemistry.

Refractive Index Sensor Based on Fano Resonances in Plasmonic Waveguide With Dual Side-Coupled Ring Resonators

A refractive index sensor based on Fano resonances in metal-insulator-metal （MIM） waveguides coupled with rectangular and dual side rings resonators is proposed. The sensing properties are numerically simulated by the finite element method （FEM）. For the interaction of the narrow-band spectral response and the broadband spectral response caused by the side-coupled resonators and the rectangular resonator, respectively, the transmission spectra exhibit a sharp and asymmetric profile. Results are analyzed using the coupled-mode theory based on the transmission line theory. The coupled mode theory is employed to explain the Fano resonance effect. The results show that with an increase in the refractive index of the fill dielectric material in the slot of the system, the Fano resonance peak exhibits a remarkable red shift. Through the optimization of structural parameters, we achieve a theoretical value of the refractive index sensitivity （S） as high as 1160 nm/RIU, and the corresponding sensing resolution is 8.62 × 10 -5 RIU. In addition, the coupled MIM waveguide structure can be easily extended to other similar compact structures to realize the sensing task and integrated with other photonic devices at the chip scale. This work paves the way toward the sensitive nanometer scale refractive index sensor for design and application.

Plasmonic Refractive Index Sensors Based on One-and Two-Dimensional Gold Grating on a Gold Film

In this paper,we propose two kinds of composite structures based on the one-and two-dimensional(1D&2D)gold grating on a gold film for plasmonic refractive index sensing.The resonance modes and sensing characteristics of the composite structures are numerically simulated by the finite-difference time-domain method.The composite structure of the 1D gold semi-cylinder grating and gold film is analyzed first,and the optimized parameters of the grating period are obtained.The sensitivity and figure of merit(FOM)can reach 660 RIU/nm and 169 RIU-1,respectively.Then,we replace the 1D grating with the 2D gold semi-sphere particles array and find that the 2D grating composite structure can excite strong surface plasmon resonance intensity in a wider period range.The sensitivity and FOM of the improved composite structure can reach 985 RIU/nm and 298 RIU-1,respectively.At last,the comparison results of the sensing performance of the two structures are discussed.The proposed structures can be used for bio-chemical refractive index sensing.