| Surface Plasmon Polariton(SPP)is a transverse magnetic mode(TM)surface wave that formed by the interaction of photons with free electrons on a metal surface.It can propagate at the metal-insulator interface and the field strength is perpendicular to the interface.The field strength decays exponentially in the insulator on both sides perpendicular to the interface direction,and the energy is maximum at the interface.Surface Plasmon Wave(SPW)can break through the photon restriction by the diffraction limit,realize the local limitation of the sub-wavelength scale and has a wide range of applications in spectroscopy,nanophotonics,imaging,and biosensing.When the metal is very thin,about tens of nanometers,it is called short-range surface plasmon polariton(SRSPP)if the refractive index of the insulators on both sides of the metal is different,the oscillation weakens,the loss increases,and the propagation distance is greatly reduced.It is called long-range surface plasmon polariton(LRSPP)if the insulators on both sides of the metal have the same refractive index,the surface plasma waves at the upper and lower interfaces of the metal-insulator will be coupled to each other,and the oscillations will be strengthened,which can propagate a longer distance.Optical sensors based on LRSPP waveguides mainly work on the wavelength of 1310 nm and 1550 nm,and are mostly discrete unit devices.Further research is needed on integration with light sources and detectors,and low-cost detection.This article mainly studies optical waveguide devices based on the surface plasmon effect,including LRSPP straight waveguide refractive index sensors and input / output grating couplers.In this paper,the finite-difference eigenmode(FDE)method is used to study the optimal conditions for detecting refractive index of the LRSPP waveguide at 850 nm and the optimal geometric parameters of the waveguide are determined.The Lift-off method is used to fabricate the LRSPP straight waveguide to test the optical transmission characteristics.In order to integrate the waveguide sensor with the light source and the detector,an input / output grating coupler based on a polymer waveguide grating is theoretically designed.This paper mainly researches systematically from the following aspects: 1.LRSPP straight waveguide refractive index sensorLRSPP straight waveguide refractive index sensor is a bulk refractive index sensor composed of a gold waveguide embedded in a UV-sensitive polymer SU-8.At the excitation wavelength of 850 nm,the size of the gold waveguide was simulated and optimized by the FDE method.When the width of the gold waveguide is 1,1.5,2,2.5,and 3 ?m,the thickness of the gold waveguide is 15,20,25 and 30 nm,the refractive index range of the analyte is 1.562 to 1.586,and the refractive index step is 0.004,Coupling loss(CL),mode power attenuation(MPA),and sensitivity(S)of the sensor were simulated.The results show that the sensitivity of the sensor is 195.69 and 187.68 dB / RIU / mm at 850 nm when the width of gold waveguide is 1.5 ?m and the height is 25 nm and the refractive index of the analyte is less than or greater than the refractive index of SU-8.A series of 1.5 ?m wide and 25 nm thick LRSPP gold waveguides were prepared by the Lift-off method,the size and surface roughness of the gold waveguide were characterized by atomic force microscope.The insertion loss(IL)of the LRSPP straight waveguide of 1.88,2.40 and 2.60 mm was tested by an established optical system and the results show that the transmission loss is about 13.09 dB / mm,which is similar to the simulation result of 12.01 dB / mm,indicating that the sensor design has better experimental feasibility.2.Input / output grating couplerIn order to solve the problems of large fiber mode and mode field size mismatch in the traditional vertical coupling method,which leads to large coupling loss,an efficient polymer waveguide grating coupler with directionality enhancement is proposed.The upper cladding layer is air,and the waveguide core layer is a polymer SU-8 with a refractive index of 1.58.In order to increase the refractive index contrast,the lower cladding layer is a fluorinated polymer CYTOP with a refractive index of 1.33.Based on the diffraction effect of the grating,the grating parameters are calculated and optimized using the finite-difference time-domain(FDTD)method to enhance the intensity of the forward diffracted light perpendicular to the direction of the incident / exit light.To prevent light energy leaking to the substrate,a gold film reflector is introduced under the grating coupler and the part of transmitted light is reflected back to the coupling grating,thereby improving the coupling efficiency;a grating reflector connected in series with the coupling grating can further increase the coupling efficiency.With the enhancement of directionality,an optimized coupling efficiency of 51.31% can be obtained at the wavelength of 1550 nm.The 62 nm-wide 1-dB bandwidth that covers the wavelength range from 1520 to 1582 nm is favorable for wide band operation. |
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