Research On Improving The Performance Of Grating Coupled SPR Sensors

The surface plasmon resonance(SPR)sensor is a label-free and highly sensitive optical sensor.Its principle is that when incident light excites SPR at the interface between the metal and the medium,the refractive index of the medium can be obtained with one-to-one correspondence to the resonance wavelength.Current commercial SPR sensors are all based on prism coupling.Although the sensitivity is very high,the device is bulky and expensive.With the maturity of nano-fabrication processes and people's urgent need for miniaturization of sensor devices,grating coupled SPR sensors have emerged.Current grating coupled SPR sensors have problems of low sensitivity and inability to detect opaque solutions,which limit the popularization and application of such sensors.In this paper,through theoretical analysis and simulation calculations,the grating structure is optimized and the sensitivity of the sensor is improved effectively.In the experiment,a variety of methods are used to prepare grating sensor chips with different periods,and the corresponding SPR sensors are built.A new back-illuminated grating coupled SPR sensor is proposed,which successfully detected colored and opaque solutions.The research details are as follows:(1)Considering the sensitivity of the sensor,the full width at half maximum and the depth of the SPR absorption peak,a new evaluation parameter FOM*is defined.According to FOM*,three grating coupled SPR sensors with different structures are optimized.Through the finite difference time domain method,the influences of grating period,grating depth,metal film thickness and duty cycle on the performance of SPR sensors based on sinusoidal gratings,continuous metal film rectangular gratings and discontinuous metal film rectangular gratings are analyzed.The results show that FOM*of the optimized grating coupled SPR sensor can reach up to 123.70.The corresponding sensitivity is 568.46 nm/RIU,the average full width at half maximum is 3.31 nm and the average absorption peak depth is 72.03%.(2)We theoretically analyze the sensitivity limit of grating coupled SPR sensors,provide design methods to reach the maximum sensitivity and the way to increase the sensitivity limit.The theoretical analysis shows that metal gratings with periods as large as possible should be used and the incident angle should be set close to 90° in order to maximize the sensitivity for a certain detecting wavelength.Moreover,the sensitivity limit increases as the detecting wavelength increases.Experimentally,we prepare three gratings with periods of 314 nm,1470 nm and 6733 nm by stripping commercial optical discs or photolithography.The measured sensitivities of the sensors based on these gratings are 319.96 nm/RIU,1477.74 nm/RIU and 2077.26 nm/RIU,respectively.The sensitivity achieved in this article is much higher than existing ones due to the use of design method following the theoretical analysis.(3)A back-illuminated grating coupled SPR sensor is proposed to measure the refractive index of colored and opaque solutions.Polydimethylsiloxane is used to imprint DVD-R discs to obtain a transparent grating substrate,on which a 35 nm gold film is evaporated to make a sensor chip.Different from the traditional grating coupled SPR sensor,the incident light in this paper is incident on the metal layer from the substrate side,and the SPR at the interface between the metal and the measured solution is excited.This structure can effectively avoid the partial absorption of incident light when passing through the solution.By measuring the absorption and reflection spectra of different concentrations of glucose solution,blue engine coolant,green antirust cutting fluid and white antirust emulsified oil,the designed sensor is proved to be able to detect colored and opaque solutions.The optimization design method proposed in this paper can greatly improve the sensitivity and detection limit of the grating coupled SPR sensor.The grating preparation method we used has the advantages of simple steps,low cost,and good SPR excitation effect.The sensor we designed can be widely used in the fields of medical diagnosis,environmental monitoring and biological protein detection.