Research On Optical Biosensor Based On Guided Mode Resonance

Optical refractive index(RI)sensors capable of high sensitivity,immunity to electromagnetic interference,non-destructive,non-contact,real-time montoring and multiplexing capability are preferred for detecting low molecular-weight substances such as glucose and biotin of the antibody-antigen interactions,and adenine,thymine,and so on of the binding reactions between complementary DNA strands in the field of biochemistry.The optical RI sensors demodulate by measuring the resonant wavelength,or angle,light intensity,and interferometric phase.There is less transmission loss of incident wave for the guided mode resonance(GMR),so ultra-narrow half-width modulation of resonance angle or wavelength can be realized,and then high resolution is achieved by using the GMR sensor.In this paper,the GMR are studied based on the equivalent medium theory(EMT),rigorous coupled wave method(RCWA)and finite-difference time-domain method(FDTD),which includes the following as:A subwavelength grating for highly-sensitive RI sensing is proposed.The light-coupling condition of the grating covered by the liquid to be detected is sensitive to changes in RI of the liquid.The influence of the grating period and thickness on the coupling is studied.At the large angle of incidence,it is found the effective RI of the grating slab is varied with the incidence angle,and for which,then the coupling orignated from the GMR impacted by such variation is revealed.To scan incidence angle at a fixed wavelength of 623.8nm,the calculated results indicate the sensor sensitivity is mainly depended on the period,while the resolution is controlled by the thickness.After the period and thickness have been optimized in sequence,with that an averaged sensitivity of249.70/RIU and resolution ranging from 0.1° to 0.3° are obtained,which of the grating sensor are better than that of the SPR sensor who has high sensitivity supported by the substrate with low RI.An optical refractive index(RI)sensor,possessing the excellent sensing characteristics in sensitivity,resolution and SNR,is proposed by using a composite subwavelength grating(CSG).The CSG has a sandwich structure: the interlayer is a low RI grating layer(LRGL),and the two sides are arranged with the high RI grating layer(HRGL).The light-coupling condition of the SCG is sensitive to RI changes of the liquid sample.The calculated results indicate that the sensitivity mainly depends on the SCG period,and the resolution and SNR can be improved by regulating thickness and position of the LRGL.With optimizing these parameterts,finally,an averaged sensitivity of ?273°/RIU and resolution ranging from 0.1° to 0.15° are given by this CSG,there is the minimal refelectance of ?1% adjacent to the resonant peak.A surface relief grating(SRG)structure supported by dielectric film on metal substrate is designed.It is proved that the structure can be used for high quality factor optical RI sensing.The influences of grating period,thickness,substrate thickness and dielectric film thickness on the resonant coupling of the SRG sensor are studied.The calculation results show that the sensitivity of this sensor is mainly determined by the period,and the resolution is controlled by the thickness of the grating substrate.After optimizing the period and thicknesses,the sensitivity is about 110°/RIU,and the resolution is 0.00003°.