Absorption Sensing And Photoelectric Optical Bistability Based On The Symmetrical Metal-cladding Waveguide

Symmetrical metal-cladding waveguide(SMCW)and three layers of the dielectric slab waveguide have the same structure,which make up by coupling layer,waveguide layer and the substrate layer.But in symmetrical metal-cladding waveguide,both the coupling layer and the substrate layer are metallic material.Usually,the metallic material is precious metal such as gold or silver,whose real part of the dielectric constants is a big negative in optical frequency region.This characteristic makes the guide have many unique properties.Such as the effective refractive index region of guided mode is very large.So this kind of waveguide allow free-space coupling and allow gas and solution to be the waveguide layer.What's more,these ultra-high guided modes of sub-millimeter scale waveguide have polarization independence,slow wave property and field enhanced effect.It's also extremely sensitive to the refractive index and thickness of waveguide layer,and the wavelength of incident light.All of these make it have a potential application in bio-chemical sensor.The attenuated total reflection curve of symmetrical metal cladding waveguide shows that,when the imaginary part of dielectric constant of waveguide layer has a small change,it will lead to a notable change in the minimum reflectivity.It is found that the thickness of the metal coupling film is a critical factor.With a thin metal coupling film,the minimum reflectivity decreases at first,reaches zero,then increases gradually when extinction coefficient of the sample increases.While in another case,the minimum reflectivity always increases as increasing extinction coefficient of the sample.Based on these characteristics,a novel optical scheme for absorption sensing is proposed.The theory shows that the slope of the minimum reflectivity curve becomes steeper and thus the sensitivity becomes higher as increasing the thickness of coupling layer.However,a very high sensitivity comes at a price of a narrower sensing range.And the minimum reflectivity is a uniform function of the extinction coefficient of sample when the thickness of the coupling metal layer being more than the critical thickness.For equilibrium consideration,the SMCW with a critical thickness of metal coupling film can be employed in the actual experiment.The experimental measurements of the methylene blue concentration confirm the sensitivity analysis well,and obtain a high sensitivity.The attenuated total reflection curve of symmetrical metal cladding waveguide also shows that,the resonance absorption peak will shift when the real part of dielectric constant of waveguide layer changed.That to say,when we fix the incidence angle and the incident wavelength,the reflectivity will changes as the refractive index or thickness of waveguide layer changes,so we have an photoelectricoptical bistable device based on symmetrical metal cladding waveguide.The device uses lithium niobate crystal as the waveguide layer of symmetrical metal cladding waveguide.Where we transform part of reflected light to voltage signal,and use an amplifier to enlarge it.Then load it on the lithium niobate crystal.The inverse piezoelectric effect and electro-optic effect will provide a photoelectric hybrid feedback mechanism.We have realization of optical bistability in a very low intensity of incident light.The numerical calculation shows that the closer the incident angles to the resonance angle,the lower the threshold value of bistability.