Investigation Of Whispering Gallery Modes Microcavity Gas Sensor

Microresonators that support the whispering gallery mode(WGM)have been used as high-sensitivity sensors for many years due to their ultra-high Q factor and very small mode volume.For example,microspheres,microdisks or microrings have been reported for a variety of applications,namely temperature and biochemical sensing.Since the dielectric material itself acts as a microcavity,where light is bound by continuous total internal reflection.These devices have optical quality factors greater than 105,and WGM-based microcavity sensors typically provide higher resolution,which has been used for the detection of individual nanoparticles.In the past decade,a relatively new WGM resonator,microbubble,consisting of a thin spherical shell,can support the whispering mode.Compared with the traditional solid WGM resonator,the microbubble not only inherits the solid resonator.Many of the features,sensing can also occur on the inner surface of the structure,while the WGM can still be excited by an external coupler(e.g.a tapered fiber)through the evanescent field.Due to its thin-walled structure,the optical mode produces a strong evanescent field with its inner surface interacting with the outer surface.When the core of the microbubble is filled with a suitable fluid,most of the optical modes are greatly affected by changes in the surrounding refractive index.This forms the basis of very sensitive microfluidic sensing.At present,there are some unavoidable shortcomings of carbon dioxide(CO2)gas sensors based on optical fiber measurement,such as low sensitivity,long response time and small detection range.Therefore,we need to explore a carbon dioxide gas sensor that overcomes the above disadvantages and is simple and easy to manufacture,such as WGM microbubbles and solid polystyrene(PS)microspheres.In this paper,we propose two high-sensitivity CO2 gas sensors with different structures,with WGM microbubbles and PS microspheres as research objects.The first sensor realizes high-sensitivity CO2 gas sensing based on the polymer PHMB coated microbubble inner wall;the second sensor,based on the polymer material functionalized PS microsphere integrated into the micro-flow chip CO2 high-sensitivity gas sensor;they were experimentally confirmed.The specific research work can summarize the following two aspects:(1)We proposed a CO2 gas sensor based on PHMB coated WGM microbubble resonator and verified it experimentally.First,we use a heat and pressure method to make microbubbles using two reverse arc discharges focused on a microcapillary;The inner wall of the microbubbles is coated with a layer of PHMB by a filling and sintering process;The WGM microbubble cavity is characterized by a microscope and a scanning electron microscope image;The transmission spectrum is obtained by coupling with a tapered fiber.A typical WGM resonance is observed,and the variation of Q value and FSR is analyzed.Then,based on the characteristics of WGM resonance formation,the resonance shifts with the change of the effective refractive index of the WGM microbubble core.The different concentrations of CO2 are injected into the microbubble resonator to observe the spectral changes.The experimental results show that as the concentration of carbon dioxide increases,a blue shift appears in the spectrum.In addition,a high sensitivity of 0.46 pm/ppm and a good linear relationship were obtained in the measurement range of 200-700 ppm.By using COMSOL Multiphysics to establish the electric field distribution simulation of the resonant wavelength in the microbubble wall and changing the wall thickness,it is theoretically verified that almost no light extends toward the inner wall surface.Finally,different concentrations of gas were injected into the microbubbles to explore the dynamic response of the sensor and accurately estimate its response and recovery time;the different types of gases were injected into the microbubbles to verify their selectivity;the same concentration and different time periods were tested to observe the repeatability and stability.The sensor has the advantages of high sensitivity,simple structure,easy manufacture,and low cost.Optimize the thickness of the functional layer by using a microbubble resonator with a higher quality factor and a deeper extinction ratio,and the size of the microbubble waveguide is adjusted for a given operational polarization to achieve a stronger interaction between the optical mode and the functional layer.(2)We fixed dye-doped PS microspheres coated with PHMB material in a microfluidic chip made of polydimethylsiloxane(PDMS),and proposed a novel structure of CO2.The gas concentration sensor was confirmed by experiments.In the experiment,the pulsed laser is focused by the microscope lens focusing method onto the surface of the microfluidic chip,and is excited to the surface of the microsphere through the chip,and the typical WGM laser generated by the microsphere is generated.By injecting different concentrations of CO2 gas into the chip,the WGM resonance wavelength changes in real time,and the experimental results show a higher sensitivity(for mode TE2041)of 7.36 pm/ppm.At the same time,the experimental results show that the micro-flow carbon dioxide gas sensor is reversible and reproducible,and the sensitivity remains almost unchanged for a relatively long period of time.The time from when the gas enters the channel to when the resonant wavelength changes,that is,the response time of the sensor is about 25 s.The research in the experiment can be proved as a concept and can be extended to the field of higher quality WGM microcavity gas sensing.