Experimental Research On Liquid Raman Detection Based On Photonic Crystal Fiber

Raman spectroscopy is a powerful analytical tool that probes vibrational fingerprints of molecules and enables high-content analysis of composite systems of physical,chemical,and biological interests by virtue of its inherent specificity.Therefore,Raman spectroscopy is widely used in chemical detection,disease diagnosis and environmental monitoring.However,The low sensitivity of Raman spectroscopy severely limits its practical applications,which range from trace-amount molecular detection to real-time bioimaging.Surface-enhanced Raman spectroscopy(SERS),as a surface-sensitive Raman technique that enables significant enhancement of Raman signals of adsorbed molecules on an engineered surface,has been considered as a promising technique to overcome the low sensitivity of traditional Raman spectroscopy.In this paper,based on the SERS probe of PCF,an experimental study on Raman detection of liquid molecules is carried out.First,the axial distribution filling truncation method and the femtosecond two-photon polymerization method are used to achieve selective blocking of the PCF,and secondly,the liquid filling of the PCF is realized by the siphon method and the pressure difference method.Finally,the preparation method of SERS probe based on PCF with a subwavelength air core is introduced and the prepared optical fiber SERS probe is used for Raman detection of the aqueous solution of rhodamine molecule.The specific research contents are as follows:Axial distribution filling cut-off method and femtosecond two-photon polymerization method were used to achieve selective blocking of PCF air holes.The axial distribution filling cut-off method is simple to operate,does not require complicated equipment,but is only suitable for the selective sealing of air holes of different diameters;the femtosecond two-photon polymerization method requires the precise operation of the femtosecond laser and microscopy system,but it can truly achieve PCF any structure,any air hole is selectively blocked.The liquid filling of PCF air holes was achieved by siphon method and pressure difference method respectively.For different air hole sizes,two sets of pressure difference filling devices,high-pressure injection pump and nitrogen pressurization,were constructed in the experiment to achieve efficient and uniform filling of subwavelength air holes.The sub-wavelength air hole of the defective PCF core has a strong light field distribution,and it is based on the total internal reflection light guide mechanism,and liquid filling does not require complicated fiber post-processing.Filling the liquid to be tested into the air hole of the fiber core can greatly improve the interaction efficiency of the liquid to be tested and the light field.Therefore,the defective PCF has unique advantages in liquid Raman detection.In this paper,based on the physical drying method and the chemical bonding method,the SERS substrate of the defective PCF was prepared,and the experimental comparison and analysis of the Raman enhancement efficiency in the presence and absence of nanoparticles in the inner wall of the optical fiber and the presence or absence of nanoparticles in the liquid to be tested The results show that when the alloy nanoparticles are bonded to the inner wall of the optical fiber and there are gold nanoparticles in the test solution,the enhancement effect is most significant,and the lowest concentration of rhodamine solution that can be detected is 10^-8 mol/L.In order to overcome the problem that the confocal Raman spectroscopy microscope has a complex structure,a large volume,and cannot be monitored on-site in real time,a real-time liquid Raman detection device based on PCF is built based on a portable Raman spectrometer.The same detection limit as the confocal Raman spectrometer is achieved.