Effect Of The Infrared Properties Of Sodium Nitrate And Ethanol By Photonic Crystals

Infrared （IR） absorption spectroscopy can be endowed with a level of chemical specificity by probing the vibrational modes associated with the various molecular bonds, and then is used for the analysis of structures of the molecules and the quantitative detections. IR absorption spectroscopy is with the advantages which low cost, high analysis precision, detection speed, repeatability, use less and so on. However, solvent like water and ethanol, as an essential component in measurement of solution, presents the major obstacle. The absorption of their OH groups （bending and/or stretching vibration）, which have strong absorption on infrared light, can overlap some signal around.The 2D photonic crystal（PC） of nickel （Ni）, which has properties of filtering and surface plasmonic effects, can enhance the IR absorption of the samples. In this letter, we examined the effect of the PC on the IR absorption of sodiμm nitrate and ethanol by means of FTIR spectroscopy. The main contents are as following:First of all, the rigorous coupled-wave method was used to study the infrared spectral characteristics of 2D photonic crystal of Ni. Through the simulation calculations, we examined the effect of periods, the sizes of apertures, the thickness of films and angles of incidence of light on the transmission characteristics of photonic crystal. The results of simulation calculations showed that:the period of the photonic crystal could produce some phenomena such as redshift, split, changes of intensities. Especially, there had a transmitted minimum at wavelength of the period. Moreover, the sizes of apertures and thickness of films had no influence on shift of resonance peaks, but widths and transmission intensities of the transmitted minimμm could change. Furthermore, with the increase of angle, the transmission spectra also shifted, splited and intensities changed accordingly. At vertical incidence of infrared light, the transmission intensities of two-dimensional photonic crystal（Ni, P=10μm） was minima at 1000cm^-1、2100cm^-1, and was maximμm at 1500-1600cm^-1.These was basically same with theoretical results, and have important theoretical significance to design photonic crystal filters.Secondly, we examined the effect of the PC on the IR absorption of sodiμm nitrate and ethanol by means of FTIR spectroscopy, when the PC was considered as a filter. First of all, we examined the effect of the PC on the IR absorption of sodium nitrate. The experimental results indicated that the intensities of antisymmetric stretching vibrations of nitrate at 1300 cm^-1～1500cm^-1 were improved obviously by the effect of photonic crystal. What is more, with the angle increased, the intensity at 1350cm^-1 decreased, while the intensity at 1400cm^-1 increased. And then we also examined the effect of the PC on the IR absorption of ethanol. The results showed that IR intensities of C-H bending vibrational peaks (1300～1500 cm^-1) and stretching vibrational peaks (2800～3000 cm^-1) of ethanol are increased. In this case, the PC is considered as a filter, which is used to regulate the intensities of IR light. Hence the PCs have no influence on the positions of vibrational peaks of sodiμm nitrate and ethanol.Finally, we studied the changes of IR spectra when there had ethanol on the surface and in the hole of the PC. The experimental results showed that this case can make the IR transmission spectra shift, split and intensities change because the refractive index of holes was changed. And then we recorded time-series IR spectra of ethanol on PC during evaporation. The bending vibrational peak of CH (1200cm^-1～1500cm^-1) disappeared first, and then OH stretching vibration (3200cm^-1) and C-O stretching vibration (1050 cm^-1) disappeared in turn. The peak of CH3 antisymmetric stretching vibration (2970cm^-1) disappeared in the end. It proved that the photonic crystal can not only modulate the intensities of IR light, but also selectively enhance the intensities of IR absorption.The infrared absorption enhancement of photonic crystal, especially selective enhancement, can improve the detection limit of infrared spectroscopy, which has practical value for detecting low concentration samples in water or ethanol.