Evanescent wave absorption spectroscopy

Ruddy, V. MacCraith, B. D. Murphy, J. A., Evanescent wave absorption spectroscopy using multimode fibers, J. Appl. Phys. 1990, 67, 6070 6074... 

Modified fiber-optic-based sensors can be used for sensing pollutants, explosives, drugs, pharmaceuticals, and miscellaneous organics (Yeh et al. 2006). Optical fibers coated with porous silica can be used to detect the presence of chlorinated hydrocarbons. Alternatively, these compounds can also be detected using fiber-optic-coupled surface plasmon resonance methods. Aromatic compounds were detected by evanescent wave absorption spectroscopy. Suitably modified fiber-optic array tips can be used to detect presence of explosive materials (Wolfbeis 2000). 

On-chip analyte transport can be realized by integrating microfluidic flow systems or miniaturized gas chambers with the sensor device [69]. Hu et al. demonstrated a microfluidic sensor device monolithically integrated with planar Ge-Sb-S ChG waveguides [70]. Quantitative chemical sensing via evanescent wave absorption spectroscopy was demonstrated using the microfluidic device. 

Figure . Top Standard absorption measurement Bottom evanescent wave absorbance spectroscopy.

In these sensors, the intrinsic absorption of the analyte is measured directly. No indicator chemistry is involved. Thus, it is more a kind of remote spectroscopy, except that the instrument comes to the sample (rather than the sample to the instrument or cuvette). Numerous geometries have been designed for plain fiber chemical sensors, all kinds of spectroscopies (from IR to mid-IR and visible to the UV from Raman to light scatter, and from fluorescence and phosphorescence intensity to the respective decay times) have been exploited, and more sophisticated methods including evanescent wave spectroscopy and surface plasmon resonance have been applied. 

In conclusion, when a WGM is excited in a dielectric microresonator, its evanescent component provides a convenient probe of the microresonator s surroundings. Various ways to implement evanescent-wave sensing have been devised, but the emphasis of this chapter has been on microcavity-enhanced absorption spectroscopy. The techniques described here have broad applicability, can even be used with broadband sources, and lend themselves well to further enhancement methods. We are looking forward to continuing our development of these sensors. 

One can distinguish between methods in which absorption of the evanescent surface wave in different wavelength regions is measured (these are often called attenuated total reflection methods), and methods which use the evanescent wave to excite other, spectroscopic phenomena, like fluorescence and Raman scattering or light scattering. As the methods of conventional fluorescence spectroscopy have been shown to be exceptionally successful in studies of proteins and other biopolymers, their evanescent surface-sensitive counterparts will be reviewed first. 

Water exhibits very strong absorption bands in the mid-IR region, which generally precludes its use as a solvent for IR spectroscopy. Aqueous samples can be analyzed by so-called ATR-spectroscopy (Fahrenfort, 1961 Harrick, 1979), see Sec. 6.4. However, ATR detection limits are often too large. Therefore further effort is necessary to exclude water and to enrich the organic compound in the area where the evanescent wave is penetrating the sample. 

ESCA (electron spectroscopy for chemical analysis) = XF(E)S ESR = electron spin resonance Euler s theorem 1.2.28, 1.2.14a evanescent waves 1.7.75, 1.18, 2.54 EXAFS = extended X-ray absorption fine structure exchange (adsorption from binary mixtures) 2.1, 2.3, 2.4 constant (2.3.16]... 

The use of infrared spectroscopy in the Earth and environmental sciences has been widespread for decades however, until development of the attenuated total reflectance (ATR) technique, the primary use was ex situ material characterization (Chen and Gardella, 1998 Tejedor-Tejedor et al., 1998 Degenhardt and McQuillan, 1999 Peak et al., 1999 Wijnja and Schulthess, 1999 Aral and Sparks, 2001 Kirwan et al., 2003). For the study of environmental systems, the strength of the ATR-Fourier transform infrared (FTIR) technique lies in its intrinsic surface sensitivity. Spectra are collected only from absorptions of an evanescent wave with a maximum penetration depth of several micrometers from the internal reflection element into the solution phase (Harrick, 1967). This short optical path length allows one to overcome any absorption due to an aqueous phase associated with the sample while maintaining a high sensitivity to species at the mineral-water interface (McQuillan, 2001). Therefore, ATR—FTIR represents a technique capable of performing in situ spectroscopic studies in real time. 

With attenuated total reflection spectroscopy, the light absorption by the electrolyte solution and the cell window is no obstacle. The probe beam enters a crystal transparent for infrared light. It is directed to the outer surface of the crystal, which is coated with a thin layer of the electrode material under investigation. The beam is reflected, but a small part (the evanescent wave) penetrates the surface and thus can probe species located immediately on the electrode surface. The returning beam contains exactly this information. As discussed below (p. 91) in detail, this approach shows also serious limitations. 

According to the Lambert—Beer s law the absorbance is linear with the absorption coefficient, concentration c and volume of the probed sample. Assuming that the absorption coefficient, concentration and cross-section do not change, it can be supposed that the absorbance of a sample is linear with thickness. This is actually true for most data recorded using transmission or similar sampling techniques. In the case of ATR spectra the exponential decay of the evanescent wave has to be taken into account if the band area and film thickness are to be correlated. For use in ATR spectroscopy, the Lambert—Beer s law can be empirically modified to ... 

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