The effect of deformation of absorbing scatterers on Mie-type signatures in infrared microspectroscopy

Abstract

Mie-type scattering features such as ripples (i.e., sharp shape-resonance peaks) and wiggles (i.e., broad oscillations), are frequently-observed scattering phenomena in infrared microspectroscopy of cells and tissues. They appear in general when the wavelength of electromagnetic radiation is of the same order as the size of the scatterer. By use of approximations to the Mie solutions for spheres, iterative algorithms have been developed to retrieve pure absorbance spectra. However, the question remains to what extent the Mie solutions, and approximations thereof, describe the extinction efficiency in practical situations where the shapes of scatterers deviate considerably from spheres. The aim of the current study is to investigate how deviations from a spherical scatterer can change the extinction properties of the scatterer in the context of chaos in wave systems. For this purpose, we investigate a chaotic scatterer and compare it with an elliptically shaped scatterer, which exhibits only regular scattering. We find that chaotic scattering has an accelerating effect on the disappearance of Mie ripples. We further show that the presence of absorption and the high numerical aperture of infrared microscopes does not explain the absence of ripples in most measurements of biological samples.

Domes and Semi-Capsulesas Model Systems for Infrared Microspectroscopy of Biological Cells

“…However, when L > 0, these whispering gallery modes die out since the deformed shapes do not support standing waves along the boundary. This is corroborated by the results reported in reference 13 . For polarization of the electric field parallel to the elongated axis, see Appendix B.…”

“…To calculate the total amount of radiation removed from the forward direction, either by absorption or by scattering, we looked at the dimensionless extinction efficiency, Q ext , defined as the ratio of the extinction cross section and the geometric cross section of the sample 10 . The following equation shows the relation between the apparent absorbance and the extinction efficiency 12,13 :…”

“…In infrared absorbance measurements, pollen grains display close to perfect Mie-scattering signatures, with both wiggles and ripples 12 . Ripples are sensitive to the shape of the scatterer, and it has been shown that shape deformations can suppress or even eliminate ripples in infrared spectra 13 .Therefore, in addition to direct observation with optical microscopy, the presence of ripples is further indication that pollen grains retain their spherical shape.…”

Attenuated Total Reflection (ATR) Micro-Fourier Transform Infrared (Micro-FT-IR) Spectroscopy to Enhance Repeatability and Reproducibility of Spectra Derived from Single Specimen Organic-Walled Dinoflagellate Cysts

“…35 These scattering effects cause spectral distortions in terms of absorption band intensities, band positions, relative band proportions, and baseline effects and thus obscure the true absorbance spectrum. 36– 50 For some sample setups and geometries, these spectral distortions can be partly quantified and corrected with model-based correction algorithms which often need pure absorbance spectra as a reference. 37,51–59 In the past few years, deep learning approaches have made their advance in FT-IR spectroscopy.…”

Effects of the coupling of dielectric spherical particles on signatures in infrared microspectroscopy

“…Some studies have been performed on the interpretation of scattering characteristics of infrared microscopy spectra of biological samples 11 , 12 . Since biological cells or tissues are not always spherical samples, the effect of the deformation of biological samples on the Mie-type signatures and the chaotic scattering caused by sample deformation has been studied in 11 . It is also very common that the sample is not a single particle.…”