Theory of Mid-infrared Absorption Microspectroscopy: II. Heterogeneous Samples

Abstract

Fourier transform infrared (FT-IR) spectroscopic imaging combines the specificity of optical microscopy with the spectral selectivity of vibrational spectroscopy. There is increasing recognition that the recorded data may be dependent on the optical configuration and sample morphology in addition to its local material spectral response, but a quantitative framework for predicting such dependence is lacking. Here, a theory is developed to relate recorded data to the spectral and physical properties of heterogeneous samples. The modeling approach combines optical theory through rigorous coupled wave analysis with modeling of sampling geometry and sample structure. The interplay of morphology and dispersion are systematically explored using increasingly sophisticated samples to illustrate the dependence of the detected optical intensity on the spatial sample structure. Predictions of spectral distortions arising from the sample structure are quantified, and experimental validation of the developed theory is performed using a microfabricated standard from a commercial photoresist polymer. The developed framework forms a basis for understanding sample induced distortions in spectroscopic IR microscopy and imaging.

Microspectroscopy as applied to the study of wood molecular structure

“…Light scattering and effects due to heterogeneity within samples not only lead to loss of spatial information but may also cause band shifts. The latter effect due to resonant Mie scattering is typically observed on very small objects and sample margins (Bassan et al 2009;Davis et al 2010Davis et al , 2011.…”

Medical Applications of Infrared Spectral Imaging of Individual Cells

“…May it suffice here to state that these effects are due to the morphology of the sample; in particular, spherical particles exhibit these effects particularly strongly. However, as demonstrated theoretically by Bhargava and coworkers [11,12], any sample with strong gradients of the refractive index (for example, a straight edge of a particle boundary) will exhibit these effects as well. At present, there exist several computational approaches to correct these phenomena, based on either an iterative numerical process [14], a step-wise approximation method [15], or a phase-correction approach that was suggested as early as 2005 [9,16,24].…”

BIM-Sim: Interactive Simulation of Broadband Imaging Using Mie Theory

“…In addition, many plotting algorithms are available for creating Mie scattering plots for specific cases such as coated [13] and multi-layered spheres in shaped beams [14]. Finite element methods are generally used for non-spherical particles [15], while general solutions are known for fibers [16,17]. These computational methods are often based on the BHMIE method proposed by Bohren and Huffman [8].…”

“…Recent methods have been proposed for creating a general framework for simulating scattering in microspectroscopy [16,19], with the goal of correcting artifacts. These methods evaluate cross-sections of the EM field near the sample.…”