Spectral Depth Profiling of Arbitrary Surfaces by Thermal Emission Decay—Fourier Transform Infrared Spectroscopy

Abstract

We report a new spectroscopic technique that combines step-scanning Fourier transform infrared spectroscopy with opto-thermal transient emission radiometry (OTTER) in order to provide near-surface depth-resolved spectra in the range 700–1800 cm−1. It works nondestructively, without contact, with samples of arbitrary shape and size, without requiring prior preparation. The depth of surface probed depends on the thermal diffusivity of the sample; for organic materials it is ∼10 μm. With homogeneous samples, absolute absorption coefficients can be measured. With two-layered samples, the technique proved able to distinguish between the spectral properties of the top layer and the substrate and to estimate the thickness of the top layer. We present a theoretical analysis with the main design features of the instrumentation and software, together with studies of homogeneous and layered samples, to validate the methods and illustrate the potential of the technique for practical applications.

Raman spectroscopy: techniques and applications in the life sciences

“…Various infrared spectroscopic depth profiling techniques exist, but they are usually limited to the range of 1 to 500 microns [382][383][384][385][386][387]. Initial attempts with Raman spectroscopy relied on exploiting the time-dependent exit of the Raman photons from a turbid sample, which correlated with the likely depth a photon scattered within the medium [388].…”

Photothermal Radiometry for Skin Research

“…It is interesting to point out that, immediately after soaking, this curved feature seems to disappear, which might suggest excess water absorbed during soaking has gone into different layers in hair. It is also interesting to point out that it takes more than 40 min for hair to recover to its normal hydration level, this is slower than the skin and the nail in our previous studies [10,26]. Additionally, as hair gradually loses its excess water to the ambient environment, the curved feature of the hair hydration depth profile started to re-appear.…”

“…By combining photothermal radiometry and Fourier Transform InfraRed spectroscopy (FTIR) [9,26,27] or diffraction grating [28], it is possible to achieve photothermal spectroscopy. Different from traditional FTIR, photothermal spectroscopy is depth-resolved, which means we can get the spectra information from both the skin surface and at different depth underneath the skin surface.…”

Mid‐infrared in vivo depth‐profiling of topical chemicals on skin

“…(For detailed description see Refs. (39, 40). ) A low‐power‐pulsed Er‐YAG laser (emission wavelength 2.94 μm, pulse energy ∼3 mJ, beam diameter ∼5 mm, 100 ns pulse duration, and 5 Hz pulse repetition) were used as excitation source because its emission wavelength overlaps with the strong absorption band of water in SC.…”