Advancements in quantum cascade laser-based infrared microscopy of aqueous media

Abstract

The large mid-infrared absorption coefficient of water frequently hampers the rapid, label-free infrared microscopy of biological objects in their natural aqueous environment. However, the high spectral power density of quantum cascade lasers is shifting this limitation such that mid-infrared absorbance images can be acquired in situ within signal-to-noise ratios of up to 100. Even at sample thicknesses well above 50 μm, signal-to-noise ratios above 10 are readily achieved. The quantum cascade laser-based microspectroscopy of aqueous media is exemplified by imaging an aqueous yeast solution and quantifying glucose consumption, ethanol generation as well as the production of carbon dioxide gas during fermentation.

On the role of interference in laser‐based mid‐infrared widefield microspectroscopy

“…The calculation of absorbance in the absence of an object seemingly mitigates the impact of interference since the absorption is calculated by division of 2 identical intensity patterns. In passing, we would like to note that the time averaging in pseudothermalsource microspectroscopy facilitates the pixel-to-pixel noise as well as the single-pixel noise to remain as low as shown previously [22] at approximately 1 × 10 −3 a.u. despite the introduction of scattering surfaces (see Figure 5).…”

“…In order to evaluate the performance of the plain laser source and the pseudothermal source when measuring the absorbance of an empty scene, we calculated the values for single‐pixel and pixel‐to‐pixel noise for the measurements presented in Figure (see Table ).…”

“…In order to evaluate the performance of the plain laser source and the pseudothermal source when measuring the absorbance of an empty scene, we calculated the values for single-pixel and pixel-to-pixel noise [22] for the measurements presented in Figure 5 (see Table 1). Table 1 reveals that by using the pseudothermal source, values for single-pixel and pixel-to-pixel noise tend to be slightly higher compared to when using the plain laser source.…”

“…Bottom row: picture detail of an absorption image of a tissue thin section measured at 1051 cm −1 [8]. The length of the bars amounts to 500 μm TABLE 1 Comparison of the single-pixel and pixel-to-pixel noise [22] calculated from measurements without a sample when using a plain laser source and a pseudothermal source Single-pixel noise of measurement without sample (a.u. )…”

“…At any given wavelength, the pixel-to-pixel noise corresponds to the SD of absorbance values among all pixels (see refs. [12,22]). In order to allow for a quantitative analysis, coherence properties may be calculated from the observed intensities by virtue of the Siegert relation [17,23,24]:…”