Direct mineralogical imaging of economic ore and rock samples with multi-modal nonlinear optical microscopy

Abstract

Multi-modal nonlinear optical (NLO) microscopy, including stimulated Raman scattering (SRS) and second harmonic generation (SHG), was used to directly image mineralogical features of economic ore and rock samples. In SRS/SHG imaging, ore samples generally require minimal preparation and may be rapidly imaged, even in their wet state. 3D structural details, at submicron resolution, are revealed tens of microns deep within samples. Standard mineral imaging based on scanning electron microscopy (SEM), with elemental analysis via energy dispersive X-Ray spectroscopy, was used to independently validate the mineral composition of the samples. Spatially-resolved SRS from dominant Raman-resonant bands precisely maps the locations of specific minerals contained within the samples. SHG imaging reveals locally non-centrosymmetric structures, such as quartz grains. Competing absorption and nonlinear scattering processes, however, can reduce contrast in SRS imaging. Importantly, the correlation between standard electron microscopy and multi-modal NLO optical microscopy shows that the latter offers rapid image contrast based on the mineral content of the sample.

Label-Free Multiphoton Imaging of Microbes in Root, Mineral, and Soil Matrices with Time-Gated Coherent Raman and Fluorescence Lifetime Imaging

“…Our results support the findings of previous groups that have used multiphoton SHG and SFM imaging to image minerals. 49,64,65 SHG and SFM are more prominent in minerals than in organic materials, providing additional contrast mechanisms useful in complex samples. Hence, the combination of TPEF, CARS, FLIM, SHG, and SFM could be particularly useful for studying mineral−microbe associations.…”

“…38,39,43,53 CARS overcomes the signal strength limitations of traditional spontaneous Raman scattering using multiphoton coherent excitation of molecular bonds to increase the signal strength by several orders of magnitude. 40,41 Although spontaneous Raman scattering has been previously used in geoscience, plant microbiology, and bacteriology to obtain molecular vibration information, 54−61 it has a weak signal, requires long image acquisition times of up to several hours, 62,63 and can be overwhelmed by autofluor- 42 Although CARS is a new application for soil imaging, it has been previously applied to minerals studies, 49,64 such as microfractures in a quartz crystallographic lattice. 65 Autofluorescence can interfere with CARS imaging, and therefore it is useful to correct CARS images for this background with time gating.…”

“…Second-harmonic generation (SHG) and sum-frequency mixing (SFM) are additional multiphoton processes; in the context of soil, SHG and SFM can highlight minerals with highly polarized and noncentrosymmetric molecular or crystal structure, which activates second-order nonlinear processes. SHG and SFM can also be used to image some biomolecules such as collagen, myosin, and microtubules. ,, …”

“…C–H and C–H 2 bond vibrational modes produce particularly strong CARS signals and can be used for mapping the distribution of lipids in biological samples . Although CARS is a new application for soil imaging, it has been previously applied to minerals studies, , such as microfractures in a quartz crystallographic lattice …”

Background-suppressed SRS fingerprint imaging with a fully integrated system using a single optical parametric oscillator

“…This is especially true in presence of endogenous fluorophores, in pigmented environments or more generally with any absorbing molecules. Practical examples of impeding nonlinear background include melanin in skin tissues [13], hemoglobin in blood vessels [13,17,19], chlorophyll and carotenoids absorption in plants [16,20] but also various sources of nonlinear background in mineralogical samples [15,21].…”