Abstract
Active thermal imaging is a valuable tool for the nondestructive characterization of the morphological properties and the functional state of biological tissues and synthetic materials. However, state-of-the-art techniques do not typically combine the required high spatial resolution over extended fields of view with the quantification of temperature variations. Here, we demonstrate quantitative far-infrared photo-thermal imaging at sub-diffraction resolution over millimeter-sized fields of view. Our approach combines the sample absorption of modulated raster-scanned laser light with the automated localization of the laser-induced temperature variations imaged by a thermal camera. With temperature increments ∼0.5–5 °C, we achieve a six-time gain with respect to our 350-μm diffraction-limited resolution with proof-of-principle experiments on synthetic samples. We finally demonstrate the biological relevance of sub-diffraction thermal imaging by retrieving temperature-based super-resolution maps of the distribution of Prussian blue nanocubes across explanted murine skin biopsies.
Highlights
Active thermal imaging is a valuable tool for the nondestructive characterization of the morphological properties and the functional state of biological tissues and synthetic materials
Existing approaches reaching the highest spatial resolution include fluorescence-based thermometry, where temperature is probed via its effect on the intensity, anisotropy and lifetime of the fluorescence emission of dyes, proteins and nano-constructs[13,14,15,16], and scanning thermal microscopy, that relies on the near-field interaction between a surface and a heated tip[17,18,19,20,21]
In the context of active thermography, the exploitation of the response of optical properties to a modulated change in the sample temperature is at the basis of scanning thermoreflectance microscopy[11,22,23,24] and PHoto-thermal Imaging (PHI)[7,25,26,27,28]
Summary
Active thermal imaging is a valuable tool for the nondestructive characterization of the morphological properties and the functional state of biological tissues and synthetic materials. High resolution active thermal imaging is relevant to monitor the dissipation efficiency or detect the fault of electronic devices and micro-electromechanical systems[10,11], or to perform quality controls on the thermo-conduction properties of materials[4,12] For all these applications, morphological imaging of the sample structure with resolution well below the millimeter range should be combined with the high sensitivity (∼0.1 °C) measurement of the induced temperature variations over millimeter to centimeter-sized fields of view. Scanning thermo-reflectance imaging can reach nanoscale spatial resolution and provide quantitative measurement of the induced temperature variations after sample-dependent calibration[11] It finds its best application in the characterization of electronic and opto-electronic devices[11,29]; the sample coating with a thin metal transducer layer, which is often required to improve the technique sensitivity[30,31,32], may limit the applicability to biological systems
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