High-resolution infrared imaging performance of new high numerical aperture Schwarzschild objectives

Mid-Infrared (IR) microspectroscopy is an advanced technique that enables the examination of samples providing micrometer-level spatial resolution and generating detailed chemical imaging of the samples. However, despite its advantages, spatial resolution in far-field optical microscopy and image resolution is limited by diffraction and there is a constant desire to push this limitation further. High numerical aperture (NA) objectives have been available for decades but primarily in the form of refractive lenses, however, these lack broadband capability, as they introduce chromatic aberrations. A reflective type objectives avoid this problem at the expense of lower NA and recently reflective objectives with an improved design and NA ranging from 0.7 to 0.8 were released. This study characterized the spatial resolution and signal to noise ratio (SNR) of new high numerical aperture infrared reflective objectives of NA = 0.7 (20x) and NA = 0.78 (40x) and compared their performance to the commonly used 15x (NA = 0.4) and 36x (NA = 0.52) objectives on a set of polymer and tissue samples using a benchtop IR microscope. It was found that the spatial resolution reaches values below 3 µm for the highest NA objective and approximately 6 µm for the lowest NA objective for shortest wavelengths. At the same time, the high-NA objectives provided better SNR compared to a similar magnification objective in both transmission and transflection geometry. Finally, a 3 µm PMMA sphere was successfully imaged using a conventional IR source (not a synchrotron or a laser), giving an impressive performance of the objectives.