Doubling the far-field resolution in mid-infrared microscopy

The spatial resolution in far-field mid-infrared (lambda > 2.5 mu m) microscopy and micro-spectroscopy remains limited with the full-width at half maximum of the point-spread function ca. lambda/1.3; a value that is very poor in comparison to that commonly accessible with visible and near-infrared optics. Hereafter, it is demonstrated however that polymer beads that are centre-to-centre spaced by lambda/2.6 can be resolved in the mid-infrared. The more than 2-fold improvement in resolution in the far-field is achieved by exploiting a newly constructed scanning microscope built around a mid-infrared optical parametric oscillator and a central solid-immersion lens, and by enforcing the linear polarization unidirectional resolution enhancement with a novel and robust specimen error minimization based on a particle swarm optimization. The method is demonstrated with specimens immersed in air and in water, and its robustness shown by the analysis of dense and complex self-assembled bead islands. (C) 2016 Optical Society of America