Doubling the far-field resolution in mid-infrared microscopy

Abstract

Abstract:The spatial resolution in far-field mid-infrared (λ>2.5 µm) microscopy and microspectroscopy remains limited with the full-width at half maximum of the point-spread function ca. λ/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 λ/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. Anal. Chem. 87(1), 485-493 (2015). 2. F. Lu and M. A. Belkin, "Infrared absorption nano-spectroscopy using sample photoexpansion induced by tunable quantum cascade lasers," Opt.

Spatial-domain filter enhanced subtraction microscopy and application to mid-IR imaging

“…In line with [17], the PS beads are imaged at the water/silicon interface at 3.4 µm as asymmetric depressions with strong side-lobes due to the central obscuration of the reflective objective and the large refractive index difference between Si and water [ Fig. 1(b)].…”

“…The mid-IR microscope used in these experiments has been described in [17] with the difference here that one of the two beam paths accommodates a π-step phase plate to generate the half-moon beam [ Fig. 1(a)].…”

“…The subtraction was then computed from the simulated images according to Eq. (2), as well as according to the recently introduced intensity weighted subtraction [13] and to the constant weight scheme [2][3][4][5][6][7][8][9][10][11][12]14,17]. For IWS, following [13], the simulated images were normalized to one and the subtraction performed according to 1 .…”

“…These earlier proposals envisaged the exploitation of reflective objectives typically used for imaging in the mid-IR but for which the numerical apertures (NA) remain practically limited so that the PSF with a Gaussian beam exhibits a full-width at half maximum (FWHM) that is not narrower than ca. λ/1.3 in air [16,17]. With a central solid immersion lens (c-SIL) [18,19], the mid-IR FWHM is reduced to λ/2 along the axis normal to the direction of polarization and the reconstruction of sparse specimens with a resolution of λ/2.6 was demonstrated when combining images recorded with crossed polarizations [17].…”

“…λ/1.3 in air [16,17]. With a central solid immersion lens (c-SIL) [18,19], the mid-IR FWHM is reduced to λ/2 along the axis normal to the direction of polarization and the reconstruction of sparse specimens with a resolution of λ/2.6 was demonstrated when combining images recorded with crossed polarizations [17].…”