  Mid-infrared (MIR) spectroscopy is widely recognized as a powerful,
non-distractive method for chemical analysis. However, its utility is
constrained by a micrometer-scale spatial resolution imposed by the
long-wavelength MIR diffraction limit. This limitation has been recently
overcome by MIR photothermal (MIP) imaging, which detects photothermal effects
induced in the vicinity of MIR absorbers using a visible-light microscope.
Despite its promise, the full potential of its spatial resolving power has not
been realized. Here, we present an optimal implementation of wide-field MIP
imaging to achieve high spatial resolution. This is accomplished by employing
single-objective synthetic-aperture quantitative phase imaging (SOSA-QPI) with
synchronized sub-nanosecond MIR and visible light sources, effectively
suppressing the resolution-degradation effect caused by photothermal heat
diffusion. We demonstrate far-field MIR spectroscopic imaging with a spatial
resolution limited by the visible diffraction, down to 125 nm, in the MIR
region of 3.12-3.85 um (2,600-3,200 cm-1). This technique, through the use of a
shorter visible wavelength and/or a higher objective numerical aperture, holds
the potential to achieve a spatial resolution of less than 100 nm, thus paving
the way for MIR wide-field nanoscopy.
