Mid-infrared wide-field nanoscopy

Mid-infrared (MIR) spectroscopy is widely recognized as a powerful, non-destructive method for chemical analysis. However, its utility is constrained by a micrometre-scale spatial resolution imposed by the long-wavelength MIR diffraction limit. This limitation has been recently overcome by MIR photothermal 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 MIR photothermal imaging to achieve high spatial resolution. This was accomplished by employing single-objective synthetic-aperture quantitative phase imaging with synchronized subnanosecond MIR and visible light sources, effectively suppressing the resolution-degradation effect caused by photothermal heat diffusion. We demonstrated far-field MIR spectroscopic imaging with a spatial resolution limited by the visible diffraction, down to 120 or 175 nm in terms of the Nyquist–Shannon sampling theorem or full-width at half-maximum of the point spread function, respectively, in the MIR region of 3.12–3.85 μm (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.