Long-wavelength, high-resolution microscopy using upconversion in ultra-thin crystals

In this paper, a novel approach based on frequency upconversion in ultra-thin nonlinear crystals is investigated for use in high-resolution infrared (IR) microscopy in the 5-12 µm range, an important domain for biomedical research. Traditional IR imaging encounters spatial resolution constraints due to diffraction, which are addressed via upconversion imaging using ultra-thin crystals. The present work combines a tunable IR quantum cascade laser and a short wavelength mixing laser to circumvent the classical resolution limit dictated by the Rayleigh criterion. A detailed numerical model for small signal upconversion imaging at μm-scale resolution shows good agreement with experimental data. The presented approach opens new avenues for IR applications for label-free biomedical diagnostics and spectral imaging.