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.