Absorbance modulation enables lateral superresolution in optical lithography and transmission microscopy by generating a dynamic aperture within a photochromic absorbance-modulation layer (AML) coated on a substrate or a specimen. The absorbance-modulation is the property of photochromic molecules modulated between two states. The process is therefore solely controlled by far-field radiation at different wavelengths.
The applicability of this concept to reflection microscopy has not been addressed so far, although reflection imaging exhibits the important ability to image a wide range of samples, transparent or opaque, dielectric or metallic. We will present a simulation model for absorbance-modulation imaging (AMI) in confocal reflection microscopy and it is shown that imaging well beyond the diffraction limit is feasible. Our model includes the imaging properties of confocal microscopy, reflections at the boundaries, the photochromic process and diffraction due to propagation through a subwavelength aperture.
We derive an analytical design equation which estimates the dependence of the achievable resolution on relevant parameters, such as the AML properties and the applied light powers. This equation is very similar to the corresponding equation for STED (Stimulated emission depletion) microscopy and it is helpful for a fast design of the arrangement of optical setup and AML. As rapid scanning is relevant for a short imaging duration, we further derived an estimation for the pixel dwell time. We prove the validity of these equations by comparing the estimations with the complex numerical simulations. In addition, we show that a resolution enhancement down to 1/5 of the diffraction limit is possible.