Quantum Image-Forming Theory for Calculation of Resolution Limit in Laser Microscopy

Here we show what determines the optical resolution in laser microscopy. We define the expanded resolution limit (spatial frequency cutoff ) that includes the classic Abbe definition as 2 NA/λ, where λ is the wavelength. The resolution limit can approximate‐ ly be redefined as the frequency cutoff αNA/λ, where α is the constant that depends on the optical process occurring in the sample. In the case of the optical process originat‐ ing from the linear susceptibility χ(1), the resolution limit is well known as the Abbe definition, namely, α = 2. However, when other optical processes are harnessed to form the image through laser microscopy, the resolution limit can differ. We formulate a theoretical framework that can calculate the expanded resolution limits of all kinds of laser microscopy utilizing coherent, incoherent, linear, and nonlinear optical processes.