Multiphoton, single-element detection, photon-counting structured light imaging  

We have developed a spatial frequency modulation imaging (SPIFI) system for biological imaging applications that enables enhanced resolution across contrast mechanisms including two-photon excitation fluorescence (TPEF, incoherent signal) and second- or third-harmonic generation (SHG, THG, coherent signal). The microscope architecture is such that in-line pulse optimization and characterization is possible, enabling quantitative 4D beam characterization — important for quantifying the exposure intensity when imaging within a biological specimen. Significantly, we have advanced this particular imaging modality into the photon-counting regime. For multiphoton imaging, where the excitation scales as the second order with excitation intensity (for both TPEF and SHG) we have pushed the SPIFI image to four times the spatial frequency cutoff (fc) of the optical system: 4* fc = 4NA/λ, where NA = numerical aperture, and λ is the wavelength of the excitation beam. By incorporating photon-number resolved imaging, there is the potential to push the resolution even further. A photon-number resolved count enables the possibility of processing both classical and quantum information, the fusion of which could lead to significant resolution enhancement.