Focal Field Engineered Infrared-sensitive Third-order Sum Frequency Generation Microscopy

In this work we present experimental demonstration of focal-field engineering in infrared-sensitive third-order sum frequency generation (TSFG) microscopy by utilizing beam-shaping technique. Two photons of the input mid-infrared (MIR) beam at 3000 nm are upconverted to 615 nm in the presence of a single photon at 1040 nm through the TSFG process. The focal-field engineering scheme studied here improves optical resolution and contrast of the TSFG imaging. We observe best improvement of ~43 % in the central-lobe full-width half diameter with ~35% side-lobe strength of that of the central-lobe with the use of optimum phase-mask using isolated amorphous silicon (a-Si) nano disks as the sample. We compare the contrast enhancement between the experiments and simulations as a function of varying grating pitch and find good overall agreement between the two. In addition to annular phase masks, we also demonstrate edge contrast enhancement by imaging gratings with higher-order Hermite-Gaussian beams profile generated using horizontally partitioned 0- phase profile.