Correlation-resolved sum-frequency generation microscopy of biological tissue with bright twin beams

Sum-frequency generation (SFG) driven by bright twin beams from high-gain parametric down-conversion yields a coherent component from correlated mode pairs and an incoherent component from uncorrelated mode combinations. Modal correlations can enhance coherent SFG in anisotropic nonlinear media, for example by extending the effective interaction length. Whether these correlations can also benefit image formation in biological tissue, where molecular organization and refractive index vary spatially, remains unknown. Here we address this question by performing SFG imaging of biological tissue using bright twin beams as the excitation source. Spectral decomposition yields co-registered coherent and incoherent images from a single acquisition. In mouse skeletal muscle, the coherent-component image revealed sarcomere periodicity with a Bragg-spot-to-background ratio approximately 40 times that of the incoherent-component image. In fresh human dermis, the coherent-component image showed greater Fourier power in the 0.30-0.50 m-1 band and greater fractional autocovariance decay at short spatial lags than the incoherent-component image. These results demonstrate that twin-beam correlations can enhance contrast and detectability at selected spatial frequencies in heterogeneous tissue. Correlation-resolved SFG microscopy thus offers a route to label-free characterization of tissue microstructure with bright quantum light, with potential applications in biomedical research.