Optical biopsy of fixed human skin with backward-collected optical harmonics signals

Harmonics-based optical microscopy has been widely applied in biomedical researches due to its noninvasiveness to the studied biomaterials. Due to momentum conservation consideration, most previous studies collect harmonics generation signals in a forward geometry, especially for third harmonic generation signals. However, the adopted forward transmission type geometry is not feasible for future clinical diagnosis. In this paper, first virtual biopsy based on backward propagating optical higher harmonics, combining second harmonic and third harmonic, is demonstrated in the fixed human skin specimens. In our study, third harmonic generation can provide morphologic information including the distribution of basal cells while second harmonic generation can provide distribution of collagen fibers in dermis. Therefore, this technique is ideal for future noninvasive in vivo skin disease examination without dye.