Deep-brain 2-photon fluorescence microscopy in vivo excited at the 1700  nm window

Here we demonstrate deep-brain 2-photon fluorescence microscopy in mouse in vivo, excited at the 1700 nm window. Through signal versus power measurement, we show that indocyanine green (ICG) is a promising 2-photon fluorescent dye excitable at the 1700 nm window. In order to excite ICG circulating in the vasculature in the deep brain, we employ a circular-polarization soliton self-frequency shift technique to generate energetic femtosecond pulses at 1617 nm. Combining the labeling and laser technologies, we achieve a record 2-photon fluorescence brain vasculature imaging depth of 2000 μm in vivo. Both the effective attenuation length measurement and signal-to-background ratio measurement indicate that we have reached the theoretical depth limit in 2-photon fluorescence microscopy.