  Two-photon microscopy (2PM) has become an important tool in biology to study
the structure and function of intact tissues in-vivo. However, adult mammalian
tissues such as the mouse brain are highly scattering, thereby putting
fundamental limits on the achievable imaging depth, which typically resides
around 600-800um. In principle, shifting both the excitation as well as
(fluorescence) emission light to the shortwave near-infrared (SWIR, 1000-1700
nm) region promises substantially deeper imaging in 2PM, yet has proven
challenging in the past due to the limited availability of detectors and probes
in this wavelength region. To overcome these limitations and fully capitalize
on the SWIR region, in this work we introduce a novel array of superconducting
nanowire single-photon detectors (SNSPDs) and associated custom detection
electronics for the use in near-infrared 2PM. The SNSPD array exhibits high
efficiency and dynamic range, as well as low dark-count rates over a wide
wavelength range. Additionally, the electronics and software permit seamless
integration into typical 2PM systems. Together with a fluorescent dye emitting
at 1105 nm, we report imaging depth of > 1.1mm in the in-vivo mouse brain,
limited only by available labeling density and laser power. Our work further
establishes SWIR 2PM approaches and SNSPDs as promising technologies for deep
tissue biological imaging.
