Comparison of Signal Detection of GaAsP and GaAs PMTs for Multiphoton Microscopy at the 1700-nm window

Multiphoton microscopy (MPM) enables noninvasive optical imaging into the deep tissue in animal models in vivo. Recently, 1700 nm has been demonstrated as a promising excitation window for deeper penetration. Signal depletion is currently the limiting factor for imaging depth at the 1700-nm window in MPM. As a result, efficient signal detection is an effective means to further boost imaging depth. GaAsP and GaAs photomultiplier tubes (PMTs) are commonly used for signal detection in MPM. Here, we demonstrate a comparison investigation of signal detection of a GaAsP and a GaAs PMT for several modalities of MPM at 1700 nm. The two PMTs are identical in structure, connected to the same electronics, and mounted at exactly the same place in the microscope, facilitating a fair comparison. Our results show that with a 1667-nm excitation, GaAsP PMT is more efficient for signal detection of a 3-photon fluorescence of quantum dot Qtracker 655, third-harmonic generation signal, and a 4-photon fluorescence of fluorescein, whereas GaAs PMT is far superior in detecting a second-harmonic generation signal. The measured results are in good agreement with theoretical calculations based on wavelength-dependent cathode radiant sensitivities. We expect that our results will offer guidelines for PMT selection for MPM at the 1700-nm window.