Two-photon Imaging with Silicon Photomultipliers

Abstract

Silicon photomultipliers (SiPMs) are a class of inexpensive and robust single-pixel detectors with applications similar to photomultiplier tubes (PMTs). We performed side-by-side comparisons of recently-developed SiPMs and a GaAsP PMT for two-photon fluorescence imaging of neural activity. Despite higher dark counts, which limit their performance at low photon rates (<1/μs), SiPMs matched the signal-to-noise ratio of the GaAsP PMT at photon rates encountered in typical calcium imaging experiments due to their much lower pulse height variability. At higher photon rates and dynamic ranges encountered during high-speed two-photon voltage imaging, SiPMs significantly outperformed the GaAsP PMT.Photomultiplier tubes (PMTs) detect light using a photocathode, which emits electrons upon photon absorption 2 . Emitted electrons are accelerated under high voltage in vacuum to strike a series of dynodes coated in a secondary emission material 3 . Collisions with the dynodes release additional electrons, amplifying the photon signal into a detectable current pulse. The highestsensitivity PMTs available for visible imaging use Gallium-Arsenide-Phosphide (GaAsP) photocathodes with high quantum efficiencies (QE; >40%), although these photocathodes degrade with light exposure. Alkali photocathodes are also used, which do not degrade substantially but are less sensitive. PMT output pulses have highly variable amplitudes 4 because they rely on a series of low-gain stochastic amplification steps. PMTs are the most commonly used single-pixel light detectors in microscopy.Avalanche photodiodes (APDs) are light-sensitive semiconductor diodes to which a bias voltage is applied. Photon absorption generates electron-hole pairs, which are accelerated by the bias voltage, producing additional electron-hole pairs via impact ionization. At lower bias voltages, APDs operate in 'linear mode', in which each photon produces a pulse of current. At higher bias voltages, APDs operate in Geiger mode, in which runaway ionization produces a saturating signal, causing the APD to act as a photon-gated binary switch. In Geiger mode, additional simultaneously-arriving photons do not produce additional signal. The Geiger mode avalanche is quenched by active or passive circuits 5 , which cause the bias voltage to drop below breakdown then restore it, allowing another photon to be detected. Geiger mode APDs (also called SPADs) have excellent quantum efficiency (>80%) and lower pulse height variability than PMTs, but have small sensitive areas, making single SPADs inefficient for collecting the large etendue of emission light from scattering specimens.

Imaging and Molecular Annotation of Xenographs and Tumours (IMAXT): High throughput data and analysis infrastructure

“…Following the standard notation in astronomy, we will refer to one of these digital units as a count, measured in analog-to-digital units (ADU). Knowing the gain of the instrument allows to transform back from ADU to electrons, and from there given a quantum efficiency into incident photons 23 .…”

Silicon Photomultipliers as a Low-Cost Fluorescence Detector for Capillary Electrophoresis

“…12 Several methods have emerged for deploying a SiPM in experiments for photon counting where a pulsed light source and high bandwidth analog to digital converter (ADC) or oscilloscope is used. 17,18 Low frequency or constant power applications have also begun to emerge for projects focused on providing affordable instrumentation. 13,19,20 We sought to apply a simple circuit for the SiPM detector for use with a detection apparatus and signal processing workflow designed for oncolumn epifluorescence detection.…”