Photon-Timing Detector Module for Single-Molecule Spectroscopy With 60-ps Resolution

Advanced fluorescence measurements on single molecules demand single-photon detectors with high-quantum detection efficiency, low noise, and high time resolution. We have developed a compact (82x60x30 mm) and versatile single-photon timing module (SPTM), based on a planar epitaxial single photon avalanche diodes (SPAD) working with a monolithic integrated active quenching and active reset circuit (i-AQC) and cooled by a Peltier element. The main operating parameters are computer controlled via RS-232 interface and the photon counting rate can be continuously monitored. The photon detection efficiency is 45% at 500 nm with cooling at -15 °C, the dark counting rate is 5 c/s with SPAD operating at 5 V excess bias voltage, 10 c/s operating at 10 V. The time resolution obtained with tightly focused illumination has 60-ps full-width at half-maximum. Comparative tests with the SPTM prototype and with an advanced commercially available photon counting module confirmed that the time resolution and sensitivity of the SPTM make it possible to resolve and measure even short lifetime components of a single molecule. The SPTM thus made possible experiments leading to a deeper insight into angstrom-scale structural changes of single-protein molecules.