Development of an ultrafast single photon counting imager for single molecule imaging

Ultrafast single-photon detection at high repetition rates based on optical Kerr gates under focusing

The ultrafast detection of single photons is currently restricted by the limited time resolution (a few picoseconds) of the available single-photon detectors. Optical gates offer a faster time resolution, but so far they have been applied mostly to ensembles of emitters. Here, we demonstrate through a semi-analytical model that the ultrafast time-resolved detection of single quantum emitters can be possible using an optical Kerr shutter at gigahertz rates under focused illumination. This technique provides sub-picosecond time resolution, while keeping a gate efficiency at around 85%. These findings lay the ground for future experimental investigations on the ultrafast dynamics of single quantum emitters, with implications for quantum nanophotonics and molecular physics.

Development of new photon-counting detectors for single-molecule fluorescence microscopy

Two optical configurations are commonly used in single-molecule fluorescence microscopy: point-like excitation and detection to study freely diffusing molecules, and wide field illumination and detection to study surface immobilized or slowly diffusing molecules. Both approaches have common features, but also differ in significant aspects. In particular, they use different detectors, which share some requirements but also have major technical differences. Currently, two types of detectors best fulfil the needs of each approach: single-photon-counting avalanche diodes (SPADs) for point-like detection, and electron-multiplying charge-coupled devices (EMCCDs) for wide field detection. However, there is room for improvements in both cases. The first configuration suffers from low throughput owing to the analysis of data from a single location. The second, on the other hand, is limited to relatively low frame rates and loses the benefit of single-photon-counting approaches. During the past few years, new developments in point-like and wide field detectors have started addressing some of these issues. Here, we describe our recent progresses towards increasing the throughput of single-molecule fluorescence spectroscopy in solution using parallel arrays of SPADs. We also discuss our development of large area photon-counting cameras achieving subnanosecond resolution for fluorescence lifetime imaging applications at the single-molecule level.

Detectors for single-molecule fluorescence imaging and spectroscopy

Single-molecule observation, characterization and manipulation techniques have recently come to the forefront of several research domains spanning chemistry, biology and physics. Due to the exquisite sensitivity, specificity, and unmasking of ensemble averaging, single-molecule fluorescence imaging and spectroscopy have become, in a short period of time, important tools in cell biology, biochemistry and biophysics. These methods led to new ways of thinking about biological processes such as viral infection, receptor diffusion and oligomerization, cellular signaling, protein-protein or protein-nucleic acid interactions, and molecular machines. Such achievements require a combination of several factors to be met, among which detector sensitivity and bandwidth are crucial. We examine here the needed performance of photodetectors used in these types of experiments, the current state of the art for different categories of detectors, and actual and future developments of single-photon counting detectors for single-molecule imaging and spectroscopy.