Single-photon particles are the smallest element of light. Being able to detect light at that level offers extremely sensitive imaging. But even more importantly, being able to measure exactly when a photon is detected after it is reflected or emitted from an object, reveals important information about that object. This information is key, for example, for determining the chemical specificity in fluorescence lifetime imaging. Additionally, detecting spatio-temporal correlations between single photons points the way to new imaging devices, improving the resolution of traditional imaging. The EU-supported gammaCam project demonstrated the capabilities of two CMOS imaging sensors in the laboratory, before integrating one into a prototype camera. This camera can record photons within 200 picoseconds of their arrival at the detector, enabling stunning resolution. “Alongside consolidating the technical achievements developed in a previous project, we quantified the resources required for small-scale production and market researched the most promising applications,” says project coordinator André Stefanov from the University of Bern, the project host.
Building the idea detector
While imaging technology has advanced recently, it still relies on detecting average light intensity at each pixel, within the exposure time of the camera. So, traditional cameras can only record light fluctuations that are slower than the time it takes to capture an image, usually in the millisecond range. This matters, because being able to measure light intensity fluctuations provides valuable additional information about the objects under investigation. “An ideal imaging device records both the detection time and the position of each individual photon in a beam of light illuminating an object,” explains Stefanov. “This spatio-temporal information is key to numerous imaging technologies such as time-of-flight imaging or correlated imaging.” Gated cameras are available with nanosecond exposures, but as well as being expensive they can’t detect photons arriving at different times and so don’t capture the full spatio-temporal information. GammaCam continued development of two sensors designed for quantum imaging under the SUPERTWIN project. These sensors exploited arrays of a photodetector known as a ‘single-photon avalanche diode’ (SPAD) which records the detection time of each photon with sub-nanosecond accuracy. SuperEllen has 32 x 32 pixels, while SuperAlice has 224 x 272. SuperAlice is also a versatile sensor which can either record photon detection times or simply count the photons. To continue SUPERTWIN’s evaluation of spatial correlation from quantum light, both sensors were tested with entangled photon pairs. “It was very exciting to see the first quantum light measurements. Especially given that unlike SuperEllen, SuperAlice’s many pixels also allowed more accurate characterisation of quantum light through observing long-range correlations between photons,” adds Stefanov.
A range of applications
The most likely applications for those SPAD arrays will be for advanced microscopes to investigate molecular processes critical to medicine. “Our sensors could either improve already established methods – such as light-sheet microscopy – or help develop new methods and markets. For this we need to further identify the full range of measurements they uniquely allow,” concludes Stefanov. The prototype camera is transportable and, as it is relatively easy to set up within current optical systems, can already be used in experiments. While the SuperAlice could be mass produced by the same foundry as the prototype LFoundry, the sensor still needs to be integrated into a user-friendly camera for a commercially available off-the-shelf product.
gammaCam, photon, sensor, imaging, resolution, CMOS, camera, microscope, light, picoseconds