Cylindrical microlensing for enhanced collection efficiency of small pixel SPAD arrays in single-molecule localisation microscopy

Abstract

Single-photon avalanche photodiode (SPAD) image sensors offer time-gated photon counting, at high binary frame rates of >100 kFPS and with no readout noise. This makes them well-suited to a range of scientific applications, including microscopy, sensing and quantum optics. However, due to the complex electronics required, the fill factor tends to be significantly lower (< 10%) than that of EMCCD and sCMOS cameras (>90%), whilst the pixel size is typically larger, impacting the sensitivity and practicalities of the SPAD devices. This paper presents the first characterisation of a cylindrical-shaped microlens array applied to a small, 8 micron, pixel SPAD imager. The enhanced fill factor, ≈50% for collimated light, is the highest reported value amongst SPAD sensors with comparable resolution and pixel pitch. We demonstrate the impact of the increased sensitivity in single-molecule localisation microscopy, obtaining a resolution of below 40nm, the best reported figure for a SPAD sensor.

Single-photon avalanche diode imagers in biophotonics: review and outlook

“…photosensitive) pixel area ( Figure 1(c)). Examples of SPAD-related microlens developments and sensors are presented in [19,[36][37][38][39][40][41][42]. The microlenses are typically optimized for specific applications and properties (for example collimation) of the impinging light.…”

“…Concerning the first point, Gyongy et al used the 320×240 SPAD SPCImager [24] for localization-based super-resolution imaging [42,115]. By employing microlenses, they improved the effective fill factor from 27% to 50% and used this sensitivity enhancement to demonstrate 40 nm resolution.…”

“…As the previous discussion has indicated, the fundamental differences between EMCCD, sCMOS and SPAD imagers do also need to take into account, in addition to the overall sensitivity, the different noise contributions and achievable frame rates. A theoretical comparison between these imagers, when applied to localization based super-resolution microscopy, has been [42]. (f) Comparison of differences in localization uncertainty with and without "smart" aggregation and the microlenses impact [42,115].…”

Characterization of electronic displays using CMOS single‐photon avalanche diode image sensors

“…Thanks to recent advances in CMOS technology, CMOS-SPAD arrays with FF of 50% (with microlens), pixel pitch of 8 μm, and frame rate of over 10 kfps have been demonstrated. 7,18 With over 35% quantum efficiency in the visible spectrum, a display measurement platform can be offered based on CMOS-SPAD arrays. In this manuscript, we explore the application of SPAD arrays to display measurements, a challenging area where dynamic range and imaging speed are important, and show that CMOS-SPAD arrays offer advantages over CCD, EMCCD, and CMOS technology.…”

“…Thanks to recent advances in CMOS technology, CMOS‐SPAD arrays with FF of 50% (with microlens), pixel pitch of 8 μm, and frame rate of over 10 kfps have been demonstrated . With over 35% quantum efficiency in the visible spectrum, a display measurement platform can be offered based on CMOS‐SPAD arrays.…”