Alberto Tosi

Image Scanning Microscopy with Single-Photon Detector Array

Image scanning microscopy (ISM) improves the spatial resolution of conventional confocal laser-scanning microscopy (CLSM), but current implementations reduce versatility and restrict its combination with fluorescence spectroscopy techniques, such as fluorescence lifetime. Here, we describe a natural design of ISM based on a fast single-photon detector array, which allows straightforward upgrade of an existing confocal microscope, without compromising any of its functionalities. In contrast to all-optical ISM implementations, our approach provides access to the raw scanned images, opening the way to adaptive reconstruction methods, capable of considering different imaging conditions and distortions. We demonstrate its utility in the context of fluorescence lifetime, deep, multicolor and live-cell imaging. This implementation will pave the way for a transparent and massive transition from conventional CLSM to ISM. confocal microscopy | time-resolved spectroscopy | image scanning microscopy | single-photon detector array

High count rate InGaAs/InP SPAD system with balanced SPAD-dummy approach running up to 1.4 GHz

The capability to achieve high count rates has become an imperative in the most areas where near-infrared single-photon counters are required to detect photons up to 1.7 μm. Hence, afterpulsing mitigation is a dominant theme in recent works concerning systems based on InGaAs/InP SPADs. Given the challenges inherent in reducing the density of defects that give rise to the carrier trapping events causing afterpulsing, the only viable approach is to reduce the potential number of carriers that can be trapped by limiting the charge flow per avalanche event. In this paper we present a sine-wave gating system based on the balanced detector configuration. The gate frequency is programmable in a wide range (1.0 – 1.6 GHz) for allowing synchronization with an external laser system and for exploring the best trade-off between afterpulsing and photon detection efficiency. The long-term stability can be achieved with a stable cancelation of the gate feedthrough. In this work this is guaranteed by a feedback loop that continuously monitors the residual output power at the gate frequency and adjusts the amplitude and phase of the two sinusoids fed to the SPAD-dummy couple.

Protective measurements: extracting the expectation value by measuring a single particle

In quantum mechanics, the eigenvalues and their corresponding probabilities specify the expectation value of a physical observable, which is known to be a statistical property related to large ensembles of particles. In contrast to this paradigm, we demonstrate a unique method allowing to extract the expectation value of a single particle, namely, the polarisation of a single protected photon, with a single experiment. This is the first realisation of quantum protective measurements.

Image scanning microscopy (ISM) with a single photon avalanche diode (SPAD) array detector

If a scanning illumination spot is combined with a detector array, we acquire a 4 dimensional signal. Unlike confocal microscopy with a small pinhole, we detect all the light from the object, which is particularly important for fluorescence microscopy, when the signal is weak. The image signal is basically a cross-correlation, and is highly redundant. It has more than sufficient information to reconstruct an improved resolution image. A 2D image can be generated from the measured signal by pixel reassignment. The result is improved resolution and signal strength, the system being called image scanning microscopy. A variety of different signal processing techniques can be used to predict the reassignment and deconvolve the partial images. We use an innovative single-photon avalanche diode (SPAD) array detector of 25 detectors (arranged into a 5× 5 matrix). We can simultaneously acquire 25 partial images and process to calculate the final reconstruction online.

0.16 µm BCD single-photon avalanche diode with 30 ps timing jitter, high detection efficiency and low noise

CMOS SPADs are nowadays an established imaging technology for applications requiring single-photon sensitivity in a compact form-factor (e.g. three-dimensional LIDAR imaging and fluorescence lifetime FLIM microscopy). However, we aimed at further enhance overall SPAD performances, by exploiting smart power technologies, such as the BCD (Bipolar-CMOS-DMOS) one. We achieved the present state-of-the-art SPADs fabricated in the 0.16 μm BCD technology by STMicroelectronics, attaining >60% photon detection efficiency at 500 nm, dark count rate density < 0.2 cps/μm2, and less than 30 ps FWHM timing jitter.

Optical spectroscopy in the time-domain beyond 1.1 μm: A tool for the characterization of diffusive media

Summary form only given. Time-resolved diffuse optical spectroscopy (TRS) is a valuable technique for the characterization of a variety of diffusive media like biological tissues, fruit, wood and pharmaceutical tablets. The technique is based on the injection of a short pulse (~ps) on the surface of the sample. The time-of-flight distribution (TOF) of photons detected on a different location of the surface gives information about the absorption and scattering probabilities [1]. Due to the widespread application of the technique to biological tissues, where there is a low light attenuation in the 0.6-1.1 μm range, the range beyond 1.1 μm is relatively unexplored with time-resolved techniques, also because of the difficulty to combine mode-locked continuously tunable sources with detectors having a sensitivity down to the single-photon level. Nevertheless beyond 1.1 μm there are organic compounds that contribute to interesting spectral structures like collagen, lipids, hydroxyapatite, starch, glucose and lignin. In this work we explore the application of TRS beyond 1.1 μm to the characterization of a variety of samples.