KEYWORDS: Sensors, Molecules, Imaging spectroscopy, Signal detection, Fluorescence correlation spectroscopy, Photodetectors, Temporal resolution, Single molecule spectroscopy, Point spread functions, Signal to noise ratio
Solution-based single-molecule fluorescence spectroscopy is a powerful new experimental approach with applications in
all fields of natural sciences. Two typical geometries can be used for these experiments: point-like and widefield
excitation and detection. In point-like geometries, the basic concept is to excite and collect light from a very small
volume (typically femtoliter) and work in a concentration regime resulting in rare burst-like events corresponding to the
transit of a single-molecule. Those events are accumulated over time to achieve proper statistical accuracy. Therefore the
advantage of extreme sensitivity is somewhat counterbalanced by a very long acquisition time. One way to speed up data
acquisition is parallelization. Here we will discuss a general approach to address this issue, using a multispot excitation
and detection geometry that can accommodate different types of novel highly-parallel detector arrays. We will illustrate
the potential of this approach with fluorescence correlation spectroscopy (FCS) and single-molecule fluorescence
measurements. In widefield geometries, the same issues of background reduction and single-molecule concentration
apply, but the duration of the experiment is fixed by the time scale of the process studied and the survival time of the
fluorescent probe. Temporal resolution on the other hand, is limited by signal-to-noise and/or detector resolution, which
calls for new detector concepts. We will briefly present our recent results in this domain.
Single-molecule spectroscopy is a powerful approach to measuring molecular properties such as size, brightness,
conformation, and binding constants. Due to the low concentrations in the single-molecule regime, measurements with
good statistical accuracy require long acquisition times. Previously we showed a factor of 8 improvement in acquisition
speed using a custom-CMOS 8x1 SPAD array. Here we present preliminary results with a 64X improvement in
throughput obtained using a liquid crystal on silicon spatial light modulator (LCOS-SLM) and a novel standard CMOS
1024 pixel SPAD array, opening the way to truly high-throughput single-molecule spectroscopy.
Scientific experiments often demand the detection of very weak light signals at high-speed or to precisely measure the
time of arrival of single photons. Arrays of Single-Photon Avalanche Diodes (SPAD) are ideal candidates when high
sensitivity is required together with high frame-rate or precise photon-timing resolution. We designed a linear 32x1
SPAD array using a high-voltage CMOS technology able to provide both good SPAD performance and fast electronics.
During frame acquisition all pixels work in parallel, each of them being equipped with anything necessary for photon
counting. The array architecture is capable of fully parallel operation of all pixels allowing free running acquisition at
high frame-rate. With a low-speed 10 MHz clock frequency, one pixel is read out in 100 ns while the whole array is
readout in 320 ns, corresponding to a frame-rate of 312.5 kframe/s. The frame-rate can top to 4 Mframe/s with a clock of
128 MHz. The photon timing modality employs the photon time-of-arrival information provided by each of the 32
outputs. All 32 "timing" outputs feed external Time-Correlated Photon Counting boards. The Full-Width at Half-
Maximum using very short laser pulses is 55 ps with few kcps counting rate.
Several applications require systems for 3D ranging acquisition, where both high frame-rate and high sensitivity (for
either very dark environments or opaque objects) are a must. We exploited a monolithic chip with 32 x 32 Single-Photon
Avalanche Diode smart-pixels for 3D ranging applications based on an Indirect Time-of-Flight (iTOF) technique. The
scene is illuminated by a sinusoidally modulated LED and the reflected light is acquired by the imager in different timeslots,
for measuring the phase delay of outgoing vs. incoming signal, hence computing the distance between the sensor
and objects in the scene.
All 1024 array pixels are synchronously enabled by a global gate signal, which allows photon counting in well-defined
time-slots within each frame. The frame duration is set in accordance to the desired SNR. We report on measurements
performed on chips fabricated in a standard high-voltage 0.35 μm CMOS technology, which feature 40% photon
detection efficiency at 450 nm and 20% at 650nm. The single-photon sensitivity allowed the use of just few LEDs at 650
nm and 20MHz for acquiring a scene with a maximum distance of 7.5 m, with better than 10 cm distance resolution and
frame-rates higher than 50 frames/s.
KEYWORDS: Sensors, 3D acquisition, Single photon, 3D metrology, Phase measurement, 3D image processing, Cameras, Prototyping, Distance measurement, Modulation
Three dimensions (3D) acquisition systems are driving applications in many research field. Nowadays 3D acquiring
systems are used in a lot of applications, such as cinema industry or in automotive (for active security systems).
Depending on the application, systems present different features, for example color sensitivity, bi-dimensional image
resolution, distance measurement accuracy and acquisition frame rate. The system we developed acquires 3D movie
using indirect Time of Flight (iTOF), starting from phase delay measurement of a sinusoidally modulated light. The
system acquires live movie with a frame rate up to 50frame/s in a range distance between 10 cm up to 7.5 m.
We present a high-speed Single-Photon Camera for demanding applications in biology, astrophysics,
telecommunications, 3D imaging and security surveillance.
The camera is based on a 32-by-32 array of "smart pixels" processed in a standard high-voltage technology. Every pixel
is a completely independent photon-counting channel. Sensitivity is at the single-photon level and no readout noises
affect the measure. The camera has high Photon-Detection Efficiency (PDE) in the blu/green visible spectrum (45% at
450 nm) and low Dark-Counting Rate (DCR) even at room temperature (usually lower than 2 kcps). The use of
microlenses makes it possible to further increase the effective pixel fill-factor.
The camera can be configured by means of a cross-platform user-friendly software that communicates with the camera
through a fast USB link. The integration time window may range from few tens of nanoseconds to milliseconds. The
maximum frame rate for the whole 1,024 pixels is about 100 kframe/s, while the minimum 20 ns dead-time between
frames boosts the sensor dynamic range. The camera is equipped with a standard C-Mount connector. A gating input pin
can be used to quickly gate on/off the integration.
The camera works in One-Shot mode for the maximum acquisition speed, Real-Time mode for very long measurements
and Live mode for setups alignment purposes.
Solution-based single-molecule fluorescence spectroscopy is a powerful new experimental approach with applications in
all fields of natural sciences. The basic concept of this technique is to excite and collect light from a very small volume
(typically femtoliter) and work in a concentration regime resulting in rare burst-like events corresponding to the transit
of a single-molecule. Those events are accumulated over time to achieve proper statistical accuracy. Therefore the
advantage of extreme sensitivity is somewhat counterbalanced by a very long acquisition time. One way to speed up data
acquisition is parallelization. Here we will discuss a general approach to address this issue, using a multispot excitation
and detection geometry that can accommodate different types of novel highly-parallel detector arrays. We will illustrate
the potential of this approach with fluorescence correlation spectroscopy (FCS) and single-molecule fluorescence
measurements obtained with different novel multipixel single-photon counting detectors.
Aim of the paper is to discuss design, fabrication and performances of Single-Photon Avalanche Diode (SPAD) arrays
developed at the SPADLab of Politecnico di Milano, in both custom and fully-CMOS technologies. Applications span
from 2D imagers for high sensitivity fast frame-rate (close to Mframe/s) video acquisitions, to molecular imaging, to
functional time-resolved Near-Infrared Spectroscopy (fNIRS) of organs and tissues, to Fluorescence Correlation
Spectroscopy (FCS), Fluorescence Lifetime Imaging (FLIM) with 30psFWHM photon timing resolution. Various
microelectronic single-chip detection modules and monolithic SPAD arrays will be presented and discussed.
We present the design and we discuss in depth the operating conditions of a two-dimensional (2-D) imaging array of
single-photon detectors that provides a total of 1024 pixels, laid out in 32 rows by 32 columns array, integrated within a
monolithic silicon chip with dimensions of 3.5 mm x 3.5 mm. We employed a standard high-voltage 0.35μm CMOS
fabrication technology, with no need of any custom processing.
Each pixel consists of one Single-Photon Avalanche Diode (SPAD) and a compact front-end analog electronics followed
by a digital processing circuitry. The in-pixel front-end electronics senses the ignition of the avalanche, quenches the
detector, provides a pulse and restores the detector for detecting a subsequent photon. The processing circuitry counts
events (both photon and unwelcome "noise" ignition) within user-selectable integration time-slots and stores the count
into an in-pixel memory cell, which is read-out in 10 ns/pixel. Such a two-levels pipeline architecture allows to acquire the actual frame while contemporary reading out the previous one, thus achieving a very high free-running frame rate, with negligible inter-frame dead-time. Each pixel is therefore a completely independent photon-counter. The measured Photo Detection Efficiency (PDE) tops 43% at 5V excess-bias, while the Dark-Counting Rate (DCR) is below 4kcps (counts per second) at room temperature. The maximum frame-rate depends on the system clock; with a convenient 100MHz system clock we achieved a free-running speed of 100 kframe/s from the all 1024 pixels.
We present a compact detection head based on a 32x1 array of Single-Photon Avalanche Diodes (SPAD) and associated
electronics for time tagging single photons (photon counting) with 3μs time-resolution, with high photon detection
efficiency (45% at 450nm) and sharp photon-timing resolution (55 ps). The array is composed by 32 "smart" pixels,
working in photon-counting mode, with fully parallel and synchronous acquisition. The array is driven by an FPGA able
to acquire data from the sensor and to upload them to a remote PC via an USB 2.0 link, for real-time continuous
acquisition up to 312.5 kframe/s. The module is bus-powered for convenient use with laptops, and provides also direct
timing outputs from two pixels for time-resolved measurements (photon timing).
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