In order to fulfill the requirements of many applications, we recently developed a new technology aimed at combining the advantages of traditional thin and thick silicon Single Photon Avalanche Diodes (SPAD). In particular we demonstrated single-pixel detectors with a remarkable improvement in the Photon Detection Efficiency at the longer wavelengths (e.g. 40% at 800nm) while maintaining a timing jitter better than 100ps. In this paper we will analyze the factors the currently prevent the fabrication of arrays of SPADs by adopting such a Red-Enhanced (RE) technology and we will propose further modifications to the device structure that will enable the fabrication of high performance RE-SPAD arrays for photon timing applications.
Single-molecule fluorescence spectroscopy of freely diffusing molecules in solution is a powerful tool used to
investigate the properties of individual molecules. Single-Photon Avalanche Diodes (SPADs) are the detectors of choice
for these applications. Recently a new type of SPAD detector was introduced, dubbed red-enhanced SPAD (RE-SPAD),
with good sensitivity throughout the visible spectrum and with excellent timing performance. We report a
characterization of this new detector for single-molecule fluorescence resonant energy transfer (smFRET) studies on
freely diffusing molecules in a confocal geometry and alternating laser excitation (ALEX) scheme. We use a series of
doubly-labeled DNA molecules with donor-to-acceptor distances covering the whole range of useful FRET values. Both
intensity-based (μs-ALEX) and lifetime-based (ns-ALEX) measurements are presented and compared to identical
measurements performed with standard thick SPADs. Our results demonstrate the great potential of this new detector for
smFRET measurements and beyond.
The implementation and commercialization of quantum cryptography technologies have to face some challenges related to the development of single-photon detectors operating at 1550 nm. The main requirements are: i) high detection efficiency; ii) low noise; iii) high count rate; iv) low timing jitter. Different technologies are currently available for single-photon detection at 1550 nm, but semiconductor devices (like Single-Photon Avalanche Diode, SPAD) offer a
photon detection efficiency that is inherently higher than any photocathode employed in vacuum tube detectors.
Additionally InGaAs/InP SPADs can detect single photons at 1550 nm with low noise when moderately cooled by
means of thermo-electric coolers. Consequently, InGaAs/InP SPAD can be the enabling technology of practical quantum key distribution (QKD) systems, provided that the maximum count rate is increased above 1 Mcps. The main limit is the afterpulsing effect that usually sets too long (< 10 μs) a hold-off time after each avalanche ignition.
We present the developments achieved in InGaAs/InP SPAD device design, fabrication technology and front-end
electronics, aimed at decreasing the afterpulsing effect, while not impairing photon detection efficiency and timing jitter. The new InGaAs/InP SPADs provide count rates higher than 1 Mcps and temporal response with 60 ps Full-Width at Half Maximum and very short (30 ps time constant) tail. The front-end electronics includes a wide-band pulse generator able to gate the SPAD up to 133 MHz repetition rate. Eventually, a fast avalanche-quenching scheme minimizes quenching time to less than 1 ns, thus effectively reducing afterpulsing by decreasing the total charge flowing through the junction.
In the last years many progresses have been made in the field of Silicon Single Photon Avalanche Diodes (SPAD) thanks
to the improvements both in device design and in fabrication technology. For example, the use of custom fabrication
processes has allowed a steadily improvement of SPAD performance in terms of active area diameter, Dark Count Rate
(DCR), and Photon Detection Efficiency (PDE). Although a significant breakthrough has been achieved with the recent
introduction of a new device structure capable of combining a good timing resolution with a remarkable PDE in the near
infrared region, nevertheless there is still room for further improvements.
In this paper we will discuss further modifications to the device structure enabling the fabrication of arrays with red
enhanced photon detection efficiency.
Over the past few years there has been a growing interest in monolithic arrays of single photon avalanche diodes (SPAD)
for spatially resolved detection of faint ultrafast optical signals. SPADs implemented in planar technologies offer the
typical advantages of microelectronic devices (small size, ruggedness, low voltage, low power, etc.). Furthermore, they
have inherently higher photon detection efficiency than PMTs and are able to provide, beside sensitivities down to
single-photons, very high acquisition speeds. Although currently available silicon devices reached remarkable
performance, nevertheless further improvements are needed in order to meet the requirements of most demanding timeresolved
techniques, it is necessary to face problems like electrical crosstalk between adjacent pixel, high detection
efficiency in the red spectral range, large area, low dark counting rate. Moreover to develop array with high number of
pixel became more and more important to develop all the TCSPC electronics with picosecond resolution to create a new
family of detection system for TCSPC applications. Recent advances in our research on single photon time resolved
array is here presented.
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.
Many applications require high performance Single Photon Avalanche Diodes (SPAD) either as single pixels or as small
arrays of detectors. Although currently available silicon devices reached remarkable performance, nevertheless further
improvements are needed in order to meet the requirements of most demanding time-resolved techniques.
In this paper we present a new planar silicon technology for the fabrication of SPAD detectors, aimed at improving the
Photon Detection Efficiency (PDE) of classical thin SPAD in the near infrared range while maintaining a good Temporal
Resolution (TR). Experimental characterization showed a significant increase in the PDE with a remarkable value of
40% at 800nm; a photon timing jitter as low as 93ps FWHM as been also attained, while other device performances,
such as Dark Count Rate (DCR) and Afterpulsing Probability (AP) are essentially unchanged, compared to thin SPAD.
Being planar, the new technology is also intrinsically compatible with the fabrication of arrays of detectors.
We introduce a novel SPAD device with high photon detection efficiency and good performances in terms of temporal resolution and dark count rate. The designed detectors are able to attain a PDE as high as 40% at a wavelength of 800 nm while keeping photon detection jitter below 100 ps. The device was fabricated with a suitable planar silicon technology process that allows the development of detector arrays.
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.
Remarkable advances in semiconductor technology as long as improvements in device design resulted in today's Silicon
Single Photon Avalanche Diodes (SPADs) that are widely used in many demanding applications thanks to their excellent
performance. However a lot of work is still be done in order to simultaneously meet three requirements crucial in a large
number of applications, i.e. high Photon Detection Efficiency (PDE), good timing resolution and suitability for the
fabrication of arrays.
We will report on our advances on the development of a new planar silicon SPAD with high photon detection efficiency
(PDE) and good photon timing resolution. A thick epitaxial layer allows for the absorption of a significant fraction of
photons even at the longer wavelengths, while a suitable electric field profile limits the breakdown voltage value and the
timing jitter; biased guard rings are also included to prevent edge breakdown. Preliminary results show that the new
devices can attain a PDE as high as 30% at a wavelength of 800nm while keeping photon detection jitter below 100ps.
In this review we present the instrumental and theoretical developments for functional diffuse reflectance spectroscopy at
small source-detector distances. We proposed the possibility to perform photon migration measurements at null or small
inter-fiber distances demonstrating the improvement of this novel approach in terms of achievable contrast, spatial
resolution and number of detected photons. We developed a novel system to perform time-resolved diffuse reflectance
measurement at small source detector separation based on a single photon avalanche photodiode (SPAD) operated in fast
time gated mode and a broadband fiber laser. By means of time gating it is possible to detect longer lived photons
neglecting initial ones. We show results both on homogeneous and inhomogeneous tissue phantoms demonstrating a
dynamic range of 7 orders of magnitude and a temporal range of 6 nanoseconds. Furthermore, this approach proved
valuable to detect brain activity.
KEYWORDS: Sensors, Photodetectors, Doping, Silicon, Absorption, Electric field sensors, Single photon detectors, Near infrared, Instrument modeling, Ionization
We will report on our advances on the development of a new planar silicon SPAD with high photon detection efficiency
(PDE) and good photon timing resolution. We will show that a 10μm thick epitaxial layer allows for the absorption of a
significant fraction of the incident photons even at the longer wavelengths, while a suitable electric field profile limits
the breakdown voltage value and the timing jitter. Simulations show that the new devices can attain a PDE higher than
30% at a wavelength of 800nm.
InGaAs/InP Single-Photon Avalanche Diodes (SPADs) have recently shown good performances in terms of dark count
rate and detection efficiency, making them suitable for many NIR single-photon counting applications. However, it is
mandatory to operate InGaAs/InP SPADs in optimized working conditions and in association with proper dedicated
electronics. A complete characterization of primary dark count rate, afterpulsing, detection efficiency and timing jitter is
required in order to be able to tailor the working conditions to the specific request. Moreover, very fast quenching
circuits can efficiently minimize afterpulsing, while low-jitter front-end circuits detect the avalanche pulse with high
timing precision.
Solution-based single-molecule spectroscopy and fluorescence correlation spectroscopy (FCS) are powerful techniques
to access a variety of molecular properties such as size, brightness, conformation, and binding constants. However, this
is limited to low concentrations, which results in long acquisition times in order to achieve good statistical accuracy.
Data can be acquired more quickly by using parallelization. We present a new approach using a multispot excitation and
detection geometry made possible by the combination of three powerful new technologies: (i) a liquid crystal spatial
light modulator to produce multiple diffraction-limited excitation spots; (ii) a multipixel detector array matching the
excitation pattern and (iii) a low-cost reconfigurable multichannel counting board. We demonstrate the capabilities of
this technique by reporting FCS measurements of various calibrated samples as well as single-molecule burst
measurements.
KEYWORDS: Photodetectors, Picosecond phenomena, Sensors, Signal detection, Near infrared spectroscopy, Near infrared, Silicon, Single photon detectors, Time metrology, Biomedical optics
We present a novel instrumentation for wide dynamic range optical investigations based on a time-gated silicon Single-
Photon Avalanche Diode (SPAD) in a Time-Correlated Single-Photon Counting (TCSPC) setup. The detector is gatedon
and off in less than 200 ps and kept-on for detecting photons only within short time slots. Such technique is
particularly useful in applications where a large amount of unnecessary photons precedes or follows the optical signal to
detect, such as in time-resolved NIR spectroscopy, optical mammography, and optical molecular imaging. In particular,
in time-resolved reflectance spectroscopy, when the source-detector separation is decreased, the detection electronics
easily saturates, due to the huge amount of "early" photons, diffused by superficial layers. Instead, our setup is able to
reject those photons and to detect only "late" photons, thus allowing to increase the injected power and to drastically
widen the investigation dynamic range, while remarkably speeding up the acquisition.
We acquired Instrument Response Functions (IRFs) at multiple wavelengths between 600 nm and 1000 nm and we achieved up to 108 dynamic range in a very short measurement time (few minutes). Moreover, we tested the instrumentation with SPADs of different active areas and we compared the performances.
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.
In this paper we present a physically-based model aimed at calculating the Photon Detection Efficiency (PDE) and the
temporal response of a Single-Photon Avalanche Diode (SPAD) with a given structure. In order to calculate these
quantities, it is necessary to evaluate both the probability and the delay with which a photon impinging on the detector
area triggers an avalanche. Three tasks are sequentially performed: as a first step, the electron-hole generation profile
along the device is calculated according to the silicon absorption coefficient at the considered wavelength; successively,
temporal evolution of the carriers distribution along the device is calculated by solving drift diffusion equations; finally,
the avalanche triggering probability is calculated as a function of the photon absorption point.
Validation of the model has been carried out by comparing simulation and experimental results of a few generations of
detectors previously realized in our laboratory. Photon detection efficiency has been measured and calculated for
wavelengths ranging from 400nm to 1000nm and for excess bias voltages ranging from 2 to 8V. Similarly, temporal
response has been investigated at two different wavelengths (520 and 820nm). A remarkable agreement between
experimental and simulation results has been obtained in the entire characterization domain simply starting from the
measured doping profile and without the need of any fitting parameter. Consequently, we think that this model will be a
valuable tool for the development of new detectors with improved performances.
KEYWORDS: Capacitors, Signal detection, Sensors, Time metrology, Amplifiers, Picosecond phenomena, Signal processing, Logic, Analog electronics, Transistors
In the last years Time-Correlated Single-Photon Counting (TCSPC) has increasingly been used in many different
scientific applications (e.g.: single molecule spectroscopy, fluorescence lifetime imaging, diffuse optical tomography).
Many of these applications are calling for new requests on the development of instrumentation that operates at higher
and higher conversion rates and that is able to resolve optical signals not only in the time domain, but also in wavelength,
polarization and position. To exploit the potential of parallel analysis over multiple acquisition channels, a new
generation of TCSPC devices is needed that is characterized by low size and costs. The core block of TCSPC
instrumentation is the time-interval measurement section, which can be implemented with a Time-to-Amplitude
Converter (TAC); the converter can be integrated on a single chip in order to reduce the overall size and cost of the
system. This paper presents a monolithic TAC that has been designed to achieve the high resolution, good differential
linearity and fast counting rate required in modern applications. The TAC here described is built on a commercial
0.35 μm CMOS technology, and is characterized by resolution better than 60 ps, differential nonlinearity limited to
0.5% rms and short dead-time of 80 ns. The low area occupation (1.4x1.8 mm) and minimal need for external
components allow the realization of very compact instruments with multiple acquisition channels operating
simultaneously at very high count rates.
Improving SPAD performances, such as dark count rate and quantum efficiency, without degrading the photontiming
jitter is a challenging task that requires a clear understanding of the physical mechanisms involved. In this
paper we investigate the contribution of the avalanche buildup statistics and the lateral avalanche propagation to
the photon-timing jitter in silicon SPAD devices. Recent works on the buildup statistics focused on the uniform
electric field case, however these results can not be applied to Si SPAD devices in which field profile is far from
constant. We developed a 1-D Monte Carlo (MC) simulator using the real non-uniform field profiles derived
from Secondary Ion Mass Spectroscopy (SIMS) measurements. Local and non-local models for impact ionization
phenomena were considered. The obtained results, in particular the mean multiplication rate and jitter of the
buildup filament, allowed us to simulate the statistical spread of the avalanche current on the device active area.
We included space charge effects and a detailed lumped model for the external electronics and parasitics.
We found that, in agreement with some experimental evidences, the avalanche buildup contribution to the total
timing jitter is non-negligible in our devices. Moreover the lateral propagation gives an additional contribution
that can explain the increasing trend of the photon-timing jitter with the comparator threshold.
Over the past few years there has been a growing interest in monolithic arrays of single photon avalanche diodes
(SPAD) for spatially resolved detection of faint ultrafast optical signals. SPADs implemented in CMOS-compatible
planar technologies offer the typical advantages of microelectronic devices (small size, ruggedness, low voltage, low
power, etc.). Furthermore, they have inherently higher photon detection efficiency than PMTs and are able to provide,
beside sensitivities down to single-photons, very high acquisition speeds (i.e. either high frame-rates or very short
integration time-slots). SPADs offer several advantages over other commercially available imagers. For example, CCDs
and similar imagers lack in speed because their readout process is based on a slow charge-transfer mechanisms. CMOS
APS, on the other hand, are unable to detect very faint optical signals, due to poor sensitivity and noisy electronics.
In order to make SPAD array more and more competitive it is necessary to face several issues: dark counts, quantum
efficiency, crosstalk, timing performance. These issues will be discussed in the context of two possible approaches to
such a challenge: employing a standard industrial CMOS technology or developing a dedicated technology. Advances
recently attained will be outlined with reference to both photon counting and Time correlated single photon counting
detector arrays.
Near-Infrared (NIR) picosecond pulsed light shined in biological tissues (e.g. brain, breast, muscle) offers the
opportunity for non-invasive quantitative spectroscopy and imaging. Tissue optical properties determine high attenuation
levels of optical signals and nanosecond scale dynamics. Therefore high-performance set-ups are needed. We aimed at
developing a winning photodetector-electronics pairing for a broad field of multiple-wavelengths faint-signal optical
investigations, like brain functional imaging, optical mammography, in-vivo spectroscopy, drugs characterization,
molecular imaging.
We present an electronic instrumentation based on silicon Single-Photon Avalanche Diode (SPAD) and fast-gating frontend
electronics, in a Time-Correlated Single-Photon Counting (TCSPC) set-up. Detection efficiency is very high (50% at
550 nm and 15% at 800 nm), allowing acquisition of very faint optical signals on a wide spectral range. Furthermore,
the fast-gating circuitry enables the detector very quickly (500 ps) and for user-selectable (200 ps - 510 ns) durations,
thus allowing the rejection of very intense optical signals (e.g. scattered light from more superficial layers of the tissue
under investigation) preceding useful faint signals (e.g. scattered light from sub-cellular components or coming from
"deep" tissue layers), which would be otherwise overwhelmed and made undetectable. We attain photon-counting
dynamic ranges up to 107 with photon-timing resolutions of 95 ps.
In this work we focused on time-resolved measurements in diffusive media performed at small (few millimeters)
source-detector distances in reflectance geometry. This configuration has been predicted to have better contrast,
better spatial resolution, and lower noise than the typical measurements performed at few centimeters. In our
instrumental set-up we exploited a fast-gating (rise-time < 400 ps) front-end electronics enabling a silicon Single-
Photon Avalanche Diode (SPAD) for time-correlated single-photon counting. By means of this detector, we can
acquire "late" photons of the diffused light collected 2 mm apart from the injection point. This is possible
because the fast gated SPAD rejects the huge amount of "early" photons which otherwise would saturate the detection electronic chain. The time resolution of the set-up is 100 ps. The instrument has been validated on both homogeneous and inhomogeneous (high absorbing inclusion at different depths inside) tissue phantoms with different optical properties. We obtained diffused time-resolved curves with dynamic ranges of about 107. Moreover, we demonstrated good agreement between the measured time-resolved contrasts and those calculated by Monte Carlo numerical simulations.
InGaAs/InP devices suitable as Single-Photon Avalanche Diodes (SPADs) for photon counting and photon timing
applications in the near-infrared provide good detection efficiency and low time jitter, together with fairly low darkcount
rate at moderately low temperatures. However, their performance is still severely limited by the afterpulsing effect,
caused by carriers trapped into deep levels during the avalanche current flow and later released.
We present preliminary experimental characterization of recently-developed InGaAs/InP detectors that can promisingly
be operated slightly cooled. We investigate the primary dark-count rate, taking into account both thermal generation in
the InGaAs absorption layer and trap-assisted tunnelling in the InP multiplication layer. We report on improvements
obtainable by selecting the proper operating conditions and electronic circuit solutions. The fundamental role played by
the front-end circuits in minimizing the effects of afterpulsing is assessed and demonstrated.
We report the performance of a 25 μm-diameter InGaAs/InP SPAD at 1550 nm wavelength, with dark count rate of 400
cps (count per seconds) at 175 K and just 2000 cps at 225 K, with afterpulsing showing off only below TOFF=10μs. The
photon timing resolution is 100 ps (FWHM, Full Width at Half Maximum) at 7 V of excess bias.
The development of a very-compact DNA sequencer instrument based on Single Photon Avalanche Diode (SPAD) for
microchip electrophoresis is here reported. The planar epitaxial SPAD combines the typical advantages of microelectronic
devices with high sensitivity. We present a miniaturized system based on a custom array of SPAD, purposely designed
to be compatible with Amersham Biosciences commercial markers. This system is the first example of very compact,
ultra-sensitive, portable and low cost DNA sequencer. It may represent a breakthrough in DNA sequencing system and
open the way to the development of a new category of portable low-cost apparatus.
Single photon counting (SPC) and time correlated single photon counting (TCSPC) techniques have been developed in
the past four decades relying on photomultiplier tubes (PMT), but interesting alternatives are nowadays provided by
solid-state single photon detectors. In particular, silicon Single Photon Avalanche Diodes (SPAD) fabricated in planar
technology join the typical advantages of microelectronic devices (small size, ruggedness, low operating voltage and low
power dissipation, etc.) with remarkable basic performance, such as high photon detection efficiency over a broad
spectral range up to 1 μm wavelength, low dark count rate and photon timing jitter of a few tens of picoseconds. In
recent years detector modules employing planar SPAD devices with diameter up to 50 µm have become commercially
available. SPADs with larger active areas would greatly simplify the design of optical coupling systems, thus making
these devices more competitive in a broader range of applications. By exploiting an improved SPAD technology, we
have fabricated planar devices with diameter of 200 μm having low dark count rate (1500 c/s typical @ -25 °C). A
photon timing jitter of 35 ps FWHM is obtained at room temperature by using a special pulse pick-up network for
processing the avalanche current. The state-of-the-art of large-area SPADs will be reviewed and prospects of further
progress will be discussed pointing out the challenging issues that must be faced in the design and technology of SPAD
devices and associated quenching and timing circuits.
Single-Photon Avalanche Diodes (SPADs) for near-infrared (800-1700 nm) wavelengths can be manufactured both in InGaAs/InP and in germanium. Recently, new InGaAs/InP SPADs became commercially available with good overall performances, but with the intrinsic bottleneck of strong afterpulsing effect, originated in the InP multiplication layer. At present, germanium technology is not exploited for single-photon detectors, but previous devices demonstrate lower afterpulsing even at very low temperatures and promising dark count rate when employing pure manufacturing process. In this work, we compare germanium and InGaAs/InP SPADs in terms of dark counts, afterpulsing, timing jitter, and quantum efficiency. Eventually, we highlight the motivations for considering germanium as a key material for single-photon counting in the NIR.
We present a multichannel photon counting module that exploits a monolithic array of single-photon avalanche diodes
(SPADs). The detector array consists of eight 50μm diameter SPADs featuring low dark counting rate and high photon
detection efficiency (50% at 550nm); inter-pixel crosstalk probability is as low as 2•10-3. The use of highly integrated
active quenching circuits makes it possible to design a very compact read-out circuit, yet providing eight fully
independent counting channels and gating capability. The detection module maintains the same physical dimensions of
commercially available single element modules and can be used as a plug-in replacement to add multichannel
capabilities to existing measurement setups. Full characterization of module performance is here presented.
One of the main drawbacks of Single Photon Avalanche Diode arrays is the optical crosstalk between adjacent detectors.
This phenomenon represents a fundamental limit to the density of arrays, since the crosstalk increases with reducing the
distance between adjacent devices. In the past, crosstalk was mainly ascribed to the light propagating from one detector
to another through a direct optical path. Accordingly, deep trenches coated with metal were introduced as optical
isolation barriers between pixels. This solution, however, was unable to completely prevent the crosstalk. In this paper
we present experimental evidence that a significant contribution to crosstalk comes from photons reflected internally at
the bottom of the chip. These photons can bypass trenches making them ineffective. We also propose an optical model
suitable to predict the dependence of crosstalk on the position within the array.
We demonstrate the feasibility of time-resolved diffuse reflectance at small source-detector separations using a single-photon
avalanche photodiode (SPAD) operated in time-gated mode. Photon time distributions at an interfiber distance of
0.2 cm were obtained on tissue phantoms with a reduced scattering coefficient of 10 cm-1, and an absorption coefficient
of 0.1 cm-1, with a dynamic range of 106 and collecting photons at arrival times up to 4 ns. By time-gating the initial
photons, carrying information mainly from superficial layers, it is possible to detect longer lived photons that have
explored deeper depths even at almost null interfiber distances. The proposed approach should provide higher number of
photons at any arrival time, higher contrast, and better spatial resolution as compared to longer interfiber distances.
We demonstrate the feasibility of time-resolved diffuse reflectance at small source-detector separations using a
single-photon avalanche diode (SPAD) operated in time-gated mode. Photon time distributions at an interfiber
distance of 0.2 cm were obtained on tissue phantoms with a reduced scattering coefficient of 10 cm-1, and
an absorption coefficient of 0.1 cm-1, with a dynamic range of 106 and collecting photons at arrival times up
to 4 ns. By time-gating the initial photons, carrying information mainly from superficial layers, it is possible
to detect longer lived photons that have explored deeper depths even at almost null interfiber distances. The
proposed approach should provide higher number of photons at any arrival time, higher contrast, and better
spatial resolution as compared to longer interfiber distances.
Photon counting was introduced and developed during four decades relying on Photomultiplier Tubes (PMT), but
interesting alternatives are nowadays provided by solid-state single-photon microdetectors. In particular, Silicon Single-Photon Avalanche-Diodes (SPAD) attain remarkable basic performance, such as high photon detection efficiency over a
broad spectral range up to 1 micron wavelength, low dark counting rate and photon timing jitter of a few tens of
picoseconds. In recent years SPADs have emerged from the laboratory research phase and they are now commercially
available from various manufactures. However, PMTs have much wider sensitive area, which greatly simplifies the
design of optical systems; they attain remarkable performance at high counting rate and can provide position-sensitive
photon detection and imaging capability. In order to make SPADs more competitive in a broader range of applications it
is necessary to face issues in semiconductor device technology. The present state of the art, the prospect and main issues
will be discussed.
KEYWORDS: Sensors, Signal to noise ratio, Signal detection, Electronics, Photodetectors, Quenching (fluorescence), CMOS sensors, Single photon detectors, Luminescence, Photon counting
We present the design and fabrication of 2x2 Quad-Cells and related electronics for ultra-sensitive detection and spatial
tracking of single photons in the visible wavelength range. Though four pixels do not offer the imaging capabilities of
CCDs, CMOS sensors and the like, their single-photon sensitivity enables most detection equipments demanding higher
detection efficiency, faster gated operation, and sharper time response. In fact Quad-Cells are aimed to both photon-counting
and photon-timing applications, i.e. whenever it is important to reconstruct both continuous or slow-varying
very-faint light signals (with time-slots down to 50&mgr;s) and very fast luminescence waveforms (down to 150-ps), by
having at the same time either the spatial information of the photon absorption position or the possibility to track the
luminous source. In this paper we present the design and fabrication of 2x2 Quad-Cell, with pixel diameters from 50&mgr;m
to 100 &mgr;m, developed by means of a planar microelectronic processing. They reach 55%-detection efficiency at 520-nm-wavelength
and show a broad peak, higher than 40% in the 420-660-nm range, with still more than 2% at 1,000 nm.
We demonstrate that III-V photodetectors operated with dedicated front-end electronics and cooled at sufficiently low
temperature (220 K or lower) can be exploited as Single-Photon Avalanche-Diodes (SPAD) for near-infrared photon
counting and timing. Low dark-count rate can be achieved in gated-mode operation, though InGaAs/InP SPADs are
plagued by strong avalanche carrier trapping that leads to afterpulsing.
In order to reach the best performance, we designed fast circuits for gating SPADs and properly sensing the photoninduced
avalanche pulse, cancelling spurious spikes due to gate transients thus accurately extracting photon timing
information, and reducing avalanche charge thus minimizing afterpulsing.
We report the results obtained with In0.53Ga0.47As/InP SPADs employing an integrated Active Quenching Circuit,
designed for gated-mode operation at cryogenic temperature, and a fast signal pick-up network for extracting the best
timing resolution. The joint use of a good InGaAs/InP detector and the presented electronics allows to reach low dark
count rate (below 20 kHz), low time jitter (about 40 psFWHM), high operation frequency (up to 100 kHz) with limited
afterpulsing even when the photodetector is enabled for long gate-on times (even longer than 100 ns).
Time-correlated single photon counting (TCSPC) is exploited in emerging scientific applications in life sciences, such as
single molecule spectroscopy, DNA sequencing, fluorescent lifetime imaging. Detectors with wide active area (diameter
> 100 μm) are desirable for attaining good photon collection efficiency without requiring complex and time-consuming
optical alignment and focusing procedures. Fiber pigtailing of the detector, often employed for having a more flexible
optical system, is also obtained more simply and with greater coupling efficiency for wide-area detectors. TCSPC,
however, demands to detectors also high photon-timing resolution besides low noise and high quantum efficiency.
Particularly tight requirements are set for single-molecule fluorescence analysis, where components with lifetimes of tens
of picoseconds are often met. Small photon timing jitter and wide area are considered conflicting requirements for the
detector.
We developed an improved planar silicon technology for overcoming the problem and providing a solid-state alternative
to MCP-PMTs in demanding TCSPC applications. We fabricated Single Photon Avalanche Diodes (SPADs) with 200
μm active area diameter and fairly low dark counting rate (DCR). At moderately low temperature (-25 °C with Peltier
cooler) the typical DCR is 1500 c/s and it is not difficult to select devices with less than 1000 c/s. The photon detection
efficiency peaks at 48% around 530 nm and stays above 30% over all the visible range. A photon timing resolution of 35
ps FWHM (full width at half maximum) is obtained by using our patented pulse pick-up for processing the avalanche
current.
Silicon Single-Photon Avalanche-Diodes (SPAD) are nowadays considered a solid-state alternative to Photomultiplier
Tubes (PMT) in single photon counting (SPC) and time-correlated single photon-counting (TCSPC) over the visible
spectral range up to 1 micron wavelength. SPADs implemented in planar epitaxial technology compatible with CMOS
circuits offer the typical advantages of microelectronic devices (small size, ruggedness, low voltage and low power, etc.).
Furthermore, they have inherently higher photon detection efficiency, since they do not rely on electron emission in
vacuum from a photocathode as PMT, but instead on the internal photoelectric effect. However, PMTs offer much wider
sensitive area, which greatly simplifies the design of optical systems; they provide position-sensitive photon detection
and imaging capability; they attain remarkable performance at high counting rate and offer picosecond timing resolution
with Micro-Channel Plate (MCP) models. In order to make SPADs more competitive in a broader range of SPC and
TCPC applications it is necessary to face both semiconductor technology issues and circuit design issues, which will be
here dealt with. Technology issues will be discussed in the context of two possible approaches: employing a standard
industrial high-voltage compatible CMOS technology or developing a dedicated CMOS-compatible technology. Circuit
design issues will be discussed taking into account problems arising from conflicting requirements set by various
required features, such as fast and efficient avalanche quenching and reset, high resolution photon timing, etc.
InGaAs and Germanium devices employed as Single-Photon Avalanche-Diodes (SPAD) for the infrared spectral range
must be cooled to low temperature for reducing the dark-counting rate due to thermal generation and are plagued by
strong avalanche carrier trapping. Released trapped carriers re-trigger the avalanche and generate correlated afterpulses.
This effect can be counteracted by reducing the avalanche pulse charge and by covering the trapped carrier release
transient with a hold-off time after quenching. Gated operation is employed, but simple gated passive circuits are suitable
only for short gate intervals (few nanoseconds). For longer gate times, we investigated gated operation of a SPAD under
the control of an active quenching circuit, which yielded accurate timing performance. We designed an integrated active
quenching circuit (iAQC) suitable for gated mode for operating the SPAD down to cryogenic temperature. The iAQC
senses the avalanche and swiftly quenches it, without waiting the end of the gate interval. Operation with longer gate
times (100 ns and more) is achieved with remarkably reduced afterpulsing with respect to passive gated circuits. We
expressly designed fast circuits for processing the avalanche pulse, cancelling spurious spikes due to gate transients and
accurately extracting the photon timing information, with less than 50 ps jitter.
In recent years quantum information research has lead to the discovery of a number of remarkable new paradigms for information processing and communication. These developments include quantum cryptography schemes that offer unconditionally secure information transport guaranteed by quantum-mechanical laws. Such potentially disruptive security technologies could be of high strategic and economic value in the future. Two major issues confronting researchers in this field are the transmission range (typically <100km) and the key exchange rate, which can be as low as a few bits per second at long optical fiber distances. This paper describes further research of an approach to significantly enhance the key exchange rate in an optical fiber system at distances in the range of 1-20km. We will present results on a number of application scenarios, including point-to-point links and multi-user networks.
Quantum key distribution systems have been developed, which use standard telecommunications optical fiber, and which are capable of operating at clock rates of up to 2GHz. They implement a polarization-encoded version of the B92 protocol and employ vertical-cavity surface-emitting lasers with emission wavelengths of 850 nm as weak coherent light sources, as well as silicon single-photon avalanche diodes as the single photon detectors. The point-to-point quantum key distribution system exhibited a quantum bit error rate of 1.4%, and an estimated net bit rate greater than 100,000 bits-1 for a 4.2 km transmission range.
We present the development of silicon monolithic arrays of 60 photon-counters (SPADA, Single-Photon Avalanche Diode Array) for the visible. The SPADA system is suitable for state-of-the-art Adaptive Optics operations and Fast Transient image acquisitions, at quite a fraction of the current cost of imaging arrays. The fabricated solid-state photon counters are rugged, easy to be integrated in the optical system. They are free from readout noise and provide very fast frame-rates (>10kHz, for visible corrections) and nanosecond electronic gating (for ranging the up-going laser beam).
The detection head has been integrated into an optomechanical system suitable for alignment and focusing in available astrophysics telescopes. The detection electronics includes an integrated Active Quenching Circuits for each pixel of the array. The real-time data-processing board is implemented into FPGA and DSP and is configurable for dealing with different applications: the extraction of the curvature wavefront for AO applications, and the acquisition and processing of two-dimensional images with fast frame rate. A remote host computer controls all the SPADA blocks and uploads the processed information and images. We report the optical and electrical characterization of the detectors and the associated electronics.
In this paper we report the results relative to the design and fabrication of Single Photon Avalanche Detectors (SPAD) operating at low voltage in planar technology. These silicon sensors consist of pn junctions that are able to remain quiescent above the breakdown voltage until a photon is absorbed in the depletion volume. This event is detected through an avalanche current pulse.
Device design and critical issues in the technology are discussed.
Experimental test procedures are then described for dark-counting rate, afterpulsing probability, photon timing resolution, quantum detection efficiency. Through these experimental setups we have measured the electrical and optical performances of different SPAD technology generations. The results from these measurements indicate that in order to obtain low-noise detectors it is necessary to introduce a local gettering process and to realize the diode cathode through in situ doped polysilicon deposition. With such technology low noise detectors with dark counting rates at room temperature down to 10c/s for devices with 10mm diameter, down to 1kc/s for 50mm diameter have been obtained.
Noticeable results have been obtained also as far as time jitter and quantum detection efficiency are concerned.
This technology is suitable for monolithic integration of SPAD detectors and associated circuits. Small arrays have already been designed and fabricated. Preliminary results indicate that good dark count rate uniformity over the different array pixels has already been obtained.
Linear arrays of single photon avalanche detectors (SPADs) designed for use in low light level imaging applications were fabricated using a novel planar process that is compatible with standard CMOS technology. The device characteristics for these arrays are presented here to investigate their suitability for high efficiency low light level imaging. A new scheme is proposed to eliminate the problem of optical crosstalk between pixels in the array by introducing a trench isolation process coupled with silicon-on-insulator (SOI) technology.
KEYWORDS: Sensors, Germanium, Avalanche photodetectors, Signal detection, Single photon, Photodetectors, Analog electronics, Near infrared, Luminescence, Single photon detectors
We report the first results obtained with a germanium quad-cell sensor operated in Geiger- mode regime. After a quantitative characterization of the single pixel, both in counting and in timing applications, we quantitatively assess the intensity of the optical coupling among the detectors of the cell due to secondary photon emission from hot carriers. This effect, intrinsically related to Geiger-mode operation, has been overcome by sequentially driving the pixels of the cell. A preliminary test demonstrates the tracking capabilites of the sensor. Since the single pixel can detect the arrival time of the photon with a precision better than 100ps FWHM arrays of such devices could be also employed in wavelength and timing resolved luminescence measurements in the near-infrared.
We report the photon-counting and timing performance of various Single-Photon Avalanche Diodes (SPADs) employed to detect single photons in the near-infrared wavelength range. Suitable Silicon structures achieve high quantum efficiency (70% at 800 nm) and can work up to 1.1 micrometers . Ge SPADs and InGaAs devices are sensitive up to 1.4 micrometers and 1.6 micrometers , respectively, wit ha few percent-quantum efficiencies. We compare these results with the performance of state-of-the-art photomultiplier tubes with extended near-infrared sensitivity. We also report the first results obtained with a germanium quad-cell sensor, which may be considered the first step towards the development of SPAD arrays.
We report about the development of novel circuitry for solid state photon counting devices, based on avalanche photodiodes (PC-APD), and tailored for adaptive optics applications. The recent development of EG&G SilkTM devices, has improved silicon. APDs considerably, reducing the afterpulsing effects, improving the effective Q.E., and reducing the dark current to negligible values. These new APD allow to conceive new quenching circuitry and new applications of solid state photon counters for improved adaptive optics performance.
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