**1. Introduction**

302 Advanced Photonic Sciences

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Time resolved imaging up to the single photon sensitivity is one of the most ambitious and important goals of photonics. Currently there are no commercial devices able to provide both the information on position (imaging) and arrival time of photons emitted by weak and ultra-weak sources. Only few and very expensive devices (Intensified CCD, electron bombarded CCD etc.) are able to reach the single photon detection threshold together with the possibility to collect photons in small time window (up to few ns). Unfortunately such devices are not able to provide any information on the arrival time of photons, fundamental for the recent developments on ultra-fast, time correlated optical sensing techniques.

Fluorescence-based imaging (both single and multi-photon) is the research field that has most influenced the development of fast and sensitive optical detectors. Examples of techniques in this class include Förster resonance energy transfer (FRET) (Jares-Erijman et al., 2003), fluorescence lifetime imaging microscopy (FLIM) (Becker et al., 2006), and fluorescence correlation spectroscopy (FCS) (Schwille et al., 1999). The success of these techniques, particularly FLIM, derives from the ability to characterize an environment based on the time domain behaviour of certain fluorophores with high resolution in space domain. This characterization can be done today with high levels of accuracy in 3D with minimal interference from the surroundings and almost no dependence on fluorophore concentration.

Many others scientific areas like astronomy, biophysics, biomedicine, nuclear and plasma physics etc. can benefit from a time resolved imaging device; it can improve the actual detection limits providing physical information otherwise inaccessible.

In astronomy and astrophysical science, one of the toughest problems affecting groundbased telescopes is the presence of the atmosphere, which distorts the spherical wave-front, creating phase errors in the image-forming ray paths. Even at the best sites, ground-based telescopes observing at visible wavelengths cannot achieve an angular resolution in the visible better than telescopes of 10 to 20 cm diameter, because of atmospheric turbulence alone. The cause is random spatial and temporal wave-front perturbations induced by turbulence in various layers of the atmosphere; one of the principal reasons for flying the Hubble Space Telescope was to avoid this image smearing. In addition, image quality is affected by permanent manufacturing errors and by long timescale wave-front aberrations

Time Resolved Camera: The New Frontier of Imaging Devices 305

A number of solid-state solutions have been proposed as a replacement of MCPs and PMTs using conventional imaging processes. The challenge, though, has been to meet single photon sensitivity and low timing uncertainty. To address the sensitivity problem, cooled and/or intensified CCDs (Etoh et al. 2005) and ultra-low-noise CMOS APS architectures (Kawai et al. 2005) have been proposed. Multiplication of photo-generated charges by

As an alternative to PMTs and MCPs, researchers have turned to solid-state photon counters based on avalanche photodiodes (APDs). In the last four decades, solid-state multiplication based photo-detectors have gradually evolved from relatively crude devices to the sophistication of today. Semiconductor APDs have the typical advantages of solid state devices (small size, low bias voltage, low power consumption, ruggedness and reliability, suitability to build integrated systems, etc.). Their quantum detection efficiency is inherently higher, particularly in the red and near infrared range. In APDs operating in linear mode the internal gain is not sufficient or barely sufficient to detect single photons. However, single photons can be efficiently detected by avalanche diodes operating in Geiger-mode, known as single-photon avalanche diodes (SPADs). Almost every imaging technology has one photo-detector device and the range of implementations is quite wide. In this context, SPADs have recently attracted significant interest thanks to their relative simplicity and ease

In the last decades, the possibility to build a silicon photo-sensor suitable for single photon counting applications was investigated by several research teams (Cova et al. 1983 and references therein). The original idea, firstly proposed by R.J. Mc Intyre since the 1960s, was to implement a semiconductor photodiode with characteristics suitable for the triggered avalanche operation mode, known as Geiger mode of operation, and therefore able to detect

Carriers generated by the absorption of a photon in the p–n junction, are multiplied by impact ionization thus producing an avalanche. APDs can reach timing uncertainties as low as a few tens of picoseconds thanks to the speed at which an avalanche evolves from the initial carrier pair forming in the multiplication region. An APD is implemented as photodiode reverse biased near or above breakdown, where it exhibits optical gains greater than one. When an APD is biased below breakdown it is known as proportional or linear APD. It can be used to detect clusters of photons and to determine their energy. When biased above breakdown, the optical gain becomes virtually infinite (see fig. 1). Thus, with relatively simple ancillary electronics, the APD becomes capable of detecting single photons. The APD operating in this regime is called single-photon avalanche diode - SPAD. If the primary carrier is photo-generated, the fast leading edge of the avalanche pulse marks the arrival time of the detected photon. After the avalanche is triggered, the current keeps flowing until the avalanche is quenched by lowering the bias voltage down to breakdown voltage or below. The bias voltage is then restored in order to detect another photon. This

operation requires a suitable circuit that is usually referred to as a quenching circuit.

renewed interest in recent years due to the versatility of their applications.

Individual detectors and detector arrays based on the SPAD technology have received

impact ionization has also been used in CCDs (Hynecek et al. 2001).

of fabrication.

single photons.

**2. Single photon avalanche diodes** 

introduced by mechanical, thermal, and optical effects in the telescope, such as defocusing, decentring, or mirror deformations generated by their supporting devices.

Adaptive optics (AO) and Natural Guide Star (NGS) are the solutions to these problems: a deformable mirror is inserted in the light path of the telescope, and its control signal is based on measurement of the incoming wave-front, performed by a suitable high-sensitivity and time resolved imaging detector. NGS adaptive optics suffers from the fact that it is not always possible to find a suitable NGS close enough to the portion of sky under investigation. Normally only 1% of the sky has an available NGS for current AO systems. The most promising way to overcome the lack of sufficiently bright natural reference stars is the use of artificial reference stars, also referred to as laser guide stars (LGS). These are patches of light created by the back-scattering of pulsed laser light by sodium atoms in the high mesosphere, or in the case of Rayleigh LGS, by molecules and particles located in the low stratosphere. Such an artificial reference star can be created as close to the astronomical target as desired, if will be available a wave-front sensor measuring the LGS wave-front in order to correct the atmospheric aberrations on the target object.

In plasma science, even 60 years after the invention of the laser, we witness a rapid development of systems generating electromagnetic pulses with extreme parameters such as duration, wavelength, peak power, and focused intensity.

The employment of solid-state laser materials allows the generation and subsequent amplification of light pulses as short as a few optical cycles only. When combined with the technique of chirped pulse amplification - CPA (Strickland et al. 1985) where the laser pulses are temporally stretched and recompressed before and after their amplification, table-top laser systems reaching peak powers of several tens or hundreds of Terawatt (1 TW = 1012 W) can be realized. At such huge intensities, the rapidly oscillating electric field of the laser pulse reaches peak values exceeding the atomic fields binding the electrons to the positively charged nucleus by several orders of magnitude. It is due to this fact that all kinds of matter when exposed to laser light shot under such extreme conditions are almost instantaneously ionized and a plasma – sometimes called the "4th state of matter" – is formed. Within such plasma, the interaction between the charged constituents mediated by the long-range Coulomb interaction governs the behaviour and the evolution of the plasma. This gives rise to a large magnitude of effects that makes the generation and application of plasmas a fascinating field of current research in physics.

In this context, the use of a time resolved imaging devices to get information on the spatial and temporal evolution of laser generated plasma is fundamental to improve the actual level of knowledge.

Thus far, in many time-resolved and/or high-sensitivity applications the detectors of choice have been photomultiplier tubes (PMTs) and multichannel plates (MCPs) (McPhate et al. 2005). While these devices can reach time uncertainties of a few tens of picoseconds (MCPs), usually are bulky, fragile, sensitive to electromagnetic disturbances (especially PMTs) and mechanical vibrations, require high supply voltages (2–3 kV) and are costly devices, particularly the high-performance models. Moreover multi-pixel images are not possible without bulky setups and expensive equipment. Thus, high sensitivity and/or timeresolved imaging has been relegated to applications requiring important investments for optical and detector equipment.

introduced by mechanical, thermal, and optical effects in the telescope, such as defocusing,

Adaptive optics (AO) and Natural Guide Star (NGS) are the solutions to these problems: a deformable mirror is inserted in the light path of the telescope, and its control signal is based on measurement of the incoming wave-front, performed by a suitable high-sensitivity and time resolved imaging detector. NGS adaptive optics suffers from the fact that it is not always possible to find a suitable NGS close enough to the portion of sky under investigation. Normally only 1% of the sky has an available NGS for current AO systems. The most promising way to overcome the lack of sufficiently bright natural reference stars is the use of artificial reference stars, also referred to as laser guide stars (LGS). These are patches of light created by the back-scattering of pulsed laser light by sodium atoms in the high mesosphere, or in the case of Rayleigh LGS, by molecules and particles located in the low stratosphere. Such an artificial reference star can be created as close to the astronomical target as desired, if will be available a wave-front sensor measuring the LGS wave-front in

In plasma science, even 60 years after the invention of the laser, we witness a rapid development of systems generating electromagnetic pulses with extreme parameters such as

The employment of solid-state laser materials allows the generation and subsequent amplification of light pulses as short as a few optical cycles only. When combined with the technique of chirped pulse amplification - CPA (Strickland et al. 1985) where the laser pulses are temporally stretched and recompressed before and after their amplification, table-top laser systems reaching peak powers of several tens or hundreds of Terawatt (1 TW = 1012 W) can be realized. At such huge intensities, the rapidly oscillating electric field of the laser pulse reaches peak values exceeding the atomic fields binding the electrons to the positively charged nucleus by several orders of magnitude. It is due to this fact that all kinds of matter when exposed to laser light shot under such extreme conditions are almost instantaneously ionized and a plasma – sometimes called the "4th state of matter" – is formed. Within such plasma, the interaction between the charged constituents mediated by the long-range Coulomb interaction governs the behaviour and the evolution of the plasma. This gives rise to a large magnitude of effects that makes the generation and application of plasmas a

In this context, the use of a time resolved imaging devices to get information on the spatial and temporal evolution of laser generated plasma is fundamental to improve the actual

Thus far, in many time-resolved and/or high-sensitivity applications the detectors of choice have been photomultiplier tubes (PMTs) and multichannel plates (MCPs) (McPhate et al. 2005). While these devices can reach time uncertainties of a few tens of picoseconds (MCPs), usually are bulky, fragile, sensitive to electromagnetic disturbances (especially PMTs) and mechanical vibrations, require high supply voltages (2–3 kV) and are costly devices, particularly the high-performance models. Moreover multi-pixel images are not possible without bulky setups and expensive equipment. Thus, high sensitivity and/or timeresolved imaging has been relegated to applications requiring important investments for

decentring, or mirror deformations generated by their supporting devices.

order to correct the atmospheric aberrations on the target object.

duration, wavelength, peak power, and focused intensity.

fascinating field of current research in physics.

level of knowledge.

optical and detector equipment.

A number of solid-state solutions have been proposed as a replacement of MCPs and PMTs using conventional imaging processes. The challenge, though, has been to meet single photon sensitivity and low timing uncertainty. To address the sensitivity problem, cooled and/or intensified CCDs (Etoh et al. 2005) and ultra-low-noise CMOS APS architectures (Kawai et al. 2005) have been proposed. Multiplication of photo-generated charges by impact ionization has also been used in CCDs (Hynecek et al. 2001).

As an alternative to PMTs and MCPs, researchers have turned to solid-state photon counters based on avalanche photodiodes (APDs). In the last four decades, solid-state multiplication based photo-detectors have gradually evolved from relatively crude devices to the sophistication of today. Semiconductor APDs have the typical advantages of solid state devices (small size, low bias voltage, low power consumption, ruggedness and reliability, suitability to build integrated systems, etc.). Their quantum detection efficiency is inherently higher, particularly in the red and near infrared range. In APDs operating in linear mode the internal gain is not sufficient or barely sufficient to detect single photons. However, single photons can be efficiently detected by avalanche diodes operating in Geiger-mode, known as single-photon avalanche diodes (SPADs). Almost every imaging technology has one photo-detector device and the range of implementations is quite wide. In this context, SPADs have recently attracted significant interest thanks to their relative simplicity and ease of fabrication.
