Explore this comprehensive introduction to the foundations of photodetection from one of the leading voices in the field
The newly revised Photodetectors: Devices, Circuits and Applications delivers a thoroughly updated exploration of the fundamentals of photodetection and the novel technologies and concepts that have arisen since the release of the first edition twenty years ago. The book offers discussions of established and emerging photodetection technologies, including photomultipliers, the SPAD, the SiPM, the SNSPD, the UTC, the WGPD/TWPD, the QWIP, and the LT-GaAs. New examinations of correlation measurements on ultrafast pulses and single-photon detectors for quantum communications and LiDARs have also been added.
Each chapter includes selected problems for students to work through to aid in learning and retention. A booklet of solutions is also provided. The book is especially ideal for students and faculties of Engineering, with an emphasis on first principles, design, and the engineering of photodetectors. Issues in the book are grouped through the development of concepts, as opposed to collections of technical details.
Perfect for undergraduate students interested in the science or design of modern optoelectronics, Photodetectors: Devices, Circuits and Applications also belongs on the bookshelves of professors teaching PhD seminars in advanced courses on photodetection and noise, as well as engineers and physicists seeking a guide to an optimum photodetection solution.
SILVANO DONATI is Emeritus Professor in the Department of Industrial Engineering and Informatics, Faculty of Engineering at the University of Pavia. He has been the Founder and first Chairman of the Photonics Society Italy Chapter, and is an IEEE Fellow, an OSA Fellow and a Member of SPIE.
Photodetectors can be broadly defined as those electronic devices yielding an output electrical signal in response to, and as a replica of the input light signal. They are a key element in virtually any optoelectronic system and application, paralleling in importance the role of sources. Indeed, whenever the system performance is described by a signal-to-noise ratio, increasing the source power by a factor K is equivalent to reducing the noise of the photodetector by the same amount.
The birth of photodetectors can be dated back to 1873 when W. Smith discovered photo conductivity in selenium, although Nobili and Macedonio Melloni already in 1829 observed that a thermocouple is sensitive to the warm body of a human passing by, what today we would describe as detection of infrared by a thermopile.
Progress was slow until 1905, when Einstein explained the newly observed photoelectric effect in metals, after Planck solving the blackbody emission puzzle with the introduction of the quanta hypothesis. Applications and new devices soon flourished, pushed by the dawning technology of television. In 30 years, photoelectric vacuum tubes covering all the fields of detection were developed, including the orthicon - the father of image pickup devices, the image converters and intensifiers, and the photomultiplier which technically survives today as the easy way to single-photon detection and a key device for probing elusive nuclear particles like solar neutrinos (see Fig.P-1). Worth to mention, in the 1950's Weimer, Forgue, and Goodrich at RCA invented the vidicon, a beautifully simple alternative to the complex orthicon, that soon became the undisputed workhorse of television cameras before being superseded by CCDs forty years later. After World War II, semiconductor devices were invented, a result of the improved understanding of solid-state physics along with adequate control of their technology. As a technical fallout, a number of semiconductor photo detectors have been developed with improved performance, spectral coverage, reliability and size. These have made possible new applications in instrumentation and communications.
Zworykin and Morton, the celebrated fathers of videonics, on the last page of their legendary book Television (1939) concluded that: 'when rockets will fly to the moon and to other celestial bodies, the first images we will see of them will be those taken by camera tubes, which will open to mankind new horizons'.
Their foresight became a reality with the Apollo and Explorer missions showing the landscape of the moon. More recently (1997), the CCD camera aboard the 2.4-m Hubble Space Telescope delivered a deep-space picture (see Fig.P-2), the result of 10 days integration, featuring galaxies of the 30th magnitude - an unimaginable figure even for astronomers of our generation. The next effort is the 6.5-m Webb Telescope due to launch in spring 2021. This instrument will feature a 25,000x25,000 pixel InSb CCD to detect the middle infrared and observe high redshift sources, looking back even closer to the big-bang age. Thus, photodetectors continue to open to mankind the most amazing new horizons.
We can distinguish two classes of photodetectors (Table 1-1) according to the handling of the received power: Single-element photodetectors are those yielding an output signal proportional to the total (or integral) power collected by the active area, and Image photodetectors are those with an active area physically or virtually divided in a string (1-D) or array (2-D) of individual elements (also called pixels from picture element), whose signals can be properly organized to be readable at the electrical output port. Pickup tubes, CCDs, and image converters or intensifiers belong to the image class.
Photodetectors are further classified according to the type of optical to electrical conversion effect. The most important is the photoelectric effect, by which a photon (or quantum of energy h?) is absorbed by the material with the release of an electron-hole pair.
If the photogenerated electron is further emitted out of the material, becoming available for collection or multiplication, the device is called a photoemission device, or one based on the external photoelectric effect. This is the class of vacuum photodiodes, photomultipliers, and photocathode-based pickup and image intensifier tubes. Instead, if no emission takes place but the photogenerated carriers are available for current circulation in the external circuit, we talk of an internal photoelectric effect or device, and we find all the semiconductor pho- todetectors: junction and avalanche photodiodes, phototransistors, photoresistances, etc.
The above classification has a practical importance: the emitted photoelectron is more valuable than one internal to the material, because it may be better singled out with respect to non-photogenerated electrons, that is, the dark current of the device. In addition, the emitted photoelectron can be multiplied by other electrodes with little added noise, as it actually is by means of the dynodes in a photomultiplier.
This is the reason why a photoemission device like the photomultiplier has been the unsur-passed detector of single photons, easily reaching the quantum-limit regime even at very low levels of input power. However, recent developments in SPAD devices (Chapter 6) have much progressed the solid-state counterpart of photomultipliers and added new functionalities like the image capability, as well as the superconducting nanowire (Chapter 7) that has unequalled speed of response.
Table 1-1 Synopsis of Photodetectors and their Spectral Ranges
SINGLE ELEMENT IMAGE
- - photoemission devices (or external photoelectric devices)
vacuum photodiode gas photodiode photomultiplier pickup tubes image intensifiers and converters
- - internal photoelectric devices
semiconductor photodiode avalanche photodiode, SPAD phototransistor (BJT, FET) photoresistance CCDs
thermocouple (or photopile)
thermistor (or bolometer)
pyroelectric uncooled IR FPA
- weak interaction detectors
photon drag, Golay cell
point contact diode
Photoemission devices share the drawbacks of vacuum-tube technology (high-bias voltages, fragility, unsuitability for batch manufacture) and are limited to rather a narrow spectral range of operation, from UV to near infrared.
Conversely, internal photoelectric devices cover a much wider spectral range, from deep UV to far infrared, albeit with different semiconductors, each optimal for a ?-interval not wider that an octave. The big advantage of solid-state components is that they are compact, rug- ged, reliable, manufactured in batches, and require low bias voltages.
Synonymous with photoelectric device (both internal and external) is the term quantum detector, in reference to the conversion mechanism.
Still another important class of photodetectors is that of thermal detectors. These detectors are based on a two-step process: radiation is first dissipated in an absorber and the subsequent small increase of temperature is measured with a conventional electronic temperature sensor. Though they have a much lower sensitivity as compared with photoelectric detectors, thermal detectors offer a very wide spectral range of operation, covering UV to far IR in a single unit and with a nearly flat response. This feature has used to advantage in power meters and for the easy calibration of detectors in different spectral ranges. Recently, integrated thermal detectors have been exploited to realize third-generation uncooled focal-plane arrays for handheld infrared cameras (see Fig.P-3 and Chapter 8).
Lastly, a lot of indirect and weak-interaction effects that have been discovered and proposed, especially in the spectral range of the far and extreme IR. The photon drag detector, the Golay cell, and the photo-electromagnetic detector are just few examples. We will not treat them in this book, because of performance or reproducibility not fully satisfactory from the engineering point of view. Instead, new effects based on the superconducting properties of materials have been demonstrated viable, and the SNSPD (Chapter 7) is an example.
From the perspective of applications, photodetectors are usually understood, and most frequently used, as the devices for the conversion of a radiant signal into an electrical signal, for data communications or performing measurement, and in this case, operation is in the small-power signal regime.
Recently, however, photodetectors have become important also for power conversion or photovoltaics, since solar cells (Chapter 9) are manufactured on large scale with cheap technologies and attain good conversion efficiency, and the cost of...