07-06-2021 | | By Robin Mitchell
Photomultipliers are critical for detecting extremely weak light signals, but are often bulky and use old technology such as vacuum tubes. Why are photomultipliers important, what have the researchers demonstrated, and how could it enable new applications?
Photomultipliers are devices that can amplify a weak light signal into a strong one so that it is more easily detected. A classic example is true night vision; a normal camera cannot see in the dark as the light hitting objects are far too faint, but the photomultiplier in a night vision set can amplify these weak signals to create an image.
But creating a photo-multiplying device is not easy, and these devices need to have a degree of repeatability to be useful. The most common design uses a vacuum tube with multiple charged plates that will emit more electrons upon colliding with an electron (i.e. multiply). At the start of the tube is a photocathode that “converts” an incoming photon into a free electron which goes on to hit the first plate. With each successive plate, the number of electrons grows exponentially, and thus a single photon causes a large current flow.
While vacuum tube photomultipliers work well, they are large and expensive. Like most electronics, it would be more practical if a photomultiplier could be designed using modern solid-state electronics, but so far no such device exists. Avalanche photodiodes do exist, but these only provide one amplification stage and can be susceptible to noise.
Recently, engineers from the University of Texas and University of Virginia announced the development of photodiode devices that can multiply incoming light signals multiple times like a photomultiplier. While the concept of such a device has been around since the 80s, it has only been possible to create such a device recently due to the need for advanced technologies.
The new device creates a staircase-like conversion stage whereby an incoming photo releases a single electron, and this electron then continues to release another electron. Those two electrons continue to release four electrons in total following a 2n pattern. The device consists of a single-pixel sensor whose size is close to that of standard pixels. The ability to create non-random multiplication enables the sensor to amplify weak signals while reducing noise accurately.
According to the researchers, the ability to create such devices stems from researchers being able to accurately grow and deposit layers of different materials and crystals on top of each other. Then, the electrons can interact with the different grown layers, and it is the structure that enables the amplification of free electrons.
While such diodes could be useful in low-light cameras, one area, in particular, that is of interest to researchers is LiDAR. The ability to accurately and reliably amplify low-light levels can enable LiDAR that is not only noise-free but has extended ranges. Such improvements to LiDAR would help emerging technologies such as autonomous driving systems need LiDAR to detect distant objects. But it is not just autonomous vehicles that would benefit; any application involved with LiDAR would benefit. For example, any ranging application would be able to measure greater distances while 3D mapping systems would be more sensitive to reflections from objects.
Other photodiode applications that could benefit from such a device also include optical communication systems. Optic fibre cables used for communication suffer from signal degradation, and as such require repeaters. The use of highly sensitive photodiodes would allow for fewer of these relays to be used, and the additional sensitivity can also help to increase the bandwidths of such systems.