PMTs and what they’re good for
7 years ago DTU Electro and the Japanese company Hamamatsu Photonics joined forces to develop a new class of photodetectors based on an old-school concept: The photomultiplier tube (PMT).
Classic PMTs are an enabling component in a range of modern technologies.
They are e.g. used in nuclear medicine imaging where they detect the signal originating from a radioactive contrast liquid in the patient, thus enabling diagnosis and treatment of e.g. heart disease and cancer. PMTs are also used in space exploration, such as on the James Webb telescope, to provide fine guidance of the entire telescope and allowing it to maintain a high degree of pointing precision when photographing galaxies far away.
Photomultiplier tubes (PMTs) are a type of photodetector that have been used for over 60 years without major updates. This is because they rely on basic physics that haven’t changed.
Here’s how PMTs work: they have a vacuum tube with a special surface called a photocathode on one end. When light hits the photocathode, it releases an electron into the vacuum. This electron is then multiplied by accelerating it and making it hit a metal wall inside the tube, which releases more electrons. This multiplication process can happen up to 10 times, turning one electron into 100 million electrons. These electrons create an electrical current, which indicates that a photon (a particle of light) started the process.
Because the process happens in a vacuum, there is very little noise, even at room temperature. However, the photocathode needs to release an electron into the vacuum to start, which requires light in the near infrared range or higher. This means PMTs can't work with mid- or far-infrared light.
Until now!
A new class
Researchers at DTU Electro have discovered that by using a specially designed metasurface instead of the traditional photocathode, a PMT can detect light at any frequency, from microwaves to visible light.
The classic photocathode works using the photoelectric effect, where light causes electrons to be emitted. The new metasurface, however, uses a different process called light-driven field emission. In this process, the incoming electromagnetic radiation is focused on tiny spots on the metasurface. At these spots, the energy is so intense that it causes electrons to jump from the metasurface into the vacuum through quantum tunnelling.
By adjusting the design of the metasurface, this effect can be tuned to work with any frequency of light.
In essence, this breakthrough allows Hamamatsu Photonics to take an existing class of photodetectors and only exchange a small part of it to suddenly being able to operate it in completely new frequency regimes. Seeing as Hamamatsu Photonics has hundreds of PMTs in the catalogue and more than 1000 people involved in the company’s production, this can lead to huge commercial opportunities in the market.
Perspectives
We now have the ability to use these detectors for new things, and we are still exploring where they should go. Fundamental research is a no-brainer.
The new PMTs also seem to work well in the inspection of semiconductor materials and 2D materials – here, they can be used to map the quality of electrical properties of materials with unprecedented precision, which is key to creating high-speed electronics such as faster transistors.
For certain types of gases, e.g CO2, these PMTs could be ideal for environmental monitoring by observing fluctuations in gas concentration with extremely high sensitivity. Time will tell what other applications we will find.
This project demonstrates that cross-border collaborations like ours with Hamamatsu Photonics are not only where innovation is created but also realised into impact.
