Gentle terahertz light Switzerland: In the limelight
Researchers from the Paul Scherrer Institute (PSI) succeeded in visualising terahertz light with commercially available camera technology. This opens up many new opportunities for applications in areas like security or medicine.
Terahertz light is a potentially very effective method to uncover hidden structures. It effortlessly penetrates paper, plastics or textiles and makes objects covered by these materials visible. And even though its penetration depth in biological tissue is only a few millimetres, it has one feature that makes it particularly interesting for medical diagnostics: unlike X-ray radiation, it does not damage the tissue.
The reason is that terahertz light is composed of relatively low-energy particles (photons). This property also makes terahertz light an important tool in scientific research, because it can trigger processes without the trigger leaving behind any traces. This is utilised for example in researching new materials for magnetic data storage, where flashes of terahertz light enable scientists to magnetise an analysed material or to change its optical properties virtually lightning-fast.
Too gentle for robust sensors
However, as convenient the low photon energy of terahertz light might be, it has also caused a few problems, because so far it could only be visualised with bolometers. Bolometers are not only expensive, but also very sensitive to environmental influences - particularly to heat. Thus holding a hand too close to the sensor might already distort the results. Also, the sensor's image resolution is relatively low.
CCD sensors, which not only ensure a razor sharp image in smartphone or video cameras, but are also widely used in scientific research, so far could not be used for terahertz light because the light was too weak for the robust sensors - they simply did not respond to it.
High intensity for sharp images
Thanks to a stronger terahertz laser developed at the PSI, Christoph Hauri and his team were now able to overcome the CCD sensors' sensitivity threshold. In contrast to previous terahertz laser sources, the terahertz laser developed at PSI is characterized by a particularly high intensity, explains Christoph Hauri.
With their experiment, the researchers were able to demonstrate that commercially available CCD sensors can be used to make the PSI's intensive terahertz light visible. This is an important technological milestone: «Now that terahertz light is intensive enough to be able to visualise it with a regular CCD sensor, we can get images with a resolution 25 times better than the bolometer's», says a happy Mostafa Shalaby, who conducted the experiment. This is because the CCD has an approximately five times higher pixel density than the bolometer. And being able to use CCD sensors provides another major advantage: Its sensitivity to environmental influences such as heat is negligible.
No longer groping in the dark
The intensive PSI terahertz laser was specially developed for future applications on the SwissFEL. The free-electron laser SwissFEL is currently being built at the PSI and will be put into operation from the end of 2016. It will produce X-ray light pulses with the properties of laser light. That terahertz light can now be visualised with CCD sensors will provide various benefits, as the terahertz lasers will be used in combination with the SwissFEL's X-ray light. For example, to research new materials for magnetic data storage, a terahertz laser pulse will trigger a change in the magnetisation of a sample of the examined material. A few femtoseconds later, the sample will be X-rayed with the SwissFEL's X-ray laser pulse. Thus, researchers can find out what has happened within the sample during these femtoseconds.
The researchers are particularly interested in the fact that CCD sensors can now visualise terahertz light in its experimental environment. «This allows us to record the terahertz beam's exact spatial location during the experiment», Hauri emphasises. Furthermore, the CCD sensor's refresh rate is high enough to keep up with the speed at which experiments take place on the SwissFEL. The SwissFEL fires 100 X-ray light pulses per second and with each of these, a separate experiment is conducted. After the researchers have now demonstrated that the visualisation of terahertz light with CCD sensors works in principle, their goal is to further develop this idea. «It is of course possible to tailor CCD sensors for specific research applications», says Carlo Vicario, who has carried out the experiment with Mostafa Shalaby. But the researchers also see great potential for applications outside of research. «CCD sensors are inexpensive and robust. But for widespread application, there would also have to be marketable terahertz lasers of sufficient intensity. But since CCD sensors not only perform better but also cost less than one-tenth of a bolometer, they will surely quickly gain a foothold in the rapidly growing discipline of Terahertz science», Hauri is convinced.