Gentle terahertz light Switzerland: In the limelight

| Editor: Lisa Saller, Lisa Saller

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.

PSI researchers Christoph Hauri, Carlo Vicario and Mostafa Shalaby (left to right) in the laser laboratory at the Paul Scherrer Institute. The terahertz laser developed at the PSI is currently the most intensive terahertz light source in the world.
PSI researchers Christoph Hauri, Carlo Vicario and Mostafa Shalaby (left to right) in the laser laboratory at the Paul Scherrer Institute. The terahertz laser developed at the PSI is currently the most intensive terahertz light source in the world.
(Photo: Paul Scherrer Institut)

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.

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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.

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