Light in research

Light plays an important role in research at PSI, whether as a laser that triggers ultrafast processes in biological molecules, special illumination in cleanrooms or the glow of gas atoms when protons hit them. In this gallery, we highlight five different spectral colours.

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Red 

The red laser light is used here at PSI’s newest large research facility, the X-ray free-electron laser SwissFEL. Mirrors direct it onto samples, such as biomolecules, to put them in an excited state. The extremely rapid processes thereby induced are recorded by very short pulses of exceedingly brilliant X-rays. The result is an extreme slow-motion film of the triggered dynamics, vividly illustrating the atomic movements of these important building blocks of life. This fundamental research leads, for example, to new insights into light-harvesting proteins and processes involved in photosynthesis.

Yellow 

Yellow light is essential when preparing to transfer microstructures onto wafers – circular discs made of a semiconductor material such as silicon that are used to manufacture microchips. First, the wafers are cleaned, and foreign atoms are introduced to improve their electrical properties. Afterwards a photosensitive resist layer is applied, into which circuits will be etched using masks and light. During the application of the photoresist, no blue or ultraviolet light may be present. This is why lighting fixtures in the PSI cleanrooms at the adjacent Park Innovaare have been coated with filter films, producing the yellow light you see reflected off the wafer.

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Green 

The green laser light is used at the PSI Center for Proton Therapy, where cancer patients receive treatment. Proton beam radiation therapy is particularly gentle on surrounding healthy tissue. Since the shape and location of a tumour can change during the course of the multi-week therapy, low-dose computed tomography (CT) provides the basis for optimising radiation delivery. The reference lines projected onto the patient’s body by green laser light allow patients to be positioned accurately and reproducibly before each CT scan.

Blue 

The Swiss Light Source SLS uses accelerated electrons to generate X-rays of extremely high brilliance. It is used for research in fields including physics, materials science, biology, chemistry and environmental science. This versatility stems from the fact that synchrotron light is polychromatic. At around 20 experimental stations, the wavelengths required for specific experiments are selected from the spectrum. Synchrotron light predominantly lies outside the visible range; the blue light shown here belongs to the smaller portion that is visible. At the diagnostic beamline, the light is analysed to determine key machine parameters – to optimise the accelerator’s performance and ensure it is operating reliably.

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Violet 

This violet beam can be seen behind a vacuum window downstream of the PSI proton source. In the source, molecular hydrogen gas is irradiated with microwaves. This transforms the gas into plasma in which hydrogen nuclei – individual protons – can move about freely. These protons are accelerated by high voltage and form a beam. The proton beam itself emits no light. Rather, the violet colour arises from the protons colliding with residual gas, causing it to glow. Subsequently, in the main cyclotron at the heart of PSI’s proton accelerator facility, the protons are accelerated to 80 percent of the speed of light, resulting in one of the world’s most intense proton beams.

Text: Paul Scherrer Institute PSI/Christian Heid

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