Researchers develop photonic crystal light converter

Spectroscopy is the use of light to analyze physical objects and biological samples. Different types of light can provide different types of information. Vacuum ultraviolet light is useful as it can help people in a wide range of research fields, but generating that light has been difficult and expensive. The researchers created a new device to efficiently generate this special type of light using an ultra-thin film with nanoscale perforations.

The wavelengths of light you see with your eyes constitute a mere fraction of the possible wavelengths of light that exist. There is infrared light that you can feel in the form of heat, or see if you are a snake, which has a longer wavelength than visible light. At the opposite extreme is ultraviolet (UV) light that you can use to make vitamin D on your skin, or see if it’s a bee. These and other forms of light have many uses in science.

Within the UV range there is a subset of wavelengths known as vacuum ultraviolet light (VUV), so called because they are easily absorbed by air but can pass through a vacuum. Some VUV wavelengths in the region of around 120-200 nanometers are of particular use to medical scientists and researchers, as they can be used for chemical and physical analyzes of different materials and even biological samples.

However, there is more to light than a wavelength. For VUV to be really useful, it also needs to be twisted or polarized in a way called circular polarization. Existing methods of producing VUVs, such as the use of laser-powered plasma or particle accelerators, have many drawbacks, including cost, scale, and complexity. But also, these can only produce non-twisted linear polarized VUV. If there was a simple way to make circular polarized VUVs, it would be extremely beneficial. Assistant Professor Kuniaki Konishi of the University of Tokyo Institute for Photon Science and Technology and her team may have the answer.

“We have created a simple device to convert circularly polarized visible laser light into circularly polarized VUV, twisted in the opposite direction,” Konishi said. “Our photonic crystal dielectric nanomembrane (PCN) consists of a sheet made of an aluminum oxide-based crystal (ℽ-Al2O3) only 48 nm thick. It sits on a 525 micron thick silicon sheet that has 190nm wide holes within 600nm apart. “

In our eyes, the PCN membrane looks like a flat surface with no distinctive features, but under a powerful microscope the perforation pattern can be seen. It looks a bit like holes in a shower head that increase water pressure to make jets.

“When pulses of circularly polarized blue laser light with a wavelength of 470nm shine through these channels in silicon, the PCN acts on these pulses and twists them in the opposite direction,” Konishi said. “It also reduces its wavelengths to 157nm, which is within the VUV range that is so useful in spectroscopy.”

With short pulses of circularly polarized VUV, researchers can observe fast or short-lived physical phenomena on a submicron scale that would otherwise be impossible to see. Such phenomena include the behavior of electrons or biomolecules. So this new method of generating VUVs can come in handy for researchers in medicine, life science, molecular chemistry, and solid-state physics. Although a similar method has been demonstrated previously, it produced less useful longer wavelengths, and it did so using a metal-based film that is subject to rapid degradation in the presence of laser light. PCN is much more robust to this.

“I am pleased that through our PCN study, we found a useful new application for converting circularly polarized light, generating VUV with the intensity necessary to make it ideal for spectroscopy,” said Konishi. “And it was surprising that the PCN membrane was able to survive repeated bombardment of laser light, unlike previous metal-based devices. This makes it suitable for laboratory use where it can be used extensively for long periods. We made it for Basic science and I look forward to seeing many types of researchers make good use of our work. ”