Photonic crystal light converter
A new device could be a powerful tool for observation in physics and life sciences

Date:
July 22, 2020
Source:
University of Tokyo
Summary:
Spectroscopy is the use of light to analyze physical objects and
biological samples. Different kinds of light can provide different
kinds of information. Vacuum ultraviolet light is useful as it can
aid people in a broad range of research fields, but generation of
that light has been difficult and expensive. Researchers created
a new device to efficiently generate this special kind of light
using an ultrathin film with nanoscale perforations.



FULL STORY ========================================================================== Spectroscopy is the use of light to analyze physical objects and
biological samples. Different kinds of light can provide different kinds
of information.

Vacuum ultraviolet light is useful as it can aid people in a broad range
of research fields, but generation of that light has been difficult
and expensive.

Researchers created a new device to efficiently generate this special
kind of light using an ultrathin 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's infrared light
which you can feel in the form of heat, or see if you happen to be a
snake, that has a longer wavelength than visible light. At the opposite
end is ultraviolet (UV) light which you can use to produce vitamin D in
your skin, or see if you happen to be a bee. These and other forms of
light have many uses in science.

Within the UV range 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 scientists and medical
researchers as they can be used for chemical and physical analyses of
different materials and even biological samples.

However, there is more to light than a wavelength. For VUV to be truly
useful, it also needs to be twisted or polarized in a manner called
circular polarization. Existing methods to produce VUV, such as using
particle accelerators or laser-driven plasmas, have many drawbacks,
including cost, scale and complexity. But also, these can only produce untwisted linear polarized VUV. If there was a simple way to make circular polarized VUV, it would be extremely beneficial. Assistant Professor
Kuniaki Konishi from the Institute for Photon Science and Technology at
the University of Tokyo and his team may just 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," said Konishi. "Our photonic crystal dielectric nanomembrane
(PCN) consists of a sheet made from an aluminium oxide-based crystal
(?-Al2O3) only 48 nm thick. It sits atop a 525 micrometer-thick sheet of silicon which has 190 nm-wide holes cut into it 600 nm apart." To our
eyes the PCN membrane just looks like a flat featureless surface, but
under a powerful microscope the pattern of perforations can be seen. It
looks a little like the holes in a showerhead which increase the water
pressure to make jets.

"When pulses of circularly polarized blue laser light with a wavelength
of 470 nm shine down these channels in the silicon, the PCN acts on these pulses and twists them in the opposing direction," said Konishi. "It also shrinks their wavelengths to 157 nm which is well within the range of
VUV that is so useful in spectroscopy." With short pulses of circularly polarized VUV, researchers can observe fast or short-lived physical
phenomena at the submicrometer scale that are otherwise impossible to
see. Such phenomena include the behaviors of electrons or biomolecules. So
this new method to generate VUV can be useful to researchers in medicine,
life sciences, molecular chemistry and solid state physics.

Although a similar method has been demonstrated before, it produced less
useful longer wavelengths, and did so using a metal-based film which is
subject to rapid degradation in the presence of laser light. PCN is far
more robust to this.

"I am pleased that through our study of PCN, we found a new and useful application for circularly polarized light conversion, generating VUV
with the intensity required to make it ideal for spectroscopy," said
Konishi. "And it was surprising that the PCN membrane could survive
the repeated bombardment of laser light, unlike previous metal-based
devices. This makes it suitable for lab use where it may be used
extensively over long periods. We did this for basic science and I hope
to see many kinds of researchers make good use of our work."

========================================================================== Story Source: Materials provided by University_of_Tokyo. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. Kuniaki Konishi, Daisuke Akai, Yoshio Mita, Makoto Ishida,
Junji Yumoto,
Makoto Kuwata-Gonokami. Circularly polarized vacuum ultraviolet
coherent light generation using a square lattice photonic crystal
nanomembrane.

Optica, 2020; 7 (8): 855 DOI: 10.1364/OPTICA.393816 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200722093444.htm

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