Photonic Crystal Generates VUV Light at Wavelengths Suitable for Spectroscopy

Researchers at the University of Tokyo have created a tabletop device that efficiently generates circularly polarized vacuum ultraviolet (VUV) light using an ultrathin film with nanoscale perforations.

VUV wavelengths, which can be absorbed by air but can pass through a vacuum, are useful for chemical and physical analyses, especially VUV wavelengths in the region of around 120 to 200 nm. Circularly polarized light in the VUV region is important for probing the structural and electronic properties of matter. Moreover, a circularly polarized VUV coherent light enables scientists to observe the dynamics of biomolecules and electron spins in solids.

The researchers demonstrated a solid-based method for the direct generation of circularly polarized VUV coherent light using third-harmonic generation in a dielectric square lattice photonic crystal nanomembrane (PCN). The PCN consisted of a sheet made from an aluminium oxide-based crystal 48 nm thick, placed on top of a 525-μm-thick sheet of silicon that had 190-nm-wide holes cut into it 600 nm apart. Under a microscope, the pattern of perforations resembles the holes in a showerhead.

Electric field intensity (arb. unit)

The device's effectiveness relies heavily on the spacing of the holes. In this simulation, holes 600 nm apart (left) provide far greater results than holes 500 nm apart (right). Courtesy of Konishi et al.

The waveguide resonance of the PCN with fourfold rotational symmetry, irradiated by a circularly polarized fundamental beam, generated circularly polarized third harmonic at 157 nm with sufficient intensity for VUV spectroscopic applications.

“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,” professor Kuniaki Konishi said. “It also shrinks their wavelengths to 157 nm, which is well within the range of VUV that is so useful in spectroscopy.”

Circularly polarized laser light goes through the PCN device and comes out the other side as VUV polarized in the opposite direction. Courtesy of Konishi et al.

With short pulses of circularly polarized VUV, researchers can observe fast or short-lived physical phenomena, such as the behaviors of electrons or biomolecules, at the submicrometer scale. The Tokyo team’s method for generating VUV could be useful to researchers in medicine, life sciences, molecular chemistry, and solid-state physics. Although a similar method has been demonstrated before, the team said that the previous method produced less useful longer wavelengths, and did so using a metal-based film that was subject to rapid degradation in the presence of laser light. The PCN membrane was able to withstand the repeated bombardment of laser light, unlike metal-based devices, making it suitable for labs where it might be used extensively over long periods.

“We have created a simple device to convert circularly polarized visible laser light into circularly polarized VUV, twisted in the opposite direction,” Konishi said. “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.”

The research was published in Optica (www.doi.org/10.1364/OPTICA.393816).