Thinner Photonic Crystals

Scheme of the woodpile photonic crystal structure that has a similar structure as a diamond crystal but a thousand times magnified. (Source: U. Twente)

Photonic crystals are the nano­structures that can mani­pulate photons by means of an energy gap, similar to how the semiconductors in computer chips mani­pulate electronic current. It was always thought that photonic crystals should be thick and bulky to be func­tional. Scientists from the University of Tokyo, the University of Electro-Communi­cations in Tokyo, the Kyoto Institute of Technology and the University of Twente dis­covered that even very thin 3D photonic band gap crystals are powerful devices to strongly control the flow of light. The new insights lead to design rules for new opto­electronic devices for efficient telecommu­nication and computers, and thin solar cells.

“We studied carefully assembled photonic crystals with the woodpile structure”, says Takeyoshi Tajiri. “Our crystals consist of stacked arrays of rods in two perpen­dicular directions in a semi­conductor wafer such as gallium arsenide. The crystal structure is inspired by diamond gemstones.” The advanced method con­veniently allows making thin structures of only a few layers thick, between a few hundred nanometers to about one micron.

To inves­tigate their new crystals, the Japanese-Dutch team decided to measure the reflec­tivity spectra. Therefore, after fabri­cation in Japan, they were shipped to Twente for micro­scopic measure­ments. The spectra showed that the thin diamond-like photonic crystals function remarkably well: all crystals showed both high reflec­tivity and broad peaks. Remarkably, this occurred even for the thinnest crystal.

The reflected light is forbidden to enter crystals over a considerable range of wavelengths. In the Japanese-Dutch crystals, the situation is even more radical, since the light is forbidden to travel in all directions simul­taneously. Tajiri explains: “The quick formation of the for­bidden gap in our crystals is remarkable because earlier 3D crystals required a large thickness for a gap to emerge.” Satoshi Iwamoto, leader of the Japanese team, says: “Our discovery that even thin photonic crystals are powerful devices means that we can save consi­derable fabri­cation time and resources.” And Willem L. Vos, leader of the Dutch team, enthuses: “The result that even thin structures are fully func­tional is great news for applications in photo­voltaics. Here, scientists are looking for thin broadband back reflectors to improve the per­formance of thin solar cells”. (Source: U. Twente)

Reference: T. Tajiri et al.: Reflectivity of three-dimensional GaAs photonic band-gap crystals of finite thickness, Phys. Rev. B 101, 235303 (2020); DOI: 10.1103/PhysRevB.101.235303

Link: Complex Photonic Systems COPS, MESA+ Institute for Nanotechnology, University of Twente, Enschede, The Netherlands

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