Photonic Capsules for Injectable Laser Resonators

Optical microscopy image of the capsule-type laser resonator. (Source: KAIST)

A KAIST research group presented photonic capsules for injec­table laser resonators using microfluidic technology. The capsule’s diameter is comparable to a human hair and stable in gas and liquid media, so it is injec­table into any target volume. The research group headed by Shin-Hyun Kim in the Department of Chemical and Biomo­lecular Engi­neering applied an interes­ting optical property from nature. Kim, who has dived deep into photonic materials research inspired from nature such as the Morpho butterfly, used a trait of beetles this time.

Chrysina gloriosa, commonly known as the glorious beetle, shows a green color similar to leaves when illu­minated by left-handed, circularly-polarized light while showing no color with right-handed, circu­larly-polarized light. This unique optical feature helps the beetles communi­cate with each other and protects them from predators. The principle behind this interes­ting optical property of the beetles relies on helical nanostructures with left-handedness that are present on the shell of the beetles. The helical structures reflect a circularly-polarized light with the same handed­ness of the helix at the wavelength selected by the helical pitch through optical inter­ference.

Such helical nano­­structures can be arti­­ficially created using liquid crystals (LCs). LCs with a helical ar­range­­ment are referred to as cholesteric LCs (CLCs). The CLCs exhibit the polari­zation-dependent reflec­tion of light in the same manner as the beetles and have been used for various photonic appli­cations. In particular, CLCs have been cast to a film format that serves as mirror­less laser resonators, unlike conven­tional lasing systems. However, the film-type CLCs are large in size and show unidirec­tional emission, which restricts the use of CLC reso­nators in micro­environments.

To overcome these limi­tations, Kim’s group has encapsu­lated the CLCs with dual shells using micro­fluidic tech­nology. The inner shell is a water layer that promotes the alignment of LC molecules and the outer shell is an elastic polymer layer that secures capsule stability and enables reversible mechanical defor­mation. The spherical symmetry of the capsules enables omni­directional laser emissions. Moreover, laser intensity and lasing direction can be further controlled by deforming the capsules, while its wave­length remains tunable. This new type of CLC laser resonator is promising for laser treatments in various bio­medical appli­cations. Kim said, “The helical nano­structure used in the laser resonator resembles that of the shell of chrysina gloriosa. Humans learn from nature and engineer materials to create something unpre­cedented.” (Source: KAIST)

Reference: S. S. Lee et al.: Wavelength-tunable and shape-reconfigurable photonic capsule resonators containing cholesteric liquid crystals, Sci. Adv. 4, eaat8276 (2018); DOI: 10.1126/sciadv.aat8276

Link: Dept. of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology KAIST, Daejeon, Republic of Korea

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