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Arthur T Knackerbracket has found the following story:

A new way of modifying the dipole moment of cholesteric liquid crystals allows for researchers to select between the different band-edge modes experimentally for the first time.

Since lasers were first developed, the demand for more adaptable lasers has only increased. Chiral nematic liquid crystals (CLCs) are an emerging class of lasing devices that are poised to shape how lasers are used in the future because of their low thresholds, ease of fabrication, and ability to be tuned across wider swaths of the electromagnetic spectrum. New work on how to select band-edge modes in these devices, which determine the lasing energy, may shine light on how lasers of the future will be tuned.

The laser cavities are formed of a chiral nematic liquid crystal doped with a fluorescent dye. The liquid crystal creates a photonic bandgap in the laser cavity. An international team of researchers demonstrated a technique that allows the laser to electrically switch emission between the long- and short-wavelength edges of the photonic bandgap simply by applying a voltage of 20 V. They report their work this week in Applied Physics Letters, from AIP Publishing.

"Our contribution is to find a way to change the orientation of the transition dipole moment of the gain medium [the fluorescent dye] in the CLC structure and achieve mode selection between long- and short-wavelength edges without tuning the position of the photonic bandgap," said Chun-Ta Wang, an author of the paper. "We also demonstrated a polymer-stabilized CLC system, which improved the laser's stability, lasing performance and threshold voltage."

CLC lasers work through a collection of liquid crystals that self-assemble into helix-shaped patterns, which then act as the laser's cavity. These helices are chiral, meaning they corkscrew in the same direction, which allows them to be tuned across a wide range of wavelengths. While many lasers, like the laser diodes used in DVD players, are fixed at one color, many CLC lasers can be tuned to multiple colors in the visible light spectrum and beyond.

Chun-Ta Wang, Chun-Wei Chen, Tzu-Hsuan Yang, Inge Nys, Cheng-Chang Li, Tsung-Hsien Lin, Kristiaan Neyts and Jeroen Beeckman. Electrically assisted bandedge mode selection of photonic crystal lasing in chiral nematic liquid crystals. Applied Physics Letters Jan. 22, 2018 (DOI: 10.1063/1.5010880.

Original Submission