Tuning Liquid-Crystal-Lasers

Demonstration of chiral nematic liquid crystals lasing device with liquid crystals self-assembled into helices. (Source: C.-T. Wang)

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. “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 from the National Sun Yat-Sen Univer­sity, Taiwan. “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. In addition to tuning the lasing wave­length, one hot area of inquiry is in finding different ways of tuning the wavelength by switching the lasing mode from one edge of the photonic bandgap to the other. Some attempts so far have sugges­ted it is possible to switch between the long- and short-wave­length edges.

Wang’s team’s work demon­strates that this mode switching is possible by applying a direct-current electric field to the fluorescent dye, altering its order para­meter without affecting the spectral position of its bandgap. The researchers tested three mixtures by varying ratios of liquid crystals and dyes and recor­ding their laser outputs through fiber-optic spectro­metry. They found that it was possible for all the samples to shift from lasing at the short-wave­length edge to lasing at the long-wave­length edge, a shift of nearly 40 nano­meters, with as little as 20 volts. Moreover, a polymer-stabi­lized planar CLC sample was able to leverage its extra structural stabi­lity to reversibly switch between the two modes and showed improved per­formance and threshold voltage.

“There have been many calcu­lations for how to achieve this pheno­menon in this field, but to our knowledge, this is the first time it was proven experi­mentally,” Wang said. Looking ahead, Wang said widespread use of CLC lasers is still slated for the future. In the meantime, he and his team are hoping to expand our under­standing of electri­cally assisted band-edge mode selection in other types of photonic crystals. (Source: AIP)

Reference: C.-T. Wang et al.: Electrically assisted bandedge mode selection of photonic crystal lasing in chiral nematic liquid crystals, Appl. Phys. Lett. 112, 043301 (2018); DOI: 10.1063/1.5010880

Link: Dept. of Photonics, Nat. Sun Yat-Sen Univ., Kaohsiung, Taiwan • Dept. of Electronics and Information Systems, Ghent Univ., Ghent, Belgium

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