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This is a demonstration of CLC lasing device with liquid crystals self-assembled into helices. Credit: Chun-Ta Wang

A new way to modify the dipole moment of cholesteric liquid crystals may improve future laser technology.

An international team of researchers has created a new technique that allows lasers to electrically switch emission between the long-and-short-wavelength edges of the photonic bandgap by applying a relatively small voltage.

Chiral nematic liquid crystals (CLCs) are an emerging class of lasing devices that are poised to shape how lasers are used in the future due to their low thresholds, ease of fabrication and ability to be tuned across wider swaths of the electromagnetic spectrum.

“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,” Chun-Ta Wang, PhD., an author of the paper from Ghent University, said in a statement. “We also demonstrated a polymer-stabilized CLC system, which improved the laser's stability, lasing performance and threshold voltage.”

The researchers doped the laser cavities of a chiral nematic liquid crystal with a fluorescent dye to create a photonic bandgap.

CLC lasers work through a collection of liquid crystals that self-assemble into helix-shaped patterns, which then act as the laser’s cavity. The helices are chiral—meaning they corkscrew in the same direction—allowing them to be tuned across a wide range of wavelengths.

CLC lasers can be tuned to multiple colors in the visible light spectrum and beyond, unlike many lasers that are fixed at one color including the laser diodes used in DVD players.

The researchers also demonstrated the ability to switch the lasing mode from one edge of the photonic bandgap to the other by applying a direct-current electric field to the fluorescent dye.   This alters its order parameter without affecting the spectral position of its bandgap.

They tested three mixtures by varying ratios of liquid crystals and dyes and recorded their laser outputs through fiber-optic spectrometry.

The team then discovered 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.

A polymer-stabilized planar CLC sample could also 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.

The researchers are now hoping to expand the understanding of electrically assisted band-edge mode selection in other types of photonic crystals.

The study was published in Applied Physics Letters.

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