Since lasers were first developed, the demand for more adaptable lasers has only increased. Chiral nematic liquid crystals (CLCs) are emerging devices that promise to shape the way lasers are used in the future because they have lower thresholds, are easier to manufacture, and can be tuned over a wider range of electromagnetic spectra. The latest work on how to choose the band edge mode in these devices - which determines the energy of the transmitted laser - may provide hope for how the future lasers will be tuned.
The laser cavity is formed by a chiral nematic liquid crystal doped with a fluorescent dye. This liquid crystal creates a photonic bandgap in the laser cavity. A research team has demonstrated a technique that enables the laser to electrically convert the emission between the long wavelength and the short wavelength edge of the photonic bandgap only by applying a voltage of 20 volts. They reported this result in the Applied Physics Letter, which is part of the American Institute of Physics (AIP).
"Our contribution is to find a way to change the direction of the dipole moment of the gain medium (fluorescent dye) transition in the CLC structure, and realize the mode between the long wavelength and the short wavelength edge without tuning the photonic band gap position. "Choose," said Chun-Ta Wang, one of the authors of the paper. "We have also demonstrated a polymer-stabilized CLC system that improves the stability of the laser, the ability to emit laser light, and the threshold voltage."
CLC lasers act through a series of self-assembled spiral modes and act as liquid crystals that later act as laser chambers. These helical structures are chiral, meaning that they move spirally in the same direction. This allows them to be tuned over a wide range of wavelengths. Although many lasers such as laser diodes used in DVD players are fixed in one color, many CLC lasers can be tuned to a variety of colors in and out of the visible light spectrum.
In addition to tuning the laser wavelength, one area of ​​interest has been the discovery of different ways of tuning the wavelength by switching the laser mode from one photonic bandgap edge to another. To date, some attempts have shown that it is possible to switch between long and short wavelength edges. The team led by Wang confirmed that this mode conversion is possible. The method is to apply a DC electric field to the fluorescent dye so as to change the order parameters without affecting the band gap spectral position. The researchers tested the three mixtures by changing the ratio of liquid crystal to dye and recording the laser output using fiber optic spectroscopy. They found that all samples are likely to transmit lasers from the short wavelength edge to the long wavelength edge. Moreover, polymer-stabilized planar CLC samples can in turn convert the two modes using additional structural stability and exhibit improved performance and threshold voltages.
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