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  • A New Breed of Tunable Lasers
Apr 2009
by Krista D. Zanolli

CAMBRIDGE, England -- A new technology using liquid crystals in concert with a single, laser-based light source is spawning a new breed of low cost, reliable and extremely useful tunable lasers. Liquid crystal (LC) lasers offer unprecedented depths of color resolution and could soon be used in displays, medical diagnostics and sensors, according to researchers.

Polychromatic laser emission from a gradient pitch liquid crystal cell, pumped from a single optical source. (Images: University of Cambridge Department of Engineering)

Engineers at the Centre of Molecular Materials for Photonics and Electronics (CMMPE) at the University of Cambridge are developing self-organizing LC lasers that are tiny and highly tunable, and that could ultimately be produced at a fraction of the cost of other display technologies.

These new lasers work by using three separate red, green and blue (RGB) sources, which have a very narrow spectral linewidth compared to the broadband RGB sources from other displays, such as liquid crystal displays (LCD), plasma and the latest organic light-emitting diode (OLED) displays.

Typically lasers have two mirrors and light bounces back and forth between them, however LC lasers don’t use mirrors because the optical cavity is defined by the pitch (the length of one complete twist of a crystal). By controlling the degree of twist, the researchers have discovered how to tune the output wavelength of the LC lasers.

“These polychromatic lasers are capable of narrowband emission at any wavelength in the spectrum from near ultraviolet to near-infrared,” said professor Harry J. Coles, director of the CMMPE. “When added to the ability to produce 100 x 100 arrays of such lasers, in tens of micron spots and indeed 10-micron cavity lengths, with very high conversion efficiencies of up to 60-70 percent, they open up a range of novel applications.”

Independent red, green and blue liquid crystal laser arrays.

The CMMPE researcher team led by Coles recently demonstrated a two-dimensional LC laser array consisting of RGB colors simultaneously being emitted from a single LC laser device, while being optically pumped with a single 430-nm source.

The published research suggests that LC lasers could replace the individual RGB lasers that are currently used in emerging (and expensive) laser displays. Furthermore, LC lasers are less likely to suffer from the undesirable visual property of speckle, which is typical with conventional laser display systems.

Because LC lasers are not merely restricted in their use to laser displays, researchers at CMMPE are also developing applications for their use in infrared medical diagnostic tools, telecommunication devices and holographic projection.

Liquid crystals are fast becoming an alternative medium for use as the feedback structure for a wide variety of miniature laser devices. What makes these liquid crystals attractive for lasing materials is their capability of spontaneously self-organizing themselves into photonic bandgap structures. According to the research, when combined with a gain medium, such as a fluorescent dye, the chiral liquid crystal provides sufficient feedback to generate lasing within a device of thicknesses less than a human hair.

Simultaneous red-, green- and blue-emitting liquid crystal laser array.

“Currently the lasers use a single pulsed solid-state miniature benchtop laser to pump the lens-let arrays, but we anticipate that this will be replaced by a miniature blue laser diode in the very near future,” Coles said. “The LC lasers have already demonstrated quasi-CW working at a 10-kHz repetition rate and so to the human eye appear to be a continuous ‘any wavelength’ laser.”

The study also reveals that the emission can be chosen to be at any desired wavelength across the visible range through careful control, chemically, of the macroscopic material properties. A gradient in the periodicity of the liquid crystal structure can therefore be formed, which gives rise to simultaneous different emission wavelengths across the device. Such a feature is not readily achievable with existing laser technologies.

As for the future of LC lasers, Cole added, “These ‘next steps’ include combinatorial spectroscopy, projection flat panel TVs, holographic mobile cell phones and a host of biological and medical applications. The beauty of the technology is that the lasers are based on known liquid crystal technology and therefore incredibly straightforward to fabricate.”

This research is ongoing and is part of the four-year Basic Technology Research Grant 'Cosmos' funded by the Engineering and Physical Sciences Research Council (EPSRC) to develop a new generation of micron-sized tunable coherent light sources based on ordered organic periodic structures.

“Our utopian aim is to use an organic or polymeric LED to act as the pump source to produce a miniature all organic/plastic laser,” said Coles.

Krista D. Zanolli

1. The range of frequencies or wavelengths over which radiations are absorbed or emitted in a transition between a specific pair of atomic energy levels. The full width is determined between half-power points of the line. 2. In a laser, the range of frequencies over which most of the beam energy is distributed.
liquid crystal
A type of material that possesses less geometrical regularity or order than normal solid crystals, and whose order varies in response to alterations in temperature and other quantities. Liquid crystals are characterized by phase varieties, including cholesteric, nematic and smectic. The optical properties of liquid crystals are familiar from their use in displays, known as LCDs.
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