A liquid-crystal laser is a

laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fi ...

that uses a liquid crystal as the resonator cavity, allowing selection of emission wavelength and polarization from the

active laser medium The active laser medium (also called gain medium or lasing medium) is the source of optical gain within a laser. The gain results from the stimulated emission of photons through electronic or molecular transitions to a lower energy state from a h ...

. The lasing medium is usually a dye doped into the liquid crystal. Liquid-crystal lasers are comparable in size to diode lasers, but provide the continuous wide spectrum tunability of

dye laser A dye laser is a laser that uses an organic dye as the lasing medium, usually as a liquid solution. Compared to gases and most solid state lasing media, a dye can usually be used for a much wider range of wavelengths, often spanning 50 to 100 ...

s while maintaining a large coherence area. The tuning range is typically several tens of

nanometers 330px, Different lengths as in respect to the molecular scale. The nanometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: nm) or nanometer (American and British English spelling differences#-re ...

. Woltman 2007, p. 357

Self-organization Self-organization, also called spontaneous order in the social sciences, is a process where some form of overall order arises from local interactions between parts of an initially disordered system. The process can be spontaneous when suff ...

at micrometer scales reduces manufacturing complexity compared to using layered photonic metamaterials. Operation may be either in continuous wave mode or in pulsed mode.

History

Distributed feedback lasing using

Bragg reflection In physics and chemistry , Bragg's law, Wulff–Bragg's condition or Laue–Bragg interference, a special case of Laue diffraction, gives the angles for coherent scattering of waves from a crystal lattice. It encompasses the superposition of wave ...

of a periodic structure instead of external

mirror A mirror or looking glass is an object that reflects an image. Light that bounces off a mirror will show an image of whatever is in front of it, when focused through the lens of the eye or a camera. Mirrors reverse the direction of the im ...

s was first proposed in 1971, predicted theoretically with

cholesteric A cholesteric liquid-crystal display (ChLCD) is a display containing a liquid crystal with a helical structure and which is therefore chiral. Cholesteric liquid crystals are also known as '' chiral nematic liquid crystals''. They organize in laye ...

liquid crystals in 1978, achieved experimentally in 1980, and explained in terms of a photonic

band gap In solid-state physics, a band gap, also called an energy gap, is an energy range in a solid where no electronic states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference ( ...

in 1998. A

United States Patent Under United States law, a patent is a right granted to the inventor of a (1) process, machine, article of manufacture, or composition of matter, (2) that is new, useful, and non-obvious. A patent is the right to exclude others, for a limited ...

issued in 1973 described a liquid-crystal laser that uses "a liquid lasing medium having internal distributed feedback by virtue of the molecular structure of a cholesteric liquid-crystal material."

Mechanism

Starting with a liquid crystal in the nematic phase, the desired helical pitch (the distance along the helical axis for one complete rotation of the nematic plane subunits) can be achieved by doping the liquid crystal with a chiral molecule. For light circularly polarized with the same handedness, this regular modulation of the refractive index yields selective reflection of the wavelength given by the helical pitch, allowing the liquid-crystal laser to serve as its own resonator cavity.

Photonic crystal A photonic crystal is an optical nanostructure in which the refractive index changes periodically. This affects the propagation of light in the same way that the structure of natural crystals gives rise to X-ray diffraction and that the atomic ...

s are amenable to band theory methods, with the periodic dielectric structure playing the role of the periodic electric potential and a photonic band gap (reflection notch) corresponding to forbidden frequencies. The lower photon group velocity and higher density of states near the photonic bandgap suppresses spontaneous emission and enhances stimulated emission, providing favorable conditions for lasing. If the electronic band edge falls in the photonic bandgap, electron-hole recombination is strictly suppressed. This allows for devices with high lasing efficiency, low lasing threshold, and stable frequency, where the liquid-crystal laser acts its own waveguide. "Colossal" nonlinear optics, nonlinear change in refractive index is achievable in doped nematic-phase liquid crystals, that is the refractive index can change with illumination intensity at a rate of about 10³cm²/W of illumination intensity. Most systems use a semiconductor Laser pumping, pumping laser to achieve population inversion, though flash lamp and electrical pumping systems are possible. Tuning of the output wavelength is achieved by smoothly varying the helical pitch: as the winding changes, so does the length scale of the crystal. This in turn shifts the band edge and changes the optical path length in the lasing cavity. Applying a static electric field perpendicular to the dipole moment of the local nematic phase rotates the rod-like subunits in the hexagonal plane and reorders the chiral phase, winding or unwinding the helical pitch. Similarly, optical tuning of the output wavelength is available using laser light far from the pick-up frequency of the gain medium, with degree of rotation governed by intensity and the angle between the polarization of the incident light and the dipole moment. Reorientation is stable and reversible. The chiral pitch of a cholesteric phase tends to unwind with increasing temperature, with a Order and disorder (physics), disorder-order transition to the higher symmetry nematic phase at the high end. By applying a temperature gradient perpendicular to the direction of emission varying the location of stimulation, frequency may be selected across a continuous spectrum. Similarly, a quasi-continuous doping gradient yields multiple laser lines from different locations on the same sample. Spatial tuning may also be accomplished using a wedge cell. The boundary conditions of the narrower cell squeeze the helical pitch by requiring a particular orientation at the edge, with discrete jumps where the outer cells rotate to the next stable orientation; frequency variation between jumps is continuous. If a defect is introduced into the liquid crystal to disturb the periodicity, a single allowed mode may be created inside of the photonic bandgap, reducing power leeching by spontaneous emission at adjacent frequencies. Defect mode lasing was first predicted in 1987, and was demonstrated in 2003. Woltman 2007, pp. 332–334 While most such thin films lase on the axis normal to the film's surface, some will lase on a conic angle around that axis.

Applications

* Biomedical sensing: small size, low cost, and low power consumption offer a variety of advantages in biomedical sensing applications. Potentially, liquid-crystal lasers could form the basis for "lab on a chip" devices that provide immediate readings without sending a sample away to a separate lab. * Medical: low emission power limits such medical procedures as cutting during surgeries, but liquid-crystal lasers show potential to be used in microscopy techniques and ''in vivo'' techniques such as photodynamic therapy. * Display screens: liquid-crystal-laser-based displays offer most of the advantages of standard liquid-crystal displays, but the low spectral spread gives more precise control over color. Individual elements are small enough to act as single pixels while retaining high brightness and color definition. A system in which each pixel is a single spatially tuned device could avoid the sometimes long relaxation times of dynamic tuning, and could emit any color using spatial addressing and the same monochromatic pumping source. * Environmental sensing: using a material with a helical pitch highly sensitive to temperature, electric field, magnetic field, or mechanical strain, color shift of the output laser provides a simple, direct measurement of environmental conditions.

References

Bibliography

External links