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 fir ...

that uses a

liquid crystal Liquid crystal (LC) is a state of matter whose properties are between those of conventional liquids and those of solid crystals. For example, a liquid crystal may flow like a liquid, but its molecules may be oriented in a crystal-like way. T ...

as the

resonator cavity A resonator is a device or system that exhibits resonance or resonant behavior. That is, it naturally oscillates with greater amplitude at some frequencies, called resonant frequencies, than at other frequencies. The oscillations in a resonato ...

, allowing selection of emission wavelength and

polarization Polarization or polarisation may refer to: Mathematics *Polarization of an Abelian variety, in the mathematics of complex manifolds *Polarization of an algebraic form, a technique for expressing a homogeneous polynomial in a simpler fashion by ...

from the active laser medium. The lasing medium is usually a

dye A dye is a colored substance that chemically bonds to the substrate to which it is being applied. This distinguishes dyes from pigments which do not chemically bind to the material they color. Dye is generally applied in an aqueous solution an ...

doped into the liquid crystal. Liquid-crystal lasers are comparable in size to diode lasers, but provide the continuous wide spectrum tunability of dye lasers 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 suffi ...

at micrometer scales reduces manufacturing complexity compared to using layered

photonic metamaterial A photonic metamaterial (PM), also known as an optical metamaterial, is a type of electromagnetic metamaterial, that interacts with light, covering terahertz ( THz), infrared (IR) or visible wavelengths. The materials employ a periodic, cellular ...

s. Operation may be either in continuous wave mode or in pulsed mode.

History

Distributed feedback lasing using Bragg reflection of a periodic structure instead of external

mirror A mirror or looking glass is an object that Reflection (physics), 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 ...

s was first proposed in 1971, predicted theoretically with cholesteric liquid crystals in 1978, achieved experimentally in 1980, and explained in terms of a photonic band gap 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 crystals are amenable to

band theory In solid-state physics, the electronic band structure (or simply band structure) of a solid describes the range of energy levels that electrons may have within it, as well as the ranges of energy that they may not have (called ''band gaps'' or '' ...

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 Spontaneous emission is the process in which a quantum mechanical system (such as a molecule, an atom or a subatomic particle) transits from an excited energy state to a lower energy state (e.g., its ground state) and emits a quantized amount of ...

and enhances

stimulated emission Stimulated emission is the process by which an incoming photon of a specific frequency can interact with an excited atomic electron (or other excited molecular state), causing it to drop to a lower energy level. The liberated energy transfers to th ...

, 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 The lasing threshold is the lowest excitation level at which a laser's output is dominated by stimulated emission rather than by spontaneous emission. Below the threshold, the laser's output power rises slowly with increasing excitation. Above t ...

, and stable frequency, where the liquid-crystal laser acts its own waveguide. "Colossal"

nonlinear In mathematics and science, a nonlinear system is a system in which the change of the output is not proportional to the change of the input. Nonlinear problems are of interest to engineers, biologists, physicists, mathematicians, and many other ...

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 pumping laser to achieve

population inversion In science, specifically statistical mechanics, a population inversion occurs while a system (such as a group of atoms or molecules) exists in a state in which more members of the system are in higher, excited states than in lower, unexcited energ ...

, 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 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 Surgery ''cheirourgikē'' (composed of χείρ, "hand", and ἔργον, "work"), via la, chirurgiae, meaning "hand work". is a medical specialty that uses operative manual and instrumental techniques on a person to investigate or treat a pat ...

, but liquid-crystal lasers show potential to be used in

microscopy Microscopy is the technical field of using microscopes to view objects and areas of objects that cannot be seen with the naked eye (objects that are not within the resolution range of the normal eye). There are three well-known branches of micr ...

techniques and ''

in vivo Studies that are ''in vivo'' (Latin for "within the living"; often not italicized in English) are those in which the effects of various biological entities are tested on whole, living organisms or cells, usually animals, including humans, and ...

'' techniques such as

photodynamic therapy Photodynamic therapy (PDT) is a form of phototherapy involving light and a photosensitizing chemical substance, used in conjunction with molecular oxygen to elicit cell death (phototoxicity). PDT is popularly used in treating acne. It is used cl ...

. * 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