Liquid crystal laser
Encyclopedia
A liquid crystal laser is a laser
that uses a liquid crystal
as the resonator cavity, allowing selection of emission wavelength and polarization from the active laser medium
. 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
s while maintaining a large coherence area
. The tuning range is typically several tens of nanometers. Self-organization at micrometer scales reduces manufacturing complexity compared to using layered photonic metamaterials. Operation may be either in continuous wave mode
or in pulsed mode.
of a periodic structure instead of external mirror
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
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."
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
change in refractive index is achievable in doped nematic-phase liquid crystals, that is the refractive index can change at a rate of about 1000 per 1 W/cm2 of illumination intensity. Most systems use a semiconductor 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 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.
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.
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
that uses a liquid crystal
Liquid crystal
Liquid crystals are a state of matter that have properties between those of a conventional liquid and those of a solid crystal. For instance, an LC may flow like a liquid, but its molecules may be oriented in a crystal-like way. There are many different types of LC phases, which can be...
as the resonator cavity, allowing selection of emission wavelength and polarization from the active laser medium
Active laser medium
The active laser medium is the source of optical gain within a laser. The gain results from the stimulated emission of electronic or molecular transitions to a lower energy state from a higher energy state...
. The lasing medium is usually a dye
Dye laser
A dye laser is a laser which 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. The wide bandwidth makes them particularly suitable for tunable lasers and...
doped into the liquid crystal. Liquid crystal lasers are comparable in size to diode lasers
Laser diode
The laser diode is a laser where the active medium is a semiconductor similar to that found in a light-emitting diode. The most common type of laser diode is formed from a p-n junction and powered by injected electric current...
, but provide the continuous wide spectrum tunability of dye laser
Dye laser
A dye laser is a laser which 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. The wide bandwidth makes them particularly suitable for tunable lasers and...
s while maintaining a large coherence area
Coherence (physics)
In physics, coherence is a property of waves that enables stationary interference. More generally, coherence describes all properties of the correlation between physical quantities of a wave....
. The tuning range is typically several tens of nanometers. Self-organization at micrometer scales reduces manufacturing complexity compared to using layered photonic metamaterials. Operation may be either in continuous wave mode
Continuous wave
A continuous wave or continuous waveform is an electromagnetic wave of constant amplitude and frequency; and in mathematical analysis, of infinite duration. Continuous wave is also the name given to an early method of radio transmission, in which a carrier wave is switched on and off...
or in pulsed mode.
History
Distributed feedback lasing using Bragg reflectionBragg's law
In physics, Bragg's law gives the angles for coherent and incoherent scattering from a crystal lattice. When X-rays are incident on an atom, they make the electronic cloud move as does any electromagnetic wave...
of a periodic structure instead of external mirror
Mirror
A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection...
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
Band gap
In solid state physics, a band gap, also called an energy gap or bandgap, is an energy range in a solid where no electron states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference between the top of the valence band and the...
in 1998.
A United States Patent
United States patent law
United States patent law was established "to promote the Progress of Science and useful Arts, by securing for limited Times to Authors and Inventors the exclusive Right to their respective Writings and Discoveries;" as provided by the United States Constitution. Congress implemented these...
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 crystalPhotonic crystal
Photonic crystals are periodic optical nanostructures that are designed to affect the motion of photons in a similar way that periodicity of a semiconductor crystal affects the motion of electrons...
s are amenable to band theory
Electronic band structure
In solid-state physics, the electronic band structure of a solid describes those ranges of energy an electron is "forbidden" or "allowed" to have. Band structure derives from the diffraction of the quantum mechanical electron waves in a periodic crystal lattice with a specific crystal system and...
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
Density of states
In solid-state and condensed matter physics, the density of states of a system describes the number of states per interval of energy at each energy level that are available to be occupied by electrons. Unlike isolated systems, like atoms or molecules in gas phase, the density distributions are not...
near the photonic bandgap suppresses spontaneous emission
Spontaneous emission
Spontaneous emission is the process by which a light source such as an atom, molecule, nanocrystal or nucleus in an excited state undergoes a transition to a state with a lower energy, e.g., the ground state and emits a photon...
and enhances stimulated emission
Stimulated emission
In optics, stimulated emission is the process by which an atomic electron interacting with an electromagnetic wave of a certain frequency may drop to a lower energy level, transferring its energy to that field. A photon created in this manner has the same phase, frequency, polarization, and...
, 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
Nonlinear optics
Nonlinear optics is the branch of optics that describes the behavior of light in nonlinear media, that is, media in which the dielectric polarization P responds nonlinearly to the electric field E of the light...
change in refractive index is achievable in doped nematic-phase liquid crystals, that is the refractive index can change at a rate of about 1000 per 1 W/cm2 of illumination intensity. Most systems use a semiconductor pumping laser
Laser pumping
Laser pumping is the act of energy transfer from an external source into the gain medium of a laser. The energy is absorbed in the medium, producing excited states in its atoms. When the number of particles in one excited state exceeds the number of particles in the ground state or a less-excited...
to achieve population inversion
Population inversion
In physics, specifically statistical mechanics, a population inversion occurs when a system exists in state with more members in an excited state than in lower energy states...
, 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
Order and disorder (physics)
In physics, the terms order and disorder designate the presence or absence of some symmetry or correlation in a many-particle system.In condensed matter physics, systems typically are ordered at low temperatures; upon heating, they undergo one or several phase transitions into less ordered...
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.
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 microscopyMicroscopyMicroscopy is the technical field of using microscopes to view samples and objects that cannot be seen with the unaided eye...
techniques and in vivoIn vivoIn vivo is experimentation using a whole, living organism as opposed to a partial or dead organism, or an in vitro controlled environment. Animal testing and clinical trials are two forms of in vivo research...
techniques such as photodynamic therapyPhotodynamic therapyPhotodynamic therapy is used clinically to treat a wide range of medical conditions, including malignant cancers, and is recognised as a treatment strategy which is both minimally invasive and minimally toxic...
. - 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.