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 lasers 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.
Distributed feedback lasing using Bragg reflection of a periodic structure instead of external mirrors 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."
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 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 with illumination intensity at a rate of about 103cm2/W 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.
- 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.
- Woltman 2007, p. 357
- Jacobs; Cerqua; Marshall; Schmid; Guardalben; Skerrett (1988). "Liquid-crystal laser optics: design, fabrication, and performance". Optical Society of America. doi:10.1364/JOSAB.5.001962.
- Kogelnik, H.; C.V. Shank (1971). "Stimulated emission in a periodic structure". Applied Physics Letters. 18 (4): 152. Bibcode:1971ApPhL..18..152K. doi:10.1063/1.1653605.
- Kukhtarev, NV (1978). "Cholesteric liquid crystal laser with distributed feedback". Soviet Journal of Quantum Electronics. 8 (6): 774. Bibcode:1978QuEle...8..774K. doi:10.1070/QE1978v008n06ABEH010397.
- Ilchishin, I.P.; E.A. Tikhonov; V.G. Tishchenko; M.T. Shpak (1980). "Generation of a tunable radiation by impurity cholesteric liquid crystals". Journal of Experimental and Theoretical Physics Letters. 32: 24–27. Bibcode:1980JETPL..32...24I.
- Woltman 2007, p. 310
- Kopp, V.I.; B. Fan; H. K. M. Vithana; A. Z. Genack (1998). "Low-threshold lasing at the edge of a photonic stop band in cholesteric liquid crystals". Optics Express. 23 (21): 1707–1709. Bibcode:1998OptL...23.1707K. doi:10.1364/OL.23.001707. PMID 18091891.
- Dolgaleva, Ksenia; Simon K.H. Wei; Svetlana G. Lukishova; Shaw H. Chen; Katie Schwertz; Robert W. Boyd (2008). "Enhanced laser performance of cholesteric liquid crystals doped with oligofluorene dye". Journal of the Optical Society of America. 25 (9): 1496–1504. Bibcode:2008JOSAB..25.1496D. doi:10.1364/JOSAB.25.001496.
- Lawrence Goldberg and Joel Schnur Tunable internal-feedback liquid crystal-dye laser U.S. Patent 3,771,065 Issue date: 1973
- Kuroda, Keiji; Tsutomu Sawada; Takashi Kuroda; Kenji Watanabe; Kazuaki Sakoda (2009). "Doubly enhanced spontaneous emission due to increased photon density of states at photonic band edge frequencies". Optics Express. 17 (15): 13168–13177. Bibcode:2009OExpr..1713168K. doi:10.1364/OE.17.013168. PMID 19654722.
- Yablonovich, Eli (1987). "Inhibited Spontaneous Emission in Solid-State Physics and Electronics". Physical Review Letters. 58 (20): 2059–2062. Bibcode:1987PhRvL..58.2059Y. doi:10.1103/PhysRevLett.58.2059. PMID 10034639.
- Lucchetti, L.; M. Di Fabrizio; O. Francescangeli; F. Simoni (2004). "Colossal optical nonlinearity in dye doped liquid crystals". Optics Communications. 233 (4–6): 417–424. Bibcode:2004OptCo.233..417L. doi:10.1016/j.optcom.2004.01.057.
- Khoo, I.C. (1995). "Holographic grating formation in dye- and fullerene C60-doped nematic liquid-crystal film". Optics Letters. 20 (20): 2137–2139. Bibcode:1995OptL...20.2137K. doi:10.1364/OL.20.002137. PMID 19862276.
- Khoo, Iam-Choo (2007). Liquid Crystals. Wiley-Interscience. ISBN 978-0-471-75153-3.
- Morris, Stephen M.; Philip JW Hands; Sonja Findeisen-Tandel; Robert H. Cole; Timothy D. Wilkinson; Harry J. Coles (2008). "Polychromatic liquid crystal laser arrays towards display applications" (PDF). Optics Express. 16 (23): 18827–37. Bibcode:2008OExpr..1618827M. doi:10.1364/OE.16.018827. PMID 19581971.
- Maune, Brett; Marko Lončar; Jeremy Witzens; Michael Hochberg; Thomas Baehr-Jones; Demetri Psaltis; Axel Scherer; Yueming Qiu (2004). "Liquid-crystal electric tuning of a photonic crystal laser". Applied Physics Letters. 85 (3): 360. Bibcode:2004ApPhL..85..360M. doi:10.1063/1.1772869.
- Furumi, Seiichi; Shiyoshi Yokoyama; Akira Otomo; Shinro Mashiko (2004). "Phototunable photonic bandgap in chiral liquid crystal device". Applied Physics Letters. 84 (14): 2491. Bibcode:2004ApPhL..84.2491F. doi:10.1063/1.1699445.
- Andy, Fuh; Tsung-Hsien Lin; J.-H. Liu; F.-C. Wu (2004). "Lasing in chiral photonic liquid crystals and associated frequency tuning". Optics Express. 12 (9): 1857–1863. Bibcode:2004OExpr..12.1857F. doi:10.1364/OPEX.12.001857. PMID 19475016.
- Khoo, Iam-Choo; Wu, Shin-Tson (1993). Optics and nonlinear optics of liquid crystals. World Scientific. ISBN 978-981-02-0934-6.
- Morris, S.M.; A. D. Ford; M. N. Pivnenko; H. J. Coles (2005). "Enhanced emission from liquid-crystal lasers". Journal of Applied Physics. 97 (2): 023103. Bibcode:2005JAP....97b3103M. doi:10.1063/1.1829144.
- Morris, SM; AD Ford; HJ Coles (July 2009). "Removing the discontinuous shifts in emission wavelength of a chiral nematic liquid crystal laser". Journal of Applied Physics. 106 (2): 023112. Bibcode:2009JAP...106b3112M. doi:10.1063/1.3177251.
- Ozaki, M.; M. Kasano; D. Ganzke; W. Haase; K. Yoshino (2002). "Mirrorless lasing in a dye-doped ferroelectric liquid crystal". Advanced Materials. 14 (4): 306–309. doi:10.1002/1521-4095(20020219)14:4<306::AID-ADMA306>3.0.CO;2-1.
- Huang, Yuhua; Ying Zhou; Shin-Tson Wu (2006). "Spatially tunable laser emission in dye-doped photonic liquid crystals". Applied Physics Letters. 88: 011107. Bibcode:2006ApPhL..88a1107H. doi:10.1063/1.2161167.
- Jeong, Mi-Yun; Hyunhee Choi; J. W. Wu (2008). "Spatial tuning of laser emission in a dye-doped cholesteric liquid crystal wedge cell". Applied Physics Letters. 92 (5): 051108. Bibcode:2008ApPhL..92e1108J. doi:10.1063/1.2841820.
- Woltman 2007, pp. 332–334
- Schmidtke, Jürgen; Werner Stille; Heino Finkelmann (2003). "Defect Mode Emission of a Dye Doped Cholesteric Polymer Network" (PDF). Physical Review Letters. 90 (8): 083902. Bibcode:2003PhRvL..90h3902S. doi:10.1103/PhysRevLett.90.083902. PMID 12633428. Retrieved 2011-04-29.
- Lee, C.-R.; Lin, S.-H.; Yeh, H.-C.; Ji, T.-D. (7 December 2009). "Band-tunable color cone lasing emission based on dye-doped cholesteric liquid crystals with various pitches and a pitch gradient" (PDF). Optics Express. 17 (25). Bibcode:2009OExpr..1722616L. doi:10.1364/oe.17.022616.
- "Liquid crystal lasers the size of a human hair". Physorg. December 2005. Retrieved 2011-04-09.
- "Liquid crystal lasers promise cheaper, high colour resolution laser television". Physorg. April 2009. Retrieved 2011-04-09.
- "Laser Displays: liquid-crystal laser promises low-fabrication-cost display". Laser Focus World. January 2009. Retrieved 2011-04-09.
- Palffy-Muhoray, Peter; Wenyi Cao; Michele Moreira; Bahman Taheri; Antonio Munoz (2006). "Photonics and lasing in liquid crystal materials". Philosophical Transactions of the Royal Society A. 364 (1847): 2747–2761. Bibcode:2006RSPTA.364.2747P. doi:10.1098/rsta.2006.1851.
- Woltman, Scott J.; Crawford, Gregory Philip; Jay, Gregory D. (2007). Liquid crystals: frontiers in biomedical applications. World Scientific. ISBN 978-981-270-545-7.
- Coles, Harry; Stephen Morris (2010). "Liquid-crystal lasers". Nature Photonics. 4 (10): 676–685. Bibcode:2010NaPho...4..676C. doi:10.1038/nphoton.2010.184.
- Joannopoulos, John D.; Johnson, Steven G.; Winn, Joshua N.; Meade, Robert D. (2008). Photonic Crystals: Molding the Flow of Light. Princeton University Press. ISBN 978-0-691-12456-8. Retrieved 2011-04-10.