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Liquid-crystal laser

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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 high power output and large coherence area. The tuning range is typically several tens nanometers.[1] 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.[2]

History

Distributed feedback lasing using Bragg reflection of a periodic structure instead of external mirrors was first proposed in 1971,[3] predicted theoretically cholesteric liquid crystals in 1978,[4] achieved experimentally in 1980,[5] and explained in terms of a photonic bandgap in 1998.[6][7][8] 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."[9]

Mechanism

Starting with a liquid crystal in the nematic phase, the desired helical pitch (distance along the helical axis for one complete rotation of the nematic plane subunits) can be achieved by doping with a chiral molecule.[8] 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.[7][10] If the electronic band edge falls in the photonic bandgap, electron-hole recombination is strictly suppressed.[11] 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.[12][13][14] Most systems use a semiconductor pumping laser to achieve population inversion, though flash lamp and electrical pumping systems are possible.[15]

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.[16] 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.[17][18][19] 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.[5][20][21][22] By applying a temperature gradient perpendicular to the direction of emission varying the location of stimulation, frequency may be selected across a continuous spectrum.[23] Similarly, a quasi-continuous doping gradient yields multiple laser lines from different locations on the same sample.[15] 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.[24]

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.[25][11][26]

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.[27]

Applications

  • Biomedical: 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.[28]
  • Medical: although due to the low power of emission liquid crystal laser probably would not find the use in such medical procedures as cutting during surgeries, they still a great potential to be used in "microscopy techniques employing lasers are an ideal application for tunable liquid crystal lasers; photodynamic therapy holds promise as an in vivo technique requiring carefully tuned, low-power light sources."[1]
  • 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.[28][29][30]
  • 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.[31]

References

  1. ^ a b Woltman, p. 357
  2. ^ 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. {{cite web}}: |access-date= requires |url= (help); Missing or empty |url= (help)
  3. ^ Kogelnik, H. (1971). "Stimulated emission in a periodic structure". Applied Physics Letters. 18 (4): 152. doi:10.1063/1.1653605. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  4. ^ Kukhtarev, NV (1978). "Cholesteric liquid crystal laser with distributed feedback". Soviet Journal of Quantum Electronics. 8 (6): 774. doi:10.1070/QE1978v008n06ABEH010397.
  5. ^ a b Ilchishin, I.P. (1980). "Generation of a tunable radiation by impurity cholesteric liquid crystals". Journal of Experimental and Theoretical Physics Letters. 32: 24–27. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  6. ^ Woltman, p. 310
  7. ^ a b Kopp, V.I. (1998). "Low-threshold lasing at the edge of a photonic stop band in cholesteric liquid crystals". Optics Express. 23 (21): 1707–1709. doi:10.1364/OL.23.001707. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  8. ^ a b Dolgaleva, Ksenia (2008). "Enhanced laser performance of cholesteric liquid crystals doped with oligofluorene dye". Journal of the Optical Society of America. 25 (9): 1496–1504. doi:10.1364/JOSAB.25.001496. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)}
  9. ^ Lawrence Goldberg and Joel Schnur Tunable internal-feedback liquid crystal-dye laser U.S. patent 3,771,065 Issue date: 1973
  10. ^ Kuroda, Keiji (2009). "Doubly enhanced spontaneous emission due to increased photon density of states at photonic band edge frequencies". Optics Express. 17 (15): 13168–13177. doi:10.1364/OE.17.013168. PMID 19654722. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  11. ^ a b Yablonovich, Eli (1987). "Inhibited Spontaneous Emission in Solid-State Physics and Electronics". Physical Review Letters. 58 (20): 2059–2062. doi:10.1103/PhysRevLett.58.2059. PMID 10034639.
  12. ^ Lucchetti, L. (2004). "Colossal optical nonlinearity in dye doped liquid crystals". Optics Communications. 233 (4–6): 417–424. doi:10.1016/j.optcom.2004.01.057. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  13. ^ Khoo, I.C. (1995). "Holographic grating formation in dye- and fullerene C60-doped nematic liquid-crystal film". Optics Letters. 20 (20): 2137–2139. doi:10.1364/OL.20.002137. PMID 19862276.
  14. ^ Khoo, Iam-Choo (2007). Liquid Crystals. Wiley-Interscience. ISBN 9780471751533. {{cite book}}: |access-date= requires |url= (help)
  15. ^ a b Morris, Stephen M. (2008). "Polychromatic liquid crystal laser arrays towards display applications" (PDF). Optics Express. 16 (23): 18827. doi:10.1364/OE.16.018827. PMID 19581971. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  16. ^ Maune, Brett (2004). "Liquid-crystal electric tuning of a photonic crystal laser". Applied Physics Letters. 85 (3): 360. doi:10.1063/1.1772869. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  17. ^ Furumi, Seiichi (2004). "Phototunable photonic bandgap in chiral liquid crystal device". Applied Physics Letters. 84 (14): 2491. doi:10.1063/1.1699445. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  18. ^ Andy, Fuh (2004). "Lasing in chiral photonic liquid crystals and associated frequency tuning". Optics Express. 12 (9): 1857–1863. doi:10.1364/OPEX.12.001857. PMID 19475016. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  19. ^ Khoo, Iam-Choo; Wu, Shin-Tson (1993). Optics and nonlinear optics of liquid crystals. World Scientific. ISBN 9789810209346. {{cite book}}: |access-date= requires |url= (help)
  20. ^ Morris, S.M. (2005). "Enhanced emission from liquid-crystal lasers". Journal of Applied Physics. 97 (2): 023103. doi:10.1063/1.1829144. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  21. ^ Morris, SM (2009-07). "Removing the discontinuous shifts in emission wavelength of a chiral nematic liquid crystal laser". Journal of Applied Physics. 106 (2): 023112. doi:10.1063/1.3177251. {{cite journal}}: Check date values in: |date= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  22. ^ Ozaki, M. (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. {{cite journal}}: |access-date= requires |url= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  23. ^ Huang, Yuhua (2006). "Spatially tunable laser emission in dye-doped photonic liquid crystals". Applied Physics Letters. 88: 011107. doi:10.1063/1.2161167. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  24. ^ Jeong, Mi-Yun (2008). "Spatial tuning of laser emission in a dye-doped cholesteric liquid crystal wedge cell". Applied Physics Letters. 92 (5): 051108. doi:10.1063/1.2841820. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  25. ^ Woltman, pp. 332–334
  26. ^ Schmidtke, Jürgen (2003). "Defect Mode Emission of a Dye Doped Cholesteric Polymer Network" (PDF). Physical Review Letters. 90 (8): 083902. doi:10.1103/PhysRevLett.90.083902. PMID 12633428. Retrieved 2011-04-29. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  27. ^ 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).
  28. ^ a b "Liquid crystal lasers the size of a human hair". Physorg. 2005-12. Retrieved 2011-04-09. {{cite news}}: Check date values in: |date= (help)
  29. ^ "Liquid crystal lasers promise cheaper, high colour resolution laser television". Physorg. 2009-04. Retrieved 2011-04-09. {{cite news}}: Check date values in: |date= (help)
  30. ^ "Laser Displays: liquid-crystal laser promises low-fabrication-cost display". Laser Focus World. 2009-01. Retrieved 2011-04-09. {{cite news}}: Check date values in: |date= (help)
  31. ^ Palffy-Muhoray, Peter (2006). "Photonics and lasing in liquid crystal materials". Philosophical Transactions of the Royal Society A. 364 (1847): 2747–2761. doi:10.1098/rsta.2006.1851. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

Bibliography

Further reading

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