Abstract

Optically induced nematic liquid crystal axis reorientation results in extraordinarily large nonlinear refractive index changes that could find practical applications in conjunction with cw or long-pulse lasers. We discuss the origins of these nonlinearities, and present the results of recent experimental studies of image conversion, optical limiting and sensor protection using aligned dye-doped nematic liquid crystal films in all-optical configurations. These processes are characterized by unprecedented low threshold laser powers, thus presenting nonlinear photosensitive nematic liquid crystals as promising next generation image processing and optical switching/limiting material.

© Optical Society of America

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References

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  1. I. C. Khoo Liquid Crystals: Physical Properties and Nonlinear Optical Phenomena (Wiley Interscience, NY. 1994).
  2. I. C. Khoo and S. T. Wu Optics and Nonlinear Optics of Liquid Crystals (World Scientific, River Edge, New Jersey, USA 1993).
    [CrossRef]
  3. I. C. Khoo, "Novel liquid crystalline structure for nonlinear optics," IEEE J. Quantum Electronics (USA) 32, 525 (1996).
    [CrossRef]
  4. E. V. Rodenko and A. V. Sukhov, "Optically induced spatial charge separation in a nematic and the resultant orientational nonlinearity," JETP (Russia) 78, 875 (1994).
  5. I. C. Khoo, S. Slussarenko, B. D. Guenther, Min-Yi Shih, P. H. Chen and M. V. Wood. "Optically induced space charge fields, DC voltage, and extraordinarily large nonlinearity in Dye-doped Nematic Liquid Crystals," Opt. Lett. 23, 253-255 (1998).
    [CrossRef]
  6. I.Janossy, L. Csillag and A. D. Lloyd, "Temperature-dependence of the optical freedericksz transition in dyed nematic liquid-crystals," Phys. Rev. A 44, 8410-8413 (1991).
    [CrossRef] [PubMed]
  7. S. Sato, "Photovoltaic effects in MBBA cells containing organic dyes," Jpn. J. Appl. Phys. 20, 1989-1990 (1981).
    [CrossRef]
  8. Mark T. Gruneisen and James M. Wilkes, "Compensated imaging by real-time holography with optically addresses spatial light modulators" in OSA TOPS 14, Spatial Light Modulators. Ed: G. Burdge and S. C. Esener.
  9. M. A. Kramer, C. J. Wetterer and T. Martinez, "One-way imaging through an aberrator with spatially incoherent light by using an optically addressed spatial light modulator," Appl. Opt. 30, 3319-3323 (1991).
    [CrossRef] [PubMed]
  10. John H. Hong, Frederick Vachss, Scott Campbell and Pochi Yeh, "Photovoltaic spatial light modulator," J. Appl. Phys. 69, 2835-2840, (1991).
    [CrossRef]
  11. I. C. Khoo, M. V. Wood, B. D. Guenther, Min-Yi Shih, P. H. Chen, Zhaogen Chen and Xumu Zhang, "Nonlinear optical liquid cored fiber array and liquid crystal film for ps-cw frequency agile laser optical limiting application," Opt. Express 2, 471-482, (1998); http://www.opticsexpress.org/oearchive/source/4201.htm.
    [CrossRef]
  12. I. C. Khoo, M. V. Wood, B. D. Guenther, Min-Yi shih and P. H. Chen, "Nonlinear- absorption and optical limiting of laser pulses in a liquid-cored fiber array," J. Opt. Soc. Am. B (USA) 15, 1533 (1998), USA Patent no. 5,589,101 (1996).
    [CrossRef]

Other (12)

I. C. Khoo Liquid Crystals: Physical Properties and Nonlinear Optical Phenomena (Wiley Interscience, NY. 1994).

I. C. Khoo and S. T. Wu Optics and Nonlinear Optics of Liquid Crystals (World Scientific, River Edge, New Jersey, USA 1993).
[CrossRef]

I. C. Khoo, "Novel liquid crystalline structure for nonlinear optics," IEEE J. Quantum Electronics (USA) 32, 525 (1996).
[CrossRef]

E. V. Rodenko and A. V. Sukhov, "Optically induced spatial charge separation in a nematic and the resultant orientational nonlinearity," JETP (Russia) 78, 875 (1994).

I. C. Khoo, S. Slussarenko, B. D. Guenther, Min-Yi Shih, P. H. Chen and M. V. Wood. "Optically induced space charge fields, DC voltage, and extraordinarily large nonlinearity in Dye-doped Nematic Liquid Crystals," Opt. Lett. 23, 253-255 (1998).
[CrossRef]

I.Janossy, L. Csillag and A. D. Lloyd, "Temperature-dependence of the optical freedericksz transition in dyed nematic liquid-crystals," Phys. Rev. A 44, 8410-8413 (1991).
[CrossRef] [PubMed]

S. Sato, "Photovoltaic effects in MBBA cells containing organic dyes," Jpn. J. Appl. Phys. 20, 1989-1990 (1981).
[CrossRef]

Mark T. Gruneisen and James M. Wilkes, "Compensated imaging by real-time holography with optically addresses spatial light modulators" in OSA TOPS 14, Spatial Light Modulators. Ed: G. Burdge and S. C. Esener.

M. A. Kramer, C. J. Wetterer and T. Martinez, "One-way imaging through an aberrator with spatially incoherent light by using an optically addressed spatial light modulator," Appl. Opt. 30, 3319-3323 (1991).
[CrossRef] [PubMed]

John H. Hong, Frederick Vachss, Scott Campbell and Pochi Yeh, "Photovoltaic spatial light modulator," J. Appl. Phys. 69, 2835-2840, (1991).
[CrossRef]

I. C. Khoo, M. V. Wood, B. D. Guenther, Min-Yi Shih, P. H. Chen, Zhaogen Chen and Xumu Zhang, "Nonlinear optical liquid cored fiber array and liquid crystal film for ps-cw frequency agile laser optical limiting application," Opt. Express 2, 471-482, (1998); http://www.opticsexpress.org/oearchive/source/4201.htm.
[CrossRef]

I. C. Khoo, M. V. Wood, B. D. Guenther, Min-Yi shih and P. H. Chen, "Nonlinear- absorption and optical limiting of laser pulses in a liquid-cored fiber array," J. Opt. Soc. Am. B (USA) 15, 1533 (1998), USA Patent no. 5,589,101 (1996).
[CrossRef]

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Figures (10)

Fig. 1.
Fig. 1.

Schematic depiction of the interaction of polarized laser beams with a homeotropically aligned nematic liquid crystal film [1].

Fig. 2.
Fig. 2.

Schematic depiction of photo-induced space charge generation and space field formation in the nematic liquid crystal film illuminated by a sinusoidal laser intensity. (a) in a direction along the laser, (b) along the grating direction.

Fig. 3a.
Fig. 3a.

Observed photo-voltage as a function of the illuminating beam intensity in a 25 μm hick nematic film for two different incident laser direction . Laser beam diameter is 8.0 mm. [Circles: β= 0°; dots: β = 22°]

Fig. 3b.
Fig. 3b.

Dependence of photo-voltages on temperature. Photo-insert shows faster rise time as temperature → Tc.

Fig. 4a
Fig. 4a

Shows the grating diffraction dependence on temperature for two different optical powers.

Fig. 4b.
Fig. 4b.

Observed diffracted power versus applied ac frequency for different ac voltages. Argon [λ = 488 nm] writing beam power: 2 mWatt each; beam diameter: 3 mm. He-Ne probe beam power : 5 mWatt. All beams co-polarized and incident on the liquid crystal film as extraordinary rays with β = 0.4 radian and α= 0.015 radian; sample thicknesses = 25 μm Photo shows the dynamical response of the diffracted power under dual frequency ac field of Vpp = 20 V . Diffraction on: f = 300 Hz. Diffraction off: f = 30 kHz. Time scale: 50 ms/div.

Fig. 5.
Fig. 5.

Experimental set up for incoherent to coherent image conversion. Insert is a photograph of the reconstructed coherent image. Note that this set up could also be used for wavelength conversion.

Fig. 6a.
Fig. 6a.

Experimental set up for optical limiting action using laser induced nematic liquid crystal axis realignment effect. Upper diagram shows the geometry of the twisted nematic cell. The focused spot diameter of the Argon laser on the film is 150 microns. Sample thickness: 25 microns. Dye concentration: 0.5 %.

Fig. 6b.
Fig. 6b.

Photograph of the transmitted laser beam with the liquid crystal film (i) at low laser intensity, showing no limiting effect, and (ii) above the limiting threshold, showing that the untwisting of the cell greatly attenuates the laser.

Fig.6c.
Fig.6c.

Plot of detected output power versus input laser power. Insert is an oscilloscope trace of the detected output for an input step-on cw laser showing how the later portion of the on- axis power is ‘switched’ off by the self-defocusing effect.

Equations (3)

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μ d 2 θ d t 2 + γ 1 dt + M el + M f = M appl
E ph = qνm k b T [ σ σ d 2 ] cos ( π 2 )
E ( Δσ , ε ) = E dc [ ( ( σ , ε ) ( σ , ε ) ) sin ( 2 θ ) ] 2 [ ( σ , ε ) sin ( 2 θ ) + ( σ , ε ) cos ( 2 θ ) ]

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