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.
©1999 Optical Society of America
Nematic liquid crystals (NLC) possess very broadband (0.4 μm – 12 μm) birefringence and transparency, and unusually large susceptibility to ac, dc and optical fields [1, 2]. These properties, in combination with their low cost, low power consumption and compatibility with semiconductor opto-electronics materials and technologies, have led to their ever-increasing use in many optoelectronics image sensing, display and processing devices. Nematic liquid crystals are also known for their unusually large nonlinear optical properties [1,2]. The nonlinear mechanisms range from sub-picosecond electronic hyperpolarizabilities to millisecond-second director axis reorientation effects. In particular, recent studies have revealed extraordinarily large orientational photorefractive nonlinearities in dye-or Fullerene C60- doped NLCs [3,4], and an even larger nonlinear optical effect in methyl-red doped NLC film .
As a result of these extremely nonlinear optical properties, dye-doped liquid crystals have emerged as promising image sensing and sensor protection materials. In this paper, we present a brief review of liquid crystal optical nonlinearities, and describe recent experimental studies of all-optical image conversion, optical power limiting and sensor protection.
2. Liquid crystal director axis reorientation
In analogy to reorientation by ac or static fields, an impinging optical field will realign the birefringent director axis of the nematics through the dipolar interaction [1, 2]. Consider the interaction of a linearly polarized laser beam with a homeotropically aligned nematic liquid crystal film as shown in Fig.1. The angular acceleration d2θ/dt2 of the director axis reorientation angle θ is given by a balance equation between the applied torque and the restoring torques acting on the director axis :
where μ is the moment of inertia of the liquid crystal, γ1 is the viscosity coefficient, Mel is the elastic restoring torque, Mf the flow-rotational viscous torque and Mappl is the applied torque.
The magnitude of the induced reorientation angle θ, and therefore the extraordinary index change Δn = ne(β + θ) - ne(β), depend on whether the interaction is transient or steady state, and on other boundary conditions . In the steady state, Δn in most cases can be shown to be proportional to the optical intensity I , i.e., Δn = n2 I, where n2 is the nonlinear coefficient. For purely optically induced director axis reorientation, i.e., Mappl = Mop, typical n2 values for 50–100 μm thick nematic film are on the order of 10-5 – 10-4 cm2/Watt. In some dye-doped nematic films  , the optically excited dye dopant molecules could exert an intermolecular torque Mm that is much greater than the purely optical torque Mop. The nonlinear index coefficients n2 of these processes are in the range of 10-4 – 10-3 cm2/W.
With photocharge producing dopants such as R6G dye or Fullerene C60, an even more nonlinear orientational photorefractive effect had been observed [3,4]. The basic mechanisms are illustrated by the optical wave mixing configuration shown in Fig. 2a–b. An incident optical intensity grating generates photocharges, which migrate and diffuse within the material and set up various dc space charge fields. In conjucntion with a small dc field applied across the film, these space charge fields create a torque Mph on the liquid crystal director axis, creating reorientation and large refractive index changes.
Two kinds of space charge fields have been identified. One is similar to those usually encountered in inorganic crystals or semiconductors. For an incident optical intensity grating function of the form Iop= Io (1 + m cos (qξ)), it is given by [3,4]:
where kb = Boltzmann constant, σ = illuminated conductivity, σd = dark state conductivity, υ = (D+ - D-)/(D+ + D-), where D+ and D- are the diffusion constants for the positively and negatively charged ions, respectively; m is the optical intensity modulation factor, q = 2π/ʌ is the magnitude of the grating wave vector, and ξ is the coordinate along q.
The other sources of space charge fields arise from the conductivity and dielectric anisotropies of the nematic liquid crystal. For a spatially varying director axis reorientation angle θ, the space charge fields are given by [3,4]:
where (σ∥ - σ⊥) is the conductivity anisotropy and (ε∥ - ε⊥) the dielectric anisotropy, and Edc is the applied dc field. In general, for a 25 μm thick homeotropically aligned doped film under a dc bias voltage of 1.5 V, the nonlinear index coefficients n2 observed are in the range of 10-3 – 10-2 cm2/W.
3. The most nonlinear optical effect observed to date
Recent studies  in methyl-red doped nematic films have shown that these photo-induced space charge fields can be so large that no applied dc field is needed to create observable director axis reorientation effect. This is a useful feature for practical application, as it avoids dc bias field-induced instabilities and dynamic scattering.
The much larger photo-charge and space charge field production efficiency in the methyl-red dye doped nematics manifest itself in the form of a sizable dc photo-voltage across the ITO-electrode coated windows. Fig. 3a shows the photo-voltages across a 25 μm thick methyl-red doped homeotropically aligned 5CB [Pentyl-Cyano-Biphenyl] film for two different angles of incidence [β= 0 and β = 22 degrees] of the Ar+ laser [488nm]. The polarity of the photovoltage is reversed if the light impinges from the opposite direction.
For a particular cell thickness, the photo-voltage build-up time is dependent on the optical intensity . The build up time of these photovoltages decreases from 1s to 100 ms as the incident laser intensity is raised from 1 to 3 mWatt/cm2. At 10 mW/cm2, the build up time drops to below 10 ms. These dynamics are also reflected in the observed response times of the nonlinear optical processes such as grating diffraction .
We have observed similar effects in planar, twisted as well as homeotropically aligned nematic cells. Such photo-voltaic effects have been observed before  in methyl-red doped MBBA and 5CB films. Since the photo-voltage originates from photo-charge generation and subsequent diffusion and redistribution of the ions with different mobilities, it should manifest itself in the nematic as well as the isotropic phases. This is indeed shown in Fig. 3b. As the temperature is raised towards the nematic (liquid crystal) →isotropic(liquid) transition temperature Tc, the magnitude of the photo-voltage increases, and the build-up time is shorter owing to increased ionic mobilities. Above Tc, the magnitude and the build-up speed of the photovoltages change very little with temperature, in contrast to the nematic phase.
Fig. 4a shows the grating diffraction dependence on temperature for two different Ar+ pump beam intensities. As the temperature is raised towards Tc, the reorientation effect diminishes, and vanishes in the isotropic phase. Such a dependence is typical of nematic axis reorientation effect [1,2]; it is another indication that the underlying index grating is not of thermal origin, since thermal index changes increases tremendously as the temperature approaches Tc. In previous studies using different pump and probe beam polarizations, thermal effect was also ruled out; the refractive index change at the low incident optical power/intensity level used in this study is due entirely to induced reorientation of the director axis. In general, we observe that the director axis tends to reorientate along the optical intensity gradients, so flow-reorientational effects may also contribute, although the relative contribution from this mechanism remains to be ascertained.
An important and useful feature of this effect is that it can be enhanced with an applied low frequency ac field. On the other hand, a high frequency field will quench the reorientational effects, c.f. fig. 4b. Such dependence on the ac field frequency may be useful for dual-frequecny switching/modulation application. The photo-insert in Fig. 4b shows the on-off switching dynamics of the diffraction from the film as the applied ac field frequency is switched back and forth between 300 Hz and 30 kHz, at Vpp = 20 V. In general, the on time is on the order of 12 ms, and the off time is about 17 ms. Faster response is possible at higher Vpp, or using nematics of lower viscosity and higher dielectric anisotropy.
The effective refractive index change coefficient n2 can be estimated from Fig. 4b. For this case, the grating spacing λg=30 μm [crossing angle ~ 1 degree] and the diffraction is in the Raman-Nath regime. The first order diffraction efficiency η is thus given by η ~(πΔnd/λ)2. From Fig. 4b, we note that a diffraction efficiency η = 10-3 is obtained for the case where ac voltage = 20 Vpp case. Using d=25μm, λ=488nm, we obtain Δn ~ 2x 10-4. Since the intensity of the writing beam is ~ 44 μWatt/cm2, this gives the magnitude of n2 = 6 cm2/Watt which is similar to those reported in reference  for a thinner [6 micron thick] sample. Without the ac field, the measured n2 value for a 25 μm thick film is on the order of 2 cm2/Watt.
With the use of 10 micron or thinner cells, and enhancement with a small applied ac voltage, the nonlinear coefficient n2 can be made even larger. Methyl-red doped nematic films are arguably the most nonlinear optical materials known to date. While there remain many unanswered interesting questions regarding the underlying basic mechanisms responsible for such extraordinarily large nonlinearity, we shall contend here with their application potentials. In the next section, we discuss some preliminary results on image sensing/processing processes, and all-optical switching and limiting effect.
4. Image processing
One of the most widely used image sensing device is liquid crystal spatial light modulator LCSLM[8,9]. In its simplest configuration, it employs a photoconducting semiconductor layer to transfer an input image’s intensity distribution to a corresponding spatial voltage drop across an aligned nematic film. In general, LCSLM’s sensitivity to the input light is defined by the photoconducting surface, and is typically in the tens of μWatt/cm2 range. Such sensitivity used to be unreachable in all-optical or nonlinear optical effect/device because of the smallness of the nonlinear response n2. For methyl-red doped NLC with n2 ~10 cm2/W, this has become possible.
Using the set up similar to that employed by Hong et al , c.f, Fig.5, we have demonstrated incoherent to coherent image, as well as wavelength conversion. The incoherent image bearing optical beam, at a wavelength of 488 nm, creates a spatial phase shift on the nematic film, which is sensed by a coherent He-Ne laser. Visible coherent images can be created with an input incoherent beam intensities as low as 90 μWatt/cm2. We have also employed similar optical intensity levels to demonstrate optical phase conjugation effect using a wave-mixing configuration similar to that used in reference [8,9].
In comparison to the commercial optically addressed liquid crystal spatial light modulators OASLM used in reference [8,9], dye doped nematic liquid crystal [DDNLC] films are considerably less costly to fabricate as they do not require the processing of a photoconductive film, and does not require any ac bias field. In terms of resolution capability, DDNLC films would allow 200 lp/mm or higher capability, as demonstrated in the study of diffraction efficiency dependence on grating spacing . These promising attributes of DDNLC films are potentially useful for developing a new generation spatial light modulator.
5.Optical limiting and sensor protection.
We have previously  shown that tilted homeotropically aligned film will defocus an incident Gaussian beam through self-lensing effect. Self-defocusing and optical limiting effect at unprecedented low input laser powerlevel of around 70 nanowatt were obtained. Such configuration, however, will have limited dynamic range as the defocusing effect will diminish as the reorientational effects spatially saturate at high input laser power.
We have recently succeeded in demonstrating a practical configuration that will give very high dynamic range sensor protection capability, and eye-safe clamped transmission. The liquid crystal used is methyl-red doped 5CB. We have also used methyl-red doped E7, and obtained similar results. The cell is made by sandwiching the lightly doped [1% by weight] liquid crystal between two rubbed PVA [polyvinyl alcohol] coated glasses. The director axis of the nematic liquid crystal lies on the plane of the cell windows, which are placed such that the director axis of the nematic film rotates by 90 degrees from the incident to the exit windows, c.f. fig. 6a.
A polarized Argon laser at 488 nm is used to simulate a ‘threat’ laser over a background ‘scenery’ which consists of an image of a resolution chart illuminated by a low power He-He laser, c.f fig. 6a. The polarization of the low power Argon laser follows adiabatically the director axis, and thus is rotated similarly. At high laser power, the induced space charge field in the direction normal to the plane of the window, c.f. fig. 2a, will cause the director axis to tilt towards the normal to the cell window. This changes the polarization state of the laser upon its exit from the sample. Consequently, the analyzer at the exit end will cause the output laser power to continuously decrease towards vanishing value as the incident laser power is increased.
Because of the finite absorption in the doped sample, the index will also change [decreases] from n∥ [1.70] to niso [1.62] as the cell temperature increases towards the nematic→isotropic transition temperature Tc. For 5CB, Tc is ~ 35° C. At very high cw laser power [>40 mW], the sample begins to turn isotropic, with a liquid ‘hole’ forming in the central region of the laser illuminated spot. The hole grows in size as the laser intensity is increased. In the isotropic liquid phase, the exit laser’s polarization will be orthogonal to the analyzer, and thus the transmitted laser power is also attenuated.
Both reorientation and thermal effect in such 90 degrees twisted nematic film, in combination with the analyzer at the output end, therefore, are effective in reducing the transmission of the incident laser as its power increases. Since the clamping action gets better for higher input laser power, the system thus possesses an extremely large dynamic range.
Fig. 6b shows the photographs of the transmitted resolution chart illuminated by the He-Ne laser. The ‘threat’ Argon laser gives rise to the bright spot. At high input power, the Argon laser is suppressed, and the spot practically disappears, leaving the background scenery intact and safe to view by eye. We noticed that at the onset of the limiting action, self-lensing effect due to the Gaussian beam intensity distribution also contribute significantly to the optical limiting action.
We have obtained similar results for the methyl-red doped E7 sample. The clamped output depends on the sample initial alignment qualities. Generally it is in the range of a few μWs, for input laser as high as 140 mw, c.f fig. 6c. The clamped transmission is about 3 × 10-5. The low power ‘linear’ transmission of the sample is 10 % [0.1], after losses through the polarizer and uncoated cell windows], and so this clamped transmission corresponds to a dynamic range [low power transmission/high power transmission] of over 3000.
Using millisecond laser pulses, we have determined that the response time, for the laser pulse height to switch to e-2 of the input level, is on the order of 40 milliseconds. With optimization of sample configuration and alignment properties, dopant and nematic liquid crystals choice, we expect to improve these limiting performance characteristics considerably. Nevertheless, this study shows the feasibility of constructing a practical all-optical limiter for cw -long pulse laser with low threshold and clamped output levels, and very high dynamic range. In conjunction with the nonlinear fiber array containing isotropic liquid crystal for nanosecond/picosecond limiting application , these twisted nematic films will enable the construction of optical sensor protection device covering an extremely wide temporal range [from ps -cw laser].
In conclusion, nematic liquid crystals in its various pure and doped forms possess many interesting and useful nonlinear optical responses. In particular, the highest index change coefficient among all nonlinear optical materials known to date is exhibited by Methyl-red doped films. We have discussed two promising applications, namely, image sensing/processing and sensor protection. Owing to space limitation, much of the details are relegated to longer articles elsewhere. We are currently investigating various dyes and high-birefringence and low-viscosity nematic liquid crystals in our quest to expand the useful spectral range of this effect, and to obtain faster and even more nonlinear responses.
This work is supported in parts by the Army Research Office, Air Force and the Joint Services Agile Program.
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