Photorefractive Binary System Based on Ferroelectric Liquid Crystals

May 8, 2007 - of a truly bistable, defect-free cell with low scattering is not a .... (b) a 4.8 µm thick sample of C70-doped L-15278 (1 ms pulses). I...
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J. Phys. Chem. C 2007, 111, 7834-7840

Photorefractive Binary System Based on Ferroelectric Liquid Crystals Mara Talarico and Attilio Golemme* Centro di Eccellenza CEMIF.CAL, Sezione INSTM della Calabria, Department of Chemistry, UniVersity of Calabria, 87036 Rende, Italy ReceiVed: January 17, 2007; In Final Form: March 23, 2007

In this paper, results of studies regarding the photorefractive effect in bistable surface stabilized ferroelectric liquid crystals are reported. In particular, an innovative approach toward the optical control of polarization switching in C70-doped bistable devices by means of a photogenerated space-charge field is described. The photorefractive nature of the resulting refractive index gratings is confirmed by two-beam coupling and direct phase shift measurements. The influence of an applied field and of light intensity on grating formation is also studied. The photorefractive gratings obtained by using this approach are stable, easily erasable, and rewritable. The described devices are “binary” systems, in which the refractive index switches only between two possible values. Bistable systems constitute a completely new class of photorefractive materials, with peculiar properties which distinguish them from previously developed media.

1. Introduction After being observed in ferroelectric LiNbO3 crystals1 and considered as optical damage, the photorefractive (PR) effect has been recognized as a potentially useful tool for image processing and storage. The PR effect is a refractive index modulation induced by a nonuniform light intensity in a photoconductive material with an electric field dependent refractive index.2 In the first step of the process, photogenerated charge carriers are redistributed in space by drift and diffusion to produce a nonuniform charge density which induces an internal space-charge field. The refractive index of the material is modulated by such a field to create a hologram that is phase shifted with respect to the light distribution. This phase shift distinguishes photorefractivity from all other local phenomena producing similar optical effects, such as photochromism, thermochromism, thermorefraction, and so on,3,4 and it can lead to an energy transfer between two interfering beams, which can be detected in two-beam coupling experiments. Applications of the PR effect have been proposed in different areas, including hologram storage,5,6 real-time holography7,8 and image processing,9,10 edge enhancement,11,12 and optical memories.13,14 As in many other areas of optoelectronics and photonics, recent years have seen a surging interest in the development of organic materials for PR applications,15-17 a consequence of their many advantages over inorganics: low cost, light weight, ease of processing, and possible design of molecules with tailored properties. In organic materials, refractive index modulations are induced by the space-charge field not only through second-order nonlinear optical effects (as in inorganic crystals), but also via orientational mechanisms,18 which enhance their PR performance. The understanding that the field-induced reorientation of polar chromophores is important in most highly efficient PR polymers highlighted the PR potential of liquid crystals,19 whose high spontaneous birefringence allows orientational responses using electric fields that are orders of magnitude lower compared to the fields used with amorphous materials. * Corresponding author. E-mail: [email protected].

The first investigations of the PR effect in liquid crystals were performed in doped nematics,20,21 and more recently in chiral smectic phases22-28 (SmC* and SmA*). In nematic phases, the index modulation is a consequence of the director reorientation induced by the coupling between the dielectric anisotropy and the total electric field. In SmC* and SmA* phases, the coupling is instead between the total field and a spontaneous or induced polarization, leading in principle to molecular reorientations in the microsecond time regime. However, in all cases the director reorientation was modest and the resulting values of index modulation (∆n) were much lower than those potentially achievable if the whole range of the spontaneous birefringence of the mesophase had been exploited. This is a consequence of the relatively small modulation of the applied field induced by the photogenerated field, with the resulting total field spanning only a narrow direction range. The PR performance of SmC* phases in terms of ∆n can be drastically improved by exploiting their potential bistability.29 It is well-known that, by confining a SmC* liquid crystal into cells with a gap smaller than its helical pitch, the formation of a bistable surface stabilized ferroelectric liquid crystal device (SSFLC) can be obtained.30 In the ideal case, smectic layers are perpendicular to the substrate plane and the director can adopt two stable orientations corresponding to up and down polarization, respectively, as shown in Figure 1a. The application of an electric field within the smectic planes induces the orientation of the spontaneous polarization (and thus of the director) in one of these two energetically equivalent states, depending on the polarity of the field. In bistable devices, the director orientation does not change after the field is turned off and an optical control of polarization switching via the photogenerated space-charge field would produce a high index modulation (depending on the tilt angle θ). In addition, once written the PR holograms would be stable, with no further need to supply a voltage until erasing or rewriting is required. Obtaining PR bistable SmC* devices implies the fulfillment of several requirements. The first one is the setup of a photogenerated electric field, which implies the use of either an intrinsically photoconducting liquid crystal or of proper

10.1021/jp070409b CCC: $37.00 © 2007 American Chemical Society Published on Web 05/08/2007

PR Binary System of Ferroelectric Liquid Crystals

Figure 1. (a) Schematic representation of an SSFLC device. The liquid crystal is confined between two closely spaced substrates, both coated with a rubbed nylon layer. The smectic layers are perpendicular to the substrates, and the director is tilted by an angle θ with respect to the layer normal. The symmetry of the SmC* phase allows the presence of a spontaneous polarization, which is coupled with the tilt and normal to the plane defined by the director and the layer normal. There are two stable positions of the director around the cone which correspond to up and down polarization. An applied electric field of definite polarity can be used to switch the polarization, and thus the director, between these two states, which remain stable even after the field is removed. (b) Schematic representation of the experimental setup. The SSFLC device is exposed to the interference pattern of two laser beams. Ω is the angle between the sample normal and the writing beams bisector, while δ is the angle between the normal to the smectic layers and the plane of incidence. For clarity, the dimensions of the layers are out of scale.

photosensitization. Moreover, the direction and the intensity of the resulting space-charge field must be adequate for controlling polarization switching. This last issue may be tackled by adopting a driving scheme in which the photogenerated field becomes the discriminant (but not necessarily the only) factor for polarization switching. In this paper we report how director reorientation, and the consequent refractive index modulation, can be optically induced in bistable smectic devices by a PR space-charge field. We will also show how, by considering the details of the influence of different experimental parameters on the performance of devices, a deeper understanding of this phenomenon can be reached. 2. Bistable Photoconducting SSFLC Devices The first step in the attempt to control polarization switching in bistable SmC* phases via an optical PR mechanism is the making of a bistable SSFLC device itself. Although such devices have been described in the literature long ago, the fabrication of a truly bistable, defect-free cell with low scattering is not a simple task. It requires a proper liquid crystal/surface treatment combination and often also a careful control of preparation conditions, including the cooling rate from the higher temperature phase and the application of electric fields and/or shear stresses. The first attempts were carried out by using “conventional” ferroelectric mixtures (SCE9 from Clariant and ZLI 3654 from Merck), but considering both bistability and defects, results were not satisfactory. In particular, the presence of defects generates scattering, which is highly detrimental in optical applications. Since it is well-known31 that the appearance of defects is often correlated with the layer shrinkage at the SmA*-SmC* transition, our attention focused on different materials, including newly synthesized fluorinated FLCs,32 and in particular on the experimental five-component mixture L-15278 from 3M. The partial fluorination of this material induces some interesting properties, such as the disappearance of the N* phase and the enhancement of the SmA* and SmC* phases. In fact, with the following phase transition temperatures, Cr 18 °C SmC* 57.7 °C SmA* 96.0 °C I, L-15278 offers,

J. Phys. Chem. C, Vol. 111, No. 21, 2007 7835 among others, the advantage of a SmC* phase at room temperature. L-15278 is also characterized by an unusually long pitch of 6 µm, which allows the use of wide cell gaps. The pitch was estimated by observing the spatial modulation of the unidirectional pattern obtained in 50 µm thick planar samples at room temperature. The most interesting feature of this fluorinated FLC is that it does not exhibit chevron defects since its smectic layer thickness is mostly temperature independent.33 In particular, L-15278 aligns very well on rubbed Elvamide 8023R (a nylon multipolymer resin) to give an almost ideal bookshelf structure, in contrast with what happens with SiOx or other more common aligning agents, which induce the appearance of many focal conic defects. Bistable L-15278 samples were prepared by using the following procedure: on two ITO coated glass slides a layer of Elvamide 8023R (from DuPont) was spin-coated at 5000 rpm for 25 s from a 0.5 wt % methanol solution. Before spin coating, the solution was cleaned on filters with a 0.2 µm pore size. After spin coating, the excess solvent was allowed to evaporate at room temperature. The nylon was then rubbed unidirectionally by using a velvet cloth. The glass slides were assembled into cells by using a UV-cured epoxy as a sealant. The thickness of the liquid crystal was controlled using 3-5 µm spacers (interferometric measurements were performed to check the thickness of empty cells). Before using L-15278 to pursue our objective, this liquid crystal was characterized by measuring several physical properties. Spontaneous polarization was obtained by applying a triangular wave voltage while simultaneously measuring the current through the cell.34 The reversal of the spontaneous polarization PS causes a characteristic current response, whose contribution to the total current, observed as a peak, can be easily separated. In particular, a value of PS ) 30.8 nC cm-2 was extracted. A tilt angle θ ) 22.5° was also measured at room temperature, by applying a square wave of very low frequency (0.2 - 1 Hz) so that switching could be observed directly using an optical microscope. Finally, information about how quickly L-15278 responds to an external voltage was obtained by measuring its switching time τ as a function of the applied field: a value of τ ∼ 10 µs was obtained by applying a 10 V/µm square wave at 1 kHz. Such a fast response is due to a remarkably low viscosity (30 mPa‚s), another consequence of fluorination.32 As already outlined at the beginning of this section, the most important feature to be tested was the possibility of obtaining bistable devices using L-15278. Positive and negative dc pulses were applied on well-aligned (on glass surfaces treated with Elvamide nylon) SSFLC samples placed between crossed polarizers, while following their optical responses. The resulting optical transmission shows the presence of two homogeneous stable states (see Figure 2a), demonstrating the excellent bistability of the prepared SSFLC devices. L-15278 is not an intrinsically photoconductive material, and photosensitivity at the working wavelength (532 nm) had to be induced. Two different approaches of space-charge field generation were considered: at the surfaces of the SSFLC device by coating the substrates with a photoconductor or within the LC bulk by doping. The first approach was successfully used with PR nematics35,36 and FLC spatial light modulators,37 but it had never been used before with PR FLCs. The chosen photoconductor was poly(N-vinylcarbazole) (PVK) photosensitized with 2,4,7-trinitro-9-fluorenone (TNF), and in order to align the LC a further insulating layer of alignment agent (nylon) was spincoated on the photoconducting layer. Results obtained by using this strategy were not satisfactory, since the presence of a double layer (photoconductor plus aligning agent) on the cell surface

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Figure 2. Applied voltage (i) and optical response through crossed polarizers (ii) of (a) a 1.6 µm thick sample of pure L-15278 (60 µs pulses) and (b) a 4.8 µm thick sample of C70-doped L-15278 (1 ms pulses). In both cases, the transmission states, corresponding to the two stable positions of the director, are clearly persistent after field removal.

causes a poor alignment of the liquid crystal and the lack of “classical” bistability: in these samples bistability can be observed only by applying long electric pulses (1 s), as a consequence of the presence of a strong depolarization field.38 In the second approach, L-15278 was doped with fullerene C70, preferred over the most popular C6039 for its higher solubility. A 0.2 wt % solution of C70 (Aldrich) in the liquid crystal L-15278 (3M) was obtained by dissolving both components in chloroform, stirring for 2 h, and evaporating the solvent at 60 °C for 2 h and then at 70 °C for an additional 2 h. Cells were filled by capillarity with the doped liquid crystal in the isotropic phase (100 °C) and very slowly (0.1 °C/min) cooled to room temperature to obtain a good orientation of the director. The introduction of C70 did not affect the bistability of planarly aligned SSFLC devices, as it was clearly shown by their optical transmission between crossed polarizers under the application of short dc electric pulses. Also, in this case, the presence of two homogeneous stable states after field removal is clearly visible, as illustrated in Figure 2b. However, the presence of C70 introduces a few extra defects and some additional scattering, which is nonetheless kept at acceptable levels, since the total losses can be estimated at R ∼ 60 cm-1 at 532 nm. Moreover, it also causes a slight decrease of the switching threshold, from 0.16 to 0.13 V/µm, while all the other physical properties of the liquid crystal are left unchanged. The photoconductive properties of the doped SSFLC devices were assessed by measuring the current flow through the samples, with and without illumination with a Keithley 6517A electrometer. A photoconductivity σp ) 5.3 × 10-11 Ω-1 cm-1 was obtained by applying a field E ) 0.3 V µm-1 and by irradiating with light having a power density I ) 0.4 W cm-2 at λ ) 532 nm, while the measured dark conductivity was σ ) 7.3 × 10-13 Ω-1 cm-1. No photoconductivity was detected in samples without C70. 3. Results and Discussion The device described above meets the two basic requirements of photoconductivity and bistability. However, the aim of an optical control of polarization reversal through a PR mechanism requires two additional conditions: the setup of a photoinduced space-charge field and its use as a discriminant factor for director reorientation. In organic PR materials, a level of photogeneration adequate for the establishment of a space-charge field is only obtained in the presence of high electric fields,40 although with liquid crystals lower fields may be used. In the system described here the application of an electric field (above a certain threshold) bears the additional consequence of reorienting the director, thus erasing preexisting refractive index modulations. Since both the intensity and the direction of an electric field

are relevant to director reorientation, in order to fully understand and control the field-induced effects, it is important to consider not only the orientation of the smectic layers in a SSFLC cell, but also the orientation of the whole cell with respect to the light beams that generate the intensity pattern. As shown in Figure 1a, in the experimental setup the normal to the smectic layers is within the plane of the substrate and the director can assume the two stable orientations within the plane of the substrate, defined by the angles (θ with respect to the layer normal. Such orientations correspond to spontaneous polarizations (PS, both normal to the substrate plane: only a field within the layers will induce polarization switching and director reorientation. The photogenerated space-charge field is always within the incidence plane (defined by the two writing beams), with a direction normal to the beams bisector. Whether it can contribute to polarization switching depends on the orientation of the sample, defined by two parameters: the angle Ω between the beams bisector and cell normal and the angle δ between the normal to the smectic layers and the plane of incidence, as illustrated in Figure 1b. To maximize the drift contribution toward the setup of ESC, all the experiments described here were carried out with Ω ) 60°. Under such conditions, for δ ) 0 ESC will have one component normal to the layers and one component within the layers, in the same direction as the applied field. With δ ) π/2 the whole ESC will be oriented within the layers, but with a different direction with respect to the applied field. The necessary sequence of events to achieve PR control of polarization switching must then include (a) the setup of ESC using nonuniform illumination (e.g., an interference pattern) in the presence of an applied field and (b) the use of the total resulting field to selectively reorient polarization in some areas. It is important to underline that, at least under the experimental conditions described in this work, the minimum amplitude of the applied field necessary to observe any light-induced effect was above the switching threshold. As a consequence, in contrast with the driving of common PR materials, a modulation of the applied field was necessary during the writing process: the photogeneration of ESC requires the application of a sufficiently strong external electric field for a certain time, but then the amplitude of such field must be lowered below the switching threshold in order to give to the spatially modulated ESC the role of discriminant factor for mesophase switching, writing in this way the refractive index grating. Several types of driving schemes have been tested (see Figure 3). In all of them, after the initial application of an intense field E1 for a time long enough to establish a space-charge field (induction period), the external field is lowered to a value below the switching threshold. The component of the modulated ESC

PR Binary System of Ferroelectric Liquid Crystals

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Figure 5. Photomicrograph of a C70-doped L-15278 sample after the writing process. The resulting stable director distribution is clearly visible. nl, nd, and N show the orientations of the director in the bright and dark regions and the orientation of the rubbing direction, respectively. The sample was exposed to an interference pattern with Λ ) 10 µm. “A” and “P” indicate the directions of the crossed polarizers placed before and after the sample.

Figure 3. Driving voltage schemes used in attempts to produce the PR grating. Several values of the parameters (E1, T1, E2, T2, T3) were tested for each waveform.

Figure 4. Intensities of the two writing beams under the application of a 10 V triangular voltage wave of 300 mHz on a 4.8 µm thick sample of C70-doped L-15278 during a two-beam coupling experiment. The periodicity of the interference pattern was Λ ) 20 µm. A transient energy exchange between the two writing beams, with beam amplification γ1 ∼ 1.06, occurs for nearly vanishing applied voltage.

within the layers, ESCL, will spatially modulate the intensity of the total field Etot ) E + ESCL in the layers: under the proper conditions, in some regions the total field will not be intense enough to switch polarization, which will remain in the state reached during the induction period, while in other regions polarization reversal can be achieved. Each driving scheme was applied on samples illuminated by two superimposed polarized laser beams (λ ) 532 nm), both with the same polarization plane (the overlapping region was S ∼ 1 × 10-2 cm2). A grating was formed for all the tested driving schemes (at least one order of diffraction was observed), but only in cases d-g of Figure 3 was the noise level low enough to allow the measurement of an energy exchange between the two writing beams. As an example, Figure 4 shows the result of a twobeam coupling experiment performed while applying a 10 V triangular voltage at 0.3 Hz. A transient energy exchange between the two writing beams, with beam amplifications γ1 ) I1(I2 * 0)/I1(I2 ) 0) ∼ 1.06 and γ2 ) I2(I1 * 0)/I2(I1 ) 0) ∼ 0.93, occurs for nearly vanishing applied voltage. Similar results

were obtained by applying a 10 V sinusoidal voltage at 0.1 Hz. Also in this case, at almost zero voltage applied (for both positive and negative slopes), a transient energy exchange between the two writing beams was measured with beam amplifications γ1 ∼ 1.01 and γ2 ∼ 0.93. However, to exploit the bistability of the device and write permanent gratings, different schemes, such as Figure 3f,g, were tested. In particular, the driving scheme Figure 3g gave the best results. In this case, doped L-15278 samples were illuminated by the interference pattern (with a spatial periodicity Λ ranging between 2 and 40 µm) while simultaneously applying, for a time T1, an electric field E1 which uniformly orients the director and assists charge photogeneration and transport. At the end of T1, a field E2, below the switching threshold, is applied. If the amplitude of E2 is properly chosen, the liquid crystal will reorient only in regions of space where ESCL + E2 is above the threshold, with a resulting modulation of the refractive index. To minimize depolarization processes, particularly relevant in the presence of photogenerated charges, alternate positive and negative pulses were used in scheme Figure 3g. By applying such a driving scheme, up to eight diffraction orders were obtained. Observation at the optical microscope (between crossed polarizers) of exposed cells reveals the presence of stripes, where the director is alternatively oriented along one of its two stable states (see Figure 5). The stripes are always formed along the direction defined by the photogenerated space-charge field. Once written, gratings are stable for several months if no field is applied and can be erased in ∼10 µs by applying a 10 V/µm electric field. Gratings obtained by using the driving scheme Figure 3g were characterized by measuring the first order diffraction efficiency η, defined as the ratio between the first order diffraction intensity and the intensity of the transmitted beam without grating. The experimental setup is shown in Figure 1b: two coherent laser beams with the same wavelength overlap inside the sample driven by the pulse. In particular, η measurements were performed as a function of the driving parameters E1, T1, and E2 and of the geometric parameter δ. As illustrated in Figure 6a, η increases with E1 until it reaches a plateau, probably because at a certain threshold value the processes of charge photogeneration and drift mobility are optimized. The dependence of efficiency on T1 shows instead a more complicated pattern, illustrated in Figure 6b: η increases at first, it reaches a maximum, and then it starts decreasing. The value of η is relatively constant around its maximum, for T1 between 150 and 700 ms under the experimental conditions chosen, showing that high efficiencies can be obtained even with T1 of 150200 ms. The lowering of η for high T1 might be brought back to the development of depolarization. The dependence of η on

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Figure 7. Time dependence of diffraction efficiency η. The spatial periodicity of the interference pattern was Λ ) 5 µm, while the driving scheme parameters were E1 ) 2.1 V/µm, T1 ) 500 ms, and E2 ) -0.05 V/µm. At time t ) 0.035 s the applied voltage E1 ) 2.1 V/µm is changed to E2 ) -0.05 V/µm and a grating forms.

Figure 6. First order diffraction efficiency (η) measurements as a function of the driving parameters E1, T1, and E2 of driving scheme Figure 3g and of the geometric parameter δ: (a) η dependence on the intensity of the applied field E1, with E2 ) -0.05 V/µm and T1 ) 500 ms at Λ ) 5 µm; (b) η dependence on T1, with E1 ) 2.1 V/µm and E2 ) -0.05 V/µm at Λ ) 5 µm; (c) η dependence on the applied field E2, with E1 ) 2.1 V/µm and T1 ) 500 ms at Λ ) 10 µm; (d) η dependence on δ, with E1 ) 2.1 V/µm, E2 ) -0.05 V/µm, and T1 ) 500 ms at Λ ) 20 µm.

E2, illustrated in Figure 6c, shows a relatively high efficiency even for E2 ) 0, suggesting that the space-charge field is intense enough to control polarization switching by itself, without any contribution from E2. With increasing E2, the efficiency reaches a maximum and then vanishes when the contribution of ESC becomes negligible and E2 uniformly switches the polarization. Under the experimental conditions for which the data of Figure 6c were collected, the order of magnitude of ESC can thus be evaluated at ∼0.1 V/µm. Finally, the η dependence on δ shows a drastic decrease when the layer normal does not lie within the incidence plane. Such a pattern can be explained by considering the orientation of the space-charge field with respect to the smectic layers. For simplicity, one can examine the two extreme cases, δ ) 0 and δ ) π/2. For δ ) 0, ESC will have one component normal to the layers (and thus perpendicular to the polarization) and one component within the layers, in the same direction of the applied electric field. Only the last component will affect the polarization, and thus the director, which will be switched between its two stable orientational states. For δ ) π/2, ESC is completely oriented within the layers but not in the same direction as the applied field. Thus, the polarization will be subjected to a torque that will tend to move the director away from both stable positions, inducing a pretilt angle and worsening the quality of the optically controlled switching process. An alternative explanation takes into account the different orientations of the smectic layers. In the δ ) 0° case, the space-charge field, and thus the PR grating wavevector, is perpendicular to the smectic layers and thus the borders between the domains of liquid crystal where the director is alternatively oriented along one of its two stable states are formed along the layers. In the δ ) 90° case instead, the grating wavevector is within the smectic layers and thus the borders between the domains cut through the layers. It has been recently shown33 that L-15278 exhibits an extremely weak interlayer tilt direction correlation, which means that there are weak induction and dispersion forces acting between adjacent smectic layers, much weaker than the interactions existing among molecules belonging to the same layer. It can thus be expected that the

Figure 8. Dependence of diffraction efficiency η on the spatial periodicity of the interference pattern Λ, measured with E1 ) 2.1 V/µm, T1 ) 500 ms, and E2 ) -0.05 V/µm. The number next to the symbol is the number of diffraction orders visible in each case.

switching process will be favored for δ ) 0° with respect to the case where δ ) 90°. As shown in Figure 7, a first order diffraction rise time τS ∼ 30 ms was measured by using the Figure 3g driving scheme. This is the time required for the reorientation of the director under the combined effect of ESC and E2 at the end of the induction time T1: the sum T1 + τS thus sets a total writing time TW ∼ 250 ms. The measured τS is much longer than the switching time τ ∼ 10 µs previously obtained for L-15278. The apparent inconsistency between these two values can be explained by considering that FLC switching times strongly depend on the amplitude and on the length of the applied pulse.41 Given a certain length of the applied pulse, faster switching times require higher fields. The 10 µs switching time for L-15278 was measured by applying a 10 V/µm square wave, a much higher field compared to the ones used in PR experiments (on the order of ∼0.1 V/µm). In order to have information about the maximum resolution of the gratings that can be written on our SSFLC devices, the dependence of η on the interference pattern period Λ was also measured, and results are presented in Figure 8. As expected, the efficiency decreases with Λ and we pass from eight diffraction orders observed at Λ ) 40 µm to one diffraction order observed at Λ ) 2 µm. Another important parameter that might influence diffraction efficiency is the intensity of the light used to illuminate samples. Light plays a fundamental role in the PR mechanism since it generates charges, and although other effects may be relevant, it can be expected that by increasing light intensity higher spacecharge fields may be achieved. All measurements reported above were carried out by using two writing beams of the same intensity with a total power density I ∼ 0.4 W/cm2. The efficiency dependence on light intensity is shown in Figure 9:

PR Binary System of Ferroelectric Liquid Crystals

Figure 9. η dependence on light intensity. Data obtained for E1 ) 2.1 V/µm, T1 ) 500 ms, and E2 ) -0.05 V/µm are represented by squares, while data obtained with E2 ) 0 V/µm are represented by circles. All experiments were performed at Λ ) 10 µm.

Figure 10. Intensities of the two incident beams during the grating writing process in a two-beam coupling experiment on a 4.8 µm thick sample of C70-doped L-15278. At t ) 32 ms the applied electric field E1 ) 2.1 V/µm is changed to E2 ) -0.05 V/µm. A large energy exchange is evident between the writing beams with beam coupling ratios γ1 ∼ 1.18 and γ2 ∼ 0.72.

η increases with I until it reaches a maximum for I ∼ 0.42 W/cm2 and then it decreases for higher intensities when all the parameters of the driving scheme are kept constant. This pattern reminds us of the η dependence on T1. Optical microscopic observations of samples subjected to experiments at higher light intensities explain the worsening of the diffraction efficiency: the “stripes” of liquid crystal where the director switches under the effect of ESC + E2 are wider than the regions where the director retains the initial stable orientation and the resulting refractive index pattern loses its regular square wave profile. This can be explained by assuming that exposure to higher light intensities increases charge generation, giving rise to a depolarization field playing the same role of E2.42 Thus the regions where the total field ESC + E2 (strengthened by the depolarization field) can induce mesophase switching are much wider, and asymmetrical square wave patterns of refractive index can be generated with a consequent decrease of first-order efficiency. One might then expect that, by varying the amplitude of E2, higher diffraction efficiencies could be obtained. In fact, by performing the same experiments at high intensities (up to 3 W cm-2) with an applied field E2 ) 0, efficiencies as high as ∼7% were measured (see Figure 9, circles). The PR nature of the obtained gratings was confirmed performing two different types of experiments: two-beam coupling2 and direct phase shift measurements.43-45 As already outlined in the Introduction, two-beam coupling detects possible energy exchanges between two beams interfering on a medium, and thus it is the technique of choice to assess the photorefractive nature of a hologram. As shown in Figure 10, in two-beam coupling experiments a large energy exchange between the two writing beams was obtained, with beam coupling ratios γ1 ∼

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Figure 11. Intensities of the two writing beams monitored immediately after the grating formation. At t ) 0.08 s, the sample started to be translated at a constant speed V ) 108 µm/s along the grating wavevector.

1.18 and γ2 ∼ 0.72, the highest ever measured in PR FLCs.29 From these experiments it was also possible to confirm the value of the grating formation time τ ∼ 30 ms, already determined from first order diffraction efficiency measurements. However, since the obtained gratings fall in the Raman-Nath regime,2 the observation of two-beam coupling is not necessarily due to a phase shift between light pattern and phase grating. Several direct phase shift measurements were then performed by using the moving grating technique, where samples are mechanically translated by a precision translator (OptoMike from OptoSigma), after the grating formation, along the direction of the grating wavevector, at a constant speed faster than the response time of the material (V ∼ 108 µm/s). This process changes the relative phase between the light pattern and the PR grating, resulting in intensity oscillations in both transmitted beams, which are monitored and used to determine the phase shift. A typical result is illustrated in Figure 11. A phase shift Θ ) 40° ( 10° was measured for the experimental conditions used, confirming the PR nature of our bistable photoconductive SSFLCs. Additional phase shift measurements were also performed at low light intensity. From the η dependence on I, in fact, it has emerged that different mechanisms may act in our samples for different illumination conditions (see Figure 9). At higher light intensities (as well as for long T1), in addition to the PR effect, charge accumulation at the interface between the FLC and the nylon may give rise to a spatially modulated depolarization field which can contribute to the FLC switching, together with ESC. This effect, however, would be in phase with the interference pattern and thus it would not contribute to beam coupling. In order to better understand the mechanisms acting within our system, phase shift measurements were also performed on samples exposed to a very low light intensity I ) 0.03 W/cm2. In this way the contribution from the “in-phase” mechanism could be minimized. A phase shift Θ ) 80° ( 10° was measured by using the same driving parameters as in the I ∼ 0.42 W/cm2 case, confirming that at low light intensities the phase-shifted PR mechanism is the predominant one. 4. Conclusions The PR performance of SmC* phases can be drastically improved by exploiting their bistability. In fact, by controlling polarization switching via a photogenerated space-charge field in a bistable SSFLC cell, a high refractive index modulation could be obtained, as a consequence of a wider director reorientation. In contrast with what happens in traditional PR materials, in bistable devices the director can adopt only two orientational states and only two values of refractive index are allowed. In this sense, the device illustrated in this paper can be considered a binary optical system. Moreover, once written

7840 J. Phys. Chem. C, Vol. 111, No. 21, 2007 the refractive index gratings are stable in time, with no further need to supply a voltage, unless erasing or rewriting is required. The PR origin of these gratings was confirmed by two-beam coupling experiments and direct phase shift measurements. The highest energy transfer ever measured in PR FLCs was detected in C70-doped SSFLC devices, with a typical beam coupling ratio γ1 ) 1.18 and a phase shift Θ ) 40° ( 10° between the interference pattern and the refractive index grating. The influence of the driving pulse parameters, of light intensity, and of sample orientation on diffraction efficiency was also determined in order to achieve a deeper understanding of the phenomenon. In particular, from the efficiency dependence on light intensity, it has emerged that different switching mechanisms may act in doped SSFLC devices for different illumination conditions. At high light intensities, a spatially modulated depolarization field exists, which can contribute to the FLC switching, together with the PR ESC. The depolarization contribution, however, is in phase with the interference pattern and thus it does not affect beam coupling. Acknowledgment. This work has been partially supported by MIUR under the projects “Molecular and Organic/Inorganic Hybrid Nanostructures for PhotonicssFIRB 2001” and PRIN 2004. We thank Marc Radcliffe for the generous gift of the liquid crystal L-15278 and Alfredo Pane for the preparation in the clean room of some of the cells used. References and Notes (1) Ashkin, A.; Boyd, G. D.; Dziedzic, J. M.; Smith, R. G.; Ballman, A. A.; Levinstein, J. J.; Nassau, K. Appl. Phys. Lett. 1966, 9, 72. (2) (a) Yeh, P. Introduction to PhotorefractiVe Nonlinear Optics; John Wiley & Sons: New York, 1993. (b) Solymar, L.; Webb, D. J.; GrunnetJepsen, A. The Physics And Applications Of PhotorefractiVe Materials; Oxford University Press: Oxford, 1996. (3) Eichler, H. J.; Gu¨nther, P.; Pohl, D. W. Laser-Induced Dynamic Gratings; Springer Series in Optical Sciences; Springer-Verlag: Berlin, Heidelberg, 1986; Vol. 50. (4) Brown, G. H. Photochromism, Tecniques of Chemistry; WileyInterscience: New York, 1971; Vols. I-III. (5) Staebler, D. L.; Amodei, J. J. Ferroelectrics 1972, 3, 107. (6) Oliveira, I. de; Frejlich, J.; Arizmendi, L.; Carrascosa, M. Opt. Commun. 2005, 247, 39. (7) Huignard, J. P.; Herriau, J. P. Appl. Opt. 1977, 16, 1807. (8) Kippelen, B.; Peyghambarian, N. AdV. Polym. Sci. 2003, 161, 87. (9) White, J. O.; Yariv, A. Appl. Phys. Lett. 1980, 37, 5. (10) Kippelen, B.; Sandalphon Peyghambarian, N.; Lyon, S. R.; Padias, A. B.; Hall, H. K. Electron. Lett. 1993, 29, 1873. (11) Huignard, J. P.; Herriau, J. P. Appl. Opt. 1978, 17, 2671. (12) Volk, T.; Wohlecke, M. Crit. ReV. Solid State 2005, 30, 125. (13) Owechko, Y. IEEE J. Quantum Electron. 1989, 25, 619.

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