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We report on the effects of a low-frequency electric field on the optical nonlinear response of thin dye-doped liquid crystal cells. Experimental data show that the external field allows reaching extremely high values of the optical nonlinearity without any critical control of the cell interfaces. A qualitative interpretation of the collected data, based on the light-induced modulation of the bulk voltage through surface modifications, is proposed.
©2006 Optical Society of America
1. Introduction
We have recently reported a colossal nonlinear response in 5CB doped by the azo-dye methyl-red (MR) [1], showing that, under proper experimental conditions, the nonlinear coefficient n2 can reach values higher than 103. This huge nonlinear response has been ascribed to a light-induced modification of the surface conditions indicated as SINE: Surface Induced Nonlinear Effect [2,3]. The possibility of reaching such a high level of nonlinearity opens the way to a wide use of liquid crystals for image processing operations, as it has been recently demonstrated with optical phase conjugation and wave-front correction experiments [4,5]. Unfortunately SINE is difficult to control. As already discussed [2,3], the effect originates from the surface interaction between dye and liquid crystal (LC) molecules and it strongly depends on the nature of the cell interfaces. For this reason its good reproducibility requires a high degree of control of the sample preparation process, while it is clear that a total reproducibility of the effect would be highly desirable.
In this respect, we have already demonstrated that high values of the optical nonlinearity can be obtained in a more reproducible way, simply by heating the LC cells to near the nematic-isotropic transition temperature [6]. However, this method works just in a very restricted temperature range. Here we report an experimental study of the effects of a low-frequency electric field on the optical nonlinearity of our samples. We will show that, for certain values, the external field allows reaching optical nonlinearities comparable to the colossal ones in cells that otherwise would not exhibit such a response, thus offering an alternative method to get high nonlinear levels in a reproducible way.
We have already observed that a dc field applied perpendicular to the cell substrates in MR doped 5CB samples is able to quench their huge nonlinear response, thus confirming the orientational character of the effect [2–6]. Quenching is due to the ability of the field to align homeotropically the LC molecules. The present work concerns the effects of an external field weaker than the one necessary to saturate the homeotropic orientation.
2. Experimental details
Several kind of cells have been studied. Homeotropic samples obtained by coating only one substrate with Dimethyloctadecyl[3-(trimethoxysilyl)-propyl]ammonium chloride (DMOAP), planar cells prepared by coating the glass substrates with Polivynil-alcool (PVA) and randomly oriented samples have been prepared. These latter have been obtained by using glass substrates without any surface treatment. Details on samples preparation have been given in [1]. All the cells have been filled by capillarity with a mixture of 5CB doped with 1% of MR (weight concentration). Cell thickness was controlled by means of 1 μm silica spheres.
DMOAP treated cells have been irradiated on the untreated side.
Optical gratings were written with two coherent laser beams (Nd:YVO4 laser, λ=532 nm) in a standard configuration in nontilted geometry. The beams diameters on the sample were 2 mm. The diffraction efficiency η = I+1/I0 was measured by means of a probe beam and its behaviour under the application of a low frequency (1-10 Hz) electric field, has been studied.
The analysed cells do not show any nonlinear optical response without the external bias at the low intensity levels used. (We would like to remark that SINE is not fully reproducible and our aim is to analyse the possibility of obtaining a good control of the process by means of an external field. For this reason, we are taking into account only cells in which the SINE effect does not work properly without the external field).
3. Results and discussion
Figure 1 shows the first order diffracted beam (curve a) under the application of the modulated electric field (curve b), for a randomly oriented cell. The total pump intensity is about 100 μW/cm2. All the fields are switched on at the same time and the rise time of the diffraction is on the order of 1 s. As it can be seen, the diffracted signal is modulated by the field. It reaches the maximum value (η = 15 %) each time the external bias is 1.2 V and vanishes in correspondence of the field minima and maxima. The modulation of the diffracted signal with the external field is also evident in Fig. 2, where a short real time movie is shown.
Fig. 1. First order diffracted beam (curve a) under the application of the modulated electric field (curve b), for a randomly oriented cell. The total pump intensity is 100 μW/cm2. The maximum value of the diffraction efficiency corresponding to the maxima of curve a is 15 % and it is reached each time the external bias is 1.2 V.
Fig. 2. Real time movie showing the diffraction orders modulated by an external electric field of 1 Hz frequency. (1.814 KB)
The maximum value of the signal decreases with time and decays after about 20 s. In these conditions, as shown in Fig. 3, a change of the bias polarity results in a reappearing of the signal that slowly decreases and decays after an ever shorter period. Worthy of note, the nonlinear optical coefficient n2 correspondent to the observed diffraction efficiency is of the order of 103 cm2/W, that is comparable to the value typical of the colossal optical nonlinearity [1], here obtained without any critical control of the cell interfaces nor of the cell temperature.
Fig. 3. Time evolution of the first order diffracted intensity during the application of the modulated external bias. The total pump intensity is again 100 μW/cm2.
If the pump beams are switched off, the diffracted signal relaxes in times of the order of 0.1 s. On the contrary, if the pump intensity is higher (namely around 200 μW), the signal keeps on pulsing with the external field even after the pump beams are removed.
An external field of frequency on the order of 100 Hz does not produce any effect.
If a static bias of 1.2 V is applied to the cell, the diffracted beam increases to its maximum and then decays with a decay constant of about 2 s.
Planar cells show a behaviour similar to that described, but the maximum efficiency is lower (η ≅ 5%) and is achieved with higher values of the external bias (V = 3V or 4V depending on the cells) and of the optical intensity (about 1.5 mW/cm2).
Homeotropic samples on the contrary, are not affected by the external field.
Data clearly indicates that the external bias assists grating formation in samples that otherwise would not exhibit any response to low power light. The effect is observed only with static or almost static fields which suggests that dc field-induced separation of charges is involved.
Several reorientation effects in LCs connected to charge production and redistribution have been recently reported; the main part of them requires a proper choice of the alignment layer [7] or a proper combination of LC and surfactant [8,9]. On the contrary, the effect here described can be observed in cells with bare substrates, and cells with PVA or DMOAP coatings (it also works with DMOAP-treated cells with bad homeotropic alignment), suggesting that the involved charges come from the LC+dye mixture.
It is known that an external field applied to a LC cell produces ions drift toward the electrode of opposite sing and it has been recently shown that the main effect of surface ion accumulation on the molecular reorientation, is the reduction of the effective bulk voltage [10]. Now, the voltage drop across the cell bulk is strictly dependent on the surface charge density of ions, according to the relation:
where V is the externally applied voltage, εS the surface dielectric constant, d the cell thickness and σq the surface charge density of ions collected in front of each electrode [10].
It is also known that photoinduced adsorption and/or desorption of dye molecules close to the irradiated surface are almost unavoidable processes in MR-doped samples, and the fundamental role they play in both static and dynamic director reorientation has been widely explored and demonstrated. In particular, adsorption of dye molecules on the irradiated surface induced by polarised light, is known to give rise to an easy axis parallel to the incident polarisation, whereas desorption of adsorbed molecules from the irradiated surface induces an easy axis orthogonal to the incident polarisation [11]. Recent experiments have put into evidence that both these light-induced effects are indeed already active before the creation of the easy axis and that they are responsible for the surface modifications leading to the recently reported huge nonlinearity of dye-doped nematics [12].
Our idea is that in the present case the surface density of charges is modulated by the optical field through light-induced desorption and adsorption of ions in correspondence of the interference maxima. The adsorption and desorption phenomena in fact produce a variation of the surface density of ions and are active only in correspondence of the interference maxima. Thus, they can produce a modulated ion distribution on the irradiated surface. If σq in Eq. (1) is modulated by the optical field, the bulk voltage is modulated as well following the interference pattern of the incident light. This modulated field strength gives rise to a modulated director distortion and thus to the observed optical diffraction grating. In particular, in case of light-induced desorption of surface ions, σq is lower in correspondence of interference maxima and, as a consequence, the bulk voltage is higher there since the screening effect is lower than in correspondence of interference minima. The opposite situation occurs in case of adsorption of ions on the irradiated surface. In both cases the voltage inside the LC cell is modulated and produces an orientational diffraction grating, but when desorption dominates the distortion and the diffracted signal are expected to be bigger. This is actually what we observe by reverting the pump polarization from parallel to perpendicular to the filling direction in order to make respectively desorption and adsorption the dominant processes [12].
Under this scenario, the whole process can be regarded as connected to a light-induced modification of the surface conditions, similar to the SINE effect even if, in the present case, the director distortion arises due to the external low frequency field.
The observed modulated diffraction efficiency can be explained considering that when the external bias is maximum the homeotropic alignment is saturated and the optical response is quenched. The occurrence of full homeotropic alignment has been verified by putting each sample under an optical microscope to observe in real time the effect of the external field. On the other hand, the signal decrease at each cycle and its decay to zero can be ascribed to an internal space charge field opposite to the external one. Reverting the bias polarity the signal reappears but is weaker and decays after a slower period since the irradiated surface has been already slightly modified (see Fig. 3). If the pump power is increased to typically 200 μW the light induced surface modifications are stronger and produce a stable grating that can be revealed by the external electric field, as described.
The decay observed under the application of an external static bias of 1.2 V is probably due to the total screening of the external field occurring in steady state conditions or, for planar samples, to the decrease of the bulk voltage to below the Freedericks threshold. In this respect it is worth noting that a similar decrease of the bulk voltage has been observed in 30 μm 5CB planar cells after about 8 s from the voltage application [10].
The described phenomenon is obviously not active for fields that saturate the director orientation in the homeotropic configuration.
As a final remark, it is interesting to observe that a modulation of the diffraction efficiency under the action of a low frequency electric field, has been already reported by Zhang et al. [13], who investigated a sort of surface-mediated photorefractive effect in homeotropic 5CB cells. However, the phenomenology there reported is different from that of the present work, even if surface modifications are also involved. In the paper of Zhang and co-workers, the modulation occurs also after having switched off the optical field, while in our case the two fields (optical and electrical) must be both present for the very low pump power levels we used. Moreover, the effect we have described is absent in homeotropic cell
4. Conclusions
To conclude, we have reported on the effects of a low frequency electric field on the optical nonlinear response of thin dye-doped liquid crystal cells. Experimental data show that the external field allows reaching extremely high values of the optical nonlinearity without any critical control of the cell interfaces. The application of an external field of proper value, thus offers a simple and fully reproducible way to get a colossal nonlinear response. A qualitative interpretation of the collected data, according to which the bulk voltage is modulated due to light-induced surface modifications, is proposed.
References and links
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