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Nonlinear optical refraction properties of azo dye doped nematic liquid crystals were investigated with a He-Ne laser (λ = 632.8 nm) as off-resonant light irradiation using spatial self-phase modulation method. Compared with the experimental results of resonant light (λ = 532 nm), the sample irradiated by He-Ne laser exhibited lower optical threshold and the number of diffraction rings was sensitive to the laser polarization. The mechanism of nonlinear optical response of azo dye doped nematic liquid crystals at 632.8 nm could be mainly attributed to photoisomerization effect. Moreover, the diffractions of holographic gratings recorded by He-Ne laser in azo dye doped liquid crystals were consistent with the grating formation based on photoisomerization effect.
© 2016 Optical Society of America
1. Introduction
Azo dye doped nematic liquid crystals have attracted much attention owing to their promising applications in holography display, optical storage, optical limiting and nonlinear optical devices [1–8]. The azo dye molecules in the guest-host system can undergo a reversible trans-cis-trans photoisomerization under irradiation with light of an appropriate wavelength and the molecular geometrical change during this photoisomerization process can lead to the photoinduced anisotropy of azo chromophores, which exert intermolecular torques to align liquid crystals perpendicular to the polarization of incident light [9–16]. With the aid of azo dye molecules, the magnitude of nematic liquid crystals reorientation mediated nonlinearity has been observed to increase by several orders or magnitude, resulting in the observations of numerous nonlinear optical phenomena with extremely low power [9–16]. A model [15–17] was proposed to explain optical reorientation of liquid crystals in the presence of photoisomerization process, in which the trans and the cis isomer of azo dye can be regarded as two different dye dopants that contribute with a negative and positive enhancement of azo dye-induced torque, respectively. The negative and positive torques from trans and cis isomers reorient liquid crystals perpendicular and parallel to the polarization of the pump laser, respectively. Azo dye doped liquid crystals are usually focused on the applications in the resonant region. Large absorption in the resonant region could lead to the enhancements of photoisomerization process and photothermal effect. However, large photothermal effect is not beneficial to practical application. For dynamic holographic display application, when the recording intensity of resonant light is above the critical level, the diffraction effect is gradually diminished in less than a few seconds [5]. Off-resonant light is far from the maximum absorption band of the sample. Off-resonant light may be a good candidate for the practical application due to the lower photothermal effect. In this paper, we investigate the off-resonant nonlinear optical refraction properties of Disperse Red 1 doped nematic liquid crystals with a He-Ne laser irradiation and compare the nonlinear optical response of the resonant light using spatial self-phase modulation (SSPM) method.
2. Experiments
In this experiment, the nematic liquid crystal and the azo dye were pentylcyanobiphenyl (5CB, Merck) and Disperse Red 1 (DR1, Aldrich), respectively. The ordinary and extraordinary refractive indices of 5CB were no = 1.542 and ne = 1.729, respectively. DR1 was doped into the nematic liquid crystals host at a concentration of ~0.5 wt. %. The mixture was magnetically stirred at room temperature in the dark until the DR1 was dissolved. The homogenous mixture was obtained and injected into an empty cell by capillary. The sample was sandwiched by two indium-tin oxides (ITO) glass substrates with 100 μm thick spacers, and the two ITO glass substrates were precoated with polyvinyl alcohol (PVA) and rubbed in the same direction to obtain homogeneous alignment, which was confirmed by polarizing microscope. Figure 1 shows the absorption spectra of the sample. Because the empty cell has little absorption in the visible range, the contribution of substrates on the observed absorption spectra could be neglected. The spectra exhibit strongly dichroic absorption near 500 nm. The typical maximum absorption coefficient is of the order of 900 cm−1.
Fig. 1 Absorption spectra of nematic liquid crystals doped with ~0.5 wt. % DR1 sandwiched between two substrates. ∥ and ⊥ correspond to the polarization directions of the probe light being parallel and perpendicular to the rubbing direction, respectively.
We used the standard configuration of a SSPM experiment to study the nonlinear optical refraction properties of DR1 doped liquid crystals. A linearly polarized Gusssian beam at 632.8 nm from He-Ne laser was focused normally onto the sample by a lens with a focal length of 400 mm. The radius of the laser spot on the sample was 82 μm. Half-wave plate was used to change the polarization direction of incident laser beam. The texture of the transmitted light was projected onto a screen after the sample. To compare the resonant nonlinear optical refraction properties, a CW Nd:YAG laser operating at 532 nm was used as resonant light. Similar experiments were performed on the above measurement setup. The radius of the CW Nd:YAG laser spot on the sample was 60 μm.
Holographic gratings were recorded by He-Ne laser as a practical application of off-resonant optical nonlinearity in DR1 doped nematic liquid crystals. A linearly s-polarized He-Ne laser with the diameter of 3 mm was split into two equal-intensities beams with a total intensity of 0.32 W/cm2. These two beams were overlapped on the sample with an intersection angle of ~3°. The intensities of the first-order self-diffractions were simultaneously detected by a photodetector and recorded by a digitizing storage oscilloscope.
3. Results and discussion
We used the simple but powerful method of spatial self-phase modulation method [18] to study the nonlinear optical refraction properties of azo dye doped liquid crystals. In the experiment, the laser beam has a Gaussian intensity profile, and therefore the light-induced nonlinear refractive index variation of the medium will be larger at the beam axis and diminish radially towards the beam edge. The different radial parts of the beam will interfere with each other and cause formation of diffraction rings in the far field seen on the screen. The number N of the diffraction rings can be estimated by the following expression
where is the phase shift seen by laser beam in the beam center traversing the liquid crystal cell thickness d, Δn(z) is the local refractive index change induced by the laser beam, and λ is the wavelength of the laser beam [18]. From the number of diffraction rings, the magnitude of refractive index change Δn can be calculated by Eq. (1). When the analyzed cell was irradiated by He-Ne laser, the typical SSPM patterns were easily observed on the screen. Figure 2 shows the photographs of the SSPM diffraction rings in DR1 doped liquid crystals irradiated with various intensities. The number of diffraction rings increases with laser intensity. Although the cell thickness and the spacer thicknesses were not in agreement, the difference between them was minor in the experiment. The thickness of 100 μm was used to calculate the value of Δn. When the laser intensity was about 1.09W/cm2, the number of diffraction rings N = 4 and Δn was estimated to be 0.025.Fig. 2 Photographs of spatial self-phase modulation diffraction rings in DR1 doped liquid crystals irradiated with various intensities: (a) 0.51 W/cm2, (b) 0.70 W/cm2, (c) 0.89 W/cm2, (d) 1.09 W/cm2.
The resonant nonlinear optical performances of azo dye doped liquid crystals were also investigated in the experiment. Figure 3 shows the number of diffraction rings as a function of light intensity I in DR1 doped liquid crystals under He-Ne laser and CW Nd:YAG laser irradiations. The Optical Freedericksz Transition (OFT) thresholds for the formations of the SSPM patterns were both observed by two lasers irradiations. As shown in Fig. 3, the OFT thresholds of DR1 doped liquid crystals at 632.8 nm and 532 nm are 0.28 W/cm2 and 0.62 W/cm2, respectively. Above the OFT threshold, the number of diffraction rings was strongly dependent on the incident light intensity. With the increase of laser intensity, the number of diffraction rings at 532 nm increases faster than that at 632.8 nm. The same measurements did not show any diffraction pattern for undoped sample. The results mean that the nonlinear optical responses in DR1 doped liquid crystals are from DR1 molecules.
Fig. 3 The number of diffraction rings in DR1 doped liquid crystals as a function of laser intensity under He-Ne laser and CW Nd:YAG laser irradiations.
Azo dye molecules can undergo a reversible trans-cis-trans photoisomerization under laser irradiation. During the process, molecular geometrical change of azo dye can lead to the photoinduced anisotropy. Because of the molecular interactions between azo dye molecules and liquid crystals, azo dye molecules exert intermolecular torques to align liquid crystals. The photoinduced reorientations of azo dye doped liquid crystals resulting from the photoinduced desorption and adsorption of azo dye molecules from and to the surface were reported in thin samples [19,20]. In our experiment, 100 μm thick spacers were used to prepare the homogenous cell. In this situation, the surface effect is small on the nonlinear optical response of DR1 doped nematic liquid crystals, which can be proved by the sample in an untreated cell. The difference of the threshold intensity was very small and the number of diffraction rings had no variation with irradiation of the same laser intensity. It is well known that liquid crystals tend to be reoriented parallel to the direction of light polarization when the sample is irradiated by linearly polarized light [21]. However, the photoisomerization process of azo molecules exerts torque to align liquid crystals perpendicular to the polarization of incident light. These two processes compete against each other to reorient liquid crystals. The overall photoinduced torque of the photoisomerization process is a superposition of two torques contributed from trans and cis isomers, which depends on the concentration ratio between two isomers. In the experiment, the SSPM patterns of He-Ne laser can be observed when the polarization of the incident laser is parallel to the alignment of the cell, which means the variation of refractive index is negative and the main torque to reorient liquid crystals comes from trans isomers [12].
Figure 3 shows that the OFT threshold at 532 nm is higher than that at 632.8 nm. The absorption coefficient of DR1 doped liquid crystals at 532 nm is about 20 times more than that at 632.8 nm. The high absorption at 532 nm results in the significant attenuation of incident laser intensity and photothermal effect, which affects the process of reversible photoisomerization and the reorientation of liquid crystals and leads to a higher Freedericksz threshold for 532 nm laser, especially for the thick sample [22]. It has been reported that cis isomers exhibit a new n - π* absorption in the red regions [11,12]. When the sample is irradiated by He-Ne laser, the cis isomers are isomerized back to trans isomers and a higher concentration of trans isomers can be obtained. Because of the molecular interactions between azo dye molecules and liquid crystals, trans isomers exert negative torques to align liquid crystals, which could induce the lower OFT threshold for He-Ne laser irradiation. The impacts of trans and cis isomers on photoinduced reorientation are under investigation.
Above the threshold of OFT, the photoisomerization effect and photothermal effect can both induce liquid crystal reorientation and result in the variation of diffraction rings. We observed the number of diffraction rings versus the direction of the laser polarization. For the experimental results in Fig. 4, the number of diffraction rings at 532 nm has no large variation with rotating the polarization direction of incident light. However, the number of diffraction rings at 632.8 nm is sensitive to laser polarization. There was no diffraction ring when the polarization direction of He-Ne laser was perpendicular to the alignment direction. It is reasonable to believe that the mechanisms of nonlinear optical responses of azo dye doped liquid crystals with He-Ne laser and Nd:YAG laser irradiations are different. When the sample is irradiated by incident laser of the polarization direction parallel to the alignment direction, azo dye molecules of photoisomerization effect exert intermolecular torques to align liquid crystals perpendicular to the polarization of incident laser. With changing the laser polarization to perpendicular direction, the torques contributed from trans isomers could become decreasing. Moreover, the reorientation effect should not be possible for the polarization direction perpendicular to liquid crystal orientation direction [23]. The experimental result in Fig. 4(a) indicates that the nonlinear optical mechanism of the sample at 632.8 nm may be the photoisomerization effect. For the experimental results in Fig. 4(b), no large variation of the number of diffraction rings can be observed by rotating laser. The nonlinear response seems isotropic and could be mainly attributed to photothermal effect rather than photoisomerization effect [24]. Large light absorption gives rise to local temperature variation and induces photothermal effect. With changing laser polarization to the perpendicular alignment direction, the temperature gradient of extraordinary component of the incident light could be changed to the temperature gradient of ordinary component to induce the refractive index variation.
Fig. 4 The number of diffraction rings in DR1 doped liquid crystals versus the direction of laser polarization for (a) λ = 632.8 nm, (b) λ = 532 nm.
We used He-Ne laser to record the holographic grating in DR1 doped liquid crystals. The self-diffraction was appeared using the experimental setup described in experiments section. In the experiment, DR1 molecules are excited by recording beams and performed photoisomerisation process. The photoinduced cis isomers in the bright regions of the light interference pattern are thermally excited back to trans isomers. Trans isomers end up with their optical axes perpendicular to the light polarization [9–16]. Because of the molecular interactions between DR1 molecules and liquid crystals, the liquid crystals in the bright regions will be reoriented perpendicular to the unchanged liquid crystals in the dark regions. This behavior leads to a refractive index modulation of bright and dark regions. The first-order self-diffraction intensities versus time for different DR1 concentrations are shown in Fig. 5. The diffraction intensities are strongly dependent on DR1 concentration. The DR1 concentration is associated with the rate of trans-cis isomerization of DR1 molecules in the bright regions. The results reveal that the grating amplitude depends on the rate of trans-cis isomerization of DR1 molecules. As mentioned in previous, the photoinduced reorientation is affected by the light polarization. When the polarization of recording beams is perpendicular to the alignment director of the sample, the diffraction intensity is zero. These results are in agreement with the photoisomerisation mechanism of grating formation. The maximum diffraction efficiency of 3% was measured for nematic liquid crystals doped with 2% wt. % DR1, which is twice higher than that of holographic grating recorded by CW Nd:YAG laser.
Fig. 5 First-order self-diffraction intensities of holographic gratings recorded by He-Ne laser in DR1 doped nematic liquid crystals for different DR1 concentrations with the same recording intensities of 160 mW/cm2. Inset, photograph of self-diffraction in nematic liquid crystals doped with 0.5% DR1.
4. Conclusion
In conclusion, we have investigated the nonlinear optical properties of azo dye doped liquid crystals using SSPM method with He-Ne laser as off-resonant light irradiation. The sample irradiated by He-Ne laser exhibits lower threshold than the one irradiated by CW Nd:YAG laser as resonant light. Moreover, the number of diffraction rings at 632.8 nm is sensitive to the polarization of incident laser. Based on the experimental results, the mechanism of the optical nonlinearity at 632.8 nm could be mainly attributed to photoisomerization effect. The first-order self-diffraction intensities of holographic gratings recorded by He-Ne laser are strongly dependent on azo dye concentration and the polarization of recording beams, which support the grating formation based on photoisomerization effect.
Acknowledgment
This work was supported by the National Natural Science Foundation of China (No. 11174203 and 11574211) and the fund of State Key Laboratory of Advanced Optical Communication Systems and Networks.
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