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Dynamical grating diffraction experiments and reflection/transmission polarization spectroscopy have been conducted on azo-dye doped Blue-Phase Liquid Crystal (BPLC) to investigate the mechanisms responsible for laser induced refractive index changes. The underlying mechanisms for the transient grating diffraction components are attributed to thermal indexing and lattice distortion, whereas the persistent component is due to lattice distortion/expansion caused by laser excited dye molecule isomerization. These mechanisms were distinguishable by their response dynamics and gave rise to the observed reflection spectra and photonic bandgap shift, polarization dependency and optical activity. Some preliminary studies have demonstrated the feasibility of using these mechanisms for coherent holographic and direct image writing operations.
© 2015 Optical Society of America
1. Introduction
Blue Phases (BPs) are a particular class of liquid crystalline phases that appear upon cooling from the isotropic phase (ISO) before reaching the cholesteric phase (N*) [1, 2]. In these phases, the director axes assemble spontaneously into cylinders of double twist helix that are stacked periodically in three dimensions forming a photonic crystal lattice. In the two most commonly occurring phases, BPI lattice is body-centered cubic and BPII is simple cubic. Having the characteristic lattice spacing on the order of some hundred nanometers, the photonic bandgaps of BP are generally positioned in the visible spectrum. Owing to their tightly wound director arrangement, bulk Blue Phase Liquid Crystals (BPLC’s) are therefore optically isotropic but possess large electro-optic response with faster relaxation times; in the pristine or polymer-stabilized forms, they have been fruitfully employed for various electro-optics switching and display applications [3–10]. There have been recent investigations into the all-optical equivalent of such field induced effects in BPLC, i.e. their nonlinear responses where processes are triggered by optical means rather than low-frequency electric field [11–15]. Such all-optical processes eliminate the use of electrodes and enable much larger degree of freedom in the applied field directions and interaction geometry. In particular [12–14], reported the observations of large optical Kerr effects in grating diffraction experiments, and [15] has demonstrated nonlinear transmission and all-optical switching operations with BPLC cored fiber arrays. In the investigation by Ptasinski et al. [16], methyl-red doped BPLCs have found practical application in making all-optical silicon racetrack tunable resonators; optical tuning of reflection spectra has also been demonstrated by Lin et al. [2].
In this paper, we report the results of several wave-mixing based experiments aimed at further exploring the underlying mechanisms responsible for these observed nonlinear optical effects, and their dynamical properties which remain relatively unexplored. In methyl-red (MR, an azo-dye) doped BPLC, we have characterized the grating build-up and decay times associated with the three principal contributing mechanisms for laser induced index changes: a faster acting/decaying thermal indexing effect typical of absorptive dye-doped liquid crystals, and two slowly responding (under the moderate laser intensity) lattice distortion and dilation effects. Besides index changes, these mechanisms also produce photonic bandgap modifications leading to shifts in the reflection spectra, as well as changes in the optical activity of the BPLC leading to corresponding rotation of the polarization state of the impinging laser. We have also conducted some preliminary feasibility demonstrations of coherent and image processing and direct image recording operations using these mechanisms.
2. Grating diffraction experiments
The host BPLC used in this work was formulated by blending nematics E48 and 5CB with a chiral smectic S811 in a weight ratio of 8:8:9. Upon cooling from the isotropic liquid state, the pristine mixture undergoes the following phase sequence: ISO-(31°C)-BPII-(29°C)-BPI-(23°C)-N*. Detailed temperature and phase dependence of its refractive index and photonic bandgap were illustrated in [17]. To investigate all the dye-assisted nonlinear index changing mechanisms, we had used a higher concentration of MR doping (0.5–2 wt %) with respect to our previous study [12] (where < 0.1 wt % was used to avoid thermal effect). The dyed BPLC was then filled into a 100 μm-thick glass cell. The cell was thermally controlled in either BPI or BPII phases. Since MR-BP thin film has a strong absorption band at around 530 nm; thus a 532-nm laser and a 633-nm laser were used as the pump and probe, respectively.
In the wave mixing experiment, cf. Figure 1(a), the pump laser was split into two coherent beams and combined on the sample with a crossing angle of 1.3°. In contrast to the lightly doped BPLC sample used in earlier study [12] where the diffraction signals took seconds to reach the steady state, strong side diffractions were observed within a very short time (1–2 milliseconds) upon pump beam illumination. For samples maintained in the BPI and BPII phases, the probe side diffractions exhibit a fast and a slowly developing components. When the pump beams were blocked, the probe side diffractions were observed to be diminished considerably to a longer lasting component, cf. Figure 1(b).
Fig. 1 (a) Schematic depiction of the wave-mixing experiment involving two coherent pump lasers and a probe on BPLC cell; (b) photographs of the nonlinear grating diffractions of the probe beam at pump beams-on and-off states. With pump beams on, the presence of both thermal and persistent grating index gives rise to a stronger diffraction than if the pump beams are blocked when the thermal component vanished. (Owing to the appearance of many higher orders diffractions, only the first few orders on both sides of the probe beam are shown).
The corresponding time constants for these grating relaxations depended strongly on the photoexcitation time. Specifically, millisecond-long pump led to a relaxation process in seconds; longer exposure (on the order of 10‘s of seconds to a minute) gave rise to decay times of several minutes (even several hours had been observed). To gain further insights into the dynamics of these grating formations and their relaxation, laser pulses of varying durations were employed. The results reported in the following sections are representative of the principal mechanisms observed in such methyl-red doped BPLC systems where the 532-nm pump laser used was an electronically chopped square pulse of 11 ms with an intensity I ~180 mW/cm2. This choice of pulse duration and laser intensity was adopted after some systematic investigations of the diffraction phenomena with various pulse durations and intensity levels, such that both build up to steady state, and complete relaxation that followed were measured following a single pump pulse excitation.
Figure 2(a) shows the dynamical evolution of the + 1 side-diffraction from the CW probe laser within the excitation pulse duration. The grating build-up processes all commenced with a sharp rise and peaked within 1–2 ms, for all phases (BPI, BPII and ISO). It is evident that the grating developed at a rate R differing between phases: RBPII > RBPI > RISO. These results are consistent with previous studies of the temperature dependence of the index of refraction BPLC [17] and that the fast rising component was due to thermal index effect [18]. For BPLC, the thermo-optic coefficient dn/dT was found to exhibit the following relationship: (dn/dT)BPII > (dn/dT)BPI > (dn/dT)ISO [17]. Higher dn/dT would create a greater index amplitude, hence the diffraction signal intensified more rapidly in the corresponding phases. Furthermore, for the crossing angle used, the grating constant was on the order of 45 μm which gives a thermal relaxation time constant of ~1 ms for a typical LC diffusion constant on the order of 1 × 10−3 cm2s–1. Accordingly, after the grating built up to a maximum in ~1 ms. heat diffusion began to set in and the diffraction signal dropped by four-fifths of the maximum value and subsequently leveled off to a steady state value. In our case, the nonlinear grating diffraction followed the Raman-Nath regime, so that the diffraction efficiency η ~(πdΔn/λ)2. The measured peak + 1 diffraction efficiency η ~2.5% allowed one to estimate the index grating amplitude Δn ~–3.2 × 10−4. The nonlinear index coefficient n2 defined by Δn = n2I was thus –1.8 × 10−3 cm2/W. The negative sign in Δn and n2 is due to the fact that the thermal index gradients dn/dT’s of BPLC are negative [17].
Fig. 2 Evolution of the first-order diffraction signals in (a) the first few ms during the pump-on stage and (b) an observational time interval of 600 ms beginning just before the pump-on and long after the pump was off.
Besides thermal indexing effect, another mechanism also contributed to the grating diffraction. This is manifested in the observed diffraction signal over a much longer time scale when the pump pulse is over. Figure 2(b) shows the relaxation dynamics of the induced gratings in the BPLC for samples maintained in the BPI, BPII and isotropic (liquid) phase. In comparison with the quick rise and fall recorded at the very beginning of the pump-on stage (τ ~1 ms), the subsequent relaxation when the pump pulse was over took place in an extremely slow manner. In the BPI and BPII phases, the relaxation occurred in several 102’s of milliseconds, whereas in the isotropic phase the signal decayed as soon as the pump pulse is over. The slow dynamics of the optically induced director axis reorientation and lattice distortion of the BPLC resembles its low-frequency electric-field counterpart [19–23], where the electrically deformed BP relaxes slowly with a time constant on the order of 101–102 s [20, 21]; the deformed BPI takes a much longer time than BPII to restore to the cubic state. In azo-doped liquid crystals, photo-isomerization activated intermolecular torques account for most of the orientation responses [12, 13, 24], which include: (i) director reorientation — the long axis of the trans dye molecules gradually drift away from the polarization direction of the pump during repeated isomerization, thereby exerting an uniform intermolecular torque on the neighboring mesogenic molecules; or, (ii) director disordering — orientation disturbance among the mesogenic molecules originating from the structural incongruity between the bent cis-azobenzene and the calamitic liquid crystal. Both mechanisms result in persistent grating formation.
Using such transient and memory effects of MR doped BPLC, we have conducted a simple demonstration of holographic image processing/storage operation, similar to those studies with MR doped nematic liquid crystals (NLC) [25–27]. Unlike the birefringent NLC which requires strict polarization requirements on the writing beam polarization orientation and incidence angle w.r.t. the NLC director axis, similar image processing/recording can be performed in the optically isotropic BPLC with input lasers beams of arbitrary orientation and direction of incidence. In the experiment, one of the pump beams acted as an object beam (carrying the spatial information of ‘8’, cf. Figure 3(a)) and the other pump beam (having a Gaussian distribution) acted as a reference beam. In BPI and BPII, the diffractions stayed after one of the writing beams was blocked [Fig. 3(b) and 3(c)]; the isotropic-phase (ISO) BPLC cell did not exhibit storage effect; with either reference or object beam blocked, the side diffraction signals vanished promptly [Fig. 3(d)]. For the stored grating, the reference beam acting as a reading beam reconstructed a ‘true’ image of the object ‘8’ in its + 1 order diffraction, and a ‘virtual’ image exhibiting some edge-enhancement properties in the –1 order diffraction [25–27]. The marked difference between the true image [generated in the phase matched direction and resembled the input] and the virtual image [in the non-phase matched direction] is simply due to phase distortion experienced by the latter. Probing with the object beam produced diffractions of a rippled ‘8’, representing the complex (phase and amplitude) interaction between a shaped beam (object beam ‘8’) and the stored holographic grating.
Fig. 3 Holographic image processing and storage in a MR-BPLC film in BPI, BPII and ISO phases. (a) An object beam ‘8’; (b)–(d) transmitted beams on a screen. Top roll: writing with two beams on; Middle roll: reading with reference beam; Bottom roll: reading with object beam.
3. Photonic bandgap shift and optical activity
Besides index changes, prolonged irradiation also produced changes in the photonic crystal bandgap of BPLC and therefore the reflected color from the BPLC. Figure 4(a) shows how the selective reflection of BPI in the pumped region had shifted from the unexposed greenish color to red. This was corroborated by comparing the reflection spectra of the pumped and un-pumped parts of the sample [Fig. 4(b)]. The stopband had shifted to the longer wavelength region by over 40 nm from the peak reflection at 585 nm, indicative of elongated lattice spacing.
Fig. 4 (a) Pumped (reddish region) and re-cooled greenish MR-BP film; (b) reflection spectra of the pumped and un-pumped regions; (c) red-colored laser-written text ‘NSYSU’ on a green background; (d) normalized probe intensity recorded as a function of the polarizing angle of analyzer.
It is important to point out here that this phenomenon observed in thick (~100 μm) BPLC sample was different from Liu et al.’s findings [28] on the light-driven effect in thin (~10 μm) BPLCs containing the same azo dopant (Methyl Red) and chiral constituent S811. In [28], the photonic bandgap was ‘pinned’ (temperature-invariant) after photo-aligning the BPLC by Methyl Red (MR) surface layer; the strong surface anchoring by the photo-aligned MR layer on the substrate was sufficient to dictate/maintain the lattice structure, i.e. the stopband were ‘pinned’ and remain unchanged with temperature variation. In our samples, the penetration depth of the surface anchoring was very shallow in comparison with the cell gap; the MR molecules in the bulk thus became more influential than those adsorbed on the surface. Our observations are therefore more in line with those reported in references [2, 29, 30] where the color tuning observed in the thick MR-BP film was attributed to light-induced conformational change of the MR dye. Specifically, trans-cis photo-isomerization lengthened the cholesteric pitch, hence a swell in the BPLC lattice. The lattice could stay expanded for hours in the absence of the pump beam unless it underwent temperature recycling through the isotropic phase and re-cooled to the Blue Phase [Fig. 4(a)].
Such persistent lattice distortion effect could be used for (direct) non-holographic means writing of spatial information on a MR-BP film. As shown in Fig. 4(c) a micron-scaled text ‘NSYSU’ with linewidth of about 10–20 μm was directly written using the green laser. In this quick feasibility demonstration, the uneven reflections from the written image were due to the multi-domain BPLC with randomly oriented crystallites. To improve the image quality, one may employ electric and surface treatments to create more uniform lattice plane orientation [31, 32].
As in other studies of BPLC [33], we have also observed marked changes in the optical activity due to the field induced lattice distortion/dilation. Upon traversing the un-pumped 100 μm-thick MR-BPLC cell, the linearly polarized probe (633 nm) experienced a polarization rotation of 15° counterclockwise away from the incident vector at 135° [Fig. 4(d), blue line]; i.e. the optical rotatory power of the un-pumped MR-BP (operated in the BPI) was thus calculated to be –0.15°/μm, with the rotating direction matching the handedness of the material. During photoexcitation (I ~150 mW/cm2), the optical rotary power rose slightly by –0.01°/μm [Fig. 3d, green line]; eventually when the pump was removed after prolonged illumination, we found that the redshift of the photonic bandgap had boosted the optical rotatory power to –0.30°/μm, converting the polarization from 135° to 105°. Such a change consistent with our general understanding of optical rotatory dispersion in a photonic crystal [34, 35]; the optical activity grows stronger as the probe wavelength approaches the stopband.
4. Conclusion
To conclude, the principal mechanisms responsible for nonlinear processes taking place in azo dye-doped BPLCs were experimentally investigated. From the wave-mixing experiments, it was found that dye-assisted thermal indexing and lattice distortion occurred simultaneously under the exposure but were distinguishable by their response dynamics. The build-up and decay times of the thermal indexing grating diffraction component were consistent with our findings on the temperature dependence of refractive index. The relaxation times of the intermolecular torque induced lattice distortion were analogue to its electrostriction counterpart, i.e. τ(D)BPI > τ(D)BPII, the deformed BPI takes a much longer time than BPII to restore to the cubic state. By examining the reflected color and polarization state of transmitted beam, isomerization-mediated lattice expansion was also identified as the mechanism for persistent ‘storage’ effect. Together with the unique advantages of BP over their nematic counterparts, these nonlinear effects have made dyed BPLCs possible candidates for all-optical photonic applications as illustrated by some preliminary demonstration of coherent image processing and holographic and direct image recordings.
Acknowledgement
This work is supported by the Air Force Office of Scientific Research FA9550-14-1-0297 (U.S.A.) and Ministry of Science and Technology 103-2112-M-110-012-MY3 (Taiwan). We acknowledge some technical assistance from Shuo Zhao and Cheng-Chang Li.
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