We discuss materials that reveal fundamental intercoupling of light and chirality in creation of complex structures. These materials are based on cholesteric liquid crystals (CLCs) photosensitized by azobenzene nematics. Transformation of the one-dimensional periodic structure of such CLCs into complex spatial patterns takes place on macroscopic scales, over the whole area of the CLC layer, under the influence of low power radiation including LED, ambient illumination, and sunlight. The obtained structures, with their origin in the strain of the CLC layers caused by trans-cis photoisomerization precede a shift in the bandgap position of the CLCs. The effect is observed both in red-shifting as well as blue-shifting CLCs.
© 2007 Optical Society of America
Periodicity is the way nature organizes itself in all scales and forms as evident by the hierarchy of periodic structures and periodic processes which makes up the world around us [1, 2]. In many cases, periodicity is obtained due to helical packing of chiral “building blocks”. The pitch of helical structures created by chiral molecules spans from a few nanometers in DNA  to thousands of nanometers in cholesteric liquid crystals (CLCs) . Periodicity of CLCs at the scale of visible radiation wavelengths allows using them for numerous optical applications as efficient and large photonic bandgaps which can be controlled by thermal, electrical, or optical stimuli. In the present paper we show that the helical structure of photoresponsive CLCs evolves into complex periodic patterns when exposed to low power illumination of UV-visible wavelengths, including ambient light. Light and chirality become thus fundamentally entangled in the formation of complex structures.
The phenomenon of periodic pattern formation in intrinsically layered CLCs is well known and has been studied in detail for electrical stimuli. Several monographs summarize different aspects of the problem [4-6]. Periodic structures are formed as a compromise between the influence of the electric field that reorients the molecules and elastic forces that preserve the orientation distribution within the pitch [7-9]. Such structures in CLCs have also been induced by mechanical stresses [9, 10] and by heating [7, 11].
Kitaeva and Zolot’ko observed diffraction patterns in thick layers of CLCs (~100 µm) that were attributed to periodic distortions of the director field in a laser beam . High power (~100 mW) beams were focused on the material to induce the effect which took place after nearly 60 J of input energy. The process was accompanied by thermal self-focusing, and distortions of CLC pitch were attributed to not only absorptive heating but also the photoisomerization of azoxybenzene groups, part of the CLC compounds.
Laser induced generation of a periodic lattice of submicron size defects was reported for smectic LCs with nanometer scale of the material periodicity (~3 nm) doped with trans-cis isomerizable molecules . The effect was observed in focused He-Ne laser beams of ~10 mW power and required ~1 J input energy. Computer simulations attributed the behavior to a ~0.5% shrinkage of the smectic layers due to segregation of cis-isomers between the layers .
In contrast to electrically or thermally induced processes and those taking place in focused laser beams, the generation of spatial structures in the photoresponsive CLCs used in the present work takes place on macroscopic scales, over the whole area of the CLC cell, under the influence of very low power radiation sources including LEDs, ambient illumination, and sunlight. The CLCs used in these observations contained room-temperature azobenzene nematic LCs as photosensitizing agent [15, 16].
Cholesteric structures which change their pitch due to trans-cis photoisomerization have been intensively studied and used in many applications [17-24]. Changes to the Bragg reflection wavelength (both blue shifts and red shifts) induced by UV radiation have been documented in the materials studied here . As demonstrated in , room-temperature azobenzene nematic LCs (NLCs) can be dissolved in CLCs in large quantities ~25 wt.% without strongly affecting the CLC order. Such CLCs possess therefore high photosensitivity, dλB/dI ~300 nm/(J/cm2), and wide spectral range of phototuning of their Bragg reflection band ~400 nm. We show that long-range 2-dimensional transverse structures similar to those previously induced with electrical fields form spontaneously at the early stages of irradiation, preceding the reflection wavelength shift.
2. Experimental results
The formation of these periodic spatial structures was studied in a large variety of CLCs doped by room-temperature azo NLCs. We present the results obtained for two material systems with opposite signs of photoinduced change in their pitch. Namely, cholesteric liquid crystals BL061 and MDA1445 available from Merck Ltd. were used as hosts. Clearing temperatures Tc of phase transition from CLC phase to isotropic phase are 86oC for BL061 and 94oC for MDA1445. Azo NLC 1005 (BEAM Engineering) was used for photosensitizing both host CLCs. The material 1005 is a multi-component compositions based on a series of 4- n-alkyl-4’-n-alkoxyazobenzenes which is photosensitive in a large temperature range including room temperature . The clearing temperature of azo NLC 1005 is 49oC. The concentration of nematic azo dopants in CLCs varied from 0 wt.% to 25 wt.%. The helical pitch increases in the mixtures 1005/1445 and decreases for 1005/BL061 as a result of exposure to UV radiation resulting in red and blue shift of their Bragg reflection bands, respectively. Note that the Bragg wavelength λB is determined not only by the CLC pitch, but by the average refractive index <n>=(no+ne)/2 as well, λB=<n>h, with h being the helical pitch of the CLC. The prevailing effect of photoisomerization at the early stage of illumination consists in increasing the fluctuations of molecular orientation around their average direction. Decreasing molecular order results in decreasing average value of the refractive index leading to the blue shift of the Bragg wavelength even in materials with red-shifting pitch as is the case of CLC 1005/1445 .
Two 2-mm thick plane-parallel glass substrates were coated with polyvinyl alcohol (PVA) and then unidirectionally rubbed with cloth to promote planar alignment of the LC molecules. The axis of helical structure at planar orienting boundary conditions is perpendicular to the cell substrates, whereas the molecules are oriented in the planes parallel to the substrates. Bragg reflection wavelength λB of CLC cells is 467 nm for BL061 and 468 nm for MDA1445. The helical pitch h was 292 nm for BL061 and 292.5 nm for MDA1445. The CLC layer thickness was much longer than the helical pitch and was varied by spacers from 5 to 75 µm.
Observations were performed by an Olympus inverted microscope equipped with a Hitachi CCD color camera, two polarizers, 20X and 100X objectives. The CLC cells were placed on the microscope stage between crossed polarizers (the axis of one polarizer parallel to the rubbing direction). The pattern formation dynamics was studied in a pump-probe experiment wherein CLC cells were exposed to a violet laser beam (λpump=409 nm) of 2 mm diameter at different power levels and a He-Ne laser beam (λprobe=633 nm) of 0.5 mm diameter was used to probe the transmission of the cells. The power of the red laser beam was fixed at 20 µW not to affect the material.
Figure 1 shows typical spatial structures generated by 3 mW/cm2 LED irradiation (λ=397 nm) after 15 s. The initially uniformly colored cell quickly evolves into a system of complex, periodic 2-dimensional square patterns. Large areas stretching tens of micrometers with a common orientation of square domains are observed where each area has a different orientation. Observation of the domain pattern using a 100X objective, through the thickness of the cell (parallel to the helical axis z), shows that each small domain extends through the entire cell [(Fig. 1(b)]. The difference in patterns shown in Fig. 1(b) for different focusing planes is a result of light diffraction on periodic phase grating produced by the molecular orientation pattern in CLC.
Quantitative studies were performed using a low power violet diode laser beam (λ=409 nm, I=10 mW/cm2). The spatial period, Λ, of a square grid instability in a planar CLC can be evaluated as Λ=(3K3/2K2)1/4(hL)1/2, where K3 and K2 are the elastic constants of the LC, L is the cell thickness, and h is the helical pitch of CLC structure . Interestingly, Λ does not depend on the nature of the influence.
The square root dependence of the period of the pattern as a function of the cell thickness and the CLC pitch can be described as a result of two contributions into the elastic energy of the CLC. The tilt of the LC orientation with transverse spatial period Λ requires elastic energy Fe ~Λ -2. On the other hand, the relative compression of the CLC pitch δh/h related with transverse modulation of the LC orientation is inversely proportional to the cell thickness L and increases with increasing transverse period, δh/h~Λ/L. Hence this contribution into the elastic energy Fc~(Λ/hL)2 increases proportional to Λ2 . Thus the total elastic energy is minimized at a certain value of the spatial period Λ~(hL)1/2.
The period Λ of the structures was measured from direct microscopic images as well as by measuring the diffraction angle α of a test beam (He-Ne laser, λ=633 nm) using the relationship Λ=λ/sinα. Typical diffraction patterns are shown in Fig. 1(c), 1(d) for unfocused and focused beams of the probe laser. The size of structural grid element obtained from the radius of the 1st diffraction circle for an unfocused laser beam [Fig. 1(c)] is 5.5 µm for a 20-µm thick CLC sample.
The period of the square structure as a function of concentration of the azobenzene dopant and the thickness of the CLC film is shown in Figs. 2(a) and 2(b). Photogeneration of these periodic structures was observed above a concentration of the azo NLC dopant of ~7 wt.%. The period, which changes from 4.7 to 5.6 µm over the dopant concentration range 7 wt.% to 25 wt.%, can be described by the function h1/2 typically expected in spontaneous pattern formation situations. The anticipated square root law also holds well for the thickness dependence of the texture period [Fig. 2(b)]. The smallest period equal to 2.4 µm was measured in a 5 µm-thick cell, whereas the largest period, equal to 8.4 µm was realized in a cell of 75 µm thickness, all shown visually in Fig. 2(c). The starting pitch of the CLC was h=355 nm corresponding to the Bragg wavelength λB=568 nm and was obtained in the material 1005(14 wt.%)/1445. The elastic constants for the materials under study are not known. Note, however, that the dependence of the grating period on the ratio of the elastic constants is a rather slow function. Typically, the ratio of the elastic constants K3/K2≈3, see, e.g., Merck’s datasheets of LC properties. The grating period evaluated using a typical value of the ratio K3/K2≈3 deviates by only 10 - 20% from the obtained values throughout the measurement range.
The periodic structures have their origin in the strain of the CLC layers caused by transcis photoisomerization of the photochromic molecules, accumulation of cis-isomers, and related change of the equilibrium value of the CLC pitch. This effect is rather substantial in the materials under study when subject to radiation of UV-green wavelengths as shown in Fig. 3. Note that the effect takes place in CLCs with both signs of photoinduced change of their helical pitch.
These spatial structures are formed at very low concentrations of cis-isomer molecules and require low radiation energy. These patterns appear during the first seconds of light exposure and persist till Grandjean-Cano lines, defect lines which form between two regions of different number of helical pitches, appear and move through the area (Fig. 4).
Examining the CLC transmission in time during irradiation reveals an incubation period during which the transmission does not change. This is typical for photoinduced phase transition processes in azo LC’s since a critical concentration of cis-isomers is needed to start the process . A critical concentration of cis isomers is required for initiating pattern formation as well. The decrease in CLC transmission due to diffraction (pattern formation) is clearly seen in Fig. 5(a) and was verified by direct observation. The time period τi required for starting the process of pattern formation and the evolution time τe of the pattern are shown in Fig. 5(b). Both the incubation and evolution times are inversely proportional to the power density P: τiP=52.6 mJ/cm2, and τeP=31.2 mJ/cm2 for CLC 1005(11 wt.%)/BL061. These numbers are smaller for higher concentrations of azo NLC dopant .
Summarizing, we have observed the formation of periodic transverse patterns in azo-nematic doped CLCs exposed to low power illumination. These patterns are caused by accommodation of the strains the CLC layers experience due to photoisomerization. These patterns are scalable and can diffract radiation. The present observation provides new insight in the fundamental intercoupling of light, chirality, and structure formation. There is also tremendous interest in the fabrication of multidimensional dielectric structures which can be utilized to control various optical processes including reflection, emission, propagation, and diffraction. Numerous wavelength specific structures exist in modern optics, however the development of dynamic systems whose wavelength-specific optical properties can be remotely modulated using light is a critical next step.
This work was supported by DoD SBIR program.
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