Reversible, fast, all-optical switching of the reflection of a cholesteric liquid crystal (CLC) is demonstrated in a formulation doped with push-pull azobenzene dyes. The reflection of the photosensitive CLC compositions is optically switched by exposure to 488 and 532 nm CW lasers as well as ns pulsed 532 nm irradiation. Laser-directed optical switching of the reflection of the CLC compositions occurs rapidly, within a few hundred milliseconds for the CW laser lines examined here. Also observed is optical switching on the order of tens of nanoseconds when the CLC is exposed to a single nanosecond pulse with 0.2 J/cm2 energy density. The rapid cis-trans isomerization typical of push-pull azobenzene dye is used for the first time to rapidly restore the reflection of the CLC from a photoinduced isotropic state within seconds after cessation of light exposure.
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Phototunable cholesteric liquid crystals (CLCs) consisting of photochromic molecules such as azobenzene are capable of large changes in spectral position of their reflection band when exposed to light [1–12]. Recently UV-light induced tuning of a CLC reflection notch of as much as 2000 nm has been reported . These systems are intriguing in that they use light to control light and therefore may conceivably evolve into a number of applications as passive optical elements in lasing, filtering, and displays.
Nearly all examinations of photoresponsive CLC mixtures have focused on tuning, e.g. changing the spectral position of the reflection notch upon light exposure. Optical switching in these mixtures has also been demonstrated and could be of practical importance especially in systems that are sensitive to cheap and readily available laser lines. Typically azo-based CLC mixtures have been photoresponsive to UV light. However, a number of recent reports have utilized visible laser irradiation, primarily 532 nm, to induce optical effects in these systems, including formation of concentric color domains in CLCs doped with azobenzene nematic liquid crystals (azo-NLCs) , rastering of intricate images in both the reflective and isotropic states , and nonlinear optical switching .
Highly sensitive CLC materials based on certain azo-NLCs are readily tunable to direct 532 nm irradiation [16, 17]. These CLC mixtures consist of a high concentration of room temperature azo-NLCs and small amount (~10%) of light-insensitive chiral dopants (S1011 and R1011 from EMD Industries). Due to trans-cis isomerization induced by a low intensity (1-10 mW/cm2) CW 532 nm laser beam, these materials exhibit blue shifting reflection changes that span a large portion of the visible spectrum. Under prolonged exposure, mixtures based on azobenzene NLCs may also undergo a photoinduced (e.g. isothermal) phase transition from the CLC to the isotropic phase, which has been referred to as the photoinduced isotropic state (PHI). A major limitation in the utilization of these materials as optical switches is the days-long relaxation of these materials from the PHI state back to the CLC phase. The dark relaxation of these systems is governed nearly entirely by the slow kinetics of the cis-trans isomerization of this class of azobenzenes, which typically take tens of hours [18–20].
The rate of the thermal cis-trans isomerization of molecules that contain electron donor and acceptor at both ends of the azobenzene moiety can be controlled by changing the strength of the donor and the acceptor . Azo dye molecules containing two benzene rings with donor-acceptor, or push-pull, π-π conjugation have previously been used for speeding up relaxation of a PHI state back to a NLC phase . With the quickened thermal cis-trans isomerization, as evident in , the peak absorption wavelength also shifts dramatically to the red with maximum absorbance values for trans azobenzene shifting from 365 nm to as much as 550 nm in push-pull azobenzene systems.
We report here on new photosensitive CLC formulations based on push-pull azobenzene chromophores capable of fast, all-optical switching and nearly instantaneous relaxation times.
2. Experiment and results
4-pentyl-4’-cyanobiphenyl (5CB, clearing temperature Tcl = 35°C) was used as the host NLC. The cholesteric phase was induced by adding the left-handed chiral dopant S1011 (EMD Industries). The mixture was photosensitized through the addition of a push-pull azobenzene dye 1-(2-Chloro-4-N-pentylpiperazinylphenyl)-2-(4-nitrophenyl)diazene (CPND-5, beamco.com). The synthesis and fundamental properties of dyes of this type are described in  and the references therein while the nonlinearities of this dye in NLC mixtures were studied in Refs [24–26]. The maximum absorption of trans-CPND-5 in 5CB is 471 nm. The CLC composition S1011(9.5 wt%)/CPND-5(10.9 wt%)/5CB(79.6 wt%) was tested in a cell with planar orienting glass substrates with 1.6 μm and 3 μm cell gaps. The Bragg reflection wavelength of this mixture is 635 nm.
The optical switching and relaxation of this mixture is shown in Fig. 1. The optical switching was induced with the 488 nm line of an Argon-ion laser (Ar+) of 4 mm diameter and an intensity of 31 mW/cm2. This wavelength is close to the peak absorption wavelength of the material. The reflectance of the CLC cell during and after laser exposure was monitored with a fiber optic spectrometer (Ocean Optics). As shown in Fig. 1(a), the reflection of the CLC quickly disappears upon exposure to the 488 nm laser. Within 7 seconds, the reflection of the CLC has switched – transitioning from reflective to clear as the mixture transforms from a CLC to PHI state. Due to the low clearing temperature of the host NLC (5CB), the reflection of the CLC does not tune to any appreciable degree before it switches. For reference, much faster switching has been observed in dye-doped twisted nematics, as low as 100 μs . The novelty of this mixture is shown in Fig. 1(b), which plots the reflection spectrum of the CLC in the time after the 488 nm laser is removed. Within 0.5 s, a small reflection is already evident, with complete restoration of the initial reflective properties (reflectivity, baseline transmission, bandwidth) occurring in 3 seconds. The temporal dynamics of both switching and relaxation of the CLC mixture is shown more clearly in Fig. 1(c), a plot of the reflectivity as a function of time. The notch approaches a minimum reflectivity, likely associated with the glass of the LC cell, beginning at 5 s for the conditions examined here. The laser is shuttered at 7 s, upon which the reflectivity immediately and rapidly returns, approaching the original reflectivity of the cell in just two seconds. Prior examinations of azo-based photoresponsive CLCs have observed relaxation on the order of many hours to multiple days. As such, the novel use of push-pull azobenzenes such as CPND-5 represent a major breakthrough in the development of passive optical elements capable of on-demand optical effects observed only in the presence of the light stimulus.
As might be expected, the switching ‘off’ times for the CLC reflectors is a function of cell thickness and laser power. The examination was undertaken in a simple pump-probe experiment utilizing a circularly polarized 635 nm laser as probe beam, initially reflected by the CLC cell but transmitted upon switching. The probe beam was focused onto the CLC with a lens of 300 mm focal length to sample an area smaller than the pump beam. A 532 nm laser beam 1.5 mm diameter was used as the pump. Using this pump-probe setup, the reflected power of the probe beam was monitored during and after laser exposure. As shown in Fig. 2(a), the reflection of the 633 nm laser immediate decreases as the CLC reflection is optically switched by the 532 nm laser. The reflection from the CLC is completely switched within 3 seconds in the conditions examined here. Upon removal of the 532 nm laser light, the reflection immediately restores, as evident in the increase in the reflected power of the 633 nm laser beam. Inset into Fig. 2(a) are photos of the CLC immediately after laser exposure (taken on a black background) where it is clear the reflection is gone only in the exposed area. After a short while in the dark, a second photo was captured which shows complete restoration of the reflectivity. Figure 2(b) plots the response time (measured by 90% decrease in normalized reflectivity) for 1.6 and 3 μm cells as a function of 532 nm laser beam power. Clearly, increasing laser power quickens the response time for both CLC cells. Interestingly, the thinner CLC switches faster, by as much as a factor of two when exposed to 1 mW of 532 nm irradiation. Only at low power levels less than 1 mW is there any discernable difference in the restoration times of these materials for both 1.6 and 3 μm cells (Fig. 2(c)). Increasing the cell thickness increases the relaxation time from approximately 3 s to 6 s.
The relationship between the pump laser power, cell thickness, and the switching times is related to the rate that cis isomers are generated and the overall concentration of them in the system. As has been stated, exposure of the cell to the 532 nm laser (as in Fig. 2) or the 488 nm laser (as in Fig. 1) drive trans-cis isomerization of the push-pull azobenzene chromophores utilized here. It is well known that increasing the power of the exposing irradiation will expedite the kinetics of the trans-cis isomerization, thus more quickly generating the cis isomers that serve to destroy the CLC helix and yield the all-optical switching. The higher transmission of thinner cells means that effectively the average power level of the irradiation is higher across the cell thickness, and thus, optical switching is faster in these samples (Beer-Lambert law). With regard to relaxation times, given the exposure of the cell to over 10 seconds of irradiation, higher laser power will generate more cis isomers. As such, the relaxation process, although extremely fast for all the cases examined here, shows a reverse dependence on laser power – due to the higher concentration of the cis isomer accumulation in CLC cells exposed to the higher laser powers.
The S1011/5CB/CPND-5 CLC mixtures are not only switchable with CW lasers, but pulsed irradiation as well. We have previously demonstrated tuning/switching of azobenzene based CLCs with short laser pulses  with materials where the relaxation was many hours. While the metastability has its own utility, a number of interesting opportunities exist in all-optical switching of CLCs typified by switching only in the presence of the light stimulus. This necessitates materials such as those examined here, capable of rapid reversal of the photogenerated processes in the dark. To illustrate the all-optical switching and corresponding dark relaxation of the photoresponsive CLC examined here, we studied the processes in an optical setup with a pulsed 532 nm pump beam (7.5 ns pulse duration and 3 mJ/195 mJ/cm2) and a 635 nm probe beam (e.g. within the reflection of the CLC examined here). The probe beam was circularly polarized for maximum reflection at the initial state of the CLC. The rapid material response is further illustrated in Fig. 3(b). Using an oscilloscope and photodetectors, the intensity of the pump and probe beam were monitored on an ns time scale. The response of the 3 μm-thick CLC occurs during and shortly after the pulse exposure.
Exposure to the 532 nm pulsed energy transitions a high concentration of trans isomer to cis, rapidly destroying the CLC helix through an order-disorder transition. It is not surprising that the unwinding of the helix and corresponding loss of reflectivity does not occur within the length of the pulse. Intricate thermodynamics and diffusive processes must happen as the system reorganizes into an isotropic state. However, clearly within 200 ns the CLC is optically switched.
3. Theoretical estimations
Above we noted that the thinner cell is characterized by faster switching and longer relaxation time compared to the thicker cell. For both cells, the curves describing the dependence of the switching time τ on laser beam power P are well fit by the function τ ~ 1/P. The ratio of the switching times of the thicker cell to thinner one is thus constant and equal to 1.98 while the ratio of their relaxation times is 0.6. These features can be understood proceeding from a simple presentation of the cis isomer concentration in the material:
Here q is the quantum efficiency of photoisomerization, σ is the absorption cross-section, NT is the concentration of trans isomers, I is the power density of incident light expressed in photons/cm2 s, τC is the lifetime of cis isomers. Here we ignore the diffusion process since the diffusion time, τD ~ l 2/D, where l ~ 10-4 cm is the thickness of the material and D ~ 10-6 cm2/s is the diffusion constant, is on the order of 10 ms for both layer thicknesses under examination, and is much faster than the observed processes of changing the CLC reflection.
The initial stage of the process is described primarily by the kinetics of cis isomer generation: NC = qσNTIt. Since the light intensity attenuates in the material, I = I 0exp(-αz), let us replace the intensity in Eq. (1) with its average value <I> =I 0(1-e -αL)/αL. Thus the concentration of cis isomers starts growing with the rate Nc = qσNT <I>t saturating at NC max = qσNT <I>τC The response time is inverse proportional to the average intensity, ton ~ 1/qσNT <I> and is shorter for the thinner cell. Indeed, the evaluation made using the absorption constant 5.8 103 cm-1, results in the ratio 1.4 which is a reasonably good evaluation given the simplicity of these theoretical considerations that does not take into account the complexity of the dependence of the reflection coefficient of the CLC on concentration of isomers.
In the process of relaxation after blocking the light, the concentration of cis isomers decreases according to NC = NC max e -t/τC. This concentration has to decrease to a certain low level NCLC to allow formation of the CLC state. The time required for such a drop in concentration, toff = In(NCmax/NCLC), is shorter for the thicker cell due to their smaller steady state concentration of cis isomers, NC max (L 2) < NC max (L 1) if L 2 > L 1. With no knowledge of concentration of cis isomers critical for destruction/restoration of the CLC phase, NCLC, we cannot evaluate the ratio of relaxation times for cells of different thickness. Further spectroscopic examination of the photoinduced phase behavior of the CLC mixture is necessary to explain the photochemistry behind this interesting observation.
The relaxation time
is shorter for the thicker cell due to their smaller steady state concentration of cis isomers, NC max (L 2) < NC max (L 1) if L 2 > L 1, Eq. (3). With no knowledge of concentration of cis isomers critical for destruction/restoration of the CLC phase, NCLC, we cannot evaluate the ratio of relaxation times for cells of different thickness. Further spectroscopic examination of the photoinduced phase behavior of the CLC mixture is necessary to explain the photochemistry behind this interesting observation.
In closing, we demonstrate a photoresponsive cholesteric liquid crystal (CLC) formulation capable of rapid optical switching as well as rapid reflection restoration. Optical switching is caused by a isothermal phase transition due to trans-cis isomerization of the azobenzene dye. The rapid restoration of the reflection notch is enabled by the utilization of push-pull azobenzene dyes that quickly undergo cis-trans isomerization after the removal of the exposing light source. Previous reports of optical switching/tuning in azo-based CLC mixtures report restoration times of many hours to multiple days. Due to the red-shifted absorbance spectra of the CPND-5 molecule utilized here, optical switching was triggered with both 488 (CW) and 532 nm (CW and pulsed) lasers. Switching times to pulsed irradiation are reported as low as 200 ns.
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