Integration of liquid crystal elastomer photomechanical optical devices Nathan J. Dawsona , Mark G. Kuzykb , Jeremy Nealc , Paul Luchettec , and Peter Palffy-Muhorayc a Department
of Physics and Astronomy, Youngstown State University, Youngstown, Ohio 44555, USA; b 1Department of Physics and Astronomy, Washington State University, Pullman, Washington 99164-2814, USA; c Liquid Crystal Institute, Kent State University, Kent, Ohio 44242, USA ABSTRACT Azobenzene dye-doped liquid crystal elastomers (LCE) are known to give strong photomechanical responses. We review photothermal heating actuated Photomechanical Optical Devices (PODs) and applications to systems by examining successful attempts at cascading macroscopic PODs in a series configuration. Using these results, we present some new design strategies that have the potential of miniaturizing these systems with increases in the response time and system integration. Keywords: Photomechanical effect, photothermal heating, photo-isomerization, liquid crystal elastomer, photomechanical system
1. INTRODUCTION Photomechanical effects have been studied for over a century using various types of materials. The first recorded photomechanical device was developed by Alexander Graham Bell, which was dubbed the “photophone.”1 Significant progress in photomechanical research was at a standstill until Uchino, et al, developed the Uchino walker, which was composed of a top slab with two legs.2–4 Each leg was made from a bilayer with an active and passive material that, when illuminated at the surface of the active material, would bend via the piezo electric effect induced by a charge ejection.5 In recent times, end-pumped photomechanical devices created from azobenzene-doped polymer fibers were shown to act as actuators and sensors that displayed bistability.6–8 More recently, photomechanical cantilevers made from azobenzene-doped polymers and liquid crystal elastomers (LCEs) have been shown to activate through trans-cis photo-isomerization of azobenzene moieties.9, 10 The polymer cantilevers were doped with a low concentration and end-pumped with an off-centered beam spot, which induced small strains. The the LCEs were doped with high concentrations and side pumped to illuminate the surface which induced large surface strains causing a dramatic bend. These LCE cantilevers were shown to have nonlinear attributes to their dynamic bending via nonmonotonic photostrains.11 The technological applications of this photomechanical bending phenomenon has been demonstrated in LCEs by creating a photomechanical “swimmer.”12 In this paper, we review how an LCE in the nematic phase that is doped with high concentrations of azobenzene molecules behaves when placed in a Fabry-Perot etalon.13–15 The etalon/LCE system is a type of photomechanical optical device (POD), and we demonstrate the signalling and activation of these devices in a series configuration, where multiple devices are cascaded together to create a photomechanical system. The first LCE is activated by a pump beam and probed by a second beam directed through the interferometer. The second LCE is pumped by the probe beam transmitted from first POD’s etalon, and the second POD is probed by an additional beam directed through another etalon. Although we review this proof-of-concept demonstration, one could imagine the miniaturization of these PODs to create future smart-materials. In addition to the review Further author information: (Send correspondence to N. J. D) N. J. D.: E-mail:
[email protected], Telephone: 1 330 941 7467
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Figure 1. The chemical structures of the silicon backbone, crosslinker, mesogenic sidechain, disperse orange 3 dopant chromophore, and disperse orange 11 dopant chromophore that make up the dye-doped LCEs.
of macroscopic devices, we present new design configurations that have the potential to realize much faster device response times in addition to successful the succesful miniaturization as well as implementation into a photomechanical system.
2. DETERMINING THE MECHANISM OF ACTUATION OF MACROSCOPIC DEVICES There are two mechanisms that are considered for photomechanical actuation. The first mechanism is trans-cis photo-isomerization, where light is absorbed by azobenzene molecules that relax from an excited state into a long-lived cis state. Here, the cis state has little preferred orientational axes and may easily be approximated as a sphere due to thermal tumbling near room temperature. Therefore, photo-isomerization increases the number of dopants in the cis state, which reduces the long-range orientational order in an LCE.16, 17 The second mechanism is photothermal heating, where light absorption and nonradiative relaxation increase the temperature of the LCE, and thereby reduces the long-range orientational order. In both mechanisms, the reduction in order will cause a net contraction of the bulk material along the nematic director, n. The LCEs are made of a mesogenic side-chain crosslinked to a silicon backbone at 10% crosslinking and highly doped with disperse orange 3 (DO3). To separate the mechanisms, we also introduced a control dye, disperse orange 11 (DO11), into a reference LCE. The DO11 molecule does not undergo an isomeric shape change, and therefore, cannot reduce the orientational order of the mesogens. The molecular structures of the side-chain, crosslinker, backbone, DO3, and DO11 are shown in Figure 1. The use of DO11 as a control dopant in the LCEs allegorizes the photothermal effect as the dominant mechanisms. Thus, by deduction, we shall find that photo-isomerization is a negligible contribution to the stain in a end-pumped geometry. The LCEs are initially pumped by 488nm CW laser with the direction of propagation oriented parallel to the director. The transmitted probe beam intensity measures the strain, and a thermocouple detects the change in temperature. The experimental setup of a single POD is illustrated in Figure 2a. Figure 2b shows a linear correlation (within uncertainty) between the rate of change in the average temperature of the bulk material measured in the center of the LCE and the rate of length contraction when the LCE is illuminated. The LCEs are uniformly illuminated at the surface, with the majority of heat conduction occurring at the LCE/glass interface. The heat flow along the length of the elastomer allows us to approximate the temperature profile to be linear and one-dimensional for a single heat source at one end. The control dye DO11 (which does not photoisomerize) shows the same signature as a function of time, which directs us to a conclusion that temperature change is the dominant mechanism for inducing strains in this experimental configuration. Thus, by absorbing the majority of the light near the surface of the LCE at high concentrations of dopant molecules, the material may increase in average temperature due to heat diffusion, while only a small portion is subject to an isomerizationinduced order reduction. Additional experimental results at longer wavelengths (out to 647nm) showed the same
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Figure 2. (a) The experimental setup for a single POD pumped by a CW laser at a wavelength of 488nm, and (b) the strain and temperature change as a function of time for an LCE doped with DO11 and DO3. Here, we illustrate the correlation between the change in temperature and the induced strain, which is attributed to the photothermal effect.
behavior, where all experiments showed a negligible dependence on photo-isomerization. The DO3 dopant is less absorbative at these photon energies, and so the light travels further through the material but fewer molecules are excited with a higher probability of cis-to-trans isomerization. As a second experiment to decouple the effects of photo-isomerization and photothermal heating, we painted the illuminated surface of the LCE with a thin layer of broadband absorbing materials. The light is either scattered or absorbed at the interface in this setup, where all measured strains can only be induced thermally. As we observe from Figure 3, the rate of length change in the LCE at all times is proportional to the rate of temperature increase. Note that the strain per unit change in temperature is identical to the data shown in Figure 2b. Thus, the second experiment verifies our hypothesis that photothermal heating is the dominant mechanism of the observed photomechanical effects in our end-pumped configuration. Photothermal heating is responsible for the majority of the strain due to the fact that the LCEs are endpumped. By that, we mean that the LCEs are pumped at the substrate where the director is normal to the
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interface. Therefore, all the light is absorbed near the surface by the high concentrations of dopant molecules that are necessary for photo-isomerization-induced order reduction. Because the penetration depth is much smaller than the sample length due to absorption, only a fraction of the total length of the material is illuminated. Therefore, the temperature increase throughout the entire sample from thermal diffusion produces a far greater strain than that induced by the increasing number of cis isomers in the small volume subject to the pump beam.
3. SYSTEMS OF MACROSCOPIC PODS While a single device is interesting for characterization purposes, a pair of inline devices that actuate from a preceding modulated pump intensity can be used to create a photomechanical system. The setup shown in Figure 4a shows a successful scheme for creating a cascaded system of macroscopic PODs in a series configuration. There are three structures of interest in this diagram. The first is a Fabry-Perot etalon driven by a speaker and the other two are LCE-driven PODs. Each POD is designed with parallel input beams so that the initial beam is split into three beams for the simple system. The first beam travels through the etalon, where the distance between the substrates is modulated by the speaker, and the transmitted intensity pumps the first POD. The second beam that splits from the fundamental probes the first POD, which is modulated by the LCE, where the transmitted intensity of this beam pumps the second POD. The third beam probes the second POD and is read by a photodiode detector. Although this last probe beam is sent directly to a detector, a third POD could also be included. Likewise, an additional POD could be placed after that device, and so on, to create a large scale system.
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Figure 4. (a) The experimental setup for a system of PODs in series and pumped by a CW laser modulated with a speaker driven etalon. (b) The relative probe beam intensities as a function of time for the two-POD system when the transmitted intensity of the speaker-driven etalon is a (top) square or (bottom) triangle waveform at a frequency of 1Hz.
The response of the probe beam transmitted by the POD depends on both the LCE’s response to the incident pump beam and the initial distance of separation between the mirrored surfaces of the etalon. This dependence on the initial mirror separation suggests that we characterize all PODs in a system using the LCE response, and then individualize each POD in the system by the appropriate phase of the Airy function. This method of characterization allows an additional degree of freedom that may be useful for design purposes. The response, R, of an end-pumped LCE driven by photothermal effects has previously been modeled by a sequence of three exponentials,15 3 X bi Ai e−bi (t−τ ) , (1) R (t − τ ) = i=1
where bi is the time constant and Ai the amplitude of the ith response mechanism. Note that all terms in the sequence may correspond to the same physical phenomenon, but are due to different timescales, which is explained by the heat flow through the apparatus at various boundaries. The response function describes how the input intensity, I(τ ), at time τ is related to the strain, σ(t), at a later time t according to the expression Z t σ (t) = R (t − τ ) I (τ ) dτ. (2) −∞
By combining the expression in Equation 2 and the appropriate phase, the transmitted probe intensity is expressed as −1 Iout (t) 2πL0 σ (t) 2 = 1 + F sin +δ , (3) Imax λ where F is the finesse, L0 is the initial length of the LCE, λ is the wavelength, and δ is the phase shift that depends on the initial separation of the etalon’s mirrors. Here, σ = ∆L/L0 , where ∆L is the change in length of the LCE. The tri-exponential sequence is sufficient to model the photo-induced strain of the LCE. For one set of fitting parameters over a short range of time (after transients decay and the system reaches equilibrium) the model yields the theoretical curves that match the experimental measurements. These fits to the experimental data are shown in Figure 4b. We observe the cascaded response for both triangle and square waveforms. The phase attributed to the initial length can be used to engineer PODs that have either a decrease or an increase in the output intensity when subject to the incident pump. There is, however, a drawback to the slow response of a thermally actuated device as can be seen by the waveform deformations when more PODs are added to a system at these frequencies.
4. FUTURE WORK (SEARCHING FOR MINIATURIZATION TECHNIQUES AND RAPID RESPONSES) The previous section describe a thermally activated photomechanical device, which can be cast into a system. The usefulness of the current technology is limited due to the size and speed of the devices. The first part in a strategy to surpass these limitations is through device miniaturization. By creating smaller devices, the response time will also inherently be reduced per device. An obvious choice for miniaturization is to construct a photomechanical optical fiber (PhOF). Creating PODs based on polymer-based optical fibers would allow for simple manipulation of the mesoscale characteristics that can be built on several decades of research,5, 18–22 and allow for rapid responses due to the size of the device and uniformity of the pump beam through the fiber. In addition to these qualities, this type of device architecture could allow for a single pump/probe beam design with feedback mechanisms for optical bistability and/or enhanced responses. This can be achieved by either creating Bragg gratings,23 or adapting reflectors at the coupling locations of small fiber segments.8 Since we have decided on a device geometry, the next step in designing a photomechanical device is to identify techniques for creating and enhancing the mechanisms of photomechanical actuation. The idea that nematic LCs produce large strains when their mean orientational order is only slightly perturbed makes these types of materials enticing for use in future devices. There have been many applications of liquid crystal polymers (LCPs),24–26 though for optical fiber purposes, LCP clad optical fibers can be used to increase the robustness
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Figure 5. An azobenzene dye-doped LCP in the nematic and isotropic phase, where the lightly colored species (yellow) are the mesogens and the darker colored species (red) are the azobenzene-dopant molecules. The absorbed light causes the dopants to photoisomerize, which in turn, reduces the orientational order parameter of the mesogenic constituents. If desired, an nematic ordered material can be parameterized to undergo a nematic isotropic phase transition within a temperature range for increased length contration.
of a standard optical fiber by creating a moisture and gas barrier, resisting the effects of hardening from UV radiation, and increasing thermal stability. For our purposes, however, we wish to use an LCP clad fiber by taking advantage of the large strains that are produced when the orientational order is reduced and cause our fiber to significantly contract. To achieve this goal, we must (1) find a way to couple the power from the core to the LCP cladding and (2) actively reduce the order parameter in a nematic LCP. We first discuss how to efficiently couple light traveling through a waveguide into the exterior region of the waveguide. By choosing the proper index of refraction of the LCP cladding and number of fiber modes, we may allow light to leak through the core-cladding interface. Additionally, evanescent waves such as surface gallery modes on intentionally deformed sections as well as strategies using surface plasmon resonances may also aid in
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Figure 6. (a) Optical fiber based PODs cascaded in series pumped by a laser, (b) a single LCP clad fiber with cylindrical symmetry that is either coupled to another fiber or strung into a long optical fiber and sectioned via Bragg gratings, (c) a bundle of cylindrically symmetric PODs in a parallel configuration that can change length or bend, and (d) a dye-doped LCP and undoped polymer clad optical fiber that acts as an end-pumped cantilever.
the core-to-cladding transfer of light. Once the light is transmitted through cladding, it needs to activate the LCP to cause a strain. Azobenzene molecules doped in the cladding have been shown to induce a deformation via temperature changes, but with our new design, may also give a significant contribution to order reduction from photo-isomerization as depicted in Figure 5. This newly expected contribution of photo-isomerization induced strains in the end-pumped configuration is due to the approximately uniform delivery of light along the length of the fiber. Alternate chromophores may also improve functionality although many of these dyes do not have the dramatic shape changes of azobenzene moieties. For example, diarylethene chromophores can be used as stable on/off photochromic switches due to the large energy separation in absorption between open and closed ring isomers.27 A working photomechanical device that is much smaller, faster, and more robust is the next step to making future all-optical smart materials. Once a single device is parameterized, a series of first generation miniature PODs can be strung together to create a scaled down photomechanical system. The designs shown in Figure 6 are still met with new challenges from fabrication techniques to PhOF coupling. Fabrication of individual PODs based on PhOFs, as illustrated in Figure 6a, may also present new challenges due to variations in core diameter and inhomogeneities in the LCP orientation order. Problems with PhOF coupling, however, may be overcome with simple post-processing techniques such as introducing Bragg gratings along the length of a single fiber via UV interferometry.5 Cylindrical fibers (Figure 6b) that can be grouped into a cluster of parallel photomechanical systems (Figure 6c) are easily drawn, but other geometries, such as beaded fibers for strong whispering gallery modes, may present unforseen difficulties. Future work in core/cladding transmittance in multimode optical fibers and dopant/mesogen interactions are still underway. Also underway are designs of PhOFs that maintain polarization in partially LCP clad units, where interfacial stresses from an LCP/core boundary create end-pumped cantilevers. A half LCP clad scheme is depicted in Figure 6d, where interfacial stresses in the LCP cladding cause the end-pumped photomechanical cantilever to bend.
5. CONCLUSIONS We have reviewed experiments that show how the dominant mechanism of an azobenzene dye-doped LCE originates in a photothermal effect in an end-pumped geometry. Also, we have reviewed our recent experiments that demonstrate an all-optical cascaded system of PODs using LCEs as the active component. These systems have been characterized with a tri-exponential response function of the device, where all of the data is consistent with the theory that uses only one set of parameters, including recently analyzed data. Continued work toward miniaturization of a single (pump/probe) beam and integrating this method into a LCP clad waveguide has been presented, which could potentially increase the response time of an end-pumped photomechanical device, allow feedback mechanisms, and make interactions between devices more efficient. We discuss the reasoning behind the predicted photomechanical enhancements as well as potential difficulties while creating these devices.
ACKNOWLEDGMENTS MGK and PPM thank the National Science Foundation (ECCS-1128076) for generously supporting this work. NJD and MGK thank the Air Force Office of Scientific Research (FA9550-10-1-0286) for their generous support.
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