The reflection and transmission properties of photosensitized cholesteric liquid crystals (CLCs) are examined. Introduction of mesogenic push-pull azobenzene dyes into blue and green reflective CLCs enables fast (sub-second), photoswitchable optical properties due to the overlap of the trans and cis absorption states. Upon irradiation with CW blue-green laser radiation, the bandgap reflection is erased in a fraction of a second and reversibly restored approximately one second after the blue-green laser radiation is removed. Given the strong overlap of the trans and cis absorption maxima, we believe that repeated trans-cis and cis-trans isomerization cycles induced with irradiation lead to a destruction of the ordered LC phase. The sensitivity to the irradiating wavelength scales with the wavelength-dependent absorption of the mesogenic push-pull dye. A detailed examination of the transmitted and reflected laser beams are presented as a function of power and wavelength of CW sources.
©2011 Optical Society of America
Cholesteric liquid crystals (CLCs) are fascinating materials due to their unique optical properties. In planarly aligned cells, macroscopic orientation of their helical superstructures perpendicular to the substrates leads to films which will selectively reflect the same handedness radiation as itself . When the pitch length is on the same order as visible light, vibrant, highly colored films are formed which have been examined for numerous display , sensing , and photonic applications [4–8]. A large amount of work has been performed on making this coloration dynamic , through the use of heat [10–13], light [14–21], or electrical fields [22–24]. Both tunable and switchable constructs are continuing to be explored and methodologies to increase both the response and relaxation speeds are at the forefront of this research.
The ability to use light itself as a means to control the optical properties of CLCs has recently been reviewed [9,25]. Photoresponsive CLCs have employed a variety of photochromic moieties including azobenzenes, methones, fulgides, and overcrowded alkenes. Azo-based compounds have been extensively explored due to the comparative ruggedness of the photochemistry and the ease of inducing liquid crystallinity into this class of dyes . Most prior examinations of azobenzene-based CLCs have employed the conventional photochromic structure known for its metastable cis isomer species and large difference in absorbance profile between the trans and cis isomers. Functionalizing the azobenzene chromophore with electron donating and withdrawing moieties on opposite ends is known to red shift the absorption spectra, reduce the wavelength separation between the trans-cis and cis-trans peak absorption, and greatly speed up the relaxation behavior of the cis-trans dark relaxation [27,28]. These materials enable the reduction of the relaxation times of the photoexcited state of the cis-isomer molecules to less than a second from tens of hours [29,30]. Due to their mesogenic nature, these azo dyes can be introduced into liquid crystal solutions at much higher concentrations than classic non-mesogenic azobenzene chromophores. As a result, the energy density required for observing substantial nonlinear response was reduced to ~10 mJ/cm2 for ns pulsed excitation and to several mW/cm2 for a CW laser beam operating at a green wavelength [29–31].
Using this as a design strategy to explore rapid optical switching, we recently  demonstrated a cholesteric liquid crystal (CLC) composition with a photoswitchable bandgap that restores itself in seconds after the irradiation source is turned off. The switching of the bandgap at red wavelengths was induced by laser beams operated at 488 and 532 nm wavelengths and was probed by a red beam of a He-Ne laser (633 nm). Theoretical considerations were reconciled with experimental data for different thickness cells and initial kinetic studies were performed. The fast switching is enabled in part due to cell thicknesses (5 μm or less) where surface-driven interactions due to the anchoring conditions are maximized in the order restoration process. We compliment this previous study with a detailed kinetic study here of the nonlinear optical processes for systems with blue and green reflection notches. A series of CW laser wavelengths were used to photoswitch the reflection notch on sub-second time scales and the relaxation back to the ordered state was monitored. The response as a function of power and laser irradiation wavelength is presented.
2. Experimental Details
Material compositions examined here were based on CPND series mesogenic azo dyes, 1-(2-chloro-4-N-n-alkylpiperazinylphenyl)-2-(4-nitrophenyl)diazenes, containing two benzene rings with push-pull π-π conjugation. The synthesis and fundamental properties of such dyes are described in  and references therein. While the response time of those materials depend essentially on the power/energy density of the radiation, they are characterized by cis-trans relaxation times on the order of seconds which is considerably shorter than for azobenzene materials based on symmetric conjugation . Nematic LCs 5CB (Tcl = 35°C) and E7 (Tcl ~60°C) from Merck Ltd, and mixtures thereof were utilized. Thermodynamically stable fast CLC compositions with bandgaps at green-blue wavelengths were obtained by combining chiral dopants R1011 (HTP ≈25 μm−1) and CB15 (HTP ≈7 μm−1), both from Merck Ltd. Green/blue CLC mixtures fabricated with just CB15 proved unstable, crystallizing within a day after fabrication, and thus R1011 was utilized to obtain the appropriate pitch with a minimum of CB15. Table 1 summarizes the three material compositions used in the study and their Bragg reflection wavelengths (λB) and absorption coefficients measured in the isotropic phase at the peak absorption wavelength. All cells were 5 μm thick, had planar orienting boundary conditions key to this study, and possessed a small fraction of the mesogenic push-pull azobenzene dye.
The experimental setup, schematically shown in Fig. 1 , comprises a pump laser, a set of neutral density filters for controlling the power/energy of the pump beam, a quarter waveplate for circularly polarizing the pump beam, and a beam splitter allowing the power of the beam transmitted and reflected from the CLC cell to be simultaneously measured. An argon-ion (Ar+) laser was used for obtaining CW beams at 458 nm and 488 nm wavelengths while a diode-pumped solid state laser was used when performing measurements at 532 nm.
The peak absorption of π-π* band for CPND-n azo dyes in 5CB is observed at λmax = 471 nm wavelength. Figure 2 shows the absorption spectra of trans and cis isomers of the azo dye CPND-8 in 5CB; the inset shows the core chemical structure of the CPND dye used in the study. The absorption spectrum of the cis isomers was obtained by exposing the NLC cell to a blue CW laser beam (λ = 473 nm, I = 35 mW/cm2) expanded to a size larger than the spectrometer beam. The dip in the absorption spectrum of cis-isomers seen in Fig. 2 is caused by the presence of the pump beam. Very small changes in the material absorption spectrum as a result of exposure to a blue light are evident including a slight red-shift and a slight increase in the peak absorption. These differences are relatively small compared to changes that take place in azo LCs with symmetrical π−π conjugation as a result of trans-cis photoisomerization [34–37].
The reflection spectra of the three CLC mixtures are shown in Fig. 3 . An overlay of the absorption spectra of the pure mesogenic dye and the lines for the three laser wavelengths utilized are superimposed. The three CLC mixtures were formulated to be accessible to the three distinct laser lines. Two of the three mixtures are composed of CPND-7 and one is composed of CPND-8. The photochemical response of these two compounds is indistinguishable at the concentrations employed here. The distortions apparent in the reflection spectra are due to the convolution of the dye absorption and the reflection bandgap.
Figure 4 shows the rapid destruction of the reflective phase due to the light induced formation of an isotropic phase for all three mixtures at low power densities. The dynamics of transmission and reflection was measured with an oscilloscope and power meters for irradiation with violet, blue and green wavelengths. A movie (Media 1, Media 2, and Media 3) of the CLC cell undergoing photoinduced modulation of its spectral properties is included in the Supplemental Information. In the movie (Media 1, Media 2, and Media 3), the laser wavelength can be seen as a spike superimposed on the reflection notch when turned on and the reflectivity is quickly driven to zero. The reflectance of the CLC cell during and after laser exposure was monitored with a fiber optic spectrometer (Ocean Optics) and is shown in Fig. 4 for each wavelength. More impressively, once the specific laser wavelength is turned off (dotted line), a fast return (a few seconds) to the aligned reflective phase occurs. The exposure time for all samples to respective laser beams is 10 seconds. Within this time scale, all CLC order is destroyed for all three cells.More detailed steady state examinations shown in Fig. 5 clearly show that the reflection intensity goes to zero while the transmission through the cell increases. The higher the input power, the more energy is transferred between the transmitted and reflected beams. These measurements examine only the polarized state that is the same handedness of the CLC material system. The fact that transmission and reflection are inversely proportional is an indirect confirmation that the loss of reflectivity is not due to the formation of transient scattering entities. The power (power density) thresholds of nonlinear transmission at the different wavelengths were 0.38 mW (110 mW/cm2) at 458 nm, 0.4 mW (117 mW/cm2) at 488 nm and 2.9 mW (370 mW/cm2) at 532 nm. The data in Fig. 5 relate to steady-state values whose kinetics are shown in Fig. 4.
The response times calculated at half of the steady-state values from the dynamics data for reflection and transmission are shown in Fig. 6 . The difference between response times for different wavelengths is related to the difference in absorption coefficients of the CLC compositions at the different exposure wavelengths (see Table 1). Sub-second response times are obtained at powers of only a few milliwatts.
Careful comparison of these two plots reveals that the reflectivity response times are shorter compared to that of transmission as indicated in Fig. 7 . We speculate that the change in reflection is registered as soon as the input layer of the CLC is affected. Due to averaging through the thickness of the cell, the transmission measurements are a convoluted average of each individual layer’s response. Even though thin cells were utilized, this difference is observed at low power densities but diminishes as the power density is increased.
The optical switching presented here is reproducible over many cycles as shown in Fig. 8 . Many cycles were tested with no observation of photofatigue. Figure 8 corresponds to 6.3 mW input power of the blue laser beam (488 nm).
Potential photoinduced physical mechanisms that could lead to the destruction of the bandgap include an increased cis-concentration (trans-cis effect) leading to reflection band shift and finally a photoinduced isotropic state, repetitive trans-cis and cis-trans isomerization cycles resulting in a disordering/reorientation of the helix, or local temperature increases due to laser heating. A potential increase in temperature ΔT can be evaluated as ~αIτ/ρCP, where α (cm−1) is the absorption coefficient, I (W/cm2) is the power density of the incident light, τ is the temperature relaxation time, and ρCP (J/cm3K) is the specific heat capacitance. In case absorption is caused by a dye (azodye in our case), the absorption coefficient is related to the relative concentration of the dye in the mixture as α = σc, where c is the dye concentration (c = 1 for pure dye) and σ (cm−1) is the absorption coefficient of the dye itself (at c = 1). A phase transition could take place when ΔT > Tc – To, with Tc being the clearing temperature of the NLC and To the ambient temperature, correspondingly. For a given light intensity value, this condition requires that the absorption be larger than a minimum value given by Eq. (1)Table 1, we expect negligibly small increases in temperature, ΔT ~1 K at the intensity levels utilized in this study.
The order parameter Q of the NLC doped with an azo dye is a function of both the temperature T and the concentration of the azodye c. The change in the order parameter with changing temperature and concentration near the critical point can be presented as33], radiation leads to an accumulation of cis isomers and, since if < 0, decreases the clearing temperature proportional to the azodye concentration c. Even with relatively small values of |if|, large enough concentration of the dye, c > Q0/|if|, may drive then the material through the critical state upon light influence. Non-thermal, photoinduced phase transitions could take place also with a small concentration of the dye if the impact factor is increased enough at the influence of light. This can be expected to take place in case of donor-acceptor, or push-pull, azodye molecules with a short lifetime of cis isomers, due to photoinduced repeatitive trans-cis-trans processes.
The superposition of the trans and cis absorption peaks (Fig. 2) indicates that upon exposure, repeated trans-cis and cis-trans isomerization cycles are occuring. The equilibrium ratio of cis to trans depends on the lifetime of the cis-isomer and quantum efficiencies of direct and reverse photoisomerization. Since the lifetime of the cis-isomer is quite short and the rate of trans-cis photoisomerization can be assumed to be close to the rate of cis-trans photoisomerization (same absorption peak wavelength), the trans concentration should be similar to that determined previously (Ntrans/Ncis ~3) . Taking into account that the concentration of the azo dye in the materials under study is small, ~5 wt.%, we suggest that indeed the repetitive trans-cis-trans isomerization is the driving mechanism (photokinetic) behind the optical changes rather than classic disordering due to accumulation of cis isomers due to trans-cis isomerization. The molecules of the azodye, via repetitive trans-cis-trans processes, transfer the energy of a laser beam onto the LC host decreasing its order parameter. The process is similar to a thermal effect although it is characterized by a different set of microscopic (photoisomerization efficiencies) and phenomenological parameters (the impact factor of the photoisomerization on the order parameter of the LC).
Thus, sensitizing cholesteric liquid crystal formulations with blue-green reflection notches with mesogenic push-pull azomolecules has enabled for fast photoinduced switching on sub-millsecond time scales at low powers. The strong overlap of the trans and cis molecules absorption peaks suggests that the repetitive trans-cis-trans isomerizations diminishes the LC order present in the reflective CLC phase. The result is the fast decrease in the reflectivity coupled with the commensurate increase in transmission upon irradiation. The fast inherent relaxation of the azo mesogenic molecules themselves also enables very fast relaxation back from the transparent state to the reflective state when the laser irradiation is turned off. The time scales at a given power density for different laser irradiating wavelengths scale with the wavelength dependence of the absorption. The ability to cycle between reflective and transparent states on sub-second timescales through many cycles was demonstrated.
The authors wish to acknowledge AFRL/RX and AFOSR LRIR 09RX04COR.
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