1. Introduction

Cholesteric liquid crystals self-organize, selectively forming planar structures that reflect light with the same sense of polarization as the cholesteric helix [1]. Light is reflected within the selective reflection band (photonic band gap) centered at the wavelength λ=nP, where P is the helical pitch of the chiral liquid crystal, and n=(n_e+n_o)/2 is the average refractive index of the cholesteric planes (Fig.1).

If the helical pitch is altered, the color of the sample and position of the selective reflection band will change. This constitutes the basis for building a variety of sensors able to change color in response to helical pitch changes activated by selected external factors. Cholesteric displays and temperature sensors are currently in use [1,2]. The lasing at the photonic band edge of thermotropic cholesteric liquid crystals was first observed and explained by Kopp et al. [3]. Subsequently, the number of publications devoted to chiral lasers has grown rapidly as a result of their unique properties, which include the ability to self assemble and a relatively low lasing threshold [3 –11]. For example, lasing was observed in lyotropic liquid crystals [4] and chiral blue phases [5]. Tunability is a very desirable property of chiral lasers because it greatly widens the area of their possible applications. Tunable lasers sensitive to amino acids and changes in chemical environment were demonstrated in [6]; tunable lasers controlled by UV light were recently demonstrated and studied in [7–9]. Spatially tunable lasers were designed and described in [10].

In ideal cholesteric structures doped with laser dyes, the conditions for optically pumped lasing are most favorable at the edge of the selective reflection band where the photon lifetime is the longest [3,12]. A few attempts were made to create cholesteric liquid crystals that could respond reversibly to different types of deformation. Cholesteric elastomers were demonstrated to reversibly change the position of the selective reflection band in a narrow spectral range. Finkelmann et al. demonstrated lasing in cholesteric elastomers under biaxial strain [13]. Terentjev et al. studied photonic band gap structure in cholesteric elastomers [14–16]. However, the reported shift of the selective reflection band in cholesteric elastomers with the same composition did not exceed 20% of its initial value. In this communication, we report larger color changes and mechanically tunable lasing in novel, highly viscous cholesteric materials.

2. Materials and methods

Color changing cholesteric material was prepared by mixing 70–80 wt.% of a silicone-based cholesteric liquid crystal C4745, C4754 and C4768 (Wacker Co), 30–20 wt.% of a nematic liquid crystal, MBBA (with a structure CH₃O-C₆H₄-CH=N-C₆H₄-(CH₂)₃-CH₃, Sigma-Aldrich Co.) and 0.3–0.5% of laser dye Pyrromethene 597 (Exciton Co.). Wacker silicones (see Fig. 1(a)) are cyclic polymers that form cholesteric glass when cooled to room temperature from the melt. For example, polymer C4768 has the selective reflection band centered at 680nm in the glassy state; polymer C4754, with a higher concentration of chiral cholesterol moieties, has the selective reflection band centered at 540nm; polymer C4745, with the highest concentration of chiral cholesterol moieties, has the selective reflection band centered at 440nm.The position of the selective reflection band in solid state depends on the composition of the polymer and shifts toward shorter wavelengths with increasing concentration of chiral groups. By mixing these polymers with nematic liquid crystal (MBBA, see Fig. 1(b)), it was possible to change the helical pitch of the cholesteric liquid crystal and adjust the position of the selective reflection band to any point in the spectrum from near IR to violet in the undeformed material. For example a composition of 75% MBBA Wacker 4754 and 25% of MBBA gives a selective reflection band centered around c.a. 660 nm. MBBA was added to Wacker polymers at different concentrations. The mixture was heated up to 80oC thoroughly stirred and cooled down to room temperature (22 °C±1°C). All measurements were made at room temperature. Viscosity of the system was measured either by vibrating fork method (A@D viscometer, SV-10) or by measuring the velocity gradient and force applied to two glass plates with a planar cholesteric sample sandwiched between them (plate to plate method). With an increasing concentration of polymer, the viscosity of the cholesteric liquid crystal rapidly increases. For instance, viscosity of the cholesteric mixture with 80% MBBA is about 0.1Pas; viscosity of the mixture with c.a. 30% MBBA is c.a. 5_*10³ Pas. The cholesteric liquid crystal was placed between two transparent silicone strips, one side of which was treated using a 20% NaOH water solution, and subsequently rubbed. This treatment was important for inducing a planar orientation of the cholesteric material between the strips and for increasing adhesion between the cholesteric molecules and the polymer silicone substrate. In some experiments, silicone strips were sealed at the edges, but this did not affect the color changes and lasing in any way.

 

Fig. 1 (a) Chemical structure of the Wacker polymer; increasing n/m ratio leads to a shorter helical pitch, (b) structure of MBBA molecule.

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The instrumentation included a Nd:YAG pump laser (with a pulse duration of about 6 ns and maximum power of 16 mJ), a holder for the cholesteric samples, and a double-channel fiber optic spectrometer (Ocean Optics, resolution 0.1nm) used to register the lasing emission from the samples. The pump laser was focused on a polymer strip at an angle c.a. 15°; its polarization was right handed; the diameter of the beam was c.a. 250 microns. The energy of the beam could change from 0.15 to 15 mJ. The setup used in the experiment was described in detail in previous publications [4, 6–8].

 

Fig. 2. Schematic representation of cholesteric helix, its deformation and optically pumped lasing. External force was applied to the silicone strips that resulted in the contraction of the sample in the direction perpendicular to force.

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Emission from the sample was divided by a non polarizing beamsplitter (Edmund Scientific) and then left and right circularly polarized light beams were analyzed separately. Uniaxial deformation was applied by moving the ends of the strips, which were fixed in clamps, in opposite directions (Fig. 2). This deformation is different from pure shear and results in the compression of the film in the direction parallel to the helical axis of the cholesteric film. The thickness of the sample was measured by micrometer (Mitutoyo IP 65) under a microscope in order to avoid a deformation of the plastic strips.

3. Results and discussion

The cholesteric materials discussed in this letter display a shift of the photonic band gap under uniaxial stress that does not result in increasing light scattering. Stretching deformations were applied to the materials placed between two transparent silicon strips containing a liquid crystalline mixture (Fig. 2). Stretching of the sandwiched materials induces a shift of the selective reflection band from a longer wavelength toward a shorter wavelength. The shift is accompanied by color changes, which depend on the degree of stretching and the composition of the cholesteric liquid crystal. A film with a composition (ca. 75% of Wacker C4745 and 25% of MBBA) that results in a color change from red to blue under a deformation of approximately 25% is shown in Fig. 3(a). The color was immediately and completely restored after deformation. If the film is kept stretched, then restoration of the color required six or more hours.

 

Fig. 3. (a) Color changes in a stretched film (degree of stretching changes from 0 to c.a. 25%, illumination from the front), (b) Emission from cholesteric film at different degrees of stretching (left circular polarization). The steps above the peaks indicate the position of the high energy edge of the selective reflection band. Composition of the cholesteric film is ca. 75% of Wacker C4754 and 25% of MBBA.

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The selective reflection band of the unstretched film was positioned at 640 nm (Fig. 3(b), upper curve). In the unstretched, reddish film, two emission peaks were observed in the spectrum of left-handed circularly polarized light. One peak was a broad emission centered at 572 nm corresponding to the top of fluorescence band of the pyrromethene laser dye in this matrix. This peak narrowed with increasing intensity of the pumping beam, which most likely results from random lasing with incoherent feedback, that returns light scattered on films defects and inhomogeneities back to the film [17,18]. Small peaks at the right shoulder of the narrowed emission peak may indicate that some light is coherently scattered by non-planar cholesteric domains reflecting left circularly polarized light at a shorter wavelength. Cholesteric microdomains act as small disoriented micro-mirrors. The fine structure at the low energy shoulder of the random lasing peak can easily be observed in Fig. 3(b). These small peaks are a fingerprint of random lasing [17–19]. Their position does not change as the energy of the pump beam increases. The other peak, at c.a. 640 nm, corresponds to the high energy edge of the selective reflection band schematically shown as a step in Fig. 3(b) (upper part). The amplitude of this peak is much smaller than the amplitude of the random lasing peak at 572 nm. Under conditions of stretching, the left-circularly polarized lasing emission from the polymer film shifts towards the blue end of the visible spectrum following the position of the selective reflection band (Fig. 3(b), lower curves). The left-handed emission in stretched film displays a number of sharp peaks with the intensity increasing at shorter wavelengths (right-handed emission does not display any sharp peaks). Interestingly, these left- handed multimode lasing peaks occur at the expense of decreasing intensity of the random lasing peak at 572 nm. The intensity increases in the band edge lasing modes positioned close to 572 nm because the selective reflection band shifts toward the peak of the laser dye emission with increased quantum efficiency of emission. If there were no amplification of emission at the edge of the selective reflection band, the emission at 600 nm would have been smaller than the emission at 572 nm by a factor of c.a. 100.

Cholesteric film with a composition of ca. 73% Wacker C4754 and 27% of MBBA in a planar state has the selective reflection band centered at 750nm. Deformation of the sample results in a spectral shift of the selective reflection band toward 475 nm. The percentage change of the selective reflection band position as a function of deformation is plotted in Fig. 4. During the deformation cholesteric liquid crystal retain a planar orientation due to the strong anchoring.

 

Fig. 4. Changes of thickness (squares) and shift of the selective reflection band (circles) as a function of deformation (t₀ is initial thickness before deformation; Δt is the thickness change under the deformation, λ₀ is the initial wavelength; Δλ is the shift of the selective reflection band; λ₀=750nm, t₀~12µm).

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We suggest the following mechanism as a potential cause for the observed color changes and the shift of the selective reflection band in these deformable, cholesteric lasers. The extension of silicone strips results in a stress imposed on cholesteric liquid crystal in a direction of cholesteric helices (perpendicular to the surface of the film). A contraction of cholesteric sample develops along the same direction. In our experiments the deformation of the sample occurred during 1–5 seconds, which corresponds to the rate of deformation ranging between 0.1cm/s to 1cm/s. High viscosity of the matrix (γ=5*10²-5*10³ Pas) results in a very low relaxation rate of cholesteric pitch that is roughly proportional to $\frac{K}{\gamma P}={10}^{-5}-{10}^{-4}\mathrm{cm}⁄s$ (where the elastic constant K=10^-11N and helical pitch P=700nm; for a detailed discussion of relaxation phenomena see, for example, [20]). Since this rate is much smaller than the rates of deformation of the cholesteric samples, the contraction of the sample and stress lead to elastic decrease of cholesteric pitch. The dominant role of the thickness contraction in the color changes and the shift of the selective reflection band is seen in Fig. 4, where changes in the thickness of the sample and the selective reflection band shifts are compared. Uniaxial deformation of the sample leads to a thinner layer of cholesteric liquid crystal and cholesteric pitch follows the trend. The maximum change of sample thickness is about 40% that corresponds to the shift of the selective reflection band of about 35%.

Interestingly, the shortening of helical pitch does not result in the formation of dislocations when the thickness of the sample changes by half pitch (like it happens, for example in Crandgean-Cano wedge [1]). It seems plausible to assume that the formation of dislocations is suppressed by high viscosity of cholesteric matrix. Some contribution to helical pitch changes can also arise from the changes of order parameter of the chiral dopants. A detailed qualitative model that takes these effects and viscoelastic nature of deformation into account will be published elsewhere. Materials with an initial position of the photonic band gap at 1100 nm and different viscosities were recently synthesized and are currently being studied in our laboratories. We would like to note that it is possible to create a family of cholesteric materials with lower viscosity and relaxation rate comparable to the specific rate of deformation. In this case color changes will depend on the rate of deformation.

4. Conclusions

In conclusion, we propose a new type of highly viscous cholesteric material that changes color under deformation and serves as a reversible, tunable laser, sensitive to uniaxial deformations. This material is different from rubber-like cholesteric elastomers as it is the viscoelasticity that determines its behavior. The largest demonstrated shift of the selective reflection band was ca. 250nm, and the largest shift of the lasing wavelength was 80nm. The use of several lasing dyes dissolved in the matrix should result in a much larger shift of the lasing wavelength. By adjusting the viscosity of the materials, it is possible to create sensors sensitive to the velocity of deformation. Such materials could find a number of applications, not only as sensors, but also as active and tunable lasers.