1. Introduction

Photonic band-gap materials and devices which exhibit an ordered structure with periodic change of dielectric constants have attracted much attention from both theoretical and practical point of view. A Cholesteric Liquid Crystal (CLC) is formed by rod-shaped molecules and chiral molecules: the rod-shaped molecules show a nematic-like order in planes, but the nematic director direction forms a helical structure in the perpendicular direction [1]. Since LCs are highly birefringent media, the helical structure gives a periodic modulation of the refractive indices, consequently establishing a one-dimensional photonic stop band centered at λ = np, where p is the helical pitch and n the average refractive index n = (n_e + n_o)/2, where the extraordinary and ordinary indices of refraction are denoted by n_e and n_o respectively. The width of the selective reflection band Δλ is equal to pΔn, where Δn = n_e - n_o is the birefringent of a nematic layer perpendicular to the helix axis. The supermolecular helical structure and the selective reflection of circularly polarized light [2] are unique properties that allow to consider the cholesteric as a resonator in laser emission from dye-doped luminescent molecules.

The idea to build up a laser by using a CLC was patented several years ago by Goldberg and Schnur [3] and lasing from dye-doped CLC was first obtained by Ilchishin et al. [4]. Years later, considering the CLC as a medium with a photonic band, Kopp et al. explained the observed laser emission from dye-doped thermotropic liquid crystals [5]. This approach has stimulated observations and investigations in several chiral materials. So far, low threshold laser action has been demonstrated not only in thermotropic [6, 7, 8] and lyotropic CLCs [9] but also in ferroelectric liquid crystals (Smectic C^*) [10], blue and TGB phases [11, 12], cholesteric polymers [13] and more recently glass forming cholesteric phase [14]. The ability to change the position of the selective reflection range modifying external factors provides the possibility to frequency tunable lasers. The change of the helical pitch was demonstrated in different works, by changing relevant parameters like chiral dopant concentration [15], temperature [16], electric field [17].

A different possibility is provided by CLC elastomers. It is possible, in fact, to incorporate mesogens and chiral dopant molecules as side-groups in a cross-linkable polymer backbone: this cholesteric rubber has the optical properties of the CLC and the mechanical properties of rubber, and tunability can be achieved with mechanical stress. This system has been studied theoretically [18, 19] and experimentally [20, 13, 21, 22]. Several attempts were made to optimize the lasing condition and the performance characteristics: an enhancement of the lasing efficiency was recently observed using CLC reflectors in dye-doped cholesteric liquid crystals lasers [23, 24, 25] and assembling a multilayered system that allows the separation of the cholesteric liquid crystal from the active medium.

We obtained lasing from a device, described in [22], composed of two identical cholesteric elastomers deposited onto silicon elastomers (for mechanical support), with a dye-containing isotropic elastomer “sandwiched” between the two CLC layers. The problem with this system is the presence of many lasing modes inside the transmission band-gap. A way to circumvent multimode lasing was studied by Chilaya et al. [26] in a conventional LC cell and it consists of creating a special defect inside the reflection band, where the laser emission is more likely to occur and therefore selecting just one mode out of many others. This is practically achieved by using two cholesteric mirrors whose band-gaps are slightly shifted. A “notch” appears in the middle of the band-gap and selects a single-mode emission.

In this work, we want to investigate the possibility of achieving the shifted band-gap configuration in a device entirely made by elastomers, in order to achieve mechanically tunable lasers with a single-mode emission.

2. Materials and methods

The cholesteric liquid crystal (CLC) elastomer material consists of a polysiloxane backbone 4-pentylphenyl-4’-(4-buteneoxy) benzoate (the mesogenic unit) and cholesterol pentenoate (the chiral dopant) as side-chain groups. The UV crosslinker used for polymerization was 3-methyl-3-oxetane-(11-undecenemethenether). The preparation and synthesis of these materials are extensively described in [22] and references therein. The intermediate dye-doped layer consists of a silicone elastomer (SYLGARD 184, from Dow Corning) doped with Rhodamine B (Aldrich), whose fluorescence spectrum ranges from 540nm to 650nm, with a peak around 590nm. A basic silicone elastomer was also used as a mechanical support for the films.

The alignment of each layer of CLC polymer was achieved following the method described in [22]. For the lasing experiments, a three-layer structure was assembled putting a dye-doped silicone elastomer between two CLC elastomer, each mounted on a silicone support, as shown in Fig. 1.

 

Fig. 1. Scheme representing the layered structure: two silicone elastomers are used as mechanical support for CLC elastomers. The more internal layer is a dye-doped isotropic polymer.

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Single-layer CLC films on silicone elastomer support, or assembled 3-layer systems, were uniaxially stretched with a mechanical stretching device with a micrometric screw. Transmission spectra of the CLC elastomers were measured with a USB 4000 Ocean optics spectrometer was used. The incident light was circularly polarized by a combination of a linear polarizer and a Fresnel rhombus [21]. The light was sent through a pinhole to the sample and the transmitted light was guided by an optical fibre to the monochromator. Before every measurement, a proper reference was taken with a superposition of three silicone elastomers mounted on the stretching device.

Lasing emission of the 3-layer structure was performed as described in detail in reference [22]. A 532nm pulsed laser beam (pulse duration ≈ 4ns) was obtained from frequency-doubled Nd:YVO4 laser (AOT). The pumping laser beam was focused on the sample surface at 35° incidence and the emission from the sample was detected by a spectrometer (USB 4000; Ocean optics), perpendicular to the sample surface.

3. Results and discussion

In liquid crystal cells the practical way to shift the band-gap of a CLC is modifying the composition of the liquid crystal, i.e. changing the concentration of chiral dopant [26]. The same thing can be done also for CLC elastomers, but this always requires the synthesis of new polymers and it is very time consuming. The rubbery properties of elastomers allow a much easier way to shift the band-gap simply by mechanical stretching of one cholesteric layer with respect to the other: we decided to test both these methods.

 

Fig. 2. Left panel: tuning of the notch in the shifted band-gap assembly where the two CLC elastomers had different composition (13.5% and 10% chiral dopant). The strain-dependent position of the notch is indicated by vertical lines. Right panel: wavelength shift of the notch as a function of the stretching. The fit is obtained with a power law where the exponent is -0.36, therefore in the expected range between -0.5 and -0.25 (colors online).

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The first crucial point to address is to know whether the notch is as tunable as the single cholesteric layer. In particular, the relevant quantity which needs to be measured is the relationship between the uniaxial stretching, given by the ratio between the length l along the x direction and the initial length λ_x = l/l ₀(which one can control during the experiments) and the effective strain in the perpendicular direction (along the helix) which is proportional to the wavelength blue-shift and can be measured as λ_z = λ - /λ ₀, where λ ₀ is the wavelength of the band-gap center for the unstretched sample. It was shown in [21, 22] that the relationship between these two quantities is well represented by a power law, whose exponent depends on the degree of crosslinking of the rubber and is included between -1/2 and -1/4. This relationship between the strain in two perpendicular directions is predicted also by the theory [19].

In order to measure this dependence for a three-layer device in the shifted band-gap configuration, an isotropic layer without laser dye was “sandwiched” between two CLC elastomers, and the overall spectrum was recorded. The dye-doped layer was not included in the measurement because the sample would not be transparent enough and the transmittance spectrum would be very hard to measure.

First, a stretching experiment was performed on a more “conventional” assembled device where the two CLC elastomers have a different composition in terms of chiral dopant. In order to achieve a good and visible notch, 10% and 13.5% chiral dopant was used. The tuning of the notch for this system is shown in Fig. 2. The wavelength shift of the notch as a function of the stretching follows the same power law as the band-gap center of the single CLC layer [22] (where the measured exponent for a partially crosslinked sample was -0.38). It is also possible to observe that the centers of the two band-gap get progressively closer as the sample is stretched. Therefore, for a lasing application, it may be important to limit the stretching to the range where the notch is still clearly visible.

We then tried to assemble a device where two cholesteric mirrors were stretched by a different amount. To achieve this, one of the cholesteric layers was mounted on the stretching device and stretched by 13%, then the isotropic layer and the second unstretched CLC was placed on top of it and secured with tape. The whole device was then stretched more and spectra were collected. The transmission band-gap clearly showed a “notch” in the center. The results are shown in Fig. 3. Also in this case the tuning follows the same power law as the band-gap center, the exponent being -0.36.

 

Fig. 3. Left panel: tuning of the notch in the shifted band-gap assembly. The value of the stretching is referred to the initially unstretched layer. The other layer was previously stretched by 13%. Right panel: wavelength shift of the notch as a function of the stretching. The fit is obtained with a power law where the exponent is -0.36 (colors online).

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A 12μm thick isotropic layer containing Rhodamine B was used as active medium for lasing application. Figure 4 shows the laser peak blue-shifting with progressive stretching, obtained by stretching a device made of two layers with different concentration of chiral dopant. The peaks are well defined and the tuning of the laser is achieved. The lasing threshold in this experiment was unexpectedly very high, about 100 times the one that was previously measured, but the laser peaks are mostly single-mode and this seems a very promising strategy to circumvent the problem of multimode lasing. From Fig. 4 it is evident that the lasing only occurs when the stretching of the device is more than 100%. On the one hand, this could be a possible reason why the threshold is so high (the notch is less defined for a high strain value); on the other hand, this ensures that the notch band-gap works even for high values of stretching. This is an important consideration, because, choosing the opportune dye mixture, the elastomers could be tunable over a large range of wavelengths. A more precise tuning of the concentration of chiral dopant in the polymer and the dye emission is also likely to solve, at least partially, the problem of the high threshold.

 

Fig. 4. Example of laser emission obtained with a three-layer device in the shifted band-gap configuration being progressively stretched. In the figure it is possible to observe the mechanical tuning of about 5 nanometers. The inset shows the wavelength tuning as a function of the sample strain.

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4. Conclusions

We have shown that two cholesteric elastomers films act as deformable and mechanically tunable “mirrors”, allowing an isotropic dye-doped layer in between to generate tunable emission in the resulting optical cavity. In order to overcome the problem of the multimode lasing and obtain instead a single mode emission, the band-gaps of the two cholesteric liquid crystal elastomers were shifted one with respect to the other. This was achieved either by modifying the composition of the CLC polymer, varying the percentage of chiral dopant, or by differentially stretching the two elastomers. In both cases, a “notch” in the center of the band-gap was observed, and the wavelength change of the notch as a function of uniaxial stretching followed the same power law as the single band-gaps. Single mode lasing was obtained from such system, even though the threshold was higher and the tunability range reduced. These problems could be solved, or partially solved, with further experiments and an optimization of the system. For example, the dye Rhodamine B does not have a very high quantum yield, and the emission wavelength is too blue-shifted compared to the notch in the resting position. Therefore, further experiments will be performed using a different fluorescent dye and changing the composition of the CLC elastomer in order to obtain a blue-shifted reflection band. The fact that the notch band-gap also works at high values of stretching suggests the possibility of introducing more than one dye in the sample in order to extend the tunability range.