1. Introduction

The presence of a selective reflection band, whose wavelength can be changed smoothly in a wide range, in cholesteric liquid crystals (CLC), activated by dye, has lead to the creation of frequency-tunable lasers. Because of a selective reflection band existence, CLC can be considered as one-dimensional photonic gap media [1]. At present the tunability of mirrorless distributed feedback dye doped CLC laser can be achieved exploiting different methods, for istance: changing the chiral dopant concentration, varying the temperature, applying a mechanical stress or an electric field, stimulating the photo transformation of the constituent molecules or finally, preparing CLC cells with pitch gradient [2–9]. In the case of cells with pitch gradient, a tuning range of 300 nm was achieved [9]. On the other side, several attempts were made to optimise the lasing conditions and performance characteristics. It was shown that doping cholesteric liquid crystals with polymeric dyes improves the order parameter and allows low threshold lasing [10]. An enhancement of laser emission due to a photonic defect mode in a dye doped CLC polymer network was observed in [11]. The dependence of the lasing threshold on dye concentration and sample thickness was studied in [12]. It was found that the system has a shallow lasing threshold minimum and can operate efficiently in the range of dye concentrations of 0.3–2.4 wt% and sample thickness of 10–50µm. It was also shown that lasing behavior depends on polymer concentrations in polymer dispersed liquid crystals (PDLCs) [13], showing a remarkable change of lasing characteristics increasing polymer concentration in PDLCs. Investigations on the influence of photo polymerization on lasing in CLC was made in [14]. Also it was found that emission efficiency depends on temperature [15]. Introducing defects in a dye doped polymeric network and polymeric CLC enhanced fluorescence and laser emission [16,17,18]. Recently the enhancement of lasing efficiency was observed in dye doped CLC laser using a CLC reflector [19].

One of the main drawbacks of CLC lasers limiting their technological application is their low stability. This is connected with two phenomena occurring under the influence of a powerful pumping: gradual deformation of the CLC layer planar orientation and degradation of the luminescent dye molecules.

In this paper we describe a method to enhance lasing stability in CLC laser. The problem of dye molecules degradation is common to conventional dye lasers as well. To solve this problem, in these lasers the dye solvent is circulating continuously through the laser chamber avoiding the saturation effects from pumping [20]. Since in CLC dye doped lasers it is impossible to make the dye circulate separately from the CLC structure, to improve the stability of lasing emission we have rotated the whole CLC cell. In this case an improvement of stability in both dye molecules and CLC structure can be expected.

2. Experimental

A conventional CLC laser cell with dimensions approximately 2×2 cm², with a thickness around 40 µm, was placed on a specially designed holder providing rotation of the cell around the axis perpendicular to the plane of the cell (Fig. 1).

 

Fig. 1. Sketch of the experimental set-up

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The rotation speed of the CLC cell was approximately 100 revolutions per minute and the radius of the circle described by the pumping beam was 5–7 mm. As pumping laser a nitrogen laser, Model VSL-337ND-S (Spectra Physics) was used. The pulse wavelength and duration were 337 nm and 4 ns respectively. The laser beam was focused by a lens (f=10 cm) to reduce the spot size on the cell to few hundreds of micrometers. The pumping beam hits the sample at ~45° with respect to the cell normal (a usual geometry for this kind of experiment). The repetition rate of pulses was 7 Hz and the average pumping power was 45 µW. A power meter (Thermo Oriel Instruments) was used to monitor laser emission. It must be noticed that an average power, averaged during several seconds of measurement (not per single pulse), was measured.

CLC mixtures were composed of nematic ZLI-6816 and optically active dopant MLC-6247 (Merck Ltd, Darmstad). The helical pitch in each mixture was set to provide lasing near the wavelength of dye fluorescence maximum. The following luminescent dyes were used:

1. UVITEX (2,5 -2,5-thiophenediylbis(5-tert-butyl-1,3-benzoxazole);

2. Oxazine700 (3,5,6-tetrahydro-1H,4H-hinolizino[9,9a,1-bc]benzo[i] phenoxazinon-13);

3. DCM (4-Dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran, Exciton).

All of these dyes effectively absorb the nitrogen laser pumping beam. DCM was used as a sensitizer in a Forster coupling effect with Oxazine700 as an emitter. The following dye doped CLC mixtures were investigated:

1. 99.4%[75.5%ZLI-6816+24.5%MLC-6247]+0.6%Oxazine 700

2. 99.4%[63.5%ZLI-6816+36.5%MLC-6247]+0.6%UVITEX

3. 99.2%[76.5%ZLI-6816+23.5%MLC-6247]+0.6% DCM+0.2%Oxazine 700.

The time stability of laser emission power for rotating and motionless cells was investigated. The measurements showed that the lasing power of motionless cell decreases significantly during half a minute (Figs. 2–5). For the rotating cell during the same time no noticeable change in the output power is detected. Other investigations showed that strongly orienting surfaces improve the stability and efficiency of lasing. In cells, whose plates were covered with polyimide (LQ1800, Hitachi Chemicals) and containing UVITEX dye, the output power did not change during two hours (Fig. 2), while the cell without the orienting polyimide layer showed lower initial power and lower stability (Fig. 3).

 

Fig. 2. Average output power for a 20 µm cell coated with polyimide and containing 0.6% UVITEX dye. The laser emission wavelength is at 443 nm.

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Fig. 3. Average output power for a 20 µm cell not coated with polyimide and containing 0.6% UVITEX dye. The laser emission wavelength is at 443 nm.

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However, when the Oxazine was used as dye in a cell with polyimide layers a decrease of the output power during one hour was observed (Fig. 4).

 

Fig. 4. Average output power for a 20µm cell coated with polyimide and containing 0.6% Oxazine 700 dye. The laser emission wavelength is at 623 nm.

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As it could be expected, the cell with Forster transfer effect showed the highest output power, but during one hour the output power slowly decreased (Fig. 5).

 

Fig. 5. Average output power for a 20µm cell coated with polyimide and containing 0.6% DCM and 0.2% Oxazine 700 for Foster transfer effect. The laser emission wavelength is at 615 nm.

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3. Conclusions

Sufficient slackening of degradation of the planar CLC structure and the luminescent dye in the CLC lasers was achieved rotating the CLC cell. An increase of lasing stability was demonstrated. The operating time of the laser increases by several times if compared with the fixed cells. A rotational movement of the cell was used because of its easy implementation. Degradation of the material in this case occurs along a narrow ring while the most part of the cell area remains unused. Applying more complex movements to the cell with respect to the pumping beam, allows to use the entire area of the cell and to obtain, as a result, a longer operating time of the laser. The remaining lasing instability for a moving cell is mainly expressed by the laser fluctuations which, are due to the spatial homogeneity of the CLC structure.