1. Introduction

The Cholesteric Liquid Crystal (CLC) phase is a nematic phase with a self-organized periodical helical arrangement that acts as a one dimensional periodic structure. In CLCs the period of the structure is equal to half the pitch p of the helix, and for light propagating along the helical axes, p = λ₀ / n, where λ₀ is the wavelength of the maximum reflection or the middle of the Photonic Band Gap (PBG) and n is the average of the refractive indices defined as: n = (n_e + n_o) / 2, where n_e and n_o are the extraordinary and ordinary indices of refraction respectively. The full width at half maximum of the PBG equals to Δλ = pΔn, where Δn = n_e − n_o is the birefringence of a nematic layer perpendicular to the helix axis. CLCs possess several unique properties: 100% selective reflection of circularly polarized light and the ability to change their selective reflection wavelength changing external or internal factors (electric and electromagnetic fields, temperature, local order) [1–4]. Due to these properties, CLCs are of practical interest for various applications. As an example, the use of a CLC layer as a resonator-mirror in conventional lasers was carried out long time ago [5,6]. Moreover, if the CLC consists of luminescent molecules or if it contains a luminescent dopant it becomes possible to build a dye doped (DD) CLC laser [7–9]. Lasing from DD-CLC was obtained for the first time by Ilchishin et al. (1980) [10] and the consideration of the reflection band as a photonic band gap, many years later, has stimulated the investigation of lasing in several different systems. An enhancement of lasing efficiency was observed using a CLC reflector in a dye doped CLC laser [11] or assembling a multilayer system that allows the separation of the cholesteric liquid crystal from the active medium [12,13]. Moreover, lasing can be obtained also from other liquid crystalline phases similar to the cholesteric one and lasing can be tuned applying an electric field [14,15]. Recently, lasers at different wavelengths were also obtained using CLC spherical microshells [16–18].

Here, we present for the first time a strategy to obtain multicolored lasing from cholesteric liquid crystals. In particular, we obtained two laser lines separated by 25nm. The novelty of this system relies on the peculiar cell design. We have used a four layers cell, one layer containing the photoluminescent dye doped solution and the other three layers containing the cholesteric liquid crystals mixtures that act as mirrors. In ref [12,13]. we have shown that lasing in a three layered structure, two cholesteric mixtures sandwiching a dye and glycerol mixture, can be considered as a defect mode type and multimode lasing within the photonic band gap with several emission peaks can be obtained. To achieve single mode lasing, two CLCs with different pitches can be used, obtaining laser emission at the wavelength in which the two PBGs overlap. In the present case, dual mode lasing was obtained, the first laser line at a wavelength in which the first and second cholesteric PBGs overlapped and the second laser line at the wavelength in which the second and third cholesteric PBGs overlapped.

Additionally, applying an electric field one of the two laser lines can be switched off.

Lasers that emit at several wavelengths, as Krypton and Argon lasers, are extremely useful in scientific research and for many applications in medicine and in optics.

2. Experimental

Cholesteric liquid crystal mixtures were prepared using the nematic liquid crystal ZLI-1939 and the chiral dopant MLC-6247, both available from Merck. Three mixtures with different concentrations of the optically active dopant were prepared: CLC (1) 72.5% wt. ZLI-1939 + 27.5% wt. MLC-6247, CLC (2) 74% wt. ZLI-1939 + 26% wt. MLC-6247 and CLC (3) 75.5% wt. ZLI-1939 + 24.5% wt. MLC-6247. As luminescent dye Rhodamine 6G (Sigma Aldrich), dissolved in glycerol, was used.

In Fig. 1(a) the transmission spectra of each CLC layer (blue – CLC (1), green – CLC (2) and red – CLC (3) and in Fig. 1(b) the spectrum of dye emission (black curve) are shown. The PBGs of the CLC layers were shifted in such a way that only the edges of the band gaps overlapped. This design allows to determine with high accuracy the spectral position of laser emission lines.

 

Fig. 1 (a) Transmission spectra of the three cholesteric layers: blue – CLC (1), green – CLC (2) and red – CLC (3). (b) Photoluminescence spectrum of Rhodamine 6G.

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To obtain a multicolor laser, a peculiar cell geometry was used. The cell consisted of four layers: one dye/solvent layer and three CLC layers (Fig. 2). Each layer was confined between transparent glass plates. The whole cell was prepared using five glasses separated by Teflon spacers setting the thickness of each layer. All the CLC layers had a thickness of 6 microns and the thickness of the dye solution was set to 200 microns. To obtain a good planar orientation of the CLC layers, the corresponding surfaces were coated with PVA (Polyvinyl Alcohol, Sigma Aldrich) and unidirectionally rubbed. PVA was prepared in water at a concentration of 0.5% in weight. The cells were infiltrated by capillarity with the CLC mixtures. The characteristic oily streak texture was obtained.

 

Fig. 2 Scheme of the four layer cell.

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For lasing experiments, the second harmonic of a Q-switched Nd:YAG laser (Continuum, Surelite II) was used as pumping light source. The pulse wavelength, width, and repetition rate were 532nm, 4ns, and 10 Hz, respectively. The laser beam was attenuated and focused by a lens to reduce the spot of the laser beam on the cholesteric cell to a few hundreds of micrometers. The pumping intensity was about 5 mJ cm⁻². The pump beam irradiated the sample at an angle of 45° with respect to the cell normal. An optical fiber, coupled to a spectrometer (AVASPEC-2048, Avantes) collected the light emitted from the sample, perpendicularly to the glass plates. Light could be collected from both sides of the cell.

When the pumping beam hit the cell, lasing was observed. The emission from the cell highlighted the presence of two stable laser lines (violet peaks in Fig. 3). One line was observed in the region where the longer wavelength edge of the PBG of CLC (1) overlapped the shorter wavelength edge of the PBG of CLC (2). The second line is observed in the region where the longer wavelength edge of the PBG of CLC (2) overlapped the shorter wavelength edge of the PBG of CLC (3). The distance between laser lines was 25nm. In principle, this distance can be tailored according to the position of the PBGs and the emission spectrum of the photoluminescent dye. The emitted laser beams intensity was 0.37mJ cm⁻², with an efficiency of 7.4%.

 

Fig. 3 PBGs of the cholesteric layers and laser lines intensities. The shorter wavelength laser peak on the violet line corresponds to the pumping light source, the other two laser lines with larger intensities correspond to the emission from the four layers sample cell. As expected, the emitted laser wavelengths are positioned where the edges of the PBGs overlap.

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In Fig. 4 a photograph of the laser beam spot and the cell is shown. Lasing emission can be controlled applying an electric field. To observe the switching off of one of the laser beams, the cell preparation was slightly changed in order to apply a DC voltage to the third cholesteric layer. For this purpose, we used ITO covered glass plates to confine the CLC (3) mixture (Fig. 2). Applying an increasing DC voltage, the cholesteric order decreased, distorting the supramolecular helical structure. This effect produced a modification of the PBG of the third cholesteric layer as shown by Fig. 5(a).

 

Fig. 4 Laser emission from the four layer sample cell and picture of the two colors laser spot.

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Fig. 5 (a) transmission spectra of the CLC (3) varying the applied voltage. Lasing from the four layer cell at different electric voltages: 0V/μm (b), 5V/μm (c), and after 1 min from the removal of the electric field (d).

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We observed a broadening of the PBG at 1V/μm and a significant reduction at 2 V/μm. At 5 V/μm the PBG completely disappeared. Since the third cholesteric layer no longer worked as a reflector, to visualize the laser lines the fiber optic spectrophotometer had to collect light from the side of the first cholesteric layer. Figure 5(b) shows that without applying an electric field both laser lines are present and that one of them disappears when an electric field of 5V/μm is applied (Fig. 5(c)). In fact, once the third cholesteric PBG is destroyed, the only surviving laser line is the one at the wavelength in which the first and the second cholesteric PBGs overlap. After the electric field is removed, the cholesteric order is slowly restored. Figure 5(d) was recorded one minute after the removal of the electric field and the two laser lines were visible again.

3. Conclusion

We presented a novel cell design to obtain multicolor lasing exploiting the selective reflection properties of cholesteric materials. This result is achieved using three cholesteric liquid crystal mirrors in combination with a photoluminescent layer. The three cholesteric mirrors are prepared so that their photonic band gaps have overlapping edges. Further, applying an electric field, one of the laser lines can be switched off. This is due to the electrically induced perturbation of the local cholesteric order in one of the CLC mixtures. Other laser lines can be added simply increasing the number of layers and choosing the proper dyes and they can, eventually, be switched off by applying an electric field. Therefore, this cell is the basic element of a single compact device able to produce and control several laser beams at the same time and along the same propagation direction with possible applications for instance in biology [19], medicine [20], data recording and storage [21] or advanced microscopy.