Open Access
Add to BibSonomyAbstract
Cellulose derivatives, because of their molecular structure and chirality, can self-assemble to form spatially periodic cholesteric liquid crystal phases. We have synthesized and produced solid cross-linked cholesteric cellulose based films optimized to provide high reflectivity. Since these films are self-assembled photonic bandgap materials, they may be expected to show distributed feedback lasing. By doping samples with fluorescent dyes and optically pumping thin films of these materials, we were able to demonstrate, to the best of our knowledge, for the first time, mirrorless band-edge lasing in cellulose derivatives.
© 2013 Optical Society of America
1. Introduction
Cellulose is the most common organic compound on earth, constituting about a third of all plant matter. Because of its ubiquity and importance, it has been extensively characterized and studied [1]. Cellulose is a polysaccharide, a linear polymer with the chemical formula (C6H10O5)n, where 102 < n < 104. Colloidal suspensions of cellulose can form liquid crystal phases, and liquid crystalline properties may be preserved in solid cellulose films [2–8]. Especially by reacting the hydroxyl groups of cellulose with suitable reagents (like isocyanates), cellulose derivates can be prepared, which form lyotropic cholesteric phases [4–8].
Low molecular weight cholesteric liquid crystals, consisting of rod-like chiral molecules, form spatially periodic structures, as shown in Fig. 1.Molecules tend to be parallel to one another in planes perpendicular to the helix axis; the direction of average orientation varies linearly with position along the helix axis. Since the resulting dielectric tensor is spatially periodic, these structures exhibit photonic bandgaps [9]. Since propagation of one of the eigenmodes is forbidden in the band, high reflectivity (~50%) is observed for wavelengths within the bandgap.
If light is emitted due to fluorescence in the bulk, photons with wavelengths in the bandgap are confined, and the material acts as a distributed cavity. If sufficient gain is provided, such materials can lase at the band edge, where propagation is allowed. Distributed feedback lasing, first predicted in 1971 [10], has been demonstrated in cholesteric liquid crystals [11–14].
Although mirrorless lasing in low molecular wt. liquid crystals is an exciting phenomenon with many potential applications, considerable advantages are offered by solid liquid crystals; that is, by liquid crystal elastomers and networks [15]. These include mechanical and thermal stabilization of the bandgap structure by cross-linking [16], and the ability to incorporate dyes and other gain media after the distributed feedback structure has been formed [17]. Although lasing has been demonstrated in cholesteric liquid crystal elastomers [18], lasing from solid liquid crystal films has nonetheless proven to be extremely challenging. This is due, in part, to the lack of structural uniformity. Unlike defect-free solid crystals or monodomain liquid crystals, liquid crystal elastomers and networks are soft solids whose polymeric component introduces structural heterogeneities.
Cellulose based cholesteric liquid crystal networks are particularly appealing for distributed feedback applications because of the ready availability of starting materials. In addition lyotropic cholesteric phases of substituated cellulose possess only a rather small birefringence. The fiber are thus very transparent and do not scatter much. Their inherent structural inhomogeneities, however, make their use as distributed cavity hosts extremely challenging. In this paper, we describe the development and realization of cholesteric cellulose films which show, to the best of our knowledge, for the first time, distributed feedback lasing.
2. Sample materials
The liquid crystalline host in our experiments consists of a 46% w/w solution of a cellulose tricarbanilate in an acrylate as “reactive (=polymerizable)” solvent (see Fig. 2). These solutions are known to lead to nice cholesteric films after polymerization16. The optical quality of these films had, however, been still poor [[ 16,19]. To improve the optical quality of the cholesteric films we tried to optimize the conversion of the hydroxylgroups up to 100% (3 urethane bonds per repeating units). Furthermore we reduced the viscosity of the lyotropic cholesteric phase by optimizing the acrylate used as solvent and by adding additives to reduce intramolecular H-bonding (see Fig. 2). The details of this work are described in [19].
Fig. 2 Chemical structures of the constitutents. CTC = cellulose tricarbanilate, TFAA = trifluoroacetic acid, TFE = trifluoro ethanol, EGMEA = ethylene glycol methyl ether acrylate, Lucirin TOP = photoinitiator
The cellulose was purchased as Avicell PH101 from Sigma Aldrich and was reacted three times with 3-(trifluoromethyl)phenyl isocyanate to yield an all substituted cellulose tricarbanilate (CTC) [16,19]. It was dissolved in 54% w/w ethylene glycol methyl ether acrylate (EGMEA). Additionally, small amounts (several mg per 500 mg CLC, depending on the desired wavelength of selective reflection) of trifluoroacetic acid (TFAA) or trifluoro ethanol (TFE) were added to interrupt intermolecular hydrogen bonds and prevent polymer aggregation [19,20]. A reduction of polymer aggregation increases the cholesteric alignment and hence the reflectance of the film. The amount of TFAA or TFE can be varied; increasing the TFAA concentration blue-shifts the reflection band up to 150 nm.
By optimizing all parameters (see [19]) it is possible to create polymerizable films, in which – after crosslinking – 50% of the light within the band gap is reflected perpendicular to the surface (see Fig. 3). This corresponds to a perfectly oriented cholesteric phase. The photoinitiator Lucirin TPO (1% w/w with regard to the monomer) was added to the solution prior to the addition of the CTC. In this case, the amount of TFAA was reduced by 50% in order to have the photonic bandgap at the same position as in the unpolymerized samples. During the polymerization, the bandgap shifts ~30nm towards the blue. Curing is achieved by shining a 500W UV light source in the 320nm – 380nm range on the sample for one minute.
Two dyes were used as gain materials: Pyrromethene 597 (P597) dye from Exciton Inc., Dayton OH, and Potomac Yellow (PY) dye from DayGlo Color Corp., Cleveland OH.
Due to the low solubility of Potomac Yellow, samples with PY were prepared by dissolving the dye (0.1-1%wt.) in the acrylic solvent EGMEA, using an ultrasonic bath. Residual solids were removed with centrifugation prior to mesogen addition. The mixture was stirred mechanically for at least three hours to obtain a highly viscous unpolymerized orange liquid crystal with a blue/green reflection color as shown in Fig. 4. These samples were allowed to equilibrate undisturbed for at least 24 hours prior to lasing experiments.
Samples with P597 were prepared by swelling the polymerized films in a saturated solution of P597 in toluene. The residual toluene was removed in vacuum. Polymerized liquid crystal samples with the dye P597 appear red, with orange reflectance, as shown in Fig. 5.
Films with these two dyes were prepared using glass microscope slides which have been subjected to several strokes of unidirectional rubbing with cellulose tissue. After rubbing, the liquid crystal was placed between the slides, which were pressed together. The film thickness was controlled by polyester foil stripes between the glass slides. In our experiments, we used 25μm and 50μm thick unpolymerized samples with PY dye and 90μm thick polymerized samples with P597 dye. The reflection band peaks varied slightly (5-10nm) from sample to sample.
Measurements were first carried out on unpolymerized samples with PY dye, and then on polymerized samples with P597 dye.
3. Experimental results
In order for distributed feedback lasing to occur in cholesteric liquid crystals, there must be an overlap between the fluorescent emission of the gain material and the reflection band of the host [21].
The reflection band maxima of the early cellulose samples were in the vicinity of 505nm, and for this reason, the dye PY, with fluorescence maxima at 506nm was chosen as the gain medium. The excitation and emission spectrum of PY is shown in Fig. 6, the chemical structure is shown in Fig. 7.
Fig. 6 Excitation and emission spectra of Potomac Yellow (PY) dye in cellulose. The response to excitation was measured at λem = 530nm, while excitation for the emission spectrum was at λex = 450nm.
To see if distributed feedback lasing could be produced, the sample was optically pumped, and the emission was measured with a spectrometer. The schematic of the experimental setup is shown in Fig. 8.
Fig. 8 Experimental setup. Pump pulses from a nanosecond laser were focused on the cellulose tricabanilate sample. Laser emission was measured with a spectrometer. The same experimental setup, was used throughout. Details are given in the text.
The pump source was an Opolette 355 II-UV Opotek tunable laser, with 6ns pulses at λ = 450nm. The intensity of the laser was controlled with the polarizer and analyzer; the pump energy was measured with a Molectron Optimum 4001 energy meter. The beam diameter from the laser was 1cm; the focal lens of Lens 1 was 20cm. The beam waist at the sample position was 300μm.
The angle of indidence of the pump beam was 45°; the sample reflectance was measured with an Ocean Optics UV-Vis U2000 spectrometer, and the emission spectrum was measured with a Fluorolog 3 Horiba Jobin-Yvon spectrofluorometer fitted with a Hamamatsu PMT R928P.
The reflection band of cellulose film samples is similar to low molecular weight cholesteric liquid crystals with low birefringence [22]. In both unpolymerized samples with PY dye, distributed feedback lasing was observed; this is the first observation of distributed feedback lasing in cellulosic materials. The laser emission was near the middle, slightly towards the high energy edge of the reflection band, Fig. 9.The intensity of the fluorescent emission was measured to determine the lasing threshold. The emission intensity vs. pump energy is shown in Fig. 10.
Fig. 9 Reflection band and emission spectrum from two unpolymerized samples with PY dye. The pump beam is visible at 450nm. For both samples, laser emission is near the high-energy band edge.
Fig. 10 Emission intensity from the 25μm thick sample with PY dye as function of pump energy. Pump pulses were 6ns wide at λ = 450nm. The lasing threshold is 60μJ/pulse.
Lasing experiments were next carried out on polymerized cellulose films with the dye P597. The reflection band maxima of the polymerized films were shifted towards the red relative to unpolymerized samples, and were in the vicinity of 569nm. For this reason, the dye P597, with fluorescence maxima at 580nm was chosen as the gain medium. The excitation and emission spectrum of P597 is shown in Fig. 11, the chemical structure is shown in Fig. 12.
Fig. 11 Excitation and emission spectra of Pyrromethene (P597) dye in cellulose. The response to excitation was measured at λem = 565nm, while excitation for the emission spectrum was at λex = 532nm.
Lasing experiments were carried out as before, the schematic of the experimental setup is shown in Fig. 8.
The pump source was a frequency doubled Coherent Nd:YAG laser, with 7.5ns pulses at λ = 532nm. The intensity of the laser was controlled with the polarizer and analyzer; the pump energy was measured with a Molectron Optimum 4001 energy meter. The beam diameter from the laser was 0.9cm; the focal lens of Lens 1 was 12cm . The beam waist at the sample position was 10μm. The angle of incidence of the pump beam was 45°; the sample reflectance was measured with an Ocean Optics UV-Vis U2000 spectrometer, and the emission spectrum was measured with a Jobin Yvon-Spex TRIAX 550 spectrometer.
In the polymerized samples with P597 dye distributed feedback lasing was also observed. The laser emission was near the middle of the reflection band.
As discussed in the introduction, the reflection band exists for only one of the eigenmodes – for right circularly polarized light. Laser emission from the sample was right circularly polarized as well; this was verified with circular polarizers (contrast ratio 500:1). Right circularly polarized reflection and emission from a 90μm thick polymerized sample with P597 dye is show on Fig. 13.
Fig. 13 Right circularly polarized (RCP) reflectance and emission from a 90μm thick polymerized sample with P597 dye. Laser emission is near the center of the reflection band. The reflected light and lasing are right circularly polarized.
To demonstrate lasing, it is necessary to show line narrowing as well as a lasing threshold. The intensity of the fluorescent emission was measured to determine the lasing threshold. Figure 14 shows narrowing, while Fig. 15 shows the lasing threshold.
Fig. 14 Emission from a 90μm thick polymerized sample with P597 dye for different pump energies (μJ/pulse) showing line narrowing.
Fig. 15 Emission from a 90μm thick polymerized sample with P597 dye for different pump energies. Pump pulses were 7.5ns wide at λ = 532nm. The lasing threshold is 1.8μJ/pulse.
In cholestereric liquid crystals with large birefringence, the reflection band for thick samples, where the sample thickness consists of many periods of the cholesteric helix, the reflection band is typically more rectangular in shape, with well-defined band edges. In such a system, the density of states is sharply peaked at the band edges; a standing wave exists with the electric field vector parallel to the direction of average molecular alignment at the low energy band edge, and perpendicular to it at the high energy band edge. Lasing therefore occurs near the band edge in such systems. Here, due to the smaller birefringence of cellulosic materials, photon confinement is effectively reduced. The density of states is expected to be broadened, with the maximum near the peak of the reflection band. This is consistent with our observations of lasing near the peak of the reflection band in samples with PY dye as well as with the dye P597.
Laser linewidth is significantly narrower in the polymerized samples with the P597 dye. The reason for this is not entirely clear. We conjecture that it may be due to a more uniform sample structure/composition; this conjecture needs to be verified. We have studied linewidth narrowing as function of pump energy in polymerized samples with P597 dye. As shown in Fig. 14, the emission linewidth narrows markedly with pump power; it changes from Δλ ≈10Åto Δλ ≈5Å with a ~3% increase in pump energy. The narrowest linewidth measured was 3.4Å.
Samples with the P597 dye pumped at λ = 532nm showed a clear lasing threshold at 1.8μJ/pulse. Although this is significantly less than the 60μJ/pulse threshold for samples with the PY dye, we remark that the fluence at threshold is comparable in both cases, since the beam waist at the sample with the PY dye is ~300μm, while it is ~10μm for the samples with P597. We note, however, that the samples with P597 are significantly thicker than those with PY. The threshold is expected to depend on sample thickness [21], and we plan to study the dependence of the lasing threshold on sample thickness in cellulose samples in the future.
4. Summary
We have produced liquid crystalline films of cellulose tricarbanilate, and carried out lasing experiments in unpolymerized samples containing the dye Potomac Yellow (PY) and Pyrromethene 597 (P597). We have observed lasing in both types of samples, when pumped with 6ns and 7.5ns pulses at λ = 450nm and λ = 532nm in the μJ regime as evidenced by emission threshold, directional emission, and line narrowing. Although the chiral liquid crystallinity of cellulose2 and cellulosic materials3 is well known, to our knowledge this is the first time that distributed feedback lasing has been demonstrated in these materials. This was made possible by strongly improving the quality of orientation of cholesteric films from cellulose tricarbanilates [19]. Because its ubiquity and easy availability, cellulose may therefore be useful for lasing and other photonic applications in the future.
References and links
1. M. Jarvis, “Chemistry: cellulose stacks up,” Nature 426(6967), 611–612 (2003). [CrossRef] [PubMed]
2. D. G. Gray, “Chiral nematic colloidal suspensions and films of cellulose,” Abstr. Pap. Am. Chem. S. 219, U273 (2000).
3. Y. Geng, P. L. Almeida, S. N. Fernandes, C. Cheng, P. Palffy-Muhoray, and M. H. Godinho, “A cellulose liquid crystal motor: a steam engine of the second kind,” Sci Rep 3, 1028 (2013). [PubMed]
4. M. Müller, R. Zentel, and H. Keller, “Solid opalescent films originating from urethanes of cellulose,” Adv. Mater. 9, 159–162 (1997). [CrossRef]
5. S. Tseng, G. V. Laivins, and D. G. Gray, “The propanoate esterof (2-hydroxypropyl)cellulose: a thermotropic cholesteric polymer that reflects visible light at ambient temperatures,” Macromolecules 15(5), 1262–1264 (1982). [CrossRef]
6. S. N. Bhadani and D. G. Gray, “Liquid crystal formation form the benzoic acid ester of hydroxypropulcellulose,” Macromol. Rapid Commun. 3(6), 449–455 (1982). [CrossRef]
7. Y. Nishio, T. Yamane, and T. Takahashi, “Morphological studies of liquid-crystalline cellulose derivatives,” J. Polym. Sci., Polym. Phys. Ed. 23(5), 1043–1052 (1985). [CrossRef]
8. C. Zhao and B.- Cai, “UV-initiated solidification of liquid crystalline ethylcellulose/acrylic acid films and bands formed in the process,” Macromol. Rapid Commun. 16(4), 323–328 (1995). [CrossRef]
9. P. Palffy-Muhoray, W. Cao, M. Moreira, B. Taheri, and A. Munoz, “Photonics and lasing in liquid crystal materials,” Philos Trans A Math Phys Eng Sci 364(1847), 2747–2761 (2006). [CrossRef] [PubMed]
10. H. Kogelnik and C. V. Shank, “Stimulated Emission in a Periodic Structure,” Appl. Phys. Lett. 18(4), 152–154 (1971). [CrossRef]
11. Liquid Crystal Microlasers, ed. L. M. Blinov, R. Bartolino, (Transworld Research Network, Trivandrum, 2010).
12. B. Taheri, P. Palffy-Muhoray, and H. Kabir, Cuyahoga Falls, Feb. 18–19 ALCOM Symposium: Chiral Materials and Applications (1999).
13. W. Haase, F. Podgornov, Y. Matsuhisa, and M. Ozaki, Nanophotonic Materials, Photonic Crystals, Plasmonics and Metamaterials. (Wiley-VCH, Weinheim, 2008), Chap. 13.
14. H. Coles and S. Morris, “Liquid-crystal lasers,” Nat. Photonics 4(10), 676–685 (2010). [CrossRef]
15. Liquid Crystal Elastomers: Materials and Application, Advances in Polymer Science 250, ed. W. H. de Jeu (Springer, Heidelberg, 2012).
16. M. Müller and R. Zentel, “Cholesteric phases and films from cellulose derivatives,” Macromol. Chem. Phys. 201(15), 2055–2063 (2000). [CrossRef]
17. A. Muñoz, M. E. McConney, T. Kosa, P. Luchette, L. Sukhomlinova, T. J. White, T. J. Bunning, and B. Taheri, “Continuous wave mirrorless lasing in cholesteric liquid crystals with a pitch gradient across the cell thickness,” Opt. Lett. 37, 2904–2906 (2012). [CrossRef] [PubMed]
18. H. Finkelmann, S. T. Kim, A. Muñoz, P. Palffy-Muhoray, and B. Taheri, “Tunable mirrorless lasing in cholesteric liquid crystalline elastomers,” Adv. Mater. 13(14), 1069–1072 (2001). [CrossRef]
19. D. Wenzlik and R. Zentel, “High Optical Quality Films of Liquid Crystalline Cellulose Derivatives in Acrylates,” Macromol. Chem. Phys. 214(21), 2405–2414 (2013). [CrossRef]
20. A. B. Shipovskaya, G. F. Mikul'skii, and G. N. Timofeeva, “Structurization and optical activity in cellulose triacetate modified with trifluoroacetic acid vapor,” Russ. J. Appl. Chem. 77(1), 148–153 (2004). [CrossRef]
21. W. Cao, A. Marino, G. Abbate, P. Palffy-Muhoray, and B. Taheri, “Lasing thresholds of cholesteric liquid crystals lasers,” Mol. Cryst. Liq. Cryst. (Phila. Pa.) 429(1), 101–110 (2005). [CrossRef]
22. D. K. Yang, J. L. West, L. C. Chien, and J. W. Doane, “Control of reflectivity and bistability in displays using cholesteric liquid-crystals,” J. Appl. Phys. 76(2), 1331–1333 (1994). [CrossRef]
