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Random lasing in a dye doped cholesteric liquid crystal polymer solution
Optical Materials 31 (2008) 375–379
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Random lasing in a dye doped cholesteric liquid crystal polymer solution Benqiao He a,c, Qing Liao b, Yong Huang c,* a
Tianjin Key Laboratory of Fiber Modification and Functional Fiber, School of Material Science and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300160, China State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Beijing National Laboratory of Molecular Science, Institute of Chemistry, CAS, Beijing 100080, China State Key Laboratory of Polymer Physics and Chemistry and Joint Laboratory of Polymer Science and Material, Beijing National Laboratory of Molecular Science, Institute of Chemistry, CAS, Beijing 100080, China b c
a r t i c l e
i n f o
Article history: Received 18 October 2007 Received in revised form 23 May 2008 Accepted 24 May 2008 Available online 14 July 2008 PACS: 42.25.Fx 78.45.+h 81.20.Fw
a b s t r a c t Random lasing in rhodamine 6G (R6G) doped ethyl-cryanoethyl cellulose [(E-CE)C]/acrylic acid (AA) cholesteric liquid crystal (LC) solution without scattering particles was studied. The effects of concentration of (E-CE)C/AA solution and the thickness of the sample on the random lasing were investigated. The random laser with coherent feedback occurs in (E-CE)C/AA anisotropic solution, while only amplified spontaneous emission (ASE) is observed in (E-CE)C/AA isotropic solution and AA solvent. The random laser also occurs in the (E-CE)C/poly(acrylic acid) (PAA)/R6G solid film with cholesteric structure through quick polymerization of AA. The experimental results suggest that the cholesteric LC domains play a very important role in this random lasing. Ó 2008 Elsevier B.V. All rights reserved.
Keywords: Cellulose Cholesteric liquid crystal polymer Optical scattering Random lasing Rhodamine 6G
1. Introduction In recent years, random laser has been increasing interests due to its extraordinary properties, such as omnidirectional emission and tiny dimension [1–4]. Random laser does not require a regular cavity but instead usually depends on scattering particles, such as fine laser crystal powders and fine glass powders, which can trap the light within the gain medium for a longer average path length and then the light will be amplified to produce random laser. In the 1980’s, Markushev et al. observed lasing in Nd-doped laser crystal powder [5,6]. They found that a single particle could serve as a laser resonator. In the early 1990, Lawandy et al. reported stimulated emission from a laser dye solution containing titanic dioxide (TiO2) powders [7]. Cao et al. firstly observed the discrete random laser peaks from the suprathin film of zinc oxide (ZnO) powder with a diameter of 100 nm [1] and from laser dye solution containing ZnO particle [8], which was believed to be the laser with coherent feedback. Very recently, the random lasing from laser dyes in nematic liquid crystal (NLC) solution with fine powders [9–12] or ordered NLC solution without fine powders [13] was investigated. The threshold [9–11] and the wavelength [12] of laser emission in * Corresponding author. Tel.: +86 10 68597350; fax: +86 10 68597356. E-mail address: [email protected] (Y. Huang). 0925-3467/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2008.05.014
the liquid crystal solutions can be tunable by changing the solution temperature that leads to the phase transition of nematic liquid crystal solution. The scattering particles, which result in strong scattering, play a crucial role in random lasing [7–12]. But the scattering particles can be deposited in the solution after some time, which is detrimental to the stableness of random laser. To solve this problem, solidification of solutions containing laser dye and fine particles was introduced [14,15]. That is, the fine powders were fixed in the solid film. A cholesteric LC solution may be another simpler candidate for fabrication of stable random laser. The cholesteric LC contains helical domains in which the orientation vector successively turns a small torsion angle from one layer to the next one along the helical axis. These helical domains can selectively reflect some wavelength light (k). The relationship between wavelength and pitch (P) of domains can be described by the following equation: kmax = nPsinh. Where kmax is maximal wavelength of the selective reflection; n is the average refractive index of mesophase; h is an angle between the incident light and ordered molecular layer. And the domains have different refractive index from the isotropic system [10]. These could result in strong scattering of light [10]. Therefore, it is possible to produce the random lasing in a cholesteric LC/laser dye system without solid powder.
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In our previous studies, investigations on the cholesteric LC behavior of the ethyl-cyanoethyl cellulose/acrylic acid [(E-CE)C/ AA] LC solution had been undertaken [16–20]. Similar to other polymer lyotropic cholesteric LC solutions, the (E-CE)C/AA solutions show the behavior of cholesteric LC when their concentration are above the critical concentration of 33 wt.%. In the present paper, we report a new random lasing system that contains laser dye rhodamine 6G (R6G) in lyotropic cholesteric LC (E-CE)C/AA solutions without any solid powder. The effects of the concentration of the cholesteric LC polymer solution and the thickness of the samples on the lasing are investigated and the laser action is discussed based on the cholesteric LC properties. 2. Experimental (E-CE)C, as a cholesteric LC polymer, [19] was obtained by the reaction of ethyl cellulose with acrylonitrine. The degree of substitution for ethyl was about 2.1, and for cyanoethyl was about 0.4, determined through elemental analysis (CHN-O-RAPID, Heraeus, Germany). AA was a chemically pure reagent and not further purified. The laser dye R6G (chloride salt, laser grade, Lambda-Physik) was firstly dissolved into AA with a concentration of 8 10 3 mol/ L. Then a desired amount of (E-CE)C was added into the R6G/AA solution. Three (E-CE)C/AA concentrations with 20, 35, and 46.5 wt.% were prepared. The solutions were stored in the darkness for more than two weeks at room temperature and the homogeneous solutions were formed. The solution with a concentration of 20 wt.% is isotropic; the solutions of 35 and 46.5 wt.% are anisotropic, i.e., cholerteric LC solutions. The solution films of different thickness were sandwiched between two glass wafers and then sealed by using solid olefin. The thickness of solution films was controlled by PET spacer. The polymerization of R6G/(E-CE)C/AA solution films was carried out according to the literature [19]. Briefly, 4 wt.% (with respect to the solvent AA) initiator, benzoin ethyl ether, was added into AA. The solution films were polymerized in an ultraviolet chamber equipped with a 250 W high-intensity mercury arc lamp for 4 min. A schematic diagram of experimental set-up to produce random laser was shown in Fig. 1. A pulsed frequency second Nd:YAG laser (532 nm, 8 ns in pulse width, Tempest 300, New Wave Research) was used to measure the photoluminescence (PL) of the sample at room temperature. The strongly attenuated pump laser was focused on the samples to an area with a diameter of 0.5 mm by adjusting the distance between sample and convex lens. The PL
mirror
Nd:YAG
lens
sample
532 nm filter lens
Computer CCD Fig. 1. A schematic diagram of the experimental set-up.
light was collected in a front scattering geometry and was focused onto the entrance slit of the polychromator. The laser (532 nm) after transmitting the sample was filtered by 532 nm filter. The PL spectra were recorded with a liquid-nitrogen-cooled charge coupled device (model SPEC-10-400B/LN, Roper Scientific Research, NJ).
3. Results and discussion Fig. 2 shows the emission spectra of R6G doped (E-CE)C/AA solution with different concentrations at an excitation energy of 80 lJ. The dye concentration is fixed at 8 10 3 mol/L. The sample thickness is 560 lm. When the R6G is dissolved into the AA solvent or 20 wt.% (E-CE)C/AA solution, namely, isotropic systems, only spectral narrowing is observed even in higher pumping energy as shown in Fig. 2a and b. Under optical pumping, the dye molecules have a broad gain spectrum. The gain length, which is the distance that a photon travels before generating another photon, is shortest at the peak of the gain spectrum. As the pump intensity increase, the gain length decreases. When the shortest gain length becomes equal to the average path length that a photon travels in the gain medium, on average every photon generates a second photon before leaving the solution film. Then the photon density at the frequency of gain maximum builds up quickly. Apart from the gain maximum, the gain length is still longer than the average path length. The photon cannot generate a second photon. Therefore, the drastic increase of photon density only at the gain maximum results in narrowing the emission spectrum. This process in nature should be amplification of spontaneous emission (ASE). When the (E-CE)C/AA concentration is 35 or 46.5 wt.% (in Fig. 2c and d, respectively), several discrete lasing peaks with a minimal full width at half-maximum (FWHM) of 0.3 nm appear in the emission spectra. The spectrally integrated intensity sharply increases as the increase of the excitation intensity as shown in the insert of Fig. 2d. A threshold behavior (about 20 lJ) is observed in the 46.5 wt.% solution. The same threshold behavior (a value of 27 lJ) is also observed in the 35 wt.% solution. These are commonly seen for random lasers with coherent feedback. The differences of emission property among the samples should be related to the concentration of (E-CE)C/AA. From our previous paper, [19] the (E-CE)C/AA solution showed LC behavior when the concentration was above the critical concentration (about 33 wt.%). The LC polymer chains can be helical arrangement and form cholesteric domains. Every LC domain can reflect some wavelength light. And the refractive index of cholesteric LC domains is different from the isotropic system, which decreases diffusion constant [10,11]. These result in very strong scattering within the films. Therefore, the average path length increases in the anisotropic solution films. When the average path length is equal to the gain length, a random laser occurs. The appearance of the discrete laser peaks indicates the existence of different spatial resonances in these cholesteric LC systems. Such resonances can be caused by recurrent light reflection of LC domains to form the closed loop path of light [8]. The different frequencies of laser peaks result from the different resonant modes formed by closed loop. The closed loops should be easily formed in cholesteric LC solution due to multiple scattering resulting from the selective reflection and low diffusion constant in LC solution [10]. It should be noted that the lasing mentioned above is different from that in a dye doped cholesteric liquid crystal solution with perfect planar structure (that is, all cholesteric domains align in the same direction, which shows a narrow reflective peak with a FWHM of less than 50 nm [21]). A single lasing peak with a line-
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Fig. 2. Emission spectra of R6G in (a) AA, (b) 20 wt.%, (c) 35 wt.% and (d) 46.5 wt.% (E-CE)C/AA. The insert in the Fig. 2d is the function of integral intensity as the pump pulse energy. The sample thickness is 560 lm. The pump pulse energy is 80 lJ.
width of less than 1 nm occurred at the edge of the selective reflection band of the perfect planar structure [21–24]. The perfect planar structure actually acted as a regular cavity, and this laser was similar to the regular laser in principle. In our system, though a strong reflective peak can be observed in the (E-CE)C/AA solution film of 46.5% or higher [19], the FWHM of the peak is often far above 100 nm, which suggests that many defects exist in the planar structure [19,20]. Therefore, it is difficult to form a single laser resonant mode to produce a single lasing peak. In the isotropic solution systems of AA and 20 wt.% (E-CE)C/AA solution, there is no reflection and a higher diffusion constant [10], which result in much weaker scattering compared with anisotropic systems. The average path length hardly exceeds the gain length. Therefore, only ASE occurs even under higher pump intensity. Next, we investigate the dependence of lasing on the thickness of the sample. Fig. 3 shows the evolution of emission spectra as the thickness of the sample with a concentration of 46.5%. When the thickness is 10 lm, only fluorescent spectrum with a FWHM of 27 nm is observed. Even the pump energy further increases to 160 lJ, only spectral narrowing occurs. When the thickness of the sample is over 60 lm, several discrete random laser peaks with a FWHM of about 0.2–0.8 nm are observed. And the threshold is decreased from 30, to 26, to 25 and to 20 lJ with increasing the thickness of the samples from 60, to 160, to 260 and to 560 lm, respectively. In a random laser, the overall gain depends on the volume of a sample while the total losses depend on its surface area [1,10]. There exists a critical volume above that the gain becomes larger than the losses and random laser occurs. In case of slab geometry, the volume of a sample is proportion to the thickness. Therefore, when the sample thickness (for example 10 lm) is less than the critical thickness, no laser occurs even in the higher pump energy.
When the sample thickness is larger than the critical value, the random laser occurs. Furthermore, the light wave can keep a longer time inside the thicker sample for the gain to become more efficient. Therefore, there is a smaller threshold for a thicker sample. It is also found from Fig. 3 that the gain-band center is redshifted from 562 to 569 nm with the thickness of the samples from 60 to 560 lm. This may result from the reabsorption of the emission light. This property can be used to tune laser wavelength [15]. AA is a polymerizable solvent. The (E-CE)C/AA solution containing photo-initiator can be quickly photo-polymerized to keep the cholesteric structure [16–20]. The random lasing is investigated in the (E-CE)C/PAA/R6G solid film as shown in Fig. 4a. The random lasing still occurs due to existence of helically structural domains in the solid film. But the threshold value is slightly higher than that in the solution, up to 40 lJ (in Fig. 4 b), which is probably due to the deteriorative changes of cholesteric structure after photopolymerization that degrades the reflection ability of the cholesteric domains [19]. In summary, we reported random lasing in a dye doped cholesteric LC polymer solution without any solid powder. The LC domains with helical structure in R6G/(E-CE)C/AA solution have selective reflection of wavelength and low diffusion constant and then result in strong multiple scattering, which plays important roles in the random lasing. Random lasing is evident as a very small linewidth of 0.2 nm, an abrupt increase of emitted intensity with the increase of pump energy and a critical volume existing, as is commonly seen for random laser based on laser dye. This laser may find application as a tiny or big-area laser with all kinds of forms because all kinds of forms with different sizes can be easily made from the concentrated (E-CE)C/AA solution and kept the forms and inner structure through quick polymerization.
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Fig. 3. Emission spectra from R6G/(E-CE)C/AA solution when the thickness of the samples are 10, 60, 160, 260, 560 lm for a–e, respectively. The (E-CE)C concentration is 46.5 wt.% and the pump intensity is 80 lJ.
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Fig. 4. (a) Emission spectra of the R6G/(E-CE)C/PAA solid film. The thickness of sample is 560 lm. The (E-CE)C concentration is 46.5 wt.% and the pump intensity is 80 lJ. (b) The function of the integral intensity as the pump energy.
B. He et al. / Optical Materials 31 (2008) 375–379
Acknowledgements Grants from National Natural Science Foundation of China (Grant Nos. 50473057, 20374055 and 50521302) and Chinese Academy of Sciences (Grant No. KJCX2-SW-H07) in support of this research are gratefully acknowledged. References [1] H. Cao, Y.G. Zhao, S.T. Ho, E.W. Seelig, Q.H. Wang, R.P.H. Chang, Phys. Rev. Lett. 82 (1999) 278. [2] A.L. Burin, M.A. Ratner, H. Cao, S.H. Chang, Phys. Rev. Lett. 88 (2002) 093904. [3] C.W. Beenakker, J.C. Paasschens, P.W. Brouwer, Phys. Rev. Lett. 76 (1996) 1368. [4] D. Wiersma, Nature 406 (2000) 132. [5] V.M. Markushev, V.F. Zolin, C.M. Briskina, Sov. J. Quantum. Electron. 16 (1986) 281. [6] V.M. Markushev, N.E. Ter-Gabrielyan, C.M. Briskina, V.R. Belan, V.F. Zolin, Sov. J. Quantum. Electron. 20 (1990) 773. [7] N.M. Lawandy, R.M. Balachandran, A.S.L. Gomes, E. Sauvain, Nature 368 (1994) 436.
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H. Cao, J.Y. Xu, S.H. Chang, S.T. Ho, Phys. Rev. E 61 (2000) 1985. D.S. Wiersma, S. Cavalieri, Nature 414 (2001) 708. D.S. Wiersma, Mol. Cryst. Liq. Cryst. 375 (2002) 15. S. Murai, K. Fujita, T. Hirao, K. Nakanishi, K. Hirao, Opt. Mater. 9 (2007) 949. Q. Song, S. Xiao, X. Zhou, L. Liu, L. Xu, Y. Wu, Z. Wang, Opt. Lett. 32 (2007) 373. G. Strangi, S. Ferjani, V. Barna, A. De Luca, C. Versace, N. Scaramuzza, R. Bartolino, Opt. Express 14 (2006) 7737. Y. Ling, H. Cao, A.L. Burin, M.A. Ratner, X. Liu, R.P.H. Chang, Phys. Rev. A 64 (2001) 63808. E. Tikhonov, V.P. Yashchuk, O. Prygodjuk, V. Bezrodny, Solid State Phenom. 106 (2005) 87. S.H. Jiang, Y. Huang, J. Appl. Polym. Sci. 50 (1993) 607. S.H. Jiang, Y. Huang, J. Appl. Polym. Sci. 49 (1993) 125. Y. Huang, S.H. Jiang, J.R. Shen, Polym. Bull. 34 (1995) 203. L.G. Wang, Y. Huang, Macromolecules 37 (2004) 303. L.G. Wang, Y. Huang, Macromolecules 35 (2002) 3111. P.V. Shibaev, K. Tang, A.Z. Genack, V. Kopp, M.M. Green, Macromolecules 35 (2002) 3022. S. Furumi, Y. Sakka, J. Nanosci. Nanotechnol. 6 (2006) 1819. Y. Zhou, Y. Huang, Z. Ge, L.P. Chen, Q. Hong, T.X. Wu, S.T. Wu, Phys. Rev. E. 74 (2006) 061705. J. Schmidtke, W. Stille, H. Finkelmann, S.T. Kim, Adv. Mater. 14 (2002) 746.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Random lasing in a dye doped cholesteric liquid crystal polymer solution Benqiao He a,c, Qing Liao b, Yong Huang c,* a
Tianjin Key Laboratory of Fiber Modification and Functional Fiber, School of Material Science and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300160, China State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Beijing National Laboratory of Molecular Science, Institute of Chemistry, CAS, Beijing 100080, China State Key Laboratory of Polymer Physics and Chemistry and Joint Laboratory of Polymer Science and Material, Beijing National Laboratory of Molecular Science, Institute of Chemistry, CAS, Beijing 100080, China b c
a r t i c l e
i n f o
Article history: Received 18 October 2007 Received in revised form 23 May 2008 Accepted 24 May 2008 Available online 14 July 2008 PACS: 42.25.Fx 78.45.+h 81.20.Fw
a b s t r a c t Random lasing in rhodamine 6G (R6G) doped ethyl-cryanoethyl cellulose [(E-CE)C]/acrylic acid (AA) cholesteric liquid crystal (LC) solution without scattering particles was studied. The effects of concentration of (E-CE)C/AA solution and the thickness of the sample on the random lasing were investigated. The random laser with coherent feedback occurs in (E-CE)C/AA anisotropic solution, while only amplified spontaneous emission (ASE) is observed in (E-CE)C/AA isotropic solution and AA solvent. The random laser also occurs in the (E-CE)C/poly(acrylic acid) (PAA)/R6G solid film with cholesteric structure through quick polymerization of AA. The experimental results suggest that the cholesteric LC domains play a very important role in this random lasing. Ó 2008 Elsevier B.V. All rights reserved.
Keywords: Cellulose Cholesteric liquid crystal polymer Optical scattering Random lasing Rhodamine 6G
1. Introduction In recent years, random laser has been increasing interests due to its extraordinary properties, such as omnidirectional emission and tiny dimension [1–4]. Random laser does not require a regular cavity but instead usually depends on scattering particles, such as fine laser crystal powders and fine glass powders, which can trap the light within the gain medium for a longer average path length and then the light will be amplified to produce random laser. In the 1980’s, Markushev et al. observed lasing in Nd-doped laser crystal powder [5,6]. They found that a single particle could serve as a laser resonator. In the early 1990, Lawandy et al. reported stimulated emission from a laser dye solution containing titanic dioxide (TiO2) powders [7]. Cao et al. firstly observed the discrete random laser peaks from the suprathin film of zinc oxide (ZnO) powder with a diameter of 100 nm [1] and from laser dye solution containing ZnO particle [8], which was believed to be the laser with coherent feedback. Very recently, the random lasing from laser dyes in nematic liquid crystal (NLC) solution with fine powders [9–12] or ordered NLC solution without fine powders [13] was investigated. The threshold [9–11] and the wavelength [12] of laser emission in * Corresponding author. Tel.: +86 10 68597350; fax: +86 10 68597356. E-mail address: [email protected] (Y. Huang). 0925-3467/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2008.05.014
the liquid crystal solutions can be tunable by changing the solution temperature that leads to the phase transition of nematic liquid crystal solution. The scattering particles, which result in strong scattering, play a crucial role in random lasing [7–12]. But the scattering particles can be deposited in the solution after some time, which is detrimental to the stableness of random laser. To solve this problem, solidification of solutions containing laser dye and fine particles was introduced [14,15]. That is, the fine powders were fixed in the solid film. A cholesteric LC solution may be another simpler candidate for fabrication of stable random laser. The cholesteric LC contains helical domains in which the orientation vector successively turns a small torsion angle from one layer to the next one along the helical axis. These helical domains can selectively reflect some wavelength light (k). The relationship between wavelength and pitch (P) of domains can be described by the following equation: kmax = nPsinh. Where kmax is maximal wavelength of the selective reflection; n is the average refractive index of mesophase; h is an angle between the incident light and ordered molecular layer. And the domains have different refractive index from the isotropic system [10]. These could result in strong scattering of light [10]. Therefore, it is possible to produce the random lasing in a cholesteric LC/laser dye system without solid powder.
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In our previous studies, investigations on the cholesteric LC behavior of the ethyl-cyanoethyl cellulose/acrylic acid [(E-CE)C/ AA] LC solution had been undertaken [16–20]. Similar to other polymer lyotropic cholesteric LC solutions, the (E-CE)C/AA solutions show the behavior of cholesteric LC when their concentration are above the critical concentration of 33 wt.%. In the present paper, we report a new random lasing system that contains laser dye rhodamine 6G (R6G) in lyotropic cholesteric LC (E-CE)C/AA solutions without any solid powder. The effects of the concentration of the cholesteric LC polymer solution and the thickness of the samples on the lasing are investigated and the laser action is discussed based on the cholesteric LC properties. 2. Experimental (E-CE)C, as a cholesteric LC polymer, [19] was obtained by the reaction of ethyl cellulose with acrylonitrine. The degree of substitution for ethyl was about 2.1, and for cyanoethyl was about 0.4, determined through elemental analysis (CHN-O-RAPID, Heraeus, Germany). AA was a chemically pure reagent and not further purified. The laser dye R6G (chloride salt, laser grade, Lambda-Physik) was firstly dissolved into AA with a concentration of 8 10 3 mol/ L. Then a desired amount of (E-CE)C was added into the R6G/AA solution. Three (E-CE)C/AA concentrations with 20, 35, and 46.5 wt.% were prepared. The solutions were stored in the darkness for more than two weeks at room temperature and the homogeneous solutions were formed. The solution with a concentration of 20 wt.% is isotropic; the solutions of 35 and 46.5 wt.% are anisotropic, i.e., cholerteric LC solutions. The solution films of different thickness were sandwiched between two glass wafers and then sealed by using solid olefin. The thickness of solution films was controlled by PET spacer. The polymerization of R6G/(E-CE)C/AA solution films was carried out according to the literature [19]. Briefly, 4 wt.% (with respect to the solvent AA) initiator, benzoin ethyl ether, was added into AA. The solution films were polymerized in an ultraviolet chamber equipped with a 250 W high-intensity mercury arc lamp for 4 min. A schematic diagram of experimental set-up to produce random laser was shown in Fig. 1. A pulsed frequency second Nd:YAG laser (532 nm, 8 ns in pulse width, Tempest 300, New Wave Research) was used to measure the photoluminescence (PL) of the sample at room temperature. The strongly attenuated pump laser was focused on the samples to an area with a diameter of 0.5 mm by adjusting the distance between sample and convex lens. The PL
mirror
Nd:YAG
lens
sample
532 nm filter lens
Computer CCD Fig. 1. A schematic diagram of the experimental set-up.
light was collected in a front scattering geometry and was focused onto the entrance slit of the polychromator. The laser (532 nm) after transmitting the sample was filtered by 532 nm filter. The PL spectra were recorded with a liquid-nitrogen-cooled charge coupled device (model SPEC-10-400B/LN, Roper Scientific Research, NJ).
3. Results and discussion Fig. 2 shows the emission spectra of R6G doped (E-CE)C/AA solution with different concentrations at an excitation energy of 80 lJ. The dye concentration is fixed at 8 10 3 mol/L. The sample thickness is 560 lm. When the R6G is dissolved into the AA solvent or 20 wt.% (E-CE)C/AA solution, namely, isotropic systems, only spectral narrowing is observed even in higher pumping energy as shown in Fig. 2a and b. Under optical pumping, the dye molecules have a broad gain spectrum. The gain length, which is the distance that a photon travels before generating another photon, is shortest at the peak of the gain spectrum. As the pump intensity increase, the gain length decreases. When the shortest gain length becomes equal to the average path length that a photon travels in the gain medium, on average every photon generates a second photon before leaving the solution film. Then the photon density at the frequency of gain maximum builds up quickly. Apart from the gain maximum, the gain length is still longer than the average path length. The photon cannot generate a second photon. Therefore, the drastic increase of photon density only at the gain maximum results in narrowing the emission spectrum. This process in nature should be amplification of spontaneous emission (ASE). When the (E-CE)C/AA concentration is 35 or 46.5 wt.% (in Fig. 2c and d, respectively), several discrete lasing peaks with a minimal full width at half-maximum (FWHM) of 0.3 nm appear in the emission spectra. The spectrally integrated intensity sharply increases as the increase of the excitation intensity as shown in the insert of Fig. 2d. A threshold behavior (about 20 lJ) is observed in the 46.5 wt.% solution. The same threshold behavior (a value of 27 lJ) is also observed in the 35 wt.% solution. These are commonly seen for random lasers with coherent feedback. The differences of emission property among the samples should be related to the concentration of (E-CE)C/AA. From our previous paper, [19] the (E-CE)C/AA solution showed LC behavior when the concentration was above the critical concentration (about 33 wt.%). The LC polymer chains can be helical arrangement and form cholesteric domains. Every LC domain can reflect some wavelength light. And the refractive index of cholesteric LC domains is different from the isotropic system, which decreases diffusion constant [10,11]. These result in very strong scattering within the films. Therefore, the average path length increases in the anisotropic solution films. When the average path length is equal to the gain length, a random laser occurs. The appearance of the discrete laser peaks indicates the existence of different spatial resonances in these cholesteric LC systems. Such resonances can be caused by recurrent light reflection of LC domains to form the closed loop path of light [8]. The different frequencies of laser peaks result from the different resonant modes formed by closed loop. The closed loops should be easily formed in cholesteric LC solution due to multiple scattering resulting from the selective reflection and low diffusion constant in LC solution [10]. It should be noted that the lasing mentioned above is different from that in a dye doped cholesteric liquid crystal solution with perfect planar structure (that is, all cholesteric domains align in the same direction, which shows a narrow reflective peak with a FWHM of less than 50 nm [21]). A single lasing peak with a line-
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Fig. 2. Emission spectra of R6G in (a) AA, (b) 20 wt.%, (c) 35 wt.% and (d) 46.5 wt.% (E-CE)C/AA. The insert in the Fig. 2d is the function of integral intensity as the pump pulse energy. The sample thickness is 560 lm. The pump pulse energy is 80 lJ.
width of less than 1 nm occurred at the edge of the selective reflection band of the perfect planar structure [21–24]. The perfect planar structure actually acted as a regular cavity, and this laser was similar to the regular laser in principle. In our system, though a strong reflective peak can be observed in the (E-CE)C/AA solution film of 46.5% or higher [19], the FWHM of the peak is often far above 100 nm, which suggests that many defects exist in the planar structure [19,20]. Therefore, it is difficult to form a single laser resonant mode to produce a single lasing peak. In the isotropic solution systems of AA and 20 wt.% (E-CE)C/AA solution, there is no reflection and a higher diffusion constant [10], which result in much weaker scattering compared with anisotropic systems. The average path length hardly exceeds the gain length. Therefore, only ASE occurs even under higher pump intensity. Next, we investigate the dependence of lasing on the thickness of the sample. Fig. 3 shows the evolution of emission spectra as the thickness of the sample with a concentration of 46.5%. When the thickness is 10 lm, only fluorescent spectrum with a FWHM of 27 nm is observed. Even the pump energy further increases to 160 lJ, only spectral narrowing occurs. When the thickness of the sample is over 60 lm, several discrete random laser peaks with a FWHM of about 0.2–0.8 nm are observed. And the threshold is decreased from 30, to 26, to 25 and to 20 lJ with increasing the thickness of the samples from 60, to 160, to 260 and to 560 lm, respectively. In a random laser, the overall gain depends on the volume of a sample while the total losses depend on its surface area [1,10]. There exists a critical volume above that the gain becomes larger than the losses and random laser occurs. In case of slab geometry, the volume of a sample is proportion to the thickness. Therefore, when the sample thickness (for example 10 lm) is less than the critical thickness, no laser occurs even in the higher pump energy.
When the sample thickness is larger than the critical value, the random laser occurs. Furthermore, the light wave can keep a longer time inside the thicker sample for the gain to become more efficient. Therefore, there is a smaller threshold for a thicker sample. It is also found from Fig. 3 that the gain-band center is redshifted from 562 to 569 nm with the thickness of the samples from 60 to 560 lm. This may result from the reabsorption of the emission light. This property can be used to tune laser wavelength [15]. AA is a polymerizable solvent. The (E-CE)C/AA solution containing photo-initiator can be quickly photo-polymerized to keep the cholesteric structure [16–20]. The random lasing is investigated in the (E-CE)C/PAA/R6G solid film as shown in Fig. 4a. The random lasing still occurs due to existence of helically structural domains in the solid film. But the threshold value is slightly higher than that in the solution, up to 40 lJ (in Fig. 4 b), which is probably due to the deteriorative changes of cholesteric structure after photopolymerization that degrades the reflection ability of the cholesteric domains [19]. In summary, we reported random lasing in a dye doped cholesteric LC polymer solution without any solid powder. The LC domains with helical structure in R6G/(E-CE)C/AA solution have selective reflection of wavelength and low diffusion constant and then result in strong multiple scattering, which plays important roles in the random lasing. Random lasing is evident as a very small linewidth of 0.2 nm, an abrupt increase of emitted intensity with the increase of pump energy and a critical volume existing, as is commonly seen for random laser based on laser dye. This laser may find application as a tiny or big-area laser with all kinds of forms because all kinds of forms with different sizes can be easily made from the concentrated (E-CE)C/AA solution and kept the forms and inner structure through quick polymerization.
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Fig. 3. Emission spectra from R6G/(E-CE)C/AA solution when the thickness of the samples are 10, 60, 160, 260, 560 lm for a–e, respectively. The (E-CE)C concentration is 46.5 wt.% and the pump intensity is 80 lJ.
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Fig. 4. (a) Emission spectra of the R6G/(E-CE)C/PAA solid film. The thickness of sample is 560 lm. The (E-CE)C concentration is 46.5 wt.% and the pump intensity is 80 lJ. (b) The function of the integral intensity as the pump energy.
B. He et al. / Optical Materials 31 (2008) 375–379
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