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Abstract
Random lasers (RLs) offer new functionalities inaccessible with conventional lasers, such as an alterable shape and an easy integration with flexible optoelectronic devices. Here, we demonstrate a stretchable and threshold tunable RL by modulating the order degree of the nematic liquid crystal (NLC) that is caused by the alignment of polymer chain under tensile force. The lasing thresholds show a “U” shape curve variation trend, which is attributed to the competition between the partial orientation of the NLC molecules and the reduction of the dye and NLC densities. The results are further confirmed by the power Fourier transform (PFT) spectrum analysis. This work evokes deeper understanding of the effect of order degree on RLs and extends the applications of polymer polyvinylidene fluoride (PVDF) on tunable RLs.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Tunable lasers fabricated on flexible substrates are urgently needed for flexible optoelectronic devices. Random lasers (RLs) are one kind of omni-directional coherent light sources in which the optical feedback is based on the disorder-induced multiple scattering effect instead of a specific cavity. RLs have great potential applications in many fields, such as speckle-free laser imaging, environment lighting, remote sensors, medical diagnostics and integrated devices [1–6]. Compared with conventional lasers, RLs do not need regular resonant cavities such as Fabry-Perot or DFB cavity that usually rely on lithography or nanofabrication process [7–10]. On one hand, it implies simple preparation process, lower fabrication cost, and in particular, the easily flexible manufacture and shape control. On the other hand, the absence of well-defined optical cavities also means that the lasing modes and threshold cannot be tuned by adjusting the length of the optical cavities as conventional lasers [11].
To generate flexible tunable RLs, the scattering media as well as the gain material should be combined with flexible substrates. The flexible substrates or/and scattering media ought to have certain degrees of controllable freedom so that their scattering properties can be tuned easily by outer fields in real time. Nematic liquid crystals (NLCs) possess excellent tunable scattering properties [12–15]. The spontaneous fluctuations of the local dielectric tensor and the refractive index can induce multiple scattering as light propagating through NLCs. Moreover, the scattering ability of NLCs can be well controlled by external electric, optic or thermal fields [16–18]. In recent years, the optically, thermally and electrically controlled RLs based on NLCs are intensely studied [19–21]. However, these NLC RLs rely on hard substrates, such as liquid crystal cells [22–24] and tubes [25–27]. The lack of flexibility limits their potential applications.
Polymers provide a choice for realizing flexible laser with tunability in terms of threshold, wavelength and polarization [28,29]. The tunable RLs by stretching or bending the nanowire/nanoparticle embedded in polymers have been recently reported. Zhai and co-authors demonstrated a wavelength-tunable RL (from 558 to 565 nm) by stretching silver nanowires embedded in polydimethylsiloxane (PDMS) [30]. Liao et al. reported a stretchable label-type RL using the composite consisting of polymethyl methacrylate (PMMA) and zinc oxide nanoparticles as active materials [31]. Lee et al. showed a flexible RL fabricated on the polyethylene terephthalate (PET) substrate [32]. The lasing wavelength can be controlled by simply bending the PET substrate decorated with the silver nanoprisms (NPRs). However, the variation of threshold or lasing wavelength originates from the change of gain and scattering media density in these works.
In this paper, we select dye Pyrromethene-597 (PM597) and NLCs as gain and scattering media, respectively. Choosing the polyvinylidene fluoride (PVDF) as RL flexible substrate, we demonstrate a thresholds tunable and stretchable polymer film RL. Compared with the flexible substrates used in previous works, the polymeric matrix PVDF possesses high break elongation index, excellent thermal and hydrophobic stability [33]. We find that the lasing threshold of the PVDF films containing NLC (PVDF-NLC) shows a “U” shape variation trend. The threshold firstly decreases before the final tensile length reach to 70% longer than the initial one and then increases as further to continue stretching. It is quite different from the lasing emission of previous works [32,34], in which the thresholds monotonously increase with the tensile length. As a comparison, the RL from dye-doped PVDF films containing gold nanoparticles (PVDF-Au) is also studied. The “U” shape curve variation trend of the RL threshold with stretching length is attributed to the competition between the partial orientation of the NLC molecules and the decrease of the dye and NLC densities with tensile length. The stretching reorients the NLC molecules along the stretching direction, making photons transfer along a closed loop more easily and thus form an annular micro-cavity. With further stretching the films, the decreasing of the concentration of dye and NLC molecules in turn gives rise to the increase of the lasing thresholds. This work gives an insight on the effect of order degree on the RL forming and provides a convenient method to tune the RL thresholds.
2. Sample preparation and experimental setup
In this experiment, a simple scraping method is used to fabricate the dye doped PVDF-NLC films by two steps (Fig. 1(a)). Step 1: The mixed powder of PVDF (1g) and dye PM597 (0.03g, Exciton) was first dissolved in N,N-Dimethylformamide (DMF) with the concentration of 100 mg mL−1 and stirred for about 8 hours to make it be dissolved completely. The NLC (0.2 g, Cr. – 10°C – Nematic – 63°C – Iso) was then added into the dye doped PVDF solution and stirred for 10 minutes on the revolving stage. Step 2: The precursor solution was first dropped onto the flat glass with an area of 10 mm×10 mm. By scraping coating, it formed a viscous solution layer with uniform thickness of 1 mm. The solution film was put onto a heating stage at 45°C to accelerate the evaporation of the DMF. After that the flexible composite film containing NLC droplets and dye molecules was formed. Peeling off from the glass, we obtained the stretchable freestanding films with 3 times elongation at most. The absorption and photoluminescence (PL) spectra of the film are shown in Fig. 1(b). The dye doped PVDF films containing gold nanoparticles (Au NPs) with diameter of ∼25 nm were prepared by the similar method.
Fig. 1. (a) Schematics of the sample preparation. The white bars are NLC molecules randomly distributed in PVDF films. (b) Absorption (black line) and PL spectrum (red line) of the films.
The experimental setup is shown in Fig. 2(a). The sample is pumped by a 532 nm laser beam output from a frequency-doubled Q-switched Nd:YAG laser. The duration and repetition of the laser pulse are ∼8 ns and 10 Hz, respectively. The pump laser is separated into two sub-beams by a polarizing beam splitter (PBS). One beam is monitored by the energy meter and the other is used to pump the sample. The incident laser beam is focused on the central area of the fabricated sample in a strip shape of 1-cm length and 20-µm width by a cylindrical lens along the normal line of the sample. The output lasing emission is measured by a fiber optic spectrometer (AvaSpec-ULS2048) with a spectral resolution of 0.11 nm.
Fig. 2. (a) Experimental setups of the lasing measurement. (b) Emission spectrum of the dye doped PVDF films without and (c) with NLC molecules. (d) The peak intensity and the FWHM of the emission spectra as a function of the pump energy from the dye doped PVDF-NLC films.
3. Results and discussion
Figures 2(b) and 2(c) depict the emission spectra from the dye doped PVDF film without and with NLC molecules. It is not difficult to find that only broad emission spectra with full width at half maximum (FWHM) ∼40 nm are observed even at the pump energy as high as ∼16 mJ/cm2 when NLCs are absent. However, the RL emission appears when NLC is added. It is observed that the radiation spectrum exhibits obvious threshold characteristics, that is, a certain critical value of the pump energy when the emission spectrum starts to produce discrete sharp random laser radiation peaks, and the spectral line width decreases significantly. We can find that the PL spectra are dominated by spontaneous emission at low pump energy and narrow lasing peaks appear with increasing the pump energy. Figure 2(d) plots the peak intensity and FWHM of the emission spectra as a function of excitation energy. The lasing threshold is about 2.87 mJ/cm2 and the FWHM of the lasing peaks is about 0.34 nm, corresponding to a Q value of ∼2000. These results indicate that NLCs play a pivotal role in the formation of RL. The pure PVDF polymer films are nearly transparent in entire visible light range, which results in light transmission with low scattering and little lose. The random distribution of the NLC molecules can enhance the scattering strength inside the films and induce the strong multiple scattering for light. In addition, the film can also act as a waveguide structure because the polymer has a much higher refractive index than that of the air. Light within certain space angle can be confined in the polymer film effectively. The multiple scattering caused by NLCs as well as the reflection of the films surface localize the light and induce the RL emission.
The PVDF possesses outstanding mechanical properties. Figures 3(a) and 3(b) present the RL spectra and lasing thresholds and their variation trend from the dye-doped PVDF-NLC films under stretch length of 0%, 50%, 70% and 100%, respectively (from bottom to top). Here, we define stretch length as the ratio of the difference between the final and the initial length to the original one. The insets in Fig. 3(a) illustrate the corresponding photographs of the stretched films. With stretching, the lasing emission wavelength range keeps stable. It is interesting to find that the thresholds show a “U” shape curve variation trend as depicted by the brown line in Fig. 3(b) if we take the stretch length as the horizontal axis and the lasing threshold as the vertical axis. The thresholds decrease monotonously from 2.39 mJ/cm2 to 0.32 mJ/cm2 when the film is stretched from 0% to 70%. When the film is further stretched, the lasing threshold begins to increase.
Fig. 3. (a) RL spectra and (b) the emission intensity as a function of excitation energy for dye-doped PVDF-NLC films under different stretch length of 0%, 50%, 70% and 100% (from bottom to top). The inset in (a) shows corresponding photographs of the films under different stretch length. (c) The spectral evolutions of the unstretched dye-doped PVDF-Au films. (d) The emission intensity as a function of excitation power for dye-doped PVDF-Au films under different stretch length of 0%, 30%, 50% and 100% (from bottom to top). The brown lines in (b) and (d) depict threshold changes when the films are stretched.
To understand the reason for the threshold change, we replace the NLC to Au NPs and study the RL properties of the dye-doped PVDF-Au films. The Au NPs are selected because they not only can provide multiple scattering and plasmonic enhancing to guide the formation of RL, but also their optical properties do not change when the film is stretched. Figure 3(c) shows the emission spectra of the unstretched films at different pump energy. RL emission with low threshold of ∼0.62 mJ/cm2 is observed. The low threshold is attributed to the strong scattering and localized electric field enhancement induced by local surface plasmon resonance. Figure 3(d) presents the evolution of the output intensity versus the pump energy of the films under different stretch length of 0%, 30%, 50% and 100% (from bottom to top). It can be found that the threshold rises monotonously from 0.66 mJ/cm2 to 2.77 mJ/cm2 as the film from unstretched state to 50% stretching state. Lasing emission cannot be observed when the film is stretched two times longer than its original length.
The laser threshold from the PVDF film embedded with NLCs decreases with the increase of the stretching length while the opposite trend is observed from the film containing gold nanoparticles. The different variation trends of lasing thresholds are owing to the orientation change of the NLC molecules and the density decrease of the NLC as well as gain media. Figure 4 schematically shows the mechanism of lasing threshold change of dye doped PVDF-NLC and PVDF-Au films. PVDF has five distinct crystalline phases, known as α, β, γ, δ and ɛ. Different crystalline phases have different chain conformations. For β-phase PVDF, the direction of the polymer chains has uniaxial orientation. The PVDF films fabricated by melt crystallization method most obtain α-phase. By drawing the original α-phase films, the β-phase will be obtained [35–37]. For the unstretched PVDF-NLC films, the NLC droplets distribute randomly in the PVDF matrix. The continuous unidirectional stretching will induce the α-phase PVDF transfer to β-phase, resulting in crystallites orientate along the stretching direction. The orientation of the polymer chains will induce the long axis of NLC molecules in droplets to orient along the same extension direction, making the NLCs molecules distribution from random to partially ordered. Compared with the totally disordered systems, the increased nematic order parameter results in lower lasing thresholds. It means that increased ordered systems motivate photons from spontaneous emission to radiate into lasing modes [38].
Fig. 4. Illustration of the distribution of the Au NPs and NLCs in dye-doped PVDF films before and after stretching. Yellow ball: Au NPs; red bar: dye molecules; grey bar: NLC molecules; purple line: PVDF chains.
In addition, stretching will make the scatters and gain density reduce, which causes the phenomenon of a higher lasing threshold. The competition between NLC partial ordering and the gain media density reducing induces the lasing thresholds to decrease firstly and then increase with stretching the films. As for the PVDF films containing Au NPs, the stretching just gives rise to the decrease of the Au NP and dye concentration, which results in the increase of the lasing thresholds.
Fig. 5. The power Fourier transform (PFT) of the lasing emission spectra shown in Fig. 3(a) for dye-doped PVDF-NLC films under stretched length of 0%, 50%, 70% and 100%.
To gain further insight into the random cavities of the dye-doped PVDF-NLC films under different stretch length, we perform a power Fourier transform (PFT) analysis for the lasing spectra from the films under stretch length of 0%, 50%, 70% and 100%, as shown in Fig. 5. The equivalent resonant cavity of RL can be assessed by the formula of
where n and d1 represent the refraction index of gain medium and the first peak in PFT curve, respectively. Based on that, the refractive index of polymer PVDF in the visible range is about 1.42, the first sharp peak in the PFT spectra give the cavity length L of 10.13 µm, 9.1 µm, 5.20 µm and 6.24 µm for the film stretched 0%, 50%, 70%, 100%, respectively. This trend is well consistent with the threshold change which indicates that the higher order degree of NLC induced by tensile force shortens the length of the equivalent cavity, which provide an easier access for photons to form closed loops that corresponds to a lower threshold.4. Conclusion
In summary, we have successfully demonstrated a flexible and stretchable coherent RL from the dye-doped PVDF films containing NLCs and Au NPs. Compared with the thresholds that monotonously increase with the tensile length for the PVDF-Au films, the thresholds of PVDF-NLC film show a “U” shape curve variation trend, which first decreases and then increases with stretching length. The competition between the partial ordering of NLC molecules and the density reduction of gain media and NLCs induces this change. It is confirmed by the Fourier transformed spectrum analysis. These results will be useful for deeper understanding of the effect of order degree on the random lasing formation. We expect that our work extends the applications of flexible materials and provides a simple method for tuning the threshold of flexible RLs.
Funding
National Natural Science Foundation of China (11474021, 61705009).
Acknowledgments
The authors would like to express our heartfelt thanks to professor Haizheng Zhong (School of Materials Science & Engineering, Beijing Institute of Technology) for his support of polymer materials and measuring instruments.
Disclosures
The authors declare no conflicts of interest.
References
1. B. Redding, M. A. Choma, and H. Cao, “Speckle-free laser imaging using random laser illumination,” Nat. Photonics 6(6), 355–359 (2012). [CrossRef]
2. H. Cao, R. Chriki, S. Bittner, A. A. Friesem, and N. Davidson, “Complex lasers with controllable coherence,” Nat. Rev. Phys. 1(2), 156–168 (2019). [CrossRef]
3. D. S. Wiersma, “The physics and applications of random lasers,” Nat. Phys. 4(5), 359–367 (2008). [CrossRef]
4. R. C. Polson and Z. V. Vardeny, “Random lasing in human tissues,” Appl. Phys. Lett. 85(7), 1289–1291 (2004). [CrossRef]
5. M. Hohmann, D. Dörner, F. Mehari, C. Chen, M. Späth, S. Müller, H. Albrecht, F. Klämpfl, and M. Schmidt, “Investigation of random lasing as a feedback mechanism for tissue differentiation during laser surgery,” Biomed. Opt. Express 10(2), 807–816 (2019). [CrossRef]
6. S. Chang, W. Liao, Y. Liao, H. Lin, H. Lin, W. Lin, S. Lin, P. Perumal, G. Haider, C. Tai, K. Shen, C. Chang, Y. Huang, T. Lin, and Y. Chen, “A white random laser,” Sci. Rep. 8(1), 2720 (2018). [CrossRef]
7. J. Herrnsdorf, B. Guilhabert, Y. Chen, A. Kanibolotsky, A. Mackintosh, R. Pethrick, P. Skabara, E. Gu, N. Laurand, and M. Dawson, “Flexible blue-emitting encapsulated organic semiconductor DFB laser,” Opt. Express 18(25), 25535–25545 (2010). [CrossRef]
8. T. Zhai, Y. Wang, L. Chen, and X. Zhang, “Direct writing of tunable multi-wavelength polymer lasers on a flexible substrate,” Nanoscale 7(29), 12312–12317 (2015). [CrossRef]
9. P. Gorrn, M. Lehnhardt, W. Kowalsky, T. Riedl, and S. Wagner, “Elastically tunable self-organized organic lasers,” Adv. Mater. 23(7), 869–872 (2011). [CrossRef]
10. K. Suzuki, K. Takahashi, Y. Seida, K. Shimizu, M. Kumagai, and Y. Taniguchi, “A continuously tunable organic solid-state laser based on a flexible distributed-feedback resonator,” Jpn. J. Appl. Phys. 42(3A), L249–L251 (2003). [CrossRef]
11. H. Cao, Y. G. Zhao, S. T. Ho, E. W. Seelig, Q. H. Wang, and R. P. H. Chang, “Random laser action in semiconductor powder,” Phys. Rev. Lett. 82(11), 2278–2281 (1999). [CrossRef]
12. S. Ferjani, V. Barna, A. De Luca, C. Versace, and G. Strangi, “Random lasing in freely suspended dye-doped nematic liquid crystals,” Opt. Lett. 33(6), 557–559 (2008). [CrossRef]
13. S. Ferjani, L. Sorriso-Valvo, A. De Luca, V. Barna, R. De Marco, and G. Strangi, “Statistical analysis of random lasing emission properties in nematic liquid crystals,” Phys. Rev. E 78(1), 011707 (2008). [CrossRef]
14. Z. Shang, L. Deng, and Y. An, “Influence of fiber and MnCl2 on mode and threshold of random lasing in random gain systems,” Opt. Express 25(26), 32522–32531 (2017). [CrossRef]
15. L. Wang, Y. Wan, L. Shi, H. Zhong, and L. Deng, “Electrically controllable plasmonic enhanced coherent random lasing from dye-doped nematic liquid crystals containing au nanoparticles,” Opt. Express 24(16), 17593–17602 (2016). [CrossRef]
16. S. Ferjani, A. De Luca, V. Barna, C. Versace, and G. Strangi, “Thermo-recurrent nematic random laser,” Opt. Express 17(3), 2042–2047 (2009). [CrossRef]
17. C. Lee, J. Lin, B. Huang, S. Lin, T. Mo, S. Huang, C. Kuo, and H. Yeh, “Electrically controllable liquid crystal random lasers below the fréedericksz transition threshold,” Opt. Express 19(3), 2391–2400 (2011). [CrossRef]
18. C. Lee, J. Lin, B. Huang, T. Mo, and S. Huang, “All-optically controllable random laser based on a dye-doped liquid crystal added with a photoisomerizable dye,” Opt. Express 18(25), 25896–25905 (2010). [CrossRef]
19. A. K. Tiwari, L. Pattelli, R. Torre, and D. S. Wiersma, “Remote control of liquid crystal elastomer random laser using external stimuli,” Appl. Phys. Lett. 113(1), 013701 (2018). [CrossRef]
20. G. Palermo, U. Cataldi, L. De Sio, T. Bürgi, N. Tabiryan, and C. Umeton, “Optical control of plasmonic heating effects using reversible photo-alignment of nematic liquid crystals,” Appl. Phys. Lett. 109(19), 191906 (2016). [CrossRef]
21. J. Lin, J. Huang, J. Wu, S. Tsay, and Y. Chen, “Electrically controllable random lasing from dye-doped nematic liquid crystals within a capillary fiber,” Opt. Mater. Express 8(9), 2910–2917 (2018). [CrossRef]
22. C. Lee, S. Lin, J. Guo, J. Lin, H. Lin, Y. Zheng, C. Ma, C. Horng, H. Sun, and S. Huang, “Electrically and thermally controllable nanoparticle random laser in a well-aligned dye-doped liquid crystal cell,” Opt. Mater. Express 5(6), 1469–1481 (2015). [CrossRef]
23. C. Chang, C. Kuo, H. Sun, S. Lin, C. Chang, and S. Huang, “All-optically controllable nanoparticle random laser in a well-aligned laser-dye-doped liquid crystal,” Opt. Express 24(25), 28739–28747 (2016). [CrossRef]
24. S. Lin, P. Chen, Y. Li, C. Chen, J. Lin, Y. Chen, S. Tsay, and J. Wu, “Manipulation of polarized random lasers from dye-doped twisted nematic liquid crystals within wedge cells,” IEEE Photonics J. 9(2), 1–8 (2017). [CrossRef]
25. J. Lin and Y. Hsiao, “Manipulation of the resonance characteristics of random lasers from dye-doped polymer dispersed liquid crystals in capillary tubes,” Opt. Mater. Express 4(8), 1555–1563 (2014). [CrossRef]
26. J. Lin, Y. Hsiao, B. Ciou, S. Lin, Y. Chen, and J. Wu, “Manipulation of random lasing action from dye-doped liquid crystals infilling two-dimensional confinement single core capillary,” IEEE Photonics J. 7(3), 1–9 (2015). [CrossRef]
27. J. Wang, Y. Zhang, M. Cao, X. Song, Y. Che, H. Zhang, H. Zhang, and J. Yao, “Platinum-scatterer-based random lasers from dye-doped polymer-dispersed liquid crystals in capillary tubes,” Appl. Opt. 55(21), 5702–5706 (2016). [CrossRef]
28. L. Ye, C. Zhao, Y. Feng, B. Gu, Y. Cui, and Y. Lu, “Study on the polarization of random lasers from dye-doped nematic liquid crystals,” Nanoscale Res. Lett. 12(1), 27 (2017). [CrossRef]
29. C. Chen, H. Huang, H. Jau, C. Wang, C. Wu, and T. Lin, “Polarization-asymmetric bidirectional random laser emission from a twisted nematic liquid crystal,” J. Appl. Phys. 121(3), 033102 (2017). [CrossRef]
30. T. Zhai, J. Chen, L. Chen, J. Wang, L. Wang, D. Liu, S. Li, H. Liu, and X. Zhang, “A plasmonic random laser tunable through stretching silver nanowires embedded in a flexible substrate,” Nanoscale 7(6), 2235–2240 (2015). [CrossRef]
31. Y. Liao, Y. Lai, P. Perumal, W. Liao, C. Chang, C. Liao, S. Lin, and Y. Chen, “Highly stretchable label-like random laser on universal substrates,” Adv. Mater. Technol. 1(6), 1600068 (2016). [CrossRef]
32. Y. Lee, C. Chou, Z. Yang, T. B. H. Nguyen, Y. Yao, T. Yeh, M. Tsai, and H. Kuo, “Flexible random lasers with tunable lasing emissions,” Nanoscale 10(22), 10403–10411 (2018). [CrossRef]
33. L. Li, M. Zhang, M. Rong, and W. Ruan, “Studies on the transformation process of PVDF from alpha to beta phase by stretching,” Rsc Adv. 4(8), 3938–3943 (2014). [CrossRef]
34. Y. Lee, T. Yeh, Z. Yang, Y. Yao, C. Chang, M. Tsai, and J. Sheu, “A curvature-tunable random laser,” Nanoscale 11(8), 3534–3545 (2019). [CrossRef]
35. A. B. Da Silva, C. Wisniewski, J. V. A. Esteves, and R. Gregorio, “Effect of drawing on the dielectric properties and polarization of pressed solution cast β-PVDF films,” J. Mater. Sci. 45(15), 4206–4215 (2010). [CrossRef]
36. J. J. Wang, H. H. Li, J. C. Liu, Y. X. Duan, S. D. Jiang, and S. K. Yan, “On the α → β transition of carbon-coated highly oriented pvdf ultrathin film induced by melt recrystallization,” J. Am. Chem. Soc. 125(6), 1496–1497 (2003). [CrossRef]
37. W. Lu, X. Wu, S. Huang, L. Wang, Q. Zhou, B. Zou, H. Zhong, and Y. Wang, “Strong polarized photoluminescence from stretched perovskite-nanocrystal-embedded polymer composite films,” Adv. Opt. Mater. 5(23), 1700594 (2017). [CrossRef]
38. G. Strangi, S. Ferjani, V. Barna, A. De Luca, C. Versace, N. Scaramuzza, and R. Bartolino, “Random lasing and weak localization of light in dye-doped nematic liquid crystals,” Opt. Express 14(17), 7737–7744 (2006). [CrossRef]
