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We demonstrate mirrorless lasers based on all organic nanostructure fabricated by seven- and nine-beam interference using low contrast material, holographic polymer dispersed liquid crystals (H-PDLC). A finite-difference time-domain (FDTD) simulation is used to study the transmission of quasicrystal. The wavelengths of lasing peak are determined by both of local structure of quasicrystal that the pumping light experienced as well as the photoluminescence of laser dye doped. Features of mirrorless laser from quasicrystal based on H-PDLC include directional light source, low threshold, simple fabrication process, low cost and tunability. These properties make H-PDLC photonic quasicrystal promising for a new type of all organic miniature lasers.
© 2016 Optical Society of America
1. Introduction
Photonic quasicrystals [1], possess no translation symmetry but have orientational symmetry [2], which can be fabricated by holography lithography [3]. Similar to photonic crystals, the optical bandgap exists in the photonic quasicrystals [4–7].Organic materials, holographic polymer dispersed liquid crystals (H-PDLCs) have received substantial attention in fabricating photonic crystal/quasicrystals owing to their easy processing and tunable optoelectronic properties [8–10]. Due to low index contrast of H-PDLC, the position of partial optical bandgap along particular direction in photonic quasicrystal plays a critical role in lasing generation, even there is no complete bandgap exist due to low index contrast.
Several studies on lasing for dye-doped 2D H-PDLC quasicrystals have been reported. M. S. Li et al reported multimode lasing from the microcavity of an octagonal quasicrystal [11]. In our previously study, we have investigated the lasing from Penrose quasicrystal with excellent linear polarization and low threshold [12], as well as the temperature effects [13]. However, the studies on lasing from quasicrystals formed by seven- or nine-beam interference have never been reported. In this letter, we demonstrate mirrorless lasers based on all organic nanostructure fabricated by seven- and nine-beam interference using low contrast material. A finite-difference time-domain (FDTD) simulation is used to study the transmission of quasicrystal. The photonic bandgaps can be used to explain the mechanism of lasing from quasicrystal nanostructure obtained. The wavelengths of lasing peak are determined by both of local structure of quasicrystal that the pumping light experienced as well as the photoluminescence of laser dye doped. The mirrorless laser is directional with simple fabrication process, low cost and possible tunability due to the excellent optoelectronic response of liquid crystals. These properties make holographic polymer dispersed liquid crystal (H-PDLC) photonic quasicrystal promising for a new type of all organic miniature lasers.
2. Experiments
In our experiment, the liquid crystal (LC)/prepolymer mixture consisted of 65 wt% monomer, trimethylolpropane triacrylate (TMPTA), 8 wt% cross-linking monomer, N-vinylpyrrollidone (NVP), 0.8 wt% photoinitiator, Rose Bengal (RB), 1 wt% coinitiator, and 1.2 wt% lasing dye, 4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl- 4H-pyran (DCM) (PM 560 was replaced for quasicrystal formed by nine-beam interference), all from Sigma-Aldrich, and 24 wt% liquid crystal, E7 (no = 1.5216 and ne = 1.7462), from Merck. The mixture was sandwiched in a cell, which was assembled by two pieces of indium tin oxide (ITO) coated glass. The organic nanostructure was fabricated through a seven-beam (or nine-beam) interference generated by a specially designed prism. The prism is made by fused silica with a refractive index of 1.46. The bottom plane of prism is an equilateral heptagon (or enneagon) with an equal side and an equal internal angle, and the side-bottom plane angle of the prism is set to Φ = 60°. θ represents the angle between the beam and vertical z axis. A collimated and linearly polarized Ar + laser (Coherent, I-306C) beam (its electric field along the x direction) with wavelength of 514.5 nm was impinged on the prism (perpendicular to the bottom plane) and split into seven beams with wave vectors of kn (n = 1~7, or n = 1~9 for nine beams), as shown in Fig. 1(a). The wave vectors of seven beams (or nine beams) can be expressed as:
where p = 7 or 9, n is an integer from 1 to p, k = 2πneff/λ is the unit wave vector, λ is the optical writing wavelength in the air, and neff is the effective index of recording material. Here we have θ1 ~θ7 = θ = 23.6°, λ = 514.5 nm, and neff = 1.524.
Fig. 1 (a) Prism used for seven-beam generation and seven-beam interference configuration. θ represents the angle between the beam and vertical z axis. Φ represents the side-bottom plane angle of the prism. (b) A laser beam impinging on the prism will split into seven beams and interference with each other, the interfered pattern will be recorded on the liquid crystal cell filled with LC/polymer mixture, or sample, which is adhered on the bottom of prism through index-match liquid. (c) Optical setup of lasing. A Q-switch frequency-doubled Nd: YAG pulsed laser operating at 532 nm is used to pump the sample with two-dimensional quasicrystal nanostructure. A cylinder lens is used to shape laser beam to a narrow line.
As shown in Fig. 1(b), the interference pattern produced at the bottom surface of the prism in the overlapped area of seven beams (or nine beams) was recorded by a cell filled with LC/prepolymer mixture according to the polymerization induced phase separation. As the seven beams (or nine beams) possess the same phase, the intensity distribution of the interference pattern is given by:
where l and m are integers from 1 to p (p = 7 or 9), E is the electric field, r = (x, y, z) is the position vector, and […] denotes the real part of the argument. The exposure intensity of each beam was ~9 mW/cm2 and the exposure time was 120 s. For the lasing generation, a Q-switched frequency-doubled Nd:yttrium-aluminum- garnet (Nd:YAG) pulsed laser (Quantel, Q-smart 450) operating at 532 nm, with a pulse duration of 10 ns and a repetition rate of 10 Hz, was focused by a cylinder lens and incident on the surface of the sample in a shape of narrow line, as shown in Fig. 1(c).The simulated interference pattern in the x-y plane of quasicrystal formed by 7- or 9-beam interference is shown in Figs. 2(a) and 2(d), respectively. The red- and blue-colored region represents high and low intensities, corresponding to the polymer-rich and LC-rich region, respectively. In our simulation, we used λ = 514.5 nm and n = 1.524 (estimated according to the ratio of the monomer and LC). The surface morphology of the formed quasicrystal nanostructure was firstly immersed in ethonal for 12 hours to remove LC and then observed by an atomic force microscopy (AFM), where the dark and bright regions represent LC and polymer respectively, as shown in Figs. 2(b) (7-beam) and 2(e) (9-beam). The diffraction image of the fabricated quasicrystal, which is generated by a green laser with wavelength of 532 nm, is depicted in Figs. 2(c) and 2(f), confirming the existence of sevenfold quasicrystals with 14 or 18 diffraction points. Normally, the pattern fabricated by N-beam interference will exhibit 2N fold diffraction points [3].
Fig. 2 (a) Simulated intensity distribution of the seven-beam interference pattern. Scale bar: 4 μm. (b) AFM image of the surface morphology of seven-beam interference pattern. Scale bar: 2 μm. (c) Diffraction image of seven-beam interference pattern. (d) Simulated intensity distribution of the nine-beam interference pattern. Scale bar: 4 μm. (b) AFM image of the surface morphology of nine-beam interference pattern. Scale bar: 2 μm. (c) Diffraction image of nine-beam interference pattern.
3. Results and discussion
The output lasing signal was collected by a spectrometer at room temperature. In our experiment, the pump light was shaped to a line, which was represented by the green band and illustrated in Figs. 3(a) (7-beam) and 3(c) (9-beam), respectively. The pump light was carefully adjusted to cross the center of formed quasicrystal nanostructure. The output lasing obtained from quasicrystal formed by 7-beam or 9-beam was directional and shown in Figs. 3(a) and 3(c), respectively. Figures 3(b) and 3(d) show the lasing spectra generated from quasicrystal formed by 7 and 9 beams, respectively. In Fig. 3(b), laser peak with wavelength of 617 nm was depicted with pump energy up to 0.121 mJ/pulse. In Fig. 3(d), multi-peak of lasing with wavelengths of 562, 564, 566, 568, and 571 nm can also be found with pump energy increases up to 0.072 mJ/pulse. The lasing intensity and line width at lasing peaks verse pump energy for seven- and nine-beam interference quasicrystals are shown in Figs. 4(a) and 4(b), respectively. The lasing thresholds at 617 nm for seven-beam case and at 564 nm for nine-beam cases were measured to be 0.027 mJ/pulse and 0.025 mJ/pulse, respectively. In both cases, the line width of lasing peaks narrow with increase of pump energy. The line widths of 617 nm and 564 nm both reduce to less than 1 nm when the pump energies were beyond the thresholds. It is noticed that the threshold is much lower than that achieved in the 2D photonic crystals fabricated by H-PDLC (0.070 mJ/pulse) [14].
Fig. 3 (a) Schematic of the lasing generated along x axis and cross the center of quasicrystal formed by seven-beam interference. (b) Spectrum of lasing from quasicrystal formed by seven-beam interference measured from 0.028 to 0.121 mJ/pulse. (c) Schematic of the lasing generated along x axis and cross the center of quasicrystal formed by nine-beam interference. (d) Spectrum of lasing from quasicrystal formed by nine-beam interference measured from 0.028 to 0.121 mJ/pulse.
Fig. 4 (a) The intensity and line width verse pumping energy (a) 617 nm for seven-beam formed quasicrystal, the threshold is 0.027 mJ/pulse. (b) 564 nm for nine-beam formed quasicrystal, the threshold is 0.025 mJ/pulse.
The laser dyes used in quasicrystal formed by 7-and 9-beam were DCM and PM 560. The photoluminescence of DCM and PM560 in our sample centers around 610 nm and 560 nm, the final output lasing should be determined by both of the photoluminescence of doped laser dye and local structure of quasicrystal the pumping light experienced.
To fully understand the behind mechanism of lasing in quasicrystal formed by seven-beam interference, a simulation was carried by a commercial finite-difference time-domain (FDTD) software package Lumerical. For quasicrystal formed by 7-beam interference, the calculated transmittance along x axis was implemented in the range of 0.5-0.7 μm, as shown in Fig. 5(a). A band-gap was found here and centered at 610 nm. The lasing peak was around 617 nm that was at the band edge where the group velocity was quite small and laser action was highly possible due to the field enhancement of the stimulated emission. This kind of band-gap was similar to that of one-dimensional photonic crystal formed by cholesteric liquid crystals. For quasicrystal formed by 9-beam interference, the simulated transmittance, with band-gap centered at 540 nm, was shown in Fig. 5(b). The lasing peaks were in range of 560-571 nm, and the position of lasing peaks also located at the band edge that favored the generation of stimulated emission. The band edge position and the laser dye doped determined the wavelength of output lasing in quasicrystal nanostructure both for 7- and 9-beam interference.
Fig. 5 Calculated transmittance spectrum along x axis of quasicrystal formed by (a) seven-beam and (b) nine-beam interference. The band gap centers at 610 nm and 540 nm, respectively. The lasing position is illustrated by blue region that is located at the band edge.
4. Conclusion
In summary, mirrorless lasers based on all organic nanostructure fabricated by seven- and nine-beam interference using low contrast material are demonstrated. The output and threshold property of lasing are investigated. A finite-difference time-domain (FDTD) simulation is used to study the transmission of quasicrystal. The wavelengths of lasing peak are determined by band edge position in quasicrystal as well as the photoluminescence of laser dye doped. The features of mirrorless laser from quasicrystal based on H-PDLC include directional light source, low threshold, simple fabrication process, low cost and tunability. These properties make holographic polymer dispersed liquid crystal (H-PDLC) photonic quasicrystal promising for a new type of all organic miniature lasers.
Acknowledgments
This work is supported by National Natural Science Foundation of China (NSFC) (Project Nos. 61405088), Basic Research of Shenzhen Science and Technology Innovation Council (Project Nos. JCYJ20150601155130435), Returned Overseas Research Project Start-up Grant of Ministry of Education.
References and links
1. D. Shechtman, I. Blech, D. Gratias, and J. W. Cahn, “Metallic phase with long-range orientational order and no translational symmetry,” Phys. Rev. Lett. 53(20), 1951–1953 (1984). [CrossRef]
2. E. Rotenberg, W. Theis, K. Horn, and P. Gille, “Quasicrystalline valence bands in decagonal AlNiCo,” Nature 406(6796), 602–605 (2000). [CrossRef] [PubMed]
3. S. P. Gorkhali, J. Qi, and G. P. Crawford, “Switchable quasi-crystal structures with five-, seven-, and ninefold symmetries,” J. Opt. Soc. Am. B 23(1), 149–158 (2006). [CrossRef]
4. Y. S. Chan, C. T. Chan, and Z. Y. Liu, “Photonic band gaps in two dimensional photonic quasicrystals,” Phys. Rev. Lett. 80(5), 956–959 (1998). [CrossRef]
5. G. J. Parker, M. E. Zoorob, M. D. B. Charlton, J. J. Baumberg, and M. C. Netti, “Complete photonic bandgaps in 12-fold symmetric quasicrystals,” Nature 404(6779), 740–743 (2000). [CrossRef] [PubMed]
6. W. Man, M. Megens, P. J. Steinhardt, and P. M. Chaikin, “Experimental measurement of the photonic properties of icosahedral quasicrystals,” Nature 436(7053), 993–996 (2005). [CrossRef] [PubMed]
7. B. Freedman, G. Bartal, M. Segev, R. Lifshitz, D. N. Christodoulides, and J. W. Fleischer, “Wave and defect dynamics in nonlinear photonic quasicrystals,” Nature 440(7088), 1166–1169 (2006). [CrossRef] [PubMed]
8. T. J. Bunning, L. V. Natarajan, V. P. Tondiglia, and R. L. Sutherland, “Holographic polymer-dispersed liquid crystals (H-PDLCs),” Annu. Rev. Mater. Sci. 30(1), 83–115 (2000). [CrossRef]
9. R. Sutherland, V. Tondiglia, L. Natarajan, S. Chandra, D. Tomlin, and T. Bunning, “Switchable orthorhombic F photonic crystals formed by holographic polymerization-induced phase separation of liquid crystal,” Opt. Express 10(20), 1074–1082 (2002). [CrossRef] [PubMed]
10. S. P. Gorkhali, J. Qi, and G. P. Crawford, “Switchable quasi-crystal structures with five-, seven-, and ninefold symmetries,” J. Opt. Soc. Am. B 23(1), 149–158 (2006). [CrossRef]
11. M. S. Li, A. Y. Fuh, and S. T. Wu, “Multimode lasing from the microcavity of an octagonal quasi-crystal based on holographic polymer-dispersed liquid crystals,” Opt. Lett. 37(15), 3249–3251 (2012). [CrossRef] [PubMed]
12. D. Luo, Q. G. Du, H. T. Dai, H. V. Demir, H. Z. Yang, W. Ji, and X. W. Sun, “Strongly linearly polarized low threshold lasing of all organic photonic quasicrystals,” Sci. Rep. 2, 627 (2012). [CrossRef] [PubMed]
13. D. Luo, Q. G. Du, H. T. Dai, X. H. Zhang, and X. W. Sun, “Temperature effect on lasing from Penrose photonic quasicrystal,” Opt. Mater. Express 4(6), 1172–1177 (2014). [CrossRef]
14. D. Luo, X. W. Sun, H. T. Dai, H. V. Demir, H. Z. Yang, and W. Ji, “Two-directional lasing from a dye-doped two-dimensional hexagonal photonic crystal made of holographic polymer-dispersed liquid crystals,” Appl. Phys. Lett. 95(15), 151115 (2009). [CrossRef]
