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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 9 — Apr. 26, 2010
  • pp: 9496–9503
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Photoerasable and photorewritable spatially-tunable laser based on a dye-doped cholesteric liquid crystal with a photoisomerizable chiral dopant

Shih-Hung Lin, Cang-Yi Shyu, Jui-Hsiang Liu, Po-Chih Yang, Ting-Shan Mo, Shuan-Yu Huang, and Chia-Rong Lee  »View Author Affiliations


Optics Express, Vol. 18, Issue 9, pp. 9496-9503 (2010)
http://dx.doi.org/10.1364/OE.18.009496


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Abstract

This study investigates, for the first time, a photoerasable and photorewritable spatially-tunable laser using a dye-doped cholesteric liquid crystal (DDCLC) with a photoisomerizable chiral dopant (AzoM). UV illumination via a photomask with a transmittance-gradient can create a pitch gradient in the cell such that the lasing wavelength can be spatially tuned over a wide band of 134nm. The pitch gradient is generated by the UV-irradiation-induced gradient of the cis-AzoM concentration and therefore the induced gradient of the cell HTP value, resulting in the spatial tunability of the laser. Furthermore, the laser has advantages of photoerasability and photorewritability. The spatial tunability of the laser can undergo more than 100 cycles of photoerasing and photorewriting processes without decay or damage.

© 2010 OSA

1. Introduction

Photonic crystals (PCs) have spatially-periodic structures with a greatly modulated refractive index and therefore consequent photonic bandgaps (PBGs) [1

1. E. Yablonovitch, “Inhibited spontaneous emission in solid-state physics and electronics,” Phys. Rev. Lett. 58(20), 2059–2062 (1987). [CrossRef] [PubMed]

,2

2. S. John, “Strong localization of photons in certain disordered dielectric superlattices,” Phys. Rev. Lett. 58(23), 2486–2489 (1987). [CrossRef] [PubMed]

]. Because of these photonic band structures, PCs can be used in numerous attractive applications, including PC fibers, waveguides, filters, switches, sensors, multiplexers, super prisms, lasers and others [3

3. J. Broeng, D. Mogilevstev, S. E. Barkou, and A. Bjarklev, “Photonic crystal Fibers: A New Class of Optical Waveguides,” Opt. Fiber Technol. 5(3), 305–330 (1999). [CrossRef]

10

10. M. Lončar, T. Yoshie, A. Scherer, P. Gogna, and Y. Qiu, “Low-threshold photonic crystal laser,” Appl. Phys. Lett. 81(15), 2680–2682 (2002). [CrossRef]

,12

12. V. I. Kopp, B. Fan, H. K. M. Vithana, and A. Z. Genack, “Low-threshold lasing at the edge of a photonic stop band in cholesteric liquid crystals,” Opt. Lett. 23(21), 1707–1709 (1998). [CrossRef]

18

18. G. Petriashvili, M. A. Matranga, M. P. De Santo, G. Chilaya, and R. Barberi, “Wide band gap materials as a new tuning strategy for dye doped cholesteric liquid crystals laser,” Opt. Express 17(6), 4553–4558 (2009). [CrossRef] [PubMed]

].

Spatially-tunable DDCLC lasers with a pitch gradient have received increasing interest in the recent decade [14

14. A. Chanishvili, G. Chilaya, G. Petriashvili, R. Barberi, R. Bartolino, G. Cipparrone, A. Mazzulla, R. Gimenez, L. Oriol, and M. Pinol, “Widely tunable ultraviolet-visible liquid crystal laser,” Appl. Phys. Lett. 86(5), 051107 (2005). [CrossRef]

18

18. G. Petriashvili, M. A. Matranga, M. P. De Santo, G. Chilaya, and R. Barberi, “Wide band gap materials as a new tuning strategy for dye doped cholesteric liquid crystals laser,” Opt. Express 17(6), 4553–4558 (2009). [CrossRef] [PubMed]

]. The pitch gradient in a CLC cell is established mainly by successive capillary injections of DDCLC mixtures with different chiral concentrations and laser dye types, by creating a temperature gradient using a temperature controller, or by exploiting a wedged cell. These methods have some shortcomings, including non-erasability, non-rewritability of the pitch gradient, or the need to use a temperature controller or multiple types/concentrations of chiral agents and laser dye types. Consequently, this work develops a photoerasable and photorewritable spatially-tunable laser based on a DDCLC film that contains a photoisomerizable chiral dopant (AzoM). Under UV illumination through a photomask with a transmittance-gradient on the cell, a pitch gradient in the cell can be easily generated such that the wavelength of the edge lasing emission can be spatially tuned over a wide band of 134nm in a length of 28mm on the cell. The pitch gradient in the cell is generated when the UV light induces a concentration gradient of cis-AzoM, such that the helical twisting power (HTP) value of the cell depends on the position, making the laser spatially tunable. Furthermore, the spatially-tunable characteristics of the laser can be photoerased by the irradiation of one uniform green beam and photorewrited by the UV-irradiation-gradient method.

2. Sample preparation and experimental setups

A CLC host with left chirality is prepared by mixing 74.2wt% NLC (MDA-03-3970, from Merck) and 12.5wt% chiral material (S811, from Merck). The CLC host is then mixed with 12.6wt% photoisomerizable chiral agent (AzoM, left-handedness) and two laser dyes, 4-dicyanmethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM, from Exciton) and pyrromethene 567 (P567, from Exciton), in proportions 0.5 and 0.2wt%, respectively, to form a DDCLC mixture with AzoM. Details of the synthesis of AzoM can be found elsewhere [19

19. J.-H. Liu, P.-C. Yang, Y.-K. Wang, and C.-C. Wang, “Optical behaviour of cholesteric liquid crystal cells with novel photoisomerizable chiral dopants,” Liq. Cryst. 33(3), 237–248 (2006). [CrossRef]

,20

20. J.-H. Liu and P.-C. Yang, “Synthesis and characterization of novel monomers and polymers containing chiral (−)-menthyl groups,” Polymer (Guildf.) 47(14), 4925–4935 (2006). [CrossRef]

]. A mixture of DDCLC with AzoM is injected into the empty cell and uniformly dispersed to form a 4cm-long × 0.5cm-wide planar DDCLC cell with AzoM. The empty cell is pre-fabricated by combining two indium-tin-oxide glass slides, separated by two narrow 23μm-thick plastic strips (cell gap). Homogeneously-aligned PVA films are individually pre-coated on the glass slides of the cell and pre-rubbed in the same direction.

Figure 1
Fig. 1 Experimental setup for measuring lasing spectra of DDCLC cell at various positions of the cell. The DDCLC cell (cell gap: d) is fixed on a sample holder, which is removable on a translational stage, such that the incident single pumped pulse can excite the cell at various cell positions. The spectrometer probe is placed at a distance of l = 1cm behind the cell to receive lasing emission along the cell normal. PBS: polarizing beam splitter, λ/2: half-wave plate for 532nm.
displays the experimental setup for measuring the lasing spectra in the visible region at various cell positions. The pulse light source that is employed to pump the DDCLC cell is a Q-switched Nd:YAG second harmonic generation (SHG) pulse laser (wavelength: 532nm) with a pulse duration of 8ns and a pulse energy, E. A single pumped pulse is focused using a lens (focal length = 20cm) on the cell at an incident angle of approximately 20° and the generated lasing signal, which is emitting perpendicular to the CLC planes of the cell, can be measured. A half-wave plate (λ/2 for 532nm) and polarizing beam splitter (PBS) are placed in front of the lens to modify the pulse energy of the laser beam that impinges on the cell. To analyze the measured lasing signal at a certain cell position, the corresponding reflection spectrum (in the visible region), which indicates the CLC band structure at that position, is also measured. Both the lasing and the reflection spectra of the cell are recorded using a fiber-optic probe of a fiber-based spectrometer system (USB2000-UV-VIS, Ocean Optics, optical resolution: ~1.4) that is placed at a distance of l = 1cm behind the cell. Notably, the DDCLC cell is fixed on a sample holder on a translation stage, which can be moved to enable the spatial tuning feature of the obtained lasing and corresponding reflection spectra of the cell to be investigated.

3. Results and discussion

A pitch gradient in the AzoM-added DDCLC is pre-formed before the lasing and reflection spectra of the cell are measured. As displayed in Fig. 2(a)
Fig. 2 (a) Pitch gradient in the AzoM-added DDCLC cell formed upon irradiation by one uniform UV beam with an intensity of 3mW/cm2 for 20min through a transmittance-gradational photomask. (b) Gradient of transmittance of UV light through the photomask from 0 to 100% as cell position changes from x = 0 to x = 40mm. Reflection images of one white beam from AzoM-added DDCLC cell (c) without and (d) with a pitch gradient.
, an expanded and collimated UV beam with a uniform intensity of 3mW/cm2 passes through a rectangular photomask (5.08cm-length × 2.54cm-width) with a position-dependent transmittance to illuminate the AzoM-added DDCLC cell for 20 minutes by the UV-irradiation-gradient method. Figure 2(b) reveals that the transmittance of the UV light via the photomask monotonically increases as the cell position shifts from x = 0 to x = 40mm. Figures 2(c) and 2(d) present homogeneous and inhomogeneous image colors of a white beam reflected from the cell before and after UV irradiation, respectively. Apparently, in Fig. 2(d), the CLC reflection band (CLCRB) of the cell can be observed by naked eye gradually to red-shift from green to red as x increases. Figure 3(a)
Fig. 3 Red-shifts of (a) measured CLCRB and (b) measured lasing signal at LWE as position of AzoM-added DDCLC cell varies from x = 6 to x = 34mm. The grey curve represents the fluorescence emission spectrum of the cell in the isotropic state.
directly shows the gradual red-shift in the measured CLCRB as the cell position shifts from x = 6 to x = 34mm, establishing a CLCRB gradient. Experimental results in Figs. 2(d) and 3(a) reflect the formation of the pitch gradient on the cell by the UV irradiation. Since a separate experiment demonstrated the pitch-invariance of the DDCLC cell without AzoM following the above UV irradiation (data not shown), the UV-induced pitch gradient on the AzoM-added DDCLC cell, shown in Fig. 2(d), is certainly established by the effect of the UV-excited AzoM on the cell pitch. The mechanism is described as follows.

Figure 4(b) presents the absorption and fluorescence emission spectra of the DDCLC cell in the isotropic state; the latter is distributed mostly between 540 and 700nm. Based on photonic band-edge lasing theory for a 1D PC-like planar DDCLC, lasing can occur at the edges of the CLCRB of the cell if the region of the spontaneously emitted fluorescence effectively overlaps the band edges [12

12. V. I. Kopp, B. Fan, H. K. M. Vithana, and A. Z. Genack, “Low-threshold lasing at the edge of a photonic stop band in cholesteric liquid crystals,” Opt. Lett. 23(21), 1707–1709 (1998). [CrossRef]

,13

13. V. I. Kopp, Z.-Q. Zhang, and A. Z. Genack, “Lasing in chiral photonic structures,” Prog. Quantum Electron. 27(6), 369–416 (2003). [CrossRef]

]. Since the fluorescence emission spectrum of laser dyes (540-700nm) entirely overlaps the region of the spectrum associated with the CLCRB gradient of the AzoM-added DDCLC cell with the pitch gradient [Fig. 3(a)], a spatially-tunable edge lasing emission can be stimulated. Figure 3(b) presents the experimental results: the lasing emission can be excited at the long wavelength edge (LWE) of each position-dependent CLCRB when stimulated by a single pumped pulse with E = 18μJ/pulse. Accordingly, the lasing wavelength at the edge can be spatially tuned from 553 to 687nm at cell positions from x = 6 to x = 34mm. The gray fluorescence spectrum curve of the cell in the isotropic state in the region of 500-700nm is also displayed in Fig. 3(b). Apparently, the strongest lasing emission (~596 nm) does not exactly occur at the wavelength (~575 nm) at which the fluorescence emission of the cell is highest. This is possibly due to the weak re-absorption effect of the fluorescence photons with the wavelength of ~575nm, where the tail of the absorption spectrum curve of the cell overlaps [Fig. 4(b)].

Figures 5(a)
Fig. 5 (a)-(f) Lasing patterns with lasing wavelengths of 553 to 687 nm (green to deep red) at the LWE of the CLCRB obtained as pumping position of AzoM-added DDCLC cell increases from x = 6 to x = 34mm.
5(f) show the obtained color-changeable lasing patterns (from green to deep red) on the screen, as well as the corresponding lasing wavelengths (from 553 to 687nm) as the pumping position of the cell is varied from x = 6 to x = 34 mm. Hence, a large spatially-tunable range of lasing wavelengths with a width of 134nm in a length of 28mm on the cell, based on the AzoM-added DDCLC laser with a UV-induced pitch gradient, can be obtained. As shown in Fig. 4(b), lasing emission does not easily occur outside the range 553-687nm because of the strong loss caused by the strong re-absorption of fluorescence at λ<553nm or the negligible fluorescence emission at λ>687nm.

This study also elucidates the photoerasability and thermal stability of the formed pitch gradient and, thus, the spatial tunability of the laser. A pitch gradient is produced from x = 6 to x = 34mm in the AzoM-added DDCLC cell in response to UV-irradiation-gradient method for 40min. The wavelength at the LWE (λLWE) of the measured CLCRB of the cell varies from 561 to 745nm as the cell position varies from x = 6 to x = 34mm, as displayed in Fig. 6(a)
Fig. 6 Photoerasibility and thermal relaxation of the pitch gradient and thus the associated spatial tunability of laser. A pitch gradient is pre-written in the cell by the UV-irradiation-gradient method, yielding a position-dependent wavelength distribution at the LWE of the measured CLCRB (black curves in (a) and (b)). (a) The wavelengths at the LWEs at all cell positions shift to a single value (black curve → red line) as the duration of irradiation of one uniform green beam with an intensity of 20mW/cm2 on the cell with a pitch gradient increases from 0 to 90s. (b) The wavelength at the LWE at each cell position slowly relaxes to the original value (black curve → red line) as the relaxation time increases from 0 to 38hr.
(black curve). This result is similar to that shown in Fig. 3(a). Next, an expanded and uniform green beam (from a diode-pumped solid state laser, output power <1W, λ: 532nm) with an intensity of 20mW/cm2 is applied to irradiate the cell with the pitch gradient for tG = 0 to tG = 90s, such that the black curve gradually shifts to the red line. Therefore, λLWE at x>19mm and x<19mm gradually blue-shift and red-shift, respectively, to an identical value, 581nm, as tG increases from 0 to 90s. This result reflects the fact that the pitch gradient and thus the spatial tunability of the DDCLC laser mentioned above can be erased in 90s. The erasing time may be reduced by increasing the irradiated intensity of the green beam. This photoerasibility of the spatial features of the DDCLC laser is attributable to the green-beam-induced photoisomerization reaction of AzoM in the cell. In the cis-AzoM-rich region (x>19mm), the green beam can cause the cis-AzoM to enter the trans-state at a high cistrans isomeirzation rate, causing the resultant HTP value to increase such that both the CLCRB and its LWE exhibit a large blue-shift. In contrast, in the trans-AzoM-rich region (x<19mm), the green beam transform the trans-AzoM into the cis-state at a low transcis isomerization rate, reducing the HTP value, such that both the CLCRB and its LWE are slightly red-shifted. Under continued irradiation of the green beam, a uniform dynamic equilibrium with a high concentration of trans-AzoM and a low concentration of cis-AzoM in the whole cell can be achieved such that the λLWE at each cell position can eventually reach a stable value.

4. Conclusion

This work demonstrates, for the first time, a photoerasable and photorewritable spatially-tunable laser that is based on a DDCLC cell doping with a photoisomerizable chiral dopant (AzoM). UV irradiation through a photomask with a transmittance-gradient can establish a pitch gradient in the AzoM-added DDCLC cell such that the generated edge lasing wavelength can be spatially tuned over a wide band from green to deep red (553 to 687nm) in a cell length of 28mm. The pitch gradient is formed by the UV-irradiation-induced gradient of the concentration of the cis-AzoM and therefore the induced gradient of the HTP value of the cell, resulting in the spatial tunability of the DDCLC laser. Additionally, the laser is photoerasable and photorewritable. The spatial tunability of the laser can endure more than 100 cycles of photoerasing and photorewriting processes without decay or damage. The total thermal relaxation of the spatial-tunability of the laser does not take as long as 38 hours. Other methods, such as polymer-network stabilized method, will be employed to improve the thermal stability of the tunable laser in the future work.

Acknowledgments

The authors would like to thank the National Science Council of the Republic of China, Taiwan (Contract No. NSC 97-2112-M-006-013-MY3) and the Advanced Optoelectronic Technology Center, National Cheng Kung University, under projects from the Ministry of Education for financially supporting this research. We greatly appreciate Ted Knoy for editorial assistance and Jui-Hsiang Liu et al. for offering the photoisomerizable chiral agent.

References and links

1.

E. Yablonovitch, “Inhibited spontaneous emission in solid-state physics and electronics,” Phys. Rev. Lett. 58(20), 2059–2062 (1987). [CrossRef] [PubMed]

2.

S. John, “Strong localization of photons in certain disordered dielectric superlattices,” Phys. Rev. Lett. 58(23), 2486–2489 (1987). [CrossRef] [PubMed]

3.

J. Broeng, D. Mogilevstev, S. E. Barkou, and A. Bjarklev, “Photonic crystal Fibers: A New Class of Optical Waveguides,” Opt. Fiber Technol. 5(3), 305–330 (1999). [CrossRef]

4.

J. Tervo, M. Kuitinen, P. Vahimaa, J. Turunen, T. Aalto, P. Heimala, and M. Leppilhalme, “Efficient Bragg waveguide-grating analysis by quasi-rigorous approach based on Redheffer’s star product,” Opt. Commun. 198(4-6), 265–272 (2001). [CrossRef]

5.

P. Tran, “Optical switching with a nonlinear photonic crystal: a numerical study,” Opt. Lett. 21(15), 1138–1140 (1996). [CrossRef] [PubMed]

6.

R. W. Ziolkowski and T. Liang, “Design and characterization of a grating-assisted coupler enhanced by a photonic-band-gap structure for effective wavelength-division demultiplexing,” Opt. Lett. 22(13), 1033–1035 (1997). [CrossRef] [PubMed]

7.

T. D. James, A. C. Greenwald, E. A. Johnson, W. A. Stevenson, J. A. Wollam, T. George, and E. W. Jones, “Nano-Structuredd Surfaces For Tuned Infrared Emission For Spectroscopic Applications,” Proc. SPIE Opt. 2000. Photonics West, San Jose, CA, 22–28. January (2000).

8.

B. S. Song, S. Noda, and T. Asano, “Photonic devices based on in-plane hetero photonic crystals,” Science 300(5625), 1537 (2003). [CrossRef] [PubMed]

9.

P. St. J. Russell, “Photonic crystal fibers,” Science 299(5605), 358–362 (2003). [CrossRef] [PubMed]

10.

M. Lončar, T. Yoshie, A. Scherer, P. Gogna, and Y. Qiu, “Low-threshold photonic crystal laser,” Appl. Phys. Lett. 81(15), 2680–2682 (2002). [CrossRef]

11.

J. P. Dowling, M. Scalora, M. J. Bloemer, and C. M. Bowden, “The photonic band edge laser: A new approach to gain enhancement,” J. Appl. Phys. 75(4), 1896–1899 (1994). [CrossRef]

12.

V. I. Kopp, B. Fan, H. K. M. Vithana, and A. Z. Genack, “Low-threshold lasing at the edge of a photonic stop band in cholesteric liquid crystals,” Opt. Lett. 23(21), 1707–1709 (1998). [CrossRef]

13.

V. I. Kopp, Z.-Q. Zhang, and A. Z. Genack, “Lasing in chiral photonic structures,” Prog. Quantum Electron. 27(6), 369–416 (2003). [CrossRef]

14.

A. Chanishvili, G. Chilaya, G. Petriashvili, R. Barberi, R. Bartolino, G. Cipparrone, A. Mazzulla, R. Gimenez, L. Oriol, and M. Pinol, “Widely tunable ultraviolet-visible liquid crystal laser,” Appl. Phys. Lett. 86(5), 051107 (2005). [CrossRef]

15.

Y. Huang, Y. Zhou, and S.-T. Wu, “Spatially tunable laser emission in dye-doped photonic liquid crystals,” Appl. Phys. Lett. 88(1), 011107 (2006). [CrossRef]

16.

K. Sonoyama, Y. Takanishi, K. Ishikawa, and H. Takezoe, “Position-sensitive cholesteric liquid crystal dye laser covering a full visible range,” Jpn. J. Appl. Phys. 46(36), L874–L876 (2007). [CrossRef]

17.

M.-Y. Jeong, H. Choi, and J. W. Wu, “Spatial tuning of laser emission in a dye-doped cholesteric liquid crystal wedge cell,” Appl. Phys. Lett. 92(5), 051108 (2008). [CrossRef]

18.

G. Petriashvili, M. A. Matranga, M. P. De Santo, G. Chilaya, and R. Barberi, “Wide band gap materials as a new tuning strategy for dye doped cholesteric liquid crystals laser,” Opt. Express 17(6), 4553–4558 (2009). [CrossRef] [PubMed]

19.

J.-H. Liu, P.-C. Yang, Y.-K. Wang, and C.-C. Wang, “Optical behaviour of cholesteric liquid crystal cells with novel photoisomerizable chiral dopants,” Liq. Cryst. 33(3), 237–248 (2006). [CrossRef]

20.

J.-H. Liu and P.-C. Yang, “Synthesis and characterization of novel monomers and polymers containing chiral (−)-menthyl groups,” Polymer (Guildf.) 47(14), 4925–4935 (2006). [CrossRef]

OCIS Codes
(140.3490) Lasers and laser optics : Lasers, distributed-feedback
(140.3600) Lasers and laser optics : Lasers, tunable
(160.3710) Materials : Liquid crystals
(230.3720) Optical devices : Liquid-crystal devices

ToC Category:
Optical Devices

History
Original Manuscript: March 3, 2010
Revised Manuscript: April 16, 2010
Manuscript Accepted: April 20, 2010
Published: April 21, 2010

Citation
Shih-Hung Lin, Cang-Yi Shyu, Jui-Hsiang Liu, Po-Chih Yang, Ting-Shan Mo, Shuan-Yu Huang, and Chia-Rong Lee, "Photoerasable and photorewritable spatially-tunable laser based on a dye-doped cholesteric liquid crystal with a photoisomerizable chiral dopant," Opt. Express 18, 9496-9503 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-9-9496


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References

  1. E. Yablonovitch, “Inhibited spontaneous emission in solid-state physics and electronics,” Phys. Rev. Lett. 58(20), 2059–2062 (1987). [CrossRef] [PubMed]
  2. S. John, “Strong localization of photons in certain disordered dielectric superlattices,” Phys. Rev. Lett. 58(23), 2486–2489 (1987). [CrossRef] [PubMed]
  3. J. Broeng, D. Mogilevstev, S. E. Barkou, and A. Bjarklev, “Photonic crystal Fibers: A New Class of Optical Waveguides,” Opt. Fiber Technol. 5(3), 305–330 (1999). [CrossRef]
  4. J. Tervo, M. Kuitinen, P. Vahimaa, J. Turunen, T. Aalto, P. Heimala, and M. Leppilhalme, “Efficient Bragg waveguide-grating analysis by quasi-rigorous approach based on Redheffer’s star product,” Opt. Commun. 198(4-6), 265–272 (2001). [CrossRef]
  5. P. Tran, “Optical switching with a nonlinear photonic crystal: a numerical study,” Opt. Lett. 21(15), 1138–1140 (1996). [CrossRef] [PubMed]
  6. R. W. Ziolkowski and T. Liang, “Design and characterization of a grating-assisted coupler enhanced by a photonic-band-gap structure for effective wavelength-division demultiplexing,” Opt. Lett. 22(13), 1033–1035 (1997). [CrossRef] [PubMed]
  7. T. D. James, A. C. Greenwald, E. A. Johnson, W. A. Stevenson, J. A. Wollam, T. George, and E. W. Jones, “Nano-Structuredd Surfaces For Tuned Infrared Emission For Spectroscopic Applications,” Proc. SPIE Opt. 2000. Photonics West, San Jose, CA, 22–28. January (2000).
  8. B. S. Song, S. Noda, and T. Asano, “Photonic devices based on in-plane hetero photonic crystals,” Science 300(5625), 1537 (2003). [CrossRef] [PubMed]
  9. P. St. J. Russell, “Photonic crystal fibers,” Science 299(5605), 358–362 (2003). [CrossRef] [PubMed]
  10. M. Lončar, T. Yoshie, A. Scherer, P. Gogna, and Y. Qiu, “Low-threshold photonic crystal laser,” Appl. Phys. Lett. 81(15), 2680–2682 (2002). [CrossRef]
  11. J. P. Dowling, M. Scalora, M. J. Bloemer, and C. M. Bowden, “The photonic band edge laser: A new approach to gain enhancement,” J. Appl. Phys. 75(4), 1896–1899 (1994). [CrossRef]
  12. V. I. Kopp, B. Fan, H. K. M. Vithana, and A. Z. Genack, “Low-threshold lasing at the edge of a photonic stop band in cholesteric liquid crystals,” Opt. Lett. 23(21), 1707–1709 (1998). [CrossRef]
  13. V. I. Kopp, Z.-Q. Zhang, and A. Z. Genack, “Lasing in chiral photonic structures,” Prog. Quantum Electron. 27(6), 369–416 (2003). [CrossRef]
  14. A. Chanishvili, G. Chilaya, G. Petriashvili, R. Barberi, R. Bartolino, G. Cipparrone, A. Mazzulla, R. Gimenez, L. Oriol, and M. Pinol, “Widely tunable ultraviolet-visible liquid crystal laser,” Appl. Phys. Lett. 86(5), 051107 (2005). [CrossRef]
  15. Y. Huang, Y. Zhou, and S.-T. Wu, “Spatially tunable laser emission in dye-doped photonic liquid crystals,” Appl. Phys. Lett. 88(1), 011107 (2006). [CrossRef]
  16. K. Sonoyama, Y. Takanishi, K. Ishikawa, and H. Takezoe, “Position-sensitive cholesteric liquid crystal dye laser covering a full visible range,” Jpn. J. Appl. Phys. 46(36), L874–L876 (2007). [CrossRef]
  17. M.-Y. Jeong, H. Choi, and J. W. Wu, “Spatial tuning of laser emission in a dye-doped cholesteric liquid crystal wedge cell,” Appl. Phys. Lett. 92(5), 051108 (2008). [CrossRef]
  18. G. Petriashvili, M. A. Matranga, M. P. De Santo, G. Chilaya, and R. Barberi, “Wide band gap materials as a new tuning strategy for dye doped cholesteric liquid crystals laser,” Opt. Express 17(6), 4553–4558 (2009). [CrossRef] [PubMed]
  19. J.-H. Liu, P.-C. Yang, Y.-K. Wang, and C.-C. Wang, “Optical behaviour of cholesteric liquid crystal cells with novel photoisomerizable chiral dopants,” Liq. Cryst. 33(3), 237–248 (2006). [CrossRef]
  20. J.-H. Liu and P.-C. Yang, “Synthesis and characterization of novel monomers and polymers containing chiral (−)-menthyl groups,” Polymer (Guildf.) 47(14), 4925–4935 (2006). [CrossRef]

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