Electrically controllable laser based on cholesteric liquid crystal with negative dielectric anisotropy

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Electrically controllable laser based on cholesteric liquid crystal with negative dielectric anisotropy

Tsung-Hsien Lin and Hung-Chang Jau

Institute of Electro-optical Science and Engineering, National Cheng Kung University, Tainan, Taiwan 701, Republic of China

Ching-Hsu Chen and Yi-Jan Chen

Department of Physics, National Cheng Kung University, Tainan, Taiwan 701, Republic of China Tai-Huei Wei and Chen-Wei Chen

Department of Physics, National Chung Cheng University, Chia-Yi, Taiwan 621, Republic of China Andy Y.-G. Fuh^a兲

Department of Physics and Institute of Electro-optical Science and Engineering, National Cheng Kung University, Tainan, Taiwan 701, Republic of China

共Received 16 August 2005; accepted 2 December 2005; published online 10 February 2006兲 This work examines a planar cholesteric liquid crystal 共CLC兲 cell with a negative dielectric anisotropy, doped with laser dye, as an electrically tunable one-dimensional photonic crystal laser device. The lasing wavelength is demonstrated to be tunable by applying a voltage. Additionally, lasing can be switched on and off changing the frequency of the applied voltage. Wavelength tuning caused by the shift of the reflection band of CLC is attributed to the electrohydrodynamical effect in the negative dielectric cell. © 2006 American Institute of Physics. 关DOI:10.1063/1.2168259兴

Photonic crystals共PCs兲, which have a periodic dielectric structure with a periodicity in the range of optical wave- lengths, have attracted considerable attention because of their fundamental importances and practical applications.¹ The propagation of light in such PCs is analogous to the well-known wavelike propagation of electrons in a crystal- line band-gap structure, such as a semiconductor. The density of states of light has a narrow singularity near the photonic band edge, or equivalently, the group velocityvgof the band edge tends toward zero, so lasing action is expected at the band edge if the gain inside the photonic band-gap material is high enough.^2,3

Planar cholesteric liquid crystals 共CLCs兲共Ref. 4兲 with self-organized helical structures can be considered to be a one-dimensional photonic band-gap material. When the band edge overlaps the emission spectrum共gain bandwidth兲 of the laser dye, and the optical pumping is sufficiently strong, lasing of dye-doped CLC is observed at the band edges.^5–15The CLC in the system is a distributed cavity host and laser dye is the active material. The central wavelength of the reflection band is the product of the mean refractive index of the liquid crystal 共LC兲 and the pitch of CLC, ␭=np, and the reflection bandwidth is the product of the refractive index anisotropy 共⌬n⬅ne− n₀, where ne共n0兲 is the extraordinary 共ordinary兲 refractive index of a LC兲 and the pitch, i.e., ⌬␭

=⌬np. Right-handed circularly polarized laser output is ob- served when the sample cell has a right-handed CLC. The benefit of a dye-doped CLC laser system is that its lasing wavelength can be tuned by varying the pitch of CLC. Over the past few years, several methods have been developed to control the lasing action in a dye-doped CLC system. For instance, changing the concentration ratio of the chiral dopant,¹¹using a concentration gradient,¹⁵and external con-

ditions, such as mechanical stress,^7–9 temperature,¹⁰ electric field,¹¹ and optical field, were able to control the laser operation.^12,13

We previously reported dye-doped CLC lasing by an optical method.^16,17Adding a tunable chiral monomer or a chiral azo dye enables the lasing wavelength to be tuned when the sample cell is exposed under ultraviolet light. Other groups have reported an electric method;¹¹ however, it is difficult to tune the laser wavelength by electrical method.

This work reports the control of dye-doped CLC lasing by applying an electric field. The experiments show that the dye-doped CLC laser wavelength can be tuned by changing the applied dc voltage. Furthermore, the laser can be switched on/off by controlling the frequency of an applied ac voltage.

A negative dielectric anisotropy CLC sample was pre- pared by mixing a chiral material共S811, Merck兲 with negative dielectric anisotropy 关⌬␧⬅␧储−␧_⬜⬍0, where ␧储共␧_⬜兲 is the dielectric constant of a LC with its director axis being parallel共perpendicular兲 to the applied E field兴 nematic LCs 共95% MLC6608 and 5% ZLI2293, Merck兲 in an appropriate ratio共⬃67:33兲. Following homogeneous mixing, ⬃0.5 wt%

of laser dye, 4-共dicyanomethylene兲-2-methyl-6-共4- dimethlyaminostryl兲-4H-pyran 共Exciton兲 was dissolved in the cholesteric host. Notably, the Bragg reflection edges of the CLC sample had to be within the emission spectrum of the laser dye to cause lasing. The final compound was in- jected into an empty cell that had been made from two glass plates coated with indium-tin oxide and separated by a 15␮m thick spacer. The surfaces of the glass plates were coated with a polyimide and rubbed to promote the homogeneous alignment of the cell. The sample thus formed was a planar CLC whose helical axis 共parallel to the z axis兲 was perpendicular to the surface of the cell.

Figure 1 presents the transmission spectrum of a sample to which dc voltages of 0, 100, and 200 V were applied along

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the helical axis. Clearly, the central wavelength of the reflection band is blueshifted when a dc voltage is applied. The threshold voltage for the shifting of a band is around 30 V.

Most importantly, the spectral edges are very sharp and the band shift is distinct. This band shift characteristic differs from that of a homogeneously aligned CLC system that exhibits positive dielectric anisotropy. In the case of positive dielectric anisotropy, the reflection band is distorted and the blueshift in the red wavelength region is almost negligible.¹⁸ When a dc field is applied along the helical axis in planar CLC texture, the sample tends to undergo conical deforma- tion, so the reflection band is strongly distorted. Applying a weak dc voltage slightly blueshifts the central wavelength in the green wavelength region, but the reflection edges are not sharp.¹⁸ The blueshift is caused by the pitch contraction¹⁹ and the periodic distortion of the planar texture,^20,21accord- ing to a calculation that accounts for only the elastic defor- mation energy and the electric field energy. However, an electric field applied along the helical axis further stabilizes the CLC planar texture when the dielectric anisotropy is negative. The blueshift in Fig. 1 is believed to be caused by the electrohydrodynamical effect. Helfrich theory^22,23 indi- cates that when a field is applied along the helical axis of a planar CLC cell with a negative dielectric anisotropy and a positive anisotropic electric conductivity, the induced distortion of the LCs causes the segregation of space charges. The space charges interact with the electric field, causing the LCs to flow. The flow is accompanied by a shear stress and then the shear applies a torque on the LC molecules. The shear- induced torque tends to alter the direction of the preferred axis and so reacts to the orientation pattern to form the sinu- soidal periodic distortion of a planar CLC cell. Such a periodic distortion is responsible for the blueshift of the CLC reflection spectrum. The shift depends on the extent of the distortion. The band shift presented in Fig. 1 can be used to realize a tunable CLC laser.

Figure 2 presents the transmission spectra of a CLC cell when 100 V ac with frequencies of 0, 30, and 50 Hz, is applied. The central wavelength of the reflection band is not shifted. However, the reflection edge is broad and decreases as the ac frequency is increased to 50 Hz. The decline in the transmission and the broadening of the reflection band are caused by dynamic scattering of the sample. In fact, dynamic instability is clearly observed under a polarized optical mi- croscope. Therefore, CLC can be switched on共off兲 by varying the frequency of the applied voltage between 0 and 50 Hz.

The pumping source is a single pulse of second- harmonic generation 共␭=532 nm兲 from a Q-switched Nd:YAG laser. The duration of the Q-switched pulse is around 8 ns. A focusing lens with a focal length of 10 cm was used to focus the pump laser on to the sample at an angle of incidence of 45°. The diameter of the pumping spot on the sample cell is estimated to be approximately 500␮^m.

The CLC cell lased in the direction of the normal of the surface. A detector connected to a spectrometer records the lasing output from the cell.

Figure 3 presents the fluorescence spectrum of the dye- doped CLC sample pumped by a single pulse when the pumping intensity is less than the lasing threshold共⬃1␮^J兲.

Clearly, the dye’s emission in the photonic band of CLC between 605 and 643 nm is reduced, but is enhanced at the edges of the reflection band. Notably, the lasing threshold of the long wavelength edge is lower than that of the short- wavelength edge, so lasing is expected at the long- wavelength edge of the reflection band.

Figure 4 shows the lasing spectra of the CLC sample when a dc voltage of 0 or 150 V is applied. The lasing wavelength is 643 nm when 0 V is applied and 629 nm when 150V is applied. Figure 5 plots the lasing wavelength of the sample against the applied dc voltage. The blueshift of the reflection band causes a blueshift of the lasing wavelength.

Further increasing the dc voltage ends the laser action because it breaks the quasi-planar texture.

FIG. 1. Transmission spectra of a CLC sample with negative dielectric anisotropy when dc voltages of 0, 100, and 200 V are applied along the helical axis.

FIG. 2. Transmission spectra of the same CLC sample as yielded the results in Fig. 1, when 0, 30, and 50 Hz 100 V_ACis applied.

FIG. 3. Emission spectrum of dye-doped CLC sample pumped by a single pulse with an intensity below the lasing threshold共⬃1␮J兲.

FIG. 4. Lasing spectra of a CLC cell when 0 and 150 V dc are applied.

061122-2 Lin et al. Appl. Phys. Lett. 88, 061122共2006兲

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When the applied field is switched from dc to 50 Hz ac, while the amplitude is maintained at 100 V, the lasing disap- pears, because the electrohydrodynamic instability causes the dynamic scattering of the sample, reducing the transmission and broadening of the reflection band, as presented in Fig. 2.

Therefore, the CLC lasing can be switched on共off兲 by applying a dc共ac兲 voltage.

In conclusion, this work establishes the feasibility of a CLC laser with both electrically tunable and switchable ca- pabilities. CLC with negative dielectric anisotropy exhibits an electrohydrodynamic effect when an electric field is applied. The lasing can be switched on共off兲 by applying a dc 共ac兲 voltage. Additionally, the lasing wavelength can be tuned by altering the applied dc voltage.

The authors would like to thank the National Science Council共NSC兲 of the Republic of China 共Taiwan兲 for finan- cially supporting this research under Contract No. NSC 93- 2112-M-06-016.

1J. D. Joannopoulos, R. D. Meade, and J. N. Winn, Photonic Crystals-

Molding the Flow of Light共Princeton University Press, Princeton, NJ, 1995兲.

2J. P. Dowling, M. Scalora, M. J. Bloemer, and C. M. Bowden, J. Appl.

Phys. 75, 1896共1994兲.

3V. I. Koop, B. Fan, H. K. Vithana, and A. Z. Genack, Opt. Lett. 23, 1707 共1998兲.

4P. G de Gennes and J. Prost, The Physics of Liquid Crystals共Clarendon, Oxford, 1995兲.

5A. Munoz, P. Palffy-Muhoray, and B. Taheri, Opt. Lett. 26, 804共2001兲.

6L. S. Goldberg and J. M. Schnur, U.S. Patent No. 3,771,065共November 6, 1973兲.

7H. Finkelmann, S. T. Kim, A. Munoz, P. Palffy-Muhoray, and B. Taheri, Adv. Mater.共Weinheim, Ger.兲 13, 1069 共2001兲.

8T. Matsui and R. Ozaki, Appl. Phys. Lett. 81, 3741共2002兲.

9J. Schmidtke and W. Stille, Adv. Mater.共Weinheim, Ger.兲 14, 746 共2002兲.

10M. Ozaki, M. Kasano, D. Ganzke, W. Hasse, and K. Yoshino, Adv. Mater.

共Weinheim, Ger.兲 14, 306 共2002兲.

11S. Furumi, S. Yokoyama, A. Otomo, and S. Mashiko, Appl. Phys. Lett.

82, 16共2003兲.

12A. Chanishvili, G. Chilaya, and G. Petriashvili, Appl. Phys. Lett. 83, 5353 共2003兲.

13S. Furumi, S. Yokoyama, A. Otomo, and S. Mashiko, Appl. Phys. Lett.

84, 2491共2004兲.

14P. V. Shibaev, J. Madsen, and A. Z. Genack, Chem. Mater. 16, 1397 共2004兲.

15A. Chanishvili, G. Chilaya, G. Petriashvili, R. Barberi, R. Bartolino, G.

Cipparrone, A. Mazzulla, and L. Oriol, Adv. Mater.共Weinheim, Ger.兲 16, 791共2004兲.

16A. Y.-G. Fuh and T.-H. Lin, Opt. Express 12, 1857共2004兲.

17T. -H. Lin, Y. J. Chen, C. H. Wu, and A. Y.-G. Fuh, Appl. Phys. Lett. 86, 161120共2005兲.

18G. Chilaya, G. Hauck, H. D. Koswig, G. Petriashvili, and D. Sikharulidze, Cryst. Res. Technol. 32, 401共1997兲.

19C. J. Gerritsma and P. V. Zanten, Phys. Lett. 37A, 47共1971兲.

20R. B. Meyer, Appl. Phys. Lett. 12, 281共1968兲.

21W. Helfrich, Appl. Phys. Lett. 17, 531共1970兲.

22W. Helfrich, J. Chem. Phys. 51, 4092共1969兲.

23W. Helfrich, J. Chem. Phys. 55, 839共1971兲.

FIG. 5. Variation in lasing wavelength of the sample when dc voltage is applied.

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