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This letter addresses a dye-doped planar cholesteric cell as a one-dimensional photonic crystal, which can be lased at the band edges of the photonic band gap. The effect of the composition of the material and the thickness of a cholesteric cell (CLC) on the lasing action, and the photo-control of the lasing frequency, are experimentally investigated. Adding a tunable chiral monomer (TCM) allows the CLC’s reflection band to be tuned by varying the intensity and/or exposure time of the UV curing light, enabling the lasing frequency of the CLC sample to be tuned.
©2004 Optical Society of America
The use of liquid crystals as photonic crystals has been actively investigated. Photonic crystals (PCs), which consist of a periodic dielectric structure with a periodicity in the range of optical wavelengths, have attracted great attention, because of their potential fundamental and practical uses.1 Analogous to electronic bandgaps in semiconductors, photonic crystals have a band gap. The group velocity vg of the photonic band edge (PBG) is real and tends towards zero, so the gain is greatly enhanced at the edge of the photonic band.2, 3
Planar cholesteric liquid crystals (CLCs) with a pitch that is comparable to the optical wavelength can be considered to be a one-dimensional photonic bandgap material. When linearly polarized light propagates into a right-handed CLC cell along the helical axis, right-handed circularly polarized light is reflected if the wavelength of light λ equals np, λ=np, where n is the average refractive index of a liquid crystal and p is the pitch of the helix.
The lasing action can be expected at the edges of the stop band in the one-dimensional photonic crystal (1D PC), in which the density of states for light exhibits a narrow singularity. Equivalently, the group velocity approaches zero for light at the band edges. Dye-doped CLC, which is regarded as 1D PC, has been observed to exhibit laser activity at the band edges.4, 5 The main role of the CLC in these laser systems is as a distributed cavity host for active materials, such as DCM laser dyes used in the experiment performed herein. Only a few reports have investigated the control of laser action in the dye-doped CLC system using the external stimuli of mechanical stress,6 temperature7 and electric fields.8 This letter reports results obtained from studies of lasing effects in dye-doped CLC cells, for various material compositions and thicknesses of a CLC cell. The tuning of the lasing frequency has also been experimented on by adding a tunable chiral material (TCM).
A right-hand CLC sample was prepared by mixing a chiral material with a nematic liquid crystal (E7, Merck) in an appropriate ratio. The Bragg reflection edges of the CLC sample should be within the emission spectrum of the laser dye to induce lasing from a dye-doped CLC sample. The chiral system herein was composed of a blend of CB15, CE1 and R1011 in a ratio of 3:3:1 by weight. Additionally, a small amount of pre-polymer (RM257, Merck) was added. After homogeneously mixing, ~0.5 wt % of a laser dye, 4-(dicyanomethylene)-2-methyl-6- (4-dimethlyaminostryl)-4H-pyran (DCM, Exciton), was dissolved in the cholesteric host. The compound was injected into an empty cell that was made from two glass plates coated with indium-tin oxide (ITO) and separated by a 25μm-thick spacer. The surface of the glass plates was coated with a polyimide and rubbed to promote a homogeneously aligned cell. The formed sample was a planar CLC whose helical axis was perpendicular to the surface of the cell. The sample was finally UV-cured to polymerize the RM257 into polymer networks, which were employed to stabilize the planar texture.
The pumping source of the DCM-doped CLC cells was a single pulse of the second harmonic generation (SHG) (λ=532 nm) of a s-polarized Q-switched or a mode-locked Nd:YAG laser. The durations of the mode-locked and Q-switched pulses were approximately 30 ps and 8ns, respectively. The pumping laser was focused on to the sample with a diameter ~ 300 μm at an angle of incidence of 45° to the normal of the surface. The CLC cells were lased in the direction of the surface normal. A detector connected to a spectrometer, monitored the lasing intensity from the cell.
Figure 1 presents the lasing pattern obtained from the DCM-doped CLC to which had been added ~ 4wt% polymer using a Q-switched Nd:YAG pulse with an intensity of 13μJ. The lasing frequency was measured to be ~ 600 nm, corresponding to the longer wavelength edge of the stop band of the CLC, and the polarization was right-handed circular. Notably, a CLC sample without polymer-stabilization was pumped using a Q-switched Nd: YAG laser pulse. No stabilized lasing was observed because the pumping laser heats the cell, disturbing its structure and, therefore, the Bragg reflection was broken. When the polymer was added in the form of a polymer network to stabilize the CLC texture,9, 10 lasing was clearly observed. Fig. 1 demonstrates the existence of a ring structure. Such a ring pattern is caused by the coherence of the output light, which is evidence of the lasing action. When the pumping source was switched to the mode-locked Nd:YAG laser (with a pulse duration of ~30ps, and an intensity of ~1μJ), lasing was observed easily without the addition of a polymer, because the effect of heating was much weaker.
Fig. 1. Lasing pattern of the dye-doped cholesteric liquid crystal cell stabilized with polymer. The left green light is the pumping light.
Figure 2 depicts the reflection band of CLCs with different chiral concentrations without the addition of polymer. The cell gap of these CLC samples was 12μm. It is seen from Fig. 2 that the reflection band shifts to the shorter wavelength as the chiral concentration is increased. This is reasonable since the pitch length of the formed CLC decreases as the chiral dopant increases. Figure 3 shows the variations of the lasing wavelength with the concentration of the chiral dopant and the polymer added to the CLC samples. The pumping source was the mode-locked Nd:YAG laser with an intensity of ~1μJ. The reflection band edge of the CLC sample at the long wavelength was made within the emission spectrum of the dye that at the short wavelength was not. Figure 3 clearly indicates that the lasing wavelength is shifted to shorter wavelengths as the concentration of the chiral dopant increases, clearly because the pitch length decreases as the chiral concentration increases. The lasing wavelength is also shifted toward longer wavelengths as the polymer concentration increases. When the concentration of the polymer is increased, the formed polymer networks are expected to become denser, generating more domains of imperfect planar texture. Consequently, the band gap is widened. Accordingly, the lasing wavelength at the band edge increases.
Fig. 3. Variations of the lasing wavelength with the concentration of the chiral material and the polymer added to the sample. The pumping source was the SHG of a mode-locked Nd:YAG laser pulse with an intensity of ~1μJ.
Figure 4 gives the reflection spectrum of CLCs with different cell gaps. The thicker the CLC cell has the more pitch layers in the sample, and, thus, its band edges of reflection spectrum are sharper. A wedge CLC cell without adding polymer, as depicted in Fig. 5 was designed to determine the effect of the thickness of the cell on the lasing action. The spacing between the cells at one side was zero, and that on the other side was 50μm. The angle between the two glass substrates was ~4.2×10-4 rad. Observations were made of the lasing action caused by pumping the wedge cell at different sites with a single mode-locked Nd:YAG laser pulse with an intensity of ~1μJ. The variance in pitch is known to be much larger in the thin region than in the thick one.11 For example, the four-layer region exhibits p/4 variance, while the nine-layer region exhibits p/9 variance. Figure 6 displays the lasing spectrum from the wedge cell pumped at sites at three thicknesses ~ 2μm, 3μm and 10μm Figure 6 demonstrates that the lasing spectrum exhibits a multi-spike structure associated with sites at thicknesses of 2μm and 3μm (Fig. 6(a) and Fig. 6 (b)), but exhibits a single-spike structure at the site at the 10μm-thickness (Fig.6 (c)). These results are caused directly by the variation in the pitch across the various thicknesses, as described above. It should be noted that the intensity scale used in Figs. 6(a) and 6(b) is relatively smaller than that of Fig. 6(c). Thus, the former two curves corresponding to the lasing at the sites with smaller cell gaps (2 and 3μm) have a larger background than that of the latter one which is the lasing in the site at the 10μm-thickness.
Fig. 6. Lasing spectrum from the wedge CLC cell at various cell gaps; (a) 2μm, (b) 3μm and (c) 10μm.
Finally, a tunable chiral material (TCM, (-)-bornyl methacrylate12, 13) was added. This TCM is a chiral monomer with the same chirality as the aforementioned chiral system. The pitch (P) of a cholesteric LC can be controlled by varying the concentration of the chiral dopant. It is given by (at concentrations under 10%)14
where β0 (C0) and βTCM (CTCM) are the twist powers (concentration) of the chiral and TCM, respectively. Accordingly, C0 and CTCM can be adjusted to tune the reflection band. Upon irradiating the mixture with UV light, the TCM becomes polymerized, and CTCM decreases with time. This results in an increase in the pitch P, which, in turn, causes an increase in the lasing wavelength.
In this part of the experiment, the CLC’s long reflection band edge at ~640nm was obtained before adding TCM. TCM was added to shift the CLC’s reflection band edge to 605nm. Figure 7 shows the variation of the lasing frequency of the CLC with the duration of UV irradiation (10 mW). As expected, curing the sample for 350 s increases the lasing wavelength from 605nm to 640nm. Thereafter, the TCM is completely polymerized and the lasing wavelength has returned to the initial value.
Fig. 7. Variation of the lasing wavelength from a CLC cell to which TCM has been added, with duration of UV irradiation.
In conclusion, the lasing characteristics of DCM-doped planar choselteric cells were studied. Polymer must be added to the sample for lasing when it is pumped by a Q-switched (ns) Nd:YAG laser pulse. The polymer forms networks to stabilize the texture of the cell's Bragg reflection during the pumping that heats and disturbs the Bragg-reflection characteristics of the sample. Using a pico-second pulse as the pumping source eliminates the effect of heating. For lasing a single-spike pulse, the cell should be sufficiently thick to have ~10 layers (pitch). Adding a TCM to the sample enables the lasing wavelength to be tuned by varying the curing time.
Acknowledgments
The authors would like to thank the National Science Council (NSC) of the Republic of China (Taiwan) for financially supporting this research under Contract No. NSC 91-2112-M-06-019.
References and links
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