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Mode competition of two-lasing modes at the photonic bandedge from dye-doped cholesteric liquid crystal lasing was studied by the alternation of temperatures. The increase or decrease of the wavelengths from photonic bandedges versus the alternation of temperature is attributed to the variation of helical twist power (HTP) and thus it shows the completely different result by choosing two of different nematic liquid crystals (MDA-981602 and MDA-3970). At certain temperature, the intensity contrast and slope efficiency between long and short emission lasing peaks were dominated from the experienced gain or loss of laser for the position of the photonic bandedge. By the linear combination of these two lasing modes with different emission wavelengths and intensity contrast at distinct temperature, the wide tuning of the output colors can be revealed from the CIE chromaticity diagram and thus it has opportunity to be used in the display technology in the near future.
© 2014 Optical Society of America
1. Introduction
Cholesteric liquid crystals (CLCs), with advantages such as easily fabrication and unique optical properties, have been widely applied in many fields including display industry, biomedical diagnostic, and electo-optical modulator, etc [1–4]. To produce CLCs, chiral materials are mixed into nematic LCs (NLCs) and thus the rod-like LC molecules self-assembly align and rotate regularly along the helical axis owing to the character of helical twisting power in chiral materials. Therefore, the refractive index of CLC molecules in cell changes periodically which can be considered as a one-dimension photonic crystal (1D PC). Due to selective reflection property of photonic bandgap (PBG) from CLCs, circular polarized incident light with the same handedness as the CLC helix is reflected whereas the opposite handedness transmits the cell completely and thus it can be used as color filter, optical switch, and can also be regarded as a resonant cavity [4]. Lasing behavior in dye-doped CLCs (DDCLCs) at two-edges of PBG has been theoretically predicted by Dowling et al because the group velocity of photon approaches zero to cause the accumulation of photon density and lower down the lasing threshold [5]. In the past decades, mirrorless lasing at two edges of PBG in DDCLC has been experimentally demonstrated and received considerable attention [6–9]. Owing to the variation of pitch at different oblique angle, Lee et al. had investigated color cone lasing in dye-doped CLCs and showed the tunabilty by the birefringences dependent characteristics [8, 9].
In comparing to traditional laser, one of the superior advantages from the DDCLC laser is that the output characteristics can be simply adjusted by external factors, such as electric field, mechanical stress and temperature [10–15]. A reversible switching of lasing action has been reported in DDCLC cell by the external electric filed to change the period of helical pitch and deform the photonic bnadgap [11]. In order to tune the emission wavelength from DDCLC laser by the electric filed, a relative large voltage about 150 V is applied on a negative dielectric CLC cell for 14 nm variations [12]. On the contrary, the emission wavelength of DDCLC laser reveals wide tunability over 60 nm by the slightly change of temperature as the concentration of doping chiral is over the solubility of the LCs [14]. As temperature alternation, the slope efficiency of DDCLC laser was changed correspondingly that had been attributed to the variation of the efficiency of laser dye and the quality factor of the resonator [15]. Although two lasing peaks at two edge of DDCLC has been observed in previous work [8], there still has no investigation about the interaction of these two emission peaks as the temperature variation [8]. In this work, we report the occurrence and shift of two bandedge lasing after tuning of PBG in dye-doped CLC by the alternation of temperature. Besides, the intensity contrast of these two bandedge emission peaks will compete with each other due to experienced gain and loss.
2. Experimental setup
In this work, the CLC mixtures were prepared by doping 0.5 wt% Pyrromethene 597 (Exciton inc.) laser dye as gain medium. Two different CLC mixtures (sample I and sample II) were produced by using the same left hand chiral material S811 into two different NLCs, in which the sample I was produced by doping 24 wt% S811 into 75.5 wt% MDA-981602 (NLC, Clearing point: 109 °C, Merck inc.) and the sample II was prepared by adding 21 wt% S811 into 78.5 wt% MDA-03-3970 (NLC, Clearing point 90°C, Merck inc.). These two DDCLC mixtures were uniformly mixed in a small vessel, which was put on the hot plate with high enough temperature to make LC transit into isotropic phase, and then fill into an empty glass cell by the capillary effect. The homemade sandwich cell was constructed by two separating ITO-glass plates using 50 μm thickness plastic spacers. The inner sides of the two glass plates were coated by polyimide (PI) and then rubbed in anti-parallel direction. To confirm the existence of the photonic bandgap in our DDCLCs, reflectance spectrum was required that used the white light (DH-2000-BAL, Ocean Optics Inc.) as measured source and spectrometer (EPP-2000, stellar-net Inc.) to obtain the transmission light through the sample. As shown in inset of Fig. 1, when a white light source was incident on the CLC cell, the left circularly polarized light is reflected by the CLC cell comprising of left-handed chiral (S811) and then the right circularly polarized light transmitted through the sample completely.
Fig. 1 Schematic setup for bandedge lasing in DDCLC laser. Inset shows the alignment of the LC molecule in the cell and the selective reflection of light by the CLC.
The schematic setup for bandedge lasing generation from DDCLC is shown in Fig. 1. The DDCLC cell was optically pumped by a linear polarized frequency-doubling Q-switched Nd:YAG laser with central wavelength at 532 nm. The excited pulses with 10 Hz repetition rate and 2.2 ns pulsewidth were spatially expanded three times by two lenses (L1 and L2) with focal length of 2.5 cm and 7.5 cm, respectively. After collimating by two reflection mirrors M1 and M2, the pump beam was focused on the CLC cell through the convex lens (L3) with focal length of 20 cm. A λ/4 wave plate was used to alter the polarization of pump beam from linear polarized light into circular polarization. In this experiment, the DDCLC cell was mounted in a copper block with water cooler or heater to change the temperature of CLC. Besides, a thermal sensor was attached in one side of CLC cell to show the operation temperature. A spectrometer (HR-4000, Ocean Optics Inc.) with a fiber tip was used to measure the emission spectra from DDCLC laser.
3. Results and discussion
After excitation of sample I (MDA-981602) by the Q-switched Nd:YAG laser, the left circular polarized lasing beam was excited whose time trace were measured by the high speed detector and displayed on the oscilloscope. Figure 2 shows the time trace of the output pulse from DDCLC (blue squares) that reveals almost the same duration with the Q-switching Nd:YAG laser (black circles). After fitting with the Eq. (1) from [16], the pulse duration from DDCLC is estimated to be about 2.4 ns. With the energy of excited pulse at 40 μJ/pulse, the measured normal emission lasing spectrum (green curve, θ = 0°), transmission spectrum (blue curve) and projected lasing beam on the screen for the DDCLC from at different temperatures are shown in Figs. 3(a)–3(d) and the red curve represents the fluorescence spectrum of lasing dye (PM597). Like previous report, rob-like LCs in our DDCLC cell are periodically rotated along a helical axis to result in continuously change of refractive index so that a 1D PC can be formed with two lasing peaks at two bandedge of PBG after pumping. In addition, the projected lasing beam on the screen reveals multi-color cone caused by the variation of pitch at different oblique angle (θ) which is defined by the emerged ring relative to the cell normal [9]. Unlike anticipation that two steep edges were observed from the transmission spectrum for the 1D PC, only long wavelength bandedge was revealed and short wavelength bnadedge was disappeared instead of a dip around 532 nm resulted from absorption of the fluorescence molecules as shown in Fig. 3. While the temperature was maintained at 11°C, two lasing peaks at 636.3 nm (long wavelength lasing (LWL) peak) and 563 nm (short wavelength lasing (SWL) peak) were excited simultaneously with FWHM around 0.86 nm as shown in Fig. 3(a). Therefore, the projected lasing beam displayed two overlap color spots (green and red) in the central part (inset of Fig. 3(a)). The intensity of the SWL peak was stronger than the LWL peak since it was near gain maximum of the lasing dye. Similarly, the highest spontaneous emission peak of PL spectrum from PM597 is about 580 nm (red curve in Fig. 3). Thus, a small peak around 580 nm (green curve) after excitation by the Q-switched laser is attributed to the random lasing in the DDCLC caused by some disordering of LCs to produce light scattering of in the cell [17]. Due to the blueshift of the photonic bandedge as the temperature increase to 17°C, two emission peaks varied toward shorter wavelength simultaneously as shown in Fig. 3(b). Besides, the intensity at SWL peak became weaker than that occurring at LWL peak caused by the loss from absorption of laser dye at the shorter wavelength. Thus, the projected green spot on the screen became unapparent as shown in inset of Fig. 3(b). A further blueshift of two emission peaks were exhibited in Fig. 3(c) if we increased the temperature of DDCLC cell at 21°C and then the shorter emission peak was almost hard to be seen and disappeared completely in Fig. 3(d) while the temperature was further increase to 27°C. In Figs. 3(a)–3(d), the intensity contrast between long and short lasing wavelengths changed obviously to indicate mode competition between two bandedge lasing modes due to their experienced gain and loss when the photonic bandedge varied toward shorter wavelength as temperature increased.
Fig. 2 The time trace of the output from DDCLC (blue squares) laser, Q-switching Nd:YAG laser (black circles) and the fitting curve(red line).
Fig. 3 The measured lasing spectrum (green curve) and transmission spectrum (blue curve) and the corresponding output beam (inset) for sample I with temperature at (a) 11°C, (b) 17°C, (c) 21°C and (d) 27°C. The red curve shows the fluorescent spectrum of PM597.
Besides, we investigated the output emission intensity versus excitation pulse energy for LWL (blue squares) and SWL (red circles) peaks from DDCLC at two different temperatures (11°C (Fig. 4(a)) and 17°C (Fig. 4(b))). By the linear fitting, two slopes, lower and higher lower slopes, were obtained in this figure to show the spontaneous emission at lower excitation and the stimulated emission while the excited energy was above certain threshold value. At 11°C, the SWL peak reveals higher slope efficiency than that of LWL peak since the fluorescence molecule has higher gain at short wavelength. However, as the temperature increases to 17°C, the slope efficiency of LWL emission peak became stronger than that of SWL peak because of the enlarged loss caused by the absorption of the fluorescence dye. As temperature variation in Figs. 4(a) and 4(b), the alternation of slope efficiency between two bandedge lasing modes was also the other proof to show the obvious mode competition between these two lasing emission peaks.
Fig. 4 The output intensity versus pump energy of DDCLC for LWL peak (blue squares) and SWL peaks (red circles) at (a) 11°C and (b) 17°C (Solid lines are linear fitting lines).
Figure 5 illustrates the variation of LWL and SWL emission peaks as we change the temperature of CLC cell. Unlike previous report in DDCLC with discontinuous stepwise shift in the laser wavelength [13], the wavelengths of both emission peaks decrease continuously as temperature increase. The variation of the wavelengths of LWL and SWL (λL and λS) are determined by the bandedge of 1D PC with the formula [18]:
where ne and no are ordinary and extraordinary refractive index of the liquid crystal and P is the helical pitch of CLC that can be determined by the equationin which HTP and C are helical twist power and concentration of doping lasing dye. The shift of bandedge versus temperature relates to the temperature gradient of the ordinary index and extraordinary index (dno/dT and dne/dT) as well as the helical pitch P. As report, the temperature gradient of ordinary index shows increase tendency with the temperature in previous result [19, 20] that can’t be the reason to cause the shift toward the short wavelength for the SWL peak at higher temperature. Besides, Huang et al. have tried to tuning the photonic band gap by the temperature while the doping chiral in NLC is beyond the maximum solubility [5]. In their report, the shift of the photonic band edge is very sensitive to the temperature due to the variation of doping concentration. In this work, we have measured the variation of long (red solid squares) and short (red open circles) wavelengths bandedge of CLC versus the doping concentration of the S811 in NLC (MDA-981602) as shown in Fig. 6. The blue and red curves are fitting lines using Eqs. (1) and (2) in which the values of ne and no from MDA 981602 are 1.7779 and 1.5113, respectively, and the value of HTP is 10.8 μm−1. Both long and short edges showed the decline tendency with doping concentration and no obvious saturation situation was observed while the used concentration of the S811 was as high as 38%. Thus, we believe that the concentration of chiral about 24 wt% doesn`t change obviously as temperature increase so that the variation of the HTP might be the main reason to change the photonic bandedge in our DDCLC cell. The variation of the HTP versus the temperature was calculated from the wavelength of long emission peak by the Eq. (1) and Eq. (3) and set the refractive index and concentration as fixed values. As temperature increases from 11°C to 27°C, the helical twist power (blue triangles) of the dye-doped CLC ascends slowly from 11.64 μm−1 to 11.95 μm−1 as shown in Fig. 5.Fig. 5 The variation of wavelength for LWL (red solid squares) and SWL (red open circles) and estimated helical twist power (blue triangles) versus temperature for sample 1.
Fig. 6 The dependence of wavelengths for LWL (red squares) and SWL (black circles) on concentration of S811 into MDA-981602.
We also study the temperature dependent bandedge lasing from DDCLC by doping 21 wt% of chiral S811 into the other NLC (MDA-3970, sample II) with the energy of excited pulse at 40 μJ/pulse as shown Fig. 7. Dissimilar to the previous result shown in Fig. 3, both photonic bandedges were simultaneously redshifted as temperature increased. At lower temperature about 15°C, only one emission peak, with central wavelength at 588 nm, was excited at short photonic bandedge (Fig. 7(a)). Because the short bandedge was near the maximum gain of fluorescent dye, the contrast of obtained gain between short and long emission modes was relative large (red curve in Fig. 7(a)) so that all the gain was depressed by the short bandedge lasing mode. Thus, longer bandedge lasing mode didn’t have opportunity to grab enough gain for lasing. In addition to the central orange color spot, the corresponding projected beam on the screen (inset of Fig. 7(a)) revealed two outer rings. When the temperature was increased to 30°C, the emission peak shifted toward longer wavelength about 601 nm in accompanying with the intensity decrease due to deviation of the maximum gain. It is noted that the other emission peak at long photonic bandedge was generated at 657 nm after further increase of temperature at 40 °C as shown in Fig. 7(c). The generation of long emission peak was due to the reason that the gain gradient diminish at longer wavelength so that gain contrast between long and short wavelength bandedge became smaller and thus the longer emission peak had more chance to obtain gain. As temperature at 45°C, the intensity of both emission peaks increased suddenly as shown inset Fig. 7(d) that was attributed to disappearance of the inner ring and therefore the central spot could acquire more gain (inset of Fig. 7).
Fig. 7 The measured lasing spectrum (green curve) and transmission spectrum (blue curve) and corresponding output pattern (inset) for sample II with temperature at (a) 15°C, (b) 30°C, (c) 40°C and (d) 45°C. The red curve shows the fluorescent spectrum of PM597.
The variation of the emission peaks versus the temperature for DDCLC of sample II (using MDA-3970 and S811 in Fig. 7) are described in Fig. 8. In contrast to the sample I in Fig. 5, the measured long and short bandedge emission peaks in Fig. 8 shifted toward longer wavelengths as temperature increased from 15° C to about 50° C. As mentioned previously, the lasing peaks from DDCLC are generated at the two bandedge of the PBG that can be estimated by the Eqs. (1), (2) and (3). Thus the wavelength of lasing peaks from DDCLC are determined by the parameters including refractive index, concentration of the lasing dye and the helical twist power. However, the extraordinary refractive index ne decreases [19, 20] and the concentration increases with the temperature to result in the shift of photonic bandgap toward the short wavelength that can`t explain the shift of photonic bandgap toward the long wavelength shown in Fig. 7. Therefore, the shift of the PBG of DDCLC in Figs. 3 and 7 are resulted from the variation of the HTP using two different kinds of NLCs. As temperature increases, the pitch between LC molecular planes varies that is due to geometric change of two different NLCs to result in the shift of PBG toward the long wavelength for MDA 981602 (blue curve in Fig. 3) and the short wavelength for MDA 3970 (blue curve in Fig. 7). The shift of PBG and variation of HTP can also be demonstrated from the reflection colors of sample I and sample II at different temperature using white light source as shown in Fig. 9(a). The reflection color varies as temperature increase from yellow to green for sample I (Figs. 9(b)–9(d)) to show the shift of PBG toward short wavelength and from yellow to red for sample II (Figs. 9(e)–9(g)) to indicate the shift of PBG toward long wavelength.
Fig. 8 The variation of wavelength for LWL (red solid squares) and SWL (red open circles) and estimated helical twist power (blue solid triangles) versus temperature for sample II.
Fig. 9 (a) Photography of the reflection color from DDCLC at different temperature using white light source. The reflection light from simple I (MDA 981602) with temperature at (b) 11°C, (c) 17°C, and (d) 27°C. The reflection light from sample II (MDA 3970) with temperature at (e) 15°C, (f) 30°C, and (g) 45°C.
In considering the short emission peak with Eqs. (2) and (3), the calculated values of HTP of the DDCLC (sample II) reveals declined tendency from 11.76 μm−1 to 10.96 μm−1 as shown in Fig. 8. In order to show the temperature tunability of output color for the two bandedge lasing, we calculated the Commission Internationale de L’Eclairage (CIE) chromaticity diagram from the central spot of output beam at various temperatures that are represented at the color coordinates in Fig. 10. Because the colors of lasing peaks from sample I (solid symbols) and sample II (open symbols) have high color purity, they are located at the edge of the chromaticity diagram at different temperature. In addition, the output emission from sample I reveals wide tenability than the sample II because simultaneously excited of green and red colors.
Fig. 10 The calculated CIE chromaticity diagram for lasing emission peak from sample I (solid symbols) and II (open symbols) under different temperatures.
4. Conclusion
In this work, we investigate the temperature tuning of photonic band gap of dye-doped cholesteric crystal to observe the mode competition of two bandedge lasing. For one of the DDCLC (MDA-981602), two of bandedge emission peaks, green and red colors, were excited simultaneously at low temperature and shift toward the short wavelength at higher temperature. The intensity contrast and slope efficiency of long and short emission peaks changed at different temperature that was attributed by the experienced gain and lose at position of photonic bandege. Due to increase of absorption from fluorescent molecule, the short emission peak disappeared completely at higher temperature. For the other DDCLC (MDA-3970), only short emission peak could be excited at lower temperature around the maximum gain of fluorescent dye and shifted toward the longer wavelength at higher temperature caused by the shift of the photonic band gap. Due to decrease of gain gradient, the longer emission peak was excited at higher temperature because it had chance to grab some gain from active medium. The shift of the photonic bandedge versus temperature is attributed to the variation of the HTP whose variations are estimated from experimental result. Finally, we plot the CIE chromaticity diagram and demonstrate that the wide tuning of the output color from DDCLC laser can be achieved by the variation of temperature due to proper combination of the long and short emission peak with different central wavelengths and intensity contrast.
Acknowledgments
This work is financially sponsored by National Science Council in Taiwan, R.O.C., under the grant no. NSC 102-2112-M-027-001-MY3 and NSC 102-2221-E-027-103.
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