1. Introduction

The unique advantage of lasers associated with liquid crystals (LC) is the highly flexible controllability of their lasing characteristics or performances when subjected to external stimuli using optical, electrical, and/or thermal methods; such advantage is a result of the flexible controllability of the LC structure. Considerable research has been conducted on these lasers in the last decade [1–12]. Lasers based on dye-doped cholesteric liquid crystals (DDCLCs) are the most attractive area among the topics in this research not only because of their easy fabrication and unique photonic band structure but also because of the high tunability of their lasing wavelength and high performance of their lasing features, such as low energy threshold [7–12].

CLCs exhibit a large one-dimensional (1D) modulation of the refractive index, in which rod-like LC molecules may self-organize and rotate continuously along the helix axis through the interaction of the LCs with chiral molecules, which in turn leads to the formation of a 1D planar CLC structure [13]. Such a planar CLC can be considered a 1D photonic crystal (PC) with certain photonic bandgaps (PBGs). Within these PBGs, the incident optical eigenmode wave with the same handedness as the CLC helix is completely reflected, and the other wave with the opposite handedness transmits entirely. The spontaneously emitted fluorescence in a DDCLC is inhibited within the PBGs and is enhanced at the PBG edges. The effective multi-reflection of the fluorescence at the band edges principally lead to a very small group velocity and a large density of photonic state (DOS), which in turn results in a long dwelling time inside the CLC microresonator [13]. Under the distributed feedback of the active DDCLC multilayer in the multi-reflection process, both the spontaneous and stimulated emission rates of the fluorescence at the band edges can be amplified to obtain a high gain exceeding loss and therefore induce a low-threshold lasing emission [13].

Researchers have recently reported a unique laser phenomenon, the so-called color cone lasing emission (CCLE), which is based on single-pitched planar DDCLC cells [14]. These lasers may individually generate both a wide-banded (continuous distribution of lasing wavelength) and a conically symmetrical lasing emission at the same time. The lasing wavelength at the long wavelength edge (LWE) of the reflection band continuously decreases as the cone angle continuously increases. This lasing phenomenon is more distinct than those based on other previously developed lasers, including the traditional DDCLC laser, the solid, liquid, and gas laser commodities. In the traditional DDCLC laser, at most two edge lasing signals can simultaneously emit along the cell normal [7–10]. Although DDCLC lasers have been thoroughly studied after Kopp et al. developed the first DDCLC laser in 1998 [7], the CCLE effect could not be easily discovered and detected until the authors identified its existence and announced a series of related investigations in recent years [14–19]. The present work investigates the performance evolution of the CCLE laser in DDCLC cells at different fabrication conditions to fully realize the CCLE phenomenon and determine the key factor(s) that may influence its generation in the self-organizing process of the CLC. The experimental results show that the energy threshold (E_th) and relative slope efficiency (η_s) of the lasing signal emitted at each cone angle in the CCLE decreases and increases, respectively, with an increase in the waiting time based on the homogeneously-aligned DDCLC cell. The findings of this study show that the degree of the structural perfection of the CLC significantly influences the performance of the obtained CCLE. Through the pretreatment of a simple rapid annealing processing (RAP) on the aligned cell, the waiting time of the CLC for obtaining an optimum CCLE can be significantly shortened sixfold compared with that based on the same aligned cell with no RAP pretreatment. The surface alignment on the cell also has a necessary function in generating CCLE. With no surface alignment in the DDCLC cell, no CCLE can occur no matter whether the cell has underwent the RAP or how long the waiting time is.

2. Sample preparation and experimental setups

The DDCLC mixture used in this study consists of a nematic LC, MDA-03-3970 (n_e = 1.6309, n_o = 1.4987 at 20 °C) (from Merck), left-handed chiral dopant, S811 (from Merck), and laser dye, 4-dicyanmethylene-2-methyl-6-(p-dimethylaminostyryl)-4-H-pyran (DCM, from Exciton). The mixing ratio of MDA-03-3970:S811 in the CLC mixture is 77.4:22.6 wt.%. The concentration of DCM doping in the CLC mixture is approximately 0.5 wt.%. Each empty cell is fabricated by combining two indium-tin-oxide-coated glass slides with the dimensions of 2.5 cm × 2 cm and is then separated using 33 μm-thick plastic spacers. Two types of empty cells with and without homogenously rubbed alignment layers are prepared. The uniformly mixed DDCLC is then injected into the two types of empty cells to form two types of DDCLC cells. One cell undergoes the surface alignment treatment, whereas the other cell does not.

This work exploits two experimental setups in measuring the transmittance and lasing emission spectra of the DDCLC at different cone angles. Experimental setups and related techniques for measurement can be found in the authors’ pioneering study of CCLE, which are not repeated here for considerations of space [14]. Each DDCLC cell may be stimulated through an incident pulse train, from a Q-switched Nd:YAG SHG pulse laser with a wavelength of 532 nm, pulse duration of 8 ns, repetition rate of 10 Hz, and pulse energy of E (μJ/pulse) at an incident angle of roughly 10° from the cell normal (N). The lasing signals of the generated CCLE emitted from the cell at a distance of roughly 3.5 cm from the excited spot on the cell are measured for seven randomly selected cone angles (θ = 0°, 20°, 25°, 30°, 35°, 40°, and 45°) from N. To compare the measured lasing signal emitted at a specific cone angle, the transmittance spectrum of the cell, which indicates the CLC band structure in the visible region, is measured at that same cone angle. Both the lasing emission and the transmittance spectra of the cell at each cone angle are measured using a fiber-based spectrometer system (USB2000-UV-VIS, Ocean Optics, optical resolution: ~1.4 nm).

3. Results and discussion

3.1 Evolution of the lasing emissions based on a homogeneously-aligned DDCLC cell

We define a waiting time t_W to investigate the time-dependent performances of the obtained CCLE measured at different cone angles (θ = 0°, 20°, 25°, 30°, 35°, 40°, and 45°) in the aligned DDCLC cell (cell A). The CLC molecules tend to self-organize into a perfect planar texture if the t_W increases. We also define t_W = 0 as the moment at which the fabrication of the DDCLC cell is completed. We randomly select eight different waiting time points (t_W = 0, 24, 48, 72, 96, 144, 192, and 216 hr) to stimulate the DDCLC cell using the pumped pulse laser with an energy of E = 15 μJ/pulse. Figures 1(a) and 1(b) show the obtained lasing patterns displayed on the screen (left) and the measured lasing emission and transmittance spectra (right) at seven different angles, θ = 0°, 20°, 25°, 30°, 35°, 40°, and 45° at t_W = 0 and 216 hr, respectively. A strong lasing signal can occur at t_W = 0 but only along N (at θ = 0°) at the LWE of the PBG measured at 0°, and the lasing peaks found at the nonzero oblique angles are relatively weak. Therefore, the CCLE effect at t_W = 0 is weak, which decreases the recognition of its existence [Fig. 1(a)]. A strong CCLE whose lasing band is continuously distributed from 648 nm to 605 nm at the LWE of the PBG continuously measured at the cone angles from 0° to 35° can be obtained as the t_W is extended to 216 hr [Fig. 1(b)]. The increase in the blue-shift of the lasing emission in the CCLE at t_W = 216 hr with increased oblique angle has been explained in detail in Ref [14].

 

Fig. 1 Obtained lasing pattern (left) and transmittance and lasing emission spectra (right) based on the homogeneously-aligned DDCLC cell measured at cone angles from 0° to 45° as the cell is excited using the pumped pulses with E = 15 μJ/pulse at (a) t_W = 0 hr and (b) t_W = 216 hr, respectively.

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As displayed in Fig. 1(b), a dramatically bright lasing ring in the formed CCLE pattern can be easily seen at 35°. Experimental findings in previous reports [14–18] or in the present experiment have shown that the generation of the 35° lasing ring is associated with the location of the lasing wavelength (λ_ring). This is located right at the edge-overlapping wavelength positions of the LWE of the stop band for 35° and of the SWE of the stop band for 0° [i.e., λ_LWE(35°) = λ_SWE(0°)], based on the DDCLC cells with good planar textures. The edge-overlapping effect can lead to the enhancement in the DOS, which increases the spontaneous emission rate of the dyes in the wavelength of λ_LWE(35°) = λ_SWE(0°). In turn, this phenomenon dramatically improves the lasing intensity and decreases the energy threshold for the lasing signals emitted at 35°. Detailed explanation and discussion about the occurrence of the bright lasing ring at 35° can be found in Ref [18].

Figures 2(a) to 2(g) show the variations of the lasing emission intensity with the pumped energy at θ = 0°, 20°, 25°, 30°, 35°, 40°, and 45° measured at t_W = 0 hr to 216 hr. No lasing emission occurs at θ = 40° and 45° because no energy threshold can be found at the two angles [Figs. 2(f) and 2(g)] at all times. This result is attributable to the re-absorption of the fluorescence photons by the laser dyes at the LWE of PBG of θ ≥ 40° as the LWEs for θ ≥ 40° enter the absorption region of the dye. However, Figs. 2(a) to 2(e) show that corresponding energy thresholds exist at θ = 0°, 20°, 25°, 30°, and 35° at t_W = 0 hr to 216 hr, which implies that the lasing emission can occur at θ ≤ 35° at all times. Although the CCLE can occur at all times, the naked eye cannot easily detect CCLE at the early stages (e.g., t_W = 0) because the energy thresholds are all high and the lasing intensities of lasing signals are all low at nonzero angles. The experimental results in Fig. 2 are summarized as the plots of the energy thresholds and the relative slope efficiencies, which are defined as the ratio of the lasing intensity in the y-axis to the pumped energy in the x-axis for the lasing output of the CCLE versus t_W at the cone angles 0°, 20°, 25°, 30°, and 35°. These results are displayed in Figs. 3(a) and 3(b); the energy threshold and relative slope efficiency of the lasing signal at each angle in the CCLE decreases and increases, respectively, as t_W increases. This result implies that the performance of the CCLE laser significantly improves when the waiting time of the CLC increases from 0 hr to 216 hr. To identify the cause of the improvement of the CCLE performance with increasing time, the temporal evolution of the CLC structure in the aligned cell at t_W = 0, 24, 48, 72, 144, and 216 hr are observed under a polarizing optical microscope (POM) with crossed polarizers, as shown in Figs. 4(a) to 4(f), respectively. The number of the oily streaks in the CLC decreases as the t_W increases. A previous investigation of Zhang confirmed this monotic evolution of the defect lines [20]. These oily streaks occur during the diffusion of the DDCLC mixture into the empty cell. These defect lines have a high free energy in the CLC, and they tend to self-organize until they gradually shrink from the LC regions near the surface of the bulk region with increasing t_W under the anchoring effect of the anisotropic surface alignment. The existence of oily streaks divides the CLC cavity into various domains. When the fluorescence photons propagate into the CLC cavity, they will be scattered by the oily streaks because of the boundaries or interfaces between the homogeneously planar domains. Therefore, the presence of oily streaks with higher volume densities leads to more boundaries and discontinuous regions inside the CLC cavity, resulting in stronger scattering. When the t_W increases, the volume density of the oily streaks decreases; thus, the perfection degree of the CLC planar structure increases from multi-domain to a single-domain planar texture. This condition may induce a decrease in the loss of the CLC cavity because of a decrease in the light scattering of the multi-domain and an increase in the dwelling time of the fluorescence photons confined in the cavity, which improves the lasing performance of the CCLE. After the t_W increases to 216 hr, the CLC itself self-organizes into a nearly perfect planar structure [Fig. 4(f)], leading to the nearly optimum performance of the CCLE laser based on the aligned cell. In other words, both the energy threshold and the relative slope efficiency of each lasing signal in the CCLE measured at different angles are optimized during long-term self-organization.

 

Fig. 2 Evolution of the intensity variation of the lasing emission output of the aligned DDCLC cell with the pumped energy in the CCLE at different angles of (a) 0°, (b) 20°, (c) 25°, (d) 30°, (e) 35°, (f) 40°, and (g) 45°. The symbols ●, ●, ●, ● ●, ●, ●, and ○ represent related data corresponding with t_W = 0, 24, 48, 72, 96, 144, 192, and 216 hr, respectively.

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Fig. 3 Variations of (a) the energy threshold and (b) the relative slope efficiency of the CCLE based on the aligned DDCLC cell with the waiting time measured at five various angles, 0°, 20°, 25°, 30°, and 35°.

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Fig. 4 Images of the CLC structures observed under POM with crossed polarizers based on the aligned DDCLC cell at different waiting times of t_W = (a) 0, (b) 24, (c) 48, (d) 72, (e) 144, and (f) 216 hr.

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3.2 Evolution of the lasing emissions in a nonaligned DDCLC cell

The time-dependent performance of the lasing emissions based on a DDCLC cell with no homogenously rubbed alignment layer (cell B) is measured to further determine the influence of the surface alignment on the performance of the DDCLC laser. Figures 5(a) to 5(c) show the measured lasing emission spectra at E = 15 μJ/pulse and the transmission spectra of the nonaligned DDCLC cell (shown in the middle of Fig. 5) at various cone angles (θ = 0° to 45°) at t_W = 0, 72, and 216 hr, respectively. A strong lasing peak can occur only at θ = 0° but not at all angles of θ ≠ 0° at E = 15 μJ/pulse. The variations of the lasing output intensity with the pumped energy measured at 0° and the representative nonzero angle, 35°, need only be examined to determine if the CCLE can occur based on the nonaligned DDCLC cell at t_W = 0 hr – 216 hr for various pumped energies. Figures 6(a) and 6(b) show the relevant results. Obviously, by the comparison between the experimental results shown in Figs. 2 and 6, the E_th (η_s) for the lasing signal of 0° that occurs at each waiting time point of t_W from 0 hr to 216 hr based on the nonaligned cell is higher (lower) than that based on the aligned cell. The lasing emission cannot even be generated at the representative nonzero angle of 35° in this long period (216 hr). These results reflect that the CCLE effect cannot occur and only the residue of the performance-decayed normal lasing signal presents based on the nonaligned DDCLC cell no matter how long the waiting time is.

 

Fig. 5 Emission pattern on the screen behind cell B (left), transmittance and lasing spectra of the CCLE (middle) measured at various angles of θ = 0° to 45°, and images of the CLC texture observed under POM with crossed polarizers in cell B (right) at t_W = (a) 0, (b) 72, and (c) 216 hr. The cell is pumped using incident pulses with energy of 15 μJ/pulse. The zoom with enhanced contrast in the POM image, as shown in (c), indicates that the CLC exhibits a fine focal-conic-like texture even at t_W = 216 hr.

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Fig. 6 Evolutions of the intensity variation of the lasing emission of the nonaligned DDCLC cell with the pumped energy measured at (a) 0° and (b) 35°. The symbols ●, ●, ●, ●, ●, and ● represent related data measured at t_W = 0, 24, 48, 72, 144, and 216 hr, respectively.

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The hazy donut-like pattern of the fluorescence emission output, as seen on the screen from the pumped nonaligned cell (shown in the left of Fig. 5), implies that a strong light scattering effect is sustained at t_W = 0 hr to 216 hr; this effect significantly suppresses the generation of the CCLE in the nonaligned cell at all times. The source of the sustained strong light scattering can be demonstrated by observing the CLC structure of the nonaligned cell under the POM with crossed polarizers. The corresponding POM images (right of Fig. 5) show that the number of the oily streaks gradually decreases as the time increases; this result is similar to that based on cell A introduced in Sec. 3.1. However, the CLC seems to exhibit a fine focal-conic-like texture, in which the orientation of the LCs non-uniformly fluctuates in 3D directions; this orientation is observed when each POM image of the cell at different times [as displayed in the inset of Fig. 5(c)] is locally enlarge to reveal the detailed texture. This texture may be the source of the sustained light scattering of the fluorescence emission. The weak anchoring force from the nonaligned plane surface can induce the gradual shrinking of the oily streaks near the surface, but this force is not strong enough to influence the orientation of the LCs in the fine focal-conic-like texture which is distributed deeply in the bulk region. This force is responsible for the sustained nonoccurrence of the CCLE in cell B because of the large loss in the strong scattering in the inefficient resonator filled with the sustained focal-conic-like defects. In contrast to cell B, cell A has a similar shrinking evolution of oily streaks but does not possess the fine focal-conic-like texture at all times, which leads to discrepancies in the lasing emissions and associated evolutions of the two cells. Therefore, the surface alignment is necessary for generating the CCLE in the DDCLC.

3.3 Comparison of the lasing emissions based on an aligned and a nonaligned DDCLC cell with RAP pretreatment

Developing a method that can significantly shorten the waiting time is important for further investigations and applications because the time the aligned DDCLC takes to achieve an optimum CCLE is too long (216 hr). This work pretreats the DDCLC cell before the examination of the lasing experiment using RAP. Once the fabrication of the DDCLC is complete, the cell is heated to 80 °C, which is slightly lower than the clearing point (about 84 °C) at 30 °C/min, and is then cooled down to room temperature for 2 min. This cycle is repeated five times. The experimental results show that the RAP method can speed up the self-organizing process of the CLC from an imperfect to a perfect texture. The viscosity of the CLC mixture is low under high temperature; thus, the defects annihilate rapidly, as reported in a previous study [21]. Two other aligned and nonaligned DDCLC cells, which are both pretreated with RAP, are also prepared to compare the DDCLC cells A and B. The two cells are labeled as cells C and D, respectively. After RAP, cells C and D are placed at room temperature for 36 hours for the CLC to self-organize into a stable state. The associated POM images for cells C and D are shown in Figs. 7(a) and 7(b), respectively. Figures 7(c) and 7(d) show the measured lasing spectra (obtained through the excitation of the incident pulses with E = 15 μJ/pulse) and transmission spectra of cells C and D, respectively, at various angles. The performance of cell C at t_W = 36 hr is almost comparable to that of cell A at t_W = 216 hr, but the performance of cell D at t_W = 36 hr exhibits no improvement relative to that of cell B at t_W = 216 hr. The lasing patterns of cells C and D are also shown in Figs. 7(c) and 7(d), respectively. The reason for the different performances of cells C and D can be determined from the POM images of cells C and D, as shown in Figs. 7(a) and 7(b), respectively. The POM image in Fig. 7(a) shows that the texture of cell C at t_W = 36 hr is that of an almost perfect single-domain planar CLC, which is similar to cell A at t_W = 216 hr. Thus, the scattering caused by the defects is weak, and the CCLE characters of cell C are similar to those of cell A. Figure 7(b) shows that no oily streak defects exist in the POM image of cell D at t_W = 36 hr. However, the CLC texture inside is filled with small multi-domains of fine focal-conic defects instead of a perfect single-domain planar structure. The structure of the multi-domains can cause a strong scattering and induce a severe loss, resulting in the poor lasing performance of cell D.

 

Fig. 7 Images of the CLC structures observed under POM with crossed polarizers with a pretreatment of RAP at t_W = 0 and 36 hr based on (a) cell C and (b) cell D. The lasing pattern on the screen and the lasing and transmittance spectra of cells C and D are shown in (c) and (d), respectively.

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Figure 8 shows a comparison of the variations of the lasing intensity for θ = 0° and 35° with the pumped energies based on cells A and B at t_W = 216 hr and based on cells C and D at t_W = 36 hr. Apparently, the t_W for obtaining an optimum optical efficiency of the CCLE can be significantly shorten from 216 hr based on an aligned cell without RAP pretreatment (cell A) to 36 hr based on an aligned cell with RAP pretreatment (cell C). However, no CCLE effect occurs at all times based on not only cell D, which underwent RAP but had no surface alignment, but also cell B (with surface alignment but with no RAP pretreatment). The experimental results in Fig. 8 show that surface alignment is necessary for generating the CCLE and that RAP pretreatment can considerably shorten the waiting time needed to obtain a CCLE with the best lasing performance.

 

Fig. 8 Variations in the lasing emission with the pumped energy measured at (a) 0° and (b) 35° based on the DDCLC cells A, B, C, and D.

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Figure 9 further shows the variations of the lasing intensity of the CCLE with the pumped energies at other oblique angles of 20°, 25°, and 30° based on cell A at t_W = 216 hr and cell C at t_W = 36 hr. This is presented to explain that the t_W for obtaining an optimum CCLE for cell A is reduced sixfold via the RTA pretreatment by considering the lasing emissions not only at 0° and 35° but also at other oblique angles. Apparently, the lasing features of the CCLE at 20°, 25°, and 30° based on cell A at t_W = 216 hr are almost consistent with those based on cell C at t_W = 36 hr. Consequently, the results displayed in Figs. 8 and 9 indicates that the RAP pretreatment in the aligned DDCLC cell can effectively shorten the waiting time for obtaining an optimum CCLE of the aligned DDCLC cell by sixfold.

 

Fig. 9 Variations in the lasing emission output with the pumped energy measured at (a) 20°, (b) 25°, and (c) 30° based on the DDCLC cells A and C (full and empty dots, respectively).

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4. Conclusion

This work investigates for the first time the influence of the perfective degree of the DDCLC planar structure on the optical efficiencies of the CCLE in DDCLC cells. In summary, the optical efficiencies of the obtained CCLE increase as the waiting time increases after completing the preparation of the DDCLC cell with surface alignment. This result occurs because of the decrease of the amount of oily steak in the multi-domain texture; thus, the induced scattering decreases with increasing the waiting time under the anchoring effect of the surface alignment. The optical efficiencies of the lasing emission obtained from the cells without surface alignment are poorer than those from the cells with surface alignment. This result is attributable to the fact that, without the anchoring effect of the surface alignment, the cells are always filled with unapparent and small focal-conic-like defects in the bulk of the cell; thus, no CCLE can be measured because of the caused strong scattering. The waiting time required to obtain an optimum efficiency of the CCLE of the DDCLC cell with surface alignment can be reduced significantly (sixfold) through RAP pretreatment. However, RAP cannot improve the optical efficiency of the lasing emission based on the DDCLC cell without surface alignment. Thus, our results confirm that the perfect single-domain planar texture is the necessary condition for the appearance of the highly efficient CCLE.