1. Introduction

Since Il’chishin et al. [1] first reported on a lasing operation with a dye-doped cholesteric liquid crystal (CLC) in 1980, many researchers have studied laser devices made from various liquid crystal (LC) materials with chirality. Because LC materials are very sensitive to delicate external stimuli, such as temperature [2–8], an external electric field [3,9–11], mechanical stress [12,13], and light [14–16], this sensitivity has been applied to laser wavelength tuning. As a result of many brilliant strategies by scientists, laser generation and laser tuning have been achieved in the broad visible spectral wavelength range with cholesteric liquid crystals (CLCs) [1,2,5–15], liquid-crystal blue phases [3,4], smetic liquid crystals [17], and polymeric liquid crystals [16,18–20]. These have the unique optical characteristics of mirrorless lasing, a low threshold, and a micro-scale size [21–23].

Above all, because the CLCs are manageable and well developed, they have been widely applied to the generation of lasing and laser tuning. The CLC has a self-organized periodic helical structure that is composed of a nematic liquid crystal and chiral material. The periodic length of the helical structure, or the pitch p, is determined based on the chiral material’s concentration relative to the nematic liquid crystal. A selective Bragg reflection occurs at the photonic band gap (PBG),${\lambda }_B=n\times p$, with a bandwidth of Δλ = p × Δn, where $n=\sqrt{(n_{II}^2+n_{\perp }^2)/2}$ is the average refractive index and $\Delta n=n_{II}-n_{\perp }$is the birefringence of the nematic molecules [21,22]. Near the photonic band-edge of the dye-doped CLC, the density of the states (DOS) is sharply enhanced and the spontaneous emission is proportional to the DOS [24–30]. Therefore, lasing operation and tuning are enabled near the photonic band-edges by controlling the helical pitch of the CLC, not only with the external stimulations mentioned above, but also by the introduction of a defect layer [28], the dye dopant concentration, and a change in the chiral dopant concentration [29,30].

In our previous study, we achieved continuous laser tuning in the full visible spectral range with less than 1 nm tuning resolution by introducing a spatial pitch gradient in the wedge cell [18,27,31]. The optical properties of the laser lines and fluorescence [32], the stability for external stimulations and the stability for temporal, thermal, and strong light [18,33] were verified. A general CLC and a UV-curable polymerized CLC were employed; however, in general CLC case, the well-developed pitch gradient of the wedge CLC cell does not have long-term stability, and in the polymerized CLC case, a long time is needed to develop continuous pitch gradient [18].

In this paper, we report on the principle of laser tuning in a CLC as well as active continuous laser tuning by a spatial temperature control in a wedge cell. This represents advancement over our previous CLC results [18,27,31]. First, we clearly discover the reason for discontinuous laser tuning in a parallel CLC cell by temperature change. Through in situ observation of the lasing process in CLC using a CMOS system, we found that, when some energy is transferred to the CLC by temperature change, the change in pitch begins at the boundaries of the CLC domains and then spreads like a rising tide.

Second, active tuning has been studied by a control of light [34,35], heat [36], and DC electric field [3,9–11]. Especially using trans-cis photoisomerization, reversible phototuning is achieved in broad wavelength range over 70nm [35] and rapid phototuning speed of ~21nm in 148 ms [34]. However due to the boundary condition of the parallel CLC cell, their tuning behavier can't help discontinuous tuning with 7~10 nm wavelength. And in our previous study because at room temperature, when a wedge cell is made by a dye doped CLC, laser is tunable only just about 7~8 nm wavelength [26], so not only to expand the tuning range of laser wavelength but also to actively tune with fine tuning resolution, our new idea in this work is a forming spatial temperature gradient along a wedge CLC cell. By combining the concepts of a temperature gradient with the wedge cell, we achieved continuous laser tuning over a 105 nm spectral range with about 0.2 nm tuning resolution. It should be noted that there is no aging effect, because the CLC cell contains only one chiral concentration and it depends only on temperature. The tuning wavelength range can be actively changed by two temperature controllers connected to the cell. This strategy presents another way of achieving practical CLC laser devices.

2. CLC cell fabrication and lasing measurements

A CLC cell with a left-handed helix is made from one mixture of a nematic liquid crystal (ZLI2293; ~74 wt%, Merck), a chiral dopant (S811; 24 wt%, Merck), and two laser dyes (DCM [4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4Hpyran] and LDS698, both in ~1 wt%, Exciton). As an alignment layer, polyimide SE-5291 (with a pre-tilt angle of ~7°, Nissan Chemical Korea Co. Ltd., Korea) was employed.

For this experiment, we made some CLC cells by the capillary method: a parallel cell (P-cell; ~34 μm thickness) and a wedge cell (W-cell; with a thickness change from 14.25 μm to 30.51 μm over a lateral distance of ~1.0 cm). In order to control and develop the temperature gradient on the CLC cells, two temperature controllers were connected to both ends of the P-cell (and W-cells), individually. Figure 1 shows a schematic diagram of the spatially developed pitch gradient in the wedge cell that could be achieved when the temperature gradient and inclination of the wedge cell are well matched. When a high (low) temperature is applied to the thin (thick) side and a low (high) temperature is applied to the thick (thin) side of the cell, we say that a positive (negative) temperature gradient has developed in the wedge cell. The positive temperature gradient of the W-cell means the pitch gradient caused by a temperature. When the positive temperature gradient matches the helical pitch determined by the wedge cell thickness, the Granjean texture stripes of the W-cell at room temperature [26] will be changed to a continuous color change stemming from a continuous pitch change [31].

 

Fig. 1 Schematic diagram of a continuous CLC pitch gradient developed in the W-cell by the inclination of the cell commensurate with a positive temperature gradient.

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The helical pitches of the CLC wedge cell were quantized by the number of half-turns based on the boundary condition (BC) [22]; alternatively, because of the strong surface anchoring energy, the number of pitch/2 between two substrates should satisfy the geometrical thickness of the cell. The characteristics of the pitch gradient formed in the wedge CLC cells are specifically described in [27,31].

A Q-switched Nd:YAG laser (7 ns pulse, 10 Hz) with 355 nm light was employed as an optical pumper. The pumping laser beam with ~2 mm beam diameter was focused by a lens (focal length ~20 cm), and the beam waist (w) at the focal point was w = λ/sinθ = 71 μm (where λ = the pump beam wavelength and sin θ = 1/200). It was expected that a smaller pump spot size would provide a narrower line width with a higher spectral resolution. To attain strong absorption from the optical pumping [37], the focused pump beam was incident obliquely on the sample (incidence angle range 10°~40°). Accordingly, the beam size on the sample could be increased up to ~1.5 times.

We can certify the pump beam size in Fig. 2(e6) from a CMOS image of the CLC cell.

 

Fig. 2 CMOS camera images at a fixed position of the P-cell and its PBG and laser peak spectra by the spectrophotometer; (a) 31.7${}^{o}C$, (b) 32.2${}^{o}C$, (c) 25.4${}^{o}C$, (d) 23.4${}^{o}C$, (e) 20.6${}^{o}C$, (f) 20.5${}^{o}C$, (g) 20.4${}^{o}C$, and (h) 20.2${}^{o}C$. Main (M) and sub (S) boundaries of the CLC domains move by thermal energy transfer (Visualization 1).

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3. Experimental results and discussion

To the best of our knowledge, this work is the first time that in situ laser tuning behavior has been inspected in detail using a CMOS video camera (Net IC4203cu, with zoom lens set). In order to understand the relationship between the lasing wavelength and the pitch change at the CLC cell due to temperature change, first we observed one fixed point of the CLC cell that was exposed by the pump beam using the CMOS camera. At the same time, the generated laser emission and PBG of the measuring point of the CLC cell were measured by a spectrophotometer (0.36 nm resolution; HR 2000 + , Ocean Optics, USA) [26]. The temperature of the CLC cell was controlled by two ST540 digital controllers (Nova) and thermoelectric modules. There could be a 2~5 ${}^{o}C$ temperature deviation between the actual temperature of the measuring point in the CLC and the temperature sensor.

Figure 2 shows CMOS camera images of the CLC domains in the P-cell, and their PBG and laser peak spectra by the spectrophotometer. In this figure, the abbreviations PV and SP indicate the real temperature of the cell by the temperature sensor and the temperature set by the temperature controller, respectively. In Fig. 2(a) and 2(b), above 31.7${}^{o}C$isotropic dots appear gradually and then the domain boundaries disappear. PBG also disappears in the isotropic state (Fig. 2(b)). Some defects are caused by dye aggregation inside the domains (Fig. 2(a) and 2(b)). The CLC phase was recovered as the temperature goes down. In the CLC phase, there are main boundaries (M) of the domains which are thick and clear, and sub-boundaries (S) of the domains which are thin and weakly colored (Fig. 2(c)). Within a domain, the CLCs have the same pitch. The adjoining domains may have the same pitch or one slightly different (from ~1 nm up to ~8 nm) for a given condition. This is because the pitch of the CLC is complexly determined by temperature, the surface condition of the alignment layer, the thickness of the cell, and the chiral molecules [8]. Between CLC zones with different pitches, the boundaries appear naturally in a microscopic molecular space. This is why the CLC phase has such domain textures (Figs. 2 and 3(e)). It should be noted that the pump beam position in Fig. 2 is fixed in laboratory coordinates. We can see that for a 2${}^{o}C$ temperature change in Figs. 2(c) and 2(d), M moves ~80 μm in 2 min and 15 sec, and the laser wavelength changes from 651 nm to 659 nm. In Fig. 2(d), across the M, two domains were pumped simultaneously and one laser beam (659 nm) is generated, so the two neighboring domains have same pitch. In some other cases, the neighboring domains have different pitch (see Fig. 3(c) and 3(d)). We could see more specific and detailed dynamics of the CLC for the temperature change in Figs. 2(e)-2(h) (see Visualization 1). For a 0.4 ${}^{o}C$ temperature change in 51 sec, S spreads out and passes through one position which is pumped by a 355 nm beam, while at the same time the pitch and generated laser beams change discontinuously, from (624.44 nm, 673.64 nm) to (631.11 nm, 681.14 nm). Looking closely at Fig. 2(e), it is seen that zones A and B are divided by S. When S passes through the pumped position in half, or zone A expands and zone B decreases in Fig. 2(g), two lasing beam sets are simultaneously generated that correspond to the pitch of zone B (624.44 nm, 673.64 nm) and the pitch of zone A (631.11 nm, 681.14 nm). The moving velocity of S corresponds to the velocity of the discontinuous pitch change. The velocity of S is far faster than that of M (see Visualization 1). This discontinuous pitch change is due to BC, or the helical pitches of the CLC cell were quantized with the number of half-turns [8,31]. Below, Fig. 4(a) shows the laser lines as a function of temperature at the fixed position of the P-cell in Fig. 2.

 

Fig. 3 (a, c, e) CMOS camera images of the generated laser beam at the W-cells which have a positive temperature gradient. (b, d) Their PBG and laser peak spectra by the spectrophotometer, respectively. (e) Juxtapositions of 10 pieces of CLC texture images for each different lateral position, with 2 mm intervals.

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Fig. 4 (a) Laser lines as a function of temperature at a fixed position of the P-cell. (b) Laser lines as a function of spatial position at the W-cell at room temperature. (c) Laser lines as a function of spatial position at the W-cell with positive temperature gradient (Visualization 2). (d) Laser lines as a function of spatial position at the W-cell with negative temperature gradient.

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In order to overcome the discontinuous laser tuning behavior in the parallel CLC cell (Fig. 4(a)), our new strategy was to develop a positive temperature gradient on the wedge cell (W-cell; see Figs. 1, 3 and 4(c)). Figure 3 shows CMOS camera images of the generated laser beams and the CLC domains 3(a) and 3(c) of the W-cell, and their respective PBGs and laser peak spectra by the spectrophotometer 3(b) and 3(d). When the pump beam is exposed to one domain, one laser beam is generated 3(a) and 3(b), and when the pump beam is exposed to two domains, two laser beams are generated 3(c) and 3(d). According to our first close-up observation at the lasing instant by the CMOS camera, it appears like a bomb exploding (Fig. 3(a) and 3(c); also see Visualization 2). Figures 3(e1)-3(e10) show juxtapositions of 10 pieces of CLC texture images for each different lateral position, with 2 mm intervals. The color of the CLC textures changes smoothly according to the continuous positive temperature gradient, from 15.2 ${}^{o}C$ (Fig. 3(e1), at the thick side) to 29.3 ${}^{o}C$ (Fig. 3(e10), at the thin side) on the W-cell (see Fig. 1). That can lead to laser tuning behavior (Fig. 4(c)).

Figure 4 shows the difference in lasing behavior between the parallel CLC cell (P-cell) [8] and the wedge CLC cell (W-cell) for a change in temperature. Figure 4(a) shows the discontinuous laser tuning as a function of temperature at a fixed position of the P-cell (see also Fig. 2) [8]. At first, the temperature of the P-cell was raised from room temperature to 32${}^{o}C$ (solid squares), and then it decreased to ~16 ${}^{o}C$ (open squares) at a rate of ~1${}^{o}C$/min. As the temperature falls (or increases), within a certain temperature range the wavelength of the laser line does not change; then suddenly it is redshifted (or blueshifted) discontinuously to a long wavelength in stages, with an interval about 4~8 nm. The laser wavelength generated at low energy band edge is governed by ${\lambda }_L=n_{II}\times p$, where p is the pitch and $n_{II}^{}$ is the effective extraordinary refractive index which is parallel to the director of bulk LC and determined by the specific combination of nematic LC (ZLI2293), chiral material (S811), laser dyes (DCM and LDS698), and BC [8,27,38]. The redshift behavior of lasing caused by a temperature decrease is due to effects of the refractive index; $n_{II}$ increases as the order parameter increases by the temperature decrease [8]. In sequence, the discontinuous laser tuning behavior could be understood as a competition between the BC and thermal energy. For a small temperature change, the BC effect holds a prominent position to some degree, and eventually the BC effect gives in to the thermal energy effect, repeatedly. We found out that as the thermal energy is moved into (or outside) the domain of CLC, a discontinuous pitch change begins at the boundary of the CLC domain and expands like incoming waves. According to the pitch change, at the same time the laser wavelength immediately shifts discontinuously (see Visualization 1).

Figure 4(b) shows a cyclical laser tuning in 5~9 nm spectral range as a function of spatial position at the W-cell at room temperature. The wedge cell is well developed over the entire cell except for some defects (see Figs. 2(a), 2(b) and 3(e)) around the thick zone from 0 mm to 4 mm in the X-position. The defects result from the aggregation of dyes and disrupt the laser generation. We observe cyclical jumps in the lasing wavelength, where two simultaneous laser generations occur. The pitch correspond to the original pitch and the maximally dilated pitch by BC. This behavior is typical of the lasing tendency of a wedge cell [26,31]. The laser peaks were acquired by 50 μm X-position movements with the spectrophotometer.

Figure 4(c) shows the laser lines as a function of spatial position at the same X-positions as in Fig. 4(b) with 50 μm intervals for the W-cell. When the temperature of the cell converted from room temperature to the positive temperature gradient distribution (see Fig. 1) along the W-cell, only just about 7 nm laser tuning range at room temperature is extremely widened. In the Fig. 4(c), as the pumping position moves from 0 mm (15.2${}^{o}C$) to 17 mm (29.2${}^{o}C$), continuous laser tuning is achieved from 585 nm to 690 nm over 105 nm with a highly fine tuning resolution of ~0.2nm (wavelength see also Fig. 5(a)). This highly improved tuning behavior is result from the constructive cooperation between the optical property of the CLC for the temperature and wedge cell structure. By the Fig. 4(a) result, the effective pitch length of the CLC is increased (decreased) at low (high) temperature. And according to our previous study [26], the pitch length at the thick (thin) side of the wedge cell is increase (decrease) by the surface BC and the thickness difference of the wedge cell gives 7~9 nm lasing wavelength difference. So, we could achieve that with forming the positive temperature gradient by applying low temperature on the thick side and applying high temperature on the thin side of the W-cell, the tuning range could be constructively increased and finely tuned. Although there are cyclical jumps in the lasing wavelength, between the two jumps the spatial tuning wavelength resolution is$\Delta \lambda \approx$1 nm with 50 μm spatial intervals (see also Fig. 5(b)), and $\Delta \lambda \approx$0.2 nm with 10 μm intervals (see also Fig. 5(c)). If the pitch gradient caused by a temperature gradient matches the helical pitch determined by the wedge cell thickness, then continuous tuning could be achieved without jumping. The few missing laser peaks around zone A in Fig. 4(c) result from the defects by laser dye aggregation. Because the CLC helical structure is destroyed by the defects, lasing is impossible there (see Figs. 2(a), 2(b), 3(e1)-3(e3), and the Visualization 2).

 

Fig. 5 Laser line spectra and PBGs of the W-cell with positive temperature gradient: (a, b) by 50 μm X-position movements, (c) by 10 μm X-position movements, (d) PBG and laser peaks change of a parallel CLC cell as a function of temperature.

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On the contrary, we could also guess that when forming the negative temperature gradient by applying high temperature on the thick side and applying low temperature on the thin side of the W-cell, the two thermal effect of the CLC and the effect of the thickness change in the wedge cell are doing counterwork each other, so the total tuning range could be more destructively decreased than that of the positive temperature gradient. And although the laser wavelength increase in the overall temperature range, on the other hand the laser wavelength decrease between narrow temperature range. Figure 4(d) shows the laser lines as a function of spatial position at the same X-positions as in Fig. 4(c) for the W-cell with negative temperature gradient. Interestingly, as the temperature of the zone goes down, the tuning laser wavelength is redshifted, while between two neighboring jumps the tuning laser wavelength is blueshifted due to the BC of the thickness slope of the W-cell.

Figure 5 shows the laser line spectra and PBGs of the W-cell with a positive temperature gradient by spatial movement along the X-axis (Fig. 1). For the 50 μm X-position movement the laser wavelength is blueshifted about 1 nm (a and b), and for the 10 μm X-position movement the laser wavelength is blueshifted about 0.35 nm (c). In Fig. 5(b), we can see that the laser peak shift is more sensitive than the PBG movement for the spatial movement. This is because the lasing wavelength is determined by the pumped area (pump beam size is ~100 μm, Fig. 3); however, the PBG spectrum is determined from the averaged value by more than the large area in our measuring system. In Fig. 5(a) and 5(d), as the temperature increases the PBG is blue shifted in both the W-cell and the P-cell with 13um thickness. But the moving behavior is very different in the two cases; in the W-cell case PBG is shifted highly continuously but in the P-cell case PBG is shifted discontinuously. The position of PBG (${\lambda }_B=n\times p$,$n=\sqrt{(n_{II}^2+n_{\perp }^2)/2}$) is determined by average refractive index(n) and pitch(p). And as the temperature increases the $n_{II}^{}$ change of the LC material is monotonically decreased [8], so from the PBG change behavior we can estimate the pitch change of the CLC.

4. Conclusion

By in situ investigating the lasing process of CLC laser devices with a CMOS camera system, for the first time, the laser tuning mecanism in CLC was clearly verified. We determined that the discontinuous laser tuning of laser peak wavelengths in a parallel CLC cell resulted from the discontinuous pitch change by thermal energy transfer. The dynamics of pitch change was so fast and occurred in accordance to the pitch change; at the same time, the laser wavelength immediately shifted discontinuously. Furthermore, we could ascertain why the CLC phase has such domain textures; it is because the neighboring domains could have 1~8 nm difference in pitch, so boundaries naturally appear between them. And in this work, we also proposed a new idea to combine wedge cell structure with temperature gradient. With this new strategy we developed an efficient and simple active tunable laser array. By forming a spatial temperature gradient along the wedge CLC cell, only just about 7 nm continuous laser tuning range at room temperature is highly widened over the 105 nm wavelength range with a highly fine tuning resolution of ~0.2 nm. More importantly, this laser array has no aging effect. The tuning wavelength range of lasing could be adjusted by using a temperature controller. This simple and efficient method provides a method for practical CLC laser device applications.