1. INTRODUCTION

Since Il’chishin et al., first reported a thermal tuning of laser wavelength with a dye-doped cholesteric liquid crystal (CLC) [1], in order to realize a thin film tunable laser, CLCs, as well as many other chiral liquid crystalline phases, smetic liquid crystals [2], polymeric liquid crystals [3,4], and liquid- crystal blue phases [5,6], have been widely studied over the decades. They possess unique physical characteristics of low threshold, mirrorless lasing, microscale size [7], easy fabrication, and fine and wide laser tunability [8,9].

In particular, because CLCs are relatively affordable ingredients and manageable, they have frequently been employed as laser cavities [7–9]. CLCs have a self-organized periodic helical nanostructure that consists of birefringent nematic liquid crystals (NLC) and chiral materials. The CLC has a photonic bandgap (PBG), ${\lambda }_B=p\times n=n/(HTP\times C)$, with a bandwidth of $\mathrm{\Delta }\lambda =p\times \mathrm{\Delta }n$, where the helical pitch $p$ is the length for one full rotation of the director around the helix axis, HTP is the helix twist power, $C$ is the concentration of chiral material, n=$\sqrt{(n_e^2+n_o^2)/2}$ is the average refractive index, and $\mathrm{\Delta }n=n_e-n_o$ is the birefringence of the extraordinary ($n_e$) and ordinary ($n_o$) refractive indices of liquid crystal (LC) [10,11]. The high-energy band edge (HE-edge) and low-energy band edge (LE-edge) of the PBG are determined by ${\lambda }_h=n_o\times p$ and ${\lambda }_l=n_e\times p$, respectively. Selective Bragg reflection occurs within the PBG for circular polarized light that has the same handedness as the CLC helix, meaning that PBG could act as a laser cavity. Near the PBG edges of the CLC, the sharply enhanced density of the states (DOS) leads to lasing [12–15]. Therefore, laser tuning is enabled near the PBG edges by modulating the helical pitch and refractive indices of the CLC with external stimulation, such as temperature [5,6,16], external electric field [5,17–19], light [20–22], the introduction of a defect layer [23], dye dopant concentration, or a change in the chiral dopant concentration [24,25].

In our previous study, we achieved active fine laser tuning over a 105 nm spectral range with about 0.2 nm tuning resolution by combining a temperature gradient and a CLC wedge cell structure with left-handed helicity [8]. Furthermore, there is no aging effect because the CLC array that we employed has only one chiral molecular concentration. To the best of our knowledge, combining temperature gradient and wedge cell structure is one of the most efficient laser tuning strategies by fine laser tuning ($\sim 0.2\mathrm{nm}$ tuning resolution) in broad spectral ranges (over 100 nm) [8], in addition to electric field method [26,27] and light pumping method [20,28,29]. There are many kinds of CLCs that have different optical characteristic properties: positive or negative CLCs with either right-handed or left-handed helicity [30,31]. For the practical application of the strategy of combining a temperature gradient and a CLC wedge cell structure, we need to know more about the laser tuning mechanism of each material by temperature change. Because laser tuning is enabled near the PBG edges by modulating the helical pitch and refractive indices of the CLC, studying the PBG’s behavior of CLCs in response to thermal stimulation is absolutely necessary in order to understand the laser tuning mechanism.

In this paper, we realized active fine laser tuning in a broad spectral range. In order to specifically understand the laser tuning mechanism, we studied the temperature dependence of optical properties of two kinds of positive CLC samples with left-handed and right-handed helicity. From the study, we find out that the tuning range of lasing and PBG could be actively changed by temperature, and in order to achieve maximum tuning range, chiral molecular dopant concentration, thickness, thickness gradient, and temperature gradient on the wedge cell should be properly matched. In both CLC samples with left-handed or right-handed helicity, when the temperature was increased, the PBGs were blueshifted while the PBG widths were decreased. Our study results revealed that the refractive indices of CLCs were decreased and the chiral molecule’s solubility were increased when temperature was increased by using chiral materials with high molecular anisotropy.

2. CLC CELL FABRICATION AND EXPERIMENTAL CONDITION

First, to experiment with the dependence of laser tuning on temperature, we fabricated two kinds of CLC mixtures: an L-CLC, by mixing NLC mixture ZLI2293 (75.5 wt. %, Merck) and chiral dopant S811 (24.5 wt. %, Merck), which derives a left-handed helix; and an R-CLC, by mixing ZLI2293 (74 wt. %) and chiral dopant R811 (26 wt. %, Merck), which derives a right-handed helix. The helix twist power (HTP) of the S811 and the R811 in ZLI2293 were slightly different. In order to achieve the maximum PBG tuning ranges, the concentration ratios of the chiral materials and the temperature gradients on the wedge cell of the two kind samples were also slightly different. The appropriate concentration and temperature gradient of the cell in the visible spectral range by temperature control was dependent on physical properties of the applied CLC mixtures of NLC and chiral materials, although temperature-independent pitch invariance has been observed in some CLC samples [32].

In order to get a laser gain, we added two laser dyes (DCM [4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4Hpyran] and LDS698, both in $\sim 1\mathrm{wt}.\%$, Exciton) to the CLC mixtures. As an alignment layer of the CLC, we employed rubbed polyimide SE-5291 (pretilt angle of $\sim 7°$, Nissan Chemical Korea Co. Ltd., Korea) layers on ITO plates. Using them, we made some wedge CLC cells with a thickness change from 20 to 25 μm over a lateral distance of $\sim 18\mathrm{mm}$. We filled wedge CLC cells of right-handed helicity (WR-cells) and left-handed helicity (WL-cells) by the capillary method with R-CLC and L-CLC, respectively. Second, in order to study the optical properties of the PBG depending on temperature, we fabricated two parallel CLC cells having $\sim 25\mathrm{\mu m}$ thickness, PL-cells that were filled with L-CLC, and PR-cells that were filled with R-CLC. We also fabricated four PC-cells ($\sim 12\mathrm{\mu m}$ parallel) with molecular concentrations of the S811 at 25.5 wt. %, 26.5 wt. %, 27 wt. %, and 27.5 wt. %, respectively.

In order to control and form the temperature gradient over the wedge CLC cells, Fig. 1 shows that we connected two ST540 digital temperature controllers (Nova) and thermoelectric modules to both ends of the wedge cell. Two thermocouples were set on the sample surface with lateral distance of $\sim 18\mathrm{mm}$. The temperatures of the two points were monitored and kept constant by two individual temperature controllers. We monitored the laser tuning behavior by a CMOS video camera system (Net IC4203cu, with zoom lens set) and spectrophotometer (0.36 nm resolution; HR 2000 +, Ocean Optics, USA).

 

Fig. 1. Schematic diagram (a) of the cycling pitch change in the wedge cell at room temperature and (b) the pitch gradient with positive temperature gradient on the wedge cell.

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Figure 1(a) shows a schematic diagram of the cycling pitch change in the wedge cell at room temperature. In order to satisfy the boundary condition of the surface tension and surface anchoring energy, etc. at each point of the cell, the helical pitch numbers of the CLC are quantized by a half-pitch between two substrates [9,11,15]. Therefore, on increasing the cell thickness along the $X$-direction, the pitch cyclically changes continuously from $p_1$ to $p_2$, and then a sudden pitch jump occurs from $p_2$ to $p_1$, so Cano dislocation lines appear at the jumping positions [see Fig. 1(a) and inset of Fig. 2(a)] [14].

 

Fig. 2. CMOS camera images and a laser peak spectrum of the WR-cell: (a) juxtapositions of eight pieces of CLC texture at room temperature (the inset is a polarized microscope photo of the Cano dislocation lines), and (b) juxtapositions of eight pieces of CLC texture when the positive temperature gradient was formed along the WR-CLC cell. (c) Generated laser beam on the CLC texture. (d) PBG and generated laser peak spectra of (c) by the spectrophotometer.

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Figure 1(b) shows the spatially developed pitch gradient in the wedge cell by applying a high (low) temperature to the thin (thick) side and applying a low (high) temperature to the thick (thin) side of the cell [8]. In other words, we develop a positive (negative) temperature gradient in the wedge cell. When the positive temperature gradient matches the inclination of the wedge cell, we can develop a spatially continuous pitch gradient, so the cycling color change of the Cano dislocation lines at room temperature [inset of Fig. 2(a)] will change to a continuous color change [8,9,14,15].

As a pumping laser, we employed third-harmonic-generated 355 nm light from a $Q$-switched Nd:YAG laser (7 ns pulse, 10 Hz, Spectra Laser, USA). A lens (focal length $\sim 20\mathrm{cm}$) focused a pump beam of about $\sim 2\mathrm{mm}$, and the beam waist ($w$) at the focal point was about 71 μm ($w=\lambda /\sin \theta$, where $\lambda$ = the pump beam wavelength and sin $\theta =1/200$). In order to attain strong absorption [33], we set the pump beam to oblique incidence with incidence angle range of 10°–40° so that the pump beam diameter could be increased up to 100 μm [Fig. 2(c)].

3. EXPERIMENTAL RESULTS AND DISCUSSION

In order to study the laser tuning mechanism in the wedge cell through temperature change, we compared the CLC textures and PBGs of the two kinds of CLC cells, the WR-cell with right-hand helicity, and the WL-cell with left-hand helicity, at room temperature state and at a temperature-gradient-formed state. Figure 2(a) shows juxtapositions of the eight pieces of CMOS camera images having orange color over the whole WR-cell. The inserted polarized microscope photograph shows the Cano dislocation lines of the typical wedge cell property; compare with Fig. 1(a) [14]. However, Fig. 2(b) shows that when the positive temperature gradient develops along the wedge direction of the cell, the color of the CLC textures changes smoothly from orange tinged with yellow to orange along the temperature gradient from 17.7°C to 26°C between two temperature sensors with a lateral distance of $\sim 18\mathrm{mm}$.

The continuous color change of the texture denotes the continuous change of pitch. There could be a 3°C–5°C temperature deviation between the actual temperature and the temperature sensor. There are some defects in the CLC textures resulting from dye aggregation. For temperature change from 17.5°C to 27°C, the WL-cell also shows a similar color change. Figure 2(c) shows CMOS camera image of the just-generated laser beam on the CLC texture. We inspected the laser tuning behavior with a CMOS camera system [Fig. 2(c)], and at the same time, the generated laser emission and PBG at the laser measuring point on the CLC texture are measured by spectrophotometer [Fig. 2(d)].

Spatially moving the pump beam by 50 μm along the $X$-direction (Fig. 1) of the wedge cells, we acquired the generated laser beams by spectrophotometer. For the temperature change, Figs. 3(a) and 3(b) show laser tuning behavior of the two kinds of wedge CLC cells of the WL-cell and WR-cell, respectively. At room temperature, as a function of the spatial position, the WL-cell and WR-cell show a cyclical laser tuning in the 5–9 nm spectral range. From the results, we could see that the wedge cells are well fabricated over the entire cell, except for some defects [see Figs. 2(a) and 2(b)], which result from dye aggregation and hinder laser generation. There are cyclical jumps in the lasing wavelength along the wedge direction in accordance with the pitch change of the wedge cell; see Fig. 1(a).

 

Fig. 3. Laser peaks as a function of spatial position of the (a) WL-cell and (b) WR-cell; at room temperature (□) or with positive temperature gradient along the wedge direction (○). The blue dashed lines are the SPGs at the $X$-positions of $\sim 2\mathrm{mm}$ (1 and 2), $\sim 6\mathrm{mm}$ (3 and 4), and $\sim 14\mathrm{mm}$ (5 and 6), respectively.

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When a positive temperature gradient formed along the wedge cell [see Fig. 1(b)], the laser lines were acquired at the same $X$-positions of the cells at room temperature, respectively.

By forming a positive temperature gradient, in both cases the laser tuning range is much extended from $\sim 7\mathrm{nm}$ at room temperature to $\sim 48\mathrm{nm}$ and $\sim 76\mathrm{nm}$ for the WL- and WR-cell, respectively. Results of spatial pitch gradient [SPG (nm/mm)] of wedge cells at the $X$-position near 2 mm (blue dashed lines 1 and 2), 6 mm (3, 4), and 14 mm (5, 6) are shown in Fig. 3.

The SPG of each $X$-position of the wedge cell at room temperature was almost maintained after the temperature gradient was developed. The change to low (or high) temperature caused a SPG shift to the long (or short) wavelength. As shown in Figs. 3(a) and 3(b), a valid SPG should be at 4.7 nm/mm in the wedge cell to avoid wavelength jump during spatial laser tuning. It has been reported that the tuning ranges of lasing and PBG by temperature control depend on physical properties of the CLC used [8,32]. The thickness and thickness gradient of the wedge cell and temperature gradient are also important parameters for the tuning range compared to laser tuning range of the WL-cell results or the reference range [8].

Although there are cyclical jumps in the tuning wavelength, between the two jumps they were finely tuned with $\sim 0.36\mathrm{nm}$ resolution for 50 μm movement [see Fig. 4(b)]. Naturally, decreasing the moving displacement and beam size of the pump beam would increase the laser tuning resolution. By carefully adjusting the cell thickness, temperature gradient, and slope of the wedge cell, we could ideally match the pitch gradient caused by the temperature gradient with the helical pitch determined by the wedge cell thickness. Thus, we could achieve continuous tuning without jumping and could expand the laser tuning range more broadly. In our previous study with a WL-cell, we achieved continuous laser tuning from 585 to 690 nm over 105 nm with a very fine tuning resolution of $\sim 0.2\mathrm{nm}$, although there are cyclical jumps in the laser tuning range [8].

 

Fig. 4. Laser line spectra and PBGs of the WR-cell with a positive temperature gradient by 50 μm $X$-position movements.

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Figure 4(a) shows all the laser line spectra and PBGs of Fig. 3(d). In the WL-cell case, the laser tuning behavior was similar. This highly expanded tuning range results from the constructive cooperation between the optical property of the CLC by temperature and the boundary condition of the wedge cell structure. Figure 4(b) shows laser wavelength tuning with $\sim 0.36\mathrm{nm}$ resolution for 50 μm movement in the WR-cell. Laser wavelengths of all data were determined by average wavelength value at the full width with half of each maximum peak intensity.

Another important experimental result is that in both right-handed (WR-cell) and left-handed (WR-cell) circular helix cases, when moving to the high-temperature position of the cell, PBGs and lasing wavelengths are blueshifted. In order to understand this laser tuning behavior, we studied the change of the PBG depending on the temperature because the lasing wavelength in CLC is determined by the PBG edge.

Figures 5(a) and 5(b) show PBGs of the PL-cell that has left-handed helicity and PR-cell that has right-handed helicity at 17.5°C (black line), 21.3°C (red line), 25°C (blue line), and 29°C (green line), respectively. We measured the PBGs at fixed position. As the temperature of the cells rose, the position of the PBGs blueshifted, and the PBG width measured from the full width at half-maximum of the PBG peak decreased; see Figs. 5(a), 5(b), and 6(a). We calculated the PBG width of Fig. 6(a) and the high-energy (HE-) and low-energy (LE-) PBG edge [Fig. 6(b)] from the data of Figs. 5(a) and 5(b). In both the PR- and PL-cell cases, the temperature dependence of the PBGs was very similar to each other. Their similar behaviors with temperature dependence could be due to the following factors: (1) the same NLC matrix (ZLI2293) was used; (2) appropriate chiral molecular concentrations were employed to have the maximum tuning range of the PBG in a similar visible spectral range by temperature control. The only difference was the HTP. Other optical properties were almost same.

 

Fig. 5. PBGs at the 17.5°C (solid black line), 21.3°C (dash dotted red line), 25°C (dot blue line), and 29°C (solid green) of the (a) PL-cell and (b) PR-cell. (c) PBGs depending on the chiral molecular concentration of the PC-cell. Black line, 25.5 wt. %; red line, 26.5 wt. %; blue line, 27 wt. %; green line, 27.5 wt. %.

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Fig. 6. (a) PBG bandwidth of the PL-cell and the PR-cell by temperature change. (b) PBG low- and high-energy band of the PL-cell and the PR-cell by temperature change. (c) PBG position as a function of temperature (open square and open circle) and chiral molecular (S811) concentration (solid star).

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When their optical properties with temperature change were examined in detail, temperature change resulted in a greater shift of the HE-edge compared to that of the LE-edge [Fig. 6(b)]. We could explain this by the temperature dependence of the nematic mixture’s refractive indices. The LE- and HE-edges are determined by $n_o\times P(\text{pitch})$ and $n_e\times P$ and the behavior of $n_e$ is different from that of $n_o$; with the increase in temperature, generally $n_e$ decreases greatly, while $n_o$ decreases less [34,35].

On the other hand, this blueshift of the PBG on the temperature increase could partially be explained by the research of Kutulya et al. [36], where chiral dopants with high molecular anisotropy molecules will cause the cholesteric helix to twist, while chiral dopants with low-anisometric molecules will cause the cholesteric helix to untwist with increasing temperature. Increasing the dopant molecular anisotropy by means of $\pi$-elecronic skeleton extension affects most significantly the quantitative characteristic of the pitch dependencies [36]. Thus, we could conclude that the S811 and the R811 have high molecular anisotropy so that as temperature increases, the cholesteric helix twists and the PBGs of the WL-cell and the WR-cell are blueshifted.

On the other hand, as temperature increases, chiral dopant solubility could slightly increase, although there is somewhat of a material dependence [37]. Figures 5(c) and 6(c) show experimental results of the PBG shift depending on the chiral molecular concentration of the PC-cells filled with L-CLC. As the chiral molecular concentration increases, the PBG of the CLC cell blueshifts. The PBG behavior depending on concentration is very similar to that of temperature dependence; see Figs. 5(a), 5(b), and 5(c). Furthermore, in Fig. 6(c), about 1 wt. % increase of the chiral dopant (S811) solubility could cause an $\sim 30\mathrm{nm}$ wavelength blueshift. Therefore, in this experiment, a small increase in the chiral molecular concentration may also cause the blueshift. Hence, we believe that the PBG blueshift of the WR- and WL-cell on temperature increase results from the combined effect of three factors: the decrease of the refractive indices, the high anisotropy of the chiral molecules (S811 and R811), and the increase of the chiral molecular solubility.

4. CONCLUSION

We realized active fine laser tuning in a broad spectral range with dye-doped wedge CLC samples. In order to understand the laser tuning mechanism, we studied the temperature dependence of the optical properties of the photonic bandgap of the CLCs with left-handed and right-handed helicity. Our study results revealed that the spatial pitch gradient of each $X$-position of the wedge cell at room temperature was almost maintained after a temperature gradient was developed. In addition, the tuning range of lasing and PBG depended on physical properties of the CLC used. The tuning range could be actively controlled by temperature. In order to achieve the maximum tuning range, the chiral dopant concentration, thickness, thickness gradient of the wedge cell, and temperature gradient on the wedge cell should be properly matched. In both our CLC samples with left-handed or right-handed helicity, the PBGs were blueshifted and the widths of the PBGs were decreased. The dynamic behaviors of their PBGs were almost the same when temperature was increased. Our study results revealed that the employed chiral materials have high molecular anisotropy, the refractive indices of CLCs were decreased, and the chiral molecule’s solubility was increased by temperature increase. This temperature dependence study of the CLCs could be useful in the development of practical active tunable CLC laser devices.