1. Introduction

Cholesteric liquid crystal (CLC) is a typical phase in LCs. In such phase, because of the Van-de-Waals interaction between molecules, LCs always form a self-assembled helical structure with the pitch of several hundred nanometers, and produce the periodic refractive index modulation for the circular polarized light, resulting in a photonic band-gap. When some fluorescence dyes are doped into the CLC systems and the emission spectrum of the dye overlaps one edge of the band-gap, the photon density of states (DOS) at the edge reaches to the maximum. Thus, the laser is pumped out. This is the dye-doped liquid crystal band-edge laser [1,2]. Such kind of laser possesses many advantages, e.g. simple fabrication, low lasing threshold, and electrically-controllable. Therefore, it has attracted abundant attention from theoretical and experimental viewpoints [3–6]. For instance, Lin et al. demonstrated a phototunable laser by irradiating a photoisomerized material doped CLCs with the ultra-violet (UV) source. The helical pitch of CLC can be changed after the exposure due to the trans-cis isomerization of the dopant [7]; Huang et al. obtained a spatially tunable laser by forming a one-dimensional temperature gradient along the CLC cell, or by utilizing the solubility gradient of the chiral dopant across the cell [8,9]. In recent, Yoshida and his colleagues demonstrated a fast electrically tunable wavelength-swept laser by adopting the frequency mismatch between the pump source and the modulation signal, and the tuning range of 11 nm was reported [10]. Yu et al. developed a novel CLC with electrically tunable pitch with a non-constant distribution in space across the cell. The lasing wavelength can be tuned for about 33 nm by applying 324-V-voltage [11]. Other researchers used the polymerized CLC that sealed in a wedge cell to realize the spatial tuning of wavelength, about 70-nm-tuning-range was reported [12,13]. Such polymerized CLC is beneficial for the stabilization of helical structure, while losses the electrically tunability.

So far there have been a lot of works conducted on the wavelength tunability of CLC laser [10–14], while the work focused on emission energy modulating is rare. Considering the low energy consumption, easy energy tuning, and its potential applications on laser display, integrated optics, etc, the energy tuning is also significant. Furumi et al. reported an energy-electrically-controllable laser based on CLC photonic band gap (PBG). By applying 20-V-voltage, the lasing can be switched off [15]. However, as the conventional CLCs, the helical structure is always unstable under external field, and after the field is removed, the structure may either be destroyed to form the focal conic state with large light scattering, or recovered to the original state in a very long time (normally needs several hours). Therefore, in consideration of helix stabilization and good modulability, polymer stabilized CLC (PSCLC) is a suitable choice. Shibaev et al. conducted polymer stabilization to obtain a stable cholesteric negative liquid crystal. By applying an electric field across the cell, the domains’ planar orientation could be improved, and thus the laser emission is enhanced. They reported that the emission energy is electrically modulated in a small range from 0.90~1.03 (arb. units) [16]. As most of reported works, UV-light initiation systems are often used in polymer stabilization, however such initiation systems always lead to the deformation of PBG, and affect the lasing performances in further. Besides, the UV-light may also result in the deterioration of LCs. Hence, a suitable initiation system should be reconsidered. In addition, another key issue of polymer stabilization, the relationship between the monomer concentration and the lasing or the electrically tunable performances, is not clear at present, so it is urgent to carry out related studies.

In this paper, we fabricated an energy-electrically-modulated CLC laser by using a kind of high dielectric constant LCs. Lasing energy can be controlled with a low voltage of less than 10 V. Polymer stabilization with a well-selected visible light initiation system was proposed and implemented to maintain the PBG of CLC as well as keep the modulability. The materials and the related experimental methodologies are expressed in section 2. In section 3, the monomer concentration dependent characteristics of lasing threshold, the performances on the modulability, the hysteresis and the response time are discussed.

2. Materials and experiments

In our experiments, we prepared five sets of samples, within which the monomer concentration in descending order were respectively 12.9, 9.3, 7.0, 4.7 and 0 wt%. The CLC system was prepared by mixing a kind of high dielectric constant nematic liquid crystal XH-07 (Δn = 0.169, Δε is tested as 33.3 @ 20°C, 1kHz, supported by Xianhua Co., Ltd.) with the right-handed chiral agent R811 (supported by Merck, Germany). The concentration of R811 in CLC was ~21.5 wt% to guarantee the long wavelength band edge overlaps with the emission spectrum of dye (at ~600 nm). Monomers for polymer stabilization were mixed by PTPTP_n (containing PTPTP₆ and PTPTP₂, which were synthesized by our lab, with the weight ratio of 1:1) and 2-ethylhexyl acrylate (2-EHA), with the weight ratio of 2:1. Chemical structure of PTPTP_n was presented in details in our previous papers [17,18]. In addition, a small amount of photoinitiator (Irgacure 784, supported by BASF) was added to initiate the polymerization with the visible source, 532-nm-laser. In our work, 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostryl)-4H-pyan (DCM, supported by Aldrich) was selected as the laser dye, and was doped into the mixture with a small amount of 0.5 wt%. The whole mixture was stirred at 45°C for about 1 hour, capillary filled into the 15-μm-thick cell, and then slowly cooled down to room temperature in a darkroom. Finally a sample with well-defined Grandjean alignment was formed. The samples were exposed to the continuous-wave 532-nm-laser for 10 min, and the intensity was about 5.5 mW/cm² (this intensity is not sufficient to produce the lasing). Thus the CLC alignment was maintained by polymer. The transmission spectra measured before and after exposure were compared to ensure that the stop-band was not damaged after the polymerization.

The lasing characteristics and the electrically modulated performances of the samples were tested with the setup depicted in Fig. 1(a) . As shown, a linear-polarized second-harmonic Q-switched Nd:YAG pulsed laser (Beamtech Co., Ltd. Canada) was used as the pump source to excite the dye-doped CLC samples. The wavelength is 532 nm, pulse width and repetition rate are 6 ns and 1 Hz, respectively. In order to avoid reflection caused by PBG, a quarter wave plate (QW) was used to convert the linear polarization of pump beam into a left-handed circular polarization. The beam was then equally divided into two parts by a beam splitter (BS), of which one was incident pump beam, and the other was received by the energy meter to conveniently detect the energy of pump beam. The pump beam was focused to a small spot of ~150 μm diameter by passing a convex lens with 25cm focal length. The emitted laser from the samples was collected by a fiber connected USB spectrometer (Avaspec-2048 from Avantes) after transmitting a color filter (CF) which having a passing band from 550 to 650 nm to filter out the scattered pump light. A voltage was output by the signal generator and applied on the sample, so that the modulability of the CLC laser can be measured.

 

Fig. 1 (a) The schematic experimental setup for the lasing and measurement. A: attenuator; BS: beam splitter; CF: color filter; L: lens; P: polarizer; QW: quarter wave plate; (b) Experimental setup for measuring the response time. A: attenuator; D: high sensitive Si-based; detector; E: expander; L: lens; S: sample; LED: 550-nm-LED source.

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The response time was tested with the setup shown in Fig. 1(b). The testing light was expanded and impinged on the sample. A square wave signal with a voltage of 9 V and a frequency of 1 kHz was applied on the sample to control the transmission intensity. Then the transmission light was received by a high sensitive Si-based detector and analyzed with the oscilloscope. The testing light source used here is a monochromatic LED with the wavelength of 550 nm.

3. Results and discussions

To obtain a stable well-defined Grandjean alignment of CLC sample and a good modulated performance of CLC laser, polymer stabilization was carried out for the first step. As mentioned above, the UV-initiation system is not suitable for the polymer stabilization of such dye-doped CLC sample, so we adopted the visible light initiation system, in which polymerization can be induced by the continuance-wave 532-nm-laser. However, first of all, we fabricated the PSCLC sample by using the UV-initiation system, for studying the effect of such initiation on the PBG and the reason of that. The 365-nm LED UV lamp was selected as the exposure source to cure the sample. After UV polymerization, we found that the reflection band of the samples was broadened (shown in Fig. 2(a) ), and the effect of PBG almost disappeared. To make clear the reason for the change of PBG, the absorption of initiator and DCM at 365 nm was tested and compared. As shown in Fig. 2(b), the absorption of DCM is larger than that of initiator at 356-UV position; while at 532 nm position, the absorption decreases to less than a half of that at 365 nm. So that, the disappearance of PBG may be caused by the larger absorption of DCM at 365 nm, which may lead to the attenuation of UV intensity as the light transmits through the sample. Thus the light intensity gradient forms and introduces a pitch gradient, so the reflection band is broadened. Some similar phenomenon was also reported and discussed in previous work [19]. Due to the broadening, the light resonance and amplification cannot be obtained in such structure.

 

Fig. 2 (a) Transmission spectra of the sample before and after UV curing; (b) Absorbance of DCM and photoinitiator with 0.02 wt% concentration; (c) Transmission spectra of the sample before and after 532-nm-laserirradiation.

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Consequently, we tried to change the irradiation source as the continuance-wave 532-nm-laser to avoid the light intensity gradient, since the absorption of DCM at 532 nm is much lower than that at 365nm (as the absorbance shown in Fig. 2(b)). Figure 2(c) compared the transmission spectra, which was respectively measured before and after the cell was cured by a 532-nm-laser. We can find that there is almost no evident change on the edge of stop band. The center and the width of the reflection band are almost the same, which indicates that the influence of polymerization on the planar alignment is very small. These facts approve the feasibility of visible-light-curing to realize the polymer stabilizations as well as a good PBG effect.

The laser emission thresholds (LETs) of all cured samples (with different monomer concentration) were measured. The pump energy versus laser emission energy of the samples was tested to determine the LETs. For instance, Fig. 3(a) shows the result for the sample with 4.7 wt% monomer. No obvious lasing emission is detected until the pump energy reaches 2.04 μJ/pulse; as the energy exceeds this threshold, the emission energy linearly increases with the rising of pump energy. Figure 3(b) gives the LETs corresponding to the monomer concentration. Being similar with that reported previously [20], the LET slightly decreases with the increasing of the monomer concentration. It is found that the LET decreases to 1.86 μJ/pulse as the monomer concentration is 9.3 wt%. We consider that the reasons for this tendency can be attributed to a stable and better helix in CLC, which improves the optical gain, when the monomer concentration is higher. It is noteworthy that although increasing the monomer concentration can lead to the drop of LET, it does not necessarily mean that the higher monomer concentration is the lower LET will be. Our experiments also found that much higher monomer concentration (>12.9 wt%) may destroy the well-defined helical CLC alignment, and thus affect the monochromaticity of the laser.

 

Fig. 3 (a) Pump energy dependent laser emission energy (sample with 4.7 wt% monomer concentration); (b) Tested LETs for the samples. LETs for samples with 0, 4.7, 7.0 and 9.3 wt% monomer is about 2.16, 2.04, 1.95, 1.86μJ/pulse.

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The electrically modulated performances of the samples are studied. The voltage applied on the sample usually leads to realignment of the LCs and results in the deformation of CLC helical structure. So the emission energy will decrease as the increasing of the voltage. At the first, the voltage dependent laser emission energy of the samples was tested. The samples without polymer stabilization (0 wt% monomer) or with lower monomer concentration (4.7 wt%) were not stable. When a voltage applied, the Grandjean alignment will be disturbed, the scattered focal conic state appears instead [21]. Besides, the LCs will always be hard to recover to the original alignment as the voltage is removed. So the voltage-versus-emission energy curves in these two cases were not given here. However, samples with slightly higher monomer concentration, e.g. 7.0 and 9.3 wt%, show a good modulated characteristic. As the green curves given in Figs. 4(a) and 4(b), the sharp decline of emission energy appears as the voltage exceeds the threshold. The threshold voltage (V_th) and saturation voltage (V_S) for the two samples are almost the same. V_th is 7.3 V for the sample containing 7.0 wt% monomer, and 7.4 V for that of 9.3 wt% monomer; V_S is 8.3 V for the former and 8.2 V for the latter.

 

Fig. 4 Voltage-dependent laser emission energy for the samples with (a)7.0 wt% and (b) 9.3 wt% monomer concentration. (c) The PBGs at different applied voltage and the spectrum of the emission laser. (d) Voltage-dependent LETs. (e) Laser patterns at the voltage of 7.4, 7.8 and 8.2 V.

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The reason for such low modulated voltages is analyzed from two aspects. For one thing, in our experiments, a kind of high dielectric constant (Δε) LCs was adopted. In general, the drive electric field of CLC device is usually inversely proportional to Δε of LCs. We analyzed the composition of the LCs used in the experiment, and found the LCs is mainly composed by the cyano-biphenyl based materials. The cyano-group usually leads to a large dipole moment and polarisability of LC molecules, which are proportional to Δε according to Maier-Meier theory [22]. In addition, some amounts of nitro-amino tolane with large push-pull effect were also found. Such push-pull effect enhances the dielectric anisotropy of the molecules and increases the Δε to about 60, as previously reported [23]. Doping such molecules into the LC mixture is an effective way to enlarge the Δε of the mixture. We tested the capacitance-voltage (C-V) characteristic of the LCs that we used. The result shows a large Δε of 33.3, which is about 3 times of the commercial LCs; and a very low threshold voltage of about 0.4 V, which is only a half of the conventional used LCs [24]. Thus, high dielectric constant of LCs is one of main reasons that lead to the lower modulated voltage of CLC laser. For another thing, we tested the transmission spectra as a voltage was applied on the sample. As shown in Fig. 4(c), the stop-bands at V_th(7.4 V), and V₅₀ (defined as the voltage corresponding to the half emission energy, 7.8 V) almost coincide with that in the case of V = 0; while at the V_S(8.3 V), an evident widening is found. Such results indicate that the emission laser can be totally turned off only if the helix suffers from a small structural deformation. To study it in further, the voltage-dependent LETs was tested. As shown in Fig. 4(d), LET slightly increases when the applied voltage is lower than 6.5 V, however an evident rising appears as the voltage exceed 6.5 V. When the voltage increases to 8 V, the LET rises to about 7 μJ/pulse, and only a very small emission energy can be detected in this case according to Figs. 4(a) and 4(b); while as the voltage reaches the V_S, 8.3 V, LET becomes too large to depict in Fig. 4(d) (as the sharp dash line shown), and no lasing can be found even if the applied voltage is continually increased. The results indicate that the pump energy is not a decisive condition for the lasing. When the PBG is destroyed, the lasing disappears regardless the energy of pump source. So the other main reason for the lower modulated voltage may be related with the sensitivity of emission on the PBG structure. Apart from those, it is also found that the position of emission laser, which locates at the long-wavelength edge of the PBG (~600 nm), does not change with the voltage.

The polymer composite LC materials usually have more or less hysteresis effect, so the emission energy associated with voltage ramp-down was explored. As the blue-color line shown in Figs. 4(a) and 4(b), the hysteresis is obvious in the two samples, and is enhanced with the increasing of monomer concentration. The width of hysteresis loop for 7.0 wt%-monomer-sample (ΔV = 0.4 V) is less than a half of that for 9.3 wt%-monomer-sample (ΔV = 0.9 V). Figure 4(e) shows the far-field pattern of emission laser during the voltage is applied. When the voltage is close to the threshold (7.4 V), the laser pattern is red and bright, and the far-field interference ring can be discerned, which indicates the high coherence of the emission laser. As voltage increases to 7.8 V, the energy of laser is weakened obviously. And if the voltage rises continually to the saturation value, 8.2V, the laser is turned off. Thus the electrically-energy-modulability of the CLC laser is realized.

As a kind of external-field-modulated device, the response characteristic is an important aspect for the applications in laser display, holography and other optical systems. Thus the response time was tested through the setup depicted in Fig. 1(b). The reason for selecting 550-nm-LED as the testing source is that such wavelength locates out of the CLC stop band in the case of low applied voltage. However if the applied voltage is higher enough (9 V that we used in the testing), the wavelength will be contained in the stop band due to the widening of the band mentioned above. Therefore, the transmission intensity will be decreased when applying the voltage. We tested the rise and decay times of the samples with 7.0 wt% and 9.3 wt% monomer, respectively.

Figures 5(a) and 5(b) directly show that the rise time for the sample with 7.0 wt% monomer is 15 ms, and the decay time is much longer, 104 ms, as similar as the conventional nematic LCs; for the sample with 9.3 wt% monomer, the rise time is extended to 29 ms and the decay time is shortened to74 ms, due to the stronger anchoring energy in the system [23]. To shorten the response time in further, raising the drive voltage may be a simple and feasible way.

 

Fig. 5 Results for the response time. (a) 7.0 wt% monomer concentration, rise time: 15 ms, decay time: 104 ms; (b) 9.3 wt% monomer concentration, rise time: 29 ms, decay time: 74ms.

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4. Conclusions

In conclusions, we have prepared a kind of electrically modulated dye-doped CLC laser. Polymer stabilization with a visible light initiation system was adopted to maintain a well-defined Grandjean alignment as well as the modulability of CLC laser. The emission energy can be modulated by a very low voltage(less than 10 V) which can be matched with integrated circuit. The reasons for such low voltage are ascribed to the high dielectric constant of LCs and sensitivity of the emission on the PBG structure. In addition, the emission energy is very sensitive to the applied voltage, which shows a potential application in the field of sensor. The effect of monomer concentration on the laser emission threshold, hysteresis, and the response time were discussed. The results indicate that sample with high monomer concentration usually possesses lower LET and faster response; however the hysteresis also enhanced simultaneously.