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Spatially tunable lasing emissions from dye-doped heavy chiral concentration liquid crystals (DD-HCCLCs) were investigated in this work. After thermal control of HCCLCs to below the phase transition temperature, the central wavelength of the photonic bandgap (PBG) was sensitively related to the working temperature, which could be tuned widely with the relatively small temperature variation owing to the phase transition from the cholesteric (Ch) to the smectic (Sm) phase. By means of Keating’s theory, the change in helical pitch, obtained from the transmission spectrum, versus temperature can be well fitted to derive the phase transition temperature of the smectic A phase. As the pump beam was focused on different locations of the DD-HCCLC cell with a one-dimensional temperature gradient, we demonstrated spatially tunable lasing peaks over one hundred nanometers from 556 nm to 671 nm. The experimental result regarding the spatially tunable lasing property shows that the DD-HCCLC laser could be a promising light source in display technology.
© 2015 Optical Society of America
1. Introduction
Owing to selective reflection, photonic crystals (PCs) with a photonic bandgap (PBG) have been widely investigated and applied in the area of the electro-optical and display industry. Inside PCs, the refractive index reveals spatially periodical alternation that can be easily realized in different kinds of liquid crystals (LCs) with chirality, such as cholesteric liquid crystals (CLCs) [1], chiral smectic liquid crystals [2], and blue phase liquid crystals [3]. The LCs with chirality can therefore be used as wavelength selection and tunable devices such as optical filters, wavelength-division multiplexing (WDM), projective displays and the output coupler in LEDs, etc. The other promising utilization of PCs, for instance, dye-doped cholesteric liquid crystal (DD-CLC) laser has been theoretically proposed by Kopp et al. [4] and experimentally demonstrated [1] owing to the enhancement of the photon density at the edge of PBG with zero group velocity.
For the practical utilization of the DD-CLC laser in display technology, wide and fast tuning of the emission wavelength is needed, which has been achieved by means of the mechanical stress [5], electric field [6], temperature [7], and so on. In contrast to the wavelength tunable DD-CLC laser whose structure of PBG could be easily destroyed by applying a relatively strong electric field [8]. The thermal tuning method provides an inexpensive substitution [9, 10]. However, the temperature modulation of PBG in DD-CLC lasers is sometimes impractical because of the relatively long response time that is restricted by the thermal conductivity of LCs. On the contrary, the spatially tunable method provides a superior replacement with several advantages, such as high stability, time-saving and wide spectrum tunability. In previous reports, spatially tunable DD-CLC lasers have been achieved by creating a pitch gradient through temperature [11], chiral dopant concentrations [12], geometry of the cell [13], and exposed different intensity of UV irradiation [14].
As the temperature increases, the wide tuning of PBG for the negative dielectric CLC from near IR to visible has been exhibited by Natarajan et al. [15]. This shift is attributed to the smectic-cholesteric (Sm-Ch) pretransitional effect instead of the temperature dependent solubility of the chiral agent reported by Huang et al. [7]. Besides, Tzeng et al. revealed the extensive shift of the reflection notch toward longer wavelengths by the thermal control of the CLCs below the phase transition temperature [16]. In this work, to the best of our knowledge, we have first experimentally demonstrated the spatial tuning of laser emissions from dye-doped heavy chiral concentration LCs (DD-HCCLCs) by the pretransitional effect from the Ch to the Sm phase. Over a 100 nm shift of the lasing wavelength has been observed by slightly shifting the location of focused pump beam on the cell with the a one-dimensional temperature gradient.
2. Sample preparation and experimental setup
The various chiral nematic liquid crystals (Chiral NLCs) were produced by adding different concentrations of the left handed chiral molecules (S811, HTP = 10.8um−1) into the NLCs (MDA-98-1602, ne=1.777 and no=1.5113, clear point 109 °C, Merck Inc.). Besides, 0.5 wt% PM597 (Pyrromethene 597, Exciton inc.) was used as an active medium. The LC mixtures were uniformly mixed in a small vessel and heated by hot plate to let the LCs operate in the isotropic (Iso) phase. The homemade glass cell was constructed by two separated ITO-glass plates through 25 μm plastic spacers. Inside the cell, anti-parallel rubbed polyimide (PI) was coated on both plates. Then, the LC mixtures were injected into an empty glass cell by the capillary effect. In order to confirm the shift of PBG versus temperature, we measured the transmission spectrum by using the white light source (LS-1, Ocean optics inc.) and the spectrum meter (USB4000XR, Ocean optics Inc.). Furthermore, we studied the phase transition of the LC mixtures by the reflective polarization optical microscopy (RPOM) and the differential scanning calorimetry (DSC) technique.
The schematic setup for the band-edge lasing generation from DD-HCCLCs is shown in Fig. 1. The DD-HCCLC cell was optically pumped by using a linearly polarized frequency-doubling Q-switched Nd:YAG laser with central wavelength at 532 nm. The excited pulses of 10 Hz repetition rate and 2.2 ns pulse duration were spatially expanded by two lenses (L1 and L2) of focal lengths 2.5 cm and 7.5 cm, respectively. After collimation by two reflection mirrors M1 and M2, the pump beam was focused on the DD-HCCLC cell through the convex lens (L3) of focal length 15 cm. The estimated spot size on the LC cell was around 5 μm. Besides, the produced LC cells were mounted in a water cooling copper block to control the working temperature. A thermal sensor was contacted on one side of the cell for monitoring the working temperature. A spectrometer (HR-4000, Ocean Optics Inc.) with a fiber tip was used to measure the emission spectra of the DD-HCCLC laser.
Fig. 1 Schematic setup for band-edge lasing in DDCLC laser. Inset shows the photograph of the Q-switched Nd:YAG laser.
3. Results and discussion
The characteristic of temperature dependent phase transition from chiral NLCs with different concentrations of S811 was studied by the DSC measurement as shown in Fig. 2(a). When the concentration of S811 was normal (16 wt% and 25 wt%), the LCs were operated in the Ch phase within our measured temperature range. At this state, 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 PC with the PBG. Besides, two dips can be obviously seen at two distinct temperatures, TI and TS, from chiral NLCs with heavy concentrations of chiral agent (>34 wt%). When the working temperature was controlled between TI and TS, the LCs were operated in the Ch phase. The LCs would transit from the Ch to the Iso phase as the temperature was increased above TI. As we cooled the sample below the lower temperature dip (TS), the LCs finally transited to the smectic A (SmA) phase. At this state, the arrangement of the LC molecules would become parallel to form a layer structure and then the PBG disappeared. The phase transition temperatures TI and TS as function of the doping concentration of the chiral agent are shown in Fig. 2(b). It is noted that two phase transition temperatures (TI and TS) get close to each other with the increment in chiral agent concentration. Thus, the phase transition from the Ch to the Sm phase will occur at a higher temperature when the concentration of the chiral agent increases.
Fig. 2 (a) The measurement of differential scanning calorimetry (DSC) from chiral NLCs with different concentrations of S811, and (b) the values of phase transition temperature (TS and TI) versus chiral concentrations.
Owing to the selective reflection of the PBG, only the left handed circular light with the same handedness of the CLC helix would be reflected. Thus, the transmission of CLCs decreases apparently within a certain wavelength range to show a PBG. In Fig. 3(a), the temperature dependent shift of PBG from chiral NLCs with heavy (42.5 wt%) and normal (25 wt%) S811 concentrations are shown from the transmission spectrum. For the chiral NLCs with heavy concentration of S811 (42.5 wt%), the location of PBG can be shifted obviously from near UV to near IR with only a 3.3 °C temperature variation when the working temperature was below the phase transition temperature (TS=15 °C). With a further decrease in temperature below 12.2 °C, the LCs would transit to the Sm phase and cause the disappearance of PBG. The fact can be further approved by measuring the IR spectrum meter (NIR Queset, Ocean Optics Inc.) with a spectral range from 900 nm to 2200 nm. However, the shift of PBG versus temperature was almost indiscernible for the chiral NLCs with a normal concentration of chiral agent (25 wt%), as shown in the inset of Fig. 3(a).
Fig. 3 (a) Temperature dependent shift of PBG from the Chiral NLCs with concentration of S811 about 42.5 wt% and about 25 wt% shown in the inset, and (b) variation of pitch as a function of the temperature from Chiral NLCs with different concentrations of S811 and the fitting curves from Keating‘s theory. The inset shows the pitches obtained from the lasing emission peaks and the theoretical estimated curves from Eq. (2).
In the previous report [1], the central wavelength (λc) of PBG from CLCs can be theoretically predicted by the formula:
Here, navg = (no + ne)/2 is the average refractive index with no and ne being the ordinary and extraordinary refractive index, respectively; P is the helical pitch of a CLC cell which can be estimated with the relation P=1/(HTP*C). Here, HTP and C are the helical twisting power and the concentration of the chiral agent. Using the central wavelength (λc) of PBG from the transmission spectrum and Eq. (1), the helical pitch P from chiral NLCs with different chiral concentrations versus temperature T in kelvin (K) are shown in Fig. 3(b). For the chiral NLCs with normal chiral concentration (25 wt%, 31 wt%, and 34 wt%), the pitch only shows a slight redshift as the temperature decreases. On the contrary, the pitch varies apparently as the temperature decreases below a certain value for the chiral NLCs with heavy concentrations of S811 (≥40 wt%).The variation in pitch versus temperature can be fitted by using Keating’s theory [17] as temperature T > TA:
where TA is the transition temperature of the SmA phase, β is a measure of the temperature sensitivity of pitch P, and the scaling factor γ depends on the anharmonicity factor, interplanar distance and molecular moment of inertia [15, 17]. In Fig. 3(b), the pitch variation versus temperature (solid symbols) can be well fitted by the theoretical curves (solid lines) from Eq. (2) to obtain the fitting parameters γ, β and TA for the chiral NLCs with heavy concentrations of S811 about 40 wt%, 42.5 wt% and 45 wt%, respectively, in Table. 1. As the concentration of S811 increases from 40 wt% to 45 wt%, the parameters of γ reveal monotonic decrease and TA increase from 6.93 °C (280.08 K) to 14.40 °C (287.55 K), whose trend is consistent with the measurement from DSC. However, the obtained values of TA listed in Table 1 are lower than the values of TS measured by the DSC. It is recognized that the LCs begin phase transition from the Ch to the Sm phase when the working temperature is below TS. As the temperature approaches TA, the pitch tends to be infinity from Eq. (2) which indicates the LCs in the Sm phase. Thus, it means that the LCs were manipulated in the pretransitional state as the working temperature was operated between TS and TA. In this state, the PBG of chiral NLCs becomes relatively sensitive to the working temperature. Thus, the HCCLCs can be used as thermal sensor, temperature tunable mirror, and lasing emission tunable laser and so on.
Table 1. Fitted parameters obtained from temperature related pitch variation by the fitting of Keating‘s theory
When the one edge of the HCCLC cell (42.5 wt% S811) was mounted on the water cooling copper block with a fixed temperature of 10 °C, the one-dimensional temperature gradient was produced on the cell as shown in Fig. 4(a). It reveals the spatially dispersed color distribution from red to blue. Besides, the LCs at different locations would be operated in a distinct state that can be revealed by an RPOM. In Fig. 4(b), the RPOM image displays the oily streaks texture to demonstrate the Ch phase for the HCCLCs at higher temperatures. As the working temperature decreased, the color of the RPOM varied from blue to red (Fig. 4(b)) which is consistent with the shift of PBG toward the long wavelength in Fig. 3(a). In contrast to the report from Huang et al. [7], we did not observe aggregation or chain-like structures from the chiral NLCs with heavy chiral concentration in RPOM. Thus, the shift of PBG versus the temperature from the HCCLCs in this work is due to the phase transition from the Ch to the Sm phase instead of the solubility of the chiral agent. With the assistance of the Thermoelectric Cooling Chip, the central wavelength distribution of the PBG from red to blue would be achieved around 1 s. However, the lasing emission from DD-HCCLCs can be excited instantaneously after pump by the Q-switched laser.
Fig. 4 (a) The photograph of HCCLCs mounted on the water cooling copper block with a one-dimensional temperature gradient, and the image of RPOM at different locations of HCCLCs to show (b) the oily streaks texture in the Ch phase with color change from blue to red, and (c) a phase transition from Ch, TGB to the SmA phase with the fan-shaped texture as the temperature decreases.
After a further decrease in temperature, the twist grain boundary (TGB) phase was observed on the right of Fig. 4(c) with a rather small operation temperature range [18]. Thus, the pretransitional state including the Ch and TGB phases can be observed between TS and TA. Finally, the LCs would transit to the SmA phase whose corresponding RPOM image showed the fan-shaped texture (right in Fig. 4(c)). In the SmA phase, the selective reflection band would vanish so that PBG disappeared completely in our measured transmission spectrum.
After doping laser dye (PM597) into the HCCLCs, the DD-CLC laser [1] can be generated after pump by the Q-swithced laser whose experimental setup is shown in Fig. 1. Owing to the temperature gradient, the position dependent PBG can be used to produce a spatially tunable laser by slightly shifting the location of the pump beam on the DD-HCCLCs (42.5 wt% S811) below the phase transition temperature TS. Because the lasing range of LC cell was pretty small, below 4 mm as shown in Fig. 4(a), we mounted the sample on the translation stage so that switch time is dominated by the moving speed of translation stage. Actually, even faster tuning of lasing wavelength can be achieved by the light deflection devices. The moving direction is shown in the Fig. 4(a). Because the HCCLCs are relatively sensitive to the temperature, the thermal control is very important. In order to precisely control the environmental temperature, the room temperature was fixed at 20 °C. With pump energy around 7 μJ, the central wavelength (λp) of the lasing peak would be shifted from 556 nm to 671 nm as the pump beam was focused on the different locations of the DD-HCCLC cell (42.5 wt% S811) in Fig. 5(a). The FWHM of lasing peak was around 0.7 nm. Owing to dislocation of chiral LCs, the defect mode could be excited to generate additional peaks beside the main peaks as shown in Fig. 5(a). Because the defect mode lasing would be excited randomly on LC cell, it is not the main topic that we concern in this work and will be discussed in the following task.
Fig. 5 (a) Laser emission peaks obtained by shifting the focused pump beam on different locations of the DD-HCCLCs and the blue dash curve shows the fluorescent spectrum of PM597, and in-out characteristics of DD-HCCLCs with emission wavelength at 580 and 604 nm, respectively. (b)–(e) The different colors of the projected lasing beam on the screen.
In considering the lasing peaks (λp), the pitches of LCs, obtained by using Eq. (1), were shown in the inset of Fig. 3(b) (open green diamond: 42.5 wt% S811). With the assistance of the theoretical fitting curve (solid green line) from Eq. (2), we can deduce that the working temperature of the DD-HCCLC laser changes from 12.34 °C (285.49 K) to 13.41 °C (286.56 K). It demonstrates that wide detuning of the lasing wavelength over 100 nm can be reached with a 1.07 °C temperature variation. The spatially tunable of lasing emissions can also be observed from the other DD-HCCLCs (40 wt% S811) whose estimated pitch (open pink triangle) and theoretical fitting curve (solid pink line) are also shown in the inset of Fig. 3(b).
Besides, the in-out characteristics of all the emission peaks from DD-HCCLC laser in Fig. 5(a) can be measured by the integrated intensity from emission spectrum at different pulse energy. In order to clarify the plot, only the output intensity versus the pulse energy with λp = 580 (green circles) and λp = 604 nm (brown squares) are shown in the inset of Fig. 5(a). As excited energy increases, two slopes can be obtained by the linear fitting to show the spontaneous emission and stimulated emission. Besides, the slope efficiency with λp = 580 nm would be slightly higher than λp = 604 nm because of the higher gain of laser dye. The gain would be saturated at higher pump energy for the lasing peak λp = 580 nm. The threshold of pulse energy for DD-HCCLC laser is estimated to be 2.7 μJ/pulse and 2.3 μJ/pulse for the long (604 nm) and short wavelengths (580 nm), respectively. Besides, the lasing threshold would become a little bit higher as lasing emission wavelengths were far from fluorescence emission peak. Figures 5(b) to 5(e) also show the projected laser beams from green to red colors on the screen.
4. Conclusion
In conclusion, we have demonstrated the spatially tunable lasing emissions by the thermal control of a dye-doped HCCLC cell to produce a one-dimensional temperature gradient. Different from the solubility in previous reports, the mechanism was based on the pretransitional effect of LCs from the cholesteric to the smectic phase, which can be demonstrated by the measurement of differential scanning calorimetry and reflective polarization optical microscopy. By the fitting of Keating’s theory, the phase transition temperature of smectic A phase was estimated from the temperature dependent pitch variation. As the concentration of S811 was about 42.5 wt%, the tuning wavelength from 556 nm to 671 nm can be achieved by slight movement of the focused pump beam on the LC cell whose one edge was thermal control at 10 °C. Using the estimated pitch from the lasing peaks at different temperatures, we deducted the shift of lasing emissions over one hundred nanometers only by a 1.07 °C temperature variation. In this work, the intriguing physical mechanism and optical property of DD-CLC laser were discussed which could have opportunity to be used in practical application such as display technology in the near future
Acknowledgments
This work was supported by the National Science Council of Taiwan, Republic of China, under grant NSC 102-2112-M-027-001-MY3.
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