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The resonance characteristics of random lasers from dye-doped polymer dispersed liquid crystals (DD-PDLCs) within capillary tubes were investigated. After adding a monomer (NOA65) into the liquid crystal mixtures, the emission spectra from the capillary tube revealed multiple emission spikes with narrow emission linewidth due to enhancement of the light scattering. Besides, the number of emission spikes, full width of the half maximum (FWHM), and lasing threshold from the DD-PDLCs were determined by the density and grain size of the polymer clusters within the PDLC mixtures through the alternation of the monomer concentration. Furthermore, the lasing performance of the DD-PDLCs in the capillary tube can be controlled by temperature. At low temperature, more emission spikes at long wavelengths were excited, and the laser revealed a relatively high Q-factor, accompanied with relatively low-threshold pump energy. It is because the increase in the birefringence of the liquid crystal molecules efficiently enhanced multiple recurrent light scattering.
© 2014 Optical Society of America
1. Introduction
Random lasers, unlike conventional lasers with an optical mirror cavity, were produced by adding some amplifying medium into the random distribution media, and thus light diffusion could be efficiently reduced within the disordered materials [1]. Compared with regular lasers, the random laser has a compact cavity configuration and peculiar optical properties such as low lasing threshold, multiple emission lasing wavelengths, and broad solid angle output that have attracted much attention from scientists and physicists for specific applications [2–4]. For the random laser, the light trap was produced by the recurrent multiple light scattering within the disordered materials so that spatial coherence would be reduced to produce speckle-free full-field imaging [5]. In addition, Polson et al. have also reported the utilization of random lasers in biomedical diagnosis [6]. Up to now, various diffusive materials such as semiconductor powders [7–10], polymers [11, 12], single mode fibers [13], and biological materials [14] have been reported to generate random lasers.
In the display industry, liquid crystals (LCs) have been widely used because of their easy and fast modulation by the electric field. After doping of the chiral materials, dye-doped cholesteric LCs (DD-CLCs) with a one dimension photonic crystal structure have been experimentally demonstrated to generate band-edge lasing [15, 16]. Besides, the characteristics of random lasers from dye-doped LCs have also attracted much attention [17–23]. Lin et al. have demonstrated that the coherent random lasing behavior in DD-CLCs within oriented cell [17]. As the liquid crystal with the nematic phase, the arrangement of the molecules is along a common axis, which is termed the nematic director. However, some disordered alignment of the nematic LCs (NLCs) resulting from environmental factors such as temperature and multiple light scattering would be produced in the DD-NLCs which reduces light diffusion and forms the weak localization of light [18, 19]. In polymer dispersed liquid crystals (PDLCs), light scattering could be greatly enhanced after curing the monomer in the LCs to produce submicron- to micron-sized droplets of liquid crystal dispersed in a polymer matrix. Experimentally, the random lasing behavior in dye-doped PDLCs (DD-PDLCs) within a cell constructed with two parallel glass plates has been experimentally reported; however, the intensity contrast between the emission spike relative to the amplitude of the spontaneous emission is lower [24]. After adding additional nanoparticals, such as the silver or ZnO, into the DD-PDLCs, the characteristics of random lasers reveal higher intensity and lower threshold [25, 26].
For the practical application of random lasers, external manipulation of the resonance property is necessary. This has been done by changing the structure size [27, 28], limiting the excitation area [29, 30], and varying the optical filling factor [31]. Ardakani et al. have numerically investigated the semiconductor-based and superconducting-based random lasers whose emission spectrum can be widely tunable by the magneto-optical Voigt effect and temperature, respectively [32, 33]. In comparison with the other scattering materials, one of the superior advantages of the lasing behavior of the dye-doped LC laser is that its output property reveals a temperature-dependent characteristic [19–21], and it can also be modulated with magnetic and electric fields [23, 34]. Besides, an all-optically controllable random laser based on doping azo dye into the DD-PDLCs with nano-sized LC droplets has been reported [35]. Unlike the DD-PDLCs in a glass cell with only one dimensional confinement, in this experiment, we investigated the characteristics of random lasers from DD-PDLCs within two dimensional confinement capillary tubes. Furthermore, the resonance properties of DD-PDLCs including the lasing threshold and the number of emission spikes were shown to depend upon the doping concentration of the polymer molecules, and could be controlled by temperature.
2. Sample preparation and experimental setup
In this work, dye-doped liquid crystal (DD-LC) mixtures were prepared in a small vessel by doping 0.5 wt% of laser dye (PM-597) as the gain medium into the NLCs (MDA98-1602). Different kinds of DD-LC mixtures were studied, including one without monomer (Sample I) and the others with an additional doping of 10 wt%, 15 wt %, 20 wt%, 25%, 30 wt% and 35 wt% monomer (norland optical adhesive 65, NOA65) into the mixtures (Sample II, Sample III and Sample IV, Sample V, Sample VI, and Sample VII, respectively). After immersing the capillary tubes with inner diameter of 110 μm into the small vessel and heating it with a hot plate, the DD-LC mixtures were infiltrated into the capillary tubes via the capillary effect. Then, the capillary tubes, filled with dye-doped monomer dispersed LCs, were exposed to UV light to make the monomer (NOA65) solidify into the polymer. The two ends of the capillary tubes were sealed by curing with UV glue. In order to understand the arrangement of the LC mixtures in the capillary tubes, these prepared samples were also measured by a polarized optical microscope (POM).
The schematic setup of the random lasing generation from the DD-PDLCs in the capillary tube is shown in Fig. 1(a) and the photograph is shown on the right. Here, we fixed the produced capillary tubes onto a three-dimensional translation stage that was radially excited by a frequency doubling Q-switched Nd:YAG laser (NL200 series, EKSPLA Inc.) with a central wavelength of 532 nm. The pump pulses, with a 50 Hz repetition rate and 2.2 ns pulse duration, were focused on the inside of the capillary tube by a cylindrical lens with focal length f = 5cm. The axial emission spectrum from the capillary tube was collected by the fiber tip and measured by the optical spectrometer (resolution about 0.3 nm, Ocean Optics Inc.). After excitation, the light scattering and diffusion within the LC mixtures in the capillary tube are shown in the schematic plot in Fig. 1(b). The emission light, generated from the gain medium, was scattered by the LCs in arbitrary directions due to its birefringence property. In addition, the scattering strength of the emission light was enhanced due to the increase in the scattering centers between the interface of the LCs and the polymer clusters. Owing to the recurrent multiple light scattering within the DD-PDLCs, the diffusion coefficient was efficiently decreased to form light localization. Thus, a closed loop was produced when the traveling light waves returned to their starting position to generate the constructive interference that is equivalent to an optical resonant cavity. Along these light paths, multiple emission spikes from the random lasers would be excited as the experienced gain exceeds the loss.
Fig. 1 (a). Schematic illustration of the experimental setup of the random laser from the capillary tube infilling of the DD-PDLCs and photograph shown on the right, and (b) the description of the multiple recurrent light scattering in the capillary tube comprised of the random alignment of LCs, monomer (NOA65), and laser dye (PM597).
3. Results and discussion
Figures 2(a)-2(h) illustrate the room temperature photoluminescence (RT-PL) spectrum of the laser dye (PM597, black curve in Fig. 2(a)) with CW pumping and the RT stimulated emission spectra of the DD-LCs in the capillary tubes (Figs. 2(b)-2(h)) with excitation by the Q-switched Nd:YAG laser. As shown in Fig. 2(a), the RT-PL from the laser dye (PM597) through the CW laser pumping shows a relatively broad spectrum extending from 550 nm to above 700 nm, with a peak wavelength of around 588 nm. Through excitation of the pump pulse, the emission spectra of the DD-LCs without doping of the polymer in the capillary tube (Sample I, Fig. 2(b)) revealed that the full width of the half maximum (FWHM) is about 11.36 nm, which was narrower than the PL spectrum with CW pumping in Fig. 2(a). With the additional doping of the monomer into the DD-LC mixtures (Sample II to Sample VII), several discrete emission spikes with narrower FWHM around or below 1 nm were excited on the top of the spontaneous emission spectrum as shown in Figs. 2(c)-2(h). The number of emission spikes and the intensity contrast of the lasing spikes relative to the amplitude of the spontaneous emission increased as the doping concentration of NOA65 increased from 10 wt% (Sample II, Fig. 2(c)) to 20 wt% (Sample IV, Fig. 2(e)). However, the intensity and number of emission spikes decreased as the doping concentration of the NOA65 above 20 wt% (Figs. 2(f)-2(h)). Table 1 shows the peak wavelength (λp), FWHM (Δλ) and the estimated Q-factor (λp/Δλ) of Sample I (without NOA65) and the narrowest emission spikes generated from Sample II to Sample VII within the capillary tubes. Comparing with these samples with different monomer doping concentrations, Sample IV (20 wt% NOA65, Fig. 2(e)) reveals the narrowest FWHM of about 0.52 nm and the largest Q-factor of about 1134.6.
Fig. 2 (a) The RT-PL of laser dye (PM597) and the RT stimulated emission spectra from the capillary tube through exciting the Q-switched laser that contains the DD-LC mixtures (b) without monomer, and with (c) 10 wt%, (d) 15 wt%, (e) 20 wt%, (f) 25 wt%, (g) 35 wt% and (h) 35 wt% monomer doping concentrations. The POM pictures of the DD-PDLCs are shown on the right of each diagram.
Table 1. The peak wavelength (λp), FWHM and estimated Q-factor of the emission spectrum from the DD-LCs without monomer (Sample I) and the narrowest emission spikes from the DD-PDLCs with different concentrations of NOA65 (10 wt%, 15 wt%, 20 wt%, 25 wt%, 30 wt% and 35 wt% NOA65).
The images of the polarization optical microscope (POM) from these samples are shown in the inset of Fig. 2. Without the PI coating on the inner surface of the capillary tube, the alignment of the LCs are relatively random so that a bright image can be seen when the capillary was put in parallel on the polarizer of the POM as shown in the right side of Fig. 2(b). Although the birefringence of the anisotropic LCs can cause light scattering, the relative irregular alignment and rotation of the LCs make the whole property of DD-LCs in the capillary similar to the isotropic phase. This would be the reason why no narrower emission spikes occurred from the DD-LCs within the capillary, as shown in Fig. 2(b). After adding the additional monomer to the DD-LC mixtures, some dark domains were shown in the POM (right side in Figs. 2(c)-2(h)) that was resulted from the solidification of the monomer to form the polymer clusters. Thus, the light scattering within the PDLCs in the capillary tube was greatly enhanced and resulted in light localization. It was recognized to be the main reason for the generation of the multiple emission spikes as the concentration of NOA65 increased in Figs. 2(c)-2(h). Although rich light scattering was induced by virtue of dispersed polymer clusters into the LC mixtures to generate tiny emission spikes, the number of the emission spikes decreased as the monomer concentration increased beyond 20 wt% owing to the increase of scattering loss within the PDLCs (the corresponding POM shown in Figs. 2(f)-2(h)).
Figure 3(a) shows the RT emission spectra from Sample II (10 wt% NOA65) as the pump energies (EP) increase from 5 μJ/pulse to 50 μJ/pulse. With lower pump energy (EP = 5 μJ/pulse), the measured spectrum only reveals a broad spontaneous emission with the FWHM of about 30 nm. As the pump energy rose above 30 μJ/pulse, some narrower emission spikes began to be excited on the top of the spontaneous emission. In Fig. 3(a), two of the emission spikes with peak wavelengths of 593 nm and 596 nm were clearly seen using EP = 50 μJ/pulse (black curve). To characterize the random laser, Fig. 3(b) depicts the peak intensity and FWHM of the emission spectra shown in Fig. 3(a) with center wavelength around 593 nm as a function of the pump energy. The variation in peak intensity increased slowly with lower excitation of pulse energy, but enhanced dramatically when the EP exceeded the value of 35 μJ/pulse. After linear fitting, two red lines with different slopes are obtained to show the spontaneous emission at lower excitation and the stimulated emission when the pump energy is above a certain threshold value of 25.6 μJ/pulse. In contrast, the variation tendency of the FWHM versus pump energy is completely distinct from the output emission intensity. At low pump energy, the bandwidth of the broad emission spectrum decreased significantly as the excitation energy of the pump pulse increased. When the EP is above 35 μJ/pulse, narrower emission spikes are generated with the slightly pump energy dependent linewidth. It is noted that the positions of the slope efficiency change and the occurrence of the narrower linewidth are almost same pump energy as shown in Fig. 3(b).
Fig. 3 (a) Evolution of the RT emission spectra of the DD-PDLCs in the capillary tube (Sample II, 10 wt% NOA 65) as the pump energy increases from 5 μJ/pulse to 50 μJ/pulse, and (b) the peak intensity (red triangles) and the FWHM (blue squares) of the emission spectra as a function of pump energy.
Figure 4(a) shows the evolution of the RT emission spectra versus the pump energy from Sample IV (20 wt% NOA65) which reveals some different characteristics compared to those shown in Fig. 3(a). One may notice that a great number of emission spikes with narrow linewidth are excited at lower pump energy of about 20 μJ/pulse. After further increase of the pump energy, the intensities of the emission peaks are obviously amplified, but the amplitude of the spontaneous emission does not vary noticeably. With Ep = 50 μJ/pulse, further increase in the amplitude of longer wavelength emission spikes can be clearly seen (black curve in Fig. 4(a)). The variation of intensity and FWHM, from the emission spectra of Sample IV with center wavelength around 592 nm, versus pump energy is shown in Fig. 4(b). Similar to Fig. 3(b), two slopes are revealed (spontaneous emission and stimulated emission) after linear fitting of output intensities versus pump energy (red lines in Fig. 4(b)). Besides, the FWHM (blue squares) of the emission spectrum decreases from 30 nm at Ep = 5 μJ/pulse to below 1 nm at Ep = 20 μJ/pulse, as shown in Fig. 4(b). In addition, the positions of the slope efficiency variation and the occurrence of the narrowed emission spikes are also at the same pump energy of about 20 μJ/pulse, which is smaller than the obtained in Sample II. It is recognized that the light scattering centers from the polymer cluster were increased, as shown by the POM in Figs. 2(c) and 2(e), while we increased the concentration of the monomer (NOA65) in the DD-LCs.
Fig. 4 (a) Evolution of the RT emission spectra of the DD-PDCL in the capillary tube (Sample IV, 20 wt% NOA65) as pump energy increases from 5 μJ/pulse to 50 μJ/pulse, and (b) the peak intensity (red triangles) and the FWHM (blue squares) of the emission spectra as a function of pump energy.
Subsequently, we studied the effect of the working temperature on the evolution of the lasing spectra from Sample IV (20 wt% NOA65) by the increase of temperature from 11°C to 50°C, as shown in Fig. 5(a). When the working temperature was cooled to 11°C, a number of emission peaks with narrow linewidth and a widely distributed spectrum range were observed (black curve in Fig. 5(a)). As the working temperature increased from 19°C to 41°C, the number of emission peaks reduced and the intensity contrast between the lasing spikes and the spontaneous emission decreased owing to the increase in the spontaneous emission amplitude and suppression of the lasing peaks. Further enhancement of the spontaneous emission strength and suppression of the emission spikes could be seen if we continuously increased the working temperature. In this work, the increase in the random lasing spikes at lower temperatures is attributed to the enlargement of the birefringence of the LCs, i.e., the difference between the ordinary and extraordinary refractive indices, resulting in rich multiple light scattering.
Fig. 5 (a) Variation of emission spectra of the DD-PDLCs in the capillary tube (Sample IV, 20 wt% NOA 65), and (b) the peak number below 1 nm (red triangles) and the FWHM (blue squares) when the working temperature increases from 11 to 50°C.
Figure 5(b) illustrates the number of narrower emission spikes below 1 nm and the measured FWHM of the emission spectra as a function of temperature. Various narrower emission spikes smaller than 1 nm could be excited on the top of the spontaneous emission spectrum when the temperature dropped below 30°C (Table 2).As temperature increased, the FWHM broadened slowly and the intensity of the emission spikes decreased. Only two emission spikes with narrower linewidth below 1 nm were excited when the temperature increased to 30°C. Above 30°C in Fig. 5(b), the measured FWHM (blue squares) suddenly increased from sub-nanometer to around 10 nm owing to the light diffusion increase within the PDLCs. When the temperature increased to 50°C, the FWHM slightly decreased from 9.8 nm to 6.5 nm because of the reduction in light scattering so that the emission spectrum became more concentrated within a narrower range. Table 2 shows the peak wavelength (λp), FWHM of the emission spectrum from Sample IV, and the estimated Q-factor of the random laser. The largest Q-factor of about 1255.6 was obtained when the working temperature was 11°C.
Table 2. The peak wavelength (λp), FWHM and estimated Q-factor of the emission spectrum from the DD-PDLCs capillary tube (Sample IV) at different temperatures
Finally, the random lasing characteristics of the DD-PDLCs in the capillary tube from Sample IV (20 wt% NOA65) at 11ºC are shown in Figs. 6(a) and 6(b). The evolution of the emission spectra as a function of pump energy in Fig. 6(a) shows that multiple emission spikes could be generated even with relatively low pump energy. As the excitation energy of pump pulse increases, the number of emission spikes increases accompanying with the wavelength extending toward the longer range due to re-absorption of laser dye. Generally, the redshift of the emission spectrum for the dye laser in the scattering sample is observed under higher pump energy [29]. This phenomenon is resulted from the absorption of the blue fluorescence by the dye molecules with higher energy level and then re-emitted spectrum whose photon energy is lower and the wavelength distribution is longer than reabsorbed light. This reabsorption phenomenon would occur recurrently to cause emission light continuously move toward long wavelength at higher pump energy.
Fig. 6 When the working temperature was set at 11°C, (a) evolution of emission spectra of the DD-PDLCs in the capillary (Sample IV, 20 wt% NOA65) as pump energy increased from 5 μJ/pulse to 50 μJ/pulse, and (b) the intensity (red triangles) and the FWHM (blue squares) of the emission spectra as a function of pump energy.
At 11 °C, the variation in the intensities (red triangles) and the FWHM (blue squares) of the emission spectra from Sample IV with center wavelength around 590 nm (Fig. 6(a)) is summarized in Fig. 6(b). In this measurement, only one slope is revealed in Fig. 6(b) because the DD-PDLCs reveal relative low threshold of pump energy at low temperature. When the pump energy is below 5μJ/pulse, we cannot obtain the intensity of the emission spectrum from capillary owing to relatively lower signal to noise ratio of the emission spectra measured from our spectrum meter. After linear fitting of the output intensities from the Sample IV versus the pulse energy, a lower threshold of pump energy of about 3.8 μJ/pulse was obtained in comparison with the threshold value Ep = 15 μJ/pulse at room temperature in Fig. 4(b). In addition, the narrower linewidth below 1 nm from the emission spikes could be excited even at lower excitation energy of the pump pulse (blue squares in Fig. 6(b)). At a lower temperature (11°C), the generation of a random laser in the DD-PDLCs within the capillary with a lower threshold of pump energy was attributed to the increase in the birefringence of the LC. Thus, multiple recurrent light scattering would be increased to reduce the diffusion coefficient of light propagation within the PDLCs.
4. Conclusion
The manipulation of the random lasing characteristics of dye-doped polymer dispersed liquid crystals in capillary tubes was investigated in this work. Compared with the DD-LCs without monomer in the capillary tube, multiple emission spikes with narrower emission linewidths emerged on the top of the spontaneous emission after adding the monomer (NOA65) into the dye-doped LC mixtures, and thus the light scattering between the interface of the LC and polymer was enhanced. Through excitation, many closed-loop light tracks could be formed to produce light localization, thus taking over the role of the traditional cavity. As the doping concentration of the monomer increased from 10 wt% to 20 wt%, the random laser revealed a lower threshold of pump energy and generated more emission spikes owing to the increase of the polymer clusters so that light diffusion in the DD-PDLCs was reduced. Nevertheless, the amplitude of the emission spikes diminished at an even higher concentration of monomer (above 20 wt% NOA65) because of the scattering loss increased in the PDLC mixtures. Through the excitation of the DD-PDLCs, the abrupt rise in the slope efficiency and the occurrence of narrower emission spikes were almost at the same pulse energy, which is one of the proofs indicating the occurrence of a random laser. Finally, at a lower temperature, we demonstrated that the occurrence of a random laser with a relatively low threshold of pump energy of about 3.8 μJ/pulse and a high Q-factor of about 1255.6 owing to the increase in the birefringence of the LCs caused the efficient light scattering. Besides, more emission spikes at long wavelengths would be excited due to increase of Q-factor and reabsorption of the laser dye.
Acknowledgment
This work is financially sponsored by the National Science Council of Taiwan, R.O.C with grant no. NSC 102-2112-M-027-001-MY3. We gratefully acknowledge the helpful discussions with Prof. Wu at NTUT on several points in this paper.
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