Study on the Polarization of Random Lasers from Dye-Doped Nematic Liquid Crystals
© The Author(s). 2017
Received: 7 October 2016
Accepted: 9 December 2016
Published: 11 January 2017
Random lasers from dye-doped nematic liquid crystal (DDNLC) cells with different rubbing methods were observed due to different random ring cavities that were formed. Through constructing cells with different rubbing methods on the forward and backward surfaces of light-emitting sides, we can get two random laser beams with different polarization directions from one DDNLC cell at the same time, and the polarization direction is along the rubbing direction of the light-emitting sides. Additionally, the influence of external electric field on the polarization degree of random lasers was also studied.
Random lasers get gain from multiple scattering. Compared with conventional lasers, the resonant cavity in random lasers is built on recurrent multiple scattering instead of two mirrors with high reflectivity. The fluorescence photons can be repeatedly multi-scattered in random directions as they propagate in an active medium where the scattering particles or domains are distributed disorderly. The recurrent multiple scattering can form incoherent feedback or coherent feedback, when the scattering photons propagate along a close circuit . Random laser can be obtained light pumped or electrically pumped . With advantages of small size, cheapness, flexible shape, and some others , random lasers can be widely used in temperature sensing , document encoding, material marking, high-density optical data storage , tumor diagnosis [6, 7], liquid crystal display , integrated optics , liquid flow monitoring, and other areas .
In recent years, random lasers have been developed using liquid crystals (LCs) as a scattering material. As a typical anisotropic material, the LC has a lot of optoelectronic applications because of its unique optoelectronic properties. The LCs can be divided into nematic phase, cholesteric phase, and smectic phase. The choosing of different LC materials can achieve different emission characteristics of LC random laser [10–13]. Such random lasers possess peculiar merits of flexible controllability or tunability in their lasing characteristics (e.g., energy threshold or lasing wavelength) by thermal [14, 15], electric [16, 17], and optical  approaches. This is because the orientation of LCs with large anisotropies and its macroscopic physical properties, e.g., the refractive index and dielectric tensor, can be easily modified externally. Additionally, it is known that sample parameters affect the optical properties of the LC cells in the field of liquid crystal display  and these properties such as polarization can be controlled through changing the parameters. Nematic liquid crystal (NLC) is a kind of anisotropic material, which has the properties of birefringence. The change of temperature will affect the birefringence and light scattering of NLC. NLC molecules are aligned in parallel along the long axis of the molecule, and the liquid crystal molecules are in the shape of a rod. Therefore, the nematic liquid crystal molecules are easily affected by the applied electric field.
The polarization state of emission spectrum is an important characteristic for the random laser. In 2004, Wu et al. investigated RLs in a two-dimensional rod array. Their results showed that due to the strong scattering of the random lasers, the transverse magnetic-polarized lasing component had a lower threshold than the transverse electric (TE)-polarized component . At the same year, Gottardo et al. used extremely anisotropic scattering from small droplets of liquid crystals to create and manipulate polarized random laser emission . In 2012, it was found that random lasers in organic dye solutions can be linearly polarized using the anisotropic adsorption of the dye molecules . In , random laser emitted from DDNLCs was investigated, and any arbitrary linear polarization of RLs can be obtained by rotating the nematic liquid crystal sample. These researches have blazed a way in polarization study in different structure systems for us.
In this paper, the polarization of random lasers from DDNLCs was studied through changing the rubbing methods of the LC cells and the external electric field. By using different rubbing methods on the light-emitting sides, random laser with different polarization directions can be obtained from both forward and backward surfaces of one cell at the same time. With increasing the electric field intensity, the polarization degree of random lasers was reduced clearly.
Figure 1b shows the sketch of the experimental setup. The sample was pumped by a frequency doubled Nd:YAG laser system (PowerLite Precision II 8010) with 532-nm wavelength, 10-Hz repetition rate, and 8-ns pulse width. The strength of the pump laser can be changed by adjusting the polarization direction of the Glan prism group. The diameter of laser pump spot was about 20 μm. Random lasers were emitted from both the forward and backward sample surfaces. The emitting signal is collected by optical multichannel analyzer (OMA) with spectral resolution of 0.1 nm.
Results and Discussion
In previous reports [27–29], ZnO nanostructures exhibit a whispering gallery mode (WGM) type of resonance in which the hexagon-shaped nanostructure can support the lasing modes. The photons are confined by the total internal reflection at the ZnO-air boundary, where lasing occurs in WGMs with a high Q factor. Different with the WGM lasing which the emission peaks can be attributed to the confined resonant modes inside the hexagon-shaped structure, which is nicely reproduced in Fig. 4c of , the optical feedback of the random lasing is provided by the multiple scattering of LC molecules. On the other hand, the threshold of random lasing depends on the excitation area , actually because the numbers of LCs are different under various excitation areas, which have an effect on scattering strength and pump efficiency of random laser. The number of LC molecules can also be adjusted by changing the thickness of the samples, which can affect the threshold of random laser. As shown in our previous work , the influence of the LC cell gap on random laser energy threshold was studied, and a wedge cell with TSRS rubbing method was made. The results of the experiment show that the scattering strength is different in the distinct thickness of LC cells, which lead to the changes of random laser threshold. However, the pump threshold of WGM is not affected by the change of the excitation area in spite of the increasing number of peaks . Based on the above analysis, the possibility of WGM laser is excluded. It is worth noting that the emission spectrums have almost same frequency spacing. Based on the study of Cao , this Fabry-Pérot cavity-like emission in dye-doped nematic liquid crystals can be understood in the way that the random lasers can be regarded as random distributed feedback lasers ; the quasimodes are formed mainly by the feedback from weak scattering particles near the system boundary, resulting in quasimodes with almost regular frequency spacing, and similar experimental results have been verified and explained in our previous work .
This phenomenon can be explained by the anisotropic adsorption of the dye molecules. Random lasers in organic dye solutions can be highly linearly polarized by choosing a highly viscous solvent for the anisotropic adsorption of the dye molecules . There is guest-host effect in dye-doped nematic liquid crystals. According to this effect, rod-shaped dye molecules as “guest” molecules aligned along the direction of the rod-shaped “host” liquid crystal molecules [37, 38]. Dye molecules will tend to absorb light with the polarization direction along the long axis of the dye molecules . When the dye molecules release the energy again, the polarization direction of the emitted light is along the long axis of the dye molecules, which is the direction of the nematic liquid crystal . Due to the rubbing methods controlling the alignment of liquid crystals, the polarization of the emitted light is influenced by the rubbing direction. In the experiment, the anchor force formed from the rubbing behavior is largest for the liquid crystal molecules near the cell surface while is smallest for that in the central of the cell. The inner liquid crystal molecules, which keep away from the cell surface, are not along the nematic director decided by the rubbing direction due to the weak anchor force. However, anchor force will make the alignment of liquid crystal molecules along the orientation direction near the alignment layer, and the light paths can form different loops, as previously mentioned. When the pump light propagates through the cells and produces the random laser, the light with the polarization direction along the rubbing direction will get the largest gain. However, it is hard to get any gain by the light whose polarization direction is not along the rubbing direction. So, the polarization direction of the emitted random laser will gain along the rubbing direction of the cells due to the polarization-dependent optical gain effect. This phenomenon exist in both the forward random laser and backward random laser. This effect can be used to get different random laser beams with different polarization directions from one cell at the same time, which is useful in liquid crystal display and some other fields that polarized light is needed.
The polarization degree of the random laser under different voltage
In conclusion, the influence of rubbing methods and external electric field on the polarization of random laser from dye-doped nematic liquid crystals cells is studied in this paper. Random lasers with linear polarization can be obtained from both forward and backward surfaces of the DDNLC cells and the polarization direction is along the rubbing direction of the light-emitting side. Two random laser beams with different polarization directions from one cell can be obtained through constructing cells with different rubbing methods on the forward and backward light-emitting sides at the same time. In addition, increasing external electric field intensity can reduce the polarization degree of random lasers and change the polarization direction of the random lasers. The results reported in this paper can be used in liquid crystal display and some other fields which need light with tunable polarization.
This work was supported by the National Natural Science Foundation of China under Grant Nos. 11174160, 11474052, and 11274062.
CZ carried out the experiments. CZ and YF drafted the manuscript. LY conceived the study and participated in its design. LY and BJ participated in the design of the study and performed the analysis. YX participated in the measurements. YC and YL supervised the overall study. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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