Elsevier

Optics Communications

Volume 502, 1 January 2022, 127410
Optics Communications

Analysis of laser radiation performance of fingerprint-texture cholesteric liquid crystal device

https://doi.org/10.1016/j.optcom.2021.127410Get rights and content

Highlights

  • •

    In this paper, the fingerprint-texture cholesteric liquid crystal device we fabricated can still generate laser light even if the pitch is a few microns.

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    The single-mode emission was obtained at the side of the device, which means it can be coupled with other devices more easily.

  • •

    The lasing emission wavelength changes non-monotonically with gradually increasing electric field, and the observed spectral tuning of the lasing emission was reproducible and reversible.

Abstract

Low-cost tunable lasers have broad application prospects in optoelectronics. In this paper, a laser device based on fingerprint-textured cholesteric liquid crystal (CLC) was designed and fabricated, in which a narrow linewidth single-mode laser output on the side of the device was realized. The planar helical structure of the “fingerprint-texture” of CLC was obtained by the combined effect of externally applied electric field and substrate surface anchoring. Under the pumping light of a Nd:YAG solid-state pulsed second harmonic laser with the wavelength of 532 nm, the lasing emission wavelength of our CLC device changes non-monotonically with gradually increasing electric field, which first shifting from 568.5 nm to 609.2 nm and then shifting from 609.2 nm to 586.2 nm. And the observed spectral tuning of the lasing emission was reproducible and reversible. The analysis shows that the output laser radiation was the result of the Bragg reflection feedback provided by the fingerprint-texture.

Introduction

Dye-doped cholesteric liquid crystal (CLC) lasers have unique physical and optical properties, which are sensitive to external factors such as electric field [1], [2], light field [3], [4], mechanical stress [5], and temperature [6], [7]. Tunable lasers can be achieved by controlling the arrangement of liquid crystal molecules in real-time, In addition, they have the advantages of small size and low cost.

The textures of typically studied tunable CLC lasing devices mainly include two types. In the planar texture state, the band-edge liquid crystal laser has the selective reflection characteristics, which can prevent the light with a specific wavelength from passing through [8]. The laser device in the focal conic texture has laser output in all directions, and the lasing wavelength has a certain randomness [9]. The fingerprint texture can be regarded as a special case of the focal conic texture, created by applying external electric field with a constant frequency to the CLC sample.

In the fingerprint-texture state, the device can still produce laser light even if the pitch (the thickness corresponding to the rotation angle 2π of the LC director around the helical axis) is a few microns. Compared with the planar state and the focal conic state devices that only operation in nanometer scale, the integration of fingerprint texture is stronger. And the liquid crystal molecules in the fingerprint-texture state can spontaneously form the periodic changes of the refractive index and causes Bragg reflection.

By rationally designing the material composition and structure of the fingerprint-texture device, the researchers had realized the electronically controlled laser radiation direction switching of the tunable distributed feedback (DFB) laser with high Bragg reflection in the in-plane orthogonal direction. The laser linewidth was less than 0.5 nm, and the multi-wavelength laser emission, single-wavelength emission and random laser emission were successfully achieved under relatively low external voltage [10].

Compared with the most common methods for realizing periodic modulation of refractive index such as gratings formed with secondary processing of surface lithography [11] and liquid crystal/polymer grating [12], the tuning range of the fingerprint-texture CLC devices is larger, and the production and application are more flexible.

In this paper, a CLC laser based on fingerprint-texture was designed and fabricated. The laser radiation spectrum was measured and analyzed, and the laser radiation mechanism of the device was discussed in depth. Experiment showed that the emission wavelength of our laser system could be tuned by external electric field.

Section snippets

Experiment

Fingerprint-textured CLC devices were designed and fabricated. The production process was as follows: Two polyimide (PI) coated glass substrates with the same thickness and rubbing direction were used to build a parallel oriented liquid crystal cell. The liquid crystal mixture composed of nematic liquid crystal TEB30A (clearing point 61 °C, extraordinary refractive index ne=1.692, ordinary refractive index no=1.522, Δn = 0.17), chiral agent S-811 and laser dye DCM (λmax,abs=481nm) (mass

Electro-fingerprint-texture

In order to prove that the applied electric field causes transition of the CLC layer into the fingerprint texture, a voltage was applied to the device for different durations at 26 oC. The texture images were observed under a polarizing microscope (OLYMPIS BX-51) with crossed polarizers were shown in Fig. 2(a-i)-(d-i). In addition, the grating diffraction effect experiment was performed on the device to verify that the formed fingerprint-texture has a periodically modulated refractive index (

Conclusions

In this paper, the laser radiation characteristics of a fingerprint-textured CLC device were studied. Under the combined effect of the electric field torque and the anchoring force on the substrate surface, the original planar CLC texture of the device could be reversibly transformed into the fingerprint texture. When the fingerprint-texture sample was pumped by the 532 nm pulsed laser, single-mode lasing emission was detected at the side of the device, with the FWHM of the lasing peak less

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

Thanks for reviewers very much for their thoughtful comments that helped improve this paper substantially. This work was supported by the National Natural Science Foundation of China (No. 61705145); Liaoning Provincial Innovative Talents of University (No. LR2016079); Natural Science Foundation of Liaoning Province of China (No. 20180550330); Research Innovation Team Construction Project of Shenyang Ligong University (No. SYLUTD2020) and High Level Construction Projects (No. SYLUXM202107).

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