Dynamically manipulated lasing enabled by a reconfigured fingerprint texture of a cholesteric self-organized superstructure

Wenbin Huang ab, Cong-long Yuan b, Dong Shen b and Zhi-gang Zheng *b
aCollege of Physics, Optoelectronics and Energy & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou, 215006, China
bDepartment of Physics, East China University of Science and Technology, Shanghai, 200237, China. E-mail: zgzheng@ecust.edu.cn

Received 12th May 2017 , Accepted 22nd June 2017

First published on 22nd June 2017


Laser emission based on an electrically reconfigured fingerprint texture of a cholesteric liquid crystal helical superstructure is achieved by judiciously designing the composition of the device material and the device structure. Unlike the common lasing, resulting from photonic bandgap behaviour of photons having a certain wavelength in a self-organized liquid crystal helix with a common planar arrangement, the lasing shown herein is caused by a combination of the high-order distributed feedback resulting from periodic refractive index modulation in fingerprint stripes and the waveguide effect of the sandwiched cell structure. Furthermore, this laser, with a narrow bandwidth of less than 0.5 nm, can be switched between two orthogonal emission directions; the emission wavelength as well as the lasing mode (i.e., single-mode, multi-mode and random-mode) can be manipulated readily with a comparatively low external electric stimulation owing to the variety of configurations and reconfigurations possible for such a soft helix. In addition, the lasing can be electrically turned on and off. This study not only corroborates the lasing possibility of a cholesteric liquid crystal helix showing fingerprint texture with a helical pitch significantly exceeding the short pitch required in common photonic band edge lasing, but also simultaneously introduces potential applications in photonic integration and others processes.


Introduction

Laser emission based on a self-organized soft cholesteric liquid crystal (CLC) superstructure is thought to provide unique coherent light sources with several attractive properties, such as compactness, flexibility and bio-compatibility, which have scientific and industrial significance in micro- and nanophotonics.1–3 The self-organized soft CLC superstructure presents a reflection band gap in impinged circular polarized light with the same rotation sense as CLC helix, i.e., selective reflection. After doping the gain medium, the CLC helix furnishes a resonator necessary for laser emission;4–6 this helix structure exhibits conspicuous advantages compared to photolithography, which is commonly used for the manufacturing of laser resonators. With external pumping, laser emission normally occurs at the long-wavelength edge of the reflection band, where the group velocity of photons approaches zero, i.e., photonic band edge (PBE) lasing; this means that emission wavelength is dependent on the helical pitch of CLC according to the relationship of λL = neP. Herein, λL and P are the emission wavelength at the long-wavelength edge and the pitch length respectively and ne is the extraordinary refractive index of the liquid crystal (LC). Considerable efforts in recent decades have been devoted to achieving a wavelength-tuneable PBE laser by manipulating the CLC pitch through external stimulations, such as electric field, light, temperature and mechanical forces.7–18 The emission intensity modulation of a CLC laser, caused by stimuli-induced deformation of the helix, has been realized.19,20 In addition, a nematic LC layer can be inserted into the CLC as a defect layer to improve lasing performance.21 The working threshold can also be reduced by utilizing the Forster energy transfer mechanism.22 Random-mode lasing induced by random resonation of photons in a light-scattered system formed by a phase separated system containing polymers and a CLC has been recently reported.23–25

From another aspect, single-mode lasing from a holographic polymer dispersed liquid crystal (HPDLC) film doped with gain medium has been achieved in the past decade.26,27 A laser resonator has been formed by double-beam holography exposure of a polymer dispersed liquid crystal (PDLC) syrup, producing periodic LC-rich and polymer-rich layers on account of photo-induced phase separation, thereby realizing refractive index modulation, which is necessary for distributed feedback (DFB) lasing. Several types of fluorescent dyes have been investigated as gain media to increase lasing efficiency.27,28 Contemporarily, the structure of the resonator has been improved using a transmission HPDLC film (i.e., the refractive index periodicity is perpendicular to cell normal) to overcome the significant drawback of a short gain length of the original reflection HPDLC film (i.e., the refractive index periodicity is parallel to cell normal).29–31 Commonly, the mono-chromaticity (i.e., bandwidth) of the HPDLC-based laser is one order of magnitude narrower than that of the CLC laser (several nanometers);7–9,12,14 however, the tunability of the former is weaker than that of the latter due to strong interface anchoring between the LC and polymer network.32 In addition, similar to the structure of the reflection HPDLC film, the elongation in the gain length for a CLC laser is a significant factor for improving lasing performance.33 Therefore, a uniform lying helix of CLC, i.e., the orientation of the helical axis is uniform and parallel to the substrates of the LC cell, enabling an extension of the gain length, is more desirable.34 However, achieving such an arrangement is challenging owing to the short pitch (∼300 nm) requirement to match the PBE of CLC with the fluorescent spectrum of the gain medium for generating a PBE laser, which leads to more arrangement defects and thereby degrading lasing performance.35 Considering this aspect, an electric field is applied to maintain a complicated and subtle equilibrium among the external field, free energy of CLC and the surface anchoring, thus generating a uniform lying helix arrangement;36 moreover, polymer stabilization is required to maintain such an arrangement.37

Herein, inspired by previous research, a DFB CLC laser with sub-nanometer bandwidth and better tunability is achieved. Distinct from the common CLC laser based on the photonic bandgap effect, this laser is obtained from the high DFB order of an arrangement exhibiting specific fingerprint texture (i.e., a straight stripe texture resulting from a homogeneous planar aligned cell) due to periodic refractive index modulation. Such a specific arrangement is facilitated by a low and sustainable voltage applied across the sample. Furthermore, the texture can be reconfigured by changing the voltage slightly among a defect-free fingerprint texture exhibiting straight stripes that orient parallel to the alignment direction, a similar straight stripe texture with slight defects but orienting perpendicular to the alignment direction, and a fingerprint texture with heavy defects and curved stripes. Therefore, such a DFB CLC laser can be manipulated by applying an electric field not only on the emission wavelength and direction, but also on the emission mode (i.e., single-mode, multi-mode and random-mode). Such DFB lasing based on a soft reconfigurable helical CLC with better emission performance and tunability, which has not been previously demonstrated to the best of our knowledge, can aid the development of an advanced stimuli-directed microminiaturized laser device and expand the existing photonic applications.

Experiments

DFB lasing based on the CLC helix results from the high-order coherent back scattering of photons due to refractive index modulation of the CLC fingerprint texture and the waveguide effect of the LC cell (the average refractive index of CLC, nCLC, is 1.607 and the refractive index of the alignment polyimide layer coated on cell substrates is 1.516). This DFB lasing is characterized by lateral laser emission in the direction of refractive index periodicity, which is distinct from common surface emission based on PBE of CLC. The wavelength of the emitted laser is determined by Bragg's law, 2neffΛ = , where λ denotes the emission wavelength, M represents the DFB order, neff is the effective refractive index of the medium, which can be calculated through the waveguide theory (see ESI), and Λ is the period.

CLC was composed of commercial eutectic nematic LC TEB300 (no = 1.522, ne = 1.692; Slichem Co., Ltd, China) and the commonly used chiral agent R811 (Merck); in addition, about 0.5 wt% of fluorescent dye 4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM, Sigma-Aldrich) was added as the gain medium. The concentration of R811 was closely investigated and determined as 3.5 wt% (the corresponding helical pitch was ∼2.6 μm) to ensure an orthogonal transformation of the stripe orientation as well as a better tunability of such a helical arrangement. The mixture was homogeneously stirred for around 1 hour at the clearing point and then injected into an empty cell via capillary action, followed by slow cooling to room temperature (∼25 °C). Two indium tin oxide (ITO)-coated glass plates were spin-coated with a polyimide layer and rubbed to form anti-parallel planar alignment, and then they were assembled together and sealed by epoxy glue to prepare the empty cell while the cell gap was maintained by 4.0 μm spacers. The sample was stimulated by a 1 kHz square-wave signal with the voltage adjusted from 0 to 10.0 V. The straight fingerprint texture was observed under a polarizing optical microscope (POM, LV100POL, Nikon) with crossed polarizers and monitored by a polarized helium–neon (He–Ne) laser.

Lasing performances of the sample were analysed using a 532 nm linearly polarized second-harmonic switched neodymium-doped yttrium aluminium garnet (Nd:YAG) pulsed laser with a repetition rate of 10 Hz and a pulse duration of 8 ns (Fig. 1). The pump beam was focused by a cylindrical lens (CL1). A slit was placed at the focal plane of CL1 to filter the stray light and ensure intensity uniformity in the pumping source. This was followed by expansion and re-focusing on the sample by another cylindrical lens (CL2). A set of neural density filters (NDFs) were placed to adjust pump energy. A beam splitter (BS) was placed between the NDFs and sample for a real-time detection of pump energy. Laser emission was detected at the lateral of the sample (i.e., perpendicular to the stripe direction in the sample plane) using a high resolution (∼0.29 nm) fiber coupled spectrometer (FCS, Avaspec-ULS2048-USB2). A long pass filter (CF, cut-off wavelength: <600 nm) was inserted between the sample and the fiber detector to eliminate the effect of the pump beam on spectrum detection.


image file: c7tc02076g-f1.tif
Fig. 1 Optical setup of laser emission pumping and detection. PS: pumping source; CL: cylindrical lens; NDFs: neural density filters; BS: beam splitter; OEM: optical energy meter; CF: colour filter; FCS: fiber coupled spectrometer.

Results and discussions

The CLC displayed a common planar texture without arrangement defects when the voltage was absent, indicating a vertical alignment of the helical axis to cell substrates (Fig. 2a-i). A He–Ne laser can pass through the sample without diffraction (Fig. 2a-ii). When a 2.0 V voltage was applied on the cell, the planar texture transformed to alternate bright-and-dark straight stripes, perpendicular to the alignment of cell substrates, at an interval of ∼4.4 μm due to electric-field induced Helfrich deformation38,39 of the CLC helix (Fig. 2b-i). A significant defect segment, denoted by the red dashed circle, resulted from the dynamic equilibrium among the twisted free energy of the helix, the surface anchor of the cell and the external electric-field stimulation. The significant diffraction of the impinged laser (Fig. 2b-ii) can be attributed to periodic refractive index modulation of the straight stripe texture. As the voltage was sustained for 2–3 minutes, this texture gradually transformed to another similar straight stripe texture with a narrower interval of ∼2.56 μm (Fig. 2c-i). Notably, the stripe period was approximately equal to the pitch length of CLC in a bias-free situation, but the orientation of stripes was parallel to the alignment direction, i.e., orthogonal to the direction of the former stripes (see ESI). Moreover, the corresponding diffraction pattern (Fig. 2c-ii) changed with not only the distance between the adjacent orders, but also the distribution of the diffraction spots. Such transformation is ascribed to the rearrangement of CLC from the metastable Helfrich deformed helix to the stable lying helix. An almost randomly oriented stripe texture with more arrangement defects was obtained by applying a 10.0 V voltage on the sample to unwind the helix, followed by a switching to a sustained 2.0 V voltage (Fig. 2d-i); correspondingly, the crescent shaped diffraction pattern was obtained as a result of defect-induced light scattering (Fig. 2d-ii).
image file: c7tc02076g-f2.tif
Fig. 2 Voltage reconfigured optical textures of the CLC helix and the corresponding diffraction patterns in the following situations: (a) no voltage is applied; (b) voltage of 2.0 V is applied; (c) 3 minutes after applying 2.0 V voltage; (d) a 10.0 V voltage is applied, followed by a sudden decline to 2.0 V. Red dashed circles denote the regions of defect. Alignment direction (R) is labelled by the white double arrow. Scale bar is 20 μm. Orthogonal yellow double arrows indicate the optical axes of the crossed polarizers.

The sample exhibiting straight stripe orientation perpendicular to the alignment direction (Fig. 2b-i) was pumped by the 532 nm pulsed laser with energy density of each pulse being 1.0 mJ cm−2. Three distinguishable narrow emission peaks with central wavelengths of 603.1 nm, 605.4 nm and 607.3 nm were presented in the spectra (Fig. 3a). The corresponding DFB order of the emission, i.e., M, deduced in accordance with eqn (S1) and (S2) (see ESI), was 22. The band-widths of the emission peaks were narrower than 0.5 nm. Such multi-mode lasing is predicted to result from a small number of defects under this situation, which may influence the surrounding periodicity of the stripes. The emission intensity increased significantly after the density of pumping energy exceeded a certain value, 0.25 mJ cm−2, known as the laser emission threshold (LET); moreover, a slight increase in the case of the density of pumping below LET was caused by spontaneous light emission with a wide spectral band (Fig. 3b).


image file: c7tc02076g-f3.tif
Fig. 3 Lasing performance of the straight stripe texture resulting from Helfrich deformation of CLC. (a) Emission spectra show three discrete narrow emission peaks with their band-widths at about 0.45 nm; density of pumping energy herein is 1.0 mJ cm−2; (b) density of pumping energy vs. emission intensity.

The transformation of such a straight stripe texture, from a perpendicular orientation with less defects to a defect-free parallel orientation with respect to the surface alignment, accompanied by a shrinkage of the stripe interval led to switching of the lasing direction and wavelength. Furthermore, the periodicity of the stripes could be elongated by increasing the applied voltage, exhibiting a slight elongation from the original 2.56 μm at 2.0 V (Fig. 4a) to 2.75 μm at 3.0 V (Fig. 4c), followed by a conspicuous widening from 2.95 μm at 3.5 V (Fig. 4d) to 3.8 μm at 4.0 V (Fig. 4f). Further increase in the voltage resulted in an obscure stripe texture caused by a gradual decrease in refractive index modulation. Such stripes were removed completely as the voltage exceeded 5.0 V due to the unwinding of the CLC helix. Notably, a straight stripe with weak brightness existed between two adjacent bright stripes when the applied voltage was 4.0 V (Fig. 4f), which resulted from the effect of surface alignment anchoring.


image file: c7tc02076g-f4.tif
Fig. 4 POM textures of defect-free straight stripes with gradual increase in applied voltage. (a) 2.0 V at the initiation (period: 2.56 μm); (b) 2.5 V (period: 2.6 μm); (c) 3.0 V (period: 2.75 μm); (d) 3.5 V (period: 2.95 μm); (e) 3.75 V (period: 3.5 μm); (f) 4.0 V μm. The scale bar is 20 μm.

Such an electrically reconfigured CLC helix facilitated the switching and manipulation of the lasing direction, mode and wavelength. As the pumping beam with invariable output energy impinged on the sample exhibiting a defect-free straight stripe texture (Fig. 4b), a sharp single emission peak (i.e., single lasing mode, the corresponding DFB order was 13) with a central wavelength of 612.7 nm was observed (blue curve in Fig. 5a), which was significantly distinct from the aforementioned multi-mode lasing generated in stripes with less defects. The lasing wavelength determined by the stripe period could be manipulated conveniently through electric stimulation, displaying a spectral red-shift from 612.7 nm to 628.3 nm with a constant DFB order (Fig. 5a) when applied voltage was increased from 2.5 V to 2.7 V. The driving voltage is about one order of magnitude lower than that of the commonly reported tuneable CLC laser,9,36 which implies remarkable significance in the applications in photonic chips.40 Continual enhancement of voltage could not induce further red-shifting of the laser, but a contrary blue-shifting was induced on account of increasing order of DFB with the elongation of stripe period. For instance, the lasing wavelength shifted back to ∼612 nm when the applied voltage rose up to 3.0 V; however, the corresponding DFB order increased to 14 synchronously. Subsequently, the wavelength showed a re-red-shift with a further increase in voltage, leading to wavelength scanning of the laser, which is achieved in the common DFB laser and is desirable but challenging in certain specific optical systems. The LET exhibited a slight increase with enhancement of the electric stimulation; in contrast, the slope efficiency (defined in Fig. 5b) decreased. Such interesting dependencies resulted from the extension of the stripe period, which attenuated the coupling coefficient of the DFB system, thereby weakening lasing efficiency.41


image file: c7tc02076g-f5.tif
Fig. 5 Electric tunability of laser emission. (a) Spectral red-shift of laser emission, from 612.7 nm (blue curve), passing through 620.8 nm (green curve) to 628.3 nm (red curve), with the enhanced applied voltages of 2.5 V, 2.6 V and 2.7 V. The band-widths are 0.31 nm, 0.35 nm and 0.34 nm, respectively. The DFB order M is 13. (b) The corresponding dependency between density of pumping energy and emission intensity. Inset shows magnified view near the LETs. Slope efficiency is defined as the slope of the fitted line as the density of pumping energy exceeds the LET (i.e., tan[thin space (1/6-em)]θ).

A random oriented curved stripe texture of the CLC was achieved by unwinding the helix with an applied voltage of 10.0 V, followed by switching the voltage to 2.0 V. Emission spectra displayed numerous sharp peaks, with their band-widths of 0.5–0.8 nm, randomly distributed in a broad spectral range from 605 nm to 615 nm, which produced a typical characteristic of random lasing (Fig. 6a). However, distinct from the commonly reported LC random lasers resulting from random closed feedback loops of photons in a scattered LC system, such as nanoparticle doped LC42 and PDLC,25 random lasing herein may be attributed to defect induced periodicity fluctuation as well as light scattering in the helical system. However, such many arrangement defects induced by randomly oriented stripes affected the lasing efficiency, indicating a higher LET and a lower slope efficiency (Fig. 6b). Moreover, the lasing can be turned off by removing the voltage, which leads to the recovery of CLC to a stable planar arrangement in a bias-free environment.


image file: c7tc02076g-f6.tif
Fig. 6 Lasing performance of a randomly oriented stripe texture. (a) Spectrum of laser emission; the peak band-width varies between 0.5 nm and 0.8 nm. Density of pumping energy herein is 1.0 mJ cm−2 (b) emission intensity vs. density of pumping energy.

Conclusions

In conclusion, DFB lasing in a CLC helix presenting refractive-index modulated fingerprint stripe texture and confined in a judiciously designed wave-guided LC cell was demonstrated with respect to performance and electric tunability. Distinct from the common CLC laser induced by the effect of photonic bandgap of a CLC helix with a pitch of several hundreds of nanometers, the laser generated herein resulted from the combination of high-order distributed feedback based on a periodic stripe texture with a large period of several microns as well as the waveguide effect of the cell after consideration of the refractive index of the CLC and the alignment film coated on cell substrates. Furthermore, the electric-directed reconfiguration of the CLC helix endowed the lasing with multiple tunabilities, exhibiting an orthogonal transformation of the lasing direction; an electrically-switchable lasing mode among multi-mode, single-mode and random-mode; manipulation of the lasing wavelength. As desired, the lasing can be turned on and off by applying and removing the external voltage. This study corroborates that CLC lasing can be generated not only in the planar arranged helix with a sub-micron helical pitch, but in other arbitrary CLC helical arrangements showing a fingerprint optical texture with a period of several microns or longer. The versatile tunabilities of this laser achieved by a comparable low voltage opens extensive possible applications in micro-nano-photonics, photonic chips and other related fields.

Acknowledgements

We are grateful for the financial support of the Natural Science Foundation of China (NSFC) (Grant No. 61435008, 61575063, 61505131), Shanghai Rising-Star Program (No. 17QA1401100), Jiangsu Provincial Natural Science Foundation of China (Grant No. BK20150309), the China Postdoctoral Science Foundation (Grant No. 2015M571816), and the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions.

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Footnotes

Electronic supplementary information (ESI) available. See DOI: 10.1039/c7tc02076g
These authors contributed equally to this work.

This journal is © The Royal Society of Chemistry 2017