Symmetric Continuously Tunable Photonic Band Gaps in Blue-Phase

May 27, 2019 - Chiral doped liquid crystals have been used to form one-, two-, .... The center wavelength of the PBG in the BPs relies on the gradient...
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Symmetric continuously tunable photonic bandgaps in bluephase liquid crystals switched by an alternating current field Xiao-Wei Du, De-Shan Hou, xuan li, Dong-Peng Sun, Jiong-Fang Lan, Ji-Liang Zhu, and Wenjiang Ye ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04577 • Publication Date (Web): 27 May 2019 Downloaded from http://pubs.acs.org on May 31, 2019

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Symmetric Continuously Tunable Photonic Bandgaps in Blue-Phase Liquid Crystals Switched by an Alternating Current Field Xiao-Wei Du,‡ De-Shan Hou,‡ Xuan-Li, Dong-Peng Sun, Jiong-Fang Lan, Ji-Liang Zhu,* and WenJiang Ye Department of Applied Physics, Hebei University of Technology, Tianjin 300401, China Abstract: Symmetric continuously tunable three-dimensional liquid photonic crystals have been investigated using self-organized blue phase liquid crystal films. The photonic bandgap in the overall visible spectrum can be tuned continuously, reversibly, and rapidly as change of the applied electric field. After driven by the applied field, four-time enhancement of the reflectivity results in more vivid reflection colors. A lasing emission of tuning working wavelength has been demonstrated by using the dye-doped blue phase liquid crystal film. With the advantage of fast response speed, no alignment layer, large-scale electrically shift of the photonic bandgap, and macro optical isotropy, this selfassembled soft material has many potential applications in high performances reflective full-color display, 3D tunable lasers, and nonlinear optics. KEYWORDS: photonic crystal, blue phase, photonic band gap, full-color reflective display, lasing emission 1. INTRODUCTION Photonic crystals (PCs) of periodic modulated dielectric constant are attractive optical nanostructure materials, which can be applied in controlling and manipulating the light flow.1-3 Therefore, the PCs have enormous potential applications in PC fibers,4 PC sensors,5, 6 thin-film optics, nanolasers,7-9 and full-color reflective displays.10, 11 The functional PC materials of urgent demand can be achieved by changing the external stimuli, for example mechanical force,12, 13 thermal energy,14, 15 electric field,1618

light,19-21 and magnetic field.22, 23 Among the various external stimuli, electric field is the easiest

method to turn the photonic band-gap or lattice structure of PCs.24 Much effort has been taken to fabricate the stimuli-responsive PCs in the past two decades, such as layer-by-layer stacking techniques using micro-fabrication tools, electrochemical etching, laser-beam-scanning chemical

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capor deposition, holographic lithography, and self-assembly of monodisperse colloidal particles. However, some bottlenecks still need to be overcome, including slow response time, limited shift of bandgap, low light efficiency, hysteresis, and especially the obstacle in the actual technical application in implementation. Chiral doped liquid crystals have been used to form one, two, or three-dimensional (3D) PCs based on the self-assembled liquid crystal molecular. Compared with one- or twodimensional PCs, the complete photonic bandgap of 3D-PCs can regulate photons travel in all directions of space. Therefore, 3D-PCs are becoming increasingly important, but the cumbersome and complex production procedures greatly hinder the development of 3D-PCs.25 Abundant substantial progress have been made in the fabrication of 3D-PCs through lithography and etching, but how to achieve mass production and simplify the production procedures remains a challenging but exciting task. As one of soft artificial 3D-PCs, blue phase (BP) liquid crystals (BPLCs), exist selective Bragg reflections of light in the visible wavelength due to their spontaneous, self-assembled, 3D cubic structures.26, 27 So far, three types of BPs are reported, including BPIII of isotropic symmetry, BPII of simple cubic structure, and BPI of body centered cubic structure.28,

29

Compared with the

aforementioned techniques, the 3D-PCs fabricated with BPLCs have strength of simple, fast, and low cost. More importantly, the BPLC possess greatest strengths in soft PC area for their superior flexibility.30-33 The recent progress by applying electric field as the external stimuli opens an important door toward the wide tunable range of photonic band gap (PBG) of 3D-PC BPLCs. Heppke and Lu reported for the first time the effect of electric field on Bragg reflection wavelength of pure BPLCs and polymer-stabilized (PS) BPLCs,34,

35

respectively. Recently, Lin demonstrated the PBG of PS-

BPLC could be shifted and extended by a low direct current (DC) field,36 then asymmetric tunable PBG modulated by a DC voltage polarity was proposed using PS-BPI.37 These important achievements give us great inspiration to further investigate the influence of electric field effect on self-assembled BP 3D-PCs. However, using DC electric field to drive BP will have adverse effects, including the increase of electrical conductivity and the resultant electrohydrodynamic instabilities, which lead to phase transition from BP to N*. Once the phase transition occurs, even if the electric field was removed, the liquid crystals cannot reconstruct the initial cubic structure of BP. Compared to the DC electric field, the displacement of the PBG induced by an alternating current (AC) electric field hardly attracts people's attention in the 3D-PC BP owing to the limited PBG tunability.

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Herein, a wide electric field tuning and fast response of PBG is experimentally demonstrated in BPLCs by applying an external AC voltage. The defect structure among the double twist cylinders (DTCs) in BPLCs constitute unstable regions with higher free energy, so the BPLCs have a fragile periodic-cubic structure and is easily distorted by applying an external electric field, resulting in displacement of Bragg reflection wavelength. The BP film exhibits a field-induced reversible PBG tuning over the visible region across 470 nm to 645 nm. In addition, a liquid crystal laser of continuously tunable wavelength has been demonstrated by using dye-doping BPLC materials. This soft artificial material holds great promise for use in optoelectric devices. 2. RESULTS AND DISCUTION The BPLC consists of commercially available materials (Experimental Section). To achieve a shift reflection band gap, a delicate balance should keep between the DTCs and disclinations in BPLCs. Table I shows two optimized components of the BPLC samples. The components were taken by weight percentage. Table 1. Materials descripts of the blue phase liquid crystal mixture. LC host

Chiral dopant

Monomers

HCBP-006

R5011

RM257

12A

Sample I

89.8%

3.2%

3.5%

3.5%

Sample II

89.2%

3.8%

3.5%

3.5%

The phase transition of samples (Figure S1) was investigated by a temperature-controlled stage and a polarizing optical microscope (POM). The sample I was heated up to an isotropic at 62 0C, and then cooling the sample to 60.5 0C, the red color-spotted BP appeared. Upon further decreasing the temperature to 57.6 0C, the red spots BP gradually turned to the yellow spots and the sizes of these BP spots grew step by step. When the sample I was cooled to 55.6 0C, a vivid color of green can be observed and the small spots grew gradually to big platelet textures. Holding the temperature at 55.6 0C

for a few minutes, the platelet textures of BP would become larger. When finally cooled the cell to

54.4 0C, the phase transition from BP to N* appeared because of the appearance of fingerprints texture induced by defects in N* of liquid crystals. Figure S1 (b) shows the phase transtration of sample II. The BP could be clearly identified from 60.5 0C to 54.6 0C. When cooling the sample to 60.5 0C, the

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blue color-spotted BP appeared and the sizes of these BP spots grow gradually with further decrease the temperature. When the sample was cooled to 55.4 0C, the color of platelet textures of BP begin to change from blue to green. When the temperature was 54.6 0C, the BP changed gradually into N*. Due to the random alignment of the lattices, many independently dispersed platelet textures reflecting green (blue) color were observed without any alignment layer under the POM. The lattice was also confirmed by examining Kössel diffractions (Figure 1b). The 4-fold symmetry in Sample I arose from the lattice of a [110]. However, the edgy of 4-fold symmetry was blur due to the scattering losses generated from the lattice distortion (Figure 1c) in the sample without alignment layer.

Figure 1. The shifted PBG of BPLC. (a) Normalized reflection spectra of the sample at different AC fields. (b) The platelet textures and Kössel diagrams of BP at turn-off state and return to turn-off state after a cycle of driven by 1 kHz AC field. (c) Schematics diagrams of BPLC for underlying mechanism at E=0. To study the PC behavior of the BPLC, effects of AC field on BPs are as follows. The reflection band gap of the BPLC samples underwent a redshift because of the positive dielectrically of the liquid crystal host. In 1.6 V/um AC field of 1 kHz, due to the lattice extending along the field axis,38 the reflection network band of BP sample I underwent shift 84 nm from 556 nm to 640 nm in 20 s as

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shown in Figure 1(a). To our knowledge, 1.6 V/µm is the lowest electric field for large-scale adjustment of BPLCs PBG. After turning off the applied electric field, the reflection band relax to the original with the same time, which means that the BP did not disorder into the N* under the applied AC field. Applied AC field of 1.7 V/um changed the BP crystallites quickly into N*. Once the phase transition occurred, rebuilding the original BP will take a long time, even if the electric field was removed. After the AC field was turned off, the platelet textures of sample were more uniform than the original (Figure 1b). The reflect color looks more clear and vivid, which is recorded both visually and spectrally. By examining the Kössel diffractions can help to confirm the lattice behavior and obtained the results therefore suggest that the distribution of the lattice spacing become more homogeneous after driven by the uniform vertical AC electric field and the reflection increased to 5 times as much as before applying the electric field. When the device was cycle driven more than one time, the reflection at turn-off state kept the same, even the applied AC field reached to 1.7 V/um, because the lattice was more stable after the uniform AC field driven. To further demonstrated this phenomenon, figure S2 shows the reflection of the sample I in a cell consisted with two glass substrates coated with a layer of vertical alignment. The strong anchoring energy of the vertical alignment layer helped the lattice along the vertical direction, the same as the applied vertical AC electric field. Thus, the platelet textures in this cell were more uniform and the reflect color looks more clear and vivid. This state of affairs is also experimentally consistent with other directions.39 On the one hand, the alignment layer only effect on the surface in several hundred nanometers made the reflection slightly lower than that driven by vertical AC electric field. For another, the strong anchoring energy suppress the lattices elongation along the electric field, resulting in the low reflection and a higher electric field of 2.5 V/µm. The band gap underwent a wider redshift from 556 nm to 645 nm (figure S2). The reflection band of the sample with vertical alignment layer cannot relax to its original because of the emergence of a metastable BP state that had been identified by the simulation and experiment.40, 41

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Figure 2. Frequency effect on the PBG for BPI and BPII. The inset a) shows the effect of frequency on the center Bragg reflection wavelength of BPI; b) is the typical polarizing optical microscope image of BPI; c) and d) are the typical polarizing optical microscope images of BPII. The BPs of cubic structure formed with DTCs can be consider as a LC elastomer, which could be deformed by external electric field. The center wavelength of PBG in the BPs relies on the gradient of effective electric field (∇𝐸) applied on the LC molecules and the thermal energy. The smaller ∇𝐸, the larger deformation, resulting in a wider PBG change of BPs. To achieve a smaller ∇𝐸 needs to decrease the dielectric constant of BPLC mixture. Reference 42 has demonstrated that increasing the frequency of applied AC field was an effective way to decrease the dielectric constant of liquid crystal mixtures.[42] Figure 2 gives the information of the center wavelength of PBG and the frequency of applied AC field. At 1.6 V/um of 100 Hz, the center wavelength of PBG was shifted 61 nm, which was narrow than 84 nm that at 1.8 V/um of 1 kHz. As the frequency increased to 10 kHz from 1 kHz, the change of center wavelength of PBG decreased due to that the phase transition in the disclinations core from isotropic to the N* driven by external electric field. Besides decreasing the dielectric constant, as the frequency increases, the temperature increased because of the dielectric heating of the liquid crystal material. If the frequency was larger than 10 kHz, the change of center wavelength of the PBG become wider with the frequency increasing. Under a AC field of 23.7 V/um with 160 kHz (the electric field limit of our equipment), the center wavelength of the PBG was reversibly shifted to

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more than 110 nm in the absence of N* because the thermal energy induced by the dielectric heating of the liquid crystal material could suppress the phase transition induced by the external electric field.

Figure 3. Redshifted of the PBG: (a) Time-resolved microscopic pictures. (b) Kössel diagrams at AC field of 2.1 V/um and frequency of 1 kHz. (c) Schematics of the AC induced the shift of band-gap in BPII. Figure 2 also reveals the redshift of the PBG of sample II with different electric field and frequency. In the off state, the BP in sample II with (100) crystal lattice conformed by the Kössel diffractions (Figure 3a and 3b). Compare to BPI, the threshold electric field of BPII was higher than that of BP I because of the high ratio of the chiral dopant. If an AC field with 2.2 V/µm of 1 kHz was applied, the reflected wavelength would undergo a redshift from 470 nm to 560 nm because of the electrostriction,43 as shown in Figure 2 and 3c. After the first cycle driven, the reflection spectra of the BP II was more stable, resulting in a wider redshifted of center wavelength of the PBG. When the sample II was driven by an AC field with 2.3 V/µm of 1 kHz, the center wavelength of the PBG of BP II was redshifted to more than 100 nm. The reversible shifting of the PBG from the original position was repeatable for many times by applying the AC electric field. A method widely used to study BPs, Kössel diagram, was used to verify the types of BPII further. For the BP II of the simple cubic structure,

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the Kössel rings occurred in particular direction by illuminated with a monochromatic light.21The Kössel diagrams listed in Figure 3a shown that all the images under POM with different times driven by the AC field corresponds to (100) lattices of BP II (Figure 3b). They indicate that no phase change during the switching process. Under the applied AC field, the response of BP II is faster than that of BP I on account of the small chiral pitch of the BPII. As shown in Figure 2, we also investigated the frequency effect on the center wavelength of the PBG. As the frequency increases, the center wavelength of the PBG will be redshifted further under a higher AC field due to the decreased dielectric constant. Unlike BPI, no peak valley was observed during the frequency increase process due to the perfect balance the dielectric heating and the electrostriction.

Figure 4. a) The spectrum of the liquid crystal mixture including reflection, fluorescence, and laser emission. b) Emission spectrum of the sample I as increment of the pump energy. c) Threshold energy and FWHM versus pump energy. And d) the shift of lasing emission wavelength depending on the AC field.

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A continuously tunable lasing emission was demonstrated by mixed fluorescent dyes with the above BPLC system. To achieve the lasing emission in BPLC sample doped with PM597, the sample was pumped by a frequency-doubled Q-switched Nd:YAG laser of 10-ns pulses at 532 nm.. As shown in Figure 4, the dissolved PM597 had a broad fluorescence peak from 524 nm to 630 nm, in both the isotropic and BP phases of the host. A sharp laser peak with a full width at half-maximum (FWHM) of 2 nm and a lorentzian lineshape was measured at the edge of the PBG. Because a clear threshold was observed, the emission was considered as a mirrorless lasing. The lasing threshold was 1.6 mW/cm2 at 580.5 nm for the PM597-dopted sample I. Compare with previous work,7 the lower threshold of our sample was attributed to the lower scattering. After polymerization, the main causes of scattering were as follows: the polymer network that may made the lattice distortion of the cubic nanostructures and the refraction index mismatch of the polymer and DTCs in BPLC. The lasing emission spectrum closely corresponded with the reflection band of the PM597-dopted sample as change of the AC field. When the applied field increases from zero to 1.5 V /um, the laser moves about 20 nm to the longer wavelength, which is roughly consistent with the change of wavelength reflection band edge. After turning off the AC field, the laser reback to its original. The electrically controllable BP lasers were fabricated by the electrically switchable PBG of the BPLC. Therefore, the arbitrarily operation of the PBG by AC field with any dimension is of significance for development of 3D continuously tunable lasing emission. 3. CONCLUSIONS In summary, we proposed a soft BPLC film of large-scale PBGs shifting under a low-AC-field. The PBG can be tuned rapidly, continuously and reversible under electronic field due to the electrostriction. By switching the frequency of the applied AC field, the dielectric heating and the electrostriction can achieve a perfect balance to enlarge the redshifted of the BPI and BP II. During the switching process, the BPLC film is of 15 nm FWHM because no phase transition occurred, resulting in gorgeous and vivid appearance colors encompassing almost the entire visible spectrum. Moreover, an electrically continuously switchable lasing emission was proposed in the dye-doped BPLC material. This soft artificial 3D photonic crystal holds great potential applications in the field of optoelectronics. 4. EXPERIMENTAL SECTION 4.1. Materials. The used materials consisted of a nematic mixture (HCBP006, Δn=0.158, Δε=34.2 at 1 kHz and 298 K), chiral dopant (R5011), mesogenic diacrylate (RM257), and nonmesogenic acrylate

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(C12A). All the materials were supported by Jiangsu Hecheng Display Technology Co., Ltd. In the Supporting information, figure S3 listed the chemical structures of the used materials. PM597 (Exitone) is a fluorescence dyes to help laser emission. The constituent concentration of BPLC mixture were HCBP006/R5011/RM257/12A=86.5/3.3/4.8/4.8/0.6. To achieve the laser emission, the dye was mixed with the BPLC mixture. The samples were sandwiched between two glass substrates, whose inner surfaces were treated indium-tin-oxide homogenously. The cell-gap was ~10 μm apart by spacers. The BPLC mixture was infiltrated into the cells at an isotropic. 4.2. Optical Measurements. The microscopic pictures were observed under a POM (Olympus, BX51). A temperature-controlled stage with accuracy of 0.1 0C/min (Instec, ACS402) was used to detect the transition temperatures. Transmission and reflection spectrums were obtained by a spectrometer (USB2000+, Ocean Optics). The Kössel diagram was taken by a charge-coupled device that connected to the POM. In the Kössel diffraction study, the light source was a 488 nm light with a bandwidth of ~10 nm. The Kössel ring was seen by an oil-immersion objective (100×) under a Bertrand lens inserted microscope. A function generator (Tektronix, MDO3024) supplied the AC field. The pump light source was an Nd:YAG second harmonic generation pulse laser (Beamtech, Dawa-100) of 10 ns pulses at 532 nm. The waist of the pump beam focused on the sample was about ~120 um. The output lasing spectrum was detected by the USB2000+. 

ASSOCIATED CONTENT

Supporting Information. The following files are available free of charge. Phase sequence of BPI and BPII, AC-field effect on reflection spectra, and materials (PDF) 

AUTHOR INFORMATION

Corresponding Author *E-mail:

[email protected].

Author Contributions ‡X.-W.

Du and D.-S Hou contributed equally.

Notes

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The authors declare no competing financial interest. 

ACKNOWLEDGMENTS

This work was supported by the National Science Foundation of Hebei province of China (Grant no. A2017202004) and the National Natural Science Foundation of China (NSFC) (11504080). The authors also gratefully acknowledge Prof. Zhi-Gang Zheng from East China University of Science and Technology for inspiring discussions of the Kössel diagrams. 

References

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Symmetric Continuously Tunable Photonic Bandgaps in Blue-Phase Liquid Crystals Switched by an Alternating Current Field Xiao-Wei Du, De-Shan Hou, Xuan-Li, Dong-Peng Sun, Jiong-Fang Lan, Ji-Liang Zhu, and WenJiang Ye

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