Chromophores with side isolate groups and applications in improving the poling efficiency of second non-linear optical (NLO) materials

Accepted Manuscript Chromophores with side isolate groups and applications in improving the poling efficiency of second non-linear optical (NLO) materials Qiuli Cheng, Xiaoyu Shi, Chuangyang Li, Yuhang Jiang, Zuosen Shi, Jiawei Zou, Xuesong Wang, Xibin Wang, Zhanchen Cui PII:

S0143-7208(18)31526-2

DOI:

https://doi.org/10.1016/j.dyepig.2018.11.001

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DYPI 7145

To appear in:

Dyes and Pigments

Received Date: 10 July 2018 Revised Date:

15 October 2018

Accepted Date: 1 November 2018

Please cite this article as: Cheng Q, Shi X, Li C, Jiang Y, Shi Z, Zou J, Wang X, Wang X, Cui Z, Chromophores with side isolate groups and applications in improving the poling efficiency of second non-linear optical (NLO) materials, Dyes and Pigments (2018), doi: https://doi.org/10.1016/ j.dyepig.2018.11.001. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Chromophores with side isolate groups and applications in improving the poling efficiency of Second Non-linear Optical (NLO) materials

Xuesong Wang,a Xibin Wang,c Zhanchen Cui*,a,b a

State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin

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University, Changchun 130012, P. R. China. b

Department of Chemistry and Pharmacy, Zhuhai College of Jilin University, Zhuhai 519041,

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People’s Republic of China. c

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Qiuli Cheng,a Xiaoyu Shi,a Chuangyang Li,a Yuhang Jiang,a Zuosen Shi,*,a Jiawei Zou,a

State Key Laboratory on Integrated Opto-Electronics, Jilin University Region, Changchun

130012, P. R. China.

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Abstract

In this paper, alkyl groups with different lengths are introduced to the two-dimensional spindle-type chromophores as the steric isolators to improve the poling efficiency and long term

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stability of the poled polymers for second-order nonlinear optical materials. The structure and thermal properties of alkyl functionalized chromophores are characterized by the 1H-NMR, FT-

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IR, UV-Vis, DSC, TGA, et al. The chromophores with different alkyl groups present extremely similar maximum absorbance wavelength (480nm), which means they have very similar firstorder hyperpolarizability. Guest-host polymer doped with chromophores has been prepared. Those materials display different glass transition temperatures and good thermal stability. According to the electro-optic coefficients studies and poling efficiency results, chromophores with longer alkyl side chain groups exhibit better poling efficiency and larger electro-optic coefficients. Especially for the chromophores with octyl group, the poling efficiency increases up

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to 3 times compared to the reference chromophore 2-(3-{2,5-Phenylmethanol-4-[4(dimethylamino)styryl]-styryl-5,5-dimethylcyclohex-2-enylidene)malononitrile (STC), while the corresponding electro-optic coefficient of the poled polymers increases up to 4 times, indicating

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a significant effect of the isolate groups on improving the poling efficiency.

Keywords: Nonlinear optics; Chromophores; Poling efficiency; Electro-optical (EO)

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1. Introduction

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Over the past two decades, second-order nonlinear optical materials attracted much and deep research due to its enormous and potential applications in high speed electro-optical (EO) modulators [1-10]. Inorganic NLO materials, such as LiNbO3, K3PO4, GaAs, etc., were developed and used much more widely because of their excellent stabilities [11-13]. However, poled polymers for second-order nonlinear optical (NLO) materials were considered to be the

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ideal material for the next generation high-speed electro-optical (EO) modulating devices, due to the advantages of molecule variable, low micro-wave dielectric constant, high modulating

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frequency (>100G), ultra-low drive voltage (<1V), ease of processing molding and integration into optical electronic devices [9, 14-23]. In general, the first-order hyperpolarizability of the

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chromophores, poling efficiency and alignment stability greatly affected their non-linear optical properties [24-30]. Up to now, people have made great efforts and obtained great progress on the preparation and application of poled polymer NLO materials. Some of them were even commercialized and displayed excellent performances. However, one of the main barriers which limited their widely use was the bad long-term stability of the chromophores in the materials [27, 31-36].

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In our previous works [37-39], we synthesized a series of polymers containing spindle-type chromophores and studied their structure-properties relationship. We found that these poled polymer NLO materials exhibited excellent orientation stabilities, since these chromophores had

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larger freedom of rotation volume and stereo-hindrance effect, which could effectively decrease the intermolecular aggregations. Theoretically, decreasing the interaction of the molecules and enhancing the poling efficiency represent a key point to improve the second-order NLO property

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for poled polymers. Therefore, chromophores with “isolation groups” structures [40, 41], spherical-like hyperbranched polymers or dendrimers [42-44] and many other types of

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chromophores [45] were designed and synthesized to reduce the dipole-dipole interactions. Based on the previous research, in this study, by introducing alkyl chains with different structures and length, we synthesized alkyl chain modified “spindle-type” chromophores STC, STC-tBu, STC-C4 and STC-C8 (Fig. 1). The alkyl chains increased the steric hindrance and

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improved the poling efficiency [46-50]; we further studied their effect on the NLO properties before and after poling by the UV absorption measurements, and it was found that chromophores with octyl chains had better poling efficiency. Finally, the electro-optical (EO) properties of the

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poled polymers were tested by simple reflection method [51], the results indicated that the r33 values increased with the prolong of the side chains, the film containing STC-C8 displayed

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largest output modulation signal (Im33 =220 mV), which was up to 4 times than STC (50 mV).

2. Experimental 2.1. Materials.

4-(hydroxymethyl) phenyl boronic Acid (99%) was obtained from Sun Chemical Technology Co., Ltd. Potassium tert-butoxide and tetrakis(triphenylphosphine) palladium (0) (99.8%) were

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supplied from Aladdin. Toluene and sodium carbonate were from Beijing Chemical Co. (Beijing, China). Ethyl acetate and other reagents were purchased from Tianjin Tiantai Refine Chemicals

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was prepared according to our previous literatures [38].

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Co., Ltd (Tianjin, China). All available reagents were used without further purification. The STC

2.2. Synthesis

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Fig. 1. Structures of spindle-like chromophores STC, STC-tBu, STC-C4 and STC-C8.

Synthesis of the chromophores. 2,5-dibromobenzene-1,4-dicarbaldehyde and 1,3,3-

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trimethyl-5-dicyanovinyl-1-cyclohexene (TDC) were synthesized using the literature procedures [52]. The specific synthesis of the chromophores is showed in Scheme 1. Details of the synthesis

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is described in the supporting information.

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Scheme 1. Synthetic scheme of the chromophores.

Preparation of host-guest doped NLO thin films. To prepare the polymer films, the alkyl

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group modified chromophores (STC-tBu, STC-C4 and STC-C8) were dissolved in cyclopentanone with a 0.15 g mL-1 weight concentration. The solutions were filtered through a 0.2 mm pore size filter and spin coated onto indium tin oxide (ITO) glass substrates at 1000 rpm.

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The residual solvent was removed by heating the films in a vacuum oven at 70°C overnight. 2.3. Characterization

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The 1H NMR spectra were taken on a Bruker AVANCE NMR spectrometer at a 500 MHz resonance frequency for 1H, and tetramethylsilane (TMS) was used as the internal standard. IR absorption was recorded on an AVATAR 360 FT-IR spectroscope. Ultraviolet-visible absorption spectra were obtained by spectrophotometer (SHIMADZU) UV-3100 spectrophotometer. The

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glass transition temperature was determined by using differential scanning calorimetry (DSC) measurement taken on a NETZSCH 4 instrument at a scan rate of 10 K min-1 in the temperature range of 50-300 °C under nitrogen. The decomposition temperatures were measured using

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Perkin-Elmer TGA 7 analyzer from 50 °C to 800 °C at a heating rate of 10 K min-1 under nitrogen atmosphere. The electro-optical (EO) coefficient was determined by simple reflection

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method according to our previous report [53].

3. Results and discussion 3.1. Synthesis and structural characterization

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Spindle-type chromophores STC-tBu, STC-C4 and STC-C8 were prepared from three different alkyl isocyanates and STC respectively by an addition reaction (Scheme 1). The

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structure of chromophores was characterized by 1H NMR and FT-IR. (Fig. 2-3).

Fig. 2. The 1H-NMR spectrum of chromophores.

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The 1H-NMR spectra of compounds are displayed in Fig. 2 and showed quite considerable

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spectral date. Compared to the chromophore STC, new chemical shifts, originated to the proton of alkyl group, had been found in the spectra of STC-tBu, STC-C4 and STC-C8. In addition, the FT-IR spectra of the chromophores shown in Fig. 3 revealed a peak at about 2220 cm-1 attributed to cyano group. The stretching vibration of the N-H displayed around 3340 cm-1 and the C=O appeared at approximately 1710 cm-1 except the chromophore STC. The absorbance peak of phenyl group appeared at about 1550 cm-1. The two peaks at 2930 cm-1 and 2846 cm-1 were attributed to the absorption of methyl and methylene group respectively.

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Fig. 3. FT-IR spectra of chromophores.

3.2. Thermal properties of the chromophores

Thermal stability of chromophores was investigated by thermogravimetric analysis (TGA) under nitrogen atmosphere. As shown in Fig. 4, the temperatures of 5% weight loss (Td) for all

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the chromophores were about 200 °C; STC-C4 and STC-C8 exposed slightly higher thermal property than STC, however, the stability of STC-tBu was weakened, which were likely due to the introduction of tertiary butyl structure. Fig. 5 showed the DSC thermograms of

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chromophores. Chromophores STC-C4 and STC-C8 emerged melting peak at 195 °C and

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86 °C, respectively. This appearance could be contributed to the introduction of an alkyl group that formed barrier effect, leading to a reduction in inter-molecular forces and an increase of the molecular flexibility simultaneously, thus the chromophores melted before decomposition. There were two endothermic peaks for STC-tBu, combining with the TGA curve, which related to two decomposition processes of the chromophore.

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Fig. 4. TGA thermograms of the chromophores in nitrogen at the heating rate of 10 K min-1.

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Fig. 5. DSC thermograms of the chromophores in nitrogen at the heating rate of 10 K min-1.

3.3. UV-Vis spectra

The UV-Vis absorption spectra of four chromophores measured in a series of different polar solvents are presented in Fig. 6. The charge transfer band demonstrated that the four chromophores had similar absorption peak shape and at the same time when the same solvent is used they exhibit quite similar absorption peaks, including close wavelengths of maximum absorption and extinction coefficient. This phenomenon is attributed to the barrier of methylene

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which completely affected electro-optic activity of chromophores due to the barrier effect and flexible structure. Chromophore molecules presented bathochromic shift effect with increasing polarity of the solvent (from CH2Cl2 to CHCl3), with the polarity of solvent further increased

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(from CHCl3 to CH3CN), the wavelengths of maximum absorption showed hypsochromic shift. This phenomenon was associated with the molecules interaction with solvent which showed

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different charge separation.

Fig. 6. UV-Vis absorption spectra of chromophores in different solvents: a) STC, b) STC-tBu,

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c) STC-C4, d) STC-C8.

3.4. Structural simulation

Fig. 7 depicted a MM-minimized structure which could intuitively represent the effect of the alkyl group on the molecular structure of chromophores. Owing to the introduction of alkyl groups, the free volume of the chromophore molecules increased (Fig. 7a, front view). Moreover, due to the bond angle of carbamate linkage between methylene and alkyl groups (calculated

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results), the alkyl groups extended beyond the dipole plane, which hindered molecular interactions more effectively (Fig. 7b, lateral view). The branched-chain structure of tert-butyl group and the length of the n-octyl group were more conducive to weaken the intermolecular

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interaction.

Fig. 7. Structure Simulation of Chromophores (MM2 method): a) front view of chromophores, b) lateral view of chromophores.

3.5. Nonlinear optical properties For exhibiting an electro-optic effect, the chromophore units in the films must be noncentrosymmetric. We utilized high electric field poling to orient the chromophores along the applied electric field and thus producing a noncentrosymmetric alignment. Poling conditions

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were as follows: at 140 °C and applying a high dc voltage 5.5 kV at the tungsten wire across the films for about 1 h at the gap distance 1.0 cm. Finally, the temperature was lowered to ambient

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temperature with the electric field still applied.

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Fig. 8. UV-Vis absorption spectra of the STC/PMMA doped film before and after poling.

Fig. 9. UV-Vis absorption spectra of the STC-C8/PMMA doped film before and after poling.

To explore the noncentrosymmetric alignment of the chromophores in the films, the absorption spectra of hybrid films STC and STC-C8 before and after poling were compared. Fig. 8 and 9 showed the UV-Vis absorption spectra of the guest-host doped film before and after poling. It can be seen that polarization leaded to a reduction in absorption intensity, and the

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intensity of absorption peaks of STC-C8 changed more obviously than STC. According to the absorption change, the order parameter (Ф values) for the films could be measured as following

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equation: Ф=1-A1/A0

where A0 and A1 were the absorbance of the unpoled and poled films, respectively. The

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calculation of Ф values were summarized in Table 1. The STC-C8/PMMA doped film exhibited quite high Ф values, up to 0.17, comparing with STC/PMMA doped film (Ф=0.06). The Ф

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values tested were well in accordance with their r33 values. Table 1. Characterization data of chromophores. λbmax (nm)

STC

471.0

485.0

STC-tBu

471.5

STC-C4

469.5

STC-C8

472.0

Im33 (mV)

Ф

Tdc (℃)

r33 (pm/V)

50

0.06

205

0.54

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λamax (nm)

55

0.08

190

0.58

485.0

70

0.10

220

0.71

487.0

220

0.17

230

2.13

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485.5

The maximum UV-Vis absorption wavelength of chromophores in acetone

b

The maximum UV-Vis absorption wavelength of chromophores in trichloromethane

c

The 5% weight loss temperature of chromophores measured by the TGA analyses at nitrogen

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a

condition at the heating rate of 10 K min-1 The EO coefficients of the poled guest-host doped films were measured by the Teng-Man technique at 1310 nm[54, 55]. According to these literatures, due to the complication of the testing system, which would result in errors, we did the test by referring to a standard sample

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(GaAs crystal, r41=1 pm/V) under the same testing conditions. r33 was calculated according to the following equation reported in literature[56]. 3 = . 2

.

⁄

−

2

− ⁄

−

1

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r r

where n is the refractive index at 1,310 nm, Im is the intensity of output modulation signal, and Vm is the alternating current voltage applied to the sample. θ is the incident angle, Im41 and Vm41 is

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= 16.835 ×

− 0.5

⁄

2

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r

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the parameters of GaAs. Therefore, the equation (1) could also be displayed as follows:

According to equation (2), because all of the films were tested under the same conditions, the value of r33 could be compared by using Im33 (Vm33=20.4V). The testing results (Table 1) showed that the values of Im33 and Ф of the chromophore molecules with alkyl structure were higher than

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that of STC molecule (50 mV), which indicated that alkyl modification made the poling efficiency more effectively. The Im33 and Ф values of STC-tBu (55 mV) was similar to the value of STC, which might be due to the large volume and relative rigidity of the t-butyl group. The

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STC-C8 showed higher Im33 (220 mV) than the STC-C4 (70 mV). The reason was that the STCC8 possesses longer length alkyl and greater proportion of flexible components.

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More noteworthy was that different alkyl groups in the lateral group of the chromophores could impact the poling efficiency and the electro-optic activity. In Fig. 10, temporal stability of the chromophores was measured by being exposed in air at room temperature for several months. Although the order parameter Ф values of chromophores declined slightly, the poling efficiency trend had not changed. Overall, all of the temporal stability were satisfactory.

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Fig. 10. Temporal stability of chromophores.

4. Conclusion

In summary, a series of two-dimensional spindle-like chromophores with different length of alkyl groups on the conjugate bridge were designed and synthesized. By comparing the electro-

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optic (EO) coefficients (r33) of the poled polymers by doping chromophores in PMMA, the results showed that the chromophores with longer alkyl groups and linear alkyl group had better poling efficiency and larger electro-optic coefficients. Moreover, two-dimensional spindle-like

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chromophores had good temporal stability. Therefore, designing and optimizing chromophore molecules through modifying the blocking groups might be a new way to improve the poling

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efficiency of the chromophores and enhance the second-order nonlinear optical properties of electro-optic polymers.

Acknowledgments

This article was financially supported by the National Natural Science Foundation of China (grant no. 21374039, 21204028, 51673081). The authors would like to acknowledge those who make contributions to this article.

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[53] Zhang X, Ming L, Shi Z, Jin R, Wang X, Yan Y, et al. Novel photo-cross-linkable polymer bearing spindle-type chromophores for second-order non-linear optical materials. J Mater Sci. 2011;46(13):4458-64. [54] Chi HL, Dong HP, Herman WN. Analysis of multiple reflection effects in reflective measurements of

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Highlights

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We synthesized a series of two-dimensional spindle-like chromophores with

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Two-dimensional spindle-like chromophores had good thermal stabilities and met the requirements of the poling process.

Chromophores with longer alkyl groups and linear alkyl group had better

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different length of alkyl groups.

The poling efficiency of chromophores with octyl group increased up to 3 times and the electro-optic coefficient of poled polymers increased up to 4 times

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compared to STC.

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long-term stability, poling efficiency and larger electro-optic coefficient.