Electro-optical characteristics of holographic polymer dispersed liquid crystal gratings doped with nanosilver Menghua Zhang, Jihong Zheng*, Kun Gui, Kangni Wang, Caihong Guo, Xiaopeng Wei, and Songlin Zhuang Engineering Research Center of Optical Instrument and System, Ministry of Education, Shanghai Key Laboratory of Modern Optical System, University of Shanghai for Science and Technology, No. 516 JunGong Road, Shanghai 200093, China *Corresponding author: [email protected] Received 8 July 2013; revised 13 September 2013; accepted 14 September 2013; posted 1 October 2013 (Doc. ID 193403); published 21 October 2013
We report on the synthesis and characteristics of a holographic polymer dispersed liquid crystal (H-PDLC) switchable grating based on nano-Ag particles. The influence of doping different concentrations of nano-Ag on the diffraction efficiency, driving voltage, and response time of the H-PDLC grating is investigated. The best grating characteristics were achieved with 0.05% nano-Ag doping. Calculated and experimental results reveal that the improvement of the characteristics is likely due to the surface plasmon effect of nano-Ag. © 2013 Optical Society of America OCIS codes: (050.1950) Diffraction gratings; (160.3710) Liquid crystals; (160.4236) Nanomaterials; (240.6680) Surface plasmons. http://dx.doi.org/10.1364/AO.52.007411
1. Introduction
In recent years, electrically switchable holographic polymer dispersed liquid crystal (H-PDLC) has been widely studied [1–5]. Compared with ordinary nematic liquid crystal (LC) devices, PDLC devices offer several advantages, such as unwanted polarizers during the fabrication of PDLC devices, the possibility of reduced power consumption and weight with increased brightness, and optical contrast that requires no extra optical elements. Therefore, the technology of H-PDLC has abundant application prospects in many domains, such as zoom lenses, filters, optical information storage, photorefractive materials, etc. [6–8]. Conventional H-PDLC grating suffers from low diffraction efficiency, high switching voltage, and long response time due to many factors in fabrication, 1559-128X/13/317411-08$15.00/0 © 2013 Optical Society of America
such as the mass ratio of the materials, the intensity and time of exposure, the temperature of the environment, the humidity levels, etc. [9–11]. Various methods have been adopted to address these issues. For example, diffraction efficiency has been improved by adding cross-linking agent NVP to the materials, and switching voltages have been reduced by the addition of monomers, including the element fluorine [12], surfactant molecules [13], etc. Of increasing importance is the use of nanoparticles with PDLCs. Nanoparticles provide good electro-optical contrast and form strong networks in LC layers that can prevent pouring effects in the cells. To further optimize the properties of H-PDLC grating, researchers have doped various nanoparticles with PDLCs [14–16]. This study investigates the effect of doping H-PDLCs with nano-Ag particles. First we model the effect of different concentrations of nano-Ag on the surface morphology of fabricated PDLC gratings and provide an analysis of the surface plasma resonance of 1 November 2013 / Vol. 52, No. 31 / APPLIED OPTICS
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nano-Ag. We experimentally verify the simulated results and analyze the spectral and electro-optical characteristics of the H-PDLC first. Compared to a grating sample without nano-Ag, we have made H-PDLC gratings with higher diffraction efficiency and better electro-optical properties by optimizing the concentrations of nano-Ag. The superior properties obtained include a reduction in switching voltage from 2.35 to 1.23 V∕μm, a diffraction efficiency increase from 55% to 95%, and a response time reduction from 750 to 150 μs. 2. Experiment A.
Fabrication of H-PDLC Gratings Doped with Nano-Ag
The materials system for fabricating the PDLC gratings consisted of a photoinitiator (RB, Aldrich Inc.), a synergistic photoinitiator (NPG, Aldrich Inc.), a cross-linking agent (NVP, Aldrich Inc.), a surfactant (S-271, Chemistry Inc.), an acrylic monomer (EB8301, UCB Inc.), and nematic LCs (TEB50, Beijing Tsinghua Yawang Liquid Crystal Material Co., Ltd.) with a mass ratio of 0.15:0.4:10:10:45:35 and total mass of 3 g. Nano-Ag (diameter 80 nm, Beijing Nachen S&T Ltd.) in different quantities (0.75, 1.5, 3, and 6 mg) was added to the materials. Then the new hybrid material was heated by an ultrasonic emulsification instrument in dark conditions for about 2 h. Fabrication of the PDLC material with nano-Ag doping was complete when it had been stable for 24–48 h in a darkroom. To form the LC cells, the material was heated to about 40°C and sandwiched between two pieces of ITO coated glass. The space between the two pieces of glass was controlled to 17 μm. Then the LC cells were exposed in an interference 532 nm laser for about 60 s. The power exposed on the samples was 22 mw∕cm2 , and the ambient temperature was kept between 25°C and 30°C. The experimental setup is shown in Fig. 1. All Bragg diffraction gratings were recorded with the same spatial frequency, and the grating pitch was around 1100 nm.
Mirror
Laser
Beam expander
Beam splitter
Mirror
B. Characterization Investigation of H-PDLC Gratings Doped with Nano-Ag
We began by modeling a key property of nano-Ag in various concentrations and then experimentally studied it. More specifically, our methods entailed (1) investigation of spectra and surface plasma resonance; we calculated the surface plasma resonance peak of nano-Ag by establishing a theoretical model. Experimentally, samples with different concentrations of nano-Ag were illuminated by a halogen and deuterium light with the angle of incident light perpendicular to the LC cell. The transmission spectra were recorded by a spectrometer (Ocean Optics USB4000) for the purpose of finding the plasma resonance peak of nano-Ag. (2) Surface morphology analysis: a field emission scanning electron microscope (FE-SEM) was used to observe the nano-Ag doped H-PDLC specimen and analyze surface morphology changes caused by adding nano-Ag within H-PDLC grating. (3) Investigation of the optical and electro-optical properties of the nano-Ag doped H-PDLC grating, specifically, its diffraction efficiency, driving voltage, response time, and polarization dependence. 3. Establishment of Model
Localized surface plasmon resonance (LSPR) has been widely studied within optical material systems doped with metal nanoparticles such as Ag nanoparticles. As the nanoparticles are illuminated, there is a collective oscillation among the free electrons located on the surface of the particles. When the frequency of incident light is similar to the oscillation frequency of these free electrons [17], the electromagnetic field close to the surface of the nanoparticles intensifies. A strong resonance absorption peak is observed on the spectrum that indicates the location where LSPR just occurred. It is quite important to find the spectrum peak, and it has been reported that Hutter and Fendler observed a LSPR phenomenon at the tip of Ag nanowires using 532 and 820 nm incident laser beams [18–20]. In order to find the plasma resonance peak of Ag nanoparticles in H-PDLC, we built a model with the software CST (Cst Microwaves Studio). We suppose that the distribution of Ag nanoparticles in the PDLC material system is homogeneous. It has been calculated that the space between two Ag nanoparticles and the diameter of the Ag nanoparticles are 2 μm and 80 nm, respectively. The permittivity of the PDLC material system measured by the dielectric constant detector (TH2820, Tonghui Inc.) is 20 times that of air. The plasma frequency and fluctuating frequency of Ag nanoparticles were set to 1.37 1016 Hz and 1∕4 1014 Hz, respectively. According to a model by Drude described as Eq. (1) [21], εω ε∞ −
Sample
Fig. 1. Experimental setup for nano-Ag doped H-PDLC grating. 7412
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ω2P ; ω iγω 2
(1)
where ωP is the plasma frequency of the free electron, and γ is the characteristic collision frequency of the
90 80 Transmission[%]
70
Ag nano- particle
60
Ag 0.2% Ag 0.1% Ag 0.05% Ag 0.025% Ag 0%
50 40 30 20 10 0 300
400
500525
600
700
800
900
Wavelength[nm]
Fig. 4. Transmission spectra of H-PDLC gratings doped with different concentrations of nano-Ag. Fig. 2. Model constructed.
B. Surface Morphology
free electron. The model constructed is shown in Fig. 2. The thickness of this plane is set as 200 nm. The blue part in Fig. 2 stands for the PDLC material, and the red dots are the Ag nanoparticles. The model is irradiated by an illuminant with wavelength range from 460 to 750 nm. Figure 3 shows that light transmittance is lowest at wavelength 517 nm, which means the wavelength correspondence to the plasma resonance peak of Ag nanoparticles is around 517 nm. 4. Experimental Results and Discussion A.
Spectral Transmittance of Material
To verify the simulated results, transmission spectra of samples with different concentrations of nano-Ag were recorded by a spectrometer (Ocean Optics USB4000). The results are shown in Fig. 4. All samples share the same indentation at the wavelength of 562 nm, which is the absorption peak of RB. In addition, there is an obvious indentation around 525 nm on the transmission spectra of the sample doped with nano-Ag compared to the case without nano-Ag. We infer this trench to be the plasma resonance peak of nano-Ag. The experimental result (525 nm) is close to the simulated one (517 nm). The transmittance value of the sample doped with nano-Ag is higher than that without nano-Ag, due to the reduced loss of light intensity caused by scattering and the absorption of the material by the light. 100
Transmittance %
90 80 70 60 50 40 420
440
460
480
500 517 540 Wavelength /nm
560
580
Fig. 3. Transmittance curve of model.
600
620
To investigate the effect of nano-Ag on the surface morphology of H-PDLC gratings, we used an atomic force microscope (AFM) to observe the grooves of H-PDLC samples. Figure 5 shows that the interface between the LC and polymer becomes smoother after doping with nano-Ag [Figs. 5(b), 5(c), and 5(d)] compared to the case without nano-Ag. When the concentrations of nano-Ag reach 0.2%, the interface becomes rough partly owing to the aggregation of Ag nanoparticles. Moreover, the grooves of Ag-doped samples are deeper. Measured results show that the groove depth of grating without nano-Ag is about 28.6 nm, while the groove depths of doped gratings are 37.3, 39.6, 36.0, and 30.8 nm, respectively, for concentrations of nano-Ag of 0.025%, 0.05%, 0.1%, and 0.2%. These results confirm that the separation of polymer and LC is optimized and becomes more exhaustive after nano-Ag doping. C.
Diffraction Efficiency
We tested the diffraction efficiency of a fabricated H-PDLC grating sample with a 633 nm laser. When the laser illuminates the samples with the Bragg angle, the diverged zeroth-order diffraction beam and first-order diffraction beam can be observed clearly. The -1st-order and second-order beams are too weak to be detected by a power meter. Figure 6 gives the diffraction results of H-PDLC gratings doped and undoped with nano-Ag. The diffraction efficiency of the H-PDLC grating without nano-Ag only reaches around 50% in our experiment [Fig. 6(a)], while the diffraction efficiency of the grating doped with nano-Ag can reach 95% when the doping concentration is 0.05% [Fig. 6(b)]. However, when the concentration ratio rises to 0.1% and 0.2%, the diffraction efficiency drops back to 80%–90%. Here, the firstorder diffraction efficiency of H-PDLC grating can be calculated as ξ I 1 ∕I 0 I 1 [22], where I 0 and I 1 are the power of the zeroth- and first-order diffraction beams, respectively. (Light absorption and scattering loss have not been taken into account in this case.) These experimental results show that the doping concentration has a great influence on the 1 November 2013 / Vol. 52, No. 31 / APPLIED OPTICS
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Fig. 5. Atomic force microscope (AFM) images of each sample doped with different concentrations of nano-Ag: (a) 0% Ag, (b) 0.025% Ag, (c) 0.05% Ag, (d) 0.1% Ag, and (e) 0.2% Ag.
diffraction efficiency of H-PDLC gratings, and only a suitable doping ratio such as 0.05% can achieve higher diffraction efficiency. 7414
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Theoretically, the diffraction efficiency of the H-PDLC grating is given by Eqs. (2) and (3) [23]:
-1st order
0th order
1st order
to different peak wavelength. As the doping concentration was raised in our experiment, some of the Ag nanoparticles aggregated into larger sized particles. This caused the plasma resonance peak to move to another location and the LSPR phenomenon to weaken, thereby lowering the diffraction efficiency. For these reasons, we believe that LSPR plays an important role within the nano-Ag doped H-PDLC gratings.
2nd order
(a)
D. 0th order
1st order
2nd order
(b) Fig. 6. Comparison of H-PDLC grating diffraction pattern: (a) doped 0% nano-Ag and (b) doped 0.05% nano-Ag.
ξ sin2
πdΔn e−2xd ; λcos θS cos θP 1∕2
Δn
n¯ LC-Rich − n¯ P-Rich ; 2
(2)
(3)
where Δn is the modulation of the refractive index, d is the sample thickness, λ is the wavelength of incident light, and x is the absorption coefficient of the material. θS and θP are the incident angles of the vector of the grating with the energy propagation direction of S waves and P waves meeting the conditions cos θS K · uS and cos θP K · uP . nLC-Rich and nP-Rich are the refractive indices of the rich LC area and the rich polymer area. According to Eqs. (2) and (3), the increase of the modulation of refractive index can improve diffraction efficiency when the wavelength of incident light and the angle between incident light and the sample are invariable. Through doping with nano-Ag, higher diffraction efficiency can be obtained, owing to much more exhaustive phase separation between polymer and LC that leads to a larger refractive index difference between grating fringes. In addition, the wavelength of the laser beam for holography recording is 532 nm, which is very close to the measured plasma resonance peak of 525 nm. Thus it is possible that a strong electromagnetic field has been generated around the nano-Ag due to the LSPR phenomenon within the doped H-PDLC material. Therefore during the holographic recording process, the LSPR helps to improve the phase separation, which leads to better optoelectrical properties and higher diffraction efficiency of the electrically controlled H-PDLC grating. The fact that a rise in doping concentration beyond 0.05% reduces diffraction efficiency to 80%–90% can also be explained by LSPR. The frequency and intensity of the surface plasmon absorption bands are highly sensitive to the size of the nanostructure [20,24]; i.e., different nanoparticle size corresponds
Threshold voltage is a critical factor to evaluate the opto-electrical characteristic of H-PDLC gratings. To test the threshold voltage, each grating sample was irradiated with a 633 nm He–Ne laser with the Bragg incident angle, and meanwhile a square wave of 2 KHz with 50% duty cycle was applied. The light through the sample is diffracted into several beams, and two detectors were set to detect the power of the light at the zeroth and first orders. Thus the light power of the zeroth order and first order are recorded with the increase of external voltage, and then the normalized diffraction efficiency can be obtained just as shown in Fig 7. Figure 7 shows the relationship between diffraction and electric field, and the threshold voltage is 2.35, 1.88, 1.23, 2.71, and 3.71 V∕μm at the concentration of nano-Ag of 0%, 0.025%, 0.05%, 0.1%, and 0.2%, respectively. The theoretical estimate of the threshold voltage is given by Eq. (4) [25]: VC
d0 σ LC Kl2 − 1 1∕2 2 ; 3a σ P ε0 Δε
(4)
where a, l, d0 , σ LC , σ P , K, and Δε are the diameter of the LC droplet, the length ratio of the two axes of the LC droplet, the sample thickness, the conductivity of the LC, the conductivity of the polymer, the elastic constant, and the dielectric anisotropy, respectively. Results show that the threshold voltage drops to the lowest when the concentration of nano-Ag is 0.05%, This can be explained by the increase in the conductivity of the polymer that occurs, as more nano-Ag
100 90 Diffraction efficiency (%)
-1st order
Threshold Voltage
80 70 60 50 Ag 0%
40
Ag 0.025%
30
Ag 0.05% 20
Ag 0.1%
10
Ag 0.2%
0 0
10
20
30
40
50
60
70
V
Fig. 7. Relationship between the diffraction efficiency and the drive voltage for each sample. 1 November 2013 / Vol. 52, No. 31 / APPLIED OPTICS
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particles are trapped in it, due to its higher viscosity compared to the LC. Higher conductivity of polymer means lower voltage is needed. This mechanism for reducing threshold voltage is similar to the carbon nanotube [26]. With the rise of the concentration of nano-Ag, the level of phase separation of the LC and the polymer is lower than 0.05%, and more LC remains in the rich polymer area, which is the same as the rich LC area. The ratio of conductivity between the rich LC area and the rich polymer area becomes larger because of more nano-Ag particles sticking in the rich LC area with the polymer, which results in a rise in the threshold voltage. Response Time
We also investigated the response time of the H-PDLC grating. The detector described in the above section receiving first-order light was connected to an oscilloscope (Tektronix TDS2012). When the samples were switched by a voltage of 100 V peak to peak with a 2 KHz square wave and 50% duty cycle, the oscilloscope showed a variable diffracted signal with the change of external voltage. The recorded response times of the samples doped with different concentrations of nano-Ag are given in Table 1. In the H-PDLC system, Eqs. (5) and (6) [25,27] are used to calculate the rise time and the fall time: τon
γ1 ; ΔεE2 kl2 − 1∕a2
(5)
γ 1 a2 ; kl2 − 1
(6)
τoff
where γ 1 is the viscosity coefficient of the LC, E is the applied electric field, K is the LC constant, a is the diameter of the LC droplet, and l is the length ratio of the two axes of the LC droplet. The rise time depends on the applied electric field and the LC droplet size. The fall time is dependent on the visco-elastic coefficient of the LC and the size of the LC droplet. The data show that after doping 0.05% nano-Ag, the response time of gratings drops to less than 1 ms. The decrease in both rise time and fall time is due to the reduction of the size of the LC droplet. Since the size of LC droplets affects many properties of the H-PDLC grating, all images of the samples were captured by a FE-SEM (Quanta 450, FEI) to
Table 1.
Rise and Fall Times of Nano-Ag Doped H-PDLC Grating with Different Concentrations
Concentrations of Nano-Ag 0% 0.025% 0.05% 0.1% 0.2%
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Rise Time 750 550 150 150 400
μs μs μs μs μs
Fall Time 800 μs 800 μs 400 μs 1.1 ms 1.3 ms
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Fig. 8. FE-SEM images of H-PDLC diffraction gratings doped with (a) 0% nano-Ag and (b) 0.05% nano-Ag.
verify the reduction of the size of the LC droplets after doping with the right concentration of nanoAg. Figure 8 shows that LC droplets after nano-Ag doping are smaller than droplets without nano-Ag. While the size of the average LC droplet without nano-Ag is around 200 nm, droplets with 0.05% nano-Ag are half that size (100 nm). F.
Polarization-Dependent Characteristic
Figure 9 shows the polarization azimuth dependence of the H-PDLC gratings doped with different concentrations of nano-Ag. The first-order diffraction efficiency is shown as a function of the incident polarization state. Electro-optical performance parameters of the H-PDLC gratings were determined by probing the samples with a p-polarized He–Ne (λ 632 nm)
100 90 Diffraction Efficiency(%)
E.
80 70 60 50 40
Ag 0% Ag 0.025% Ag 0.05% Ag 0.1% Ag 0.2%
30 20 10 0
0
20
40
60 80 100 120 Polarization azimuth angle(deg)
140
160
180
Fig. 9. Effect of probe angular polarization on first-order diffraction efficiency of the H-PDLC gratings doped with different levels of nano-Ag.
laser beam; the first-order diffraction efficiency, shown as open circles, changes with polarization azimuth angle. Using the anisotropy parameter defined as 2ζ P − ζS ∕ζP ζS [28], the extent of anisotropy for each sample is 0.74, 1.41, 1.9, 0.78, and 0.86. The markedly high anisotropy dependence exhibited by the 0.05% nano-Ag sample (1.9) suggests this material may play a key role in three-dimensional film, photography, etc. The differences in the properties of all those samples are considered to be influenced by internal structures based on the LC droplet configurations, as shown in Fig. 8. For the sample doped with 0.05% nano-Ag, the LC layer is composed of coalesced droplets with smaller size, and the periodic structure becomes obvious when viewed under FE-SEM. 5. Conclusion
This paper introduced a method to improve the optical and electro-optical characteristics of H-PDLC gratings by doping with nano-Ag. We modeled the surface plasma effect of nano-Ag and showed that it benefits for phase separation and diffraction efficiency. The threshold voltage and response time are also improved in comparison to the sample without nano-Ag. In the experiment, we have found that (1) diffraction efficiency could reach up to 95% with a doping concentration of 0.05%, (2) the lowest threshold voltage of 1.23 V∕μm could be achieved, and (3) the rise time reduces to 150 μs and the fall time reduces to 400 μs. In addition, we have investigated the polarization-dependent characteristic, and the extent of anisotropy increases from 0.74 to 1.9 by doping 0.05% nano-Ag. In short, doping nano-Ag onto a H-PDLC grating can significantly improve the optical and electro-optical characteristics of the device. And it has many potential applications, such as polariscopes, biomedical sensors, hyperspectral imaging devices, and 3D devices. This research was supported by the National Science Foundation for Young Scholars of China (grant No. 60801041), the Key Research Project from Shanghai Education Committee (14ZZ138), Shanghai Key Subject Construction funding (s30502), the Innovation Fund Project for Graduate Student of Shanghai (JWCXSL1202), and the Development Fund for Shanghai Talents (2012026). References 1. T. J. Bunning, L. V. Natarajan, V. P. Tondiglia, and R. L. Sutherland, “Holographic polymer-dispersed liquid crystals (H-PDLCs),” Annu. Rev. Mater. Sci. 30, 83–115 (2000). 2. N. H. Nataj, E. Mohajerani, J. Hossein, and A. Jannesari, “Holographic polymer dispersed liquid crystal enhanced by introducing urethane trimethacrylate,” Appl. Opt. 51, 697–703 (2012). 3. Z. Li, Z. Fan, J. Li, Y. Zhao, and Y. Sun, “Polarization microscopy study on piezo-optical effect of polymer dispersed liquid crystal films,” Acta Opt. Sin. 31, 816001 (2011). 4. Y. Jia, B. Zhang, Y.- Liu, and K. Xu, “Optimization of diffraction properties for holographic polymer dispersed liquid crystal,” Acta Phys. Sin. 52, 91–95 (2003).
5. J. Zheng, G. Sun, Y. Jiang, T. Wang, A. Huang, Y. Zhang, P. Tang, S. Zhuang, Y. Liu, and S. Yin, “H-PDLC based waveform controllable optical choppers for FDMF microscopy,” Opt. Express 19, 2216–2224 (2011). 6. P. S. Drzaic and A. M. Gonzales, “Refractive index gradients and light scattering in polymer-dispersed liquid crystal films,” Appl. Phys. Lett. 62, 1332–1334 (1993). 7. R. L. Sutherland, L. V. Natarajan, V. P. Tondiglia, T. J. Bunning, and W. W. Adams, “Electrically switchable volume gratings in polymer-dispersed liquid crystals,” Appl. Phys. Lett. 64, 1074–1076 (1994). 8. F. Yang, “New directions in recent liquid crystal studies the progress in foundational research of non-display application of liquid crystals,” Prog. Phys. 28, 107–129 (2008). 9. J. C. Lin and P. L. Taylor, “Polymerization-induced phase separation of a liquid crystal-polymer mixture,” Mol. Cryst. Liq. Cryst. 237, 25–31 (1993). 10. R. L. Sutherland, L. V. Natarajan, V. P. Tondiglia, and T. J. Bunning, “Bragg gratings in an acrylate polymer consisting of periodic polymer-dispersed liquid-crystal planes,” Chem. Mater. 5, 1533–1538 (1993). 11. M. Mucha, “Polymer as an important component of blends and composites with liquid crystals,” Prog. Polym. Sci. 28, 837–873 (2003). 12. W. Ma, H. Yang, J. Liu, Z. Ni, D. Tang, and J. Yao, “Influences of shape on extinction spectrum for metal nanoparticles,” Acta Opt. Sin. 30, 2629–2633 (2010). 13. J. Klostman, L. V. Natarajan, V. P. Tondiglia, R. L. Sutherland, T. J. White, C. A. Guymon, and T. J. Bunning, “The influence of surfactant in reflective HPDLC gratings,” Polymer 45, 7213–7218 (2004). 14. M. Jamil, F. Ahmad, J. T. Rhee, and Y. J. Jeon, “Nanoparticledoped polymer-dispersed liquid crystal display,” Curr. Sci. 01, 1544–1552 (2011). 15. O. V. Yaroshchuk and L. O. Dolgov, “Electro-optics and structural of polymer dispersed liquid crystals doped with nanoparticles of inorganic materials,” Opt. Mater. 2, 1097–1102 (2007). 16. S. Eren San, M. Okutan, O. Köysal, and Y. Yerli, “Carbon nanoparticles in nematic liquid crystals,” Chin. Phys. Lett. 25, 212–215 (2008). 17. Y. Wu, T. Xu, X. Shen, S. Dai, Q. Nie, X. Wang, B. Song, W. Zhang, and C. Lin, “Influence of silver nanoparticles on the upconversion properties in Tm3+/Yb3+ codoped bismuth germanate glasses,” Acta Opt. Sin. 31, 0730002 18. F. Gan and Y. Wang, “Breaking through the optical diffraction limits, developing the nano-optics and photonics,” Acta Opt. Sin. 31, 900104 (2011). 19. M. D. Sarkar, J. Qi, and G. P. Crawford, “Influence of partial matrix fluorination on morphology and performance of HPDLC transmission gratings,” Polymer 43, 7335–7344 (2002). 20. E. Hutter and J. H. Fendler, “Exploitation of localized surface plasmon resonance,” Adv. Mater. 16, 1685–1706 (2004). 21. S. C. Howells and L. A. Schlie, “Transient terahertz reflection spectroscopy of undoped InSb from 0.1 to 1.1 THz,” Appl. Phys. Lett. 69, 550–552 (1996). 22. R. L. Sutherland, L. V. Natarajan, V. P. Tondiglia, S. Chandra, C. K. Shepherd, D. M. Brandelik, and S. A. Siwecki, “Polarization and switching properties of holographic polymer-dispersed liquid-crystal gratings. II. Experimental investigations,” J. Opt. Soc. Am. B 19, 3004–3012 (2002). 23. G. Montemezzani and M. Zgonik, “Light diffraction at mixed phase and absorption gratings in anisotropic media for arbitrary geometries,” Phys. Rev. E 55, 1035–1047 (1997). 24. S. Link and M. A. El-Sayed, “Optical properties and ultrafast dynamics of metallic nanocrystals,” Annu. Rev. Phys. Chem. 54, 331–366 (2003). 25. B. G. Wu, J. H. Erdmann, and J. W. Doane, “Response times and voltages for PDLC light shutters,” Liq. Cryst. 5, 1453–1465 (1989). 1 November 2013 / Vol. 52, No. 31 / APPLIED OPTICS
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We report on the synthesis and characteristics of a holographic polymer dispersed liquid crystal (H-PDLC) switchable grating based on nano-Ag particles. The influence of doping different concentrations of nano-Ag on the diffraction efficiency, driving voltage, and response time of the H-PDLC grating is investigated. The best grating characteristics were achieved with 0.05% nano-Ag doping. Calculated and experimental results reveal that the improvement of the characteristics is likely due to the surface plasmon effect of nano-Ag. © 2013 Optical Society of America OCIS codes: (050.1950) Diffraction gratings; (160.3710) Liquid crystals; (160.4236) Nanomaterials; (240.6680) Surface plasmons. http://dx.doi.org/10.1364/AO.52.007411
1. Introduction
In recent years, electrically switchable holographic polymer dispersed liquid crystal (H-PDLC) has been widely studied [1–5]. Compared with ordinary nematic liquid crystal (LC) devices, PDLC devices offer several advantages, such as unwanted polarizers during the fabrication of PDLC devices, the possibility of reduced power consumption and weight with increased brightness, and optical contrast that requires no extra optical elements. Therefore, the technology of H-PDLC has abundant application prospects in many domains, such as zoom lenses, filters, optical information storage, photorefractive materials, etc. [6–8]. Conventional H-PDLC grating suffers from low diffraction efficiency, high switching voltage, and long response time due to many factors in fabrication, 1559-128X/13/317411-08$15.00/0 © 2013 Optical Society of America
such as the mass ratio of the materials, the intensity and time of exposure, the temperature of the environment, the humidity levels, etc. [9–11]. Various methods have been adopted to address these issues. For example, diffraction efficiency has been improved by adding cross-linking agent NVP to the materials, and switching voltages have been reduced by the addition of monomers, including the element fluorine [12], surfactant molecules [13], etc. Of increasing importance is the use of nanoparticles with PDLCs. Nanoparticles provide good electro-optical contrast and form strong networks in LC layers that can prevent pouring effects in the cells. To further optimize the properties of H-PDLC grating, researchers have doped various nanoparticles with PDLCs [14–16]. This study investigates the effect of doping H-PDLCs with nano-Ag particles. First we model the effect of different concentrations of nano-Ag on the surface morphology of fabricated PDLC gratings and provide an analysis of the surface plasma resonance of 1 November 2013 / Vol. 52, No. 31 / APPLIED OPTICS
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nano-Ag. We experimentally verify the simulated results and analyze the spectral and electro-optical characteristics of the H-PDLC first. Compared to a grating sample without nano-Ag, we have made H-PDLC gratings with higher diffraction efficiency and better electro-optical properties by optimizing the concentrations of nano-Ag. The superior properties obtained include a reduction in switching voltage from 2.35 to 1.23 V∕μm, a diffraction efficiency increase from 55% to 95%, and a response time reduction from 750 to 150 μs. 2. Experiment A.
Fabrication of H-PDLC Gratings Doped with Nano-Ag
The materials system for fabricating the PDLC gratings consisted of a photoinitiator (RB, Aldrich Inc.), a synergistic photoinitiator (NPG, Aldrich Inc.), a cross-linking agent (NVP, Aldrich Inc.), a surfactant (S-271, Chemistry Inc.), an acrylic monomer (EB8301, UCB Inc.), and nematic LCs (TEB50, Beijing Tsinghua Yawang Liquid Crystal Material Co., Ltd.) with a mass ratio of 0.15:0.4:10:10:45:35 and total mass of 3 g. Nano-Ag (diameter 80 nm, Beijing Nachen S&T Ltd.) in different quantities (0.75, 1.5, 3, and 6 mg) was added to the materials. Then the new hybrid material was heated by an ultrasonic emulsification instrument in dark conditions for about 2 h. Fabrication of the PDLC material with nano-Ag doping was complete when it had been stable for 24–48 h in a darkroom. To form the LC cells, the material was heated to about 40°C and sandwiched between two pieces of ITO coated glass. The space between the two pieces of glass was controlled to 17 μm. Then the LC cells were exposed in an interference 532 nm laser for about 60 s. The power exposed on the samples was 22 mw∕cm2 , and the ambient temperature was kept between 25°C and 30°C. The experimental setup is shown in Fig. 1. All Bragg diffraction gratings were recorded with the same spatial frequency, and the grating pitch was around 1100 nm.
Mirror
Laser
Beam expander
Beam splitter
Mirror
B. Characterization Investigation of H-PDLC Gratings Doped with Nano-Ag
We began by modeling a key property of nano-Ag in various concentrations and then experimentally studied it. More specifically, our methods entailed (1) investigation of spectra and surface plasma resonance; we calculated the surface plasma resonance peak of nano-Ag by establishing a theoretical model. Experimentally, samples with different concentrations of nano-Ag were illuminated by a halogen and deuterium light with the angle of incident light perpendicular to the LC cell. The transmission spectra were recorded by a spectrometer (Ocean Optics USB4000) for the purpose of finding the plasma resonance peak of nano-Ag. (2) Surface morphology analysis: a field emission scanning electron microscope (FE-SEM) was used to observe the nano-Ag doped H-PDLC specimen and analyze surface morphology changes caused by adding nano-Ag within H-PDLC grating. (3) Investigation of the optical and electro-optical properties of the nano-Ag doped H-PDLC grating, specifically, its diffraction efficiency, driving voltage, response time, and polarization dependence. 3. Establishment of Model
Localized surface plasmon resonance (LSPR) has been widely studied within optical material systems doped with metal nanoparticles such as Ag nanoparticles. As the nanoparticles are illuminated, there is a collective oscillation among the free electrons located on the surface of the particles. When the frequency of incident light is similar to the oscillation frequency of these free electrons [17], the electromagnetic field close to the surface of the nanoparticles intensifies. A strong resonance absorption peak is observed on the spectrum that indicates the location where LSPR just occurred. It is quite important to find the spectrum peak, and it has been reported that Hutter and Fendler observed a LSPR phenomenon at the tip of Ag nanowires using 532 and 820 nm incident laser beams [18–20]. In order to find the plasma resonance peak of Ag nanoparticles in H-PDLC, we built a model with the software CST (Cst Microwaves Studio). We suppose that the distribution of Ag nanoparticles in the PDLC material system is homogeneous. It has been calculated that the space between two Ag nanoparticles and the diameter of the Ag nanoparticles are 2 μm and 80 nm, respectively. The permittivity of the PDLC material system measured by the dielectric constant detector (TH2820, Tonghui Inc.) is 20 times that of air. The plasma frequency and fluctuating frequency of Ag nanoparticles were set to 1.37 1016 Hz and 1∕4 1014 Hz, respectively. According to a model by Drude described as Eq. (1) [21], εω ε∞ −
Sample
Fig. 1. Experimental setup for nano-Ag doped H-PDLC grating. 7412
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ω2P ; ω iγω 2
(1)
where ωP is the plasma frequency of the free electron, and γ is the characteristic collision frequency of the
90 80 Transmission[%]
70
Ag nano- particle
60
Ag 0.2% Ag 0.1% Ag 0.05% Ag 0.025% Ag 0%
50 40 30 20 10 0 300
400
500525
600
700
800
900
Wavelength[nm]
Fig. 4. Transmission spectra of H-PDLC gratings doped with different concentrations of nano-Ag. Fig. 2. Model constructed.
B. Surface Morphology
free electron. The model constructed is shown in Fig. 2. The thickness of this plane is set as 200 nm. The blue part in Fig. 2 stands for the PDLC material, and the red dots are the Ag nanoparticles. The model is irradiated by an illuminant with wavelength range from 460 to 750 nm. Figure 3 shows that light transmittance is lowest at wavelength 517 nm, which means the wavelength correspondence to the plasma resonance peak of Ag nanoparticles is around 517 nm. 4. Experimental Results and Discussion A.
Spectral Transmittance of Material
To verify the simulated results, transmission spectra of samples with different concentrations of nano-Ag were recorded by a spectrometer (Ocean Optics USB4000). The results are shown in Fig. 4. All samples share the same indentation at the wavelength of 562 nm, which is the absorption peak of RB. In addition, there is an obvious indentation around 525 nm on the transmission spectra of the sample doped with nano-Ag compared to the case without nano-Ag. We infer this trench to be the plasma resonance peak of nano-Ag. The experimental result (525 nm) is close to the simulated one (517 nm). The transmittance value of the sample doped with nano-Ag is higher than that without nano-Ag, due to the reduced loss of light intensity caused by scattering and the absorption of the material by the light. 100
Transmittance %
90 80 70 60 50 40 420
440
460
480
500 517 540 Wavelength /nm
560
580
Fig. 3. Transmittance curve of model.
600
620
To investigate the effect of nano-Ag on the surface morphology of H-PDLC gratings, we used an atomic force microscope (AFM) to observe the grooves of H-PDLC samples. Figure 5 shows that the interface between the LC and polymer becomes smoother after doping with nano-Ag [Figs. 5(b), 5(c), and 5(d)] compared to the case without nano-Ag. When the concentrations of nano-Ag reach 0.2%, the interface becomes rough partly owing to the aggregation of Ag nanoparticles. Moreover, the grooves of Ag-doped samples are deeper. Measured results show that the groove depth of grating without nano-Ag is about 28.6 nm, while the groove depths of doped gratings are 37.3, 39.6, 36.0, and 30.8 nm, respectively, for concentrations of nano-Ag of 0.025%, 0.05%, 0.1%, and 0.2%. These results confirm that the separation of polymer and LC is optimized and becomes more exhaustive after nano-Ag doping. C.
Diffraction Efficiency
We tested the diffraction efficiency of a fabricated H-PDLC grating sample with a 633 nm laser. When the laser illuminates the samples with the Bragg angle, the diverged zeroth-order diffraction beam and first-order diffraction beam can be observed clearly. The -1st-order and second-order beams are too weak to be detected by a power meter. Figure 6 gives the diffraction results of H-PDLC gratings doped and undoped with nano-Ag. The diffraction efficiency of the H-PDLC grating without nano-Ag only reaches around 50% in our experiment [Fig. 6(a)], while the diffraction efficiency of the grating doped with nano-Ag can reach 95% when the doping concentration is 0.05% [Fig. 6(b)]. However, when the concentration ratio rises to 0.1% and 0.2%, the diffraction efficiency drops back to 80%–90%. Here, the firstorder diffraction efficiency of H-PDLC grating can be calculated as ξ I 1 ∕I 0 I 1 [22], where I 0 and I 1 are the power of the zeroth- and first-order diffraction beams, respectively. (Light absorption and scattering loss have not been taken into account in this case.) These experimental results show that the doping concentration has a great influence on the 1 November 2013 / Vol. 52, No. 31 / APPLIED OPTICS
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Fig. 5. Atomic force microscope (AFM) images of each sample doped with different concentrations of nano-Ag: (a) 0% Ag, (b) 0.025% Ag, (c) 0.05% Ag, (d) 0.1% Ag, and (e) 0.2% Ag.
diffraction efficiency of H-PDLC gratings, and only a suitable doping ratio such as 0.05% can achieve higher diffraction efficiency. 7414
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Theoretically, the diffraction efficiency of the H-PDLC grating is given by Eqs. (2) and (3) [23]:
-1st order
0th order
1st order
to different peak wavelength. As the doping concentration was raised in our experiment, some of the Ag nanoparticles aggregated into larger sized particles. This caused the plasma resonance peak to move to another location and the LSPR phenomenon to weaken, thereby lowering the diffraction efficiency. For these reasons, we believe that LSPR plays an important role within the nano-Ag doped H-PDLC gratings.
2nd order
(a)
D. 0th order
1st order
2nd order
(b) Fig. 6. Comparison of H-PDLC grating diffraction pattern: (a) doped 0% nano-Ag and (b) doped 0.05% nano-Ag.
ξ sin2
πdΔn e−2xd ; λcos θS cos θP 1∕2
Δn
n¯ LC-Rich − n¯ P-Rich ; 2
(2)
(3)
where Δn is the modulation of the refractive index, d is the sample thickness, λ is the wavelength of incident light, and x is the absorption coefficient of the material. θS and θP are the incident angles of the vector of the grating with the energy propagation direction of S waves and P waves meeting the conditions cos θS K · uS and cos θP K · uP . nLC-Rich and nP-Rich are the refractive indices of the rich LC area and the rich polymer area. According to Eqs. (2) and (3), the increase of the modulation of refractive index can improve diffraction efficiency when the wavelength of incident light and the angle between incident light and the sample are invariable. Through doping with nano-Ag, higher diffraction efficiency can be obtained, owing to much more exhaustive phase separation between polymer and LC that leads to a larger refractive index difference between grating fringes. In addition, the wavelength of the laser beam for holography recording is 532 nm, which is very close to the measured plasma resonance peak of 525 nm. Thus it is possible that a strong electromagnetic field has been generated around the nano-Ag due to the LSPR phenomenon within the doped H-PDLC material. Therefore during the holographic recording process, the LSPR helps to improve the phase separation, which leads to better optoelectrical properties and higher diffraction efficiency of the electrically controlled H-PDLC grating. The fact that a rise in doping concentration beyond 0.05% reduces diffraction efficiency to 80%–90% can also be explained by LSPR. The frequency and intensity of the surface plasmon absorption bands are highly sensitive to the size of the nanostructure [20,24]; i.e., different nanoparticle size corresponds
Threshold voltage is a critical factor to evaluate the opto-electrical characteristic of H-PDLC gratings. To test the threshold voltage, each grating sample was irradiated with a 633 nm He–Ne laser with the Bragg incident angle, and meanwhile a square wave of 2 KHz with 50% duty cycle was applied. The light through the sample is diffracted into several beams, and two detectors were set to detect the power of the light at the zeroth and first orders. Thus the light power of the zeroth order and first order are recorded with the increase of external voltage, and then the normalized diffraction efficiency can be obtained just as shown in Fig 7. Figure 7 shows the relationship between diffraction and electric field, and the threshold voltage is 2.35, 1.88, 1.23, 2.71, and 3.71 V∕μm at the concentration of nano-Ag of 0%, 0.025%, 0.05%, 0.1%, and 0.2%, respectively. The theoretical estimate of the threshold voltage is given by Eq. (4) [25]: VC
d0 σ LC Kl2 − 1 1∕2 2 ; 3a σ P ε0 Δε
(4)
where a, l, d0 , σ LC , σ P , K, and Δε are the diameter of the LC droplet, the length ratio of the two axes of the LC droplet, the sample thickness, the conductivity of the LC, the conductivity of the polymer, the elastic constant, and the dielectric anisotropy, respectively. Results show that the threshold voltage drops to the lowest when the concentration of nano-Ag is 0.05%, This can be explained by the increase in the conductivity of the polymer that occurs, as more nano-Ag
100 90 Diffraction efficiency (%)
-1st order
Threshold Voltage
80 70 60 50 Ag 0%
40
Ag 0.025%
30
Ag 0.05% 20
Ag 0.1%
10
Ag 0.2%
0 0
10
20
30
40
50
60
70
V
Fig. 7. Relationship between the diffraction efficiency and the drive voltage for each sample. 1 November 2013 / Vol. 52, No. 31 / APPLIED OPTICS
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particles are trapped in it, due to its higher viscosity compared to the LC. Higher conductivity of polymer means lower voltage is needed. This mechanism for reducing threshold voltage is similar to the carbon nanotube [26]. With the rise of the concentration of nano-Ag, the level of phase separation of the LC and the polymer is lower than 0.05%, and more LC remains in the rich polymer area, which is the same as the rich LC area. The ratio of conductivity between the rich LC area and the rich polymer area becomes larger because of more nano-Ag particles sticking in the rich LC area with the polymer, which results in a rise in the threshold voltage. Response Time
We also investigated the response time of the H-PDLC grating. The detector described in the above section receiving first-order light was connected to an oscilloscope (Tektronix TDS2012). When the samples were switched by a voltage of 100 V peak to peak with a 2 KHz square wave and 50% duty cycle, the oscilloscope showed a variable diffracted signal with the change of external voltage. The recorded response times of the samples doped with different concentrations of nano-Ag are given in Table 1. In the H-PDLC system, Eqs. (5) and (6) [25,27] are used to calculate the rise time and the fall time: τon
γ1 ; ΔεE2 kl2 − 1∕a2
(5)
γ 1 a2 ; kl2 − 1
(6)
τoff
where γ 1 is the viscosity coefficient of the LC, E is the applied electric field, K is the LC constant, a is the diameter of the LC droplet, and l is the length ratio of the two axes of the LC droplet. The rise time depends on the applied electric field and the LC droplet size. The fall time is dependent on the visco-elastic coefficient of the LC and the size of the LC droplet. The data show that after doping 0.05% nano-Ag, the response time of gratings drops to less than 1 ms. The decrease in both rise time and fall time is due to the reduction of the size of the LC droplet. Since the size of LC droplets affects many properties of the H-PDLC grating, all images of the samples were captured by a FE-SEM (Quanta 450, FEI) to
Table 1.
Rise and Fall Times of Nano-Ag Doped H-PDLC Grating with Different Concentrations
Concentrations of Nano-Ag 0% 0.025% 0.05% 0.1% 0.2%
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Rise Time 750 550 150 150 400
μs μs μs μs μs
Fall Time 800 μs 800 μs 400 μs 1.1 ms 1.3 ms
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Fig. 8. FE-SEM images of H-PDLC diffraction gratings doped with (a) 0% nano-Ag and (b) 0.05% nano-Ag.
verify the reduction of the size of the LC droplets after doping with the right concentration of nanoAg. Figure 8 shows that LC droplets after nano-Ag doping are smaller than droplets without nano-Ag. While the size of the average LC droplet without nano-Ag is around 200 nm, droplets with 0.05% nano-Ag are half that size (100 nm). F.
Polarization-Dependent Characteristic
Figure 9 shows the polarization azimuth dependence of the H-PDLC gratings doped with different concentrations of nano-Ag. The first-order diffraction efficiency is shown as a function of the incident polarization state. Electro-optical performance parameters of the H-PDLC gratings were determined by probing the samples with a p-polarized He–Ne (λ 632 nm)
100 90 Diffraction Efficiency(%)
E.
80 70 60 50 40
Ag 0% Ag 0.025% Ag 0.05% Ag 0.1% Ag 0.2%
30 20 10 0
0
20
40
60 80 100 120 Polarization azimuth angle(deg)
140
160
180
Fig. 9. Effect of probe angular polarization on first-order diffraction efficiency of the H-PDLC gratings doped with different levels of nano-Ag.
laser beam; the first-order diffraction efficiency, shown as open circles, changes with polarization azimuth angle. Using the anisotropy parameter defined as 2ζ P − ζS ∕ζP ζS [28], the extent of anisotropy for each sample is 0.74, 1.41, 1.9, 0.78, and 0.86. The markedly high anisotropy dependence exhibited by the 0.05% nano-Ag sample (1.9) suggests this material may play a key role in three-dimensional film, photography, etc. The differences in the properties of all those samples are considered to be influenced by internal structures based on the LC droplet configurations, as shown in Fig. 8. For the sample doped with 0.05% nano-Ag, the LC layer is composed of coalesced droplets with smaller size, and the periodic structure becomes obvious when viewed under FE-SEM. 5. Conclusion
This paper introduced a method to improve the optical and electro-optical characteristics of H-PDLC gratings by doping with nano-Ag. We modeled the surface plasma effect of nano-Ag and showed that it benefits for phase separation and diffraction efficiency. The threshold voltage and response time are also improved in comparison to the sample without nano-Ag. In the experiment, we have found that (1) diffraction efficiency could reach up to 95% with a doping concentration of 0.05%, (2) the lowest threshold voltage of 1.23 V∕μm could be achieved, and (3) the rise time reduces to 150 μs and the fall time reduces to 400 μs. In addition, we have investigated the polarization-dependent characteristic, and the extent of anisotropy increases from 0.74 to 1.9 by doping 0.05% nano-Ag. In short, doping nano-Ag onto a H-PDLC grating can significantly improve the optical and electro-optical characteristics of the device. And it has many potential applications, such as polariscopes, biomedical sensors, hyperspectral imaging devices, and 3D devices. This research was supported by the National Science Foundation for Young Scholars of China (grant No. 60801041), the Key Research Project from Shanghai Education Committee (14ZZ138), Shanghai Key Subject Construction funding (s30502), the Innovation Fund Project for Graduate Student of Shanghai (JWCXSL1202), and the Development Fund for Shanghai Talents (2012026). References 1. T. J. Bunning, L. V. Natarajan, V. P. Tondiglia, and R. L. Sutherland, “Holographic polymer-dispersed liquid crystals (H-PDLCs),” Annu. Rev. Mater. Sci. 30, 83–115 (2000). 2. N. H. Nataj, E. Mohajerani, J. Hossein, and A. Jannesari, “Holographic polymer dispersed liquid crystal enhanced by introducing urethane trimethacrylate,” Appl. Opt. 51, 697–703 (2012). 3. Z. Li, Z. Fan, J. Li, Y. Zhao, and Y. Sun, “Polarization microscopy study on piezo-optical effect of polymer dispersed liquid crystal films,” Acta Opt. Sin. 31, 816001 (2011). 4. Y. Jia, B. Zhang, Y.- Liu, and K. Xu, “Optimization of diffraction properties for holographic polymer dispersed liquid crystal,” Acta Phys. Sin. 52, 91–95 (2003).
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