Hidden Gratings in Holographic Liquid Crystal Polymer-Dispersed

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Hidden gratings in holographic liquid crystal polymer dispersed liquid crystal films Luciano De Sio, Pamela F. Lloyd, Nelson V. Tabriyan, and Timothy J. Bunning ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02821 • Publication Date (Web): 26 Mar 2018 Downloaded from http://pubs.acs.org on March 26, 2018

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ACS Applied Materials & Interfaces

Hidden gratings in holographic liquid crystal polymer dispersed liquid crystal films Luciano De Sio, †*, Pamela F. Lloyd§, Nelson V. Tabiryan, Timothy J. Bunning§ Beam

Engineering for Advanced Measurements Company, Orlando, Florida 32810, USA

†Department of Medico-surgical Sciences and Biotechnologies, Sapienza University of Rome, Corso della Repubblica 79, 04100, Latina, Italy §

Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson Air

Force Base, Ohio 45433-7707, USA

KEYWORDS. Diffraction gratings; holography; liquid crystals; polymers; optics

ABSTRACT. Dynamic diffraction gratings which are hidden in the field off state are fabricated utilizing a room temperature photocurable liquid crystal monomer and nematic liquid crystal (NLC) using holographic photopolymerization techniques. These holographic liquid crystal polymer dispersed liquid crystals (HLCPDLCs) are hidden due to refractive index matching between the LC polymer and the NLC regions in the as formed state (no E-field applied). Application of a moderate E-field (5V/µm) generates a refractive index mismatch due to NLC reorientation (along the E-field) generating high diffraction efficiency transmission gratings. These dynamic gratings are characterized by morphological, optical and electro-optical

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techniques. They exhibit a morphology made of oriented LC polymer regions (containing residual NLC) alternating with a two-phase region of NLC and LC polymer. Unlike classic holographic polymer dispersed LC gratings formed with non-mesogenic monomer, there is index matching between the as-formed alternating regions of the grating. These HLCPDLCs exhibit broadband and high diffraction efficiency (≈90%) at the Bragg angle, are transparent to white light across the visible range due to the refractive index matching, and exhibit fast response times (1ms). The ability of HLCPDLCs not to consume electrical power in the off state opens new possibilities for the realization of energy efficient switchable photonic devices. Introduction Diffraction gratings (DGs) are micro/nano-meter range periodic structures which split and diffract light into several beams traveling in different directions.1 DGs combined with liquid crystals (LCs), have been studied in architectures such as self-organized cholesteric LCs2-3, micropatterned LCs4 or holographic polymer dispersed liquid crystals (HPDLCs) and polymer liquid crystal polymer slices (POLICRYPS).5-7 A particular attention has been devoted to HPDLCs/POLICRYPS because they combine the diffractive properties/ strength of conventional DGs and the stimuli-responsive nature of LCs (e.g. e-field, temperature, light). The main fabrication techniques employed to realize high efficiency DGs are e-beam lithography, photolithography and interference holography (IH).

8,9,10

Over the last twenty years, IH has been

largely used for fabricating HPDLCs/POLICRYPS, both in transmission and reflection modes by employing

composite

mixtures

of

isotropic

pre-polymers

(monomers)

and

LCs.

HPDLCs/POLICRYPS exhibit very high diffraction efficiency in the field-off state (as formed) due to the large refractive index modulation between the polymer-rich and LC- rich regions (LC droplets in the PDLC case and pure nematic LC (NLC) phase in the POLICRYPS case). 7, 11, 12-13


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The application of an electric field reduces the refractive index mismatch between the two components (polymer and LC) reducing the intensity of the diffracted beam and increasing the intensity of the transmitted beam. Although examined for many applications ranging from displays to plasmonics, a strong limitation of these systems is the need to apply an electric field when the diffraction effects are not required.

14-15, 16-17

For some applications, the transparent

state is desired for the bulk of operation time and diffraction is needed only upon demand. 18 Preliminary attempts to realize reverse-mode (or hidden) switchable gratings (non diffractive/transparent in the off state) have been reported by Kossyrev et al.19 by realizing one dimensional polymer networks within a LC host. In this case, a low molecular weight LC was mixed with a low concentration of a reactive mesogenic monomer. Due to the polymer network nature of the system (small concentration of monomer), these gratings exhibit many drawbacks including irregular morphology, high switching voltages and low on-demand diffraction efficiency. Another attempt reported by Ramsey et al.20 exploited the properties of a partially cured HPDLC structure. In their case, the response time was quite slow (190 ms) and the switching voltage was relatively high (15 V/m). Within the same framework, Goto et al. 21 have reported on the realization of a reverse-mode reflective grating by combining a photo-polymer and a dual frequency LC. The technique, which requires a high voltage during the photopolymerization process, allows the realization of reflective gratings with very low reflection efficiency (4-5%). In a different approach, Ogiwara et al. 22 have realized a temperature sensitive reverse-mode gratings by using a composition made of a reactive mesogen and NLC. However, in their paper22, the investigated system lacks of electrical tunability. Recently, we have demonstrated an ultra-fast solid state phase modulator with excellent optical and electro-optical phase modulating capability based on a mesogenic LC polymer system by dispersing a low


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molar mass LC in a room-temperature mesogenic polymeric matrix (named LCPDLC).23-24 Compatibility between the mesogenic monomer and the non-reactive LC enabled substantial concentrations of both to be utilized and for highly transparent films to be formed. Here in, we present and discuss a novel generation of hidden, switchable, thermo-responsive, broadband, highly efficient, fast, diffraction gratings with superior morphological, optical and electro-optical properties by holographically patterning this same material system. In a manner similar to the previous studies, this class of gratings is called a holographic LCPDLCs (HLCPDLCs).

Materials and method ITO-coated glass substrates were treated with a polyimide layer and rubbed for planar orientation. A cell with a 20 m gap was filled with the LCPDLC mixture (details below) by capillary action in darkness above the clearing temperature (85 ⁰C) ensuring complete transition to the isotropic state of the NLC. The LCPDLC mixture was prepared from a commercially available (from BeamCo) LC acrylate monomer PLC-20-14C that exhibits a NLC phase at room temperature.25 This enables easy initial mixing with the bulk LC and facilitates the filling of a variety of cell geometries and molds. The material has a clearing temperature of 55°C. The material was photopolymerized with a UV interference pattern (details below) adding 1wt% of Darocur-4265 photoinitiator (from Ciba). A small amount of surfactant was added to reduce the switching voltage (0.05 wt% of FS-3100, from DuPont). PLC-20-14C, perfectly miscible with a variety of LCs, was mixed with the commercially available (from MUT) NLC (6CHBT, Tc = 42.8 oC) in a 40 to 60 ratio by weight. The excellent optical quality of the sample before the recording process is evident in the polarizing optical photograph shown in Figure 1a. Indicative of excellent alignment, the optical contrast between bright (Figure 1a) and dark (Figure 1b) is


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quite large (better than 50:1) for white noncollimated light. Figure 1c and Figure 1d show the polarizing optical photographs (with the optical axis at 45⁰ and 0⁰, respectively) of the sample after the holographic recording process. The optical contrast between the bright (Figure 1c) and dark (Figure 1d) is better than 30:1. Figure 1e-h shows the corresponding polarized optical microscope view of the sample in the four cases reported in Figure 1a-d, highlighting the homogeneity of the HLCPDLC film and the absence of microscopic defects. The recording process was performed with a standard UV (Ar+ laser, λ=351nm, Icur=12mW/cm2, tcur=10min) holographic setup and the angle () between the two interfering beams (see schematic reported in Figure 1j) was set in order to have a periodicity (Λ) of about 2.6µm. The correlation between  and Λ (reported in Figure 1k) can be geometrically obtained by using the Bragg condition sketched in the same figure, where q and K are the grating and wave vectors, respectively. Experiments

a) Polarized optical microscope characterization Figure 2 is a polarized optical microscope (POM) view of the HLCPDLC sample at a series of applied voltages. In the as-formed state (with 0V/µm applied), the HLCPDLC sample appears similar to a planar aligned NLC cell due to the refractive index matching between the well aligned LC polymer and the aligned polymeric films rich in LC nano-domains (see below). By gradually increasing the amplitude of the electric field (square voltage, 1 KHz), the NLC molecules start to reorient within the phase separated domains along the field direction. This generates refractive index contrast between the LC polymer-rich and NLC-rich regions which ‘develops’ a periodic grating shown in Figure 2 (5V/um). The strength of the grating increases as the field strength is increased and saturates in above 5V/um. To check the uniformity of the


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HLCPDLC alignment, the sample was rotated while keeping the applied electric field at 5V/µm. Figure 3 shows a drastic variation of the optical contrast (better than 30:1) indicating excellent alignment and uniformity of the HLCPDLC grating. This observation is clear proof that the NLC confined between the polymer layers does not exhibit a micro-sized droplet based morphology like conventional HPDLC. b) Morphological characterization In order to understand the morphology of the HLCPDLC gratings, the samples were investigated with high-resolution scanning electron microscopy (HRSEM) from both sides: the side underwent the interference pattern irradiation (front) and the opposite (back) side. This was necessary in order to verify if the morphology of the samples is uniform across the thickness of the film. Details on the samples preparation are reported elsewhere.26 The HRSEM micrographs in Figure 4a-b (front) and Figure 4c-d (back) show a periodic structure made of walls of oriented LC polymer alternating with a polymeric like spider network containing oriented NLC domains (see schematic shown in Figure 4e). The polymer exhibits a fibril-like texture which has a preferential alignment dictated by the conditions imposed by the direction of the alignment layer. No evidence of NLC droplets is evident at this level of magnification (Figure 4a-b and Figure 4c-d) although high-resolution TEM studies on similar samples clearly show a distinct two-phase morphology. 24 No difference in morphology between the front (Figure 4a-b) and the back (Figure 4c-d) of the sample is observed, confirming that the HLCPDLC film is uniform across the thickness of the film. We also have attempted to study the cross-section of the samples by using a freeze-fracture technique. Unfortunately, the HLCPDLC film is quite fragile and we were not able to perform a detailed analysis of the cross-section of the sample without affecting its morphology. Noteworthy, these results have been obtained by recording the HLCPDLC


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gratings with an interference pattern parallel to the rubbing direction (Figure 4f). Future experiments will be devoted to understanding how the morphology of HLCPDLC gratings is affected if the angle between the interference pattern and the rubbing direction is varying between 0⁰ and 90⁰. Moreover, it is worth noting that the widths of the polymeric walls and the NLC rich regions are 0.4Λ and 0.6 Λ (Figure 4b), respectively, proportional to the initial concentration of the two main components (LC polymer and NLC). c) Spectroscopic characterization The spectral response of the sample was probed with polarized white light at the Bragg angle (5.8⁰) while increasing the electric field and monitoring the zero-order transmitted intensity. Figure 5a shows the spectral response of the sample for impinging s-polarized white light. With no field applied (0 V/µm, red curve) all the impinging light is transmitted (Figure 5b is the far field pattern) due to the refractive index matching (see schematic shown in Figure 4e) between the extraordinary index of the oriented LC polymer (ne1.65) and the average extraordinary index of the LC polymer containing NLC domains (naverage1.65). The total transmittance (Figure 5a, red curve) in the off state is relatively low (80%) because we did not remove the Fresnel reflection (15%). In fact, the spectroscopic characterization has been performed by considering air as a baseline instead of an empty glass cell (the empty glass cell introduces multireflection interference peaks due to the gap between glass substrates). By increasing the electric field, the NLC encapsulated in the LC polymer starts to reorient along the electric field direction (perpendicular to the glass plates) with a consequent mismatch (see schematic shown in Figure 5e) of the polymer extraordinary index of the LC polymer (ne1.65) and the ordinary index of the LC polymer containing NLC domains (naverage1.6). As a result, the impinging white light is


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diffracted with a gradual attenuation of the transmitted intensity (the far field diffraction pattern is shown in Figure 5c). Note: the refractive index of the LC polymer does not depend on the amplitude of the applied electric field. A detailed explanation of the reverse-mode working principle has been reported in the Supporting Information section. Note that due to the weak refractive index contrast (Δngr) dispersion (refractive indices vs wavelength), the attenuation of the transmitted intensity and therefore the diffraction efficiency is almost constant (see details below on the intrinsic absorption of the used materials) in the visible range (400nm-700nm) giving a broadband nature to the HLCPDLC samples (see the Supporting Information movie 1). According to Kogelnik's coupled-wave theory, the diffraction efficiency (DE) of a volume DG is proportional to Δngr/, which is approximatively constant in our actual case.27 The same experiment performed with impinging p-polarized white light (Figure 5d) shows that in both states (with and without electric field applied) the light is always transmitted due to refractive index matching between the ordinary index of the LC polymer rich regions (no1.6) and the ordinary index of the NLC rich domains (naverage1.6) as it is evident in schematics reported in Figure 4e and Figure 5e (p-polarization cases). As a result, there is no variation in the spectral response of the sample (the two curves overlap, Figure 5d). It is important to point out that the spectrum of the zero-order transmitted intensity reported in Figure 5(a) (0V/m) and 5(d) (0V/m) is not flat (mainly between 400-500 nm) since there is selective absorption of the UV photoinitiator in the initial curing mixture. Indeed, as already shown 28, the scattering losses are negligible (in the off state) because the bicontinuous, well aligned, submicron NLC domains (200nm-500nm) are index matched with the well oriented LC polymer. Conversely, there is an

increased blue scattering (Mie scattering) for shorter wavelengths while increasing the applied external field (from 1 V/m to 5 V/m), Figure 5(a), due to the slight refractive index mismatch


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between the LC polymer and NLC (1.65 vs 1.6). This phenomenon is not visible in Figure 5d (5V/m) due to the refractive index matching also in the on state. The switching voltage (5V/m) is relatively low (in comparison with similar system)19-20 because the HLCPDLC sample exhibits better phase separation between the two main constituents (LC polymer and NLC). It is important to clarify that in order to improve the phase separation between the LC polymer and the NLC, we have performed the photo-polymerization process by using a relatively low curing intensity and a quite long curing time (Icur=12mW/cm2, tcur=10min). In this regime, the NLC partially driven by the slow photo-initiated polymerization kinetics escapes from the growing LC polymeric walls (high intensity regions) and accumulated in the spider-network regions (low intensity regions) as shown in Figure 4a-b and Figure 4c-d. Noteworthy, this level of phase separation, thus high diffraction efficiency, can also be obtained with conventional isotropic monomers, without obtaining (of course) the reverse or hidden operation mode. A further demonstration of the tunability of the HLCPDLC sample has been obtained by varying its temperature. We have performed a spectral analysis by placing the sample in a micro-oven (by CalCtec). The results are reported in Figure 6. By increasing the temperature from 25⁰C to 44 ⁰C, the refractive index contrast is increased (due to the nematic to isotropic transition of the NLC) with a resulting attenuation of the transmitted intensity since all the light is diffracted. This behavior highlights, again, the broadband nature of the HLCPDLC grating in terms of diffractive properties over the wavelength of the impinging radiation. Remarkably, the HLCPDLC sample is water clear at room temperature (inset of Figure 6, top left) while it is totally diffractive at high temperature (inset of Figure 6, top right). The photo of the sample at high temperature (inset of Figure 6, top right) was quickly acquired after removing the sample from the hot-stage.


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d) Electro-optical properties In order to characterize the optical and electro-optical properties, a linearly polarized green laser (λ=532nm) at the Bragg angle (5.8 ⁰ ) was used as a probe to measure the zero-order transmitted intensity by rotating the polarization of the impinging probe laser beam. When the applied electric field is off (Figure 7a, red curve), the transmitted intensity is polarization insensitive due to the refractive index matching between the oriented LC polymer and the NLC-rich domains (see schematic reported in Figure 4e). When the applied field is on (Figure 7a, blue curve), there is a strong correlation between the amplitude of the transmitted intensity and the polarization direction of the impinging probe beam. This behavior can be explained by the fact that for the polarization direction parallel to the alignment direction, the impinging light experiences diffraction due to the refractive index mismatch between the extraordinary index of the aligned polymer and the ordinary index of the vertically aligned NLC domains. When the impinging polarization direction is perpendicular to the alignment direction, all the light is transmitted due to refractive index matching between the ordinary index of the aligned polymer and the ordinary index of the vertically aligned NLC domains (see schematic reported in Figure 5e). We have measured a variation of the first order diffraction efficiency (defined as the ratio between the intensity of the first diffracted order over the total transmitted intensity) from 4% to 90% (see the Supporting Information movie 2). with fast and symmetric response times (on=1ms; off=1ms) as shown in Figure 7b. It is important to point out that the measured response times (both on and off are 1ms) are comparable with typical response times of pure NLC based samples. Therefore, the presence of the LC polymer does not affect the NLC performance as in similar systems previously discussed. 20


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Conclusion Hidden diffraction gratings, named holographic liquid crystal polymer dispersed liquid crystals (HLCPDLCs) with no diffraction in the as-formed state and large diffraction efficiency in the voltage-on state were fabricated by combining a LC mesogenic monomer system and a nematic LC. After holographic photopolymerization, periodic walls of oriented LC polymer alternating with a bicontinuous phase consisting of LC polymer and nanoscale NLC domains are present. Due to refractive index matching in the as-polymerized state, HLCPDLCs are invisible (transparent) in the off state while they exhibit strong diffractive properties upon the application of a moderate E-field (5V/µm). The same behavior has been observed by increasing the temperature (no grating at room temperature and high diffraction efficiency grating at elevated temperature). Detailed morphological, optical and electro-optical characterization has been performed using polarized optical microscope, high resolution scanning electronic microscopy and dynamic spectroscopy. These HLCPDLC films exhibit fast modulation (on=1ms; off=1ms) and high diffraction efficiency (≈90%) over the entire visible range. HLCPDLCs represent a step-forward in comparison to conventional switchable diffraction gratings since they are completely hidden in the off state, allowing the realization of important photonics applications with reduced power consumption.


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Figure 1: Photos of the HLCPDLC sample between crossed polarizers before (a, b) and after (c, d) the recording process with an interference pattern. The molecular director of the HLCPDLC sample is aligned at 45° in (a, c) and at 0° in (b, d). Corresponding polarizer optical microscope (POM) view (e-h) of the four cases reported in (a-d). Photo of the HLCPDLC sample showing good transparency (i). Schematic of the UV holographic recording setup (j) along with the vectorial representation of the Bragg condition (k).


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Figure 2: Polarized optical microscope view of the HLCPDLC sample for different values of the applied electric field (from 0V/µm to 5V/µm).


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Figure 3: POM view of the HLCPDLC sample for different angles. The sample was kept with the E-field on (5V/m).


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Figure 4: HRSEM analysis of the HLCPDLC film for different magnifications from the front (a, b) and back (c, d) side of the sample along with the schematic of the HLCPDLC morphology (e). Sketch of the interference pattern orientation with respect to the rubbing direction (f). The two arrows in (e) indicate two perpendicular polarization direction (p and s) of the impinging probe light.


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Figure 5: Spectral response of the sample vs the external electric field for an impinging white light s (a) and p (d) polarized. Far field diffraction pattern without (b) and with (c) the presence of the external electric field. Schematic of the HLCPDLC morphology in the on state (e).


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Figure 6: Sample spectra vs the temperature along with a photo of the sample at 25⁰ C (top left) and at  44⁰ C (top right).


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Figure 7: Behavior of the zero-order transmitted intensity (a) as a function of the angle between the LC molecular director and the electric field direction in the linearly polarized wave impinging on the sample at the Bragg angle in the off (blue curve) and on (red curve). Electrooptical response of the zero-order transmitted intensity (b).


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ASSOCIATED CONTENT Supporting information 1) Working principle of a reverse mode diffraction grating; 2) movie 1: broadband diffractive properties of a HLCPDLC grating; 3) movie 2: monochromatic diffractive properties of a HLCPDLC grating.

AUTHOR INFORMATION Corresponding Author *[email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT Authors are grateful to O. Uskova and R. Vergara for discussions and assistance.


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16. De Sio, L.; Caputo, R.; Cataldi, U.; Umeton, C., Broad band tuning of the plasmonic resonance of gold nanoparticles hosted in self-organized soft materials. Journal of Materials Chemistry 2011, 21 (47), 18967-18970. 17. Wang, K.; Zheng, J.; Gui, K.; Li, D.; Zhuang, S., Improvement on the Performance of Holographic Polymer-Dispersed Liquid Crystal Gratings with Surface Plasmon Resonance of Ag and Au Nanoparticles. Plasmonics 2015, 10 (2), 383-389. 18. DU, Y. q. In Laser Protection Materials against Optical Intensity and Protection Mechanism, 2010 Symposium on Photonics and Optoelectronics, 19-21 June 2010; 2010; pp 1-4. 19. Kossyrev, P.; Sousa, M. E.; Crawford, G., One- and Two-Dimensionally Structured Polymer Networks in Liquid Crystals for Switchable Diffractive Optical Applications. Advanced Functional Materials 2004, 14 (12), 1227-1232. 20. Ramsey, R. A.; Sharma, S. C., Holographically recorded reverse-mode transmission gratings in polymer-dispersed liquid crystal cells. Applied Physics B 2008, 93 (2), 481-489. 21. Goto, T.; Mimura, K.; Nakata, T., Design of Holographic Polymer Dispersed Liquid Crystal materials for High Reflectivity. ITE Technical Report 1999, 23.21, 37-42. 22. Ogiwara, A.; Kakiuchida, H.; Yoshimura, K.; Tazawa, M.; Emoto, A.; Ono, H., Effects of thermal modulation on diffraction in liquid crystal composite gratings. Appl. Opt. 2010, 49 (24), 4633-4640. 23. Ouskova, E.; Sio, L. D.; Vergara, R.; White, T. J.; Tabiryan, N.; Bunning, T. J., Ultra-fast solid state electro-optical modulator based on liquid crystal polymer and liquid crystal composites. Applied Physics Letters 2014, 105 (23), 231122. 24. De Sio, L.; Ouskova, E.; Lloyd, P.; Vergara, R.; Tabiryan, N.; Bunning, T. J., Lightaddressable liquid crystal polymer dispersed liquid crystal. Opt. Mater. Express 2017, 7 (5), 1581-1588. 25. www.beamco.com. 26. Vaia, R. A.; Tomlin, D. W.; Schulte, M. D.; Bunning, T. J., Two-phase nanoscale morphology of polymer/LC composites. Polymer 2001, 42 (3), 1055-1065. 27. Kogelnik, H., Coupled Wave Theory for Thick Hologram Gratings. Bell System Technical Journal 1969, 48 (9), 2909-2947. 28. Zheng, Z.; Zhou, L.; Shen, D.; Xuan, L., Holographic polymer-dispersed liquid crystal grating with low scattering losses. Liquid Crystals 2012, 39 (3), 387-391.


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Figure 1: Photos of the HLCPDLC sample between crossed polarizers before (a, b) and after (c, d) the recording process with an interference pattern. The molecular director of the HLCPDLC sample is aligned at 45° in (a, c) and at 0° in (b, d). Corresponding polarizer optical microscope (POM) view (e-h) of the four cases reported in (a-d). Photo of the HLCPDLC sample showing good transparency (i). Schematic of the UV holographic recording setup (j) along with the vectorial representation of the Bragg condition (k). 152x103mm (300 x 300 DPI)


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Table of Contents 160x68mm (300 x 300 DPI)


Hidden Gratings in Holographic Liquid Crystal Polymer-Dispersed

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