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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 13107−13112
Hidden Gratings in Holographic Liquid Crystal Polymer-Dispersed Liquid Crystal Films Luciano De Sio,*,†,‡ Pamela F. Lloyd,§ Nelson V. Tabiryan,† and Timothy J. Bunning§ †
Beam Engineering for Advanced Measurements Company, Orlando, Florida 32810, United States 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, Dayton, Ohio 45433-7707, United States Downloaded via KAOHSIUNG MEDICAL UNIV on July 10, 2018 at 15:43:00 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: Dynamic diffraction gratings that are hidden in the fieldoff state are fabricated utilizing a room-temperature photocurable liquid crystal (LC) monomer and nematic LC (NLC) using holographic photopolymerization techniques. These holographic LC polymerdispersed LCs (HLCPDLCs) are hidden because of the refractive index matching between the LC polymer and the NLC regions in the asformed state (no E-field applied). Application of a moderate E-field (5 V/ μm) generates a refractive index mismatch because of the NLC reorientation (along the E-field) generating high-diffraction efficiency transmission gratings. These dynamic gratings are characterized by morphological, optical, and electrooptical techniques. They exhibit a morphology made of oriented LC polymer regions (containing residual NLC) alternating with a two-phase region of an NLC and LC polymer. Unlike classic holographic polymer-dispersed LC gratings formed with a nonmesogenic monomer, there is index matching between the as-formed alternating regions of the grating. These HLCPDLCs exhibit broad band and high diffraction efficiency (≈90%) at the Bragg angle, are transparent to white light across the visible range because of the refractive index matching, and exhibit fast response times (1 ms). 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. KEYWORDS: diffraction gratings, holography, liquid crystals, polymers, optics
1. INTRODUCTION Diffraction gratings (DGs) are micro/nanometer range periodic structures that 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 selforganized cholesteric LCs,2,3 micropatterned LCs,4 or holographic polymer-dispersed LCs (HPDLCs), and polymer LC polymer slices (POLICRYPS).5−7 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, and light). The main fabrication techniques employed to realize high-efficiency DGs are e-beam lithography, photolithography, and interference holography (IH).8−10 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 prepolymers (monomers) and LCs. HPDLCs/POLICRYPS exhibit a very high diffraction efficiency (DE) in the field-off state (as formed) because of the large refractive index modulation between the polymer-rich and LC-rich regions (LC droplets in the polymerdispersed LC case and pure nematic LC (NLC) phase in the POLICRYPS case).7,11−13 The application of an electric field © 2018 American Chemical Society
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.14−17 For some applications, a 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 (nondiffractive/ transparent in the off state) have been reported by Kossyrev et al.19 by realizing one-dimensional polymer networks within an LC host. In this case, a low-molecular weight LC was mixed with a low concentration of a reactive mesogenic monomer. Because of 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 DE. Another attempt reported by Ramsey and Sharma20 exploited the properties of a partially Received: February 15, 2018 Accepted: March 25, 2018 Published: March 26, 2018 13107
DOI: 10.1021/acsami.8b02821 ACS Appl. Mater. Interfaces 2018, 10, 13107−13112
Research Article
ACS Applied Materials & Interfaces
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 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). photoinitiator (from Ciba). A small amount of a 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 °C) 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 quite large (better than 50:1) for white noncollimated light. Figure 1c,d shows the polarizing optical photographs (with the optical axis at 45° and 0°, 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 microscopy (POM) 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 = 351 nm, Icur = 12 mW/ cm2, tcur = 10 min) holographic setup, and the angle (θ) between the two interfering beams (see schematic reported in Figure 1j) was set to have a periodicity (Λ) of about 2.6 μm. The correlation between θ 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.
cured HPDLC structure. In their case, the response time was quite slow (190 ms) and the switching voltage was relatively high (15 V/μm). Within the same framework, Gotoh et al.21 have reported on the realization of a reverse-mode reflective grating by combining a photopolymer 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 temperaturesensitive reverse-mode gratings by using a composition made of a reactive mesogen and an NLC. However, in their paper,22 the investigated system lacks electrical tunability. Recently, we have demonstrated an ultrafast solid-state phase modulator with excellent optical and electrooptical phase modulating capability based on a mesogenic LC polymer system by dispersing a lowmolar mass LC in a room-temperature mesogenic polymeric matrix [named LC polymer-dispersed LC (LCPDLC)].23,24 Compatibility between the mesogenic monomer and the nonreactive LC enabled substantial concentrations of both to be utilized for highly-transparent films to be formed. Herein, we present and discuss a novel generation of hidden, switchable, thermo-responsive, broad band, highly-efficient, and fast DGs with superior morphological, optical, and electrooptical properties by holographically patterning this same material system. In a manner similar to the previous studies, this class of DGs is called holographic LCPDLCs (HLCPDLCs).
3. EXPERIMENTS 3.1. POM Characterization. Figure 2 is a POM view of the HLCPDLC sample at a series of applied voltages. In the as-formed state (with 0 V/μm applied), the HLCPDLC sample appears similar to a planar-aligned NLC cell because of the refractive index matching between the well-aligned LC polymer and the aligned polymeric films rich in LC nanodomains (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 a refractive index contrast between the LC polymer-rich and NLC-rich regions, which “develops” a periodic grating shown in Figure 2 (5 V/μm). The strength of the grating increases as the field strength is increased and saturates in above 5 V/μm. To check the uniformity of the HLCPDLC alignment, the sample was rotated while keeping the applied electric field at 5 V/ 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 a clear proof that the NLC
2. MATERIALS AND METHODS Indium tin oxide-coated glass substrates were treated with a polyimide layer and rubbed for planar orientation. A cell with a 20 μm gap was filled with the LCPDLC mixture (details below) by capillary action in darkness above the clearing temperature (65 °C) ensuring a 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 an 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 (at room temperature) with a UV interference pattern (details below) by adding 1 wt % of Darocur-4265 13108
DOI: 10.1021/acsami.8b02821 ACS Appl. Mater. Interfaces 2018, 10, 13107−13112
Research Article
ACS Applied Materials & Interfaces
the conditions imposed by the direction of the alignment layer. No evidence of NLC droplets is evident at this level of magnification, (Figure 4a−d) although high-resolution transmission electron microscopy studies on similar samples clearly show a distinct twophase 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 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 0° and 90°. Moreover, it is worth noting that the widths of the polymeric walls and the NLC-rich regions are 0.4Λ and 0.6Λ (Figure 4b), respectively, proportional to the initial concentration of the two main components (LC polymer and NLC). 3.3. Spectroscopic Characterization. The spectral response of the sample was probed with polarized white light at the Bragg angle (5.8°) 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 impinging light is transmitted (Figure 5b is the far-field pattern) because of 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 (empty glass cells introduce multireflection interference peaks because of 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 diffracted with a gradual attenuation of the transmitted intensity (the far-field diffraction pattern is shown in Figure 5c). Note: the ref ractive index of the LC polymer does not depend on the amplitude of the applied electric f ield. A detailed explanation of the reverse-mode working principle has been reported in the Supporting Information section. Note that because of the weak refractive index contrast (Δngr) dispersion (refractive indices vs wavelength), the attenuation of the transmitted intensity and therefore the DE is almost constant (see details below on the intrinsic absorption of the used materials) in the visible range (400−700 nm) giving a broad band nature to the HLCPDLC samples (see the Supporting Information Movie 1). According to Kogelnik’s coupledwave theory, the DE of a volume DG is proportional to Δngr/λ, which is approximately 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 because of the 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 Figures 4e and 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 5a (0 V/μm) and 5d (0 V/μm) is not flat (mainly between 400 and 500 nm) because 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, wellaligned, submicron NLC domains (200−500 nm) are index-matched
Figure 2. POM view of the HLCPDLC sample for different values of the applied electric field (from 0 to 5 V/μm).
Figure 3. POM view of the HLCPDLC sample for different angles. The sample was kept with the E-field on (5 V/μm). confined between the polymer layers does not exhibit a microsized droplet-based morphology like conventional HPDLC. 3.2. Morphological Characterization. To understand the morphology of the HLCPDLC gratings, the samples were investigated with high-resolution scanning electron microscopy (HRSEM) from both sides: the side that underwent the interference pattern irradiation (front) and the opposite (back) side. This was necessary to verify if the morphology of the samples is uniform across the thickness of the film. Details on the sample 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 that has a preferential alignment dictated by 13109
DOI: 10.1021/acsami.8b02821 ACS Appl. Mater. Interfaces 2018, 10, 13107−13112
Research Article
ACS Applied Materials & Interfaces
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.
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). 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 to 5 V/μm), Figure 5a] because of the slight refractive index mismatch between the LC polymer and NLC (1.65 vs 1.6). This phenomenon is not visible in Figure 5d (5 V/μm) because of the refractive index matching also in the on state. The switching voltage (5 V/μm) is relatively low (in comparison with a 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 to improve the phase separation between the LC polymer and the NLC, we have performed the photopolymerization process by using a relatively low curing intensity and a quite long curing time (Icur = 12 mW/cm2, tcur = 10 min). In this regime, the NLC partially driven by the slow photoinitiated polymerization kinetics escapes from the growing LC polymeric walls (high-intensity regions) and accumulates in the spider network regions (low-intensity regions), as shown in Figure 4a−d. Noteworthy, this level of phase separation, thus high DE, 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 microoven (by CaLCTec). The results are reported in Figure 6. By increasing the temperature from 25 to 44 °C, the refractive index contrast is increased (because of the nematic to isotropic transition of the NLC) with a resulting attenuation of the transmitted intensity because all of the light is diffracted. This behavior highlights, again, the broad band 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), whereas it is totally diffractive at a high temperature (inset of Figure 6, top right). The photo of the sample at a high temperature (inset of Figure 6, top right) was quickly acquired after removing the sample from the hot-stage. 13110
DOI: 10.1021/acsami.8b02821 ACS Appl. Mater. Interfaces 2018, 10, 13107−13112
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polymer does not affect the NLC performance as in similar systems previously discussed.20
4. CONCLUSIONS Hidden DGs, named, HLCPDLCs, with no diffraction in the as-formed state and large DE in the voltage-on state were fabricated by combining an LC mesogenic monomer system and an NLC. After holographic photopolymerization, periodic walls of oriented LC polymer alternating with a bicontinuous phase consisting of an LC polymer and nanoscale NLC domains are present. Because of the refractive index matching in the as-polymerized state, HLCPDLCs are invisible (transparent) in the off state, whereas they exhibit strong diffractive properties upon the application of a moderate E-field (5 V/ μm). The same behavior has been observed by increasing the temperature (no grating at room temperature and a high DE grating at an elevated temperature). Detailed morphological, optical, and electrooptical characterization has been performed using POM, HRSEM, and dynamic spectroscopy. These HLCPDLC films exhibit fast modulation (τon = 1 ms; τoff = 1 ms) and a high DE (≈90%) over the entire visible range. HLCPDLCs represent a step-forward in comparison to conventional switchable DGs because they are completely hidden in the off state, allowing the realization of important photonics applications with reduced power consumption.
Figure 6. Sample spectra vs the temperature along with a photo of the sample at 25 °C (top left) and at ≈44 °C (top right). 3.4. Electrooptical Properties. To characterize the optical and electrooptical properties, a linearly polarized green laser (λ = 532 nm) at the Bragg angle (5.8°) was used as a probe to measure the zeroorder 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 polarizationinsensitive because of 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 because of 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 of the light is transmitted because of the 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 DE (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 (τon = 1 ms; τoff = 1 ms), as shown in Figure 7b. It is important to point out that the measured response times (both on and off are 1 ms) are comparable with typical response times of pure NLC-based samples. Therefore, the presence of the LC
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b02821. Working principle of a reverse-mode DG (PDF) Broad band diffractive properties of a HLCPDLC grating (AVI) Monochromatic diffractive properties of a HLCPDLC grating (AVI)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Luciano De Sio: 0000-0002-2183-6910 Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
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) state. Electrooptical response of the zero-order transmitted intensity (b). 13111
DOI: 10.1021/acsami.8b02821 ACS Appl. Mater. Interfaces 2018, 10, 13107−13112
Research Article
ACS Applied Materials & Interfaces Notes
(18) Du, Y.-q. In Laser Protection Materials against Optical Intensity and Protection Mechanism. 2010 Symposium on Photonics and Optoelectronics, 19−21 June 2010; Vol. 2010, pp 1−4. (19) Kossyrev, P.; Sousa, M. E.; Crawford, G. One- and TwoDimensionally Structured Polymer Networks in Liquid Crystals for Switchable Diffractive Optical Applications. Adv. Funct. Mater. 2004, 14, 1227−1232. (20) Ramsey, R. A.; Sharma, S. C. Holographically recorded reversemode transmission gratings in polymer-dispersed liquid crystal cells. Appl. Phys. B 2008, 93, 481−489. (21) Gotoh, T.; Nakata, T.; Murai, K. Design of Holographic Polymer Dispersed Liquid Crystal materials for High Reflectivity. J. Photopolym. Sci. Technol. 2000, 13, 283. (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, 4633−4640. (23) Ouskova, E.; De 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. Appl. Phys. Lett. 2014, 105, 231122. (24) De Sio, L.; Ouskova, E.; Lloyd, P.; Vergara, R.; Tabiryan, N.; Bunning, T. J. Light-addressable liquid crystal polymer dispersed liquid crystal. Opt. Mater. Express 2017, 7, 1581−1588. (25) http://www.beamco.com. (26) Vaia, R. A.; Tomlin, D. W.; Schulte, M. D.; Bunning, T. J. Twophase nanoscale morphology of polymer/LC composites. Polymer 2001, 42, 1055−1065. (27) Kogelnik, H. Coupled Wave Theory for Thick Hologram Gratings. Bell Syst. Tech. J. 1969, 48, 2909−2947. (28) Zheng, Z.; Zhou, L.; Shen, D.; Xuan, L. Holographic polymerdispersed liquid crystal grating with low scattering losses. Liq. Cryst. 2012, 39, 387−391.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Authors are grateful to O. Uskova and R. Vergara for discussions and assistance.
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REFERENCES
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