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Characterization of optically switchable holographic polymer-dispersed liquid crystal transmission gratings
Optical Materials 34 (2011) 251–255
Contents lists available at SciVerse ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Characterization of optically switchable holographic polymer-dispersed liquid crystal transmission gratings Y.-C. Su a, C.-C. Chu b, W.-T. Chang c, V.K.S. Hsiao a,⇑ a
Department of Applied Materials and Optoelectronic Engineering, National Chi Nan University, 54561 Nantou, Taiwan Department of Applied Chemistry, Chung Shan Medical University, 40201 Taichung, Taiwan c Department of Applied Chemistry, National Chi Nan University, 54561 Nantou, Taiwan b
a r t i c l e
i n f o
Article history: Received 26 May 2011 Received in revised form 28 July 2011 Accepted 18 August 2011 Available online 19 September 2011 Keywords: Holographic polymer-dispersed liquid crystal (H-PDLC) Photoisomerization All-optical switching Diffraction grating
a b s t r a c t This study characterizes the all-optical switching effect in holographic polymer-dispersed liquid crystal transmission gratings. The light-induced switching behaviors of these structures are due to the doped azobenzene-derived LC (azo-LC), which changes the refractive index of phase-separated LC within the polymer composite. This study also optimizes the polymer-dispersed liquid crystal formulation containing 15 wt.% azo-LC and 35 wt.% nematic LC to achieve a grating performance with a tunable diffraction efficiency of 78% and a fast switching-on time (0.5 s) with a relatively small light stimulus of 9 mW/cm2. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Switchable diffraction gratings [1–7] are one-dimensional periodic structures whose diffraction efficiency can be controlled by external stimuli. Liquid crystal (LC) is the most effective material for switchable diffraction gratings. The combination of the holographic technique and LC/polymer composite, which forms a holographic polymer-dispersed liquid crystal (H-PDLC) [8–19], is particularly useful for fabricating switchable diffraction elements. The H-PDLC technique exposes a mixture of prepolymer, LC, and photocuring agents to a laser interference pattern. This laser exposure eventually creates a photopolymerization-induced phase separation between polymer rich and LC rich regions in the high intensity (polymer) and low intensity (LC) regions. The refractive index (RI) modulation between the LC/polymer composite offers diffractive optics. An external stimulus, such as electrical and optical field, can then switch the H-PDLC gratings depending on the LC’s switching properties. Researchers have recently demonstrated all-optical switching of diffraction gratings using LC/polymer composite through the addition of azobenzene derivatives to H-PDLC [20–23]. In this switching mechanism, UV light irradiation causes the azobenzene derivatives to undergo a conformational change from a rod-like shape, which stabilizes the nematic LCs (NLCs) to a bent shape that ⇑ Corresponding author. Tel.: +886 49 2910960; fax: +886 49 2912434. E-mail address: [email protected] (V.K.S. Hsiao). 0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2011.08.022
disrupts the NLCs. Converting the bent shape of cis-azo into the rod-like shape of trans-azo via irradiation of another light with longer wavelength or cis-to-trans thermal isomerization reaction can drive the photoinduced isotropic (PHI) state generated by the light irradiation in the reverse direction. Upon light irradiation, the disrupted NLCs within the H-PDLC gratings create an index modulation between polymer rich and LC rich regions. This in turn makes it possible to modulate the corresponding diffraction efficiency by controlling the intensity of light irradiation. Urbas et al. were the first to demonstrate optically switchable H-PDLC reflection gratings by doping H-PDLC with azobenzene-derived LCs (azo-LCs) [20]. Huang et al. fabricated an optically switchable H-PDLC transmission grating by doping H-PDLC with azobenzene derived-methyl red [22]. It is also possible to add azo-LCs and chiral dopant to H-PDLC to fabricate optically switchable, polarization-independent transmission gratings [23]. However, those methods require high-power laser irradiation to achieve the on– off switching of gratings. The diffraction efficiency and switching response time from the gratings are also comparably lower and longer. This study characterizes and optimizes an optically switchable transmission grating made of H-PDLC with the addition of azo-LCs. It investigates how the all-optical switching efficiency and response time depends on the concentrations of NLCs and azo-LC. It also focuses on the optimization of grating performance, showing that a large tuning of diffraction efficiency (78%) and a fast switching-on time (0.5 s) under the laser light stimulus of 9 mW/cm2 can be
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achieved by a H-PDLC formula containing 15 wt.% azo-LCs and 35 wt.% NLCs. 2. Experimental Optically switchable gratings fabricated by H-PDLC technique were prepared from an optimized recipe containing 15 wt.% azo-LC (4-butyl-40 -methyl-azobenzene, BMAB), 35 wt.% NLC (MDA-00-3461) and standard concentration [23] of monomer (dipentaerythritolpenta-/hexaacrylate, DPPHA), reactive solvent (N-vinylpyrrollidone, NVP), photoinitiator (rose bengal, RB) and co-initiator (N-phenylglycine, NPG). The NLCs were purchased from Merck (Taiwan Branch) and the BMAB was synthesized according to [20], and can be commercially purchased from BeamCo Inc. All other chemicals were purchased from Arcos. Other recipes with different concentrations of azo-LCs and NLCs were also used to fabricate gratings with the same conditions of laser interference patterning: a 514 nm Ar+ laser separated into two beams. A writing power of 110 mW cm2 and time of 120 s were used for interference patterning. Grating sample cells were prepared by placing 15 lL of prepolymer mixture between two glass slides. The thickness of the grating sample was controlled by a 12 lm plastic spacer. Once recorded, the transmission gratings were characterized using the pump–probe method, in which a violet laser beam of 405 nm wavelength served as the pumping beam while a He–Ne laser served as the probe beam (Fig. 1). The initial polarization of the probe beam was controlled at 50:50 ppolarization and s-polarization. A polarization beam splitter and power meter were used to characterize the diffraction efficiency and polarization-tunable properties of fabricated grating samples. The diffraction efficiency (DE) was calculated from the ratio between the diffracted intensity to the sum of diffracted and transmitted intensities, as measured by a power meter. The transmission spectrum was measured by an Ocean Optics fiber spectrometer. The optically-controlled behavior of the grating samples was obtained by measuring the diffracted and transmitted light intensities under violet laser (405 nm) irradiation at different powers. A polarized microscope (Olympus IX 71) equipped with a CCD camera recorded the micrographs of the photochemical phase transitions in the grating samples. All optical characterizations were performed at room temperature. 3. Results and discussion The transmission spectra in Fig. 2 were recorded by a fiber-optic spectrometer using a white light source (Halogen lamp) with an
Fig. 2. Transmission spectra of optically switchable grating before (light-off) and under light irradiation (light-on).
arbitrary polarization of 11.5° (Bragg angle) to sample normal. This is a fast method for finding the peak wavelength of the probe beam with the highest DE modulation, but does not provide the absolute DE with spectral distribution [24]. Before laser irradiation, the grating exhibits a high absorption in wavelengths less than 500 nm (absorption from azo-LCs). However, grating diffraction gives rise to a relative minimum in the transmission at approximately 725 nm, and the spectrum distribution is not sharp because the grating period is large [22]. The sharper notch located at 560 nm is from the absorption of photoinitiator (RB). Upon laser irradiation, the characteristics of grating spectrum range (600–850 nm) disappear and the transmission (500–650 nm) decreases. A He– Ne laser was used to probe the optical switching of DE from the sample under the alternative on–off irradiation of pumping laser. Fig. 3a shows a picture of the diffraction pattern created by the H-PDLC grating probe using a He–Ne laser with an incident Bragg angle. Before light irradiation, the intensity of the first (+1st) order diffraction was set to maximum while the zero order (0th) transmission was minimized. Upon light irradiation, the +1st order spot size of the He–Ne laser decreased while the 0th order spot size increased. Fig. 3b shows optical micrographs of optically switchable H-PDLC gratings recorded under alternative light irradiation. The bright yellow region is the azo-LC doped polymer region, and the
Fig. 1. Schematic of the optical setup for characterizing the optically switchable H-PDLC transmission grating using a He–Ne laser (633 nm) as the probe beam and a laser diode (405 nm) as the pumping beam.
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Fig. 3. He–Ne laser diffraction pattern under alternative light irradiation (a); optical microscopic image of optically switchable grating before (light-off) and under light irradiation (light-on).
dark green region is the azo-LC doped NLC region. The layered fringes of the NLC regions became invisible upon light irradiation (Fig. 3b light-on state). Since the index difference between layered fringes generates the diffraction efficiency of H-PDLC gratings, light irradiation the generated cis-form of azo-LC disrupted the NLC host and changed the phase in LC-rich regions from nematic to isotropic. That is why the LC phase was not visible within the LC-rich regions in the H-PDLC grating. The phase transmission of LC-rich regions within the H-PDLC grating could be switched between nematic and isotropic phase using light irradiation. The video in the supporting information shows the modulation of LC phase within the H-PDLC transmission grating under the alternative laser irradiation. For H-PDLC grating, the directions of LC director within the LC droplet could be characterized by measuring the grating’s angular selectivity [9]. Fig. 4a shows the diffraction intensity dependent on the incident angle using p-polarized and s-polarized He–Ne lasers as probe beams. The zero degree of incident angle represents a Bragg angle of 11.5°. The fabricated grating is strongly dependent on the polarization of probe beam, and the diffraction intensity reaches its maximum value using p-polarized light as probe beam at the incident Bragg angle. The polarization-dependent property of H-PDLC grating is due to polymerization shrinkage during the holographic interference patterning process [25,26]. The interference pattern of high intensity regions makes the monomer becomes a polymer and compresses the phase-separated LC, rendering a uniaxial LC orientation within the droplets. The diffraction efficiency (DE) of H-PDLC gratings depends on the power of light irradiation (Fig. 4b). The grating sample was probed with a p-polarized He–Ne laser at a fixed Bragg angle. The +1st order diffraction efficiency (DE) and 0th order transmission efficiency (TE) were both plotted as function of pumping laser (405 nm laser diode) power. The TE reached its maximum when the DE was minimized. Before light irradiation, the DE was 95% and reached a relative minimum of 25% at a pumping power of 3 mWcm2. Fig. 5 schematically illustrates the physics associated with the optically switchable DE in H-PDLC gratings. The NLC used in this H-PDLC has positive optical anisotropy with ne = 1.771 and no = 1.514, where ne is the extraordinary RI and no is the ordinary RI. In this configuration, the polymer RI, npoly, is almost equal to
Fig. 4. Diffraction intensity of grating dependent on the incident angle at different polarization of probe beam (a); first order diffraction efficiency (DE) and zero order transmission efficiency (TE) as a function of pumping laser power (b).
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Fig. 5. Schematic of switching mechanism and index modulation of optically switchable H-PDLC transmission grating doped with azo-LC between light-off and light-on state. At light-off state the RI of LC droplets within the LC-rich regions mismatched with the polymer-rich regions, while in the light-on state, the generation of bend-like cis-azo disrupts NLC and made the RI matched with the polymer-rich regions.
no [9]. Before light irradiation, the axial-like LC orientation profile diffracts the p-polarized light, creating a RI difference between ne and npoly. Upon light irradiation, the cis-azo LC disrupts the NLC host and changes the RI of LC droplet from ne = 1.771 to ni = 1.60, which is the RI of isotropic LC ðni ¼ 2=3no þ 1=3ne Þ [9]. Since the RI of isotropic LC has a smaller value than the RI of anisotropic LC, the p-polarized diffraction intensity decreases upon light irradiation. The optically switchable behavior of fabricated diffraction grating was reversible under the on–off pumping light irradiation. Fig. 6 shows the DE of a grating sample fabricated using different NLC concentrations as a function of pumping laser power. The all-optical switching ability dramatically increased when using 35 wt.% NLC in the H-PDLC formulation. Table 1 shows that the tunable range of the DE, required pumping power, and recovery time all depend on the NLC concentration (note that DE returns to its original value after turning off the light irradiation). Several studies have indicates that a higher NLC concentration provides better phase separation between polymer and LC regions that the
electrical field required to generate effective switching of H-PDLC grating decreases with increase of NLC concentration [9]. However, in our case here a higher NLC concentration increases the recovery time. At a fixed concentration of azobenzene derivatives, a higher concentration of NLC may decrease the kinetics of the cis–trans thermal isomerization reaction within the LC rich regions of HDPLC grating. This study also investigates the concentration of azo-LC to optimize grating functionality. Fig. 7 shows the DE of grating samples fabricated using different azo-LC concentrations as a function of pumping laser power. A lower amount of azo-LC
Fig. 6. The first order diffraction efficiency (DE) dependent on the pumping laser power applied to the optically switchable transmission grating with different NLC concentrations.
Fig. 7. The first order diffraction efficiency (DE) dependent on the pumping laser power applied to the optically switchable transmission grating with different azoLC concentrations.
Table 1 Switching behaviors of optically switchable H-PDLC grating dependent on the NLC. LC concentration (wt.%) Tunable range of DE (%) Necessary pumping power (mW/cm2) Recovery time (s)
26 57 88 40
35 78 4 60
40 63 2 120
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induce the PHI state within the LC droplet of H-DPLC grating. A higher amount of azo-LC (30 wt.%) might be able to induce the PHI state, but a lower initial and tuning DE was obtained. The optical switching dynamics (light on–off state) of the azo-LC doped HPDLC transmission grating was investigated by varying the pumping laser power from 3 mW/cm2 to 9 mW/cm2 while the probe beam remained constant (Fig. 8). A shorter response time (1 s) of light-on state could be achieved by applying the pumping laser power of 9 mW/cm2 (Fig. 8a). The recovery time (60 s) of the light-off state is independent of the pumping laser power (Fig. 8b). The on–off switching behavior of optically switchable H-PDLC transmission grating is shown in Fig. 9. 4. Conclusion This study characterizes an optically switchable transmission grating by adding different amounts of NLC and azo-LC to the typical H-PDLC. The optimized recipe uses 15 wt.% azo-LC and 35 wt.% NLC. Upon light irradiation, the doped azo-LC undergoes a cis-totrans photoisomerization and the cis-azo disrupts the NLC host. The photo-induced phase transition within the LC-rich regions changes the RI of H-PDLC grating, and the DE decreases as the pumping power laser increases. The response time of light-on state decreased to 0.5 s by applying 9 mW/cm2 power of laser light. Acknowledgments This work is supported by the National Science Council, Taiwan, under projects No. 99-2221-E-260-024 and 98-2113-M-040-003MY2. The authors thank Kou Ryou Enterprise Corp., Taiwan for providing the spacers. References
Fig. 8. Response time of the optically switchable H-PDLC grating dependent on the power of pumping beam (a); recovery time required to achieve the original DE dependent on the initial pumping power of the laser beam.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
Fig. 9. Time dependence of diffraction intensity from the optically switchable HPDLC transmission grating when the pump light was turned on and off alternatively. The pumping power of light was 9 mW/cm2.
[21] [22] [23] [24] [25]
(5 wt.%) creates grating of higher DE, and almost no optically switching characteristic. This implies that 5 wt.% azo-LC cannot
[26]
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Contents lists available at SciVerse ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Characterization of optically switchable holographic polymer-dispersed liquid crystal transmission gratings Y.-C. Su a, C.-C. Chu b, W.-T. Chang c, V.K.S. Hsiao a,⇑ a
Department of Applied Materials and Optoelectronic Engineering, National Chi Nan University, 54561 Nantou, Taiwan Department of Applied Chemistry, Chung Shan Medical University, 40201 Taichung, Taiwan c Department of Applied Chemistry, National Chi Nan University, 54561 Nantou, Taiwan b
a r t i c l e
i n f o
Article history: Received 26 May 2011 Received in revised form 28 July 2011 Accepted 18 August 2011 Available online 19 September 2011 Keywords: Holographic polymer-dispersed liquid crystal (H-PDLC) Photoisomerization All-optical switching Diffraction grating
a b s t r a c t This study characterizes the all-optical switching effect in holographic polymer-dispersed liquid crystal transmission gratings. The light-induced switching behaviors of these structures are due to the doped azobenzene-derived LC (azo-LC), which changes the refractive index of phase-separated LC within the polymer composite. This study also optimizes the polymer-dispersed liquid crystal formulation containing 15 wt.% azo-LC and 35 wt.% nematic LC to achieve a grating performance with a tunable diffraction efficiency of 78% and a fast switching-on time (0.5 s) with a relatively small light stimulus of 9 mW/cm2. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Switchable diffraction gratings [1–7] are one-dimensional periodic structures whose diffraction efficiency can be controlled by external stimuli. Liquid crystal (LC) is the most effective material for switchable diffraction gratings. The combination of the holographic technique and LC/polymer composite, which forms a holographic polymer-dispersed liquid crystal (H-PDLC) [8–19], is particularly useful for fabricating switchable diffraction elements. The H-PDLC technique exposes a mixture of prepolymer, LC, and photocuring agents to a laser interference pattern. This laser exposure eventually creates a photopolymerization-induced phase separation between polymer rich and LC rich regions in the high intensity (polymer) and low intensity (LC) regions. The refractive index (RI) modulation between the LC/polymer composite offers diffractive optics. An external stimulus, such as electrical and optical field, can then switch the H-PDLC gratings depending on the LC’s switching properties. Researchers have recently demonstrated all-optical switching of diffraction gratings using LC/polymer composite through the addition of azobenzene derivatives to H-PDLC [20–23]. In this switching mechanism, UV light irradiation causes the azobenzene derivatives to undergo a conformational change from a rod-like shape, which stabilizes the nematic LCs (NLCs) to a bent shape that ⇑ Corresponding author. Tel.: +886 49 2910960; fax: +886 49 2912434. E-mail address: [email protected] (V.K.S. Hsiao). 0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2011.08.022
disrupts the NLCs. Converting the bent shape of cis-azo into the rod-like shape of trans-azo via irradiation of another light with longer wavelength or cis-to-trans thermal isomerization reaction can drive the photoinduced isotropic (PHI) state generated by the light irradiation in the reverse direction. Upon light irradiation, the disrupted NLCs within the H-PDLC gratings create an index modulation between polymer rich and LC rich regions. This in turn makes it possible to modulate the corresponding diffraction efficiency by controlling the intensity of light irradiation. Urbas et al. were the first to demonstrate optically switchable H-PDLC reflection gratings by doping H-PDLC with azobenzene-derived LCs (azo-LCs) [20]. Huang et al. fabricated an optically switchable H-PDLC transmission grating by doping H-PDLC with azobenzene derived-methyl red [22]. It is also possible to add azo-LCs and chiral dopant to H-PDLC to fabricate optically switchable, polarization-independent transmission gratings [23]. However, those methods require high-power laser irradiation to achieve the on– off switching of gratings. The diffraction efficiency and switching response time from the gratings are also comparably lower and longer. This study characterizes and optimizes an optically switchable transmission grating made of H-PDLC with the addition of azo-LCs. It investigates how the all-optical switching efficiency and response time depends on the concentrations of NLCs and azo-LC. It also focuses on the optimization of grating performance, showing that a large tuning of diffraction efficiency (78%) and a fast switching-on time (0.5 s) under the laser light stimulus of 9 mW/cm2 can be
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achieved by a H-PDLC formula containing 15 wt.% azo-LCs and 35 wt.% NLCs. 2. Experimental Optically switchable gratings fabricated by H-PDLC technique were prepared from an optimized recipe containing 15 wt.% azo-LC (4-butyl-40 -methyl-azobenzene, BMAB), 35 wt.% NLC (MDA-00-3461) and standard concentration [23] of monomer (dipentaerythritolpenta-/hexaacrylate, DPPHA), reactive solvent (N-vinylpyrrollidone, NVP), photoinitiator (rose bengal, RB) and co-initiator (N-phenylglycine, NPG). The NLCs were purchased from Merck (Taiwan Branch) and the BMAB was synthesized according to [20], and can be commercially purchased from BeamCo Inc. All other chemicals were purchased from Arcos. Other recipes with different concentrations of azo-LCs and NLCs were also used to fabricate gratings with the same conditions of laser interference patterning: a 514 nm Ar+ laser separated into two beams. A writing power of 110 mW cm2 and time of 120 s were used for interference patterning. Grating sample cells were prepared by placing 15 lL of prepolymer mixture between two glass slides. The thickness of the grating sample was controlled by a 12 lm plastic spacer. Once recorded, the transmission gratings were characterized using the pump–probe method, in which a violet laser beam of 405 nm wavelength served as the pumping beam while a He–Ne laser served as the probe beam (Fig. 1). The initial polarization of the probe beam was controlled at 50:50 ppolarization and s-polarization. A polarization beam splitter and power meter were used to characterize the diffraction efficiency and polarization-tunable properties of fabricated grating samples. The diffraction efficiency (DE) was calculated from the ratio between the diffracted intensity to the sum of diffracted and transmitted intensities, as measured by a power meter. The transmission spectrum was measured by an Ocean Optics fiber spectrometer. The optically-controlled behavior of the grating samples was obtained by measuring the diffracted and transmitted light intensities under violet laser (405 nm) irradiation at different powers. A polarized microscope (Olympus IX 71) equipped with a CCD camera recorded the micrographs of the photochemical phase transitions in the grating samples. All optical characterizations were performed at room temperature. 3. Results and discussion The transmission spectra in Fig. 2 were recorded by a fiber-optic spectrometer using a white light source (Halogen lamp) with an
Fig. 2. Transmission spectra of optically switchable grating before (light-off) and under light irradiation (light-on).
arbitrary polarization of 11.5° (Bragg angle) to sample normal. This is a fast method for finding the peak wavelength of the probe beam with the highest DE modulation, but does not provide the absolute DE with spectral distribution [24]. Before laser irradiation, the grating exhibits a high absorption in wavelengths less than 500 nm (absorption from azo-LCs). However, grating diffraction gives rise to a relative minimum in the transmission at approximately 725 nm, and the spectrum distribution is not sharp because the grating period is large [22]. The sharper notch located at 560 nm is from the absorption of photoinitiator (RB). Upon laser irradiation, the characteristics of grating spectrum range (600–850 nm) disappear and the transmission (500–650 nm) decreases. A He– Ne laser was used to probe the optical switching of DE from the sample under the alternative on–off irradiation of pumping laser. Fig. 3a shows a picture of the diffraction pattern created by the H-PDLC grating probe using a He–Ne laser with an incident Bragg angle. Before light irradiation, the intensity of the first (+1st) order diffraction was set to maximum while the zero order (0th) transmission was minimized. Upon light irradiation, the +1st order spot size of the He–Ne laser decreased while the 0th order spot size increased. Fig. 3b shows optical micrographs of optically switchable H-PDLC gratings recorded under alternative light irradiation. The bright yellow region is the azo-LC doped polymer region, and the
Fig. 1. Schematic of the optical setup for characterizing the optically switchable H-PDLC transmission grating using a He–Ne laser (633 nm) as the probe beam and a laser diode (405 nm) as the pumping beam.
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Fig. 3. He–Ne laser diffraction pattern under alternative light irradiation (a); optical microscopic image of optically switchable grating before (light-off) and under light irradiation (light-on).
dark green region is the azo-LC doped NLC region. The layered fringes of the NLC regions became invisible upon light irradiation (Fig. 3b light-on state). Since the index difference between layered fringes generates the diffraction efficiency of H-PDLC gratings, light irradiation the generated cis-form of azo-LC disrupted the NLC host and changed the phase in LC-rich regions from nematic to isotropic. That is why the LC phase was not visible within the LC-rich regions in the H-PDLC grating. The phase transmission of LC-rich regions within the H-PDLC grating could be switched between nematic and isotropic phase using light irradiation. The video in the supporting information shows the modulation of LC phase within the H-PDLC transmission grating under the alternative laser irradiation. For H-PDLC grating, the directions of LC director within the LC droplet could be characterized by measuring the grating’s angular selectivity [9]. Fig. 4a shows the diffraction intensity dependent on the incident angle using p-polarized and s-polarized He–Ne lasers as probe beams. The zero degree of incident angle represents a Bragg angle of 11.5°. The fabricated grating is strongly dependent on the polarization of probe beam, and the diffraction intensity reaches its maximum value using p-polarized light as probe beam at the incident Bragg angle. The polarization-dependent property of H-PDLC grating is due to polymerization shrinkage during the holographic interference patterning process [25,26]. The interference pattern of high intensity regions makes the monomer becomes a polymer and compresses the phase-separated LC, rendering a uniaxial LC orientation within the droplets. The diffraction efficiency (DE) of H-PDLC gratings depends on the power of light irradiation (Fig. 4b). The grating sample was probed with a p-polarized He–Ne laser at a fixed Bragg angle. The +1st order diffraction efficiency (DE) and 0th order transmission efficiency (TE) were both plotted as function of pumping laser (405 nm laser diode) power. The TE reached its maximum when the DE was minimized. Before light irradiation, the DE was 95% and reached a relative minimum of 25% at a pumping power of 3 mWcm2. Fig. 5 schematically illustrates the physics associated with the optically switchable DE in H-PDLC gratings. The NLC used in this H-PDLC has positive optical anisotropy with ne = 1.771 and no = 1.514, where ne is the extraordinary RI and no is the ordinary RI. In this configuration, the polymer RI, npoly, is almost equal to
Fig. 4. Diffraction intensity of grating dependent on the incident angle at different polarization of probe beam (a); first order diffraction efficiency (DE) and zero order transmission efficiency (TE) as a function of pumping laser power (b).
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Fig. 5. Schematic of switching mechanism and index modulation of optically switchable H-PDLC transmission grating doped with azo-LC between light-off and light-on state. At light-off state the RI of LC droplets within the LC-rich regions mismatched with the polymer-rich regions, while in the light-on state, the generation of bend-like cis-azo disrupts NLC and made the RI matched with the polymer-rich regions.
no [9]. Before light irradiation, the axial-like LC orientation profile diffracts the p-polarized light, creating a RI difference between ne and npoly. Upon light irradiation, the cis-azo LC disrupts the NLC host and changes the RI of LC droplet from ne = 1.771 to ni = 1.60, which is the RI of isotropic LC ðni ¼ 2=3no þ 1=3ne Þ [9]. Since the RI of isotropic LC has a smaller value than the RI of anisotropic LC, the p-polarized diffraction intensity decreases upon light irradiation. The optically switchable behavior of fabricated diffraction grating was reversible under the on–off pumping light irradiation. Fig. 6 shows the DE of a grating sample fabricated using different NLC concentrations as a function of pumping laser power. The all-optical switching ability dramatically increased when using 35 wt.% NLC in the H-PDLC formulation. Table 1 shows that the tunable range of the DE, required pumping power, and recovery time all depend on the NLC concentration (note that DE returns to its original value after turning off the light irradiation). Several studies have indicates that a higher NLC concentration provides better phase separation between polymer and LC regions that the
electrical field required to generate effective switching of H-PDLC grating decreases with increase of NLC concentration [9]. However, in our case here a higher NLC concentration increases the recovery time. At a fixed concentration of azobenzene derivatives, a higher concentration of NLC may decrease the kinetics of the cis–trans thermal isomerization reaction within the LC rich regions of HDPLC grating. This study also investigates the concentration of azo-LC to optimize grating functionality. Fig. 7 shows the DE of grating samples fabricated using different azo-LC concentrations as a function of pumping laser power. A lower amount of azo-LC
Fig. 6. The first order diffraction efficiency (DE) dependent on the pumping laser power applied to the optically switchable transmission grating with different NLC concentrations.
Fig. 7. The first order diffraction efficiency (DE) dependent on the pumping laser power applied to the optically switchable transmission grating with different azoLC concentrations.
Table 1 Switching behaviors of optically switchable H-PDLC grating dependent on the NLC. LC concentration (wt.%) Tunable range of DE (%) Necessary pumping power (mW/cm2) Recovery time (s)
26 57 88 40
35 78 4 60
40 63 2 120
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induce the PHI state within the LC droplet of H-DPLC grating. A higher amount of azo-LC (30 wt.%) might be able to induce the PHI state, but a lower initial and tuning DE was obtained. The optical switching dynamics (light on–off state) of the azo-LC doped HPDLC transmission grating was investigated by varying the pumping laser power from 3 mW/cm2 to 9 mW/cm2 while the probe beam remained constant (Fig. 8). A shorter response time (1 s) of light-on state could be achieved by applying the pumping laser power of 9 mW/cm2 (Fig. 8a). The recovery time (60 s) of the light-off state is independent of the pumping laser power (Fig. 8b). The on–off switching behavior of optically switchable H-PDLC transmission grating is shown in Fig. 9. 4. Conclusion This study characterizes an optically switchable transmission grating by adding different amounts of NLC and azo-LC to the typical H-PDLC. The optimized recipe uses 15 wt.% azo-LC and 35 wt.% NLC. Upon light irradiation, the doped azo-LC undergoes a cis-totrans photoisomerization and the cis-azo disrupts the NLC host. The photo-induced phase transition within the LC-rich regions changes the RI of H-PDLC grating, and the DE decreases as the pumping power laser increases. The response time of light-on state decreased to 0.5 s by applying 9 mW/cm2 power of laser light. Acknowledgments This work is supported by the National Science Council, Taiwan, under projects No. 99-2221-E-260-024 and 98-2113-M-040-003MY2. The authors thank Kou Ryou Enterprise Corp., Taiwan for providing the spacers. References
Fig. 8. Response time of the optically switchable H-PDLC grating dependent on the power of pumping beam (a); recovery time required to achieve the original DE dependent on the initial pumping power of the laser beam.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
Fig. 9. Time dependence of diffraction intensity from the optically switchable HPDLC transmission grating when the pump light was turned on and off alternatively. The pumping power of light was 9 mW/cm2.
[21] [22] [23] [24] [25]
(5 wt.%) creates grating of higher DE, and almost no optically switching characteristic. This implies that 5 wt.% azo-LC cannot
[26]
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