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Total internal reflection holographic gratings recorded in polymer-dispersed liquid crystals
Optics Communications 260 (2006) 192–195 www.elsevier.com/locate/optcom
Total internal reflection holographic gratings recorded in polymer-dispersed liquid crystals K. Beev b
a,*
, L. Criante b, D.E. Lucchetta b, F. Simoni b, S. Sainov
a
a Central Laboratory of Optical Storage and Processing of Information, BAS, Acad. G. Bonchev Str., bl. 101, 1113 Sofia, Bulgaria Dipartimento di Fisica e Ingegneria dei Materiali e del Territorio, Universita` Politecnica delle Marche, via Brecce Bianche, 60131 Ancona, Italy
Received 29 April 2005; received in revised form 7 October 2005; accepted 16 October 2005
Abstract We report diffraction gratings recording in polymer dispersed liquid crystals (PDLC) material with a beam suffering total internal reflection from the sample air surface. The real time diffraction efficiency kinetics as well as the polarization, voltage behavior and angular selectivity of the gratings are studied at different reconstruction geometries. An increase of three times of the diffraction efficiency in case of applied voltage is observed for one of these geometries in opposite to typical PDLC switchable gratings. Such dependence appears in most of the geometries and is connected to the specific formation of two gratings by the Stetson scheme. Ó 2005 Elsevier B.V. All rights reserved.
1. Introduction Holographic recording geometry with total internal reflected (TIR) wave was first proposed by Stetson [1] in 1967 (Fig. 1). It has become an object of interest due to its applications to photolithography [2] and optical interconnects [3]. Fig. 1 shows the typical setup for TIR holographic recording. Two gratings – one with low and one with high spatial frequency are formed by the interference between both the incident beams and between the plane and TIR wave. Both patterns exhibit the Bragg behavior. In addition a grating formed by the interference of the incident and the reflected TIR wave also exists. Since the probe beam is not Bragg matched to this grating, its presence do not affect the obtained results. According to the coupled wave theory [4], the efficiency of such holograms strongly depends on the induced modulation of the refractive index n, the gratings thickness and its modification during recording (shrinkage or swelling) and finally the incident angle of the TIR beam. *
Corresponding author. Tel.: +359 2 710018; fax: +359 2 719165. E-mail address: [email protected] (K. Beev).
0030-4018/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2005.10.044
Since the TIR beam in this recording geometry suffers a polarization dependent phase shift (during the TIR), the gratings are sensitive to the polarization of the reconstructed beam. Since two gratings are recorded in this geometry, the TIR wave behaves like a pair of reference beams during reconstruction. In the present work a homogeneous mixture of reactive monomers, liquid crystal and initiators is employed. The grating formation exhibits the following mechanism. After illumination with spatially modulated light distribution, a counter diffusion occurs. At the bright regions polymerization takes place; simultaneously the liquid crystal (LC) is forced by the growing polymer network and confined in droplets at the dark regions. This phase separation is induced by the local differences in the polymerization rates. The structure, consisting of alternating polymer rich and LC rich regions, corresponding to the interference pattern, is recognized as holographic polymer dispersed liquid crystals (HPDLC). The experimental setup shown in Fig. 1 leads to two-dimensional (2D) grating formation (Fig. 2) where the liquid crystal location is more complex compared to conventional HPDLC, which pattern is usually perpendicular to the plane of incidence of the beam.
K. Beev et al. / Optics Communications 260 (2006) 192–195
Fig. 1. StetsonÕs recording geometry 1, glass prism; 2, recording material.
Fig. 2. Scheme of the diffraction grating fringes.
One of the most attracting properties of HPDLC materials is connected with the reversible switching of the diffraction efficiency by application of AC voltage. A suitable strong electric field aligns the LC inside the droplets, in which it is confined, changing the index modulation and allowing the reversible switching of the grating. Such switchable diffraction elements are under active consideration for a number of photonic applications [5], including displays [6], switchable focus lenses [7], electrooptic filters, free space and guided-wave optical switches [8,9], electrically addressable security holograms [10]. Recently, Xianyu et al. [11,12] got electrically controlled TIR conditions by using slanted geometry during recording. This is a different technique since the obtained grating is only one. In that case the p-polarization of the reconstructing beam is trapped into the recording medium, which is acting like a waveguide. Application of strong enough electric field erases the grating and the beam (the p-polarized one) passes through the material. Other experiments are directed to integrate the HPDLCs in different devices. A HPDLC has been used in a compact laser device to switch the emission wavelength by electric field application [13]. In the present work TIR recording is used to form simultaneously 2D gratings in PDLC material. The polarization, voltage and angle behavior of the grating are studied by different reconstruction geometries.
193
geometry, as presented below. The recorded gratings can be probed using different geometries. The reconstruction has been performed by both the waves (the TIR and the plane) as well as with the conjugate – the case of illumination at the opposite side of the prism, which is versus the incident direction of the TIR beam during recording. Representing the experimental data we show in the inset the used reconstruction geometry. The pre-polymer syrup consists of a homogenized solution of the monomer dipentaerythritol hydroxy pentaacrylate DPHPA (55% in weight), the liquid crystal BL038 (32% in weight) which is a well known eutectic mixture consisting of three cyanobiphenyls and cyanoterphenyl as well as 13% of photoinitiator mixture. It consists of 5% Rose Bengal RB and 10% N-phenylglycine NPG dissolved in N-vinylpyrrollidone NVP (the percentages of RB and NPG are with respect to NVP). Indium tin oxide-coated glass substrates are employed for the cell preparation. The cells are filled by a special capillary technique. Fig. 3 illustrates the optical set-up used for holographic recording. To ensure TIR at the sample-air interface, a K8 light crown glass prism with refractive indices 1.520 and 1.515 at 514.5 and 632.8 nm, respectively is used. The angle of its base is 45°. The irradiation of the Ar-ion laser at 514.5 nm, passing through a polarizer, a beam splitter, the mirror M (for the TIR beam) and the glass prism, provides two interference patterns (Fig. 2) with spatial frequencies K1 = 3590 lines/ mm and K2 = 672 lines/mm. Index matching contact liquid is used in order to ensure good optical contact between the prism and the cell. The polarization of the recording beams is perpendicular to the plane of incidence (s) (while the polarization parallel to the plane of incidence is referred as p). Simultaneously, a He–Ne laser at 632.8 nm is used to investigate the real-time diffraction efficiency (DE) kinetics during recording. The DE is calculated as the ratio between the intensities of the diffracted and the incident beam intensities. Fig. 4 presents the DE dynamics during recording. The formation of both gratings is plausible. The faster kinetics could be referred to the high spatial frequency pattern formation due to the rapid growth of the polymer chain, typical for free-radical polymerization reactions during the curing process [14] and the shorter diffusion length.
2. Experimental It is important to note that the use of an anisotropic recording material in this setup leads to differences in the diffraction properties dependent on the reconstruction
Fig. 3. Optical setup for holographic recording: BS, beam splitter; M, mirror; P, polarizer; PM, power meter.
194
K. Beev et al. / Optics Communications 260 (2006) 192–195
12
DE [%]
10
8
6
4
2
0 0
100
200
300
400
500
600
Exposure [mJ/cm2 ] Fig. 4. Diffraction efficiency dynamics during recording monitored by s-polarized probe He–Ne beam.
illumination with the TIR wave, for two different geometries, are presented in Fig. 6. It could be assumed that the higher efficiency for the conjugated p-polarized reconstructed wave (with maximum at 65°) indicate a predominant orientation of the LC directors following the grooves of the pattern (the both gratings are slanted at the same direction – Fig. 2), thus a higher index modulation for the conjugated wave. The behavior when the reconstruction is performed with the plane wave is similar to the straight line in Fig. 6 (reconstruction with the TIR wave at the direction of the incidence beam during polymerization). The DE changes in case of an applied electric field (2 kHz) are also examined. Again, the reconstruction with the conjugate TIR wave exhibits different features compared to the rest of the configurations. The p-polarization resembles the typical for PDLC switching dependence – Fig. 7, which is a different behavior compared with the other possible geometries. At the same geometry for s-polarization is observed the maximal DE increase – about three times (Fig. 7). The typical dependence of the DE on the applied voltage for our gratings, which holds in all other cases, is shown in Fig. 8. The highest DE (up to about 16%) is obtained with the maximum value of the applied voltage (5 V/lm) during illumination with the plane wave (the case presented in Fig. 8). This behavior is opposite to the one of typical switchable PDLC gratings where the fringes are perpendicular to the plane of incidence of the beams and the reorientation of the LC induced by the electric field leads to index matching with polymer. The DE increase, observed here in most of the geometries, is connected to the specific formation of two gratings by the Stetson scheme as well as the strong anchoring of the LC at the interfaces between the polymer and LC-rich areas.
Fig. 5. Angular selectivity of the gratings. The reconstruction is performed with the plane wave. The source is a He–Ne laser (632.8 nm).
The angular behavior of the recorded gratings has been investigated. A typical dependence is reported in Fig. 5 (reconstruction performed with the plane wave). In the same figure is also reported the result of the coupled wave theory analysis for such gratings [4] at the same geometry. The experimental points (the squares in Fig. 5) are in good qualitative agreement with the theoretical calculation (full line). A typical feature is the asymmetric shape of the peak around the maximum. The experimental data shows a broader peak most probably because of the light scattering from the anisotropic LC droplets. This anisotropy along with the high surface to volume ratios typical for LC dispersions [14] leads to a different polarization sensitivity dependence on the geometry of the reconstruction wave. The results obtained in case of
Fig. 6. Polarization dependence of the diffracted beam intensity in case of reconstruction with the TIR wave during recording (solid line) and its conjugate (dot line).
K. Beev et al. / Optics Communications 260 (2006) 192–195
195
dominant orientation of the director. An anisotropy in the shape of the droplets, different in the regions where only one or both the gratings are recorded could also be assumed. Anyway a detailed analysis of these morphologies is currently under progress. 3. Conclusions Employing the StetsonÕs holographic setup allows the formation of 2-dimensional structures of alternating LC and polymer-rich regions, resulting in switchable BraggÕs gratings revealing interesting polarization and electro-optical properties. Our experiments show the potential of this material for realizing complex structures suitable for photonic crystal applications, controlled by electric field application. Fig. 7. Dependence of the DE on the applied voltage in case of reconstruction with the TIR wave versus the incidence direction during recording.
Acknowledgements This research is supported by the EU project NoE FP6PLT-511568-3DTV. It has been performed within the frame of the European STREP project MICROHOLAS and of COST P8 Action. References
Fig. 8. Dependence of the DE on the applied AC electric field; the polarization is s.
The different properties of the gratings dependent on the reconstruction geometry may suggest that the LC orientation in the final morphology is strongly influenced by the polymer network at the boundary surfaces and exhibits a
[1] K. Stetson, Appl. Phys. Lett. 11 (1967) 225. [2] R.T. Chen, L. Svadovnik, T. Aye, T. Jannson, Opt. Lett. 15 (1990) 869. [3] D. Prongue´, H. Herzig, Proc. SPIE 1281 (1990) 113. [4] P. Ehbets, H. Herzig, R. Da¨ndliker, Opt. Commun. 89 (1992) 5. [5] T.J. Bunning, L.V. Natarajan, V.P. Tondiglia, R.L. Sutherland, Annu. Rev. Mater. Sci. 30 (2000) 83. [6] Jun Qia, G.P. Crawford, Displays 25 (2004) 177. [7] L. Domash, Y.-M. Chen, B. Gomatam, C. Gozewski, R. Sutherland, et al., SPIE Proc. 2689 (1996) 188. [8] R. Sutherland, L. Natarajan, V. Tondiglia, T. Bunning, SPIE Proc. 3421 (1998) 8. [9] L. Natarajan, R. Sutherland, T. Bunning, V. Tondiglia, SPIE Proc. 3292 (1998) 44. [10] B. Epling, D. Brandelik, L. Natarajan, R. Sutherland, V. Tondiglia, T. Bunning, SAE Tech. Dig. 970253 (1997) 1. [11] H. Xianyu, J. Qi, R. Cohn, G. Crawford, Opt. Lett. 28 (10) (2003) 792. [12] J. Qi, H. Xianyu, J. Liang, G. Crawford, IEEE Photon. Lett. 15 (2003) 685. [13] D. Lucchetta, L. Criante, O. Francescangeli, F. Simoni, Appl. Phys. Lett. 84 (24) (2004) 4893. [14] P.S. Drzaic, Liquid Crystal Dispersions, World Scientific Publishing, Singapore, 1995.
Total internal reflection holographic gratings recorded in polymer-dispersed liquid crystals K. Beev b
a,*
, L. Criante b, D.E. Lucchetta b, F. Simoni b, S. Sainov
a
a Central Laboratory of Optical Storage and Processing of Information, BAS, Acad. G. Bonchev Str., bl. 101, 1113 Sofia, Bulgaria Dipartimento di Fisica e Ingegneria dei Materiali e del Territorio, Universita` Politecnica delle Marche, via Brecce Bianche, 60131 Ancona, Italy
Received 29 April 2005; received in revised form 7 October 2005; accepted 16 October 2005
Abstract We report diffraction gratings recording in polymer dispersed liquid crystals (PDLC) material with a beam suffering total internal reflection from the sample air surface. The real time diffraction efficiency kinetics as well as the polarization, voltage behavior and angular selectivity of the gratings are studied at different reconstruction geometries. An increase of three times of the diffraction efficiency in case of applied voltage is observed for one of these geometries in opposite to typical PDLC switchable gratings. Such dependence appears in most of the geometries and is connected to the specific formation of two gratings by the Stetson scheme. Ó 2005 Elsevier B.V. All rights reserved.
1. Introduction Holographic recording geometry with total internal reflected (TIR) wave was first proposed by Stetson [1] in 1967 (Fig. 1). It has become an object of interest due to its applications to photolithography [2] and optical interconnects [3]. Fig. 1 shows the typical setup for TIR holographic recording. Two gratings – one with low and one with high spatial frequency are formed by the interference between both the incident beams and between the plane and TIR wave. Both patterns exhibit the Bragg behavior. In addition a grating formed by the interference of the incident and the reflected TIR wave also exists. Since the probe beam is not Bragg matched to this grating, its presence do not affect the obtained results. According to the coupled wave theory [4], the efficiency of such holograms strongly depends on the induced modulation of the refractive index n, the gratings thickness and its modification during recording (shrinkage or swelling) and finally the incident angle of the TIR beam. *
Corresponding author. Tel.: +359 2 710018; fax: +359 2 719165. E-mail address: [email protected] (K. Beev).
0030-4018/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2005.10.044
Since the TIR beam in this recording geometry suffers a polarization dependent phase shift (during the TIR), the gratings are sensitive to the polarization of the reconstructed beam. Since two gratings are recorded in this geometry, the TIR wave behaves like a pair of reference beams during reconstruction. In the present work a homogeneous mixture of reactive monomers, liquid crystal and initiators is employed. The grating formation exhibits the following mechanism. After illumination with spatially modulated light distribution, a counter diffusion occurs. At the bright regions polymerization takes place; simultaneously the liquid crystal (LC) is forced by the growing polymer network and confined in droplets at the dark regions. This phase separation is induced by the local differences in the polymerization rates. The structure, consisting of alternating polymer rich and LC rich regions, corresponding to the interference pattern, is recognized as holographic polymer dispersed liquid crystals (HPDLC). The experimental setup shown in Fig. 1 leads to two-dimensional (2D) grating formation (Fig. 2) where the liquid crystal location is more complex compared to conventional HPDLC, which pattern is usually perpendicular to the plane of incidence of the beam.
K. Beev et al. / Optics Communications 260 (2006) 192–195
Fig. 1. StetsonÕs recording geometry 1, glass prism; 2, recording material.
Fig. 2. Scheme of the diffraction grating fringes.
One of the most attracting properties of HPDLC materials is connected with the reversible switching of the diffraction efficiency by application of AC voltage. A suitable strong electric field aligns the LC inside the droplets, in which it is confined, changing the index modulation and allowing the reversible switching of the grating. Such switchable diffraction elements are under active consideration for a number of photonic applications [5], including displays [6], switchable focus lenses [7], electrooptic filters, free space and guided-wave optical switches [8,9], electrically addressable security holograms [10]. Recently, Xianyu et al. [11,12] got electrically controlled TIR conditions by using slanted geometry during recording. This is a different technique since the obtained grating is only one. In that case the p-polarization of the reconstructing beam is trapped into the recording medium, which is acting like a waveguide. Application of strong enough electric field erases the grating and the beam (the p-polarized one) passes through the material. Other experiments are directed to integrate the HPDLCs in different devices. A HPDLC has been used in a compact laser device to switch the emission wavelength by electric field application [13]. In the present work TIR recording is used to form simultaneously 2D gratings in PDLC material. The polarization, voltage and angle behavior of the grating are studied by different reconstruction geometries.
193
geometry, as presented below. The recorded gratings can be probed using different geometries. The reconstruction has been performed by both the waves (the TIR and the plane) as well as with the conjugate – the case of illumination at the opposite side of the prism, which is versus the incident direction of the TIR beam during recording. Representing the experimental data we show in the inset the used reconstruction geometry. The pre-polymer syrup consists of a homogenized solution of the monomer dipentaerythritol hydroxy pentaacrylate DPHPA (55% in weight), the liquid crystal BL038 (32% in weight) which is a well known eutectic mixture consisting of three cyanobiphenyls and cyanoterphenyl as well as 13% of photoinitiator mixture. It consists of 5% Rose Bengal RB and 10% N-phenylglycine NPG dissolved in N-vinylpyrrollidone NVP (the percentages of RB and NPG are with respect to NVP). Indium tin oxide-coated glass substrates are employed for the cell preparation. The cells are filled by a special capillary technique. Fig. 3 illustrates the optical set-up used for holographic recording. To ensure TIR at the sample-air interface, a K8 light crown glass prism with refractive indices 1.520 and 1.515 at 514.5 and 632.8 nm, respectively is used. The angle of its base is 45°. The irradiation of the Ar-ion laser at 514.5 nm, passing through a polarizer, a beam splitter, the mirror M (for the TIR beam) and the glass prism, provides two interference patterns (Fig. 2) with spatial frequencies K1 = 3590 lines/ mm and K2 = 672 lines/mm. Index matching contact liquid is used in order to ensure good optical contact between the prism and the cell. The polarization of the recording beams is perpendicular to the plane of incidence (s) (while the polarization parallel to the plane of incidence is referred as p). Simultaneously, a He–Ne laser at 632.8 nm is used to investigate the real-time diffraction efficiency (DE) kinetics during recording. The DE is calculated as the ratio between the intensities of the diffracted and the incident beam intensities. Fig. 4 presents the DE dynamics during recording. The formation of both gratings is plausible. The faster kinetics could be referred to the high spatial frequency pattern formation due to the rapid growth of the polymer chain, typical for free-radical polymerization reactions during the curing process [14] and the shorter diffusion length.
2. Experimental It is important to note that the use of an anisotropic recording material in this setup leads to differences in the diffraction properties dependent on the reconstruction
Fig. 3. Optical setup for holographic recording: BS, beam splitter; M, mirror; P, polarizer; PM, power meter.
194
K. Beev et al. / Optics Communications 260 (2006) 192–195
12
DE [%]
10
8
6
4
2
0 0
100
200
300
400
500
600
Exposure [mJ/cm2 ] Fig. 4. Diffraction efficiency dynamics during recording monitored by s-polarized probe He–Ne beam.
illumination with the TIR wave, for two different geometries, are presented in Fig. 6. It could be assumed that the higher efficiency for the conjugated p-polarized reconstructed wave (with maximum at 65°) indicate a predominant orientation of the LC directors following the grooves of the pattern (the both gratings are slanted at the same direction – Fig. 2), thus a higher index modulation for the conjugated wave. The behavior when the reconstruction is performed with the plane wave is similar to the straight line in Fig. 6 (reconstruction with the TIR wave at the direction of the incidence beam during polymerization). The DE changes in case of an applied electric field (2 kHz) are also examined. Again, the reconstruction with the conjugate TIR wave exhibits different features compared to the rest of the configurations. The p-polarization resembles the typical for PDLC switching dependence – Fig. 7, which is a different behavior compared with the other possible geometries. At the same geometry for s-polarization is observed the maximal DE increase – about three times (Fig. 7). The typical dependence of the DE on the applied voltage for our gratings, which holds in all other cases, is shown in Fig. 8. The highest DE (up to about 16%) is obtained with the maximum value of the applied voltage (5 V/lm) during illumination with the plane wave (the case presented in Fig. 8). This behavior is opposite to the one of typical switchable PDLC gratings where the fringes are perpendicular to the plane of incidence of the beams and the reorientation of the LC induced by the electric field leads to index matching with polymer. The DE increase, observed here in most of the geometries, is connected to the specific formation of two gratings by the Stetson scheme as well as the strong anchoring of the LC at the interfaces between the polymer and LC-rich areas.
Fig. 5. Angular selectivity of the gratings. The reconstruction is performed with the plane wave. The source is a He–Ne laser (632.8 nm).
The angular behavior of the recorded gratings has been investigated. A typical dependence is reported in Fig. 5 (reconstruction performed with the plane wave). In the same figure is also reported the result of the coupled wave theory analysis for such gratings [4] at the same geometry. The experimental points (the squares in Fig. 5) are in good qualitative agreement with the theoretical calculation (full line). A typical feature is the asymmetric shape of the peak around the maximum. The experimental data shows a broader peak most probably because of the light scattering from the anisotropic LC droplets. This anisotropy along with the high surface to volume ratios typical for LC dispersions [14] leads to a different polarization sensitivity dependence on the geometry of the reconstruction wave. The results obtained in case of
Fig. 6. Polarization dependence of the diffracted beam intensity in case of reconstruction with the TIR wave during recording (solid line) and its conjugate (dot line).
K. Beev et al. / Optics Communications 260 (2006) 192–195
195
dominant orientation of the director. An anisotropy in the shape of the droplets, different in the regions where only one or both the gratings are recorded could also be assumed. Anyway a detailed analysis of these morphologies is currently under progress. 3. Conclusions Employing the StetsonÕs holographic setup allows the formation of 2-dimensional structures of alternating LC and polymer-rich regions, resulting in switchable BraggÕs gratings revealing interesting polarization and electro-optical properties. Our experiments show the potential of this material for realizing complex structures suitable for photonic crystal applications, controlled by electric field application. Fig. 7. Dependence of the DE on the applied voltage in case of reconstruction with the TIR wave versus the incidence direction during recording.
Acknowledgements This research is supported by the EU project NoE FP6PLT-511568-3DTV. It has been performed within the frame of the European STREP project MICROHOLAS and of COST P8 Action. References
Fig. 8. Dependence of the DE on the applied AC electric field; the polarization is s.
The different properties of the gratings dependent on the reconstruction geometry may suggest that the LC orientation in the final morphology is strongly influenced by the polymer network at the boundary surfaces and exhibits a
[1] K. Stetson, Appl. Phys. Lett. 11 (1967) 225. [2] R.T. Chen, L. Svadovnik, T. Aye, T. Jannson, Opt. Lett. 15 (1990) 869. [3] D. Prongue´, H. Herzig, Proc. SPIE 1281 (1990) 113. [4] P. Ehbets, H. Herzig, R. Da¨ndliker, Opt. Commun. 89 (1992) 5. [5] T.J. Bunning, L.V. Natarajan, V.P. Tondiglia, R.L. Sutherland, Annu. Rev. Mater. Sci. 30 (2000) 83. [6] Jun Qia, G.P. Crawford, Displays 25 (2004) 177. [7] L. Domash, Y.-M. Chen, B. Gomatam, C. Gozewski, R. Sutherland, et al., SPIE Proc. 2689 (1996) 188. [8] R. Sutherland, L. Natarajan, V. Tondiglia, T. Bunning, SPIE Proc. 3421 (1998) 8. [9] L. Natarajan, R. Sutherland, T. Bunning, V. Tondiglia, SPIE Proc. 3292 (1998) 44. [10] B. Epling, D. Brandelik, L. Natarajan, R. Sutherland, V. Tondiglia, T. Bunning, SAE Tech. Dig. 970253 (1997) 1. [11] H. Xianyu, J. Qi, R. Cohn, G. Crawford, Opt. Lett. 28 (10) (2003) 792. [12] J. Qi, H. Xianyu, J. Liang, G. Crawford, IEEE Photon. Lett. 15 (2003) 685. [13] D. Lucchetta, L. Criante, O. Francescangeli, F. Simoni, Appl. Phys. Lett. 84 (24) (2004) 4893. [14] P.S. Drzaic, Liquid Crystal Dispersions, World Scientific Publishing, Singapore, 1995.