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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 11 — May. 23, 2011
  • pp: 10494–10500
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In situ polarized micro-Raman investigation of periodic structures realized in liquid-crystalline composite materials

Marco Castriota, Angela Fasanella, Enzo Cazzanelli, Luciano De Sio, Roberto Caputo, and Cesare Umeton  »View Author Affiliations


Optics Express, Vol. 19, Issue 11, pp. 10494-10500 (2011)
http://dx.doi.org/10.1364/OE.19.010494


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Abstract

In situ polarized micro-Raman Spectroscopy has been utilized to determine the liquid crystal configuration inside a periodic liquid crystalline composite structure made of polymer slices alternated to films of liquid crystal. Liquid crystal, Norland Optical Adhesive (NOA-61) monomer and its polymerized form have been investigated separately. The main Raman features, used as markers for the molecular orientation estimation, have been identified. In situ polarized Raman spectra indicate that the orientation of the liquid crystal director inside the structure is perpendicular to its polymeric slices. Results show the usefulness of in situ polarized micro-Raman spectroscopy to investigate liquid crystalline composite structures.

© 2011 OSA

1. Introduction

Switchable holographic structures are promising for a wide range of applications such as imaging, information processing and diffractive optics [1

1. L. V. Natarajan, R. L. Sutherland, V. P. Tondiglia, T. J. Bunning, and W. W. Adams, “Electro-optical switching characteristics of volume holograms in polymer dispersed liquid crystals,” J. Nonlinear Opt. Phys. Mater. 5(1), 89–98 (1996). [CrossRef]

]. Liquid crystals (LCs) are useful for the fabrication of these devices thanks to their capability to change orientational order under the influence of external stimuli. In the last years, great attention has been devoted to periodic liquid crystalline composite materials like POLICRYPS (acronym of POlymer LIquid CRYstal Polymer Slices) (Fig. 1(a)
Fig. 1 (a) SEM image of a POLICRYPS structure and reference axes; (b) experimental set-up used to collect the polarized Raman spectra and reference axes. θ is the angle between the direction of polarization of the incident radiation (Y axis of the laboratory reference system) and the polymer slice of the POLICRYPS structure.
), as novel structures with a wide range of optical and electro-optical applications [2

2. A. Veltri, R. Caputo, C. Umeton, and A. V. Sukhov, “Model for the photoinduced formation of diffraction gratings in liquid-crystalline composite materials,” Appl. Phys. Lett. 84, 3492–3494 (2004). [CrossRef]

5

5. R. Caputo, A. De Luca, L. De Sio, L. Pezzi, G. Strangi, C. Umeton, A. Veltri, R. Asquini, A. d’Alessandro, D. Donisi, R. Beccherelli, A. V. Sukhov, and N. V. Tabiryan, “POLICRYPS: a liquid crystal composed nano/microstructure with a wide range of optical and electro-optical applications,” J. Opt. A, Pure Appl. Opt. 11(2), 024017 (2009). [CrossRef]

]. At present, there is the possibility of exploiting the POLICRYPS as a template to be doped with metal nano-particles for the realization of controllable metamaterials [6

6. NANOGOLD project: “Self-organized Nanomaterials for tailored optical and electrical properties” (Seventh Framework Programme Theme, NMP-2008–2.2–2, Nano-structured metamaterials grant agreement no. 228455).

].

In fact, possible electro-optical applications of POLICRYPS are due to the uniform and regular steady state alignment of the director n of the Nematic Liquid Crystal (NLC) films inside the structure; nevertheless, at our knowledge, there is no direct estimation of this orientation, at microscopic level, like the one shown in the present work, obtained by using in situ polarized Raman spectroscopy.

2. Experimental

The investigated POLICRYPS has been realized by using E7 NLC (supplied by Merk) and the monomer NOA-61 (Norland Optical Adhesive). The used standard fabrication procedure is described elsewhere [7

7. R. Caputo, L. De Sio, A. Veltri, C. Umeton, and A. V. Sukhov, “Development of a new kind of switchable holographic grating made of liquid-crystal films separated by slices of polymeric material,” Opt. Lett. 29(11), 1261–1263 (2004). [CrossRef] [PubMed]

,8

8. L. De Sio, R. Caputo, A. De Luca, A. Veltri, C. Umeton, and A. V. Sukhov, “In situ optical control and stabilization of the curing process of holographic gratings with a nematic film-polymer-slice sequence structure,” Appl. Opt. 45(16), 3721–3727 (2006). [CrossRef] [PubMed]

]. Spectroscopic investigations were performed by a Raman microprobe Jobin-Yvon Labram (spectral resolution ~2 cm−1) equipped with a CCD detector and a He-Ne laser (λ = 632.8 nm emission wavelength).

A 50x Mplan Olympus objective (Numerical Aperture 0.90) was used, focusing the laser spot to 2-3 μm of diameter. The sample grating has been written with a large pitch (6 µm), while a very thin glass (0.1 mm thick) has been used for the upper cell window. In situ polarized Raman spectra have been collected by using the experimental set-up shown in Fig. 1(b). The laser source is linearly polarized along the Y axis of the reference system (in this work, small letters x,y,z are adopted for the molecular frame, while capital X,Y,Z indicate the laboratory axes). The analyzer lets only the Y polarized component of the scattered radiation reach the instrument grating; in this way, only the YY component of the polarizability tensor is detected. The sample is placed on a goniometric rotation stage (Thorlabs) and, at the beginning, it is oriented with the polymeric slices parallel to the polarization direction of the incident light. Spectra have been collected for values of the angle θ, (formed by the light polarization direction and the polymeric slices, see Fig. 1(a)) which vary from 0° up to 180°, with steps of 15°. The Raman spectra shown in this paper are representative of a wider Raman investigation performed along many liquid crystal slices.

3. Results and discussion

NOA-61 has been investigated before and after the polymerization process (Figs. 3(a)
Fig. 3 Representative Raman spectra collected on Norland Optical Adhesive NOA-61 monomer (a) and (b) polymerized. (For a better presentation, the graphs are shown with different intensity scales).
, 3(b)). It is worth underlining that the exact composition of NOA-61 is still unknown; however it should be a mix of a mercapto-ester with acrylate monomer as described elsewhere [14

14. B. Pinto-Iguanero, A. Olivares-Perez, and I. Fuentes-Tapia, “Holographic material film composed by Norland Noa 65® adhesive,” Opt. Mater. 20(3), 225–232 (2002). [CrossRef]

]. In Fig. 3(a), reporting spectra collected on NOA-61 monomer, we notice the characteristic features of the thiol groups (S-H) at about 2582 cm−1, vinyl group (C = C symmetric stretching, non conjugated) at 1650 cm−1, and the carbonyl (C = O) of the carboxyl group COOR [15

15. H. T. A. Wilderbeek, J. H. G. P. Goossens, C. W. M. Bastiaansen, and D. J. Broer, “Photoinitiated bulk polymerization of liquid crystalline thiolene monomers,” Macromolecules 35(24), 8962–8968 (2002). [CrossRef]

] at 1746 and 1764 cm−1. At 1604 cm−1 we find the aromatic ring vibration, evidently present on the R group (unknown part) of the molecule [15

15. H. T. A. Wilderbeek, J. H. G. P. Goossens, C. W. M. Bastiaansen, and D. J. Broer, “Photoinitiated bulk polymerization of liquid crystalline thiolene monomers,” Macromolecules 35(24), 8962–8968 (2002). [CrossRef]

].

The band at 677 cm−1 is assigned to the C-S bond, while the peak at 1297 cm−1 and the bands in the region between 1400 and 1500 cm−1 are assigned to the CH3 bending; those in the range between 2900 and 3100 cm−1 are assigned to the C-H stretching [16

16. M. Claudino, M. Johansson, and M. Jonsson, “Thiol–ene coupling of 1,2-disubstituted alkene monomers: the kinetic effect of cis/trans-isomer structures,” Eur. Polym. J. 46(12), 2321–2332 (2010). [CrossRef]

]. Spectra of polymerized NOA-61 (Fig. 3(b)) show the absence of the band at 2582 cm−1 and, simultaneously, a strong reduction of the band at 1650 cm−1, indicating that the curing process involves a reaction between the thiol and the vinyl groups present in the monomer [15

15. H. T. A. Wilderbeek, J. H. G. P. Goossens, C. W. M. Bastiaansen, and D. J. Broer, “Photoinitiated bulk polymerization of liquid crystalline thiolene monomers,” Macromolecules 35(24), 8962–8968 (2002). [CrossRef]

].

In Fig. 4
Fig. 4 Representative un-polarized Raman spectra collected on the POLICRYPS sample in the range between 200 and 1100 cm−1 (on the bottom), 1100 and 1800 cm−1 (on the middle) and 1800 and 2500 cm−1 (on the top). (For a better presentation, the graphs are shown with different intensity scales).
, the micro-Raman un-polarized spectra of the POLICRYPS (polymerized NOA-61 and E7), representative of all the spectra collected on the whole sample, are shown. The liquid crystal characteristic bands at 1609 cm−1 and 2232 cm−1 and the bands at 1746 and 1764 cm−1 of the carboxyl group of the NOA-61 are still well evident. For this reason, these bands are used as markers in order to evaluate the presence and the contribution of each component to the total Raman spectra.

In situ polarized Raman spectra have been collected on the NLC films (confined between the polymer slices of the POLICRYPS, see Fig. 1) as a function of the angle θ in the range between 0° and 180° (Fig. 5
Fig. 5 Representative Polarized Raman spectra collected on the liquid crystal slices of the POLICRYPS grating at different θ values between 0° and 180°.
). Intensities of the bands at 1184, 1288, 1610 and 2233 cm−1 (respectively assigned to the aromatic C-H in plane deformation, C-C stretch of biphenyl bond and C = C stretching of biphenyl rings) increase with θ, to reach the maximum value at θ = 90°; then, a decrease occurs when θ goes from 90° to 180°.

This behavior clearly evidences a preferential orientation of the NLC director within the POLICRYPS structure [12

12. W. J. Jones, D. K. Thomas, D. W. Thomas, and G. Williams, “On the determination of order parameters for homogeneous and twisted nematic liquid crystals from Raman spectroscopy,” J. Mol. Struct. 708(1-3), 145–163 (2004). [CrossRef]

,13

13. E. W. Astrova, T. S. Perova, S. A. Grudinkin, V. A. Tolmachev, Yu. A. Pilyugina, V. B. Voronkov, and J. K. Vij, “Polarized infrared and Raman spectroscopic studies of the liquid crystal E7 alignment in composites based on grooved silicon,” Semiconductors 39(7), 759–767 (2005). [CrossRef]

,17

17. H. F. Gleeson, C. D. Southern, P. D. Brimicombe, J. W. Goodby, and V. Görtz, “Optical measurements of orientational order in uniaxial and biaxial nematic liquid crystals,” Liq. Cryst. 37(6), 949–959 (2010). [CrossRef]

21

21. A. Sanchez-Castillo, M. A. Osipov, and F. Giesselmann, “Orientational order parameters in liquid crystals: a comparative study of x-ray diffraction and polarized Raman spectroscopy results,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 81(2), 021707 (2010). [CrossRef] [PubMed]

]: at θ = 90°, the Raman intensities show a maximum because the director n is parallel to the polarization direction of the laser light (the molecular z axis corresponds to laboratory Y axis). Since, in our geometry, this corresponds to the polymeric slices being perpendicular to the polarization direction of light, it can be argued that POLICRYPS gratings are made of polymer slices regularly alternated to films of Nematic Liquid Crystal molecules whose average orientation (director n) is perpendicular to the polymeric slices. We underline that this director configuration can be directly correlated to the birefringence of the sample: in the past [22

22. R. Caputo, I. Trebisacce, L. De Sio, and C. Umeton, “Jones matrix analysis of dichroic phase retarders realized in soft matter composite materials,” Opt. Express 18(6), 5776–5784 (2010). [CrossRef] [PubMed]

], a similar result has been demonstrated by measuring the polarization dependence of a POLICRYPS diffraction grating efficiency, which is proportional to the birefringence value. In fact, as shown by Sutherland et al. [23

23. R. L. Sutherland, “Polarization and switching properties of holographic polymer-dispersed liquid-crystal gratings. I. Theoretical model,” J. Opt. Soc. Am. B 19(12), 2995–3003 (2002). [CrossRef]

25

25. K. K. Vardanyan, J. Qi, J. N. Eakin, M. De Sarkar, and G. P. Crawford, “Polymer scaffolding model for holographic polymer-dispersed liquid crystals,” Appl. Phys. Lett. 81(25), 4736–4738 (2002). [CrossRef]

], this birefringence also gives a unique signature to the electro-optical properties of the structure as a function of the polarization direction of the probe beam. However, while birefringence measurements yield macroscopic indications only, the chemical sensitivity of Raman spectroscopy allows a detailed microscopic view of the director configuration; a Raman mapping of a large area of the sample could be conveniently used to deduce its birefringence distribution. It is our intention to perform new experiments in this direction.

4. Conclusions

In situ Polarized Raman spectroscopy has been exploited to investigate the orientation of liquid crystal molecules in a POLICRYPS holographic grating. Raman spectra have been collected both on the NLC and NOA-61 (monomer and polymerized forms). Bands due to the symmetric stretching of the aromatic rings of the biphenyl molecules and the stretching of the CN groups have been selected as “markers” of the NLC orientation, since they occur along the long molecular axis of biphenyl molecules (z-axis): the orientation of the NLC director n within the POLICRYPS structure turns out to be perpendicular to its polymeric slices.

Results represent a contribute for the determination of the orientational order parameters (< P2> and < P4>) in a POLICRYPS system. Moreover, they might allow to find an exact relation between the director orientation and the FWHM of the curve obtained by plotting the intensity peak at 1610 cm−1 as a function of the angle θ. In future, different Raman analysis (Raman mapping of large area sample) are planned in order to investigate the liquid crystal birefringence distribution in POLICRYPS systems.

Above studies will support the future work concerned with the use of POLICRYPS structures as a template to be nano-doped for fabricating metamaterials.

Acknowledgments

The research leading to these results has received funding from the European Union’s Seven Framework Programme (FP7/2007-2013) under grant agreement no. 228455 [NANOGOLD].

References and links

1.

L. V. Natarajan, R. L. Sutherland, V. P. Tondiglia, T. J. Bunning, and W. W. Adams, “Electro-optical switching characteristics of volume holograms in polymer dispersed liquid crystals,” J. Nonlinear Opt. Phys. Mater. 5(1), 89–98 (1996). [CrossRef]

2.

A. Veltri, R. Caputo, C. Umeton, and A. V. Sukhov, “Model for the photoinduced formation of diffraction gratings in liquid-crystalline composite materials,” Appl. Phys. Lett. 84, 3492–3494 (2004). [CrossRef]

3.

R. Caputo, L. De Sio, A. Veltri, C. Umeton, and A. V. Sukhov, “POLICRYPS switchable holographic grating: a promising grating electro-optical pixel for high resolution display application,” J. Display Technol. 2(1), 38–51 (2006). [CrossRef]

4.

A. d’Alessandro, D. Donisi, L. De Sio, R. Beccherelli, R. Asquini, R. Caputo, and C. Umeton, “Tunable integrated optical filter made of a glass ion-exchanged waveguide and an electro-optic composite holographic grating,” Opt. Express 16(13), 9254–9260 (2008). [CrossRef] [PubMed]

5.

R. Caputo, A. De Luca, L. De Sio, L. Pezzi, G. Strangi, C. Umeton, A. Veltri, R. Asquini, A. d’Alessandro, D. Donisi, R. Beccherelli, A. V. Sukhov, and N. V. Tabiryan, “POLICRYPS: a liquid crystal composed nano/microstructure with a wide range of optical and electro-optical applications,” J. Opt. A, Pure Appl. Opt. 11(2), 024017 (2009). [CrossRef]

6.

NANOGOLD project: “Self-organized Nanomaterials for tailored optical and electrical properties” (Seventh Framework Programme Theme, NMP-2008–2.2–2, Nano-structured metamaterials grant agreement no. 228455).

7.

R. Caputo, L. De Sio, A. Veltri, C. Umeton, and A. V. Sukhov, “Development of a new kind of switchable holographic grating made of liquid-crystal films separated by slices of polymeric material,” Opt. Lett. 29(11), 1261–1263 (2004). [CrossRef] [PubMed]

8.

L. De Sio, R. Caputo, A. De Luca, A. Veltri, C. Umeton, and A. V. Sukhov, “In situ optical control and stabilization of the curing process of holographic gratings with a nematic film-polymer-slice sequence structure,” Appl. Opt. 45(16), 3721–3727 (2006). [CrossRef] [PubMed]

9.

A. R. E. Brás, T. Casimiro, J. Caldeira, and A. Aguiar-Ricardo, “Solubility of the nematic liquid crystal E7 in supercritical carbon dioxide,” J. Chem. Eng. Data 50(6), 1857–1860 (2005). [CrossRef]

10.

S.-W. Joo, T. D. Chung, W. C. Jang, M.-S. Gong, N. Geum, and K. Kim, “Surface-enhanced Raman scattering of 4-Cyanobiphenyl on gold and silver nanoparticle surfaces,” Langmuir 18(23), 8813–8816 (2002). [CrossRef]

11.

I. Nicotera, C. Oliviero, G. Ranieri, A. Spadafora, M. Castriota, and E. Cazzanelli, “Temperature evolution of thermoreversible polymer gel electrolytes LiClO4/ethylene carbonate/poly(acrylonitrile),” J. Chem. Phys. 117(15), 7373–7380 (2002). [CrossRef]

12.

W. J. Jones, D. K. Thomas, D. W. Thomas, and G. Williams, “On the determination of order parameters for homogeneous and twisted nematic liquid crystals from Raman spectroscopy,” J. Mol. Struct. 708(1-3), 145–163 (2004). [CrossRef]

13.

E. W. Astrova, T. S. Perova, S. A. Grudinkin, V. A. Tolmachev, Yu. A. Pilyugina, V. B. Voronkov, and J. K. Vij, “Polarized infrared and Raman spectroscopic studies of the liquid crystal E7 alignment in composites based on grooved silicon,” Semiconductors 39(7), 759–767 (2005). [CrossRef]

14.

B. Pinto-Iguanero, A. Olivares-Perez, and I. Fuentes-Tapia, “Holographic material film composed by Norland Noa 65® adhesive,” Opt. Mater. 20(3), 225–232 (2002). [CrossRef]

15.

H. T. A. Wilderbeek, J. H. G. P. Goossens, C. W. M. Bastiaansen, and D. J. Broer, “Photoinitiated bulk polymerization of liquid crystalline thiolene monomers,” Macromolecules 35(24), 8962–8968 (2002). [CrossRef]

16.

M. Claudino, M. Johansson, and M. Jonsson, “Thiol–ene coupling of 1,2-disubstituted alkene monomers: the kinetic effect of cis/trans-isomer structures,” Eur. Polym. J. 46(12), 2321–2332 (2010). [CrossRef]

17.

H. F. Gleeson, C. D. Southern, P. D. Brimicombe, J. W. Goodby, and V. Görtz, “Optical measurements of orientational order in uniaxial and biaxial nematic liquid crystals,” Liq. Cryst. 37(6), 949–959 (2010). [CrossRef]

18.

J. K. Lim, O. Kwon, D. S. Kang, and S.-W. Joo, “Raman spectroscopy study and density functional theory calculations of the nematic liquid crystal 4-n-pentyl-4′-cyanobiphenyl under an electric field,” Chem. Phys. Lett. 423(1-3), 178–182 (2006). [CrossRef]

19.

W. J. Jones, D. K. Thomas, D. W. Thomas, and G. Williams, “Raman scattering studies of homogeneous and twisted-nematic liquid crystal cells and the determination of <P2> and <P4> order parameters,” J. Mol. Struct. 614(1-3), 75–85 (2002). [CrossRef]

20.

E. A. Büyüktanir, K. Zhang, A. Gericke, and J. L. West, “Raman imaging of nematic and smectic liquid crystals,” Mol. Cryst. Liq. Cryst. (Phila. Pa.) 487, 39–51 (2008). [CrossRef]

21.

A. Sanchez-Castillo, M. A. Osipov, and F. Giesselmann, “Orientational order parameters in liquid crystals: a comparative study of x-ray diffraction and polarized Raman spectroscopy results,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 81(2), 021707 (2010). [CrossRef] [PubMed]

22.

R. Caputo, I. Trebisacce, L. De Sio, and C. Umeton, “Jones matrix analysis of dichroic phase retarders realized in soft matter composite materials,” Opt. Express 18(6), 5776–5784 (2010). [CrossRef] [PubMed]

23.

R. L. Sutherland, “Polarization and switching properties of holographic polymer-dispersed liquid-crystal gratings. I. Theoretical model,” J. Opt. Soc. Am. B 19(12), 2995–3003 (2002). [CrossRef]

24.

R. L. Sutherland, L. V. Natarajan, V. P. Tondiglia, S. Chandra, C. K. Shepherd, D. M. Brandelik, S. A. Siwecki, and T. J. Bunning, “Polarization and switching properties of holographic polymer-dispersed liquid-crystal gratings. II. Experimental investigations,” J. Opt. Soc. Am. B 19(12), 3004–3012 (2002). [CrossRef]

25.

K. K. Vardanyan, J. Qi, J. N. Eakin, M. De Sarkar, and G. P. Crawford, “Polymer scaffolding model for holographic polymer-dispersed liquid crystals,” Appl. Phys. Lett. 81(25), 4736–4738 (2002). [CrossRef]

OCIS Codes
(050.1950) Diffraction and gratings : Diffraction gratings
(160.3710) Materials : Liquid crystals
(160.5470) Materials : Polymers
(300.6450) Spectroscopy : Spectroscopy, Raman

ToC Category:
Optical Devices

History
Original Manuscript: April 4, 2011
Revised Manuscript: May 5, 2011
Manuscript Accepted: May 7, 2011
Published: May 12, 2011

Citation
Marco Castriota, Angela Fasanella, Enzo Cazzanelli, Luciano De Sio, Roberto Caputo, and Cesare Umeton, "In situ polarized micro-Raman investigation of periodic structures realized in liquid-crystalline composite materials," Opt. Express 19, 10494-10500 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-11-10494


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References

  1. L. V. Natarajan, R. L. Sutherland, V. P. Tondiglia, T. J. Bunning, and W. W. Adams, “Electro-optical switching characteristics of volume holograms in polymer dispersed liquid crystals,” J. Nonlinear Opt. Phys. Mater. 5(1), 89–98 (1996). [CrossRef]
  2. A. Veltri, R. Caputo, C. Umeton, and A. V. Sukhov, “Model for the photoinduced formation of diffraction gratings in liquid-crystalline composite materials,” Appl. Phys. Lett. 84, 3492–3494 (2004). [CrossRef]
  3. R. Caputo, L. De Sio, A. Veltri, C. Umeton, and A. V. Sukhov, “POLICRYPS switchable holographic grating: a promising grating electro-optical pixel for high resolution display application,” J. Display Technol. 2(1), 38–51 (2006). [CrossRef]
  4. A. d’Alessandro, D. Donisi, L. De Sio, R. Beccherelli, R. Asquini, R. Caputo, and C. Umeton, “Tunable integrated optical filter made of a glass ion-exchanged waveguide and an electro-optic composite holographic grating,” Opt. Express 16(13), 9254–9260 (2008). [CrossRef] [PubMed]
  5. R. Caputo, A. De Luca, L. De Sio, L. Pezzi, G. Strangi, C. Umeton, A. Veltri, R. Asquini, A. d’Alessandro, D. Donisi, R. Beccherelli, A. V. Sukhov, and N. V. Tabiryan, “POLICRYPS: a liquid crystal composed nano/microstructure with a wide range of optical and electro-optical applications,” J. Opt. A, Pure Appl. Opt. 11(2), 024017 (2009). [CrossRef]
  6. NANOGOLD project: “Self-organized Nanomaterials for tailored optical and electrical properties” (Seventh Framework Programme Theme, NMP-2008–2.2–2, Nano-structured metamaterials grant agreement no. 228455).
  7. R. Caputo, L. De Sio, A. Veltri, C. Umeton, and A. V. Sukhov, “Development of a new kind of switchable holographic grating made of liquid-crystal films separated by slices of polymeric material,” Opt. Lett. 29(11), 1261–1263 (2004). [CrossRef] [PubMed]
  8. L. De Sio, R. Caputo, A. De Luca, A. Veltri, C. Umeton, and A. V. Sukhov, “In situ optical control and stabilization of the curing process of holographic gratings with a nematic film-polymer-slice sequence structure,” Appl. Opt. 45(16), 3721–3727 (2006). [CrossRef] [PubMed]
  9. A. R. E. Brás, T. Casimiro, J. Caldeira, and A. Aguiar-Ricardo, “Solubility of the nematic liquid crystal E7 in supercritical carbon dioxide,” J. Chem. Eng. Data 50(6), 1857–1860 (2005). [CrossRef]
  10. S.-W. Joo, T. D. Chung, W. C. Jang, M.-S. Gong, N. Geum, and K. Kim, “Surface-enhanced Raman scattering of 4-Cyanobiphenyl on gold and silver nanoparticle surfaces,” Langmuir 18(23), 8813–8816 (2002). [CrossRef]
  11. I. Nicotera, C. Oliviero, G. Ranieri, A. Spadafora, M. Castriota, and E. Cazzanelli, “Temperature evolution of thermoreversible polymer gel electrolytes LiClO4/ethylene carbonate/poly(acrylonitrile),” J. Chem. Phys. 117(15), 7373–7380 (2002). [CrossRef]
  12. W. J. Jones, D. K. Thomas, D. W. Thomas, and G. Williams, “On the determination of order parameters for homogeneous and twisted nematic liquid crystals from Raman spectroscopy,” J. Mol. Struct. 708(1-3), 145–163 (2004). [CrossRef]
  13. E. W. Astrova, T. S. Perova, S. A. Grudinkin, V. A. Tolmachev, Yu. A. Pilyugina, V. B. Voronkov, and J. K. Vij, “Polarized infrared and Raman spectroscopic studies of the liquid crystal E7 alignment in composites based on grooved silicon,” Semiconductors 39(7), 759–767 (2005). [CrossRef]
  14. B. Pinto-Iguanero, A. Olivares-Perez, and I. Fuentes-Tapia, “Holographic material film composed by Norland Noa 65® adhesive,” Opt. Mater. 20(3), 225–232 (2002). [CrossRef]
  15. H. T. A. Wilderbeek, J. H. G. P. Goossens, C. W. M. Bastiaansen, and D. J. Broer, “Photoinitiated bulk polymerization of liquid crystalline thiolene monomers,” Macromolecules 35(24), 8962–8968 (2002). [CrossRef]
  16. M. Claudino, M. Johansson, and M. Jonsson, “Thiol–ene coupling of 1,2-disubstituted alkene monomers: the kinetic effect of cis/trans-isomer structures,” Eur. Polym. J. 46(12), 2321–2332 (2010). [CrossRef]
  17. H. F. Gleeson, C. D. Southern, P. D. Brimicombe, J. W. Goodby, and V. Görtz, “Optical measurements of orientational order in uniaxial and biaxial nematic liquid crystals,” Liq. Cryst. 37(6), 949–959 (2010). [CrossRef]
  18. J. K. Lim, O. Kwon, D. S. Kang, and S.-W. Joo, “Raman spectroscopy study and density functional theory calculations of the nematic liquid crystal 4-n-pentyl-4′-cyanobiphenyl under an electric field,” Chem. Phys. Lett. 423(1-3), 178–182 (2006). [CrossRef]
  19. W. J. Jones, D. K. Thomas, D. W. Thomas, and G. Williams, “Raman scattering studies of homogeneous and twisted-nematic liquid crystal cells and the determination of <P2> and <P4> order parameters,” J. Mol. Struct. 614(1-3), 75–85 (2002). [CrossRef]
  20. E. A. Büyüktanir, K. Zhang, A. Gericke, and J. L. West, “Raman imaging of nematic and smectic liquid crystals,” Mol. Cryst. Liq. Cryst. (Phila. Pa.) 487, 39–51 (2008). [CrossRef]
  21. A. Sanchez-Castillo, M. A. Osipov, and F. Giesselmann, “Orientational order parameters in liquid crystals: a comparative study of x-ray diffraction and polarized Raman spectroscopy results,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 81(2), 021707 (2010). [CrossRef] [PubMed]
  22. R. Caputo, I. Trebisacce, L. De Sio, and C. Umeton, “Jones matrix analysis of dichroic phase retarders realized in soft matter composite materials,” Opt. Express 18(6), 5776–5784 (2010). [CrossRef] [PubMed]
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