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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 23 — Nov. 7, 2011
  • pp: 23532–23537
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Silicon oxide deposition for enhanced optical switching in polydimethylsiloxane-liquid crystal hybrids

Luciano De Sio, Andreas E. Vasdekis, Julien G. Cuennet, Antonio De Luca, Alfredo Pane, and Demetri Psaltis  »View Author Affiliations


Optics Express, Vol. 19, Issue 23, pp. 23532-23537 (2011)
http://dx.doi.org/10.1364/OE.19.023532


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Abstract

We report an optical switch based on a diffraction grating by combining PDMS microstructures with a photo-responsive Nematic Liquid Crystal (NLC). The grating was realized via replica molding and was subsequently coated with a thin SiO layer. SiO induced a full planar alignment of the liquid crystal. The induced parallel alignment of the LC reduces the response time of the structure by approximately an order of magnitude compared to the same structures without SiO. We explored the effect of the pump intensity on the transmission properties and time response of the switch and identified a strong dependence on the probe polarization, due to the full planar alignment in this structure. The aforementioned inclusion of the SiO layer enables enhanced performance of optical devices based on the fusion of nematogens with soft and flexible substrates.

© 2011 OSA

1. Introduction

2. Sample preparation

Here, we demonstrate the possibility to employ the SiO deposition in order to induce the full-planar alignment of LC on PDMS. To obtain a good planar alignment on a non-rigid substrate (PDMS surface), we utilized a Physical Vapour Deposition (PVD) technique which consists in a vacuum deposition method that deposits thin films by condensation of a vaporized form of the material onto various surfaces. In particular, we used an evaporative deposition method (evaporator equipment is an assembling of a rotative and turbomolecular pumps provided by Edwards; while the electronic controller is given by frequency counter, quartz crystal thin film provided by Balzers) in which the material to be deposited is heated to a high vapour pressure by electrically resistive heating at low pressure. We used SiO at a pressure of 1.4*10−6 mbar, at a current of about 225 A for a deposition time of 2 minutes. In this way we obtained a SiO film on PDMS microstructure with a thickness of about 30 nm. The NLC was the E7 (Merck) doped with MR at a 2% ratio in weigh. This suitable ratio has been chosen by taking into account the solubility of MR in NLC and the light sensitivity of the mixture in the visible range. The PDMS grating was fabricated by direct electron beam lithography on SU8 (Vistec EBPG5000, 100 kV acceleration voltage, at a dose of 5 µCb/cm2) and subsequent pattern transfer to PDMS via conventional cast molding. The grating period was 2μm and its thickness approximately 1.5 µm.

3. Experimental results

In Fig. 1
Fig. 1 Polarized transmission spectrum (polarization vector along the long axes of the NLC) of the mixture realized with an optical spectrometer. Inset: photo of the mixture taken with a 20X objective.
we report the transmission spectrum of the mixture which exhibits a high absorption around 500 nm range while in the inset we show a polarized optical microscope view of the mixture in planar alignment in a glass cell treated with a thin “polyimide” layer, where a preferred planar direction was induced via rubbing.

As is shown in the inset of Fig. 1, despite the low concentration of MR (2%) dissolved in NLC some agglomerates are still present due to the low solubility of the MR in liquid-crystalline materials. In any case, in order to minimize the presence of MR agglomerate, we have selected by means of optical microscope a large area of the structure were the presence of MR clusters was negligible. Based on polarization transmission spectra the rode-shaped MR is preferentially aligned in the same direction as the NLC molecular director. In fact, the anisotropic absorption coefficient of MR exhibits, in the case of NLC full in-plane aligned, an extinction ratio of about 0.3 for light polarized along the long axes of the NLC. On the other side, its value is reduced up to 0.15 for light polarized along the short axes of the NLC. At the same time, we have investigated the absorption coefficient of MR for a sample with hybrid alignment; values are 0.26 and 0.19 for light polarized along the long and short axes of the NLC respectively. This result is a proof that in both experimental conditions (NLC full in plane and hybrid alignment) the absorption coefficients are comparable and the MR molecules are preferentially aligned in the same direction of the NLC molecules. On the other hand, this low concentration enables light responsivity of the dye-doped NLC with a low pump power density (20-30 mW/cm2). The light sensitive NLC was “sandwiched” between the PDMS microstructure treated with the SiO layer and a modified glass substrate treated with a “rubbed” polyimide layer as is shown in Fig. 2a
Fig. 2 View of the grating sample (a) and corresponding SEM image (b). In (c) is reported an optical microscope view of the sample along with the far field diffraction pattern obtained by probing with a He-Ne laser the flexible grating (d).
.

The two surfaces were brought into contact via 1.5 μm glass microspheres while the sensitive NLC was introduced by capillary flow in the isotropic phase (65 °C). The sample was slowly (0.5 deg/min) brought below the isotropic- nematic transition point (typically, down to room temperature) in order to drive the NLC orientation. The scanning electron microscopy (SEM) picture of the grating structure after the SiO deposition is shown in Fig. 2b while the Polarized Optical Microscope (POM) view of the sample infiltrated with light-sensitive NLC is shown in Fig. 2c. In order to characterize the NLC alignment, we performed optical experiments by using the probe-pump setup already described in [15

15. L. De Sio, A. Veltri, C. Umeton, S. Serak, and N. Tabiryan, “All-optical switching of holographic gratings made of polymer-liquid-crystal-polymer slices containing azo-compounds,” Appl. Phys. Lett. 93(18), 181115 (2008). [CrossRef]

]. We monitored in Fig. 3
Fig. 3 Normalized diffracted Intensity versus the polarization angle of the impinging probe beam for different values of the external pump power.
the first-order diffraction efficiency behavior versus the polarization angle of the impinging probe radiation (He-Ne laser, λ = 633 nm) for different values of the external pump power (Diode laser, λ = 532 nm). Probe light polarized orthogonal to the grating stripes (p-polarization) will experience a high index contrast (ne-npdms~0.3) while light polarized along the grating stripes will experiences a low index contrast (no-npdms~0.1). Where, we have indicated with ne, no, the extraordinary and ordinary refraction index of the NLC respectively while npdms is the refractive index of the PDMS material.

Figure 4 clearly shows the strong correlation between the variation of the diffraction efficiency and the external pump power value. At the same time, the response time is a function of the impinging pump power. This behavior can be explained by assuming the rate of concentration of photoisomerized molecules is proportional to the incident intensity.

4. Conclusions

In conclusion, we report the alignment of a NLC on a flexible PDMS substrate in planar alignment. We accomplished this by coating the PDMS diffraction grating with a thin SiO layer. This surface treatment combined with a modified cover glass enables the full in plane alignment of the NLC. All-optical experiments were carried out to characterize and monitor the versatility of the process and showed that the SiO coated gratings exhibit an enhanced time-domain performance compared with the bare PDMS surfaces.

Acknowledgments

Authors are grateful to: Dr. Giovanni Desiderio for his help in the SEM analysis. The SEM utilized in this research belongs to the equipment acquired for the realization of the Project N° 1987/37, Umeton: “costituzione del Laboratorio Italiano Cristalli Liquidi”- PON: Misura II.1 Azione a - Interventi infrastrutturali (avviso 68) funded by Ministry of Education, Universities and Research (MIUR)

References and links

1.

J. G. Cuennet, A. E. Vasdekis, L. De Sio, and D. Psaltis, “Optofluidic modulator based on peristaltic nematogen microflows,” Nat. Photonics 5(4), 234–238 (2011). [CrossRef]

2.

L. De Sio, J. G. Cuennet, A. E. Vasdekis, and D. Psaltis, “All-optical switching in an optofluidic polydimethylsiloxane: Liquid crystal grating defined by cast-molding,” Appl. Phys. Lett. 96(13), 131112 (2010). [CrossRef]

3.

D. C. Duffy, J. C. McDonald, O. J. A. Schueller, and G. M. Whitesides, “Rapid Prototyping of Microfluidic Systems in Poly(dimethylsiloxane),” Anal. Chem. 70(23), 4974–4984 (1998). [CrossRef] [PubMed]

4.

D. Psaltis, S. R. Quake, and C. Yang, “Developing optofluidic technology through the fusion of microfluidics and optics,” Nature 442(7101), 381–386 (2006). [CrossRef] [PubMed]

5.

P. G. De Gennes and J. Prost, The Physics of Liquid Crystals (Oxford Science Publications, 1994)

6.

I. C. Khoo, M. Wood, M. Y. Shih, and P. Chen, “Extremely nonlinear photosensitive liquid crystals for image sensing and sensor protection,” Opt. Express 4(11), 432–442 (1999). [CrossRef] [PubMed]

7.

T. Ikeda and O. Tsutsumi, “Optical switching and image storage by means of azobenzene liquid-crystal films,” Science 268(5219), 1873–1875 (1995). [CrossRef] [PubMed]

8.

A. Urbas, V. Tondiglia, L. Natarajan, R. Sutherland, H. Yu, J.-H. Li, and T. Bunning, “Optically switchable liquid crystal photonic structures,” J. Am. Chem. Soc. 126(42), 13580–13581 (2004). [CrossRef] [PubMed]

9.

Y. J. Liu, Y. B. Zheng, J. Shi, H. Huang, T. R. Walker, and T. J. Huang, “Optically switchable gratings based on azo-dye-doped, polymer-dispersed liquid crystals,” Opt. Lett. 34(15), 2351–2353 (2009). [CrossRef] [PubMed]

10.

D. E. Lucchetta, F. Vita, and F. Simoni, “All-optical switching of diffraction gratings infiltrated with dye-doped liquid crystals,” Appl. Phys. Lett. 97(23), 231112 (2010). [CrossRef]

11.

A. Urbas, J. Klosterman, V. Tondiglia, L. Natarajan, R. Sutherland, O. Tsutsumi, T. Ikeda, and T. Bunning, “Optically Switchable Bragg Reflectors,” Adv. Mater. (Deerfield Beach Fla.) 16(16), 1453–1456 (2004). [CrossRef]

12.

C. H. Wen, S. Gauza, and S. T. Wu, “Ultraviolet stability of liquid crystals containing cyano and isothiocyanato terminal groups,” Liq. Cryst. 31(11), 1479–1485 (2004). [CrossRef]

13.

L. De Sio, S. Serak, N. Tabiryan, S. Ferjani, A. Veltri, and C. Umeton, “Composite holographic gratings containing light-responsive liquid crystals for visible bichromatic switching,” Adv. Mater. (Deerfield Beach Fla.) 22(21), 2316–2319 (2010). [CrossRef] [PubMed]

14.

P. K. Son, J. H. Park, J. C. Kim, and T. H. Yoon, “Control of liquid crystal alignment by deposition of silicon oxide thin film,” Thin Solid Films 515(5), 3102–3106 (2007). [CrossRef]

15.

L. De Sio, A. Veltri, C. Umeton, S. Serak, and N. Tabiryan, “All-optical switching of holographic gratings made of polymer-liquid-crystal-polymer slices containing azo-compounds,” Appl. Phys. Lett. 93(18), 181115 (2008). [CrossRef]

16.

T. K. Gaylord and M. G. Moharam, “Thin and thick gratings: terminology clarification,” Appl. Opt. 20(19), 3271–3273 (1981). [CrossRef] [PubMed]

17.

L. De Sio, S. Serak, N. Tabiryan, and C. Umeton, “Mesogenic versus non-mesogenic azo dye confined in a soft-matter template for realization of optically switchable diffraction gratings,” J. Mater. Chem. 21(19), 6811–6814 (2011). [CrossRef]

18.

C. R. Lee, T. L. Fu, K. T. Cheng, T. S. Mo, and A. Y. G. Fuh, “Surface-assisted photoalignment in dye-doped liquid-crystal films,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 69(3), 031704 (2004). [CrossRef] [PubMed]

19.

E. Ouskova, Yu. Reznikov, S. V. Shiyanovskii, L. Su, J. L. West, O. V. Kuksenok, O. Francescangeli, and F. Simoni, “Photo-orientation of liquid crystals due to light-induced desorption and adsorption of dye molecules on an aligning surface,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 64(5), 051709 (2001). [CrossRef] [PubMed]

OCIS Codes
(000.2700) General : General science
(050.1950) Diffraction and gratings : Diffraction gratings
(160.3710) Materials : Liquid crystals
(230.1150) Optical devices : All-optical devices
(130.4815) Integrated optics : Optical switching devices
(160.5335) Materials : Photosensitive materials

ToC Category:
Optical Devices

History
Original Manuscript: August 10, 2011
Revised Manuscript: October 6, 2011
Manuscript Accepted: October 10, 2011
Published: November 3, 2011

Citation
Luciano De Sio, Andreas E. Vasdekis, Julien G. Cuennet, Antonio De Luca, Alfredo Pane, and Demetri Psaltis, "Silicon oxide deposition for enhanced optical switching in polydimethylsiloxane-liquid crystal hybrids," Opt. Express 19, 23532-23537 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-23-23532


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References

  1. J. G. Cuennet, A. E. Vasdekis, L. De Sio, and D. Psaltis, “Optofluidic modulator based on peristaltic nematogen microflows,” Nat. Photonics5(4), 234–238 (2011). [CrossRef]
  2. L. De Sio, J. G. Cuennet, A. E. Vasdekis, and D. Psaltis, “All-optical switching in an optofluidic polydimethylsiloxane: Liquid crystal grating defined by cast-molding,” Appl. Phys. Lett.96(13), 131112 (2010). [CrossRef]
  3. D. C. Duffy, J. C. McDonald, O. J. A. Schueller, and G. M. Whitesides, “Rapid Prototyping of Microfluidic Systems in Poly(dimethylsiloxane),” Anal. Chem.70(23), 4974–4984 (1998). [CrossRef] [PubMed]
  4. D. Psaltis, S. R. Quake, and C. Yang, “Developing optofluidic technology through the fusion of microfluidics and optics,” Nature442(7101), 381–386 (2006). [CrossRef] [PubMed]
  5. P. G. De Gennes and J. Prost, The Physics of Liquid Crystals (Oxford Science Publications, 1994)
  6. I. C. Khoo, M. Wood, M. Y. Shih, and P. Chen, “Extremely nonlinear photosensitive liquid crystals for image sensing and sensor protection,” Opt. Express4(11), 432–442 (1999). [CrossRef] [PubMed]
  7. T. Ikeda and O. Tsutsumi, “Optical switching and image storage by means of azobenzene liquid-crystal films,” Science268(5219), 1873–1875 (1995). [CrossRef] [PubMed]
  8. A. Urbas, V. Tondiglia, L. Natarajan, R. Sutherland, H. Yu, J.-H. Li, and T. Bunning, “Optically switchable liquid crystal photonic structures,” J. Am. Chem. Soc.126(42), 13580–13581 (2004). [CrossRef] [PubMed]
  9. Y. J. Liu, Y. B. Zheng, J. Shi, H. Huang, T. R. Walker, and T. J. Huang, “Optically switchable gratings based on azo-dye-doped, polymer-dispersed liquid crystals,” Opt. Lett.34(15), 2351–2353 (2009). [CrossRef] [PubMed]
  10. D. E. Lucchetta, F. Vita, and F. Simoni, “All-optical switching of diffraction gratings infiltrated with dye-doped liquid crystals,” Appl. Phys. Lett.97(23), 231112 (2010). [CrossRef]
  11. A. Urbas, J. Klosterman, V. Tondiglia, L. Natarajan, R. Sutherland, O. Tsutsumi, T. Ikeda, and T. Bunning, “Optically Switchable Bragg Reflectors,” Adv. Mater. (Deerfield Beach Fla.)16(16), 1453–1456 (2004). [CrossRef]
  12. C. H. Wen, S. Gauza, and S. T. Wu, “Ultraviolet stability of liquid crystals containing cyano and isothiocyanato terminal groups,” Liq. Cryst.31(11), 1479–1485 (2004). [CrossRef]
  13. L. De Sio, S. Serak, N. Tabiryan, S. Ferjani, A. Veltri, and C. Umeton, “Composite holographic gratings containing light-responsive liquid crystals for visible bichromatic switching,” Adv. Mater. (Deerfield Beach Fla.)22(21), 2316–2319 (2010). [CrossRef] [PubMed]
  14. P. K. Son, J. H. Park, J. C. Kim, and T. H. Yoon, “Control of liquid crystal alignment by deposition of silicon oxide thin film,” Thin Solid Films515(5), 3102–3106 (2007). [CrossRef]
  15. L. De Sio, A. Veltri, C. Umeton, S. Serak, and N. Tabiryan, “All-optical switching of holographic gratings made of polymer-liquid-crystal-polymer slices containing azo-compounds,” Appl. Phys. Lett.93(18), 181115 (2008). [CrossRef]
  16. T. K. Gaylord and M. G. Moharam, “Thin and thick gratings: terminology clarification,” Appl. Opt.20(19), 3271–3273 (1981). [CrossRef] [PubMed]
  17. L. De Sio, S. Serak, N. Tabiryan, and C. Umeton, “Mesogenic versus non-mesogenic azo dye confined in a soft-matter template for realization of optically switchable diffraction gratings,” J. Mater. Chem.21(19), 6811–6814 (2011). [CrossRef]
  18. C. R. Lee, T. L. Fu, K. T. Cheng, T. S. Mo, and A. Y. G. Fuh, “Surface-assisted photoalignment in dye-doped liquid-crystal films,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys.69(3), 031704 (2004). [CrossRef] [PubMed]
  19. E. Ouskova, Yu. Reznikov, S. V. Shiyanovskii, L. Su, J. L. West, O. V. Kuksenok, O. Francescangeli, and F. Simoni, “Photo-orientation of liquid crystals due to light-induced desorption and adsorption of dye molecules on an aligning surface,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys.64(5), 051709 (2001). [CrossRef] [PubMed]

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