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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 17, Iss. 22 — Oct. 26, 2009
  • pp: 19459–19469
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Tunable optical metamaterial based on liquid crystal-gold nanosphere composite

R. Pratibha, K. Park, I. I. Smalyukh, and W. Park  »View Author Affiliations


Optics Express, Vol. 17, Issue 22, pp. 19459-19469 (2009)
http://dx.doi.org/10.1364/OE.17.019459


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Abstract

Effect of the surrounding anisotropic liquid crystal medium on the surface plasmon resonance (SPR) exhibited by concentrated suspensions of gold nanospheres has been investigated experimentally and compared with the Mie scattering theory. The observed polarization-sensitive SPR and the red-shift in the SPR wavelength with increasing concentration of the gold nanospheres in the liquid crystal matrix have been explained using calculations based on the Maxwell Garnet effective medium theory. Agglomeration of the gold nanospheres that could also lead to such a red-shift has been ruled out using Atomic force microscopy study of thin nanoparticle-doped smectic films obtained on solid substrates. Our study demonstrates feasibility of obtaining tunable optical bulk metamaterials based on smectic liquid crystal - nanoparticle composites.

© 2009 OSA

1. Introduction

Research on metamaterials having tunable optical properties at the required spectral range, is of great current interest because of numerous potential applications. A recently proposed technique to obtain such materials is to have composites made of tunable materials. Metal nanostructures which exhibit strong plasmon resonance and liquid crystals possessing order and fluidity and having the capacity to respond to external fields that can influence their structure and properties, offer a very attractive combination. There have been some reports of combining liquid crystals and plasmonic metals involving techniques requiring complex fabrication processes. One of the earliest experiments used a silver film and liquid crystal (LC) interface and showed that a shift in the surface plasmon resonance frequency could be obtained by a voltage induced change in the refractive index of the liquid crystal [1

1. Y. Wang, “Voltage induced color selective absorption with surface plasmons,” Appl. Phys. Lett. 67(19), 2759–2761 (1995). [CrossRef]

]. A more recent design fabrication to obtain tunable negative index materials suggests the use of nanostrip pairs of silver separated by a liquid crystal layer [2

2. X. Wang, K. Do-Hoon, D. H. Werner, I. C. Khoo, A.V. Kildishev, and V. M. Shalaev, “Tunable optical negative-index metamaterials employing anisotropic liquid crystals,” Appl. Phys. Lett. 91, 1–3 (2007).

]. The refractive index tunability is achieved either by changing the orientation of the liquid crystal molecules by means of an external field or by varying the temperature.

The nematic liquid crystal formed by rigid rod-like molecules is characterized by a long range orientational order but no translational order [6

6. P. G. De Gennes, and J. Prost, The Physics of Liquid Crystals (Clarendon Press, Oxford 1995).

,7

7. S. Chandrasekhar, Liquid Crystals (Cambridge University Press, Cambridge 1992).

]. The principle axes of the molecules are oriented on an average along a direction called the director which is apolar in nature. In the smectic phase, in addition to the orientational order a one-dimensional translational order is present resulting in a layered arrangement of the molecules [6

6. P. G. De Gennes, and J. Prost, The Physics of Liquid Crystals (Clarendon Press, Oxford 1995).

,7

7. S. Chandrasekhar, Liquid Crystals (Cambridge University Press, Cambridge 1992).

]. In the smectic A phase the molecules are parallel to the layer normal with a liquid-like arrangement within the layers. Usually the nematic and smectic A phases made of rod-like molecules are uniaxial.

The possibility of obtaining refractive index tunability in a composite system consisting of a nematic liquid crystal with dispersed core-shell nanospheres has been suggested theoretically [8

8. I. C. Khoo, D. H. Werner, X. Liang, A. Diaz, and B. Weiner, “Nanosphere dispersed liquid crystals for tunable negative-zero-positive index of refraction in the optical and terahertz regimes,” Opt. Lett. 31(17), 2592–2594 (2006). [CrossRef] [PubMed]

]. Combination of the relative permittivities of the particles and the field – induced permittivity change in the liquid crystal gives rise to a tunable effective refractive index of the composite medium. However, no systematic experimental studies of such composites have been reported.

In this paper we describe our experimental studies of plasmonic effects exhibited by stable concentrated dispersions of gold nanoparticles in smectic liquid crystals [9

9. R. Pratibha, W. Park, and I. I. Smalyukh are preparing a manuscript to be called “Elasticity and layer structure stabilized colloidal nanoparticle dispersions in lamellar liquid crystals”

]. The effect of the surrounding anisotropic liquid crystal medium on the experimentally observed surface plasmon resonance wavelength (λmaxSPR) has been compared with the Mie scattering theory. The progressive red-shift in λmaxSPR observed with increasing volume fraction of the nanoparticles in the liquid crystal matrix is in agreement with calculations based on the extended Maxwell Garnet effective medium theory [10

10. V. Yannopapas and A. Moroz, “Negative refractive index metamaterials from inherently non-magnetic materials for deep infrared to terahertz frequency ranges,” J. Phys. Condens. Matter 17(25), 3717–3734 (2005). [CrossRef] [PubMed]

] and is further supported by the study of surface profiles of thin nanoparticle-doped smectic films.

2. Experimental techniques and materials

The absorption spectra were recorded using the Ocean Optics miniature fiber optic spectrometer (USB2000) in conjunction with a polarizing microscope. Atomic force microscopy studies were carried out in the tapping mode using Nanoscope III AFM from digital Instruments.

The liquid crystalline compound 4-n- octyl 4′-cyanobiphenyl (8CB, from Frinton Labs) which exhibits the phase sequence Cr 21 SmA 32 N 40.5 I (°C) has been used. The synthesis of gold nanoparticles was carried out by the conventional solution-based technique [11

11. J. J. Storhoff, R. Elghanian, R. C. Mucic, C. A. Mirkin, and R. L. Letsinger, “One-Pot Colorimetric Differentiation of Polynucleotides with Single Base Imperfections Using Gold Nanoparticle Probes,” J. Am. Chem. Soc. 120(9), 1959–1964 (1998). [CrossRef]

]. Briefly, auric acid (HAuCl4) was first dissolved in deionized water and a reducing agent such as sodium citrate was subsequently added to initiate homogeneous precipitation of metal nanoparticles. The pH and temperature were carefully controlled in order to obtain good size distribution. Spherical gold nanoparticles of mean diameter 14nm as measured with SEM were initially formed in aqueous medium. In order to improve the efficiency of distribution and stabilization in the liquid crystalline medium, the nanoparticles were first coated with the amphiphilic, nonionic polymer poly (N-vinyl-2-pyrrolidone) (PVP 10 from Aldrich). The nanoparticles were then transferred from the aqueous medium to the organic solvent ethyl alcohol, to facilitate better compatibility with the thermotropic liquid crystal used in our studies. Measured amount of the suspension was added to the liquid crystalline compound in order to obtain specific volume fractions of the PVP coated gold nanoparticles (GNPs) in the liquid crystal. The temperature was maintained at 24°C such that the liquid crystal is in the smectic A phase and the mixture of GNPs in ethyl alcohol and liquid crystal continuously stirred for about 5 hours. After evaporation of most of the ethyl alcohol the solution was filled into cells made of rubbed glass plates. The remaining alcohol was allowed to evaporate over a few hours. As the alcohol evaporated, well aligned regions of smectic A liquid crystal with a planar alignment having an overall orientation of the director along the rubbing direction were obtained. The absorption spectra in the smectic A phase were recorded at 24°C for all samples.

3. Results and discussion

The absorption spectrum with the PVP coated gold nanoparticles in ethyl alcohol was first recorded before addition of the liquid crystal. The wavelength corresponding to the absorption maximum was found to occur at 522nm. After the addition of this suspension to the liquid crystal and subsequent evaporation of the ethyl alcohol as described above, the optical textures exhibited by the liquid crystalline dispersions in the smectic A phase were observed under the polarizing microscope using the planar aligned cells. Uniform regions with planar alignment of smectic A liquid crystal were obtained. The optical textures as observed under the polarizing microscope were very similar to that obtained with pure 8CB when the concentration of GNPs was low as shown in Fig. 1a
Fig. 1 Focal conic texture observed in the smectic A phase of the LC-GNP dispersions viewed between crossed polarizers, (a) Φ = 0.21 and (b) Φ =0.54.
. However with a relatively large volume fraction (Φ) ~0.5 of the GNPs, the texture exhibited an intense green color but with the focal conic like features of the smectic A phase, remaining intact as shown in Fig. 1b.

Optical absorption spectra were then recorded using the planar aligned cells. The spectra show that there is a progressive shift of λmaxSPR to longer wavelengths with increasing volume fraction of the GNPs. For the purpose of illustration we have shown in Fig. 2
Fig. 2 Optical absorption spectra obtained with GNPs suspended in ethyl alcohol before addition of the liquid crystal and LC-GNP dispersions with Φ = 0.21 and Φ = 0.54.
the spectra for dispersions with Φ = 0.21 and Φ = 0.54 of GNPs in comparison to the spectrum obtained with the GNPs suspended in ethyl alcohol.

When the concentration of the GNPs is small the absorbance is also low leading to a marked difference in the peak absorbances for samples with Φ = 0.21 and Φ = 0.54. Though there should be a slight change in λmaxSPR due to the change in the host dielectric medium from ethyl alcohol to the smectic liquid crystal, the shift is mainly due to the coupling between SPR from individual particles which we later describe using the effective medium theory.

We have performed absorption measurements on planar aligned samples with a light beam polarized along (P=0°), and perpendicular (P=90°) to the average orientation of the director in the bulk of the sample. The two orientations of the polarizer P=0° and P=90° correspond to the refractive indexes neand n0of the liquid crystal, respectively. Taking the example of a sample with Φ =0.23 a difference of ~13 nm in λmaxSPRcould be observed for P=0° and P=90°, with the wavelength being red-shifted for P=0°, as shown in Fig. 3a
Fig. 3 (a) Optical spectra obtained with comparable volume fractions of GNPs (a) from experiment and (b) from calculations based on Mie scattering theory.
.

The red shift is often observed if the average refractive index of the medium increases. This increase can be expected, as in our planar aligned samples when the light beam is polarized parallel to the director the refractive index would mainly correspond to the higher refractive index ne. We have performed Mie scattering calculations [12

12. C. F. Bohren, and D. R. Huffman, Absorption and Scattering of Light by Small Particles (New York: Wiley-Interscience1983).

] assuming isotropic materials with refractive indexes taken to be n=1.7 and n=1.5, comparable to the extraordinary (ne) and ordinary (n0) refractive indexes of the liquid crystal 8CB used in our study. The experimentally determined refractive index of gold was used in these calculations [13

13. P. B. Johnson and R. W. Christy, “Optical Constants of the Noble Metals,” Phys. Rev. B 6(12), 4370–4379 (1972). [CrossRef]

]. As shown in Fig. 3b, a difference of 21nm is obtained in λmaxSPR for calculations with n=1.7 and n=1.5 with the wavelength being higher when n=1.7 is considered.

Mie scattering calculations show that the resonance wavelength shifts towards larger wavelengths as the particle size increases. Figure 6a
Fig. 6 Dependence of resonance wavelength on particle size ranging from r=20 to 150nm from Mie scattering calculations for refractive indexes (a) n=1.5 and (b) n=1.7.
and 6b show the dependence of scattering cross section as a function of wavelength for various particle sizes with background indexes n=1.5 and n=1.7. One possible reason for the red-shift observed in our samples with increasing concentration of the particles (Fig. 2) could be due to the agglomeration of the particles. Calculations show that a shift of the wavelength to ~630nm (for n=1.5) should correspond to a particle size of r ~60nm implying an agglomeration of approximately 600 of the r=7nm GNPs used in our study.

In order to confirm the absence of agglomeration and also probe the distribution of the nanoparticles in the LC+GNPs samples we have used atomic force microcopy [9

9. R. Pratibha, W. Park, and I. I. Smalyukh are preparing a manuscript to be called “Elasticity and layer structure stabilized colloidal nanoparticle dispersions in lamellar liquid crystals”

]. Inclusions immersed in smectic liquid crystals deform the layers resulting in a displacement of the layers along the layer normal [22

22. P. Sens and M. S. Turner, “Inclusions in thin smectic films”, J. Phys, II France 7(12), 1855–1870 (1997). [CrossRef]

25

25. G. Liao, I. I. Smalyukh, J. R. Kelly, O. D. Lavrentovich, and A. Jakli, “Electrorotation of colloidal particles in liquid crystals,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 72, 1–5 (2005).

]. When the LC-GNP dispersions are obtained in the form of thin (<100nm) films on Si substrates the nanoparticles give rise to such layer deformations resulting in raised hump like regions [9

9. R. Pratibha, W. Park, and I. I. Smalyukh are preparing a manuscript to be called “Elasticity and layer structure stabilized colloidal nanoparticle dispersions in lamellar liquid crystals”

], with each hump corresponding to an individual nanoparticle, as shown in Fig. 7
Fig. 7 AFM image of the hump like regions that correspond to layer deformations induced by the nanoparticles for a sample with Φ = 0.54.
. The AFM images show that there is no severe agglomeration of the particles and they remain stabilized and well dispersed in the smectic A phase. The nanoparticles appear to be stabilized by repulsive interactions between particles within the same layer and limited mobility across the layers forming stable dispersions in the SmA phase [9

9. R. Pratibha, W. Park, and I. I. Smalyukh are preparing a manuscript to be called “Elasticity and layer structure stabilized colloidal nanoparticle dispersions in lamellar liquid crystals”

].

The Maxwell Garnet effective medium theory can be used to characterize an inhomogeneous medium by treating the material as a homogeneous substance with an effective dielectric permittivity and effective magnetic permeability. These quantities depend on the properties of the constituents, as well as on their volume fractions and sizes. Following the extended Maxwell Garnett effective medium theory, we first calculated the polarizability of individual GNP using the Mie theory and then applied the Maxwell Garnett mixing rule to calculate the complex effective index for the composite structure [10

10. V. Yannopapas and A. Moroz, “Negative refractive index metamaterials from inherently non-magnetic materials for deep infrared to terahertz frequency ranges,” J. Phys. Condens. Matter 17(25), 3717–3734 (2005). [CrossRef] [PubMed]

]. From the imaginary part of the effective index we can obtain the expected absorption spectrum which can be directly compared with the experimental absorption spectrum.

In order to make comparisons with the effective medium theory a systematic study of the absorption profiles with increasing concentration of GNPs was carried out. Figure 8
Fig. 8 Comparison of the surface plasmon resonance wavelength for different volume fraction of GNPs from effective medium calculations (dotted line drawn as guide to the eye) and from experimental absorption spectra obtained with polarizer P=0° and P=90°.
shows a comparison between λmaxSPR obtained from the effective medium calculations and that obtained experimentally from absorption measurements, as a function of increasing volume fraction. A reasonably good agreement was observed between the trend obtained from simulations and experiment with λmaxSPR progressively increasing with increase in volume fraction of the GNPs. Due to reasons mentioned earlier the shift in λmaxSPRcorresponding to ne (P=0°) and n0 (P=90°) is not as pronounced in the experimental data as seen in the simulations. We note that the achieved GNP volume fraction is very high in this composite. It is generally very difficult to obtain well-dispersed GNPs at high concentrations because of the large van der Waals attraction between GNPs. A recent study on GNP dispersion in organic solvents achieved volume fractions of the order of 10−3 [5

5. S. Kubo, A. Diaz, Y. Tang, T. S. Mayer, I. C. Khoo, and T. E. Mallouk, “Tunability of the refractive index of gold nanoparticle dispersions,” Nano Lett. 7(11), 3418–3423 (2007). [CrossRef] [PubMed]

]. In our smectic LC-GNP composite, however, the interaction between GNPs and the surrounding liquid crystal layered structure appears to prevent aggregation of GNPs even at rather high volume fractions [9

9. R. Pratibha, W. Park, and I. I. Smalyukh are preparing a manuscript to be called “Elasticity and layer structure stabilized colloidal nanoparticle dispersions in lamellar liquid crystals”

]. This makes the smectic-GNP composite a highly promising material platform for novel metamaterial structures.

4. Conclusion

In conclusion, we have demonstrated the effect of an anisotropic medium on the surface plasmon resonance of the GNPs in LC-GNP dispersions. We have shown that the optical properties can be tuned by increasing the volume fraction of the GNPs. The observed dependence of the surface plasmon resonance wavelength on volume fraction of GNPs is in agreement with the effective medium theory. The realization of uniform dispersion of GNPs with high volume fraction enables the production of nanoparticle-based metamaterials, which provide an excellent alternative to nanolithographically fabricated metamaterials. While nanolithography tends to be slow and expensive, nanoparticle-based metamaterial can be fabricated by fast and cost-effective self-assembly methods. It is also straightforward to produce 3D metamaterial structures with nanoparticle dispersions, in contrast to the inherently 2D nature of lithographically fabricated structures. Extending our study to obtain stable dispersions of gold nanoparticles in ferroelectric smectic liquid crystals possessing inherent switching properties could enable tunable metamaterial architecture leading to interesting applications. Furthermore, the use of liquid crystal matrix automatically provides the possibility of dynamically tuning the metamaterial properties by external electric or optical fields, making the LC-GNP medium a highly promising metamaterial architecture.

Acknowledgements

References and links

1.

Y. Wang, “Voltage induced color selective absorption with surface plasmons,” Appl. Phys. Lett. 67(19), 2759–2761 (1995). [CrossRef]

2.

X. Wang, K. Do-Hoon, D. H. Werner, I. C. Khoo, A.V. Kildishev, and V. M. Shalaev, “Tunable optical negative-index metamaterials employing anisotropic liquid crystals,” Appl. Phys. Lett. 91, 1–3 (2007).

3.

G. Mie, “Beiträge zur Optik trüber Medien, speziell kolloidaler Metallösungen,” Ann. Phys. 330(3), 377–445 (1908). [CrossRef]

4.

U. Kreibig, M. Völlmer, Optical Properties of Metal Clusters (Springer-Verlag, Berlin 1995).

5.

S. Kubo, A. Diaz, Y. Tang, T. S. Mayer, I. C. Khoo, and T. E. Mallouk, “Tunability of the refractive index of gold nanoparticle dispersions,” Nano Lett. 7(11), 3418–3423 (2007). [CrossRef] [PubMed]

6.

P. G. De Gennes, and J. Prost, The Physics of Liquid Crystals (Clarendon Press, Oxford 1995).

7.

S. Chandrasekhar, Liquid Crystals (Cambridge University Press, Cambridge 1992).

8.

I. C. Khoo, D. H. Werner, X. Liang, A. Diaz, and B. Weiner, “Nanosphere dispersed liquid crystals for tunable negative-zero-positive index of refraction in the optical and terahertz regimes,” Opt. Lett. 31(17), 2592–2594 (2006). [CrossRef] [PubMed]

9.

R. Pratibha, W. Park, and I. I. Smalyukh are preparing a manuscript to be called “Elasticity and layer structure stabilized colloidal nanoparticle dispersions in lamellar liquid crystals”

10.

V. Yannopapas and A. Moroz, “Negative refractive index metamaterials from inherently non-magnetic materials for deep infrared to terahertz frequency ranges,” J. Phys. Condens. Matter 17(25), 3717–3734 (2005). [CrossRef] [PubMed]

11.

J. J. Storhoff, R. Elghanian, R. C. Mucic, C. A. Mirkin, and R. L. Letsinger, “One-Pot Colorimetric Differentiation of Polynucleotides with Single Base Imperfections Using Gold Nanoparticle Probes,” J. Am. Chem. Soc. 120(9), 1959–1964 (1998). [CrossRef]

12.

C. F. Bohren, and D. R. Huffman, Absorption and Scattering of Light by Small Particles (New York: Wiley-Interscience1983).

13.

P. B. Johnson and R. W. Christy, “Optical Constants of the Noble Metals,” Phys. Rev. B 6(12), 4370–4379 (1972). [CrossRef]

14.

G. M. Koenig Jr, M.-V. Meli, J. S. Park, J. J. de Pablo, and N. L. Abbott, “Coupling of the Plasmon resonances of chemically functionlized gold nanoparticles to local order in thermotropic liquid crystals,” Chem. Mater. 19(5), 1053–1061 (2007). [CrossRef]

15.

H. Stark, “Physics of colloidal dispersions in nematic liquid crystals,” Phys. Rep. 351(6), 387–474 (2001). [CrossRef]

16.

P. Poulin, H. Stark, T. C. Lubensky, and D. A. Weitz, “Novel colloidal interactions in anisotropic fluids,” Science 275(5307), 1770–1773 (1997). [CrossRef] [PubMed]

17.

R. W. Ruhwandl and E. M. Terentjev, “Long-range forces and aggregation of colloidal particles in a nematic liquid crystal,” Phys. Rev. E Stat. Phys. Plasmas Fluids Relat. Interdiscip. Topics 55(3), 2958–2961 (1997). [CrossRef]

18.

F. S. Y. Yeung, Y. L. J. Ho, Y. W. Li, and H. S. Kwok,“Liquid crystal alignment layer with controllable anchoring energies,” J. Display Tech 4(1), 24–27 (2008). [CrossRef]

19.

S. Y. Park and D. Stroud, “Surface-enhanced plasmon plitting in a liquid crystal-coated gold nanoparticle,” Phys. Rev. Lett. 94(21), 217401 (2005). [CrossRef] [PubMed]

20.

J. Müller, C. Sonnichsen, H. von Poschinger, G. von Plessen, T. A. Klar, and J. Feldmann, “Electrically controlled light scatterring with single metal nanoparticles,” Appl. Phys. Lett. 81(1), 171–173 (2002). [CrossRef]

21.

S. Y. Park and D. Stroud, “Splitting of surface plasmon frequencies of metal particles in a nematic liquid crystal,” Appl. Phys. Lett. 85(14), 2920–2922 (2004). [CrossRef]

22.

P. Sens and M. S. Turner, “Inclusions in thin smectic films”, J. Phys, II France 7(12), 1855–1870 (1997). [CrossRef]

23.

M. S. Turner and P. Sens, “Interactions between particulate inclusions in a smectic-A liquid crystal,” Phys. Rev. 55, R1275–R1278 (1997).

24.

C. D. Santangelo and R. D. Kamien, “Bogomol’nyi, Prasad, and Sommerfield Configurations in smectics,” Phys. Rev. Lett. 91(4), 045506 (2003). [CrossRef] [PubMed]

25.

G. Liao, I. I. Smalyukh, J. R. Kelly, O. D. Lavrentovich, and A. Jakli, “Electrorotation of colloidal particles in liquid crystals,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 72, 1–5 (2005).

OCIS Codes
(160.3710) Materials : Liquid crystals
(160.3918) Materials : Metamaterials
(160.4236) Materials : Nanomaterials

ToC Category:
Metamaterials

History
Original Manuscript: August 26, 2009
Revised Manuscript: October 4, 2009
Manuscript Accepted: October 4, 2009
Published: October 13, 2009

Citation
R. Pratibha, K. Park, I. I. Smalyukh, and W. Park, "Tunable optical metamaterial based on liquid crystal-gold nanosphere composite," Opt. Express 17, 19459-19469 (2009)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-22-19459


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References

  1. Y. Wang, “Voltage induced color selective absorption with surface plasmons,” Appl. Phys. Lett. 67(19), 2759–2761 (1995). [CrossRef]
  2. X. Wang, K. Do-Hoon, D. H. Werner, I. C. Khoo, A.V. Kildishev, and V. M. Shalaev, “Tunable optical negative-index metamaterials employing anisotropic liquid crystals,” Appl. Phys. Lett. 91, 1–3 (2007).
  3. G. Mie, “Beiträge zur Optik trüber Medien, speziell kolloidaler Metallösungen,” Ann. Phys. 330(3), 377–445 (1908). [CrossRef]
  4. U. Kreibig, M. Völlmer, Optical Properties of Metal Clusters (Springer-Verlag, Berlin 1995).
  5. S. Kubo, A. Diaz, Y. Tang, T. S. Mayer, I. C. Khoo, and T. E. Mallouk, “Tunability of the refractive index of gold nanoparticle dispersions,” Nano Lett. 7(11), 3418–3423 (2007). [CrossRef] [PubMed]
  6. P. G. De Gennes, and J. Prost, The Physics of Liquid Crystals (Clarendon Press, Oxford 1995).
  7. S. Chandrasekhar, Liquid Crystals (Cambridge University Press, Cambridge 1992).
  8. I. C. Khoo, D. H. Werner, X. Liang, A. Diaz, and B. Weiner, “Nanosphere dispersed liquid crystals for tunable negative-zero-positive index of refraction in the optical and terahertz regimes,” Opt. Lett. 31(17), 2592–2594 (2006). [CrossRef] [PubMed]
  9. R. Pratibha, W. Park, and I. I. Smalyukh are preparing a manuscript to be called “Elasticity and layer structure stabilized colloidal nanoparticle dispersions in lamellar liquid crystals.”
  10. V. Yannopapas and A. Moroz, “Negative refractive index metamaterials from inherently non-magnetic materials for deep infrared to terahertz frequency ranges,” J. Phys. Condens. Matter 17(25), 3717–3734 (2005). [CrossRef] [PubMed]
  11. J. J. Storhoff, R. Elghanian, R. C. Mucic, C. A. Mirkin, and R. L. Letsinger, “One-Pot Colorimetric Differentiation of Polynucleotides with Single Base Imperfections Using Gold Nanoparticle Probes,” J. Am. Chem. Soc. 120(9), 1959–1964 (1998). [CrossRef]
  12. C. F. Bohren, and D. R. Huffman, Absorption and Scattering of Light by Small Particles (New York: Wiley-Interscience1983).
  13. P. B. Johnson and R. W. Christy, “Optical Constants of the Noble Metals,” Phys. Rev. B 6(12), 4370–4379 (1972). [CrossRef]
  14. G. M. Koenig, M.-V. Meli, J. S. Park, J. J. de Pablo, and N. L. Abbott, “Coupling of the Plasmon resonances of chemically functionlized gold nanoparticles to local order in thermotropic liquid crystals,” Chem. Mater. 19(5), 1053–1061 (2007). [CrossRef]
  15. H. Stark, “Physics of colloidal dispersions in nematic liquid crystals,” Phys. Rep. 351(6), 387–474 (2001). [CrossRef]
  16. P. Poulin, H. Stark, T. C. Lubensky, and D. A. Weitz, “Novel colloidal interactions in anisotropic fluids,” Science 275(5307), 1770–1773 (1997). [CrossRef] [PubMed]
  17. R. W. Ruhwandl and E. M. Terentjev, “Long-range forces and aggregation of colloidal particles in a nematic liquid crystal,” Phys. Rev. E Stat. Phys. Plasmas Fluids Relat. Interdiscip. Topics 55(3), 2958–2961 (1997). [CrossRef]
  18. F. S. Y. Yeung, Y. L. J. Ho, Y. W. Li, and H. S. Kwok,“Liquid crystal alignment layer with controllable anchoring energies,” J. Display Tech 4(1), 24–27 (2008). [CrossRef]
  19. S. Y. Park and D. Stroud, “Surface-enhanced plasmon plitting in a liquid crystal-coated gold nanoparticle,” Phys. Rev. Lett. 94(21), 217401 (2005). [CrossRef] [PubMed]
  20. J. Müller, C. Sonnichsen, H. von Poschinger, G. von Plessen, T. A. Klar, and J. Feldmann, “Electrically controlled light scatterring with single metal nanoparticles,” Appl. Phys. Lett. 81(1), 171–173 (2002). [CrossRef]
  21. S. Y. Park and D. Stroud, “Splitting of surface plasmon frequencies of metal particles in a nematic liquid crystal,” Appl. Phys. Lett. 85(14), 2920–2922 (2004). [CrossRef]
  22. P. Sens and M. S. Turner, “Inclusions in thin smectic films,” J. Phys, II France 7(12), 1855–1870 (1997). [CrossRef]
  23. M. S. Turner and P. Sens, “Interactions between particulate inclusions in a smectic-A liquid crystal,” Phys. Rev. 55, R1275–R1278 (1997).
  24. C. D. Santangelo and R. D. Kamien, “Bogomol’nyi, Prasad, and Sommerfield Configurations in smectics,” Phys. Rev. Lett. 91(4), 045506 (2003). [CrossRef] [PubMed]
  25. G. Liao, I. I. Smalyukh, J. R. Kelly, O. D. Lavrentovich, and A. Jakli, “Electrorotation of colloidal particles in liquid crystals,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 72, 1–5 (2005).

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