Chapter 6 Surface plasmons induced extra diffraction band of cholesteric liquid
6.2 Surface plasmon effect induced by CLC grating environment
6.2.1 Fabrication of CLC grating with Ag nanoparticles
The nematic liquid crystal E7 was doped with the chiral agent CB 15. Both E7 and CB15 were purchased from E. Merck. The nematic phase of E7 ranged from the temperature of -100C to 60.50C. The extraordinary and ordinary refractive indices (ne
and no) were 1.7354 and 1.5175, respectively. They were measured at the wavelength of λ=644 nm and the temperature of T=200C. The helical twist power (HTP) of CB15 was +7.3 m-1 at 200C for E7. The cell gap was 6 μm. In this study, the CLC cell was constructed with d/p ~ 1 for good grating structure [12, 13], where d and p were the cell gap and the helix pitch of CLC material, respectively. In order to control the helix pitch of cholesteric liquid crystal, the weight percentage of CB15 was 2.28 %.
The CLC-grating device was assembled with two ITO glass plates. One was spin-coated with a polyimide layer and rubbed unidirectionaly for homogeneous alignment. The other was thermally deposited with a 40Å-thicked silver film. The silver film was nucleated by annealing the film at 2500C in 1 min to form the silver nanoparticles on the plate surface. For comparison, another CLC planar cell was assembled with two ITO plates. Instead of the deposition of the silver film on one plate, these two plates were spin-coated with a polyimide layer and rubbed in opposite directions for homogeneous alignment.
Figure 6.7: (a) and (b) show the SEM pictures of silver thin films before and after annealing. (c) The AFM picture of the annealed film shows the size of silver nano-particle is ~50 nm.
Figure 6.7 (a) and (b) show the scanning electronic microscope (SEM) pictures of the silver thin films on the glass plates before and after annealing, respectively.
With the application of the atomic-force-microscope (AFM) to probe the silver nano-particles on the substrate, the silver nanoparticles are approximately 50 nm in size. Figure 6.7 (c) shows the three-dimensional morphology of the silver film. The metal nano-particles in this scale can exhibit high localized plasma resonance.
Figure 6.8: (a) Cholesteric liquid crystal cell with silver nanoparticles distributed on one plate of the cell, (b) the CLC grating structure well-formed with the application of a proper voltage to the cell.
The prepared CLC material was injected into the empty cell to form a planar CLC device. The helix axis of the initial structure of CLC planar cell was parallel along the direction of z-axis as shown in Figure 6.8 (a). After applying a voltage of 2.7 V (frequency = 1 kHz) to the cell, the helix axis was reoriented to y-axis and the grating structure was well-formed in the cell as shown in the Figure 6.8(b). The periodic stripe pattern parallel to the x-axis was observed with an optical microscope.
The grating spacing inspected with the microscope was about 8 μm as shown in Figure 6.9. The grating was probed by a He-Ne laser of 632.8 nm, and the diffraction pattern was significant as shown in the insert of Figure 6.9.
Figure 6.9: Cholesteric liquid crystal grating structure was observed with an optical microscope and the diffraction pattern shown in the top (probed by the He-Ne laser of 632.8 nm).
6.2.2 Extra diffraction band induced by surface plasmon effect
The CLC grating was probed by a monochromatic beam, which was emerged from a monochromator. The white light source of the monochromator was a tungsten lamp. Lens and polarizer were used to collimate the light beam and to polarize the monochromatic light, respectively. The polarization of incident beam was set parallel to the rubbing direction. The first-order diffraction efficiencies were measured by an optical detector. The detector was set close to the test sample, thus the entire first-order diffraction could be covered by the large detector head (opening area:
1cm2). An iris was put in the front of the optical detector to exclude the other-order diffractions.
Figure 6.10: Experimental setup for measuring the first-order diffraction of the CLC grating.
The CLC grating cells were probed by the polarized-monochromatic beam in this study. The range of the probing wavelength was from 450 nm to 700 nm. The first-order diffraction efficiencies of the CLC gratings with respective to the wavelength were shown in the Figure 6.10. In addition, both samples presented oscillatory diffraction efficiencies, but apparent different diffraction efficiencies exhibited on the different treated CLC grating cells with or without silver nanoparticles distributed on the glass plate surface. Without the silver nanoparticles on the glass plate for the CLC grating cell, the distribution of high diffraction efficiencies was observed around 681 nm. The diffraction efficiencies attributed from the intrinsic CLC grating.
However, the distribution of the diffraction efficiencies of the CLC grating with the silver nanoparticles on the glass plate was distinct from that without silver nanoparticles. The distribution spectrum shows two peaks around λ = 654 nm and λ = 505 nm, respectively. At the long wavelength side around λ = 654 nm, the diffraction efficiencies in the spectrum behaved similarly to that without silver nanoparticles. The diffraction arose from the intrinsic CLC grating. The extra band of diffraction efficiencies around λ = 507 nm would contributed by the modulation of surface plasmons emerged from the silver nanoparticles.
Figure 6.11: The first-order diffraction efficiency versus wavelength of the CLC grating with and without the silver nanoparticles distributed on one plate of the cell, and the absorbance of the silver nanoparticles covered with E7 liquid crystal is shown in the insert illustration.
6.3 Theoretical model of periodic surrounding environment
It is noted that based on the theory of Raman-Nath’s diffraction, the first-order diffraction efficiency (η ) can be expressed as follows1 14, 15,
2 1 1
1 1
1 exp( 2 α ds)J (2π n d λ i α ds)
η = − Δ Δ − Δ (1) where the J1 is the first kind Bessel function, the ds is the thickness of the silver nanoparticle layer, the d is the thickness of the grating, and the λ is the wavelength.
The Δn1 and Δα1 are the modulation of refractive-index and the modulation of absorption of the grating, respectively. The diffraction efficiency is dependent on the modulation of refractive-index (Δn1) and the modulation of absorption (Δα1). In Eq.1, the diffraction efficiency is an oscillatory function of d λ. Oscillatory diffraction efficiency thus appeared in the wavelength range from 450 to 700 nm.
As for the extra enhanced diffraction efficiencies appeared around λ = 507 nm, we ascertained that the absorption characteristic due to the surface plasmons emerged from the silver nanoparticles on the glass plate of the CLC grating was determined the diffraction behavior of the grating. The absorbance of the silver nanoparticles in the
CLC grating cell was measured to estimate the modulation of absorption (Δα1). The absorbance is defined as A = - log (T/T0), where the T and T0 were the transmittances of the LC (E7) cell with or without the silver nanoparticles on the glass plate. The cells were treated in the mode of homogeneous alignment. It is noted that there exists a broad absorbance bump around 500 nm as shown in the inserted illustration in Figure 6.11. The localized surface plasmon was excited on the sliver nanoparticles, and then caused the strong light scattering around 500 nm. These surface plasmons of silver nanoparticles led to the attenuation of the incident beam in the wavelength range around λ = 500 nm. The range of high absorbance corresponded to the enhanced diffraction band in the spectrum. The localized surface plasmon effect was considered to result in the extra diffraction efficiency.
Furthermore, the silver nanoparticles were covered by a CLC grating, the incident polarization light encountered different dielectric properties at the interface between the metal film and the LC media. The characteristics of surface plasmons of the metallic nanoparticles were strongly affected by the surrounding dielectric medium. The localized surface plasmons under the grating stripes would exhibit different spectral intensity distribution. Detailed analysis should consider that the absorbance generated by the CLC grating environment is dependent of wavelength.
A theoretical model was presented to demonstrate the extra enhanced diffraction efficiencies appeared around λ = 507 nm due to the silver nanoparticles in the CLC grating cell. In the model, the modulation of absorption (Δα1) induced by CLC grating environment was estimated by the theoretical absorbance of silver nanoparticles. In the experiment, the CLC grating with a spatial periodic refraction index distribution covered on the silver nanoparticles. It was assumed that the silver nanoparticles spatially experienced the environments with refraction indices of 1.74 (n) and 1.52 (n ), respectively. TheΔα was the difference between the absorbance
spectra with respect to the n=1.74 and n=1.52. The calculation of absorbance and the diffraction efficiency of CLC grating without and with silver nanoparticloes were discussed as follows.
By heating silver film, the oblate spheroidal shape silver nanoaprticles were formed to lie on ITO substrate [17]. One neglected the interaction among spheroids, the total cross sections of silver nanoparticles in s- and p-polarization are given as follows [17-19] where theε
( )
ω is the complex dielectric response function of material, the a is the focal length of the spheroid, the θ is the angle between the incident light and x-y plane, as shown in the insert of Fig. 5. The parameter of η0=R (1−R2)1/2 determines the shape of the spheroid, the R is the ratio of the minor axis to the major axis of the spheroid. The ε1,0, and ε1,1, are the responses to polarizability component. And the0 ,
Q , and 1 Q are the associated Legendre function of second kind, respectively. They 1,1 are derived from the solution of Laplace’s equation in oblate spheroid coordinate and
one dipole approximation. The detail derivation is referred to the previous work of T.
L. Ferrell et al. [17-19].
Based on the information of total cross section, the optical absorbance A of silver nanoparticles can be written as the following expression
⎟⎠
where the N is the density of spheroids in a unit of surface area. The value of N can be determined by an analysis of scanning electron micrograph. So far, the absorbance of silver nanoparticles can be determined by the particles size, shape, and density. In addition, the environment effect on surface plasmons was important to the absorbance property of silver nanoparticles.
Since the silver nanoparticles layer was covered by top CLC grating and bottom ITO substrate, the environment effect is significant in the absorbance calculation. We assumed that the ITO substrate and the CLC grating were a thin uniform dielectric film to cover the silver nanoparticles. The following dispersion relation was applied to modify the absorbance property affected by environment effect [20].
( ) ( )
( ) 0( )
2 0( )
1,1 1,0 0 environment, respectively. The optical constant of element was found in Palik’s work and the εs was the average dielectric function of nematic liquid crystal and ITO substrate [21]. The substitute values of ε1,0andε1,1 were obtained from the solution of this quadratic equation to modify the absorbance characteristics of silver nanoparticles.Figure 6.12: Theoretical absorbance of silver nanoparticles surrounded by the environment with refractive indices of 1.52 and 1.74. The insert shows the silver nanoparticles layer was posited between the CLC grating and ITO plate. The particles experienced the environments with index of ne and no.
Figure 6.6 shows the theoretical absorbance spectra of silver nanoparticles encounted by the environment with the refractive indices of 1.52 and 1.74. The insert presents the silver nanoparticles layer are posited between the CLC grating and ITO plate. The particles experience the environments with index of ne and no. The incident angle (θ), ratio of the minor axis to the major axis (R), and focus length (a) are 00, 0.6, and 40 nm, respectively. It was noted that the absorbance was a function of wavelength. The maximum absorbance occurred at around 476 and 500 nm due to the refraction index of 1.52 and 1.74, respectively. The difference in the optical absorbance spectra of silver nanoparticles led to an amplitude modulation of Δα1 occurred in the CLC cell. Since Δα1 is strongly dependent on the wavelength and that is the cause of the extra band around 500 nm in diffraction efficiency.
Figure 6.13: Theoretical diffraction efficiencies show the extra diffraction band of CLC grating with silver nanoparticles.
Figure 6.7 shows the theoretical diffraction efficiency of CLC grating without and with the silver nanoparticles. We employed the theoretical equation (1) to draw the theoretical plot of the diffraction efficiency with respective to the wavelength, the extra diffraction band around 500 nm was exhibited for the CLC grating with silver nanoparticles in the cell. The amplitude modulation Δα1 induced by the silver nanoparticles layer in CLC grating environment was considered into the diffraction efficiency. Comparing with CLC grating without silver nanoparticles, a theoretical extra diffraction band around 500 nm was exhibited as the phenomenon of enhanced diffraction revealed in the experimental data. The broader enhanced band width and little shift of peak wavelength may be caused by the size distribution of real silver nanoparticles. Our theoretical analysis shows that the various absorption spectra of silver nanoparticles in the CLC grating leads to prominent characteristics of the extra enhanced diffraction band in the spectrum. A stringent theoretical analysis with the consideration of size distribution of silver nanoparticles, the distortion of LC alignment, and the detector sensitivity is necessary.
In summary, the extra diffraction band of the CLC grating was emerged from the excitation of localized surface plasmon on the silver nanoparticles. It was noted that the absorption characteristic of sliver nanopaticles in the CLC grating cell affected the diffraction behavior significantly. The mean absorption and the modulation of absorption determined the property of diffraction efficiencies. There exist two passed bands around λ = 654 nm and λ = 505 nm. The CLC grating device could be employed as a band pass filter to switch the diffractive light in a specific wavelength range. The enhanced diffraction efficiencies would appear in the different wavelength band, by varying the kind or the size of nanoparticles on the plate surface. The physical reason of the extra diffraction band was the modulating localized surface plasma resonance emerged from silver nanoparticles. The extra diffraction efficiency due to the localized surface plasmon emerged on the metallic nanoparticles provides potential applications for optical band pass devices.
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Chapter 7 Summary
Three works in this study have been accomplished and they are entitled with
“Real time absorbance spectra due to optical dynamics of silver nano-particles film”,
“Laser pulse induced gold nanoparticles gratings”, and “Surface plamson induced extra diffraction band of cholesteric liquid crystal”.
The accomplishments corresponding to these works, respectively, are shown as follows:
A. Real time absorbance spectra due to optical dynamics of silver nano-particles film:
A real time observation to optical dynamics of silver nano-particles film under a heating treatment from 28 to 300 ℃ is investigated by using absorbance spectra and Dark-field microscopy. It was noted that the absorbance spectra of the film varied with the heat-treating temperature and time. The peak position in the spectra curve shifted to shorter wavelength below the temperature of 250 ℃, then shifted to red band due to higher temperature treatment. Comparing with scanning electron micrograph analysis, the real time absorbance spectra exhibited a particular optical property confirmed by the dynamic dark-field optical microscopy system. The real-time absorbance spectra and dark-field micrographs analyses provide a direct and non-destructed observation to the growth evolution of metal nano-particles.
B. Laser pulse induced gold nanoparticles grating:
A laser pulse induced gold nanoparticles grating is reported in this work. A single shot of a pair of Nd-YAG laser pulses of the same polarization is directed toward a thin gold film of thickness 6 nm on a substrate of polymethyl methacrylate (PMMA).
Under the observation of scanning electron and dark-field optical microcropies, it is
noted that the morphology of the gold nanoparticles grating is dependent on the fluence of laser pulse. The spectrum of first order diffraction shows a spectral dependence, possibly due to the presence of the nano-particles of various sizes. The ablation of thin films of nano-thickness via the use of laser pulses may provide a simple and efficient method for the fabrication of nano-scale structures, including 2D arrays of nano-particles.
C. Surface plamons induced extra diffraction band of cholesteric liquid crystal grating
We investigated the diffraction behavior of cholesteric liquid crystal (CLC) grating with the surface plasmon effect was investigated. A well formed phase grating was constructed in the CLC cell. It was shown that an extra first-order diffraction band was observed around 505 nm. The physical reason of the extra diffraction band could be the surface plasma effect emerged from silver nanoparticles. The extra diffraction band due to the surface plasmon effect can offer potential applications in nano-optics, such as the optical switch function.
So many outstanding works have been published in various journals in chemical and physical area, that inspire us to pay more effort on the study of potential application based on localized surface plasmon effect. The particular optical property due to localized surface plasmon resonance of metal nanoparticles is so interesting and that can be applied in a wild application, for examples, optical integral circuit, bio-sensor, signal enhancement of nano-scale molecule. In the future, we will try to create new and interesting ideas for surface plasmon resonance effect and to do out best to realize that for the final target.
Appendix
Optical property of liquid crystal [1-4]
Liquid crystal is an intermediate state of matter between the crystalline solid and the amorphous liquid. An ordered orientation of molecules exists and appears in liquid state. This intermediate state was first observed by F. Reinitzer in 1888. He discovered that cholesteryl benzoate, a crystalline solid, becomes a turbid cloudy liquid (liquid crystals) when heated to 150 ℃. As further heating to 182℃, the liquid turns into isotropic and clear. The whole transition is shown in Figure A1. The molecular structure is a function of temperature and the sequence is reversed when cooling the substance. Actually, the cloudy intermediate phase contains domains that seem to have a crystal-like molecular structure. In liquid crystal phase, this material exhibits distinct optical property and can flow like liquid. These advantages lead to a flexibly optical modulation via applying liquid crystal as electro-modulating medium.
In general, three particular types of liquid crystals can be classified in accordance with the physical parameters to control the liquid crystalline phases. They are lyotropic, polymeric, and thermotropic. Lyotropic liquid crystals can be obtained by dissolving an appropriate concentration of this material into the solvent. The dependence of concentration determined the liquid crystal phase and that is important
In general, three particular types of liquid crystals can be classified in accordance with the physical parameters to control the liquid crystalline phases. They are lyotropic, polymeric, and thermotropic. Lyotropic liquid crystals can be obtained by dissolving an appropriate concentration of this material into the solvent. The dependence of concentration determined the liquid crystal phase and that is important