1. Introduction

Switchable holographic structures are promising for a wide range of applications such as imaging, information processing and diffractive optics [1]. 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) ), as novel structures with a wide range of optical and electro-optical applications [2–5]. 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].

 

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.

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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 [7,8]. 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

The E7 NLC is a multicomponent nematic mixture made by substituted 4-cyanobiphenyls [9] (structure formula in the inset of Fig. 2 ). The related Raman spectra, collected on a randomly aligned NLC sample, are shown in Fig. 2. The higher intensity main Raman bands are assigned to the ring deformations; assignment of such modes has been performed by taking into account that biphenyl belongs to the C_2v point group [10]; therefore, the skeletal vibrational modes of the biphenyl can be described in just four symmetry species. In particular, the bands at 1020 (υ_18a), 1184 (υ_9a), 1529 (υ_19a) and 1609 (υ_8a) cm⁻¹ are described by the A₁ symmetry representation; the mode with symmetry A₂ falls at 781 cm⁻¹ (υ_10a) while the peaks at 810 (υ₄) and 830 (υ₁₁) cm⁻¹ have B₁ symmetry and the modes with B₂ symmetry [10] are at 638 (υ_6b) and 1288 (υ₃) cm⁻¹. In addition, vibrations which fall at 1020 (υ_18a), 1184 (υ_9a), 1529 (υ_19a) and 1609 (υ_8a) cm⁻¹ occur in the molecular plane, while all other vibrations occur out of the molecular plane. Modes at 409 (γ deformation) and 2232 (symmetric stretching) cm⁻¹ are assigned to the CN group [10,11].

 

Fig. 2 Representative Raman spectra collected on randomly aligned NLC E7 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).

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The components of the optical polarizability tensor for molecules which belong to point group C_2v are, in principle, all different. However, due to the rotational isotropy around the long molecular axis (z-axis), the optical polarizability tensor, for the E7 NLC, can be represented by an oblate ellipsoid with α_zz > α_yy = α_xx, where α_xx, α_yy and α_zz indicate the components of the polarizability tensor in the molecular frame. The symmetric stretching of the aromatic rings of the biphenyl molecules and the stretching of the CN groups induce the major change of polarizability along the long molecular axis of biphenyl molecules. Therefore, if there is a preferential orientation of the director n of the NLC inside the POLICRYPS, this can be evidenced by following the behaviour of the intensities of the Raman bands, related to the above mentioned stretchings, as a function of the angle θ: the θ value where the maximum intensity of those Raman bands is obtained, clearly identifies the direction of n [12,13].

NOA-61 has been investigated before and after the polymerization process (Figs. 3(a) , 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]. 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] 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].

 

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).

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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]. 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].

In Fig. 4 , 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.

 

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).

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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 ). 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°.

 

Fig. 5 Representative Polarized Raman spectra collected on the liquid crystal slices of the POLICRYPS grating at different θ values between 0° and 180°.

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This behavior clearly evidences a preferential orientation of the NLC director within the POLICRYPS structure [12,13,17–21]: 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], 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. [23–25], 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.