Liquid switchable radial polarization converters made of sculptured thin films

1 Published paper M. Oliva-Ramírez, V.J. Rico, J. Gil-Rostra, O. Arteaga, E. Bertran, R. Serna, A.R. González-Elipe, F. Yubero, Applied Surface Science 475, 230-236 (2019) DOI: https://doi.org/10.1016/j.apsusc.2018.12.200 Liquid switchable radial polarization converters made of sculptured thin films Manuel Oliva-Ramírez1, Victor J. Rico1, Jorge Gil-Rostra1, Oriol Arteaga2, Enric Bertran2, Rosalía Serna3, Agustín R. González-Elipe1, Francisco Yubero1* 1 ICMSe (CSIC – Univ. Seville), CICCartuja, C/ Américo Vespucio 49, E-41092 Seville, Spain 2 ENPHOCAMAT (FEMAN) Group, Department of Applied Physics, Universitat de Barcelona, Marti i Franques 1, E-08028, Barcelona, Catalonia, Spain Institute of Nanoscience and Nanotechnology (IN2UB), Universitat de Barcelona, Spain 3 Institute of Optics "Daza de Valdés" C/ Serrano, 121, E-28006 Madrid, Spain. *Corresponding author email: [email protected] Abstract 2 A radial polarization converter is a super-structured optical retarder that converts a conventional linearly polarized light beam into a structured beam with radial or azimuthal polarization. We present a new type of these sophisticated optical elements, which is made of porous nanostructured sculptured single thin films or multilayers prepared by physical vapor deposition at an oblique angle. They are bestowed with an axisymmetric retardation activity (with the fast axis in a radial configuration). In particular, a Bragg microcavity multilayer that exhibits a tunable transmission peak in the visible range with a retardance of up to 0.35 rad has been fabricated using this methodology. Owing to the highly porous structure of this type of thin films and multilayers, their retardance could be switched off by liquid infiltration. These results prove the possibility of developing wavelength dependent (through multilayer optical design) and switchable (through vapor condensation or liquid infiltration within the pore structure) radial polarization converters by means of oblique angle physical vapor deposition. Keywords: q-plates, radial polarization converters, oblique angle deposition, sculptured thin films Graphical abstract 3 4 Highlights  Porous nanostructured sculptured multilayer with axisymmetric optical activity  Wavelength dependent response through multilayer optical design  Optical activity switchable through liquid infiltration within the pore structure  To be considered as thin film radial polarization converters 5 1. Introduction Sculptured thin films prepared by electron beam evaporation in an oblique angle deposition (OAD) geometry have been proposed as optically active components in photonic devices [1- 4]. By this procedure, fancy anisotropic architectures formed by columnar nanostructures with slanted, zig-zag, S-shape or helix arrangements can be achieved varying the orientation of the incoming flux of material with respect to the sample surface (e.g., through the azimuthal rotation of sample during deposition) [1]. Optical birefringence can be obtained with a bundled arrangement of nanocolumns that depicts a thin film microstructural anisotropy that can be tailored by controlling the polar and azimuthal deposition angles [5-7]. These outstanding optical properties have prompted the incorporation of this type of thin films into optically active devices such as retarders [8], tunable circular polarizers [9] lasers with tuned polarization emission [10], mirrors for circular polarized light [11-12], microfluidics liquid detection [6,13], or vapor sensing [7,14]. In the present paper, we propose the synthesis and characterization of 2D axisymmetric porous birefringent OAD thin films and multilayers that behave as radial polarization converters (also known as q-plates), a type of super-structured optical retarders where the principal optical axes rotates axially about the center of the optical element [15]. This feature confers these plates the ability to generate light beams with orbital momentum, usually called light cylindrical vector beams (CV), that are characterized by axial symmetry both in amplitude and phase, as well as a rotational symmetry distribution of polarization states. The enhanced focusing properties of CV beams are of particular interest for 3D focus engineering, optical trapping and laser machining applications [16]. Several methods are reported to generate CV beams. They can be classified into active methods, involving the direct generation of the beam from a laser cavity [17,18], and passive methods involving optical devices that modify the polarization state of a polarized light beam [19-23]. These passive methods rely on specially designed polarization-state converters that 6 commonly involve complex optical elements, such as a liquid-crystal converters [19,20], photo-aligned liquid crystal polymers [21], space-variant subwavelength gratings [22], or a spatially variable segmented half-waveplate [23] that converts light beams with spatially homogeneous polarizations into polarized vortices. Another typical example of this type of passive optical devices is an array of waveplates arranged as slices of a pie chart (eight, for example) where the optical principal axis of each slice is directed towards the center of the optical element [23]. In the present paper, we have developed a one-step method to fabricate radial polarization converters based on porous OAD thin films and multilayers. Besides analyzing the optical performance of the synthesized polarization converters, we prove the switchability of their retardance by means of the infiltration of a liquid within their open porous structure. Optofluidic switching of the optical response, a completely different approach than the typical electrical switching of this kind of devices [24], is expected to open new alternatives for the development of polarization converters with a big impact in microfluidic and similar applications. 2. Materials and methods Single layer and multilayer porous nanostructured films bestowed with axisymmetric optical activity where deposited on either fused silica plates or polished Si wafer substrates by electron beam evaporation of either TiO or SiO2 pellets at a process pressure of 3x10-4 mbar. An O2 inlet was let into the preparation chamber to get fully oxidized, and therefore transparent, TiO2 or SiO2 thin films. The base pressure of the vacuum vessel was 1x10-6 mbar and the deposition rate was about 0.1 nm/s. The angle of incidence of the vapor flux was 75º and the crucible-sample distance was about 430 mm. Azimuthal sample rotation (about 30 turns per minute with rotation axis about the center of the sample) was used to achieve an axisymmetric 2D microstructural arrangement. Figure 1 shows schemes of the top (left), front (center) and side (right) views of the particular geometrical oblique angle deposition 7 arrangement used to synthesize radial planar converters. Basically, this experimental device consisted of a slit mask (15 mm size made of a 0.5 mm thick metal foil with a 2 mm perpendicular slit) placed 30 mm from the sample location in the line of sight between substrate and crucible. The particular location and dimensions of the slit impose a partial collimation to the flux of particles that, as described in the results section, renders thickness homogeneity to the coatings while bestowing them some degree of birefringence. Figure 1. Top view (left), front view (center), and side view (right) schemes of the geometrical oblique angle deposition arrangement (dimensions are not at scale). Three types of samples (size 25x25 mm) were prepared: i) single layer TiO2 porous films (qTiO2) about 600 nm thick, ii) single layer SiO2 porous films (qSiO2) about 600 nm thick, and iii) a microcavity (qBM) formed by 2 porous Bragg mirrors of 7 layers with a stacked HLHLHLH structure (H and L refer to TiO2 and SiO2 films with high and low refractive indices, respectively) and a thicker SiO2 defect layer in between. The microcavity was designed to depict a transmission peak at about 550 nm in the middle of the reflection band of the individual Bragg mirrors. Typical thickness of individual TiO2 and SiO2 layers of the Bragg mirrors was 80 nm, while the central SiO2 layer acting as optical defect had a thickness of 180 nm. 8 Samples were characterized by SEM, transmission UV-Vis, mapping spectroscopic ellipsometry (SE) and spatially resolved Muller Matrix polarimetry. The microstructure of the qTiO2 and qSiO2 samples deposited on polished Si(100) wafers (cross sectional and planar views) was characterized by scanning electron microscopy (SEM) using a Hitachi S4800 field emission microscope. The qBM sample was also characterized by FIB-SEM using a procedure similar to that described in ref.[6]. UV-Visible transmission spectra of the qBM samples deposited on fused silica were recorded in a Varian Cary100 spectrophotometer. Lateral thickness distribution and birefringence across the SiO2 and TiO2 samples surface were characterized by spectroscopic ellipsometry using a Woollan VASE apparatus provided with an accessory for spatial mapping detection. The size of the analysis probe was about 1 mm2. Measurements were taken along an orthogonal grid of 2 mm pitch. The Ψ and Δ ellipsometric parameters were determined using a biaxial model for the individual single material anisotropic layers. Basically, the model consists of an orthorhombic optical description of the deposited films, with a main axis perpendicular to the sample surface and the other two axis at orthogonal directions parallel to the sample surface with Euler angles defined as THETA=0º, PSI=0º and PHI as free fitting parameter. PHI represents the in-plane orientation of the optical axis at the sample surface plane. The mathematical description of the refractive index for each direction was done by an effective medium approximation model (isotropic Bruggeman) that considered the mixture of either SiO2 or TiO2 compact materials and voids. For the compact materials, a Cauchy type wavelength dispersion was used to describe the dependence of the refractive index with wavelength. Percentage of voids along each direction was taken as free parameter of the spectroscopic ellipsometric analysis. The main aim of this description is to get a good and independent account of the refractive index along every direction (in the sense of being able to fit the spectroscopic ellipsometry data) 9 and, at the same time, to get a direct estimation of the “empty” space in each direction (according to the effective medium Bruggeman parametrization). The optical activity of the samples was also characterized by means of spatially resolved transmission measurements in a Mueller matrix (MM) polarimeter. This apparatus, described in detail in ref.[27], measures simultaneously all elements of the 4x4 Mueller matrix thanks to four different photoelastic modulators. The sample was mounted in a two-axis motorized sample holder that allows generating 2D Mueller matrix maps. These maps, which are presented as an image, represent an area of 15x13 mm2 that is sampled in steps of 0.5 mm and entails 896 measurements at every selected wavelength. The diameter of the probing beam was 0.8 mm. The transmission Mueller-matrix measured at each point was processed to calculate, among others, the linear birefringence (LB) defined here as 2 times the retardance in waves [26]. It is noteworthy that the optical activity is not intrinsic of the involved materials, but arises from the special microstructure of the samples. 3. Results 3.1 Film structure Three types of samples (size 25x25 mm) were prepared by electron beam evaporation at oblique geometry using a special geometrical experimental set-up (see experimental section for details): i) single layer TiO2 porous films (qTiO2), ii) single layer SiO2 porous films (qSiO2), and iii) a Bragg microcavity (qBM), formed by 2 porous Bragg mirrors and a thicker SiO2 defect layer in between, and that was designed to depict a transmission peak at about 550 nm in the middle of the reflection band of the individual Bragg mirrors. The deposited single films have a porous vertical columnar microstructure as shown in the SEM crossed section micrographs included in Figure S1 (supporting information) corresponding to the qTiO2 (left) and qSiO2 (right) samples. Development of vertical 16 sample ( = 0, m13 = m23 = 0, m33=1), independently on the orientation of the optical axes (c.f., see the white color at the center of m13 and m23 and red color at the center of m33). Getting away from the sample center, the patterns of the corresponding elements indicate that retardance increases. On the other hand, from the analysis of m23 MM element it can be concluded that the fast axis aligns horizontally at the left/right (maximum positive retardance) and vertically at the top/bottom (maximum negative retardance). Similar arguments may be used to justify the 2D intensity pattern of the m13 element, now with respect to the 45º directions. Figure 5. Spatially resolved MM polarimetry measurements evaluated at 540 nm (i.e., at the transmission peak of the microcavity). The main diagonal MM elements are scaled between 0.9 and 1.0, while the other off-diagonal MM elements are scaled between 0.4 and -0.4. Meanwhile, the weaker 2D pattern intensity of m01 and m02 MM elements can be accounted for by a linear diattenuator behavior. In this case, it holds that m01 = 2*cos2φ (where A is proportional to the difference in transmittance between the two orthogonal axes), i.e., m01 follows qualitatively the same pattern distribution than the m23 element. Similarly, under the 17 same conditions, m02 = 2*sin2φ, i.e., the same pattern shape than that of the m13 element, but with opposite sign. This indicates that, in addition to the retarder behavior of the qBM, there is also a minor difference in transmission (note the weaker intensity color) between the two optical axes. This effect is due to the different transmittance of the sample as a function of the azimuth angle of the incoming linear polarization. The qualitative description of the patterns of the off-diagonal terms is also consistent with the patterns observed for the main diagonal m11, m22 and m33 elements of the MM. The evolution of the super-structured retardance of sample qBM can be appreciated more clearly after evaluating the magnitude and orientation of the linear birefringence (LB) with the method described in ref.[26] (c.f. Figure 6). LB magnitude or retardance represents the integrated effect of birefringence acting along the path of a light beam that traverses the sample. If the incident light beam is linearly polarized, the two orthogonal components of the polarized light will exit the sample with a phase difference. Therefore, the maps in this figure show that the magnitude of the in-plane LB increases when moving away from the sample center (Figure 6-left). This effect is consistent with that observed by transmittance measurements between crossed polarizers of the qBM sample and the mapping SE analysis described above for sample qTiO2 (c.f. Figure 4). Furthermore, it is apparent that the spatial distribution of optical axes in sample qBM is axisymmetric as it is shown in Figure 6-right. Effectively, this figure shows that beyond 3-4 mm from the sample center a clear smooth change of the orientation of the optical axis takes place: it starts from a horizontal orientation of the fast axis at the top of the sample (c.f., Figure 6-right, red), to turn smoothly clockwise to successive angles of -45º (yellow), -90º (blue), 45º (green), and finally to 0º (red at the bottom of the figure). 18 Figure 6. Linear birefringence (LB) magnitude in radians (left) and orientation in degrees (right) maps evaluated at 540 nm (i.e., at the resonant peak of the microcavity) by the spatially resolved MM polarimetry measurements from Figure 5. Intensity maps in Figure 5 confirm that the optical principal axis rotate around the center of the sample. This effect takes place not only at the wavelength of the BM transmission peak, but also at other wavelengths (see Figure S4 in supporting information the plots of LB magnitude and orientation at 475 and 650 nm, i.e., off the transmission peak position of the microcavity). 3.4 Fluidic switching of the radial polarization activity of qBM samples The high porosity of the OAD thin films and multilayers (up to 50-60% of the total volume of the film [6]) provide a straightforward way for tailoring their optical properties by liquid infiltration. Therefore, the optofluidic behavior of the qBM was characterized after ethanol infiltration in its porous structure. The purpose of this experiment is to check whether the radial polarization activity of the qBM can be switched off by liquid infiltration. Figure 7 shows the variation of the LB magnitude vs wavelength in the qBM sample at a point off the center of the sample before and after water infiltration in its porous structure. A first evidence of the spectra in this figure is that water infiltration of the qBM produces a decrease in the 19 magnitude of the linear birefringence by a factor of ca. 5. This drastic change can be interpreted in the following terms: when the empty pores of the film are filled with ethanol, the optical contrast between different orientations diminished and so does its optical activity. The observed effect was reversible and the LB magnitude recovered its initial value after empting the pores (e.g., drying the BM). Similar optofluidic effects have been reported for vapor and liquid sensing using sculptured slanted or zigzag BM [7]. Figure 7. Variation of the LB magnitude vs wavelength at a point off the center of the qBM sample either empty (full line) or ethanol infiltrated (dashed line). Reversibility of this phenomenon is illustrated in Video V2 included in the supporting information showing that qBM sample recovers its performance as radial polarization converter after liquid removal (drying in our case). 4. Conclusions The strength of the optical activity of the reported OAD films acting as switchable radial polarization converters (0.35 rad or 0.056 waves) is lower than that of standard q-plates with 20 q=1 i.e., with an optical axis turned by 360º around the center of the sample and a phase retardation of PI (i.e., half-wave). Nevertheless, their planar configuration in the form of single thin films or multilayers and the possibility to operate both in transmission and reflection makes the OAD alternative very interesting for applications where manufacturing simplicity and easy integration with other components are obliged requirements. In this regard the open porosity and, therefore, the demonstrated suitability of these devices for optofluidic operation are of particular interest for microfluidic integration or for their use as responsive devices towards condensed vapors and liquids. It is also of particular interest that the developed technology allows tuning the wavelength response of the device through traditional multilayer optical design. The relatively simple one-step manufacturing procedure by physical vapor deposition constitutes an additional advantage in comparison with the rather complicated procedures of alternative devices. Acknowledgements We acknowledge the support of the European Regional Development Funds program (EUFEDER), the Spanish Ministry Economy, Industry, and Competitiveness (projects 201560E055, MAT2016-79866-R, RECUPERA 2020 and Red CONSOLIDER MAT2015- 69035-REDC). 21 Appendix A. Supplementary material The following are the Supplementary data to this article: Muller matrix elements of a pure linear retarder and pure linear diattenuator, Cross section and front view SEM micrographs of qTiO2 and qSiO2 samples. Transmittance spectra at normal incidence along a line crossing the center of the qBM sample. Spectroscopic ellipsometric fitting of the qBM multilayer at a point off the center of the sample. Linear birefringence magnitude and orientation maps evaluated at 475 and 650 nm on qBM sample by spatially resolved MM polarimetry measurements. Videos of the reflectance of the qBM sample illuminated with white polarized light analyzed with a crossed polarizer and the attenuation of the characteristic extinction cross due to ethanol infiltration within the open porosity of the qBM sample. 22 References [1] A. Lakhtakia, R. Messier, Sculptured Thin Films, Nanoengineering Morphology and Optics, SPIE Press, 2005. [2] M.M. Hawkeye, M.T. Taschuk, M.J. 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