A simple single-step method to fabricate spatially graded TiO2-SiO2 Bragg stack with rainbow colored photonic band gap is presented. The gradation in thickness of the Bragg stack was accomplished with a modified glancing angle deposition (GLAD) technique with dynamic shadow enabled by a block attached to one edge of the rotating substrate. A linear gradation in thickness over a distance of about 17 mm resulted in a brilliant colorful rainbow pattern. Interestingly, the photonic band gap position can be changed across the whole visible wavelength range by linearly translating the graded Bragg stack over a large area substrate. The spatially graded Bragg stack may find potential applications in the tunable optical devices, such as optical filters, reflection gratings, and lasers.
© 2015 Optical Society of America
Photonic crystals have been an important focus of research due to their promising applications in controlling the propagation of light in various ways [1–5 ]. One dimensional photonic crystals or Bragg stacks are commonly used in optics for precise selection of reflected wavelengths out of the incident spectrum [6–9 ]. A Bragg reflector consists of a periodic stack of layers of high and low-refractive index materials respectively. The periodic variation in the refractive index forms a photonic band gap by allowing a low transmittance condition to the incident photons of a particular wavelength range . The photons being reflected in the wavelength range of the photonic band gap or stop band give rise to the structural color of a Bragg reflector. The central wavelength of photonic band gap (λBragg) or the reflectance maximum of a Bragg reflector can be determined by the Bragg-Snell law for normal incidence [9, 10 ]:9, 10 ].
In order to obtain the photonic band gap position centered at different wavelengths of interest to meet the specific requirements, a large number of Bragg reflectors have been fabricated by using different combinations of layer thickness and the materials [6–9, 11, 12 ]. However, it can be noticed that the growth constraints involved in the fabrication techniques make it necessary to produce a separate Bragg reflector for every desired photonic band gap position. Therefore considering a wide range of applications the fabrication of a single Bragg stack with tunable photonic band gap draws significant research interest. A modification of effective refractive index of the mesoporous Bragg stacks by filling the voids with specific vapors or liquids has been observed to produce a shift in the photonic band gap [13–17 ]. Bragg stacks consist of the layers of certain polymers that swell in the liquid medium have also been reported to observe a shift in the photonic band gap [18, 19 ]. However, these methods provide the tunability of photonic band gap in a very small wavelength range. Moreover, the dependency on specific liquid or vapor mediums makes the process complicated, inconvenient, and limited for many optical applications. Recently in separate studies Liu et al. and Moein et al. have fabricated the Bragg stack with layers of spatially graded or sloping thickness using the holographic photo-patterning method [20, 21 ]. An optimized gradation of thickness has been shown to allow variation in photonic band gap position over a large range of the visible spectrum at different locations of the multi-layer structure. Gradation of thickness appears as a simple and effective method to tune the photonic band gap of a Bragg stack. However, use of a simple, cost-effective and versatile technique amenable to large area fabrication of the spatially graded tunable Bragg reflector is highly desired.
Here, we present a facile and single-step approach based on the glancing angle deposition (GLAD) to fabricate the spatially graded Bragg stack with rainbow colored photonic band gap. To accomplish the spatially graded thickness of each layer of the Bragg stack we have used a modified GLAD method. In this method, a shadowing object was placed on one edge of the rotating substrate during GLAD. A dynamic shadow casted with the object was utilized to fabricate the graded thickness inside the object’s shadow region on the substrate. It has been shown that the photonic band gap position varies linearly with lateral distance on the graded Bragg stack. A proper optimization of the thickness of alternate layers and shadowing conditions during GLAD has been successfully shown to offer the tunability of photonic band gap in whole visible wavelength range.
2. Results and discussion
Shadowing for high vapor incidence angle (80°) during GLAD results in the evolution of nanocolumns inclined in the direction of vapor flux [22–26 ]. Addition of substrate rotation in the azimuthal plane during growth ensures the uniform supply of vapor flux from all around and makes the nanocolumns grow vertically without getting inclined in any preferred direction [22, 23, 26 ]. Conventional GLAD produces thin films of uniform thickness on the substrate, but here we used the modified GLAD approach to fabricate the films with spatially graded thickness.
As illustrated in Fig. 1(a) , a block of 3 mm height was fixed near one edge of the substrate to create the geometrical shadow on the substrate. A simple trigonometry suggests that in case of the vapor flux reaching at 80° with the substrate normal, this block may form the shadow region extending up to about 17 mm from the edge when in position direct facing the vapor flux as shown in Fig. 1(a). In this shadow region, a very little or almost negligible growth could be expected. However, when the substrate is rotated in the azimuthal plane then a dynamic shadowing condition can be realized. The shadow area changes gradually with rotation, allowing the film to grow in the flux-inhibited region. Figure 1(b) shows a possible condition of substrate during rotation when the block lies in the opposite side of the path of vapor flux. In this case, there will be no effective shadow region and, therefore, the whole substrate will receive uniform vapor flux. This substrate rotation-oriented dynamic shadowing results in the spatially graded thickness inside the shadow region. It can be understood that in the shadow region vapor flux cannot be received uniformly from all directions. Therefore, the growing columns in this region may incline towards the side from which they receive the maximum vapor flux that is away from the shadowing block. Similar shadowed GLAD method has also been used earlier by Krause et al. to fabricate the spatially graded helical nanostructured films for tunable circular polarizers .
To utilize this modified GLAD technique for the fabrication of spatially graded rainbow colored Bragg stack, alternate layers of high (TiO2) and low (SiO2) refractive index materials were grown on a glass substrate. Spatially graded columnar growth of the alternate layers of these two materials is illustrated by light and dark color stripes in the Fig. 1. The refractive indices of columnar TiO2 and SiO2 films separately grown by GLAD at vapor incidence angle of 80° were measured to be 1.6 and 1.3 respectively. Considering these measured values of refractive index, the maximum thickness for TiO2 and SiO2 layers that could be observed in the unshadow region on the substrate was decided as 64 nm and 194 nm respectively. Following the Bragg-Snell relation given in Eq. (1), this set of thickness will allow the photonic band gap to be centered near the red color wavelength of 700 nm. Therefore in this case, any successive decrement in thickness of TiO2 and SiO2 layers due to the dynamic block shadowing effect can be utilized to select the photonic band gap position in complete visible wavelength range lying from about 700 nm to 400 nm.
Thickness of the fabricated spatially graded TiO2 and SiO2 multilayered stack was measured from the scanning electron microscopy (SEM) images taken at different distances from the shadowing block edge along the sample length. The thickness profile as shown in Fig. 2(a) simply illustrates the gradation in thickness of the Bragg stack. It can be noticed that within the shadow region that is supposed to be extended up to about 17 mm from the shadowing block edge the thickness increases gradually as we move away from the block. Outside the shadow region, almost a constant thickness could be observed. Some representative SEM images of the Bragg stack captured at different locations within the shadow region are shown in Fig. 2(b). It is interesting to notice that columnar films fabricated by GLAD are inherently porous. A small separation between the nanocolumns indicating the porosity may also be visible in SEM images. The low values of measured indices of TiO2 and SiO2 columnar films comparing to their bulk counterparts also suggest a reduction in density or rise in porosity for the columnar film. The layers of TiO2 and SiO2 can be identified as bright (high electron density) and dark stripes, respectively.
Existence of Ti and Si elements in each layer of a nanocolumn is confirmed by energy-dispersive X-ray spectroscopy (EDX) mapping using transmission electron microscopy (TEM) as shown in Fig. 3 . TEM specimen preparation was performed by dispersing the TiO2 and SiO2 multilayered nanocolumns onto a copper grid, and then a number of multilayered nanocolumns was randomly selected for analysis. Bright-field TEM image in Fig. 3(a) also shows that the Bragg stack consists of high (bright) and low (dark) electron density layers, and the corresponding EDX mapping images of Ti and Si elements are clearly shown in Figs. 3(b) and 3(c). The distribution of Ti and Si elements are consistent with bright and dark stripes in the TEM and SEM images.
The optical image of the spatially graded TiO2-SiO2 Bragg stack captured under white light illumination with black back-ground is shown in Fig. 4(a) . The Bragg stack reflects a beautiful graded rainbow color pattern. The dashed line on the left represents the shadowing block edge. The color wavelength increases gradually from violet (~400 nm) to red (~700 nm) on moving along a lateral direction away from the block edge. A millimeter scale is marked near the bottom to indicate the position of each color region from the block edge. The transition in color wavelength with distance from the edge is predominantly observed within the shadow region lying up to about 17 mm from the block edge. Out of this expected block shadow region, nearly constant red color could be observed. This observed spatially graded rainbow color pattern simply reflects a linear shift in position of the photonic band gap. To identify the photonic band gap position, transmittance of the Bragg stack was measured at different color regions (shown in Fig. 4(b)). The locations of transmittance measurements reported in Fig. 4(b) are chosen just to highlight the characteristic as well as the tunability of the position of photonic band gap to all major color wavelengths of the visible spectrum. The transmittance minimum in each spectrum reflects central position of the photonic band gap. In all the transmittance spectra a peak transmittance fall of up to about 68% with full width at half maximum (FWHM) of about 90 nm was observed for this 10 pair TiO2-SiO2 Bragg stack. To further demonstrate the fine tunability of photonic band gap position of the graded rainbow colored Bragg stack, the transmittance spectra were measured along the lateral direction at every 2 mm distance moving away from the shadowing block. The peak position of transmittance minimum with distance from the shadowing block is shown in Fig. 5 (black). There is almost a linear shift in peak position with the lateral distance within the block shadow region could be observed. Outside the shadow region, the peak position appears to be constant. Bragg-Snell relation given in Eq. (1) was used to verify the observed graded rainbow colors and the lateral shift in central peak position of the photonic band gap. Since the SEM images were not sufficiently clear to measure the thickness of each material layer accurately at different positions from the shadowing block. Therefore, first we measured the percentage change in total thickness of the Bragg stack for a given distance on moving towards the block edge from unshadow region. Thereafter considering the fact that each layer may undergo the same percentage change, the thickness of TiO2 and SiO2 layers was calculated. The central wavelength of photonic band gap or transmittance minimum was calculated by substituting the observed thickness of TiO2 and SiO2 layers and their respective indices in the Bragg-Snell relation. A plot of calculated wavelengths of transmittance minimum with the distance from the block edge is given in Fig. 5 (red). Since here we assumed that all layers of a given material were of uniform thickness. But in actual the thickness may vary slightly due to the growth constraints. A small variation in layers thickness could also be observed from the TEM image shown in Fig. 3. Note that the orientation of nanostructures also changes from slightly tilted to vertical on moving away from the shadowing mask. The change in orientation of structures may produce a small variation in the refractive indices of the TiO2 and SiO2 layers. This slight variation in layers thickness and refractive indices might be a possible reason behind the small deviation observed between the measured and calculated values of the peak wavelengths. However, a close resemblance in trend of the calculated values with the measured values can be observed which simply suggests that the observed graded rainbow colored pattern is in agreement to the Bragg-Snell relation for one dimensional periodic stack. The variation in thickness of alternate TiO2 and SiO2 layers may be responsible for the broadening of Bragg peak. Therefore a precise growth control to fabricate the periodic stack of uniform thickness TiO2 and SiO2 layers may help in narrowing the bandwidth. Since the bandwidth of Bragg peak also depends on refractive indices of the two materials, larger difference in refractive indices produce the narrower peak. Therefore a careful selection of two materials with well distinct refractive indices may produce the narrower band pattern. The lateral dimension of graded Bragg reflector can be controlled up to some extent by adjusting the height of shadowing block or changing the vapor incidence angle. If the height of mask is h and incident angle with substrate normal is α (for the schematic shown in Fig. 1) then the mask shadowing effect or lateral dimension of graded pattern can be obtained up to the length of h(tan α). This relation shows that the lateral dimension changes rapidly for a small change in h or α. However it was experimentally observed that the pattern size could be reduced easily by changing these two parameters, but the increase in pattern size was not possible, because, for a very high shadow block or large incidence angle a very small growth was obtained in region close to the mask which gives the Bragg peak out of the visible range. It may also be noticed that if we reduce the dimension of graded pattern then individual color regions also become small and it may be hard to distinguish them. In this study we selectively chose the parameters to get the large lateral dimension of graded film with well distinguished color regions.
3. Experimental section
Spatially graded TiO2-SiO2 Bragg stack was fabricated using the GLAD method. The high contrast in refractive index and low optical absorption in visible wavelength range make these two materials an ideal choice for Bragg stack fabrication. In GLAD method, vapor flux of atoms was allowed to incident on the glass substrate at a large incidence angle (80°) with respect to the substrate normal [22–26 ]. A linear strip of 3 mm height and 20 mm width was used as the shadowing block. It was placed near one edge of the substrate to form a large shadow region on the substrate by blocking the incoming vapor flux. Since a linear strip may produce the boarder effect with curved profile of the graded Bragg mirror, to minimize this effect glass substrate of smaller width (10 mm) was placed in center of the shadowing block. To produce the graded thickness in the shadow area, the block shadow was controlled dynamically by rotating the substrate continuously in the azimuthal plane with a speed of 1 rpm. Schematic of substrate mounting condition is shown in Fig. 1. The deposition was performed in an electron-beam evaporator with base pressure of about 2 × 10−6 Torr. The deposition rate of 4 Å/s was kept constant for both the materials. Ten pairs of TiO2-SiO2 layers were grown by alternate deposition to form a Bragg stack. The surface morphology and element analysis were performed using field emission scanning electron microscopy (FE-SEM, Philips XL30S FEG) and Energy-dispersive X-ray spectroscopy (EDX). Optical characterization was done by measuring the transmittance of the Bragg stack in the visible range. Refractive indices of TiO2 and SiO2 layers were measured by ellipsometer.
In conclusion, we have successfully demonstrated the spatially graded TiO2-SiO2 Bragg stack by using a simple and cost-effective modified GLAD method. The dynamic shadow casted with a block attached to one edge of the rotating substrate during GLAD produced the gradation in thickness of film. The spatially graded Bragg stack displays a brilliant colorful rainbow pattern and photonic band gaps covering almost the whole visible wavelength range. The observed variation in photonic band gap position with linear translation over the graded Bragg stack has been found in agreement to the Bragg-Snell relation for one dimensional photonic crystals.
The authors gratefully acknowledge supports by the Brain Korea 21 PLUS project for Center for Creative Industrial Materials (F14SN02D1707).
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