Registration and optical properties of embedded two-photon polymerized features within self-organized photonic crystals

Transcription

1 Registration and optical properties of embedded two-photon polymerized features within self-organized photonic crystals Erik C. Nelson and Paul V. Braun* Department of Materials Science and Engineering, Frederick Seitz Materials Research Laboratory and Beckman Institute, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA In this work we demonstrate the ability to define precisely positioned embedded defects in threedimensional photonic crystals using two-photon polymerization. We are able to write defects with nearperfect lattice registration at specifically defined depths within the crystal. We also show that the intrinsic point defects of the crystal can potentially be used as resonant cavities by positioning waveguide structures within one lattice constant of the defects. Finally the importance of precise defect position is demonstrated by investigating the optical properties of embedded planar defects written in a photonic crystal. The experimental data is compared to spectra calculated using the Scalar Wave Approximation (SWA) and demonstrates an excellent fit. Photonic crystals are structures characterized by a periodic modulation of their refractive index on a length scale of the order of the wavelength of light. For certain three-dimensional (3D) structures of sufficiently high dielectric contrast, a photonic band gap (PBG) may result, preventing certain frequencies of light from propagating within the crystal. 1-3 Many applications have been proposed for photonic band gap materials including optical circuits, 4-6 low-loss waveguides, 2, 7 zero-threshold lasers 3 and sensors. 8 In order to realize many of these applications engineered defect structures must be controllably positioned within the photonic crystal. While self-assembled colloidal crystals provide a simple approach to rapidly fabricating photonic crystals, the introduction of embedded defects requires additional processing, in contrast to direct fabrication approaches such as lithography or direct-laser writing where the defects are formed at the same time as the photonic crystal We have shown previously that two-photon polymerization (TPP) allows for the defined addition of defects into a 3D self assembled colloidal crystal. 12, 13 However, in order to form optically functional structures, one must not only define a feature but also position it with a high degree of registry to the lattice of the photonic crystal. 11, Additionally, while the intrinsic defect density of colloidal crystals has been substantially reduced in the past few years 17 there is a still a non-zero defect density which, in the vicinity of the feature, may degrade the optical properties. Therefore, it is desirable to avoid intrinsic colloidal crystal defects when writing features within the photonic crystal. Our previous 12, 13 demonstrations of TPP writing relied on reflectance imaging to register the feature with the photonic crystal lattice. While this enabled general placement of features, the sub-lattice constant precision necessary for optical devices was not possible due to the relatively low resolution of reflectance imaging and its inability to image planes within the crystal. Additionally, defects in the colloidal crystal (e.g. vacancies and stacking faults) generally cannot be seen using reflectance imaging and thus can not be avoided or used as an advantage, for example vacancies may operate as optical cavities. In this communication we demonstrate the ability to write embedded features with excellent lattice registration as well as image and avoid intrinsic crystalline defects. Furthermore we make clear the need for these abilities by presenting the optical effects of precise positioning of planar defect structures at different locations within the crystal. This was achieved by using Invitrogen BODIPY 630/650-X, SE, a fluorescent dye for imaging that does not interact with the TPP system. This dye does not quench in the presence of TMPTA monomer, its excitation wavelength is not absorbed by the photoinitiator, and no energy transfer from the dye to the multiphoton photoinitiator or monomer is observed. Consequently, without negatively impacting the TPP process this dye enables high resolution imaging of the photonic crystal lattice, facilitating careful TPP of optically interesting features in well-ordered regions of a colloidal crystal with lattice registration. Colloidal crystals were prepared from 730nm and 920nm diameter silica particles using a vertical evaporation technique similar to that described previously. 17 Colloidal crystals were characterized by FTIR spectroscopy using a Bruker Hyperion microscope coupled into a Bruker Vertex 70 FTIR spectrometer equipped with a 4x, 0.1 NA objective and a spot size of 187.5um. Crystal quality was evaluated by measuring the low energy reflectance peak in several

2 locations on the crystal. The crystals used in this study had reflectance values ranging from 63-81%. The TPP solution was composed of trimethylolpropane triacrylate (TMPTA) monomer (Sigma Aldrich) with inhibitor; 0.1 wt% AF-350 photoinitiator (tris[4-(7- benzothiazol-2-yl-9,9-diethylfluoren-2-yl)phenyl]amine) donated by the Air Force Research Laboratory 18 and [10 μm] Invitrogen BODIPY 630/650-X, SE (6-(((4,4-difluoro-5-(2- thienyl)-4-bora-3a,4a-diaza-s-indacene-3- yl)styryloxy)acetyl)aminohexanoic acid, succinimidyl ester). The BODIPY dye has an absorption maximum of 625nm and an emission maximum of 640nm; neither wavelength causes single-photon excitation of the photoinitiator. Confocal imaging and TPP were performed on a laser scanning confocal microscope (Leica DMIRBE with an SP2 scanhead). Reflectance and fluorescence imaging was performed with a 633nm HeNe laser. Spatially defined TPP was achieved through our published procedure. 12, 13 Point defects were written using software defined bleach points. Samples were mounted with the colloidal crystal facing a coverslip; TPP is performed through the coverslip. Imaging and ROI alignment were performed using a beam expander to fill the back aperture of the objective to improve image quality in the microscope. TPP was performed with the beam expander removed; rotational alignment was unaffected by the change because the scan field does not rotate. The absolute position of the ROI in the x-y plane (parallel to the substrate) varies with the presence beam expander and is compensated for prior to writing the feature. The position of the ROI and the resultant polymer feature are systematically not coincident in the direction of the crystal thickness; the ROI is positioned offset from the desired feature location to compensate. After TPP samples were rinsed in ethanol to remove excess monomer and dried in air. Scanning electron microscope (SEM) samples were gold-palladium coated prior to imaging; the micrographs were taken using a Hitachi S-4700 SEM. Samples were converted to silicon over three growth cycles at 325C for 15h using our published procedure. 19 When strongly reflecting substrates such as silicon are used, reflectance imaging of the first layer of the colloidal crystal at the substrate can be achieved (Fig. 1a). Fluorescence imaging results in a slightly better image (Fig. 1b). Both reflectance and fluorescence images show a line defect in the lower right corner of the image (also shown enlarged). Vacancies and line defects in fluorescence images are bright since they contain a greater quantity of dye than surrounding areas. Reflectance imaging deeper in the colloidal crystal is not effective; the images are artifacts of reflections from the colloidal crystal-substrate interface (Fig. 1c). This is made clear by the continued presence of the line defect from the first layer in Figure 1c; the fluorescence image of the same plane of the crystal (Fig. 1d) shows the line defect is not actually present. In addition, the intrinsic defects seen in Figure 1d are not seen in the reflectance image of the same crystal plane (Fig. 1c) clearly indicating reflectance imaging is not acceptable for determination of the proper location of features formed through TPP. Reflectance artifacts are clearly seen in reflectance images perpendicular to the substrate (Fig. 1e) where colloids are seen on both sides of the substrate, even though they only exist on top of the substrate. Fluorescence imaging, however, enables complete 3D imaging of the crystal, including identification of intrinsic defects (Fig. 1f). This 3D information is necessary for the formation of optically functional features. Figure 1. Laser scanning confocal microscope images of the first layer of colloids on the substrate surface using (a) reflectance and (b) fluorescence imaging; magnified images show a line defect. Images of a layer of colloids near the middle oft eh thickness of the colloidal crystal, parallel to the substrate using (c) reflectance imaging (magnified image shows ghost image of the line defect) and (d) fluorescence imaging (magnified image shows no line defect). Cross-sectional images of the colloidal crystal using (e) reflectance and (f) fluorescence imaging. (a-d) are the same scale and (e, f) are the same scale. TPP features can easily be written through the thickness of the colloidal crystal (Fig. 2a) using fluorescence imaging for alignment. SEM images (Figs. 2b and 2c) confirm the high degree of registration seen in the fluorescence image. The apparent misregistration in Figure 2c is actually due to the features having partially fallen over, most likely due to capillary forces during drying. All visible interfaces (both sides of the horizontal portion of the feature as well as the right side of the negatively sloped portion) show excellent registration. Equally important is that registration can be maintained in more than one crystal direction as the feature deviates from a straight line pattern, in this case through a 60 degree bend. The SEM images also confirm the ability of fluorescence imaging to detect crystalline defects. The fluorescence image obtained near the top of the crystal (Fig. 2a) shows a bright line defect running between the first positively sloped section of the two features as well as several point defects. The line defect, which propagates up to the top surface of the crystal, is also visible in the SEM image. In addition to straight line features, point features 1

3 Figure 2. a) Laser scanning fluorescence confocal microscope image parallel to the substrate of two TPP features drawn through the top of the crystal and into the overlying monomer solution. The image plane is within the crystal. Defects can be seen near the TPP feature as bright dots or lines. b) SEM image of the TPP features showing the high degree of lattice registration. c) SEM image of the narrower TPP feature highlighting the registration along two crystalline directions. d) SEM image of a TPP point feature placed between three colloids. smaller than 500nm in diameter can also be precisely positioned. Figure 2d shows a point defect placed in the interstitial site of the colloidal lattice. A number of designs for optically active structures in photonic crystal rely on precise placement of point defects 14, 16, and, due to their small size relative to the PC structure, precise control over their position is very important. While aligning features written through the crystal is an effective demonstration of our technique, it is important for optically interesting structures that registration can be achieved for embedded features. Fluorescence imaging enables registration of embedded TPP features with the colloidal crystal lattice while avoiding intrinsic defects (Fig. 3). Figure 3a presents an embedded polymer feature written with lattice registration near two point defects, which can be observed as bright spots to the right and left of the middle segment of the feature. While intrinsic defects may have a negative effect on optical properties, this feature was intentionally written between the intrinsic defects to demonstrate the control afforded by combining fluorescence imaging and TPP. It is clear from Figure 3b that reflectance imaging is not satisfactory to either image the TPP features or any intrinsic defects. Figure 3c shows that the feature was written within the bulk of the colloidal crystal, not at an interface where reflection imaging might still be possible. The ability to register embedded features with the lattice and position them accurately with respect to crystalline defects makes possible the idea of using the intrinsic defects as optical elements. Figure 3d presents an example of two waveguides positioned on the sides of two point defects. A number of proposed photonic crystal devices rely on waveguide-cavity structures, including proposed designs for channel drop filters. 25, 26 In such a device two waveguides are coupled for a certain frequency through a resonant cavity structure, allowing a waveguide carrying multiple signals to drop the coupled frequency into the other waveguide, leaving all other frequencies unaffected. The proposed structures require the waveguides to be positioned within a few lattice constants of the resonant cavity. Figure 3d demonstrates a simple fabrication route for such a device, using the intrinsic point defects of the colloidal crystal as the source of the resonant cavity. The size of such a cavity can be tuned by various processing techniques we have described 12, 27 elsewhere to tune the coupling as required. The waveguides are written within a lattice constant of either side of the cavity, demonstrating the control required according 25, 26 to theoretical studies. In addition to alignment within a plane of the crystal, the precise placement of embedded features with respect to the crystal thickness is also very important. This is particularly true for cases where light is coupling from outside the crystal to an embedded feature such as a cavity or planar defect. The ability of light to couple to an embedded defect as well as the degree of confinement of light in an embedded feature both strongly depend on the features location with respect to the crystal surface. Fluorescence imaging and our TPP feature fabrication process allow the precise positioning of defects in the crystal thickness in addition to in-plane crystallographic alignment. Planar defects were fabricated (Fig. 4b, c, g) positioned at 25%, 33%, 50%, 66% and 75% of the total crystal thickness (measured in distance from the substrate, Fig. 4a). The precise alignment of the planar defects within the photonic crystal can be seen clearly in Figures 4d, e, f. Figure 3. Laser scanning confocal microscope images. a) Fluorescence image parallel to the substrate of an embedded TPP feature. Two point defects in the colloidal crystal are seen as regions of high fluorescent intensity near the center of the feature. b) Reflectance image parallel to the substrate of the same feature; intrinsic crystal defects cannot be seen in the image. c) Fluorescence image perpendicular to the substrate of the cross section of a TPP feature embedded nearly halfway into the depth of the colloidal crystal. d) Laser scanning fluorescence confocal microscope image parallel to the substrate of two waveguides written beside a double point defect structure. To demonstrate the importance of feature placement in the direction of crystal thickness on the optics of defects, the reflectance was measured for the 25%, 50% and 75% planar defects and compared to theoretical reflectance data (Fig. 5). Theoretical data are Scalar Wave Approximation (SWA) plots with the defect modeled as a photonic crystal surrounded by polymer. The theory matches the data quite well for each case. It is apparent that the defects placed in the center of the crystal or closer to the substrate (Fig 5b, c) create more well defined defect modes compared to defects closer to the surface of the crystal (Fig 5d). In order to better visualize the effect of defect position on the optical properties of these structures the reflected 2

4 deeper into the crystal will be more difficult for light to couple to as most of the light is reflected after 5 to 10 layers (Fig. A2). It should also be noted that the position in term of a few percent of crystal thickness is also important to determine the frequency of the defect mode which oscillates as the defect position moves from substrate to surface. Figure 5. Experimental (thick line) and simulated (thin line) reflectance data for a bare opal (a), opal with a defect at 25% of the crystal thickness (b), defect at 50% of the crystal thickness (c) and defect at 75% of the crystal thickness (d). e) Simulated reflected intensity (colors indicate intensity) as a function of defect position and normalized frequency. All simulations are for 17 layer opal plus 0.65a thick planar defect on a Si substrate. Figure 4. a) Schematic of planar defects written at various locations within a colloidal photonic crystal. b) Defects written at 25% and 50% according to the schematic in (a). c) Defects written at 50% and 75% according to the schematic in (a). Magnified images of defects written at 25% (d), 50% (e) and 75% (f). g) Dual planar defect structure with defects at 33% and 66%. intensity was plotted as a function of frequency and defect position (Fig. 5e). It is immediately clear that defects positioned near the center of the crystal produce the most well defined defect modes, as one would expect. In agreement with the reflectance data from Figure 5a-d, the contour plot clearly shows defects positioned slightly closer to the substrate exhibit better optical properties than those positioned closer to the surface for this low refractive index contrast system. This asymmetry is a substrate effect which is strong in this system because more light can reach the substrate than in a high refractive index contrast system. In this case the effect is also increased due to the strongly reflecting silicon substrate; reflectance measurements of an identical sample on a low refractive index substrate such as glass would show a reduced asymmetry (Fig. A1). When the sample is converted to a high refractive index material such as silicon this effect will diminish because significantly less light will reach the substrate. In addition, defects placed The advantages of fluorescence imaging in conjunction with TPP are clearly apparent. Imaging through the thickness parallel to the substrate, imaging the entire cross-section perpendicular to the substrate as well as direct observation of intrinsic crystalline defects have been successfully demonstrated. Fluorescence imaging provides great flexibility in positioning embedded defects by enabling precise control over location with respect to intrinsic crystal defects as well as feature registration with the photonic crystal lattice. The optics of embedded features can be greatly affected by not only their in-plane alignment but also their position relative to the surface of the photonic crystal. The precise positioning of defects in the crystal thickness and the corresponding effects on their optical properties have been demonstrated. It is seen that the ability of light to couple to embedded defects is highly dependant on the location of the defect within the crystal thickness, making precise control over defect placement essential. The techniques demonstrated here may make possible numerous photonic devices which require specific feature placement within the photonic structure. 3

5 Acknowledgements This material is based upon work supported by the U. S. Army Research Laboratory and the U. S. Army Research Office grant DAAD This work was carried out in part in the Beckman Institute Microscopy Suite, UIUC and the Center for Microanalysis of Materials, UIUC, which is partially supported by the U.S. Department of Energy under grant DEFG02-91-ER We gratefully acknowledge Dr. L.-S. Tan (U.S. Air Force Research Laboratory) for providing the two-photon sensitive dyes. We would also like to acknowledge, from our lab, Dr. S. A. Rinne for experimental assistance, Dr. Florencio García- Santamaría for the use of his Scalar Wave Approximation code and both for insight and helpful discussions during this work. *Electronic address: While planar defects in high refractive index contrast systems are expected to show strong coupling for a very narrow range of resonant frequencies, this is typically not observed experimentally in three-dimensional photonic crystals. When the low refractive index system in this work is converted to silicon it becomes significantly more difficult for light to couple into the deeply embedded defect structures and the cavities typically have low quality factors, Q, because of the finite nature of the crystal and the cavities position near the bottom of the crystal. In this case the spectra should not show significant dips in reflectance due to coupled modes, which is what is seen experimentally (Fig. A2). The defects at 25% and 50% of the crystal thickness show strong effects on the optical properties of the system where the defect at 75% shows a distortion of the peak but less clearly defined modes. Auxiliary Information The asymmetry of the contour plot of reflected intensity as a function of frequency vs. defect position is caused by substrate effects, namely reflections from the crystal/substrate interface. If the plot is recalculated using a silica substrate (n=1.5) the asymmetry is clearly reduced as the light that penetrates the crystal is reflected from the substrate less strongly (Fig. A1). The background in Figure A1a-d is significantly lower due to the reduced substrate reflections however the defect modes are still clearly defined and the most strongly confined when the defect is in the center of the crystal. Figure A2. Experimental reflectance spectra for a colloidal photonic crystal filled with approximately 55nm of silicon from a) a bare opal, b) defect at 25% of the crystal thickness, c) defect at 50% of the crystal thickness and d) defect at 75% of the crystal thickness. References Figure A1. Simulated reflectance data for a bare opal (a), opal with a defect at 25% of the crystal thickness (b), defect at 50% of the crystal thickness (c), and defect at 75% of the crystal thickness (d). e) Reflected intensity (colors indicate intensity) as a function of defect position and normalized frequency. All simulations are for a 17 layer opal plus 0.65a thick planar defect on a SiO 2 substrate S. John, Physical Review Letters 58, 2486 (1987). J. D. Joannopoulos, P. R. Villeneuve, and S. Fan, Nature 386, 143 (1997). E. Yablonovitch, Physical Review Letters 58, 2059 (1987). S. John and T. Quang, Physical Review Letters 78, 1888 (1997). S. Noda, K. Tomoda, N. Yamamoto, et al., Science 289, 604 (2000). S.-Y. Lin, E. Chow, V. Hietala, et al., Science 282, 274 (1998). A. Mekis, J. C. Chen, I. Kurland, et al., Physical Review Letters 77, 3787 (1996). 4

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