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Printable photonic crystals with high refractive index for applications in visible light
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2016 Nanotechnology 27 115303 (http://iopscience.iop.org/0957-4484/27/11/115303) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 132.174.255.116 This content was downloaded on 08/03/2016 at 04:19
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 27 (2016) 115303 (7pp)
doi:10.1088/0957-4484/27/11/115303
Printable photonic crystals with high refractive index for applications in visible light Giuseppe Calafiore1,2, Quentin Fillot3, Scott Dhuey3, Simone Sassolini3, Filippo Salvadori3, Camilo A Mejia1, Keiko Munechika1, Christophe Peroz1, Stefano Cabrini3 and Carlos Piña-Hernandez1 1
aBeam Technologies, 22290 Foothill Blvd. St2, Hayward, CA 94541, USA Polytechnic University of Turin, Corso Duca degli Abruzzi 24, 10129, Turin, Italy 3 The Molecular Foundry, LBNL, One Cyclotron Road, Berkeley, CA 94702, USA 2
E-mail: [email protected] and [email protected] Received 2 November 2015, revised 15 January 2016 Accepted for publication 20 January 2016 Published 15 February 2016 Abstract
Nanoimprint lithography (NIL) of functional high-refractive index materials has proved to be a powerful candidate for the inexpensive manufacturing of high-resolution photonic devices. In this paper, we demonstrate the fabrication of printable photonic crystals (PhCs) with high refractive index working in the visible wavelengths. The PhCs are replicated on a titanium dioxide-based high-refractive index hybrid material by reverse NIL with almost zero shrinkage and high-fidelity reproducibility between mold and printed devices. The optical responses of the imprinted PhCs compare very well with those fabricated by conventional nanofabrication methods. This study opens the road for a low-cost manufacturing of PhCs and other nanophotonic devices for applications in visible light. Keywords: titanium dioxide, functional material, photonic crystal, nanoimprint lithography, reverse imprint, optical devices (Some figures may appear in colour only in the online journal) nanoscale resolution [11]. Conventionally, in NIL patterns are imprinted onto a resist and transferred into an under-layer, which carries the properties required by the device. This process is well-established but requires two etching steps: one is to remove the residual layer of the imprinted resist; and a second for the pattern transfer. A more convenient approach consists of directly imprinting a functional material and fabricate devices through a one-step process [12–14]. To fabricate PhCs for visible light, a material with both a high refractive index (n) and low extinction coefficient (k) in the visible range is required. High refractive index ensures tighter confinement of the optical mode in the patterned medium, and a smaller footprint, while a low extinction coefficient is required to prevent optical absorption and losses when the light travels through the material. One of the most promising materials for this purpose is titanium dioxide (TiO2), having n>2 and an excellent optical transmission (>90%) down to
1. Introduction Photonic crystals (PhCs) are structures with a periodic dielectric function that are selective to wavelength and angle of the propagating light and can exhibit photonic band-gaps [1, 2]. These structural materials have found applications in many different areas including telecommunication and laser science [1, 3–5]. In the visible range, PhCs have been fabricated for self-collimation beam propagation [6], enhanced fluorescence [7], enhanced light extraction [8], lasing [9], and non-linear optics [10]. However these applications are often limited to research laboratories due to their expensive fabrication by electron beam lithography (EBL) and plasma etching. For this reason, faster and cheaper patterning techniques have recently attracted considerable attention, in particular nanoimprint lithography (NIL). NIL is a low-cost, high-throughput nano-patterning technique with single-digit 0957-4484/16/115303+07$33.00
1
© 2016 IOP Publishing Ltd Printed in the UK
Nanotechnology 27 (2016) 115303
G Calafiore et al
Figure 1. Scheme of R-NIL process on the TiO2 hybrid material for the fabrication of 2D PhCs.
400 nm wavelength [15]. Due to the difficulty to pattern TiO2 only a few devices have been demonstrated [16, 17]. In our previous works, we have developed a hybrid organic-inorganic, imprintable TiO2 material, which proved sub-10 nm patterning resolution [18]. By using this novel material, we have demonstrated the fabrication of simple planar nanophotonic devices including waveguides, on-chip demultiplexers, and light splitters [12, 18]. In order to reach a high refractive index, the material requires an annealing step after imprint [18], which also causes severe shrinkage of the imprinted patterns, up to 70% in the vertical dimension. For in-plane measurements of optical devices this issue can be avoided by imprinting TiO2 on an auxiliary high-refractive index material, like silicon nitride (Si3N4). The perturbation of the refractive index on the surface of Si3N4 that is accomplished by patterned TiO2 creates a guided-mode inside the Si3N4 layer. However the inherent shrinkage associated with this process is not suitable to develop nanophotonic devices such as PhCs, for which the optical modes need to lay within the imprinted optical films. In this paper, we present a novel process to control the shrinkage of the imprinted photonic nanostructures and fabricate printable photonic devices with almost zero shrinkage. The process is based on a multistep reverse NIL (R-NIL) [19] of TiO2-material. We test the process to pattern two dimensional PhC slabs that exhibit resonances in the visible spectrum. Measurements of the PhCs are compared to simulations to provide evidences of the potential of the proposed approach to pattern photonic structures in the visible range with performance that is
comparable to more conventional fabrication methods, such as EBL and etching.
2. Fabrication by R-NIL of high refractive index PhCs Figure 1 shows the steps implemented to fabricate twodimensional PhCs by reverse nanoimprint onto TiO2 based optical hybrid material. The process in [20] was employed to fabricate the mold by EBL on hydrogen silsesquioxane (HSQ), with a depth of 160 nm. The mold is treated with an antiadhesive layer (gold) to create a low energy surface and facilitate pattern release. The hybrid organic-inorganic TiO2 material is spin coated on the mold and solvent content is evaporated through a soft baking at 100 °C for 1 min (figure 1.1) followed by thermal annealing at 400 °C for 10 min (figure 1.2). The thermal step degrades the organic component and transforms almost the entire material into polycrystalline TiO2. After annealing, the film is cooled down to room temperature. The annealed film is imprinted onto a glass substrate (figure 1.3) with the help of a polymeric layer (ormostamp, Micro Resist Technology) [21], which is crosslinked by Ultraviolet (UV) light (figure 1.4). The mold is finally released, resulting in a 160 nm thick PhC slab of TiO2 (figure 1.5). Contrary to our previous work where imprinted chips are annealed after demolding [12], here the hybrid material is thermally treated while still embedded in the mold which delimits the shrinkage mainly to the vertical direction; most of 2
Nanotechnology 27 (2016) 115303
G Calafiore et al
Figure 2. Characterization of the R-NIL fabrication process. (a) SEM Cross-sectional view of a mono-layer of TiO2 material on the HSQ mold after annealing. The shrinkage is still present. (b) SEM cross-sectional view of a tri-layer of TiO2 material on the mold. Imprint in conformal with the mold protrusions. (c) SEM top-view of the mold for a PhC having a lattice constant of 355 nm and a diameter of 173 nm. (d) SEM top-view of the PhC imprinted by mono-layer R-NIL. (e) SEM top-view of the PhC imprinted by tri-layer R-NIL. (f) Distribution of diameters of mold, mono-layer and tri-layer R-NIL for the PhC in (c).
the shrinkage occurs on the surface of the TiO2 hybrid layer following a path towards the mold. Figure 2(a) shows a scanning electron microscope (SEM) cross-sectional view of the mold (bottom) and hybrid material (top) after annealing (like in figure 1.2). The patterns still exhibit a severe shrinkage, which implies that conventional R-NIL is not sufficient to prevent the loss of critical dimension [19]. To overcome this shrinkage, steps 1 and 2 of the process (figure 1) are sequentially repeated three times, starting with a very thin hybrid material layer (
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My IOPscience
Printable photonic crystals with high refractive index for applications in visible light
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2016 Nanotechnology 27 115303 (http://iopscience.iop.org/0957-4484/27/11/115303) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 132.174.255.116 This content was downloaded on 08/03/2016 at 04:19
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 27 (2016) 115303 (7pp)
doi:10.1088/0957-4484/27/11/115303
Printable photonic crystals with high refractive index for applications in visible light Giuseppe Calafiore1,2, Quentin Fillot3, Scott Dhuey3, Simone Sassolini3, Filippo Salvadori3, Camilo A Mejia1, Keiko Munechika1, Christophe Peroz1, Stefano Cabrini3 and Carlos Piña-Hernandez1 1
aBeam Technologies, 22290 Foothill Blvd. St2, Hayward, CA 94541, USA Polytechnic University of Turin, Corso Duca degli Abruzzi 24, 10129, Turin, Italy 3 The Molecular Foundry, LBNL, One Cyclotron Road, Berkeley, CA 94702, USA 2
E-mail: [email protected] and [email protected] Received 2 November 2015, revised 15 January 2016 Accepted for publication 20 January 2016 Published 15 February 2016 Abstract
Nanoimprint lithography (NIL) of functional high-refractive index materials has proved to be a powerful candidate for the inexpensive manufacturing of high-resolution photonic devices. In this paper, we demonstrate the fabrication of printable photonic crystals (PhCs) with high refractive index working in the visible wavelengths. The PhCs are replicated on a titanium dioxide-based high-refractive index hybrid material by reverse NIL with almost zero shrinkage and high-fidelity reproducibility between mold and printed devices. The optical responses of the imprinted PhCs compare very well with those fabricated by conventional nanofabrication methods. This study opens the road for a low-cost manufacturing of PhCs and other nanophotonic devices for applications in visible light. Keywords: titanium dioxide, functional material, photonic crystal, nanoimprint lithography, reverse imprint, optical devices (Some figures may appear in colour only in the online journal) nanoscale resolution [11]. Conventionally, in NIL patterns are imprinted onto a resist and transferred into an under-layer, which carries the properties required by the device. This process is well-established but requires two etching steps: one is to remove the residual layer of the imprinted resist; and a second for the pattern transfer. A more convenient approach consists of directly imprinting a functional material and fabricate devices through a one-step process [12–14]. To fabricate PhCs for visible light, a material with both a high refractive index (n) and low extinction coefficient (k) in the visible range is required. High refractive index ensures tighter confinement of the optical mode in the patterned medium, and a smaller footprint, while a low extinction coefficient is required to prevent optical absorption and losses when the light travels through the material. One of the most promising materials for this purpose is titanium dioxide (TiO2), having n>2 and an excellent optical transmission (>90%) down to
1. Introduction Photonic crystals (PhCs) are structures with a periodic dielectric function that are selective to wavelength and angle of the propagating light and can exhibit photonic band-gaps [1, 2]. These structural materials have found applications in many different areas including telecommunication and laser science [1, 3–5]. In the visible range, PhCs have been fabricated for self-collimation beam propagation [6], enhanced fluorescence [7], enhanced light extraction [8], lasing [9], and non-linear optics [10]. However these applications are often limited to research laboratories due to their expensive fabrication by electron beam lithography (EBL) and plasma etching. For this reason, faster and cheaper patterning techniques have recently attracted considerable attention, in particular nanoimprint lithography (NIL). NIL is a low-cost, high-throughput nano-patterning technique with single-digit 0957-4484/16/115303+07$33.00
1
© 2016 IOP Publishing Ltd Printed in the UK
Nanotechnology 27 (2016) 115303
G Calafiore et al
Figure 1. Scheme of R-NIL process on the TiO2 hybrid material for the fabrication of 2D PhCs.
400 nm wavelength [15]. Due to the difficulty to pattern TiO2 only a few devices have been demonstrated [16, 17]. In our previous works, we have developed a hybrid organic-inorganic, imprintable TiO2 material, which proved sub-10 nm patterning resolution [18]. By using this novel material, we have demonstrated the fabrication of simple planar nanophotonic devices including waveguides, on-chip demultiplexers, and light splitters [12, 18]. In order to reach a high refractive index, the material requires an annealing step after imprint [18], which also causes severe shrinkage of the imprinted patterns, up to 70% in the vertical dimension. For in-plane measurements of optical devices this issue can be avoided by imprinting TiO2 on an auxiliary high-refractive index material, like silicon nitride (Si3N4). The perturbation of the refractive index on the surface of Si3N4 that is accomplished by patterned TiO2 creates a guided-mode inside the Si3N4 layer. However the inherent shrinkage associated with this process is not suitable to develop nanophotonic devices such as PhCs, for which the optical modes need to lay within the imprinted optical films. In this paper, we present a novel process to control the shrinkage of the imprinted photonic nanostructures and fabricate printable photonic devices with almost zero shrinkage. The process is based on a multistep reverse NIL (R-NIL) [19] of TiO2-material. We test the process to pattern two dimensional PhC slabs that exhibit resonances in the visible spectrum. Measurements of the PhCs are compared to simulations to provide evidences of the potential of the proposed approach to pattern photonic structures in the visible range with performance that is
comparable to more conventional fabrication methods, such as EBL and etching.
2. Fabrication by R-NIL of high refractive index PhCs Figure 1 shows the steps implemented to fabricate twodimensional PhCs by reverse nanoimprint onto TiO2 based optical hybrid material. The process in [20] was employed to fabricate the mold by EBL on hydrogen silsesquioxane (HSQ), with a depth of 160 nm. The mold is treated with an antiadhesive layer (gold) to create a low energy surface and facilitate pattern release. The hybrid organic-inorganic TiO2 material is spin coated on the mold and solvent content is evaporated through a soft baking at 100 °C for 1 min (figure 1.1) followed by thermal annealing at 400 °C for 10 min (figure 1.2). The thermal step degrades the organic component and transforms almost the entire material into polycrystalline TiO2. After annealing, the film is cooled down to room temperature. The annealed film is imprinted onto a glass substrate (figure 1.3) with the help of a polymeric layer (ormostamp, Micro Resist Technology) [21], which is crosslinked by Ultraviolet (UV) light (figure 1.4). The mold is finally released, resulting in a 160 nm thick PhC slab of TiO2 (figure 1.5). Contrary to our previous work where imprinted chips are annealed after demolding [12], here the hybrid material is thermally treated while still embedded in the mold which delimits the shrinkage mainly to the vertical direction; most of 2
Nanotechnology 27 (2016) 115303
G Calafiore et al
Figure 2. Characterization of the R-NIL fabrication process. (a) SEM Cross-sectional view of a mono-layer of TiO2 material on the HSQ mold after annealing. The shrinkage is still present. (b) SEM cross-sectional view of a tri-layer of TiO2 material on the mold. Imprint in conformal with the mold protrusions. (c) SEM top-view of the mold for a PhC having a lattice constant of 355 nm and a diameter of 173 nm. (d) SEM top-view of the PhC imprinted by mono-layer R-NIL. (e) SEM top-view of the PhC imprinted by tri-layer R-NIL. (f) Distribution of diameters of mold, mono-layer and tri-layer R-NIL for the PhC in (c).
the shrinkage occurs on the surface of the TiO2 hybrid layer following a path towards the mold. Figure 2(a) shows a scanning electron microscope (SEM) cross-sectional view of the mold (bottom) and hybrid material (top) after annealing (like in figure 1.2). The patterns still exhibit a severe shrinkage, which implies that conventional R-NIL is not sufficient to prevent the loss of critical dimension [19]. To overcome this shrinkage, steps 1 and 2 of the process (figure 1) are sequentially repeated three times, starting with a very thin hybrid material layer (
