- Email: [email protected]
Fabrication of coaxial plasmonic crystals by focused ion beam milling and electron-beam lithography
Materials Letters 100 (2013) 192–194
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Fabrication of coaxial plasmonic crystals by focused ion beam milling and electron-beam lithography Xiaoxiao Jiang, Qiongchan Gu, Fengwen Wang, Jiangtao Lv, Zhenhe Ma, Guangyuan Si n College of Information Science and Engineering, Northeastern University, Shenyang 110004, China
art ic l e i nf o
a b s t r a c t
Article history: Received 9 November 2012 Accepted 6 March 2013 Available online 21 March 2013
Plasmonic coaxial apertures are emerging candidates for single layer metamaterials at visible frequencies. Different mechanisms of extraordinary optical transmission can be observed from coaxial nanorings and they can support localized Fabry–Pérot plasmon modes. However, nanoring fabrication is a challenge due to the well-known difficulties of etching metals, especially for ultrasmall features with deep etching. Here, we demonstrate the fabrication of coaxial structures (nanoholes and nanoparticles) using focused ion beam milling and electron-beam lithography followed by argon ion milling, respectively. Coaxial apertures with up to 20:1 aspect ratio have been achieved using focused ion beam lithography. Single layer metamaterials with high fabrication quality are shown, which could find extensive applications in nanofocusing and imaging. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Coaxial apertures Nanoholes Nanoparticles Ion beam technology
1. Introduction Surface plasmons are electromagnetic waves propagating along metallic–dielectric interfaces and they have drawn considerable attention in the past 10 years since extraordinary optical transmission (EOT) phenomenon was first reported in 1998 [1]. Furthermore, they can take different forms, from free waves to localized oscillations, depending upon the geometry of the structure. Most recently, coaxial structures have become an important candidate for metamaterials in the visible range [2–6]. Different from the mechanisms of EOT observed from subwavelength cylindrical nanoholes, such ring-like structures can support propagating plasmon modes due to strong Fabry–Pérot resonances. The dispersion of a single coaxial structure in silver was characterized recently [7]. More theoretical investigations on coaxial structures were carried out elsewhere [8–11]. Here, we report the fabrication of coaxial nanoring structures with different techniques. Both air- and metal-filled ring apertures are realized. Patterns with good uniformity and high accuracy are shown. Using focused ion beam (FIB) lithography, highly homogeneous nanoring arrays are demonstrated with down to 15 nm gap width and 20:1 aspect ratio (300 nm total etching depth) in gold films. To obtain similar complementary patterns, electronbeam lithography (EBL) and argon ion milling techniques are applied. The patterns achieved in this work could find promising
n
Corresponding author. Tel.: þ86 13603239536. E-mail address: [email protected] (G. Si).
0167-577X/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.03.040
applications in plasmson-assisted sensing, nanofocusing, and imaging [12–20].
2. Experimental Nanoring holes fabricated via FIB milling: Quartz substrates were ultrasonically cleaned using acetone before loading into the chamber of an electron-beam evaporator (Edwards Auto 306 system). Then a gold film with 300 nm thickness was deposited with a 5 nm titanium adhesion layer by evaporation for about 40 min total evaporation time. To minimize the evaporationintroduced roughness during metal film deposition, low evaporation rates were used (less than 0.13 nm/s average deposition rate). Both titanium and gold films were deposited sequentially on quartz substrates without breaking vacuum in the evaporator chamber at a base pressure of about 3 10−7 mbar. Then pre-cleaned samples were transferred to the FIB chamber. Coaxial nanoring waveguides with different gap aperture widths were drilled using a single-beam FIB system (FIB200, FEI Corporation). To minimize redeposition and charging effects, all patterns were milled in parallel instead of serially. A probe current of 70 pA was applied with 30 kV acceleration voltage. Coaxial nanoparticles fabricated by EBL followed by argon ion milling: Alternatively, complementary counterpart patterns of metallic coaxial nanoparticles (instead of air gaps surrounded by metal) were fabricated by EBL followed by ion milling. Negative resist NEB 22 was used for pattern definition and an argon source was applied for ion milling (Microetch 1201 Ion Beam Etch System). Limited by etch rates and selectivity issues, 130 nm thick
X. Jiang et al. / Materials Letters 100 (2013) 192–194
193
Fig. 1. SEM images of coaxial waveguides with different gap widths in a gold film on a quartz substrate. (a), (b), (c) and (d), outer radii Ro ¼ 220 nm, 260 nm, 300 nm and 340 nm with fixed inner radius Ri ¼ 200 nm and period Λ ¼1200 nm.
Fig. 2. (a) Coaxial structures with 160 nm inner radius and 175 nm outer radius. (b) Cross sectional view of 15 nm gap width coaxial apertures. Top view SEM images of metallic coaxial nanoparticles fabricated by EBL and ion milling with (c) 100 nm inner radius and 200 nm outer radius and (d) 60 nm inner radius and 280 nm outer radius.
gold films with 4 nm titanium adhesion were employed here. In addition, to control the temperature in the chamber during milling, the process was run for two rounds (3 min per run) with a 30 min intervening interval to cool down the system; otherwise resists may be burned due to high resultant temperature and cannot be removed by the stripper thereafter.
3. Results and discussion One can clearly observe an increasing gap width from Fig. 1(a) to (d), corresponding to 20 nm, 60 nm, 100 nm, and 140 nm (fixed inner radius Ri ¼200 nm and varying outer radii Ro ¼220 nm, 260 nm, 300 nm and 340 nm). By design, it is feasible to obtain
a wide range of metamaterials with such geometries in the visible range. The FIB technique utilizes ions (gallium, normally) produced by a liquid–metal source to scan over a sample surface and sputter away bulk material. The quality of patterns is determined by many ion beam variables such as dosage, dwell time, beam overlap and so on. By optimizing these parameters, ultrafine features are achievable. Therefore, FIB can be expected to manufacture sharp structures and obtain high aspect ratios [21,22], especially for nanoring [23–25] geometries because more beam energy can be concentrated in the coaxial area which gives rise to remarkably reduced energy spread compared to circle-shaped nanohole milling. Fig. 2(a) demonstrates such coaxial structures with ultrasmall gap width. One inevitable drawback of FIB drilled structure is the tapered profile resulted from redeposition effects during milling.
194
X. Jiang et al. / Materials Letters 100 (2013) 192–194
high-sensitivity detecting and optical imaging. The calculated electric field distributions are shown in the inset of Fig. 3. Obviously, strong confinement of light in the nanoring cavity is observed.
4. Conclusions
Fig. 3. FDTD-simulated transmittance of rings with 15 nm gap width as a function of wavelength with the measured data for comparison. Inset: top view (top panel) and cross sectional view (bottom panel) showing the electric intensity distribution of a single ring with 15 nm gap width illuminated at 790 nm (λmax) and 675 nm (λmin).
Vertical sidewalls are preferred. Non-vertical sidewalls may cause high propagation loss and further affect the performance of fabricated devices. Thus, it is important to check the cross sections of the nanoring structures. Fig. 2(b) shows the cross sectional view of coaxial apertures with 15 nm gap width (160 nm inner radius and 175 nm outer radius). Strong contrast at the periphery of the rings is caused by redeposition during the milling process. We can also find slight damage at the ring periphery and large gaps at the top portions due to long-time milling. One should note the damage is severe once the milling time is extended and seriously tapered outlines can be generated for large apertures because of the inevitable redeposition problem. However, the sidewall is almost vertical except that the top part presents cone-shaped profile. As shown in Fig. 2(c) and (d), coaxial nanoparticles are fabricated by EBL and argon ion milling. Owning to the versatility of the EBL technique, nanoring particles with various aperture sizes are realized, corresponding to 100 nm (100 nm inner radius and 200 nm outer radius) and 220 nm (60 nm inner radius and 280 nm outer radius) gap width, respectively. Compared to FIB-drilled nanoring holes, these EBL-fabricated particles present rougher borderlines. Additionally, the sample surface is not as smooth as the one of FIB-milled holes. These fabrication imperfections may depress device performance. The divergence between calculations and experiments is also attributed to the idealized features used in simulations and imperfect structures of fabricated elements used for testing. To further study the underlying physical principles related to the coaxial apertures, we carried out finite difference time domain (FDTD) simulations (http://www.lumerical.com/). Fig. 3 plots the simulated transmittance of 15 nm gap apertures with measured data for comparison. The slight divergence between measurements and simulations is caused by the shape difference between the simulation model (perfect nanorings) and the real sample (rough surface and tapered profile). Nevertheless, the simulation results agree qualitatively with experiments. As expected, the cylindrical plasmon resonance peak is centered at around 790 nm. By controlling the incident wavelength, one can simply turn ON/OFF the Fabry–Pérot nanoring cavity, leading to useful plasmon-assisted
In summary, we have shown the fabrication of high quality plasmonic nanoring holes and particles using different manufacturing tools. Both air- and metal-filled coaxial structures were fabricated by FIB milling and EBL followed by argon ion milling. The uniform profiles of the narrow gaps were also demonstrated using FIB cross sectional milling. Although the total milling depth and aspect ratio are limited by etching rates and selectivity issues using EBL, one can realize deep etching with FIB techniques. Different geometries with various lattice shapes are thus available in a wide range of metallic materials now. The whole process is completely programmable and monolithic. Such geometries are crucial in fabricating plasmonic devices and can find extensive and promising applications in focusing and imaging.
Acknowledgments This work was supported by NEU internal funding XNB201302 and Natural Science Foundation of Hebei Province under Grant no. A2013501049. J. Lv and Z. Ma gratefully acknowledge the Fundamental Research Funds for the Fundamental Research Funds for the Central Universities (N100423005), National Natural Science Foundation of China (31170956), and Hebei Province Science and Technology Plan Projects (10276722). G. Si thanks Dr. Yan Jun Liu for discussions and simulations.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
Ebbesen TW, Lezec HJ, Ghaemi HF, Thio T, Wolff PA. Nature 1998;391:667–9. Burgos SP, de Waele R, Polman A, Atwater HA. Nat Mater 2010;9:407–12. de Waele R, Burgos SP, Atwater HA, Polman A. Opt Express 2010;18:12770–8. Poujet Y, Salvi J, Baida FI. Opt Lett 2007;32:2942–4. Baida FI, Belkhir A, Van Labeke D, Lamrous O. Phys Rev B 2006;74:205419. Rodríguez-Fortuño FJ, García-Meca C, Ortuño R, Martí J, Martínez A. Opt Lett 2009;34:3325–7. de Waele R, Burgos SP, Polman A, Atwater HA. Nano Lett 2009;9:2832–7. Baida FI, Van Labeke D. Phys Rev B 2003;67:155314. Haftel MI, Schlockermann C, Blumberg G. Phys Rev B 2006;74:235405. Haftel MI, Schlockermann C, Blumberg G. Appl Phys Lett 2006;88:193104. Orbons SM, Roberts A, Jamieson DN, Haftel MI, Schlockermann C, Freeman D, et al. Appl Phys Lett 2007;90:251107. Ono A, Kato J, Kawata S. Phys Rev Lett 2005;95:267407. Kawata S, Ono A, Verma P. Nat Photonics 2008;2:438–42. Liu Y, Bartal G, Zhang X. Opt Express 2008;16:15439–48. Shvets G, Trendafilov S, Pendry JB, Sarychev A. Phys Rev Lett 2007;99:053903. Liu GQ, Tang FL, Li L, Gong LX, Ye ZQ. Mater Lett 2011;65:1998–2000. Cai Y, Li Y, Nordlander P, Cremer PS. Nano Lett 2012;12:4881–8. Ye J, Shioi M, Lodewijks K, Lagae L, Kawamura T, Van Dorpe P. Appl Phys Lett 2010;97:163106. Tsai C, Lu S, Lin J, Lee P. Appl Phys Lett 2011;98:153108. Lehr D, Dietrich K, Helgert C, Käsebier T, Fuchs H, Tünnermann A, et al. Opt Lett 2012;37:157–9. Si G, Teo EJ, Bettiol AA, Teng J, Danner AJ. J Vac Sci Technol B 2010;28:316. Si G, Danner AJ, Teo SL, Teo EJ, Teng J, Bettiol AA. J Vac Sci Technol B 2011;29 (2):021205. Si G, Zhao Y, Liu H, Teo S, Zhang M, Huang TJ, et al. App Phys Lett 2011;99:033105. Liu YJ, Si GY, Leong ESP, Xiang N, Danner AJ, Teng JH. Adv Mater 2012;24: OP131. Korsunsky AM, Sebastiani M, Bemporad E. Mater Lett 2009;63:1961–3.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Fabrication of coaxial plasmonic crystals by focused ion beam milling and electron-beam lithography Xiaoxiao Jiang, Qiongchan Gu, Fengwen Wang, Jiangtao Lv, Zhenhe Ma, Guangyuan Si n College of Information Science and Engineering, Northeastern University, Shenyang 110004, China
art ic l e i nf o
a b s t r a c t
Article history: Received 9 November 2012 Accepted 6 March 2013 Available online 21 March 2013
Plasmonic coaxial apertures are emerging candidates for single layer metamaterials at visible frequencies. Different mechanisms of extraordinary optical transmission can be observed from coaxial nanorings and they can support localized Fabry–Pérot plasmon modes. However, nanoring fabrication is a challenge due to the well-known difficulties of etching metals, especially for ultrasmall features with deep etching. Here, we demonstrate the fabrication of coaxial structures (nanoholes and nanoparticles) using focused ion beam milling and electron-beam lithography followed by argon ion milling, respectively. Coaxial apertures with up to 20:1 aspect ratio have been achieved using focused ion beam lithography. Single layer metamaterials with high fabrication quality are shown, which could find extensive applications in nanofocusing and imaging. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Coaxial apertures Nanoholes Nanoparticles Ion beam technology
1. Introduction Surface plasmons are electromagnetic waves propagating along metallic–dielectric interfaces and they have drawn considerable attention in the past 10 years since extraordinary optical transmission (EOT) phenomenon was first reported in 1998 [1]. Furthermore, they can take different forms, from free waves to localized oscillations, depending upon the geometry of the structure. Most recently, coaxial structures have become an important candidate for metamaterials in the visible range [2–6]. Different from the mechanisms of EOT observed from subwavelength cylindrical nanoholes, such ring-like structures can support propagating plasmon modes due to strong Fabry–Pérot resonances. The dispersion of a single coaxial structure in silver was characterized recently [7]. More theoretical investigations on coaxial structures were carried out elsewhere [8–11]. Here, we report the fabrication of coaxial nanoring structures with different techniques. Both air- and metal-filled ring apertures are realized. Patterns with good uniformity and high accuracy are shown. Using focused ion beam (FIB) lithography, highly homogeneous nanoring arrays are demonstrated with down to 15 nm gap width and 20:1 aspect ratio (300 nm total etching depth) in gold films. To obtain similar complementary patterns, electronbeam lithography (EBL) and argon ion milling techniques are applied. The patterns achieved in this work could find promising
n
Corresponding author. Tel.: þ86 13603239536. E-mail address: [email protected] (G. Si).
0167-577X/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.03.040
applications in plasmson-assisted sensing, nanofocusing, and imaging [12–20].
2. Experimental Nanoring holes fabricated via FIB milling: Quartz substrates were ultrasonically cleaned using acetone before loading into the chamber of an electron-beam evaporator (Edwards Auto 306 system). Then a gold film with 300 nm thickness was deposited with a 5 nm titanium adhesion layer by evaporation for about 40 min total evaporation time. To minimize the evaporationintroduced roughness during metal film deposition, low evaporation rates were used (less than 0.13 nm/s average deposition rate). Both titanium and gold films were deposited sequentially on quartz substrates without breaking vacuum in the evaporator chamber at a base pressure of about 3 10−7 mbar. Then pre-cleaned samples were transferred to the FIB chamber. Coaxial nanoring waveguides with different gap aperture widths were drilled using a single-beam FIB system (FIB200, FEI Corporation). To minimize redeposition and charging effects, all patterns were milled in parallel instead of serially. A probe current of 70 pA was applied with 30 kV acceleration voltage. Coaxial nanoparticles fabricated by EBL followed by argon ion milling: Alternatively, complementary counterpart patterns of metallic coaxial nanoparticles (instead of air gaps surrounded by metal) were fabricated by EBL followed by ion milling. Negative resist NEB 22 was used for pattern definition and an argon source was applied for ion milling (Microetch 1201 Ion Beam Etch System). Limited by etch rates and selectivity issues, 130 nm thick
X. Jiang et al. / Materials Letters 100 (2013) 192–194
193
Fig. 1. SEM images of coaxial waveguides with different gap widths in a gold film on a quartz substrate. (a), (b), (c) and (d), outer radii Ro ¼ 220 nm, 260 nm, 300 nm and 340 nm with fixed inner radius Ri ¼ 200 nm and period Λ ¼1200 nm.
Fig. 2. (a) Coaxial structures with 160 nm inner radius and 175 nm outer radius. (b) Cross sectional view of 15 nm gap width coaxial apertures. Top view SEM images of metallic coaxial nanoparticles fabricated by EBL and ion milling with (c) 100 nm inner radius and 200 nm outer radius and (d) 60 nm inner radius and 280 nm outer radius.
gold films with 4 nm titanium adhesion were employed here. In addition, to control the temperature in the chamber during milling, the process was run for two rounds (3 min per run) with a 30 min intervening interval to cool down the system; otherwise resists may be burned due to high resultant temperature and cannot be removed by the stripper thereafter.
3. Results and discussion One can clearly observe an increasing gap width from Fig. 1(a) to (d), corresponding to 20 nm, 60 nm, 100 nm, and 140 nm (fixed inner radius Ri ¼200 nm and varying outer radii Ro ¼220 nm, 260 nm, 300 nm and 340 nm). By design, it is feasible to obtain
a wide range of metamaterials with such geometries in the visible range. The FIB technique utilizes ions (gallium, normally) produced by a liquid–metal source to scan over a sample surface and sputter away bulk material. The quality of patterns is determined by many ion beam variables such as dosage, dwell time, beam overlap and so on. By optimizing these parameters, ultrafine features are achievable. Therefore, FIB can be expected to manufacture sharp structures and obtain high aspect ratios [21,22], especially for nanoring [23–25] geometries because more beam energy can be concentrated in the coaxial area which gives rise to remarkably reduced energy spread compared to circle-shaped nanohole milling. Fig. 2(a) demonstrates such coaxial structures with ultrasmall gap width. One inevitable drawback of FIB drilled structure is the tapered profile resulted from redeposition effects during milling.
194
X. Jiang et al. / Materials Letters 100 (2013) 192–194
high-sensitivity detecting and optical imaging. The calculated electric field distributions are shown in the inset of Fig. 3. Obviously, strong confinement of light in the nanoring cavity is observed.
4. Conclusions
Fig. 3. FDTD-simulated transmittance of rings with 15 nm gap width as a function of wavelength with the measured data for comparison. Inset: top view (top panel) and cross sectional view (bottom panel) showing the electric intensity distribution of a single ring with 15 nm gap width illuminated at 790 nm (λmax) and 675 nm (λmin).
Vertical sidewalls are preferred. Non-vertical sidewalls may cause high propagation loss and further affect the performance of fabricated devices. Thus, it is important to check the cross sections of the nanoring structures. Fig. 2(b) shows the cross sectional view of coaxial apertures with 15 nm gap width (160 nm inner radius and 175 nm outer radius). Strong contrast at the periphery of the rings is caused by redeposition during the milling process. We can also find slight damage at the ring periphery and large gaps at the top portions due to long-time milling. One should note the damage is severe once the milling time is extended and seriously tapered outlines can be generated for large apertures because of the inevitable redeposition problem. However, the sidewall is almost vertical except that the top part presents cone-shaped profile. As shown in Fig. 2(c) and (d), coaxial nanoparticles are fabricated by EBL and argon ion milling. Owning to the versatility of the EBL technique, nanoring particles with various aperture sizes are realized, corresponding to 100 nm (100 nm inner radius and 200 nm outer radius) and 220 nm (60 nm inner radius and 280 nm outer radius) gap width, respectively. Compared to FIB-drilled nanoring holes, these EBL-fabricated particles present rougher borderlines. Additionally, the sample surface is not as smooth as the one of FIB-milled holes. These fabrication imperfections may depress device performance. The divergence between calculations and experiments is also attributed to the idealized features used in simulations and imperfect structures of fabricated elements used for testing. To further study the underlying physical principles related to the coaxial apertures, we carried out finite difference time domain (FDTD) simulations (http://www.lumerical.com/). Fig. 3 plots the simulated transmittance of 15 nm gap apertures with measured data for comparison. The slight divergence between measurements and simulations is caused by the shape difference between the simulation model (perfect nanorings) and the real sample (rough surface and tapered profile). Nevertheless, the simulation results agree qualitatively with experiments. As expected, the cylindrical plasmon resonance peak is centered at around 790 nm. By controlling the incident wavelength, one can simply turn ON/OFF the Fabry–Pérot nanoring cavity, leading to useful plasmon-assisted
In summary, we have shown the fabrication of high quality plasmonic nanoring holes and particles using different manufacturing tools. Both air- and metal-filled coaxial structures were fabricated by FIB milling and EBL followed by argon ion milling. The uniform profiles of the narrow gaps were also demonstrated using FIB cross sectional milling. Although the total milling depth and aspect ratio are limited by etching rates and selectivity issues using EBL, one can realize deep etching with FIB techniques. Different geometries with various lattice shapes are thus available in a wide range of metallic materials now. The whole process is completely programmable and monolithic. Such geometries are crucial in fabricating plasmonic devices and can find extensive and promising applications in focusing and imaging.
Acknowledgments This work was supported by NEU internal funding XNB201302 and Natural Science Foundation of Hebei Province under Grant no. A2013501049. J. Lv and Z. Ma gratefully acknowledge the Fundamental Research Funds for the Fundamental Research Funds for the Central Universities (N100423005), National Natural Science Foundation of China (31170956), and Hebei Province Science and Technology Plan Projects (10276722). G. Si thanks Dr. Yan Jun Liu for discussions and simulations.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
Ebbesen TW, Lezec HJ, Ghaemi HF, Thio T, Wolff PA. Nature 1998;391:667–9. Burgos SP, de Waele R, Polman A, Atwater HA. Nat Mater 2010;9:407–12. de Waele R, Burgos SP, Atwater HA, Polman A. Opt Express 2010;18:12770–8. Poujet Y, Salvi J, Baida FI. Opt Lett 2007;32:2942–4. Baida FI, Belkhir A, Van Labeke D, Lamrous O. Phys Rev B 2006;74:205419. Rodríguez-Fortuño FJ, García-Meca C, Ortuño R, Martí J, Martínez A. Opt Lett 2009;34:3325–7. de Waele R, Burgos SP, Polman A, Atwater HA. Nano Lett 2009;9:2832–7. Baida FI, Van Labeke D. Phys Rev B 2003;67:155314. Haftel MI, Schlockermann C, Blumberg G. Phys Rev B 2006;74:235405. Haftel MI, Schlockermann C, Blumberg G. Appl Phys Lett 2006;88:193104. Orbons SM, Roberts A, Jamieson DN, Haftel MI, Schlockermann C, Freeman D, et al. Appl Phys Lett 2007;90:251107. Ono A, Kato J, Kawata S. Phys Rev Lett 2005;95:267407. Kawata S, Ono A, Verma P. Nat Photonics 2008;2:438–42. Liu Y, Bartal G, Zhang X. Opt Express 2008;16:15439–48. Shvets G, Trendafilov S, Pendry JB, Sarychev A. Phys Rev Lett 2007;99:053903. Liu GQ, Tang FL, Li L, Gong LX, Ye ZQ. Mater Lett 2011;65:1998–2000. Cai Y, Li Y, Nordlander P, Cremer PS. Nano Lett 2012;12:4881–8. Ye J, Shioi M, Lodewijks K, Lagae L, Kawamura T, Van Dorpe P. Appl Phys Lett 2010;97:163106. Tsai C, Lu S, Lin J, Lee P. Appl Phys Lett 2011;98:153108. Lehr D, Dietrich K, Helgert C, Käsebier T, Fuchs H, Tünnermann A, et al. Opt Lett 2012;37:157–9. Si G, Teo EJ, Bettiol AA, Teng J, Danner AJ. J Vac Sci Technol B 2010;28:316. Si G, Danner AJ, Teo SL, Teo EJ, Teng J, Bettiol AA. J Vac Sci Technol B 2011;29 (2):021205. Si G, Zhao Y, Liu H, Teo S, Zhang M, Huang TJ, et al. App Phys Lett 2011;99:033105. Liu YJ, Si GY, Leong ESP, Xiang N, Danner AJ, Teng JH. Adv Mater 2012;24: OP131. Korsunsky AM, Sebastiani M, Bemporad E. Mater Lett 2009;63:1961–3.