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The effect of groove on the light transmission through nano-apertures
Physica E 43 (2011) 929–933
Contents lists available at ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
The effect of groove on the light transmission through nano-apertures Y.C. Kim a, D.W. Kim b, Vinaya K. Jha b, Om K. Suwal b, S.S. Choi b, a b
Department of Optometry, Eulji University, Seongnam-si, Gyeonggi-do 461-713, Republic of Korea Department of Nanoscience, Sun Moon University, 100 Kalsan-ri, Tangjeong-myun, Asan-si, Chungnam 336-708, Republic of Korea
a r t i c l e in f o
abstract
Article history: Received 11 October 2010 Received in revised form 9 November 2010 Accepted 16 November 2010 Available online 21 November 2010
A few series of metal-coated nano-apertures with and without periodic groove structures have been fabricated on several kinds of pyramid-type probes with different metal thickness. And the influence of the periodic grooves as well as the metal thickness on the light transmission has been examined. The transmitted light intensity from the sub-wavelength size aperture with surrounding grooves was observed to be stronger than that from the aperture without grooves as the apertures on metal plates. And the transmitted light intensity was observed to decrease exponentially as metal thickness increases. The details of fabrication and the light transmission measurement will be discussed with the accompanying simulation results based on finite-difference time-domain method. & 2010 Elsevier B.V. All rights reserved.
1. Introduction The high transmission through a sub-wavelength size aperture by a periodic corrugation of a metal surface surrounding the aperture has attracted much attention because of the promising application of a near field beyond the restriction of the diffraction limit. Enhancement of light transmission through a sub-wavelength aperture or slit using the coupling of light to the excitation of surface waves, or surface plasmons on metallic surfaces by adopting a periodic structure on metallic surfaces has been comprehensively investigated [1–3]. In addition to the enhancement, ‘beaming’ of the light from an aperture with smaller angular divergence has been observed and analyzed [4,5]. On the other hand, a pyramidal probe with a nano-aperture based on micromachining has been extensively studied as it can be used combined with SNOM and easily modified for multi-functional probes as well as the possibility of batch fabrication [6–8]. In this study, we fabricated several series of sub-wavelength size apertures with and without periodic grooves at the apexes of metal-coated pyramidal-type probes and examined the effect of grooves on the light transmission. Also, the effect of metal thickness on the light transmission through sub-wavelength size apertures was examined.
2. Experiments Nano-apertures used for this experiment were formed at apexes of metal-coated pyramids which were fabricated through Corresponding author.
E-mail address: [email protected] (S.S. Choi). 1386-9477/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2010.11.019
a proper sequence of semiconductor fabrication procedures described in Fig. 1. At first, a double side polished Si (1 0 0) wafer was oxidized in a furnace and 100 100 mm2 square dot pattern was patterned through a photo-lithographic process as shown in Fig. 1(a). After PR strip, an anisotropic wet etching of Si was performed using a 20 wt% TMAH solution at 80 3 C to form a hollow pyramid and the inner surfaces of the pyramid was oxidized using a furnace again as illustrated in Fig. 1(b) and (c). Back-side bulk Si etching using TMAH solution was carefully performed so that the height of protruding oxide pyramid become 10 mm as shown in Fig. 1(d). We tried to make protruding pyramid small since oxide layer composing pyramid side-walls is transparent while Si layer is opaque. Therefore, the Si layer surrounding the protruding pyramid will absorb the incident light, which can improve the signal to noise ratio of the probe, and additionally it can act as a strong supporter for the fragile probe. The thickness of the remaining Si layer is 100 mm in this case. A careful isotropic oxide etch of the protruded oxide pyramid using a HF solution will open a few 100-nm nano-aperture at the pyramid apex. Next, double metal layers of Cr/Al were coated on the outer surface only (Fig. 1(e)) or both inner and outer surfaces (Fig. 1(f)) was performed by varying Al thickness. Thin metal film was deposited by a metal sputter (MHS-1500A). RF 300 and DC 300 W were applied for both Al and Cr deposition. In this experiment, we prepared total three series of samples. For series A, 10-nm-thick Cr and 600-nm-thick Al double metal layer was coated on the outer surface only. For series B, 100-nm-Al/ 10-nm-Cr was coated on the inner surface and 600-nm-Al/10-nmCr was coated on the outer surface. Finally, 540-nm-Al/10-nm-Cr layer was coated on the inner surface and 600-nm-Al/10-nm-Cr on the outer surface for the series C. A typical SEM image of an oxide aperture and the metal-coated surface are presented in Fig. 2(a) and
930
Y.C. Kim et al. / Physica E 43 (2011) 929–933
Si
SiO2
PR
Metal
Fig. 1. Fabrication procedure for a metal-coated pyramid type probe. (a) Oxidation and patterning, (b) V-groove formation, (c) re-oxidation, (d) Si bulk etching, (e) Hole opening and metal deposition and (f) metal deposition (inner-side).
Fig. 3. (a–c) SEM images of some apertures on pyramid apexes.
Fig. 2. (a) A typical SEM image obtained after oxide hole opening and (b) SEM image after the metal coating.
(b), respectively. Fig. 2 (a) A typical SEM image obtained after oxide hole opening and (b) SEM image after the metal coating. Finally, at the apexes of the metal-coated samples, both circular nano-apertures with sub-wavelength aperture diameter and the periodic groove structures were fabricated by using a FIB system (NOVA200, FEI Co.). The ion beam energy for FIB was 30 keV and the beam size used for the drilling was 50–100 nm. Fig. 3(a)–(c) present the typical SEM images of the apertures formed at the samples belong to the series B. The aperture diameter of image (a), (b), and (c) are 230, 460, and 560 nm, respectively. As presented by the images in Fig. 2(b) and Fig. 3, the surface of metal film is flat and the
grains of the metal layer are observed to be quite small, which indicates the high quality of these samples. Fig. 4(a) presents an SEM image of an pyramidal probe with groove structures and Fig. 4(b) shows its side view. Both the depth and width of grooves were fixed as 100 nm, and the number of grooves was five for all samples. The pitch of grooves was 500 nm, considering the wavelength of the laser used for the characterization. The circular nano-apertures with and without periodic groove structures were formed at the apex of each pyramid for all three types of pyramidal probes, and the aperture diameters range from 100 to 550 nm.
3. Results and discussions With the prepared samples, we tried to measure the transmittance of apertures and analyzed the results through simulation study. We measured the light intensity transmitted through the nano-apertures by illuminating a laser beam from the inner-side of the probe, that is, from the bottom of the pyramids. For the input
Y.C. Kim et al. / Physica E 43 (2011) 929–933
Fig. 4. SEM images of an aperture with a periodic groove structure. (a) Top and (b) side view.
14
Output Intensity (µW)
12
Outer Surface Coating; Cr 10 Al 600 Inner Surface Coating; A: No, B: Al 100/Cr 10, C: A l 540/Cr 10 A With Grooves A Without Grooves B With Grooves B Without Grooves C With Grooves C Without Grooves
10 8 6
931
the samples without grooves (solid symbols) regardless of the series. Also, it decreased as the coated metal becomes thicker as expected. In order to understand the experimentally observed enhancement of the transmitted light intensity through nano-apertures when groove structure is surrounding them, a simulation analysis using a commercial simulation tool (Concerto, Vector Field) based on finite-difference time-domain (FDTD) method has been performed. For the simplicity, an aperture with a diameter of 300 nm at the center of a 400 nm-thick metal (Al) plate was considered. In this simulation, the width and depth of grooves was taken as 100 nm and the period of the grooves was 500 m. Fig. 6 shows the electric field distribution of the light transmitted through the 300-nm aperture formed on the metal plate. Image (a) and (b) shows the result obtained from a 300-nm aperture with and without grooves, respectively. The electric field profile observed from grooved structure shows some particular characteristic features compared with that from a single circular aperture, Fig. 5(b). The most important point is that the output beam tends to be beamed. The angular divergence of the propagating light intensity is wide for a simple circular aperture, on the other hand, it is relatively narrow for a grooved aperture though the distribution profile is not uniform. This indicates that the reduction of the field intensity (integrated over a plane parallel to the metal plate) along the optical axis (vertical direction) is not so rapid as the case of a single aperture. The plots presented in Fig. 7 are the electric field strength (Ex) along a horizontal axis estimated at z¼100 nm plane above the exit plane. Incident wave is propagating along z direction (optical axis) and x-polarized. The solid and dotted line present the data for an aperture without and with grooves, respectively. The curve representing Ex from the aperture with grooves shows several side peaks at the groove positions. Combined with the images in Fig. 6(a) and (b), these plots give some qualitative description on the role of grooves in light propagation. At the center of the aperture, the electric field component parallel to the exit plane (in other words, poynting vector is perpendicular to the exit surface) obtained from the aperture with grooves is slightly smaller than that from the single
4 2 0 100
200
300 400 500 Aperture Diameter (nm)
600
Fig. 5. The measured light intensity transmitted through nano-apertures with (open symbols) and without (solid symbols) periodic groove structures. The coating condition for series A, B, and C can be found in text.
laser beam, 532 nm laser beam ð 2 mWÞ from a frequency doubled Nd:YAG laser was used and the light intensity was measured by a sensitive photo-sensor located around a few mm apart from the apertures. The samples and high photo-sensor are mounted on a precision x–y stage of lateral resolution ð o1 umÞ. The experimentally measured results are summarized in Fig. 5. In this plot, triangles, circles, and squares represent the light intensity obtained from series A, B, and series C, correspondingly. Open symbols represent data obtained from samples with grooves and solid symbols from the apertures without grooves for all three series. It was observed that the transmitted light intensity from the samples with grooves (open symbols) is stronger than that from
Fig. 6. Electric field (Ex) distribution obtained from a nano-aperture (a) without grooves and (b) with periodic grooves.
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Y.C. Kim et al. / Physica E 43 (2011) 929–933
aperture, which can be found from the magnified graph in the insert (dashed circle) in Fig. 7. This manifests that a relatively larger portion of the wave transmitted through an aperture propagates parallel to the surface when grooves are surrounding the aperture compared with a single aperture case. On the other hand, apart from the center, the field strength shows sharp peaks at the points where grooves are fabricated, while there appears no particular behaviour from the single aperture sample. Above results imply that light and/or plasmons propagating parallel to metal surface is interacting with the groove structures and the scattered (or re-radiated) wave can propagates upwards and forms beamed wave, which can be clearly observable by comparing Fig. 6(a) and (b). The pioneering and the most impressive results which demonstrated and analyzed the beaming of the periodic groove structure can be found in Refs. [4,5]. Thus, the wave transmitted from an aperture with a periodic groove structure tends to propagate with high directionality even though the
300 Al Thickness = 400 nm z = 100 nm with Groove without Groove
Ex (a.u.)
250 200 40 20 0 -3
-2
-1
0 X Axis µm
1
2
3
Fig. 7. These plots present the electric field (Ex) strength along a horizontal axis at z ¼100 nm above the exit plane. The solid and dotted line present the data for an aperture without and with grooves, respectively. Dotted line shows clear side peaks near the grooves. The dashed circle corresponds to the peak points at the center of the aperture.
apertures are not formed on a plane metal plate but at the apex of a pyramidal structure in this experiment. According to the preliminary simulation results for the apertures formed at metal-coated pyramid apexes, the light transmission through a grooved aperture is estimated to be higher than that through an aperture without groove. However, the analysis is somewhat complicated because of the complex optical structure of pyramidal probe including the groove structure, and the influence of detailed structure as well as the correlation between the plasmon wavelength is under investigation. In addition, we analyzed the influence of the metal thickness on the transmitted light intensity. The plots in Fig. 8 represent the variation of the transmitted light intensity depending on the metal (Al) thickness when the aperture diameter is 300 nm. In case that a 300 nm aperture is not available, interpolated values were adopted for this plot, and we disregarded the Cr wetting layer thickness in this plot. The solid circles and squares represent the measured intensity (right y-axis) from the apertures with and without grooves, respectively. And the dotted and solid lines correspond to the exponential decay fits for both experimental data. According to the results presented in Fig. 8(a), the light intensity transmitted through sub-wavelength apertures on pyramidal apexes exponentially decreases with the metal thickness. Which is different from the theoretically calculated results obtained from one-dimensional slits, expecting a oscillatory response with the metal thickness [9,10]. In order to check this point, we performed FDTD simulation for 300-nm-diameter apertures formed on an Al metal plates with different thicknesses from 400 to 1200 nm. The open symbols in Fig. 8 show the variation of transmitted light intensity (left y-axis) depending on metal plate thickness estimated through FDTD simulation. These data were calculated from the 300-nm-diameter apertures formed on Al plates with different thicknesses from 400 to 1200 nm. In this figure, uptriangles, down-triangles, and diamonds represent the estimated values of light intensity integrated over the entire horizontal plane of z ¼500, 1000, and 2000 nm, correspondingly. Also, the dashed, dash-dotted, and dash-dot-dotted lines are the exponential decay fits corresponding to each estimated values. In spite of the structural differences; the experimental data are from apertures on pyramidal apexes and the simulated ones from apertures on metal plates, both experimental and simulated values fit fairly well
2.0 8 1.5
Experimental Data With Groove Without Groove
4
1.0
0.5
Intensity µW
Intensity (a.u.)
6
2
0
Simulation z = 500 z = 1000 z = 2000 400
0.0
-0.5 600
800 1000 Metal Thickness (nm)
1200
Fig. 8. The influence of metal thickness on the transmitted light intensity. The solid circles and squares represent the measured intensity (right y-axis) from the apertures with and without grooves, respectively. And the open up-triangles, down-triangles, and diamonds represent the simulated values (left y-axis) from apertures on Al plates with different thicknesses. The lines correspond to the exponential decay fitting results.
Y.C. Kim et al. / Physica E 43 (2011) 929–933
with the exponential decay fit. Above results, with the theoretical expectation in the references calculated from one-dimensional slits, require further detailed study on the transmission of light through sub-wavelength tunnels with different thicknesses.
4. Conclusion We fabricated a few series of nano-apertures based on metalcoated hollow oxide pyramids and investigated the light transmission through the apertures accompanying a simulation study. It was observed that the transmitted light intensity from a sub-wavelength size apertures with surrounding grooves is stronger than that from an aperture without grooves. The analysis on this phenomenon has been performed by a simulation tool based on FDTD method and it can be attributed to ‘beaming’ due to the surface plasmon effects at the groove structures as reported from the grooved-apertures formed on a metal plate. Also, it was observed that the transmitted intensity decreases exponentially with the increase of coating metal thickness and confirmed by the simulation study. This thickness dependence is different from the theoretical results obtained from one-dimensional slits and offers further investigation on this area.
933
Acknowledgements The authors wish to acknowledge the support of the Korean Ministry of Science and Technology (M10400000304-05J000030410 and M10203000042-05M0300- 04210) under the National Research Lab. project and Basic Nano Research funding project.
References [1] T. Thio, H.J. Lezec, T.W. Ebbesen, K.M. Pellerin, G.D. Lewen, A. Nahata, R.A. Linke, Nanotechnology 13 (2002) 429. [2] W.L. Barnes, A. Dereux, T.W. Ebbessen, Nature 424 (2003) 824. [3] F.J. Garcia, H.J. Lezec, T.W. Ebbessen, L. Martin-Moreno, Phys. Rev. Lett. 90 (2003) 213901. [4] L. Martin-Moreno, F.J. Garcia-Vidal, H.J. Lezec, A. Degiron, T.W. Ebbessen, Phys. Rev. Lett. 90 (2003) 167401. [5] H.J. Lezec, A. Degiron, E. Devaux, R.A. Linke, L. Martin-Moreno, F.J. Garcia-Vidal, T.W. Ebbessen, Science 297 (2002) 820. [6] M. Sasaki, K. Tanaka, K. Hane, Jpn. J. Appl. Phys. 39 (2000) 7150. [7] D.W. Kim, M.J. Park, C.H. Han, S.S. Choi, Nanotechnology 16 (2005) S304. [8] A. Vollkopf, G. Georgiev, O. Rudow, M. Muller-Wiegand, E. Oesterschulze, J. Electrochem. Soc. 148 (2001) G587. [9] E. Betzig, A. Harootunian, A. Lewis, M. Isaacson, Appl. Opt. 25 (1986) 1890. [10] R.F. Harrington, D.T. Auckland, IEEE Trans. Antennas Propag. AP-28 (1980) 616.
Contents lists available at ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
The effect of groove on the light transmission through nano-apertures Y.C. Kim a, D.W. Kim b, Vinaya K. Jha b, Om K. Suwal b, S.S. Choi b, a b
Department of Optometry, Eulji University, Seongnam-si, Gyeonggi-do 461-713, Republic of Korea Department of Nanoscience, Sun Moon University, 100 Kalsan-ri, Tangjeong-myun, Asan-si, Chungnam 336-708, Republic of Korea
a r t i c l e in f o
abstract
Article history: Received 11 October 2010 Received in revised form 9 November 2010 Accepted 16 November 2010 Available online 21 November 2010
A few series of metal-coated nano-apertures with and without periodic groove structures have been fabricated on several kinds of pyramid-type probes with different metal thickness. And the influence of the periodic grooves as well as the metal thickness on the light transmission has been examined. The transmitted light intensity from the sub-wavelength size aperture with surrounding grooves was observed to be stronger than that from the aperture without grooves as the apertures on metal plates. And the transmitted light intensity was observed to decrease exponentially as metal thickness increases. The details of fabrication and the light transmission measurement will be discussed with the accompanying simulation results based on finite-difference time-domain method. & 2010 Elsevier B.V. All rights reserved.
1. Introduction The high transmission through a sub-wavelength size aperture by a periodic corrugation of a metal surface surrounding the aperture has attracted much attention because of the promising application of a near field beyond the restriction of the diffraction limit. Enhancement of light transmission through a sub-wavelength aperture or slit using the coupling of light to the excitation of surface waves, or surface plasmons on metallic surfaces by adopting a periodic structure on metallic surfaces has been comprehensively investigated [1–3]. In addition to the enhancement, ‘beaming’ of the light from an aperture with smaller angular divergence has been observed and analyzed [4,5]. On the other hand, a pyramidal probe with a nano-aperture based on micromachining has been extensively studied as it can be used combined with SNOM and easily modified for multi-functional probes as well as the possibility of batch fabrication [6–8]. In this study, we fabricated several series of sub-wavelength size apertures with and without periodic grooves at the apexes of metal-coated pyramidal-type probes and examined the effect of grooves on the light transmission. Also, the effect of metal thickness on the light transmission through sub-wavelength size apertures was examined.
2. Experiments Nano-apertures used for this experiment were formed at apexes of metal-coated pyramids which were fabricated through Corresponding author.
E-mail address: [email protected] (S.S. Choi). 1386-9477/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2010.11.019
a proper sequence of semiconductor fabrication procedures described in Fig. 1. At first, a double side polished Si (1 0 0) wafer was oxidized in a furnace and 100 100 mm2 square dot pattern was patterned through a photo-lithographic process as shown in Fig. 1(a). After PR strip, an anisotropic wet etching of Si was performed using a 20 wt% TMAH solution at 80 3 C to form a hollow pyramid and the inner surfaces of the pyramid was oxidized using a furnace again as illustrated in Fig. 1(b) and (c). Back-side bulk Si etching using TMAH solution was carefully performed so that the height of protruding oxide pyramid become 10 mm as shown in Fig. 1(d). We tried to make protruding pyramid small since oxide layer composing pyramid side-walls is transparent while Si layer is opaque. Therefore, the Si layer surrounding the protruding pyramid will absorb the incident light, which can improve the signal to noise ratio of the probe, and additionally it can act as a strong supporter for the fragile probe. The thickness of the remaining Si layer is 100 mm in this case. A careful isotropic oxide etch of the protruded oxide pyramid using a HF solution will open a few 100-nm nano-aperture at the pyramid apex. Next, double metal layers of Cr/Al were coated on the outer surface only (Fig. 1(e)) or both inner and outer surfaces (Fig. 1(f)) was performed by varying Al thickness. Thin metal film was deposited by a metal sputter (MHS-1500A). RF 300 and DC 300 W were applied for both Al and Cr deposition. In this experiment, we prepared total three series of samples. For series A, 10-nm-thick Cr and 600-nm-thick Al double metal layer was coated on the outer surface only. For series B, 100-nm-Al/ 10-nm-Cr was coated on the inner surface and 600-nm-Al/10-nmCr was coated on the outer surface. Finally, 540-nm-Al/10-nm-Cr layer was coated on the inner surface and 600-nm-Al/10-nm-Cr on the outer surface for the series C. A typical SEM image of an oxide aperture and the metal-coated surface are presented in Fig. 2(a) and
930
Y.C. Kim et al. / Physica E 43 (2011) 929–933
Si
SiO2
PR
Metal
Fig. 1. Fabrication procedure for a metal-coated pyramid type probe. (a) Oxidation and patterning, (b) V-groove formation, (c) re-oxidation, (d) Si bulk etching, (e) Hole opening and metal deposition and (f) metal deposition (inner-side).
Fig. 3. (a–c) SEM images of some apertures on pyramid apexes.
Fig. 2. (a) A typical SEM image obtained after oxide hole opening and (b) SEM image after the metal coating.
(b), respectively. Fig. 2 (a) A typical SEM image obtained after oxide hole opening and (b) SEM image after the metal coating. Finally, at the apexes of the metal-coated samples, both circular nano-apertures with sub-wavelength aperture diameter and the periodic groove structures were fabricated by using a FIB system (NOVA200, FEI Co.). The ion beam energy for FIB was 30 keV and the beam size used for the drilling was 50–100 nm. Fig. 3(a)–(c) present the typical SEM images of the apertures formed at the samples belong to the series B. The aperture diameter of image (a), (b), and (c) are 230, 460, and 560 nm, respectively. As presented by the images in Fig. 2(b) and Fig. 3, the surface of metal film is flat and the
grains of the metal layer are observed to be quite small, which indicates the high quality of these samples. Fig. 4(a) presents an SEM image of an pyramidal probe with groove structures and Fig. 4(b) shows its side view. Both the depth and width of grooves were fixed as 100 nm, and the number of grooves was five for all samples. The pitch of grooves was 500 nm, considering the wavelength of the laser used for the characterization. The circular nano-apertures with and without periodic groove structures were formed at the apex of each pyramid for all three types of pyramidal probes, and the aperture diameters range from 100 to 550 nm.
3. Results and discussions With the prepared samples, we tried to measure the transmittance of apertures and analyzed the results through simulation study. We measured the light intensity transmitted through the nano-apertures by illuminating a laser beam from the inner-side of the probe, that is, from the bottom of the pyramids. For the input
Y.C. Kim et al. / Physica E 43 (2011) 929–933
Fig. 4. SEM images of an aperture with a periodic groove structure. (a) Top and (b) side view.
14
Output Intensity (µW)
12
Outer Surface Coating; Cr 10 Al 600 Inner Surface Coating; A: No, B: Al 100/Cr 10, C: A l 540/Cr 10 A With Grooves A Without Grooves B With Grooves B Without Grooves C With Grooves C Without Grooves
10 8 6
931
the samples without grooves (solid symbols) regardless of the series. Also, it decreased as the coated metal becomes thicker as expected. In order to understand the experimentally observed enhancement of the transmitted light intensity through nano-apertures when groove structure is surrounding them, a simulation analysis using a commercial simulation tool (Concerto, Vector Field) based on finite-difference time-domain (FDTD) method has been performed. For the simplicity, an aperture with a diameter of 300 nm at the center of a 400 nm-thick metal (Al) plate was considered. In this simulation, the width and depth of grooves was taken as 100 nm and the period of the grooves was 500 m. Fig. 6 shows the electric field distribution of the light transmitted through the 300-nm aperture formed on the metal plate. Image (a) and (b) shows the result obtained from a 300-nm aperture with and without grooves, respectively. The electric field profile observed from grooved structure shows some particular characteristic features compared with that from a single circular aperture, Fig. 5(b). The most important point is that the output beam tends to be beamed. The angular divergence of the propagating light intensity is wide for a simple circular aperture, on the other hand, it is relatively narrow for a grooved aperture though the distribution profile is not uniform. This indicates that the reduction of the field intensity (integrated over a plane parallel to the metal plate) along the optical axis (vertical direction) is not so rapid as the case of a single aperture. The plots presented in Fig. 7 are the electric field strength (Ex) along a horizontal axis estimated at z¼100 nm plane above the exit plane. Incident wave is propagating along z direction (optical axis) and x-polarized. The solid and dotted line present the data for an aperture without and with grooves, respectively. The curve representing Ex from the aperture with grooves shows several side peaks at the groove positions. Combined with the images in Fig. 6(a) and (b), these plots give some qualitative description on the role of grooves in light propagation. At the center of the aperture, the electric field component parallel to the exit plane (in other words, poynting vector is perpendicular to the exit surface) obtained from the aperture with grooves is slightly smaller than that from the single
4 2 0 100
200
300 400 500 Aperture Diameter (nm)
600
Fig. 5. The measured light intensity transmitted through nano-apertures with (open symbols) and without (solid symbols) periodic groove structures. The coating condition for series A, B, and C can be found in text.
laser beam, 532 nm laser beam ð 2 mWÞ from a frequency doubled Nd:YAG laser was used and the light intensity was measured by a sensitive photo-sensor located around a few mm apart from the apertures. The samples and high photo-sensor are mounted on a precision x–y stage of lateral resolution ð o1 umÞ. The experimentally measured results are summarized in Fig. 5. In this plot, triangles, circles, and squares represent the light intensity obtained from series A, B, and series C, correspondingly. Open symbols represent data obtained from samples with grooves and solid symbols from the apertures without grooves for all three series. It was observed that the transmitted light intensity from the samples with grooves (open symbols) is stronger than that from
Fig. 6. Electric field (Ex) distribution obtained from a nano-aperture (a) without grooves and (b) with periodic grooves.
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Y.C. Kim et al. / Physica E 43 (2011) 929–933
aperture, which can be found from the magnified graph in the insert (dashed circle) in Fig. 7. This manifests that a relatively larger portion of the wave transmitted through an aperture propagates parallel to the surface when grooves are surrounding the aperture compared with a single aperture case. On the other hand, apart from the center, the field strength shows sharp peaks at the points where grooves are fabricated, while there appears no particular behaviour from the single aperture sample. Above results imply that light and/or plasmons propagating parallel to metal surface is interacting with the groove structures and the scattered (or re-radiated) wave can propagates upwards and forms beamed wave, which can be clearly observable by comparing Fig. 6(a) and (b). The pioneering and the most impressive results which demonstrated and analyzed the beaming of the periodic groove structure can be found in Refs. [4,5]. Thus, the wave transmitted from an aperture with a periodic groove structure tends to propagate with high directionality even though the
300 Al Thickness = 400 nm z = 100 nm with Groove without Groove
Ex (a.u.)
250 200 40 20 0 -3
-2
-1
0 X Axis µm
1
2
3
Fig. 7. These plots present the electric field (Ex) strength along a horizontal axis at z ¼100 nm above the exit plane. The solid and dotted line present the data for an aperture without and with grooves, respectively. Dotted line shows clear side peaks near the grooves. The dashed circle corresponds to the peak points at the center of the aperture.
apertures are not formed on a plane metal plate but at the apex of a pyramidal structure in this experiment. According to the preliminary simulation results for the apertures formed at metal-coated pyramid apexes, the light transmission through a grooved aperture is estimated to be higher than that through an aperture without groove. However, the analysis is somewhat complicated because of the complex optical structure of pyramidal probe including the groove structure, and the influence of detailed structure as well as the correlation between the plasmon wavelength is under investigation. In addition, we analyzed the influence of the metal thickness on the transmitted light intensity. The plots in Fig. 8 represent the variation of the transmitted light intensity depending on the metal (Al) thickness when the aperture diameter is 300 nm. In case that a 300 nm aperture is not available, interpolated values were adopted for this plot, and we disregarded the Cr wetting layer thickness in this plot. The solid circles and squares represent the measured intensity (right y-axis) from the apertures with and without grooves, respectively. And the dotted and solid lines correspond to the exponential decay fits for both experimental data. According to the results presented in Fig. 8(a), the light intensity transmitted through sub-wavelength apertures on pyramidal apexes exponentially decreases with the metal thickness. Which is different from the theoretically calculated results obtained from one-dimensional slits, expecting a oscillatory response with the metal thickness [9,10]. In order to check this point, we performed FDTD simulation for 300-nm-diameter apertures formed on an Al metal plates with different thicknesses from 400 to 1200 nm. The open symbols in Fig. 8 show the variation of transmitted light intensity (left y-axis) depending on metal plate thickness estimated through FDTD simulation. These data were calculated from the 300-nm-diameter apertures formed on Al plates with different thicknesses from 400 to 1200 nm. In this figure, uptriangles, down-triangles, and diamonds represent the estimated values of light intensity integrated over the entire horizontal plane of z ¼500, 1000, and 2000 nm, correspondingly. Also, the dashed, dash-dotted, and dash-dot-dotted lines are the exponential decay fits corresponding to each estimated values. In spite of the structural differences; the experimental data are from apertures on pyramidal apexes and the simulated ones from apertures on metal plates, both experimental and simulated values fit fairly well
2.0 8 1.5
Experimental Data With Groove Without Groove
4
1.0
0.5
Intensity µW
Intensity (a.u.)
6
2
0
Simulation z = 500 z = 1000 z = 2000 400
0.0
-0.5 600
800 1000 Metal Thickness (nm)
1200
Fig. 8. The influence of metal thickness on the transmitted light intensity. The solid circles and squares represent the measured intensity (right y-axis) from the apertures with and without grooves, respectively. And the open up-triangles, down-triangles, and diamonds represent the simulated values (left y-axis) from apertures on Al plates with different thicknesses. The lines correspond to the exponential decay fitting results.
Y.C. Kim et al. / Physica E 43 (2011) 929–933
with the exponential decay fit. Above results, with the theoretical expectation in the references calculated from one-dimensional slits, require further detailed study on the transmission of light through sub-wavelength tunnels with different thicknesses.
4. Conclusion We fabricated a few series of nano-apertures based on metalcoated hollow oxide pyramids and investigated the light transmission through the apertures accompanying a simulation study. It was observed that the transmitted light intensity from a sub-wavelength size apertures with surrounding grooves is stronger than that from an aperture without grooves. The analysis on this phenomenon has been performed by a simulation tool based on FDTD method and it can be attributed to ‘beaming’ due to the surface plasmon effects at the groove structures as reported from the grooved-apertures formed on a metal plate. Also, it was observed that the transmitted intensity decreases exponentially with the increase of coating metal thickness and confirmed by the simulation study. This thickness dependence is different from the theoretical results obtained from one-dimensional slits and offers further investigation on this area.
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Acknowledgements The authors wish to acknowledge the support of the Korean Ministry of Science and Technology (M10400000304-05J000030410 and M10203000042-05M0300- 04210) under the National Research Lab. project and Basic Nano Research funding project.
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