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Strongly enhanced optical transmission through subwavelength holes in metal films
Physica B 279 (2000) 90}93
Strongly enhanced optical transmission through subwavelength holes in metal "lms Tineke Thio!,*, H.J. Lezec",#, T.W. Ebbesen!," !NEC Research Institute, 4 Independence Way, Princeton NJ 08540, USA "ISIS, Univ. Louis Pasteur, 4 rue B. Pascal, 67000 Strasbourg, France #Micrion Europe GmbH, Kirchenstrasse 2, 85622 Feldkirchen, Germany
Abstract The optical transmission through a subwavelength aperture in a metal "lm is strongly enhanced when the incident light interacts resonantly with surface plasmons on the surface of the metal. Such interactions are made allowed by a periodic corrugation of the metal surface. We apply this principle on a near-"eld surface-plasmon activated device (SPADE) fabricated on an optic "ber, and demonstrate large transmission through a subwavelength aperture. ( 2000 Elsevier Science B.V. All rights reserved. Keywords: Surface plasmons; Optical transmission; Subwavelength apertures; Near-"eld probes
1. Introduction A metal "lm is optically thick, that is, opaque to light, if the "lm thickness is signi"cantly larger than the optical skin depth. The latter is typically about 20}30 nm for optical frequencies below the bulk plasma frequency. The transmission through a single aperture in such an optically thick "lm depends on both the wavelength j of the incident light, and the diameter d of the aperture. For a single, subwavelength aperture (d;j) in an in"nitely thin, in"nitely conducting "lm, the transmission is calculated [1] to scale as (d/j)4. This severely limits the transmission e$ciency of near-"eld optical devices. However, when an optically thick metal "lm is perforated with a periodic array of subwavelength apertures, the transmission per hole no longer follows the (d/j)4 law, but is orders of magnitude larger than expected, and can even exceed unity when normalised to the area of the hole [2]. The transmission enhancement occurs [3] when the
* Corresponding author. Tel.: #1-609-951-2616; fax: #1609-951-2483. E-mail address: [email protected] (T. Thio)
incident light is resonant with a surface plasmon mode, a collective excitation of the electron density on the metallic surface [4]. On a smooth metal "lm the incident light does not interact with the surface plasmons because the energy and momentum conservation requirements are not simultaneously obeyed, but the presence of the periodic array of holes creates zone folding in a wellde"ned Brillouin zone and allows grating coupling. The surface plasmon dispersion [4] can be mapped out by measuring the transmission as a function of wavelength (or photon energy) and the angle of incidence [3]. It is not necessary to perforate the metal "lm with an array of through holes to achieve surface-plasmon enhanced transmission: Any periodic surface structure can provide the grating coupling. For instance, if a single through hole is made in a "lm of which the surface has a periodic corrugation, such as a regular array of indentations or dimples, then the transmission through the hole is also enhanced compared to that of a single hole in a metal "lm with a smooth surface [5]. The wavelength at which the enhancement occurs may be tuned by designing a proper geometry of the surface corrugation. After fabrication, the operating wavelength may be further tuned by varying the refractive index of the dielectric media directly adjacent to the metal surface [6]. This is
0921-4526/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 0 6 7 7 - 8
T. Thio et al. / Physica B 279 (2000) 90}93
because the surface plasmon dispersion relation is sensitive to the index of refraction of both the metal and the adjacent medium. The surface corrugation may be on the illuminated side of the metal "lm, or on the back side [3,5]; surface plasmons on both surfaces are equally e!ective in mediating the transmission enhancement, evincing the very strong coupling of the front and back surfaces of the metal, through the holes. Numerical calculations on similar systems [7,8] con"rm that the holes are indeed instrumental in the coupling of light to the electromagnetic modes of the perforated "lm; the oscillating electric "eld is greatly enhanced in the holes, particularly at the edges where the curvature of the metal surface is highest. Such a strong enhancement of the electric "eld is also observed in surfaceenhanced Raman scattering (SERS) on granular silver "lms where surface plasmons boost the electric "elds at the silver surface by orders of magnitude compared to the incident "elds [9]. The resonant interaction of light with surface plasmons can be very useful in a large class of surface-plasmon activated devices (SPADEs), including devices for use in optical switching and subwavelength photolithography [6]. In this paper we apply the principle of surface-plasmon enhanced transmission in an optical near"eld SPADE fabricated on the face of an untapered optic "ber, coated with a silver "lm. A 200 nm aperture fabricated in the silver "lm determines the spatial resolution of the device; the transmission through the aperture is boosted by the presence of a periodic corrugation on the surface of the "lm.
2. Experimental details The near-"eld devices were fabricated on the cleaved end of a single-mode optic "ber (see Fig. 1) with an overall diameter of 125 lm and a core diameter of 4.0 lm; the numerical aperture was NA"0.12. In the centre of the cleaved face a 21]21 array of circular indentations (see Fig. 2a) were made in the silica by focused-ion beam (FIB) milling, using a Micrion 9500 system. The central position in the array was not milled. The "ber was then coated with a 300 nm thick "lm of silver by RF sputtering, which ensures even and omnidirectional coverage, minimising the creation of pinholes from shadowing. Finally, the central hole was drilled using FIB.
3. Results and discussion Fig. 1 is an electron micrograph of a cleaved "ber end with a near-"eld device. Although impurities (probably dust particles) are visible near the edges of the cleaved face, inspection through an optical microsope reveals that no light transmitted through pinholes in those areas.
91
Fig. 1. Scanning electron microscope (SEM) image of optic "ber face with near-"eld SPADE at centre, taken with Hitachi S-4700 microscope.
A square array of indentations is located at the centre, to overlap with the core region. Fig. 2a is an FIB image of a square lattice of indentations in the bare quartz of a di!erent sample. The central portion of the "nished device is shown in Fig. 2b. Here it can be seen that the indentations are completely coated by the silver "lm, but relatively shallow dimples are still visible on the metal} air interface. The diameter of the central aperture is the same as that of the indentations, d"200 nm, and the lattice constant of the square array of dimples is a "600 nm. 0 The lattice constant of the dimple array is a parameter that can be used to tune the wavelength at which surface plasmons resonantly enhance the transmission through the device, since the surface plasmon dispersion is determined by the geometry of the periodic surface corrugation [3]. For a square lattice, and with the light incident normal to the plane of the "lm, maxima in the transmission are expected for
A
B
e e 1@2 1 2 , j (i, j)"a (i2#j2)~1@2 .!9 0 e #e 1 2
(1)
where j is the wavelength, e , e are the dielectric con1 2 stants of the metal and that of the adjacent medium, respectively, and i, j"!0, 1, 2, 2, are integers characterising the particular branch of the surface plasmon dispersion. The transmission of the near-"eld device was measured by coupling light from a diode laser (j"670 nm) into the "ber. This wavelength was chosen because it lies in a transmission peak for a square array of holes with lattice constant a "600 nm (see Fig. 2 of Ref [3]); that 0 structure is equivalent to the structure of Fig. 2b since in both cases the metal surface has a corrugation with periodicity a on both the metal}air and the metal}glass 0 interfaces. An image of the optic "ber obtained through
92
T. Thio et al. / Physica B 279 (2000) 90}93
The big advantage of the present device is that the optical waveguide remains larger than the wavelength until the light reaches the subwavelength output aperture in the silver "lm, whereas in a tapered tip the diameter of the "ber becomes subwavelength well before the light reaches the output aperture [10]. The throughput of such a tapered "ber tip follows roughly a d4 dependence [11], similar to that predicted by theory. In a surfaceplasmon activated device the transmission scales [12] as d2; the transmission enhancement therefore increases with smaller aperture diameter. For instance, a SPADE with a 50 nm aperture should give a transmission as high as 4]10~4, whereas a tapered "ber tip has typically ¹)10~7 [11].
4. Conclusion
Fig. 2. (a) FIB image of array of indentations fabricated in bare cleaved face; (b) SEM image of central aperture surrounded by array of dimples (a "600 nm); (c) optical microscope image of 0 light emerging from face of near-"eld SPADE of (b), illuminated with white light from above; red laser light coupled into the "ber emerges only from the central aperture.
We have fabricated a near-"eld SPADE on an optic "ber face consisting of an aperture in a silver "lm, the surface of which has a periodic array of dimples. The transmission ¹ of this near-"eld device is signi"cantly higher than that of a conventional device with comparable spatial resolution; moreover ¹ is expected to drop only as d2 with decreasing aperture diameter. In this respect a SPADE represents a large advantage compared to a tapered "ber tip, especially at the smallest aperture diameters. Since in near-"eld scanning applications the output aperture must be scanned in very close proximity to the sample surface, it is not so useful to have a near"eld device at the centre of a large-diameter optic "ber face. In order to make a useful near-"eld scanning device, one would have to fabricate the SPADE on a face with a smaller area. One way to achieve this is by tapering the optic "ber using the conventional laser heating techniques, but such that the resulting face is a few lm in diameter. Since only a few nearest-neighbour shells of dimples surrounding the aperture su$ce to get the full transmission enhancement [5,12], a reduction of the total surface area of the probe is a viable way to make a novel near-"eld device with exceptionally high throughput.
References an optical miscroscope (see Fig. 2c) shows that in the forward direction light is detected only from the central aperture of the near-"eld device, and none from the perimeter of the "ber. The intensity of the light emerging from the near-"eld device is 0.5 lW, compared to 90 lW transmitted through a plain, cleaved optic "ber placed in the same experimental setup. Thus the throughput of the subwavelength aperture is roughly ¹"6]10~3, about 60 times the typical ¹+1]10~4 obtained in a conventional 200 nm NSOM made with a tapered "ber tip.
[1] H.A. Bethe, Phys. Rev. 66 (1944) 163. [2] T.W. Ebbesen, H.J. Lezec, H.F. Ghaemi, T. Thio, P.A. Wol!, Nature 391 (1998) 667. [3] H.F. Ghaemi, T. Thio, D.E. Grupp, T.W. Ebbesen, H.J. Lezec, Phys. Rev. B 58 (1998) 6779. [4] H. Raether, Surface Plasmons, Springer, Berlin, 1988. [5] Daniel E. Grupp, Henri J. Lezec, T. Thio, T.W. Ebbesen, Adv. Mater. 11 (1999) 860. [6] T.J. Kim, T. Thio, T.W. Ebbesen, D.E. Grupp, H.J. Lezec, Opt. Lett. 24 (1999) 256. [7] U. Schroeter, D. Heitmann, Phys. Rev. B 58 (1998) 15419.
T. Thio et al. / Physica B 279 (2000) 90}93 [8] J.A. Porto, F.J. Garcia-Vidal, J.B. Pendry, Phys. Rev. Lett. 83 (1999) 2845. [9] M. Moskovits, Rev. Mod. Phys. 57 (1985) 783. [10] E. Betzig, J.K. Trautman, Science 257 (1992) 189.
93
[11] G.A. Valaskovic, M. Holton, G.H. Morrison, Appl. Opt. 34 (1995) 1215. [12] T. Thio, H.F. Ghaemi, H.J. Lezec, P.A. Wol!, T.W. Ebbesen, JOSA B 16 (1999) 1743.
Strongly enhanced optical transmission through subwavelength holes in metal "lms Tineke Thio!,*, H.J. Lezec",#, T.W. Ebbesen!," !NEC Research Institute, 4 Independence Way, Princeton NJ 08540, USA "ISIS, Univ. Louis Pasteur, 4 rue B. Pascal, 67000 Strasbourg, France #Micrion Europe GmbH, Kirchenstrasse 2, 85622 Feldkirchen, Germany
Abstract The optical transmission through a subwavelength aperture in a metal "lm is strongly enhanced when the incident light interacts resonantly with surface plasmons on the surface of the metal. Such interactions are made allowed by a periodic corrugation of the metal surface. We apply this principle on a near-"eld surface-plasmon activated device (SPADE) fabricated on an optic "ber, and demonstrate large transmission through a subwavelength aperture. ( 2000 Elsevier Science B.V. All rights reserved. Keywords: Surface plasmons; Optical transmission; Subwavelength apertures; Near-"eld probes
1. Introduction A metal "lm is optically thick, that is, opaque to light, if the "lm thickness is signi"cantly larger than the optical skin depth. The latter is typically about 20}30 nm for optical frequencies below the bulk plasma frequency. The transmission through a single aperture in such an optically thick "lm depends on both the wavelength j of the incident light, and the diameter d of the aperture. For a single, subwavelength aperture (d;j) in an in"nitely thin, in"nitely conducting "lm, the transmission is calculated [1] to scale as (d/j)4. This severely limits the transmission e$ciency of near-"eld optical devices. However, when an optically thick metal "lm is perforated with a periodic array of subwavelength apertures, the transmission per hole no longer follows the (d/j)4 law, but is orders of magnitude larger than expected, and can even exceed unity when normalised to the area of the hole [2]. The transmission enhancement occurs [3] when the
* Corresponding author. Tel.: #1-609-951-2616; fax: #1609-951-2483. E-mail address: [email protected] (T. Thio)
incident light is resonant with a surface plasmon mode, a collective excitation of the electron density on the metallic surface [4]. On a smooth metal "lm the incident light does not interact with the surface plasmons because the energy and momentum conservation requirements are not simultaneously obeyed, but the presence of the periodic array of holes creates zone folding in a wellde"ned Brillouin zone and allows grating coupling. The surface plasmon dispersion [4] can be mapped out by measuring the transmission as a function of wavelength (or photon energy) and the angle of incidence [3]. It is not necessary to perforate the metal "lm with an array of through holes to achieve surface-plasmon enhanced transmission: Any periodic surface structure can provide the grating coupling. For instance, if a single through hole is made in a "lm of which the surface has a periodic corrugation, such as a regular array of indentations or dimples, then the transmission through the hole is also enhanced compared to that of a single hole in a metal "lm with a smooth surface [5]. The wavelength at which the enhancement occurs may be tuned by designing a proper geometry of the surface corrugation. After fabrication, the operating wavelength may be further tuned by varying the refractive index of the dielectric media directly adjacent to the metal surface [6]. This is
0921-4526/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 0 6 7 7 - 8
T. Thio et al. / Physica B 279 (2000) 90}93
because the surface plasmon dispersion relation is sensitive to the index of refraction of both the metal and the adjacent medium. The surface corrugation may be on the illuminated side of the metal "lm, or on the back side [3,5]; surface plasmons on both surfaces are equally e!ective in mediating the transmission enhancement, evincing the very strong coupling of the front and back surfaces of the metal, through the holes. Numerical calculations on similar systems [7,8] con"rm that the holes are indeed instrumental in the coupling of light to the electromagnetic modes of the perforated "lm; the oscillating electric "eld is greatly enhanced in the holes, particularly at the edges where the curvature of the metal surface is highest. Such a strong enhancement of the electric "eld is also observed in surfaceenhanced Raman scattering (SERS) on granular silver "lms where surface plasmons boost the electric "elds at the silver surface by orders of magnitude compared to the incident "elds [9]. The resonant interaction of light with surface plasmons can be very useful in a large class of surface-plasmon activated devices (SPADEs), including devices for use in optical switching and subwavelength photolithography [6]. In this paper we apply the principle of surface-plasmon enhanced transmission in an optical near"eld SPADE fabricated on the face of an untapered optic "ber, coated with a silver "lm. A 200 nm aperture fabricated in the silver "lm determines the spatial resolution of the device; the transmission through the aperture is boosted by the presence of a periodic corrugation on the surface of the "lm.
2. Experimental details The near-"eld devices were fabricated on the cleaved end of a single-mode optic "ber (see Fig. 1) with an overall diameter of 125 lm and a core diameter of 4.0 lm; the numerical aperture was NA"0.12. In the centre of the cleaved face a 21]21 array of circular indentations (see Fig. 2a) were made in the silica by focused-ion beam (FIB) milling, using a Micrion 9500 system. The central position in the array was not milled. The "ber was then coated with a 300 nm thick "lm of silver by RF sputtering, which ensures even and omnidirectional coverage, minimising the creation of pinholes from shadowing. Finally, the central hole was drilled using FIB.
3. Results and discussion Fig. 1 is an electron micrograph of a cleaved "ber end with a near-"eld device. Although impurities (probably dust particles) are visible near the edges of the cleaved face, inspection through an optical microsope reveals that no light transmitted through pinholes in those areas.
91
Fig. 1. Scanning electron microscope (SEM) image of optic "ber face with near-"eld SPADE at centre, taken with Hitachi S-4700 microscope.
A square array of indentations is located at the centre, to overlap with the core region. Fig. 2a is an FIB image of a square lattice of indentations in the bare quartz of a di!erent sample. The central portion of the "nished device is shown in Fig. 2b. Here it can be seen that the indentations are completely coated by the silver "lm, but relatively shallow dimples are still visible on the metal} air interface. The diameter of the central aperture is the same as that of the indentations, d"200 nm, and the lattice constant of the square array of dimples is a "600 nm. 0 The lattice constant of the dimple array is a parameter that can be used to tune the wavelength at which surface plasmons resonantly enhance the transmission through the device, since the surface plasmon dispersion is determined by the geometry of the periodic surface corrugation [3]. For a square lattice, and with the light incident normal to the plane of the "lm, maxima in the transmission are expected for
A
B
e e 1@2 1 2 , j (i, j)"a (i2#j2)~1@2 .!9 0 e #e 1 2
(1)
where j is the wavelength, e , e are the dielectric con1 2 stants of the metal and that of the adjacent medium, respectively, and i, j"!0, 1, 2, 2, are integers characterising the particular branch of the surface plasmon dispersion. The transmission of the near-"eld device was measured by coupling light from a diode laser (j"670 nm) into the "ber. This wavelength was chosen because it lies in a transmission peak for a square array of holes with lattice constant a "600 nm (see Fig. 2 of Ref [3]); that 0 structure is equivalent to the structure of Fig. 2b since in both cases the metal surface has a corrugation with periodicity a on both the metal}air and the metal}glass 0 interfaces. An image of the optic "ber obtained through
92
T. Thio et al. / Physica B 279 (2000) 90}93
The big advantage of the present device is that the optical waveguide remains larger than the wavelength until the light reaches the subwavelength output aperture in the silver "lm, whereas in a tapered tip the diameter of the "ber becomes subwavelength well before the light reaches the output aperture [10]. The throughput of such a tapered "ber tip follows roughly a d4 dependence [11], similar to that predicted by theory. In a surfaceplasmon activated device the transmission scales [12] as d2; the transmission enhancement therefore increases with smaller aperture diameter. For instance, a SPADE with a 50 nm aperture should give a transmission as high as 4]10~4, whereas a tapered "ber tip has typically ¹)10~7 [11].
4. Conclusion
Fig. 2. (a) FIB image of array of indentations fabricated in bare cleaved face; (b) SEM image of central aperture surrounded by array of dimples (a "600 nm); (c) optical microscope image of 0 light emerging from face of near-"eld SPADE of (b), illuminated with white light from above; red laser light coupled into the "ber emerges only from the central aperture.
We have fabricated a near-"eld SPADE on an optic "ber face consisting of an aperture in a silver "lm, the surface of which has a periodic array of dimples. The transmission ¹ of this near-"eld device is signi"cantly higher than that of a conventional device with comparable spatial resolution; moreover ¹ is expected to drop only as d2 with decreasing aperture diameter. In this respect a SPADE represents a large advantage compared to a tapered "ber tip, especially at the smallest aperture diameters. Since in near-"eld scanning applications the output aperture must be scanned in very close proximity to the sample surface, it is not so useful to have a near"eld device at the centre of a large-diameter optic "ber face. In order to make a useful near-"eld scanning device, one would have to fabricate the SPADE on a face with a smaller area. One way to achieve this is by tapering the optic "ber using the conventional laser heating techniques, but such that the resulting face is a few lm in diameter. Since only a few nearest-neighbour shells of dimples surrounding the aperture su$ce to get the full transmission enhancement [5,12], a reduction of the total surface area of the probe is a viable way to make a novel near-"eld device with exceptionally high throughput.
References an optical miscroscope (see Fig. 2c) shows that in the forward direction light is detected only from the central aperture of the near-"eld device, and none from the perimeter of the "ber. The intensity of the light emerging from the near-"eld device is 0.5 lW, compared to 90 lW transmitted through a plain, cleaved optic "ber placed in the same experimental setup. Thus the throughput of the subwavelength aperture is roughly ¹"6]10~3, about 60 times the typical ¹+1]10~4 obtained in a conventional 200 nm NSOM made with a tapered "ber tip.
[1] H.A. Bethe, Phys. Rev. 66 (1944) 163. [2] T.W. Ebbesen, H.J. Lezec, H.F. Ghaemi, T. Thio, P.A. Wol!, Nature 391 (1998) 667. [3] H.F. Ghaemi, T. Thio, D.E. Grupp, T.W. Ebbesen, H.J. Lezec, Phys. Rev. B 58 (1998) 6779. [4] H. Raether, Surface Plasmons, Springer, Berlin, 1988. [5] Daniel E. Grupp, Henri J. Lezec, T. Thio, T.W. Ebbesen, Adv. Mater. 11 (1999) 860. [6] T.J. Kim, T. Thio, T.W. Ebbesen, D.E. Grupp, H.J. Lezec, Opt. Lett. 24 (1999) 256. [7] U. Schroeter, D. Heitmann, Phys. Rev. B 58 (1998) 15419.
T. Thio et al. / Physica B 279 (2000) 90}93 [8] J.A. Porto, F.J. Garcia-Vidal, J.B. Pendry, Phys. Rev. Lett. 83 (1999) 2845. [9] M. Moskovits, Rev. Mod. Phys. 57 (1985) 783. [10] E. Betzig, J.K. Trautman, Science 257 (1992) 189.
93
[11] G.A. Valaskovic, M. Holton, G.H. Morrison, Appl. Opt. 34 (1995) 1215. [12] T. Thio, H.F. Ghaemi, H.J. Lezec, P.A. Wol!, T.W. Ebbesen, JOSA B 16 (1999) 1743.