APPLIED PHYSICS LETTERS
VOLUME 83, NUMBER 24
15 DECEMBER 2003
Launching and decoupling surface plasmons via micro-gratings Eloise Devauxa) and Thomas W. Ebbesen Laboratoire des Nanostructures, ISIS, Universite´ Louis Pasteur, 8 alle´e Monge, BP 70028, 67083 Strasbourg, France
Jean-Claude Weeber and Alain Dereux Laboratoire de Physique, Optique Submicronique, Universite´ de Bourgogne, BP 47870, 21078 Dijon, France
共Received 24 July 2003; accepted 23 October 2003兲 Controlling separately the launching of surface plasmons and their recovery as freely propagating light is essential for the development of surface plasmon photonic circuits. With this target in mind, we have studied in the near-field the launching of surface plasmons in a well-defined direction by micro-arrays of subwavelength holes milled in a thick metal film. We show that surface plasmons can then be converted back to freely propagating light by means of another appropriately designed array. These results not only provide insight into the efficient decoupling of surface plasmons but also into their role in the enhanced transmission mechanism. © 2003 American Institute of Physics. 关DOI: 10.1063/1.1634379兴
Surface plasmons 共SPs兲 are electromagnetic waves oscillating resonantly with the free electrons at the surface of a metal film which can propagate along the surface of metals over tens of micrometers in the visible range.1–3 This has led to suggestions of SP-based photonic circuits, where light would be first converted in SPs, propagating and being processed by different components, before being converted back to freely propagating light.3 The excitation of SPs at the surface of a thick metal film can be performed by means of periodic surface corrugations such as a grating. While coupling and decoupling light to and from SPs has been widely studied for single gratings共see, for example, Refs. 4 –7兲, the launching of SPs from one grating into the flat metal surface and their subsequent backconversion to freely propagating light by another has received much less attention. To study this process in detail, subwavelength holes arrays offer the advantage that they allow coupling of light to SPs through illumination from the back side and hence avoid noise and blinding due to the incident light. Moreover, the use of optically thick metal films prevents strong radiation damping of the SPs into the substrate supporting the metal film. Here we have undertaken a near-field optical study to reveal the details of the launching and decoupling processes in separate micro-hole arrays and to further elucidate the role of SPs in the enhanced transmission.8 Periodic arrays of subwavelength holes, separated by 30–100 m, were prepared with a Dual Beam Strata 235 Focused Ion Beam in 150-nm-thick gold films 关Fig. 1共a兲兴. The films were deposited by e-beam evaporation on ITO coated BK7 subtrates. The arrays were first characterized in the far field with a spectrophotometer 共Acton monochromator兲 coupled to microscope 共Nikon TE200兲 and a CCD camera 共Princeton Instruments兲. Illuminating the arrays with a collimated white light source in normal incidence, we obtained their zero-order far-field transmission spectra 关Fig. 1共b兲兴. The near-field study of the samples has then been done a兲
Electronic mail:
[email protected]
with the setup described elsewhere,9 slightly modified here to illuminate the sample in normal incidence. Figure 1共a兲 shows a typical layout consisting of two arrays with the same parameters but of different size. The small array (11⫻11 holes) on the right was illuminated in normal incidence by a Ti–sapphire laser beam focused with an angle of ⫾8° to produce a spot in the plane of the sample with a diameter of about 10 m. Thus, adjusting the location of the incident spot allowed the excitation of only the SPs launched by this ‘‘source-array.’’ The large array on the left (41⫻41 holes) served as the ‘‘probe array.’’ The size difference was by design in order to evaluate unambiguously the
FIG. 1. 共a兲 SEM image of the first pattern we studied. Both arrays have a period of 760 nm and are separated by 30 m. The hole diameter is 250 nm. The small array on the right (11⫻11 holes) is the ‘‘source’’ array whereas the big array on the left (41⫻41 holes) is not illuminated by the laser light and acts as a ‘‘probe’’ array. Inset: zoom in on the holes of one array. 共b兲 Transmission spectrum recorded in the far-field when the sample is illuminated by a collimated source of white light.
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 0003-6951/2003/83(24)/4936/3/$20.00 4936 © 2003 American Institute of Physics 193.52.246.36 On: Mon, 24 Mar 2014 14:37:25
Appl. Phys. Lett., Vol. 83, No. 24, 15 December 2003
Devaux et al.
4937
FIG. 2. 共Color兲 共a兲 Near-field optical 共NFO兲 image of the pattern presented in Fig. 1 when the laser is focused on the small array on the right and the electric field is along the x direction. 共b兲 Detail of image 共a兲 representing propapating SPs and the edge of the left array.
propagation direction and the spatial confinement of the SPs. The period of the arrays 共760 nm兲 was chosen to give a SP mode on the air–metal interface with a transmission peak at ⬃800 nm. The peak at 815 nm 关Fig. 1共b兲兴 corresponds to the FIG. 3. 共Color兲 共a兲 NFO image of the pattern presented in Fig. 1 when the 共⫾1,0兲 mode on the air side as can be deduced from the laser is focused on the small array on the right and the electric field is along equation given in Ref. 10 that predicts approximately the the y direction. 共b兲 NFO image of the pattern with a source array of peripeaks’ positions. In the following near-field optical studies, odicity a 0 ⫽700 nm: no SPs are launched in any direction 共the electric field along the x axis兲. 共c兲 NFO image of a pattern with the probe array consisting the illumination has been done at 800 nm to match this airof grooves (a 0 ⫽760 nm and the electric field along the x axis兲. side peak. When the laser was focused on the source array, we obare launched兲 is weak compared to the total intensity transserved in the far field that the edge of the probe array lit up, mitted by the source array but we can suppose that only the even though it was not illuminated directly. Figure 2 shows SPs excited near the edge are able to escape the array withthe corresponding near-field image recorded above the patout being attenuated by multiple scattering among the holes. tern of Fig. 1共a兲. As can be seen, the source array is launchIn the above-noted experiments, the incident light was ing SPs, appearing in the near-field as a streak between the polarized with the electric field along the x direction and the two arrays, which then decouple to light upon reaching the SPs are propagating in the direction of the probe array 共Fig. probe array. By zooming in at the edge of the probe array 2兲. It is worth noticing that these SPs, locally launched by the 关Fig. 2共b兲兴, we see that the SPs are structured by an interfersource array, do not suffer spread-out while propagating. ence pattern. It is due to the interaction of the plasmon Thus, a micro-grating of nanoholes allows the local launchpropagating from the source array with its counterpropagating in a given direction of fairly well confined SPs. If the ing reflexion from the probe array. The measured periodicity polarization of the incident field is rotated by 90° 关Fig. 3共a兲兴, of the interferences correspond roughly to half the wavethe SP launching occurs mainly in the y direction 共i.e., parlength of the SPs as expected. allel to the incident polarization兲. However, the edge of the As we already mentionned, the probe array is coupling large probe array is still emitting although the signal is much out the SPs into freely propagating light. The propagation of weaker than in the previous case 共see Fig. 2兲. The fact that the SPs into the probe array is consequently rapidly attenuthere is still some SP launched in the x direction is probably ated, as one can see in Fig. 2共b兲. Although the experimental due to the lack of purity in the polarization of the incident near-field setup does not allow one to measure the directionlight. Note that controlling the SP launching direction by ality and absolute intensities, one can at least estimate the means of rotating the polarization of the incident field could efficiency of the successive conversion processes by deterbe a key feature of the micro-grating in the context of SP mining roughly the ratio between the transmitted intensity signal routing. near the edge of the source array and the intensity of the Next the influence of various structural parameters were reradiated signal of the probe array. For the configuration of studied. First the period of the source array was changed to Fig. 2, the ratio is 14%, which when corrected for the finite 700 nm. In this case, there is no mode at 800 nm incident propagation length 共i.e., dissipation兲 in the film (⬃45 m This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: wavelength, the (⫾1,0) SP mode being now located at a Ref. 3兲 is probably over 20%. The intensity of the SPs 共that 193.52.246.36 On: Mon, 24 Mar 2014 14:37:25
4938
Devaux et al.
Appl. Phys. Lett., Vol. 83, No. 24, 15 December 2003
shorter wavelength 共710 nm兲. The near-field image of such a pattern, illuminated at 800 nm, is given in Fig. 3共b兲. No SPs along the x direction nor any light escaping from the probe array situated at 30 m to the left are detected showing that one needs to match the incident wavelength with that of a transmission peak of the source array to observe the results of Fig. 2. In other words, there is a direct link between the launching of SPs and the coupling to a mode of the source array, which is yet another confirmation of the involvement of the SPs in the enhanced transmission. Finally, we replaced the holes in the probe array by 50nm-deep holes or ‘‘dimples,’’ i.e., not going through the gold film and having the same periodicity and diameter as the holes in the previous experiments 共respectively, 760 and 250 nm兲. Surprisingly, the SPs launched by the source array are not coupled out radiatively by this new probe array. It is even very difficult to record a near-field image because the propagating SPs are vanishing in the metal film without interacting with the probe array. This is in agreement with the observation that dimples surrounded by holes in an array configuration always appear as dark spots in the far field. A possible reason is that for periodic corrugation to act as efficient decouplers an eigenmode should be present in the individual corrugations themselves 共e.g., cavity mode兲. This is not the case of these dimples due to their very small dimensions 共i.e., /2兲. To test this idea we prepared new samples with long parallel grooves 共depth 50 nm, width 250 nm, length 30 m, period 760 nm兲 perpendicular to the SP propagation direction: in this case the probe array is clearly able to convert back efficiently the SP mode 关see Fig. 3共c兲兴. These observations are in agreement with recent work11 where the groove cavity modes are shown to play a key role in the enhanced transmission mechanism and with theoretical studies which show that the scattering efficiency of an indenta-
tion is strongly dependent on the existence on cavity effects.12,13 The remaining question is why subwavelength holes, unlike the dimples, are efficient scatterers for SPs. One possibility is that they allow amplitude, and hence field intensity, build-up which is considered crucial in the transmission enhancement.14 This could also play a role in their ability to decouple plasmons into propagating light in free space. These near-field results show that hole arrays can be used to locally launch SPs but that the mechanism of the decoupling processes via periodic subwavelength structures is far more complex than expected. Further fundamental studies will be necessary to optimize the design for future SP-based optical devices and sensors. Stimulating discussions with W.L. Barnes and H. Lezec are gratefully acknowledged. R. H. Ritchie, Phys. Rev. 106, 874 共1957兲. H. Raether, Surface Plasmons on Smooth and Rough Surfaces and on Gratings 共Springer, Berlin, 1988兲. 3 W.L. Barnes, A. Dereux, and T.W. Ebbesen, Nature 共London兲 424, 824 共2003兲. 4 M.C. Hutley and D. Maystre, Opt. Commun. 19, 431 共1976兲. 5 J. Moreland, A. Adams, and P.K. Hansma, Phys. Rev. B 25, 2297 共1982兲. 6 U. Schro¨ter and D. Heitmann, Phys. Rev. B 60, 4992 共1999兲. 7 P.T. Worthing and W.L. Barnes, Appl. Phys. Lett. 79, 3035 共2001兲. 8 T.W. Ebbesen, H.J. Lezec, H.F. Ghaemi, T. Thio, and P.A. Wolff, Nature 共London兲 391, 667 共1998兲. 9 J.-C. Weeber, J.R. Krenn, A. Dereux, B. Lamprecht, Y. Lacroute, and J.-P. Goudonnet, Phys. Rev. B 64, 045411 共2001兲. 10 H.F. Ghaemi, T. Thio, D.E. Grupp, T.W. Ebbesen, and H.J. Lezec, Phys. Rev. B 58, 6779 共1998兲. 11 F.J. Garcia-Vidal, H.J. Lezec, T.W. Ebbesen, and L. Martin-Moreno, Phys. Rev. Lett. 90, 213901 共2003兲. 12 J.A. Sa´nchez-Gil, Appl. Phys. Lett. 73, 3509 共1998兲. 13 J.A. Sa´nchez-Gil and A.A. Maradudin, Phys. Rev. B 60, 8359 共1999兲. 14 L. Martin-Moreno, F.J. Garcia-Vidal, H.J. Lezec, K.M. Pellerin, T. Thio, J.B. Pendry, and T.W. Ebbesen, Phys. Rev. Lett. 86, 1114 共2001兲. 1 2
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 193.52.246.36 On: Mon, 24 Mar 2014 14:37:25
