NANO LETTERS
Fabrication, Characterization, and Far-Field Optical Properties of an Ordered Array of Nanoapertures
2004 Vol. 4, No. 10 1965-1968
Arnaud Chovin,† Patrick Garrigue,† Inka Manek-Ho1 nninger,‡ and Neso Sojic*,† Laboratoire d’Analyse Chimique par Reconnaissance Mole´ culaire, UniVersite´ Bordeaux I, ENSCPB, 16 aVenue Pey-Berland, 33607 Pessac, France, and Centre Lasers Intenses et Applications, UniVersite´ Bordeaux I, 351 Cours de la Libe´ ration, 33405 Talence, France Received July 23, 2004; Revised Manuscript Received August 24, 2004
ABSTRACT An ordered array of nanometer-sized optical apertures is created on the distal face of a coherent optical fiber bundle. The fabrication steps derived from SNOM probes technology allowed to produce a high-density array of apertures with adjustable dimensions which retains the initial architecture of the bundle. Moreover, each optical nanoaperture is surrounded by a gold ring-shaped nanoelectrode. The far-field angular distribution of light intensity transmitted through the array shows a diffracting behavior which is a function of the aperture sizes. This analysis demonstrates its potential use as a near-field optical array. Such an array plays therefore a bridging role by interrelating information obtained concomitantly at the nanometer and micrometer scales. Furthermore, since the nanoring electrodes show excellent electrochemical properties, the near-field optical information can be completed by combining it with an orthogonal technique, such as electrochemistry.
Scanning near-field optical microscopy (SNOM) exploits the ability to manipulate light at the nanometric level.1,2 SNOM has been extensively applied in various fields which already used conventional diffraction-limited microscopy, but it has also opened-up new directions in biophysics, high-density optical storage, nanoscale optoelectronics, nanometer manipulation, near-field imaging, etc. This scanning approach allows to perform optical and spectroscopic imaging with a spatial resolution well-beyond the classical diffraction limit. Furthermore, the localized spectroscopic information can be correlated with the topography of the sample at the nanometric scale. In the majority of aperture-based SNOM, a subwavelength-sized light source is formed at the apex of a tapered and metal-coated optical fiber. Such a local excitation (and/or collection) probe is scanned in the near-field regime over the object surface. Thus SNOM exploits mainly the nonpropagating properties of the light transmitted through the nanoaperture. When the probe approaches the object surface, evanescent waves encode the information about subwavelength details of the sample. However, in the farfield region, only propagating waves contribute notably to the recorded signal. With a decrease in aperture dimension, the fraction of energy carried by evanescent waves increases considerably, whereas the propagating waves decrease * Corresponding author. E-mail:
[email protected]. † Laboratoire d’Analyse Chimique par Reconnaissance Mole ´ culaire. ‡ Centre Lasers Intenses et Applications. 10.1021/nl048824s CCC: $27.50 Published on Web 09/08/2004
© 2004 American Chemical Society
concurrently.3 Therefore, near and far-field radiative properties of such apertures have been thoroughly investigated in order to clearly interpret SNOM measurements. The far-field angular distribution of the transmitted intensity of linearly3-5 and circularly6,7 polarized light has been previously described and theoretically analyzed8 only for single SNOM apertures. Obermu¨ller and Karrai reported the angular dependence I(θ) of radiation diffracted by SNOM probes as a function of the aperture radius a.4 To the best of our knowledge, near-field optical characteristics of a nanoaperture array have not been reported so far. We fabricated our aperture array (Figure 1A) by adapting a methodology that derives from the preparation of classical SNOM apertures based on etched optical fibers.9-11 Pantano and Walt adapted initially a similar approach to an array format and thus to fabricate an array of ∼400-800 nm diameter apertures by gold-coating the nanotip array, embedding it in epoxy, and polishing it to remove gold from the tip apex.12,13 However, neither electrochemical properties nor optical data have been reported for these arrays.12,13 The fabrication steps of the present array were progressively optimized and reported in detail elsewhere.14-16 In brief, a nanotip array is prepared by wet chemical etching of a coherent optical fiber bundle comprising 6000 individually cladded 3-4 µm diameter optical fibers (Figure 1B). Nanotips were created at each optical fiber core by using the difference in etching rates between the GeO2-doped core
Figure 1. (A) Schematic illustration (side view) of the array of subwavelength-sized apertures. (B) Epifluorescence image collected through the nanoaperture array displayed in Figure 1C. The distal face is immersed in a solution containing Ru(bpy)32+. (C-E) Scanning electron micrographs of the distal face of different arrays with various aperture radii (C-D: a ∼ 220 nm; E: a ∼ 100 nm) at different magnifications. The arrows indicate the nanoapertures.
and the fluorine-doped clad.9,12,13 The surface of the nanotip array was then sputter-coated with a gold film and the entire array surface was insulated with an electrophoretic paint except for the tip apex (Figure 1A). To create the optical nanoapertures, the exposed gold film is removed by reacting in a gold-etching solution.16 So we produced an ordered array where each optical nanoaperture is surrounded by a gold ringshaped electrode (Figure 1A). Since the electromagnetic field decays exponentially inside the metal, the gold film serves to confine light to the tip apex as in aperture SNOM. In addition, the gold nanoring is also used as the electrode material to perform electrochemical reactions. Figure 1C shows the scanning electron micrograph of the array surface. The ordered structure of the nanoapertures and the homogeneity of the array are clearly visible. At high magnification (Figure 1D), one can observe the central aperture corresponding to the etched core, which is surrounded successively by a gold film and eventually an insulating paint layer. From the SEM image, we have estimated the radius of the aperture (∼220 nm). By changing the parameters of the electrophoretic paint deposition and the gold etching time (see Supporting Information), we have prepared arrays with different aperture sizes. Figure 1E displays a nanoaperture 1966
of ∼100 nm radius. It is difficult to precisely control the aperture size based on the conditions of electrophoretic paint deposition, the number of coatings, and the gold etching time. However, more than 60% of the nanoaperture arrays displayed well-defined electrochemical and optical characteristics. The nanoaperture arrays were characterized by epifluorescence imaging.17 The distal face of the array was immersed in a solution containing Ru(bpy)32+. Excitation light (λEXC ) 485 nm) was focused onto the proximal face of the optical fiber bundle and guided by total internal reflection in each fiber core through the bundle. The transmitted light is confined to the nanoaperture by the gold film and initiates fluorescence of Ru(bpy)32+. A fraction of the isotropic emission (λEM ) 615 nm) is collected by the same nanoaperture, transmitted by the corresponding fiber core, and eventually detected at the proximal face by a CCD camera. Figure 1B displays the epifluorescence image acquired through the array which is shown in Figure 1C. Since coherent bundles were used in the present work,18 the fluorescence intensity of each nanoaperture is individually readable.15 Furthermore, a single epifluorescence image measures simultaneously the light intensities transmitted by Nano Lett., Vol. 4, No. 10, 2004
Figure 2. Cyclic voltammogram of the nanoaperture array displayed in Figure 1C in an aqueous 5 mM Ru(NH3)63+ solution containing 1 M Na2SO4 at 10 mV s-1.
all the nanoapertures forming the array. One observes also from Figure 1B that not all nanoapertures are optically active. Indeed, there are spots where no fluorescence is detectable, because no light emerged from these fibers. This might be due to imperfect gold etching. From the epifluorescence image, we can deduce the fraction of optically active nanoapertures to be ∼90%. Eventually this figure confirms the homogeneity of the array observed by SEM (Figure 1C). The electrochemical performance of this array was investigated in an aqueous Ru(NH3)63+ solution in the presence of excess supporting electrolyte. The voltammetric response corresponding to the reduction of Ru(NH3)63+ is sigmoidal in shape and shows only a little hysteresis on the return scan (Figure 2). This nearly ideal behavior demonstrates the excellent electrochemical properties of the gold ring nanoelectrodes. Furthermore, the steady-state cyclic voltammogram is characteristic of well-behaved ultramicroelectrodes. This feature indicates that quasi-steady state diffusion occurs at each electrode. It means that the overlapping between the individual diffusional layers generated by each nanoelectrode is minimized. The nanoelectrodes forming the array can therefore be considered as diffusively independent. In other words, each gold nanoring probes electrochemically a different nanoenvironment. The optical near-field information acquired via the array can be further cross-correlated to an electrochemical signal. The schematic of the experimental setup used to measure the angular distribution of the far-field intensity transmitted through the arrays of subwavelength-sized apertures is presented in Figure 3. The axis of the optical fiber bundle is aligned along the Z-axis. The nanoapertures are centered and located in the plane XY. An He-Ne laser (λ ) 633 nm) is mounted on a goniometer and scanned around the nanoapertures in the plane XZ. The polar angle θ defines the angle between the Z-axis and the laser incident direction. The light intensity transmitted through the nanoaperture array is analyzed by a polarizer and then measured by a CCD camera. At θ ) ( 90°, the laser lies in the plane XY, whereas at θ ) 0° it faces the nanoaperture arrays. The data are collected over an angle θ stretching from 0° to 80°. Even if it would be very interesting to measure backward transmission at angles greater than 90°,4,6,8 the array geometry prevents us to explore angles exceeding 80° in this far-field configuration. Nano Lett., Vol. 4, No. 10, 2004
Figure 3. Schematic representation of the experimental setup: P, polarizer; L, laser; O, microscope objective; CCD, CCD camera.
Indeed, since the nanoapertures are located in the same plane, the shadows projected by adjacent nanoapertures at θ values approaching 90° would alter the monitored intensity data. The incoming laser radiation is linearly polarized, and Φ represents the angle between the polarization plane of the detected radiation and the plane in which the laser is scanned. At Φ ) 0°, the polarization of the incident electric field is parallel to the plane XZ scanned by the laser (p-polarization). At Φ ) 90°, the polarization is perpendicular to the analysis plane (s-polarization). The transmitted intensity is measured as a function of θ for both polarizations. Figure 4 displays the angular distribution of normalized far-field intensity transmitted trough nanoaperture arrays of different dimensions for both (A) Φ ) 0° and (B) Φ ) 90° configurations. The open squares represent the numerical aperture (NA) of the polished bundle of monomode optical fibers (i.e., without nanoapertures). The distribution is polarization-independent in this case. The nanoaperture array displayed in Figure 1D shows a clearly different shape for the angular distribution (triangles in Figure 4). The curve I(θ)/I(θ ) 0) is much broader for both polarizations. Light falling on the nanoapertures, even with angles greater than the initial bundle’s NA, is transmitted to the CCD detector. Furthermore, when the aperture radii decrease, the shape of the normalized distribution tends to widen (Figure 4). This is the typical behavior of an object whose characteristic dimensions are smaller than the wavelength. In fact, each nanoaperture in the array diffracts the light. In the ppolarization (Figure 4A), the maximum of detected intensity does not correspond to the position where the laser faces the aperture (i.e., θ ) 0°). One observes a symmetric form around θ ) 0°. In addition, the signal measured at large angles (∼60-70°) is higher when the nanoaperture radius decreases. Similar features specific to p- and s-polarizations have already been reported only for a single nanoaperture.4 Other polarization-dependent differences (i.e., backward transmission until 165° in p-polarization)4 have been described for angles that are inaccessible with an array in this far-field configuration. To circumvent this limitation, we plan to use near-field measurements19,20 to explore the backward transmission. However, the far-field diffraction pattern of the nanoaperture array is qualitatively identical to the behavior of a single optical nanoaperture in both polarizations. This proves that we have fabricated an ordered array 1967
shows also a fairly inhomogeneous statistical distribution of light intensity which reflects the scattering of the aperture radii. In summary, we report the fabrication of nanoaperture arrays of adjustable dimensions. Near-field optical behavior was established in an array format where each element is equivalent to a single nanoaperture with electrochemical capabilities. This nanostructured array keeps its imaging properties at both nanometer and micrometer scales. Indeed, each nanoaperture can probe a different near-field region, whereas the global array allows imaging simultaneously a large micrometric area. In addition, such arrays can find promising applications for nanosensors, nanomanipulation, nanolithography, and electrooptical nanodevices. Acknowledgment. This work has been supported in part by the French Ministry of Research (Program Action Concerte´e Incitative Jeunes Chercheurs) and by the Conseil Re´gional d’Aquitaine. The authors thank Gilles Lovo (BASF) for the gift of the electrophoretic paint. Supporting Information Available: Fabrication procedure and epifluorescence experimental setup are included. This material is available free of charge via the Internet at http://pubs.acs.org. Figure 4. Angular distribution of normalized far-field intensity transmitted through nanoaperture arrays of different radii. The laser is scanned (A) in the plane of polarization (Φ ) 0°) or (B) perpendicularly to the plane of polarization (Φ ) 90°). The nanoaperture radii indicated on the graph correspond to the values determined from the fwhm data (see text). Laser wavelength λ ) 633 nm.
of diffracting apertures that has the potential to be applied as a near-field probe array. Obermu¨ller and Karrai established that the full width at half-maximum (fwhm) of the far-field angular distribution is a function of the aperture size.4 They found an empirical relation between the aperture radius and the fwhm of I(θ) for s-polarization.4 However, this method is valid only for aperture diameters down to λ/6. We therefore used this approach to determine the optical aperture radii and compared them with the SEM data. For Φ ) 90°, the fwhm of the tested arrays were 65° (Figure 1D and triangles in Figure 4), 80° (disks in Figure 4), and 98° (Figure 1E and crosses in Figure 4). From Figure 4 of Karrai’s work,4 these fwhm values correspond to radii of 190, 140, and 95 nm, respectively. Experimentally measured radii from SEM images (a ∼ 220, 150, and 100 nm) were found to be in good agreement with radii determined from far-field data. The small differences can probably be explained by a relative variability of the aperture dimensions in an array. The epifluorescence image acquired through an array (Figure 1B)
1968
References (1) Dunn, R. C. Chem. ReV. 1999, 99, 2891-2927. (2) Lewis, A.; Taha, H.; Strinkovski, A.; Manevitch, A.; Khatchatouriants, A.; Dekhter, R.; Ammann, E. Nature 2003, 21, 1378-1386. (3) Petrov, N. I.; Woo, G. M.; Lee, M. B. J. Microsc. 2003, 212, 169174. (4) Obermu¨ller, C.; Karrai, K. Appl. Phys. Lett. 1995, 67, 3408-3411. (5) Seebacher, S.; Osten, W.; Veiko, V. P.; Voznessenski, N. B. Optics Lasers Eng. 2001, 36, 451-473. (6) Shin, D. J.; Chavez-Pirson, A.; Lee, Y. H. J. Microsc. 1999, 194, 353-359. (7) Shin, D. J.; Chavez-Pirson, A. Opt. Lett. 2000, 25, 171-173. (8) Drezet, A.; Woehl, J. C.; Huant, S. J. Microsc. 2001, 202, 359361. (9) Ohstu, M. Optoelectronics 1995, 10, 147-166. (10) Ohtsu, M. J. LightwaVe Technol. 1995, 13, 1200-1221. (11) Maheswari, U. R.; Monobe, S.; Ohtsu J. LightwaVe Technol. 1995, 13, 2308-2313. (12) Pantano, P.; Walt, D. R. ReV. Sci. Instrum. 1997, 68, 1357-1359. (13) Liu, Y.-H.; Dam, T. H.; Pantano, P. Anal. Chim. Acta 2000, 419, 215-225. (14) Szunerits, S.; Garrigue, P.; Bruneel, J.-L.; Servant, L.; Sojic, N. Electroanalysis 2003, 15, 548-555. (15) Chovin, A.; Garrigue, P.; Vinatier, P.; Sojic, N. Anal. Chem. 2004, 76, 357-364. (16) Chovin, A.; Garrigue, P.; Servant, L.; Sojic, N. ChemPhysChem 2004, in press. (17) Bronk, K. S.; Michael, K. L.; Pantano, P.; Walt, D. R. Anal. Chem. 1995, 67, 2750-2757. (18) Pantano, P.; Walt, D. R. Anal. Chem. 1995, 67, 481A-487A. (19) Novotny, L.; Pohl, D. W.; Regli, P. J. Opt. Soc. Am. A 1994, 11, 1768-1778. (20) Novotny, L.; Pohl, D. W.; Hecht, B. Opt. Lett. 1995, 20, 970-972.
NL048824S
Nano Lett., Vol. 4, No. 10, 2004
