Surface Plasmon Near-Field Imaging of Very Thin Microstructured

Jean Claude Weeber,* Eric Finot, Guillaume Legay, Arnaud Cathelat,. Yvon Lacroute, and Alain Dereux. Laboratoire de Physique, Universite´ de Bourgogn...
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Langmuir 2004, 20, 10179-10185

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Surface Plasmon Near-Field Imaging of Very Thin Microstructured Polymer Layers Jean Claude Weeber,* Eric Finot, Guillaume Legay, Arnaud Cathelat, Yvon Lacroute, and Alain Dereux Laboratoire de Physique, Universite´ de Bourgogne, UMR-CNRS 5027, 9 Avenue Alain Savary, BP 47870, F-21078 Dijon, France Received July 13, 2004. In Final Form: August 24, 2004 We report on the near-field imaging of microstructured polymer layers deposited on an homogeneous metal thin film on which a surface plasmon mode is excited. The microstructures in the polymer layers are designed by electron beam lithography, and the near-field imaging is performed with a photon scanning tunneling microscope (PSTM). We show that, despite their very small height, the microstructures can be conveniently imaged with a PSTM thanks to the field enhancement at the surface of the metal thin film supporting the surface plasmon. The influence of the illumination conditions on the contrast of the PSTM images is discussed. In particular, we show that both the field enhancement and the near-field intensity distribution around the microstructures depend dramatically upon the illumination conditions, leading to the conclusion that the PSTM is well suited for spatially resolved near-field surface plasmon sensing purposes.

I. Introduction The characterization of organic thin films with thicknesses of a few nanometers is not only of fundamental interest but might have direct applications for biosensing. Among the experimental techniques available for such a characterization, optical methods relying on surface plasmon resonance (SPR) are widely used. Unlike optical methods based on fluorescence, SPR does not require labels or target amplification of the biomolecular thin film.1-3 A surface plasmon (SP) is a resonant oscillation of free electrons at the interface between a metal and a dielectric.4 For noble metals such as gold or silver, surface plasmon can be excited with visible or near-infrared TM polarized (electric field parallel to the plane of incidence) light provided that the wave-vector component of the incident light parallel to the metal surface matches the wave-vector of the SP. In the so-called KretschmannRaether configuration (see Figure 1), a metal film with a typical thickness of 50 nm is evaporated onto the base of a glass prism. The matching between the incident light and the SP wave-vector can be achieved by finely tuning the angle of incidence. The angle of incidence θsp corresponding to the surface plasmon resonance is larger than the critical angle of the prism such that the electromagnetic field associated with a SP exhibits an evanescent behavior and decays exponentially in the direction perpendicular to the surface of the metal thin film. The excitation of a SP enhances simultaneously the electric field at the metal surface and the absorption of the incident beam energy. This absorption induces a * To whom correspondence should be addressed. Mail: Laboratoire de Physique de l’Universite´ de Bourgogne, Equipe Optique Submicronique, UFR Sciences et Techniques, 9, rue A. Savary, BP 47870, 21078 DIJON, France. Phone: (+33). 3.80.39.60.31. Fax: (+33).3.80.39.60.24. E-mail: [email protected]. (1) Roy, S.; Kim, J.-H.; Kellis, J. T., Jr.; Poulose, A. J.; Robertson, C. R.; Gast, A. P. Langmuir 2002, 18 (16), 6319-6323. (2) Deckert, A. A.; Lesko, J.; Todaro, S.; Doyle, M.; Delaney, C. Langmuir 2002, 18 (21), 8156-8160. (3) Zhang, Z.; Menges, B.; Timmons, R. B.; Knoll, W.; Forch, R. Langmuir 2003, 19 (11), 4765-4770. (4) Raether, H. Surface plasmons on smooth and rough surfaces and on gratings; Springer-Verlag: Berlin, 1989.

Figure 1. Schematic view of the Kretchmann-Raether configuration. A collimated beam is incident through a glass prism on which a metal thin film has been deposited. For a given angle of incidence (larger than the critical angle), the phase matching between the incident electromagnetic field and the field of the surface plasmon leads the SP excitation at the upper interface of the metal film.

pronounced dip in the reflectivity curve of the metal film. Most SPR-based sensors rely on the measurement of either the angle θsp or the metal thin film reflectivity at an angle of incidence equal to θsp. When an organic layer is deposited on the metal thin film, the excitation conditions of the SP are slightly changed, leading to a small shift of θsp. The corresponding change in reflectivity enables the change in thickness and/or index of refraction of the organic layer to be monitored in situ and in real time. This method, known as attenuated total reflection (ATR), has been widely adopted for biosensors.5-7 However, because the reflectivity provides optical information averaged over the area which is illuminated by the finite size incident beam, the ATR configuration does not allow one to investigate the spatial structure of the organic layer in the plane of the metal film supporting the SP. An improvement of the ATR method which consists of forming the image of the (5) Kaneko, F.; Saito, W.; Sato, T.; Hatakeyama, H.; Shinbo, K.; Kato, K.; Wakamatsu, T. Thin Solid Films 2003, 438-439, 108-113. (6) Kumar, A.; Kamihira, M.; Galaev, I. Yu.; Iijima, S.; Mattiasson, B. Langmuir 2003, 19 (3), 865-871. (7) Kato, K.; Kawashima, J.; Baba, A.; Shinbo, K.; Kaneko, F.; Advincula, R. Thin Solid Films 2003, 438-439, 101-107.

10.1021/la048242q CCC: $27.50 © 2004 American Chemical Society Published on Web 10/05/2004

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metal thin film surface on the chips of a CCD camera has been recently reported.8 This technique, known as surface plasmon imaging (SPR-i), offers the opportunity to observe details of the layer structure with a typical lateral resolution of a few microns.9 This approach, recently applied for biosensing,10,11 allows for example the simultaneous monitoring of a hundred DNA microspots.12 Such a spatially resolved characterization is necessary for the design of miniaturized surface plasmon sensor chips.13 Local optical characterization of organic layers by SP imaging is not only useful for downsizing SPR sensors but also for improving the understanding of the interaction between the SP and organic thin films. In this work, we operate a near-field optical microscope known as the photon scanning tunneling microscope (PSTM) to investigate, at the submicron scale, polymer layers with a thickness of a few nanometers.14-16 Electron beam lithography (EBL) has been used to create structures of nanometric depth and well-controlled geometries in the polymer films. These very thin polymer films can be viewed as models of biological layers and allow us to investigate the capabilities of the SP plasmon imaging in the context of near-field optical sensing. The limitations of this technique related to the fundamental properties of SP are discussed, thereby providing a guideline for the design of highly miniaturized metal thin film SP-based sensors. II. Experimental Background PSTM Setup. The PSTM setup used in this work is schematically shown in Figure 2. This near-field optical microscope has been homemade on the basis of an atomic force microscope (D3000, Digital Instruments, Santa Barbara, CA). It consists mainly of two parts, a scanning system comprised of a piezo-tube on which is attached an optical probe and a stage allowing the illumination of the sample. The sample is optically connected to the base of a BK7 glass prism with a refractive index matching fluid providing the mechanical stability of the sample and the refractive index continuity between the prism and the sample substrate. The illumination of the sample is performed using a collimating lens attached to a single mode fiber into which a titanium-sapphire laser beam (wavelength in a vacuum λ ) 800 nm) is injected. The collimated lens produced a parallel beam with a diameter of about 600 µm. The polarization of the incident beam (typical power ) 4 mW) is controlled by a polarizer placed after the collimator. The angle of incidence can be finely adjusted using the rotation stage on which the lensed fiber is mounted. The angle of incidence corresponding to the SP resonance is determined by measuring the minimum reflectivity using a photodetector placed in the path of the reflected beam. When the SP is excited, the optical probe approaches close to the sample surface. The optical probes used in this work are multimode fiber tips produced by a standard heat-and-pull technique. The resulting tips exhibit a typical radius of curvature of about 50 nm. After the pulling process, a few nanometers of chromium and a gold layer with a thickness of 15 nm are thermally evaporated on the fiber tips while rotating around their axis. (8) Sui, S.-F.; Zhou, Y. Sens. Actuators, B 2000, 66, 146-148. (9) Haueussling, L.; Ringsdorf, H.; Schmitt, F.-J.; Knoll, W. Langmuir 1991, 7, 1837. (10) Nelson, B. P.; Grimrsud, T. E.; Liles, M. R.; Goodman, R. N.; Corn, R. M. Anal. Chem. 2001, 73, 1-7. (11) Livache, T.; Maillart, E.; Lassalle, N.; Mailley, P.; Corso, B; Guedon, P.; Roget, A.; Levy, Y. J. Pharm. Biomed. Anal. 2003, 32, 687696. (12) Bassil, N.; Maillart, E.; Canva, M.; Le´vy, Y.; Millot, M.-C.; Pissard, S.; Narwa, R.; Goossens, M. Sens. Actuators, B 2003, 94, 313-323. (13) Furuki, M.; Kameoka, J.; Craighead, H. G.; Isaacson, M. S. Sens. Actuators, B 2001, 79, 63-69. (14) Ianoul, A.; Burgos, P.; Lu, Z.; Taylor, R. S.; Johnston, L. J. Langmuir 2003, 19 (22), 9246-9254. (15) Steiner, G.; Sablinskas, V.; Hu¨bner, A.; Kuhne, Ch; Salzer, R. J. Mol. Struct. 1999, 509, 265-273. (16) Vaccaro, L.; Schmid, E. L.; Ulrich, W.-P.; Vogel, H.; Duschl, C.; Marquis-Weible, F. Langmuir 2000, 16 (7), 3427-3432.

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Figure 2. Schematic view of the PSTM setup. The SPs are excited at the surface of the sample in the Kretchmann-Raether configuration. The intensity of the reflected beam is measured with a photodetector (PD). The metal-coated optical probe mounted on a piezo-tube is used for the detection of the optical near-field intensity due to the SP excitation (PSTM signal) and the detection of a tunnel current between the probe and the sample (STM signal). The distance between the probe and the sample is adjusted by a voltage offset (Offset V) controlling the voltage (Z Voltage) applied on the Z-electrode of the piezo-tube. Thanks to this metal coating, a bias voltage can be applied on the optical probe. If the sample is grounded, a tunnel current can thus be detected for the metal-coated fiber tip in quasi-contact with the sample surface. The tip-sample distance is then controlled by applying an offset voltage to the Z electrode of the piezo-tube. When placed close to the sample surface at a typical distance of a few tenths of nanometers, the optical probe frustrates the evanescent field of the SP and converts it into a guided mode propagating along the optical fiber. A photomultiplier tube transforms the guided optical intensity into a current which is finally amplified by a current/voltage converter (typical gain, 106). The resulting voltage is then proportional to the near-field electrical intensity at the location of the optical probe. Monitoring this voltage while the tip is scanned at a constant height above the sample surface allows one to image the near-field intensity distribution. Note that the scanning of the probe is achieved without any feedback control. Thus, prior to PSTM imaging, the tilt of the scanning plane with respect to the sample surface must be finely adjusted according to the procedure described in ref 17. Sample Preparation. The samples we consider consist of a gold thin film coated with a very thin layer of poly(methyl methacrylate) (PMMA). This polymer can be spin-coated to obtain layers with thicknesses smaller than 10 nm. In addition, the PMMA resist can be microstructured by electron beam lithography at a scale down to 100 nm. The gold film with a typical thickness of 50 nm was first thermally evaporated on a BK7 glass substrate at pressure of 10-6 Pa and a rate of 0.1 nm/s. The 950K molecular weight PMMA was then subsequently spincoated on gold. The microstructures in the PMMA layer were designed using a scanning electron microscope (JEOL 840) driven by lithography software (Elphy quantum, Raith GmbH). The development of the exposed PMMA layer was performed with a 1:3 methyl isobutyl ketone (MIBK) solution using Ultra Sound agitation during 45 s. After the development step, the thickness and the size of the PMMA structures were determined using an atomic force microscope (AFM; Nanoscope, D3100, Digital Instruments) equipped with a scanner allowing a maximum x-y scanning range of 120 µm and a z range of 5 µm in air (30% relative humidity at 25 °C). The nominal spring constants of (17) Weeber, J. C.; Lacroute, Y.; Dereux, A. Phys. Rev. B 2003, 68, 115401.

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Figure 5. Computed reflectivity curves of a 50 nm thick gold film deposited on a glass substrate (n ) 1.519). The gold film is covered with a polymer layer (n ) 1.49) with a thickness d.

III. Results and Discussion

metal film. Figure 5 shows the evolution of the metal thin film reflectivity calculated for different thicknesses of a PMMA layer (n ) 1.49) deposited on a 50 nm thick gold film. For this computation, we have used an index of refraction of the gold film (nˆ ) 0.32 + i4.878) measured by ellipsometry on our samples at a frequency corresponding to an incident wavelength of 800 nm in a vacuum. The three curves displayed in Figure 5 exhibit a reflectivity dip characteristic of a surface plasmon excitation. This dip occurs for increasing values of the angle of incidence as the thickness of the PMMA layer increases. For d < 10 nm, the angle of the SP resonance θsp is found to increase almost linearly with increasing d. Note that neither the minimum reflectivity nor the full width at half-maximum (fwhm) of the reflectivity dip is significantly affected by the polymer layer. This shows that a surface plasmon can be excited on metal thin film coated with a thin polymer layer with the same efficiency as in the case of a bare metal film. The excitation of a surface plasmon can be conveniently detected by monitoring the intensity of the light reflected by or transmitted through the metal film. Indeed, the electromagnetic field close to the surface of the metal film is dramatically enhanced at the SP resonance. Because of the roughness of the metal film, the SP is partially scattered and converted into photons propagating into the upper medium. Thus, despite the evanescent character of the SP electromagnetic field, the SP resonance can be monitored with a photodetector placed in the far-field zone in the upper medium.18 However, reliable quantitative measurements are difficult to achieve with this technique since the intensity of the scattered field depends on parameters that are difficult to control such as the metal film roughness. This limitation can be overcome if the field of the SP plasmon is directly detected. In this context, the PSTM becomes a convenient tool since this near-field microscope is specifically designed to observe evanescent fields. Figure 6 shows the typical evolution of the near-field optical signal recorded when the fiber tip approaches a bare 50 nm thick gold film on which a SP has been excited (incident wavelength in a vacuum ) 800 nm). The optical approach curve has been monitored at θsp ) 42.25° corresponding to the minimum reflectivity of the gold thin film. The metal-coated fiber tip (Cr and Au) was biased during the approach such that a tunnel current can be

Field Enhancement and Near-Field Approach Curves. The excitation of a surface plasmon leads to a large absorption of the incident light energy within the

(18) Lamprecht, B.; Krenn, J. R.; Schider, B.; Ditlbacher, M.; Salerno, M.; Felidj, N.; Leitner, A.; Aussenegg, F. R.; Weeber, J. C. Appl. Phys. Lett. 2001, 79, 51.

Figure 3. AFM images of typical microstructures fabricated by EBL in a 15 nm layer of PMMA. (a) Periodic grooves (period ) 15.0 µm, width ) 1.0 µm). (b) Double grooves (width ) 2.0 µm, distance between the grooves ) 4.0 µm).

Figure 4. Thickness of the PMMA film deposited on gold film measured by atomic force microscopy as a function of two parameters of the spin coating, namely, the PMMA concentration and the rotating speed V. silicon nitride cantilevers used (Park Scientific Instrument, Sunnyvale, CA) were approximately 0.06 N/m in contact mode. The scan rate was fixed to approximately 20 µm/s so that the feedback loop could track the topography more accurately. The AFM images of typical microstructures fabricated according to this process are shown in Figure 3. Figure 4 shows the polymer layer thickness measured by atomic force microscopy with respect to both the rotation speed V and the dilution of the PMMA resist. For example, at V )7000 rpm and for a PMMA/solvent ratio of 0.5% and 0.7%, PMMA layers with respective thicknesses of 4 and 10 nm have been obtained.

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Figure 6. Simultaneously recorded PSTM (solid line) and STM (dashed line) approach curves. The sample consists of a 50 nm thick gold film excited at the SP resonance angle by incident light with a wavelength in a vacuum of 800 nm. Z denotes the absolute distance between the metal-coated fiber tip and the surface of the sample.

Figure 7. PSTM approach curves recorded over a gold thin film (50 nm) coated with a 10 nm thick PMMA layer. On the basis of reflectivity measurements, the SP resonance for the system has been found at 43.18° (dashed curve). The dasheddotted and the solid curves have been obtained at an angle of incidence of 43.28° and 42.2°, respectively.

measured for typical tip-sample distances of a few angstro¨ms. The tunnel current recorded simultaneously with the near-field optical signal is also displayed in Figure 6. The abrupt increase in the tunnel current intensity is ascribed to the contact between the tip and the sample. For tip-sample distances larger than 200 nm, the optical near-field intensity exhibits the expected exponential behavior in agreement with the evanescent character of the SP field. This exponential increase is however replaced by an oscillation for tip-sample distances smaller than roughly 170 nm. Details of the physical origin of this behavior are out of the scope of this work; however we can assume that when the tip is approached from the sample at a distance significantly smaller than the incident wavelength, a strong electromagnetic coupling between the metal thin film and the metal-coated PSTM probe occurs. Such a coupling can perturb locally the SP resonance leading to a decrease of the near-field optical signal. The experimental approach curves indicate that the maximum SP near-field intensity is obtained not for the PSTM probe in contact with the metal thin film but for a tip-sample distance of roughly λ/4. The PSTM images presented in the following have been recorded after optimization of the near-field intensity by adjustment of the tip-sample distance such that for all these images the tip-sample distance was about 170 nm. Following the same procedure, near-field intensity approach curves have been recorded for a 10 nm thick layer of PMMA spin-coated on a 50 nm thick gold film. The minimum reflectivity has been found in this case at an angle of incidence of θsp ) 43.18°. The approach curves corresponding to 43.18°, 43.28°, and 42.2° are displayed in Figure 7. As in the case of the bare gold film, the nearfield optical signal is maximum when the probe is a few tenths of nanometers away from the sample surface. When the angle of incidence is increased slightly, the maximum of near-field intensity drops dramatically such that the SP resonance can be detected directly from the near-field signal without considering the far-field reflectivity of the system. A direct comparison of the near-field optical intensity detected over the bare (Figure 6) and the polymer-coated (Figure 7) gold film is not possible since two different PSTM probes have been used in these two experiments. However, in both cases the PSTM signalto-noise ratio is extremely high, which indicates that the field enhancement is of the same order of magnitude with or without the polymer layer.

Surface Plasmon PSTM Imaging. We consider now the PSTM imaging of microstructured PMMA layers deposited on a gold thin film on which a SP is excited. The sample we consider first consists of a gold film (thickness, 50 nm) coated with a 10 nm thick polymer layer in which grooves have been carved by electron beam lithography according to the process previously described. The first pattern is a periodic arrangement of 2.0 µm wide grooves with a length of 100 µm and a periodicity of 15 µm. The PSTM image of this pattern shown in Figure 8a has been obtained at an angle of incidence of 43.18° corresponding to the SP resonance of the multilayer system (gold film + polymer layer) and the plane of incidence parallel to the long axis of the grooves. With these illumination conditions, the PSTM response of the grooves is wide dark stripes flanked by two small-amplitude bright oscillations. Unlike the near-field response of finite width metal stripes,17 the fwhm of the optical response of the grooves is found to be much larger than their widths given by AFM measurements (Figure 8b). The near-field optical profiles of the grooves have been numerically computed using the differential method.19 Among others, this technique has proven to be reliable for the computation of the electromagnetic near-field of periodic objects that are thin compared to the incident wavelength. Details of this method are outlined in ref 20. The computed electric near-field intensity profile shown in Figure 8b has been obtained with illumination conditions similar to those of the experiment. The good agreement between the theoretical and experimental profiles demonstrates first that the PSTM allows an accurate imaging of the SP electric near-field distribution around the microstructures and second that the PSTM signal can be modeled directly from the near-field electric intensity existing around the microstructures. On the basis of these results, the contrast of the PSTM images recorded with or without a SP field enhancement can be estimated numerically. The theoretical optical profiles displayed in Figure 8c have been computed for 2.0 µm wide grooves in a 10 nm thick polymer layer deposited respectively on a glass substrate (solid line) and on a gold thin film supporting a surface plasmon (dashed line). The contrast of the optical profile is 100fold higher for the SP excited on the gold thin film compared to the microstructures deposited on the glass (19) Montiel, F.; Nevie`re, M. J. Opt. Soc. Am. A 1994, 11, 3241. (20) Vincent, P. Electromagnetic theory of gratings; Petit, E., Ed.; Springer-Verlag: Berlin, 1980; Chapter 4.

Near-Field Imaging of Polymer Layers

Figure 8. (a) PSTM image of 2.0 µm wide grooves created in a 10 nm thick PMMA layer deposited on a 50 nm gold thin film. The angle of incidence corresponds to the SP resonance of the polymer-coated metal thin film. The optical signal is normalized with respect to the higher optical intensity detected over the scanning area. The plane of incidence is parallel to the long axis of the grooves, and the incident SP travels from the top to the bottom of the image. (b) Experimental (solid) and computed (dashed) optical near-field profiles of the grooves. The fwhm of the optical profiles is much larger than the actual width of the grooves (shown by the two vertical dashed lines). (c) Optical near-field profiles computed 30 nm over the top of grooves in a 10 nm thick layer of PMMA (n ) 1.49) respectively deposited on a gold thin film supporting a SP (dashed line) or on a glass substrate (solid line). The near-field electrical intensity is normalized with respect to the incident electric field intensity.

substrate. The field enhancement related to the SP excitation at the surface of the metal thin film is then the key feature of the near-field sensing. The reflectivity curves of Figure 5 show dramatic changes in the thin film reflectivity at an angle of incidence a little over θsp. As a result, the near-field distributions

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Figure 9. PSTM images of 2.0 µm wide grooves created in a 10 nm thick PMMA layer deposited on a gold thin film for different angles of incidence θ. (a) θ ) 43.18° (SP resonance of the polymer-coated gold thin film). (b) θ ) 42.2°. (c) θ ) 41.3°. The optical signal is normalized with respect to the highest value of the PSTM signal for each image. The plane of incidence is parallel to the Y-axis.

around the polymer microstructures are expected to depend dramatically on the angle of incidence. In Figure 9, we show the PSTM images of 2 µm wide polymer grooves recorded at angles of incidence of 43.18°, 42.2°, and 41.3° corresponding to the SP resonance of the polymer-coated gold film, the SP resonance of the bare gold film, and the critical angle of the glass substrate supporting the samples, respectively. At θ ) 42.2° (Figure 9b), we mainly observe the edges of the polymer grooves, whereas bright contrast stripes reveal the existence of the microstructures at θ ) 41.3° (Figure 9c). Because inside the grooves there is no polymer on the gold film, one might expect a strong field enhancement over the grooves when the angle of incidence matches the bare gold film SP resonance. We do not observe such an enhancement on our images presumably

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Figure 11. PSTM image of a 10 µm wide groove illuminated in normal diffraction (plane of incidence perpendicular to the long axis of the groove). The white dashed rectangle shows approximately the location of the surface defect. Note that only the edge of the groove parallel to the plane of incidence leads to a significant contrast in the PSTM image.

Figure 10. PSTM images of grooves with different widths W. (a) W ) 1.0 µm. (b)W ) 0.5 µm. (c) W ) 0.25 µm. The angle of incidence is θ ) 43.18°, and the incident SP propagates from the top to the bottom of the image.

because the dispersion relations of the SP modes supported by a 2 µm wide groove and an extended bare thin film are different. Although the contrast of the near-field response of the grooves is not totally reversed when switching the angle of incidence from the SP resonance of the polymercoated thin film (Figure 9a) to the SP resonance of the bare thin film (Figure 9b), the changes in the PSTM images for these two angles of incidence are significant enough to demonstrate the feasibility of a near-field spatially resolved SP sensor relying on the PSTM imaging. To check the ability of the surface plasmon PSTM imaging to investigate submicronic structures, the width of the grooves has been reduced down to 1.0, 0.5, and 0.25 µm. The PSTM images (Figure 10) have been obtained at λ ) 800 nm for θ corresponding to the SP resonance of the polymer-coated gold film and the plane of incidence parallel to the long axis of the grooves. The optical nearfield responses of the grooves reveal well-pronounced dark

stripes in agreement with the images of the 2.0 µm wide structures. In particular, Figure 10c shows that the PSTM image is highly contrasted such that a one-dimensional surface defect with a width of only λ/3 can be easily discerned using the near-field SP imaging system. All the PSTM images previously discussed have been obtained with the plane of incidence parallel to the long axis of the grooves (conical diffraction). With the aim of studying the influence of the orientation of the plane of incidence with respect to the long axis of the microstructures, we consider now the case of the normal diffraction (plane of incidence perpendicular to the long axis of the grooves). The microstructure consists of a 10 µm wide groove with a length of more than 100 µm. Apart from the orientation of the plane of incidence which is now perpendicular to the long axis of the surface defect, the illumination conditions remain the same as for the previous experiment. The scan size of the image shown in Figure 11 has been chosen such that both the short side and a part of the long side of the rectangular groove can be observed simultaneously. The very weak contrast ripples visible at the location of the long side may be explained by the interference between the incident SP (propagating from the right to the left) and the SP backreflected by the edge of the groove perpendicular to the plane of incidence; however most of the incident SP is clearly not affected by the surface defect and travels through the groove. Such a behavior is quite surprising when compared with the contrast of the PSTM images obtained in conical diffraction. As is shown in Figure 11, the optical response of the short side edge reveals a wellpronounced black spot which means that the contrast of the PSTM images obtained in conical diffraction is mainly due to the interaction of the incident SP with the edges of the microstructures parallel to the plane of incidence. From this experiment, we can conclude that the contrast of the PSTM images of the microstructures is significantly improved if their long axis is oriented parallel to the plane of incidence and their widths are small enough (in the range of a few incident wavelengths) to allow the optical response of each side of the groove to overlap. When propagating at the interface between a dielectric and a real metal with ohmic losses, the amplitude of the SP electromagnetic field is exponentially damped with the propagation distance.4 For example, the intensity of the SP field propagating along a gold/air interface is damped by an e2 factor for a propagation distance of about 40 µm at λ ) 800 nm. One can then anticipate that the interaction length of the incident SP with the surface defect plays a key role in the formation of the PSTM images. We show in Figure 12a a PSTM image recorded over a double groove illuminated in conical diffraction. The grooves have

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Figure 12. (a) PSTM image of a double groove (W ) 2.0 µm, distance between the grooves ) 4.0 µm) engraved in a 10 nm thick PMMA layer. The plane of incidence is parallel to the Y-axis, and the incident SP propagates from the top to the bottom of the image. (b) Cross-cut along the white dashed line shown in panel a. The vertical dashed line in panel b shows the beginning of the groove. The optical signal reaches a steady level after a propagation length of about 25 µm.

a width of 2.0 µm and are separated by a side-to-side distance of 4.0 µm. Unlike the PSTM images previously discussed, the image of Figure 12a shows the beginning of each groove and thus allows one to observe the change in the near-field contrast as the propagation length of the incident SP increases. A cross-cut of the optical image along the white dashed line as displayed in Figure 12b shows that a propagation length of approximately 25 µm is necessary to reach a steady level of the optical signal. Thus, the PSTM image of the microstructures is optimum provided that their dimension in the direction parallel to the plane of incidence is not downsized arbitrarily. According to the previous experiment, these dimensions should be kept at least in the range of the 1/e2 SP field damping distance. IV. Conclusion In conclusion, we have operated a near-field optical microscope to achieve the characterization of very thin microstructured polymer layers lying on a homogeneous metal thin film supporting a surface plasmon mode. By

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exploiting the field enhancement due to the excitation of the surface plasmon, we have shown that the PSTM images of the microstructures exhibit a contrast high enough to allow the use of this type of microscope for spatially resolved SP sensing purposes provided that the near-field imaging is performed in well-defined conditions. On the basis of a simultaneous recording of the nearfield optical signal and the tunneling current when approaching the PSTM probe from the sample, we have shown that the maximum PSTM signal is obtained not for the metal-coated tip in quasi-contact with the sample but for a tip-sample distance of about λ/4 if λ denotes the incident wavelength in a vacuum. Thus, to improve the visibility of the microstructures in the PSTM images, the scanning plane must be located a few tenths of nanometers away from the sample surface. Note that high-scan-rate imaging can be performed with such tip-sample distances without degrading the tip quality since the tip is not in contact with the surface. In addition, we have shown that at the surface plasmon resonance, both the near-field enhancement and the near-field intensity distribution around the microstructures depend dramatically upon the angle of incidence. Thus, if we assume an angle of incidence that is close to the SP resonance, a local change of the surface plasmon excitation conditions causes a significant change in contrast of the metal thin film surface PSTM image. Finally, we have shown that in order to optimize the near-field response of the microstructures, the distance of interaction between the surface defect and the incident SP must be long enough in the range of the 1/e2 damping distance of the SP intensity at the incident frequency. Note that this implies that miniaturized SP sensors must have a minimum size at least in the direction parallel to the plane of incidence. On the basis of these results, we conclude that the miniaturization of SPR-based sensors can only be achieved by downsizing their (lateral) dimensions perpendicular to the propagation direction of the surface plasmon. In this context, two types of sensors can be considered. First, the gold thin film can be microstructured in order to obtain metal strips with micrometric widths on dielectric substrates. In this case, the sensor consists of metal strips that can be selectively functionalized. The second approach consists of microstructuring directly the organic layer at the surface of an extended gold thin film. This could be achieved, for example, by filling with the biomolecules of interest the grooves created by lithography in an ultrathin polymer mask (a PMMA mask in this work). In summary, we have demonstrate that the near-field spatially resolved surface plasmon imaging characterization is well suited for investigating miniaturized optical biosensors. However, to achieve this goal, the near-field imaging must be performed in a liquid environment to ensure the stability of biological materials attached to the gold film. Experiments dealing with this topic are in progress and will be reported elsewhere. LA048242Q