1. Introduction

Synchrotron radiation-based Fourier-transform infrared (FTIR) spectroscopy is a well established analytical technique at many electron storage rings worldwide since they provide stable and highly brilliant infrared (IR) radiation compared to conventional thermal sources [1,2]. Synchrotron radiation (SR) is almost ideal for spectroscopy in a wide spectral regime, which is essentially due to its continuous broadband energy spectrum [3,4]. The combination of conventional IR microscopy with FTIR spectroscopy enables chemical mapping of samples with a spatial resolution limited by diffraction. Near-field based approaches, such as scattering type scanning near-field optical microscopy (s-SNOM) overcome this limitation [5–7]. Such s-SNOM systems are based on atomic force microscopy (AFM), where additionally a focused laser beam is used to illuminate the probe. The metallic probe in turn acts under optimized conditions as an antenna which confines the incident electric field around the tip-apex thus providing a nanoscale light source for high-resolution imaging [8]. This permits the simultaneous acquisition of detailed topographic and optical information regarding the local sample properties with a spatial resolution below 50 nm [9–13]. However, for obtaining spectral information in the IR regime, several time-consuming measurements at different wavelengths of a tunable laser source are required. A set of such near-field images provides wavelength dependent information on the local chemical composition or conductivity. In order to enable broadband FTIR spectroscopy with a nanoscale spatial resolution various solutions were presented recently, such as thermal sources [14] or different laser-based coherent infrared continuum sources [15–18]. However, the low power density of these sources in certain energy ranges still poses a limit to use them for broadband nano-FTIR spectroscopy. Another high-resolution technique utilizes an AFM tip as a photothermal sensor for recording IR spectroscopic information about the sample [19]. This method enables also the study of cells under ambient conditions [20].

In this work we report on the successful coupling of a s-SNOM system with a broadband IR SR source. Compared to conventional thermal sources, e.g. a globar, the electron storage ring Metrology Light Source (MLS) provides not only continuous IR radiation in the range from the near- to the far-IR but also sufficient radiant power, as required for IR spectroscopy. In the following we demonstrate the feasibility of near-field imaging and nano-FTIR spectroscopy using SR over a broad energy range on Au-coated silicon-carbide (SiC) samples.

2. Experimental part

For the experiments described in the following we use a commercially available s-SNOM system (NeaSNOM from Neaspec GmbH, Germany) consisting of an AFM operating in tapping mode and an asymmetric Michelson interferometer. The Si-based AFM tips with a resonance frequency between 76 kHz and 263 kHz are coated with a 20 nm thin Au layer yielding a tip diameter below 50 nm.

The electron storage ring MLS is a source especially suited for generating brilliant IR and THz radiation [4]. The MLS is operated with 80 electron bunches providing a pulse repetition rate of 500 MHz with a typical bunchlength of about 25 ps. The IR radiation is coupled out of the storage ring by several planar and cylindrical mirrors yielding at the end of the beamline a collimated horizontally polarized rectangular shaped beam with dimensions of approximately 25 mm (hor.) and 10 mm (vert.). A diamond window at the end of the beamline separates the ultra-high vacuum within the storage ring [4] from ambient conditions under which the s-SNOM system is operated. The integrated beam power available for experiments within the wavelength range from 1 µm to 20 µm is about 1.95 mW for a ring current of 100 mA. In order to match the aperture of the instrumental optics the size of the beam is reduced by using two parabolic mirrors. Before focusing the IR radiation by another parabolic mirror (Fig. 1) onto the metallic tip the polarization of the incident IR radiation is converted from horizontal to vertical by using a periscope like mirror arrangement for exploiting the electric field enhancement at the illuminated tip apex. The backscattered light is analyzed with a Michelson interferometer. Sample and near-field probe are located within one arm of the interferometer while the reference light is reflected at a planar mirror in the second arm. A schematic image of the experimental setup is shown in Fig. 1. Such a configuration is denoted as an asymmetric Michelson interferometer. Due to the relatively large size of the focused SR spot an illumination of the tip shaft and the sample cannot be completely avoided, which leads usually to a strong background signal. In order to separate the weak near-field from the intensive background signal contributions the interference signal is demodulated at higher harmonics nΩ (with n > 1) of the tip’s oscillation frequency Ω. The translation of the mirror in the reference arm yields interferograms of the demodulated signal. With subsequent Fourier transformation the local spectral response from the sample can be obtained as nano-FTIR spectra. A more detailed description can be found in reference 18. The signal is detected by a liquid nitrogen cooled mercury-cadmium-telluride (MCT) detector (Teledyne Judson Technologies, United States) with a sensitivity range from about 2 µm to 12 µm.

 

Fig. 1 Experimental setup of the s-SNOM system using broadband synchrotron radiation in the IR regime from the electron storage ring MLS.

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3. Results and discussion

For demonstrating the efficient background suppression by the detection of higher harmonics nΩ of the cantilevers oscillation frequency the tip is gradually retracted from the sample surface while recording the intensity decay of the backscattered IR radiation simultaneously. This ensures that the signal detected at higher harmonics originates from near-field contributions. The reference mirror is set to the position where the simultaneous interference of the different spectral components leads to a maximum of the detected signal (white light position). The approach curves recorded on an Au surface using broadband synchrotron radiation in the IR regime at a storage ring current of about 61 mA and a tapping mode amplitude of ~60 nm for the 1st, 2nd, 3rd and 4th harmonics are presented in Fig. 2. The typical exponential decay of the near-field signal intensity can be observed for S_2,Au(ω),S_3,Au(ω), and S_4,Au(ω), where the intensity decreases to the background noise level within a distance between tip and sample of approximately 50 nm. This ensures that the signal detected at higher harmonics (n > 1) originates predominantly from near-field contributions.

 

Fig. 2 Measured optical signal amplitude as a function of the distance between tip and Au-coated sample surface for the harmonic demodulation orders n = 1, 2, 3, 4. For a better comparison the signal amplitudes are normalized to unity. During the measurement the tip oscillates with an amplitude of approximately 60 nm while the current in the storage ring is about 61 mA. (Tapping set-point upon surface contact is 90%.)

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In order to validate near-field imaging by using a broadband SR source in the IR regime, scans are performed on the surface of a 6H-SiC sample coated with a 40 nm thin Au layer. For these scans the ring current is about 60 mA and the position of the reference mirror is selected such that the interference of the spectral components leads to a maximum S₂-signal. In Fig. 3 the topography image and the simultaneously acquired S₂, S₃ and S₄ near-field intensity maps (S₁ not shown) from a 1 µm x 1 µm large surface area are presented. Despite the relatively low intensity of the SR it is still possible to obtain an optical image from the sample surface within a reasonable time of typically 20 minutes by detecting the demodulated signals up to the 4th harmonic (S₄). The up to 27 nm high surface patterns on the Au surface are indicated by the bright color in the topography image (Fig. 3(a)). These patterns appear in the corresponding near-field images S₂, S₃ and S₄ (Figs. 3(b), (c) and (d)) as optically dark features.

 

Fig. 3 Topography and corresponding intensity maps (S₂, S₃- and S₄-signal) obtained from a SiC sample coated with a 40 nm thick Au layer by using broadband SR. The dark regions in the intensity maps (b), (c) and (d) indicate surface contaminations on the Au layer and correspond to the bright spots in the topography image (a). The maximum height of these patterns is about 30 nm. The acquisition time for the images is about 20 minutes.

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This strong intensity contrast in the optical images of the 2^nd, 3^rd and 4^th harmonics presented in Figs. 3(b) and (c) is caused by the different optical properties of these adsorbates in the IR regime compared to the Au layer. The results demonstrate that by using a broadband radiation source, such as SR, IR s-SNOM imaging has the same high sensitivity and material discrimination capabilities as already reported for s-SNOM experiments performed with monochromatic IR laser radiation [9].

In Fig. 4 the topography and near-field intensity maps from a scan performed around the edge of the about 40 nm thin structured Au layer on SiC substrate are shown. The average ring current during the scanning process is again around 60 mA. The SiC specific phonon resonance in near-field investigations at about ω_SiC = 927 cm⁻¹ leads to an increased absorption due to the excitation of longitudinal optical phonons [15, 21, 22]. However, due to the relatively sharp resonance [15] the SiC surface appears optically dark compared to the Au-coated surface in the acquired near-field image (Fig. 4(b)) when using broadband SR. For Au no such resonances occur in the IR regime. The higher optical signal amplitude (S₂) for Au compared to the SiC surface is indicated by the bright area in the right part of Fig. 4(b). The dashed white lines in Figs. 4(a) and (b) represent the position of the line scans shown in Figs. 4(c) and (d). Despite the use of broadband SR for near-field imaging the spatial resolution remains comparable as also observed for monochromatic radiationsources. This confirms that the spatial resolution is not determined by the used excitation wavelength, but rather by the diameter of the tip apex. In the line scan performed across the sample surface the intensity of the optical near-field signal (S₂) changes at the position of the SiC/Au edge within a distance < 100 nm (Fig. 4(d)).The most significant advantage of using broadband IR radiation, as provided in the present case by an electron storage ring, is the capability to perform nano-FTIR spectroscopy over a broad energy range. For this purpose the reference mirror (Fig. 1) in the second arm of the Michelson interferometer can be moved over a maximum distance of 800 µm while recording simultaneously the detector intensity as a function of the optical path. Due to the broadband IR radiation the accessible frequency range for spectroscopic investigations is mainly limited by the sensitivity range of the detector used for the measurements, ranging in the present case from about 2 µm to 12 µm (5000 cm⁻¹ to 833 cm⁻¹). The interferograms (I_2,SiC(ω) and I_2,Au(ω)) recorded from the SiC and Au surface (approximate measurement positions indicated by white crosses in Fig. 4(a) and (b)) are presented in Figs. 5(a) and (b), respectively. The two broadband near-field IR spectra calculated by Fourier transformation of the corresponding interferograms are compared in Fig. 5(c). Due to the constant dielectric function of Au in the mid-IR regime [15], S_2,Au(ω) represents mainly the spectral characteristics of the experimentalsetup, which is mostly determined by the transmission of the ZnSe beamsplitter, absorption by the atmospheric environment, and the responsivity of the detector. In the near-field FTIR spectrum acquired from the SiC surface S_2,SiC(ω) (Fig. 5(c) red curve) the characteristic phonon resonance band at about ω_SiC = 927 cm⁻¹ appears [22]. The strong near-field phonon resonance at this frequency confirms the near-field signal detection from SiC [15]. At higher wavenumbers the near-field IR spectra recorded on SiC and Au (Fig. 5(c)) show a general accordance, thus also confirming, that the spectral features in this regime can be attributed mainly to the experimental setup.

 

Fig. 4 Topography (a) and corresponding near-field image (S₂-signal) (b)) obtained from a patterned Au layer on SiC substrate by using broadband synchrotron radiation. A line scan performed across the edge (position indicated by dashed white lines in topography (a) and corresponding near-field images (b)) indicates an optical signal increase (d) by the factor of 4 within a distance < 100 nm. (The acquisition time is about 20 minutes.)

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Fig. 5 Interferograms recorded on the Au (a) and SiC (b) surfaces and comparison of the near-field IR s-SNOM spectra (c) obtained by Fourier transformation of the corresponding interferograms (SiC (red curve) and Au (black curve)). The typical phonon resonance at about 927 cm⁻¹ verifies near-field signal detection. The presented spectral range is limited only by the sensitivity range of the detector (For better visibility only the center part of the interferograms is shown in (a) and (b)).

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4. Conclusion

In summary, we demonstrated near-field imaging in the IR regime by utilizing a broadband, low-intensity SR source. The use of such broadband radiation has the great advantage that the acquired FTIR spectra provide local spectroscopic information over a much wider energy range than is accessible by using conventional sources, especially IR lasers. The high spatial resolution of s-SNOM in combination with nano-FTIR spectroscopy can be applied for e.g. the characterization and mapping of chemical and structural properties of various nanosystems.