In optical imaging, a well-known limit to the resolution is due to the diffraction limit. The maximum spatial resolution achievable by a classical optical instrument such a microscope lambda/2, where lambda is the wavelength used for the observation. The Fourier amplitudes of the electromagnetic field corresponding to high spatial frequencies, which are necessary to resolve details much smaller than lambda, are contained in electromagnetic waves confined near the sample surface, in a region called the near-field, as they decay exponentially in the direction perpendicular to the surface. These evanescent waves can in principle not be detected far from the sample. This constitutes a severe limitation to the resolution in classical optical imaging, especially in the infrared where the wavelength is typically of the order of 10 µm.

Scanning near-field optical microscopy (SNOM) allows one to detect the evanescent waves in the near-field in order to produce optical images at nanometer resolution. This instrument is based on an atomic force microscope (AFM), which uses piezoelectric translations in order to image the topography of the scanned area of a sample by raster scanning a nanosized tip on it. The principle of a SNOM with a scattering tip (s-SNOM) is illustrated in figure 1, for the case of near-field imaging the surface of semi-conductor laser devices, as discussed below. While in a basic AFM experiment, only the feedback signal which controls the Z position of the tip is recorded as a function of its XY position to image the topography of the sample, in a s-SNOM experiment, the tip also serves to scatter the electromagnetic near-field. As its apex constitutes a subwavelength scatterer, the intensity of the scattered field measured as a function of the XY tip position by means of a single channel photodetector combined with collection optics (parabolic mirrors, lenses, …) produces a super-resolved optical image of the scanned area of the sample surface, with the same resolution as that of the simultaneously recorded AFM topography. Resolutions as good as a few tens of nanometers can be achieved using s-SNOM in optical images, even at infrared wavelengths.

The s-SNOM developed by the group of Dr. Yannick De Wilde at Institut Langevin, ESPCI ParisTech – CNRS in Paris, is based on piezosystem jena nanopositioning systems (figure 1). The sample is mounted on a XY piezoelectric translation PXY 100SG, which achieves displacements with a resolution of the positioning better than 2 nm over a total range of 80 µm x 80 µm in closed loop. It uses an electrochemically etched tungsten tip which is glued on a quartz tuning fork (QTF). During the measurements, the latter is excited near its mechanical resonance by means of a small piezoelectric plate. The resulting oscillations produce intermittent contacts of the tip with the sample surface at a frequency of approximately 30 KHz. The amplitude of the QTF electrical signal is used in a feedback loop which controls the tip height by means of a N-series stack actuator with a full range of 16 µm. The scanning head is placed under a high numerical aperture reflective objective aimed at collecting the infrared light scattered by the tip apex, which is then focused on a nitrogen cooled mercury cadmium telluride (MCT) detector. The output signal from the detector is measured with a lock-in amplifier using the tip oscillation frequency as a reference, in order to extract the near-field signal due to tip scattering from parasitic background contributions.

This s-SNOM is capable to image the near-field at infrared wavelengths on semiconductor devices in operation, or even to image the sole infrared thermal emission from a heated sample. As a first illustrative example, figure 2 shows images of the surface topography and the near-field which have been simultaneously recorded at the surface of a mid-infrared quantum cascade laser emitting at lambda=7.5 µm. The latter was developed by the group of Dr. Raffaele Colombelli at the Institut d’Electronique Fondamentale, Univ. Paris Sud and CNRS, in Orsay. The region of the device which has been scanned is the upper metal electrode structured in a 1.2 µm-pitch grating, whose role is to generate surface waves called surface plasmons when the laser is electrically pumped [1,2]. While the topography image reveals the grating structure, the infrared near-field image shows the spatial distribution of the electromagnetic mode on the laser cavity. These type of s-SNOM observations performed at Institut Langevin, have played a key role in the development of novel devices by the team at the Institut d’Electronique Fondamentale. Among the recent demonstrations, we can cite the generation of hybrid surface plasmons by electrical pumping [1], the coupling of surface plasmons into passive waveguides [2], and the enhancement of their confinement properties [3].

Another example of application of the s-SNOM set-up at Institut Langevin is its use for probing the infrared thermal emission in the near-field [4]. This novel mode of operation of a s-SNOM, called thermal radiation scanning tunnelling microscopy (TRSTM), can be considered as the near-field equivalent of infrared night-vision camera. The set-up allows one to measure the infrared thermal emission at the surface of a sample which is mounted on a hot plate to raise its temperature up to 200 °C. New physical phenomena such as the standing mode pattern formed by thermally excited surface plasmons, have been demonstrated in TRSTM mode. Remarkably, the instrument allows one to probe the electromagnetic local density of states, which is a quantity of fundamental interest [4]. Figure 3a, gives an example illustrating the spatial resolution of the instrument for imaging the near-field thermal emission of two materials at lambda=10 µm.

Besides near-field imaging, the group has recently combined the scanning probe with a Fourier transform infrared spectrometer, in order to develop a new type of FTIR spectroscopy capable to measure an infrared spectrum with a spatial resolution of approximately 100 nm [5]. As an example, figure 3b shows the near-field spectrum measured at the surface of a silicon carbide sample by Florian Peragut who is PhD student in the Institut Langevin group. Now that near-field spectroscopy has been demonstrated, his goal is to tweak the instrument to achieve hyperspectral imaging in the infrared. The principle will then be to acquire spectra not only at one fixed location, but at every points of an image in order to map the spectral properties of the studied sample with a sub-wavelength spatial resolution [6].

ACKNOWLEDGEMENTS:

This work was supported by LABEX WIFI (Laboratory of Excellence ANR-10-LABX-24) within the French Program “Investments for the Future” under reference ANR-10- IDEX-0001-02 PSL*.

This work has been supported by the Region Ile-de-France in the framework of C’Nano IdF, the nanoscience competence center of Paris Region.

The authors are grateful to their fruitful collaborators at the Institut d’Electronique Fondamentale (R. Colombelli and A. Bousseksou), Institut P Prime (K. Joulain), Centre de Thermique de Lyon (P.-O. Chapuis), and Laboratoire Charles Fabry, Institut d’Optique (J.-J. Greffet).

Florian Peragut and Yannick De Wilde
Institut Langevin, ESPCI ParisTech, CNRS, 75238 Paris Cedex 05, France
Email: yannick.dewilde@espci.fr

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