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Fourier-transform infrared (FTIR) spectroscopy is a widely used analytical tool for chemical identification of inorganic, organic and biomedical materials, as well as for exploring conduction phenomena. Because of the diffraction limit, however, conventional FTIR cannot be applied for nanoscale imaging. Here we demonstrate a novel FTIR system that allows for infrared-spectroscopic nanoimaging of dielectric properties (nano-FTIR). Based on superfocusing of thermal radiation with an infrared antenna, detection of the scattered light, and strong signal enhancement employing an asymmetric FTIR spectrometer, we improve the spatial resolution of conventional infrared spectroscopy by more than two orders of magnitude. By mapping a semiconductor device, we demonstrate spectroscopic identification of silicon oxides and quantification of the free-carrier concentration in doped Si regions with a spatial resolution better than 100  nm. We envisage nano-FTIR becoming a powerful tool for chemical identification of nanomaterials, as well as for quantitative and contact-free measurement of the local free-carrier concentration and mobility in doped nanostructures.

Light scattering at nanoparticles and molecules can be dramatically enhanced in the 'hot spots' of optical antennas, where the incident light is highly concentrated. Although this effect is widely applied in surface-enhanced optical sensing, spectroscopy and microscopy, the underlying electromagnetic mechanism of the signal enhancement is challenging to trace experimentally. Here we study elastically scattered light from an individual object located in the well-defined hot spot of single antennas, as a new approach to resolve the role of the antenna in the scattering process. We provide experimental evidence that the intensity elastically scattered off the object scales with the fourth power of the local field enhancement provided by the antenna, and that the underlying electromagnetic mechanism is identical to the one commonly accepted in surface-enhanced Raman scattering. We also measure the phase shift of the scattered light, which provides a novel and unambiguous fingerprint of surface-enhanced light scattering.

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We demonstrate that the local near-field vector and polarization state on planar antenna structures and in nanoscale antenna gaps can be determined by scattering-type near-field optical microscopy (s-SNOM). The near-field vector is reconstructed from the amplitude and phase images of the in-and out-of-plane near-field components obtained by polarization-resolved interferometric detection. Experiments with a mid-infrared inverse bowtie antenna yield a vectorial near-field distribution with unprecedented resolution of about 10 nm and in excellent agreement with numerical simulations. Furthermore, we provide first direct experimental evidence that the nanoscale confined and strongly enhanced fields at the antenna gap are linearly polarized. s-SNOM vector-field mapping paves the way to a full near-field characterization of nanophotonic structures in the broad spectral range between visible and terahertz frequencies, which is essential for future development and quality control of metamaterials, optical sensors, and waveguides.KEYWORDS Vector near-field mapping, near-field polarization state, phase-resolved near-field microscopy, infrared antennas, bowtie aperture, local field enhancement P lasmonic nanostructures and metamaterials enable the engineering of optical fields on the nanometer scale, which is the basis for the development of ultrasensitive molecular spectroscopy, high-resolution microscopy, and nanoscale optical circuits at frequencies ranging from the visible to the microwave spectral regime.1-4 A direct experimental access to the optical near-field distribution is critical for testing the design performance, exploring design alternatives, and verifying novel theoretical concepts. Because of their three-dimensional, complex-valued polarization state (vector field), however, nanoscale confined optical near fields are highly complex and thus are challenging experimental imaging techniques. 5,6 Generally, the local near field at a sample surface is described by a three-dimensional vector E ) (E x , E y , E z ), where each near-field component E j is characterized by both a field amplitude |E j | and a phase j .6,7 While strong field amplitudes at specific sample locations open new avenues for example in vibrational spectroscopy of single molecules, [8][9][10] it is the phase distribution that is essential for nanoscale coherent control applications.11-13 The phase difference δ ij ) j -i between individual components is thereby a fundamental quantity as it determines the polarization state of the vector near field.6 For example, a phase difference of δ ij ) 0°or 180°(for all i,j) defines linearly polarized local fields, while δ ij ) (90°and |E i | ) |E j | yields circularly polarized near fields. Engineering of the individual phases thus can be applied to control 14 the near field polarization state for novel nanophotonic applications in, e.g., quantum optics 15 or solid state physics. 16,17 To this end, it is of utmost importance to develop tools that allow for ultrahigh resolution mapping of indiv...

An unprecedented control of the spectral response of plasmonic nanoantennas has recently been achieved by designing structures that exhibit Fano resonances. This new insight is paving the way for a variety of applications, such as biochemical sensing and surface-enhanced Raman spectroscopy. Here we use scattering-type near-field optical microscopy to map the spatial field distribution of Fano modes in infrared plasmonic systems. We observe in real space the interference of narrow (dark) and broad (bright) plasmonic resonances, yielding intensity and phase toggling between different portions of the plasmonic metamolecules when either their geometric sizes or the illumination wavelength is varied.

Optical activity and circular dichroism are fascinating physical phenomena originating from the interaction of light with chiral molecules or other nano objects lacking mirror symmetries in three-dimensional (3D) space. While chiral optical properties are weak in most of naturally occurring materials, they can be engineered and significantly enhanced in synthetic optical media known as chiral metamaterials, where the spatial symmetry of their building blocks is broken on a nanoscale. Although originally discovered in 3D structures, circular dichroism can also emerge in a two-dimensional (2D) metasurface. The origin of the resulting circular dichroism is rather subtle, and is related to non-radiative (Ohmic) dissipation of the constituent metamolecules. Because such dissipation occurs on a nanoscale, this effect has never been experimentally probed and visualized. Using a suite of recently developed nanoscale-measurement tools, we establish that the circular dichroism in a nanostructured metasurface occurs due to handedness-dependent Ohmic heating.