The authors present a detection technique for scattering-type near-field optical microscopy capable of background interference elimination in the entire near-UV to far-IR spectral range. It simultaneously measures near-field optical signal amplitude and phase by interferometric detection of scattered light utilizing a phase-modulated reference wave. They compare its background suppression efficiency to other known methods and experimentally show that it provides a reliable near-field optical material contrast even in the case where both noninterferometric and homodyne interferometric detection methods fail.

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We introduce ultraresolving terahertz (THz) near-field microscopy based on THz scattering at atomic force microscope tips. Nanoscale resolution is achieved by THz field confinement at the very tip apex to within 30 nm, which is in good agreement with full electro-dynamic calculations. Imaging semiconductor transistors, we provide first evidence of 40 nm (lambda/3000) spatial resolution at 2.54 THz (wavelength lambda=118 microm) and demonstrate the simultaneous THz recognition of materials and mobile carriers in a single nanodevice. Fundamentally important, we find that the mobile carrier contrast can be directly related to near-field excitation of THz-plasmons in the doped semiconductor regions. This opens the door to quantitative studies of local carrier concentration and mobility at the nanometer scale. The THz near-field response is extraordinary sensitive, providing contrast from less than 100 mobile electrons in the probed volume. Future improvements could allow for THz characterization of even single electrons or biomolecules.

The graphene photodetector is illustrated in Fig. 1a. A monolayer graphene sheet was encapsulated between two h-BN layers 15 . The h-BN(13 nm)-graphene-h-BN (42 nm) heterostructure is placed on top of a pair of 15-nm-thick AuPd gates, which are laterally separated by a gap of 50 nm. Applying individual voltages to the gates allows for controlling independently the carrier concentrations n 1 and n 2 in the graphene sheet at the left and right sides of the gap.In Fig. 1a we also introduce the concept of THz photocurrent nanoscopy, and its application for GPs mapping. The setup is based on a s-SNOM (Neaspec), where the metal tip is illuminated with the THz beam of a gas laser (SIFIR-50 from Coherent, providing output power in the range of a few 10 mW). Owing to a lightning-rod effect, the incident field is concentrated at the tip apex yielding a THz nanofocus 16 . Once brought into close proximity of the sample, the near fields of the nanofocus induce a current in the graphene sheet, similar to IR photocurrent nanoscopy 14,17 . Recording the current as a function of the tip position yields nanoscale-resolved THz photocurrent images. For the current measurement, the graphene is contacted electrically in a lateral geometry (i.e. metal contacts were fabricated at both sides of the heterostructure, as shown in Fig. 1a). Analogously to s-SNOM 18 and scanning photocurrent nanoscopy 14, 17 , we isolate the near-field contribution to the total photocurrent, I PC , by (i) oscillating the tip vertically at frequency Ω and (ii) demodulating the detector signal at 2Ω. This 3 technical procedure is required because of the background photocurrent generated by the diffraction limited illumination spot. We achieved a spatial resolution of about 50 nm (supplementary information S1), which is an improvement of nearly 4 orders of magnitude compared to diffraction-limited THz imaging. Fig. 1b shows a photocurrent image of the photodetector, recorded at 2.52 THz (λ 0 = 118.8 µm). Choosing graphene charge carrier densities n 1 = 0.77 and n 2 = -0.77x10 12 cm -2 , we generate a sharp pn-junction in the graphene above the gap between the gates.We observe a strong near-field photocurrent, I PC , which is localized to an about 1 µm wide region centred above the gap (central part of Fig. 1b). It can be explained by a photo-thermoelectric effect: due to a variation of the local Seebeck coefficient S in graphene (generated by the carrier density gradient), a local temperature gradient (caused by the THz nanofocus at the tip apex) generates a net charge current 14,17 .Because the variation of the carrier concentration -and thus ΔS -is largest between the two gates, we expect a maximum in the photocurrent at this location. In Fig. 1b, however, we observe a slight decrease of the photocurrent between the gates. We explain it by the reduced near-field intensity when the tip is above the gap, owing to the weaker near-field coupling between the tip and the metal gates. To corroborate the photo-thermoelectric origin of the THz photocurrent, we carrie...

We demonstrate that mid-infrared surface phonon polariton propagation on a SiC crystal can be imaged by scattering-type near-field optical microscopy. From the infrared images, we measure the wave vector and the propagation length of locally excited surface phonon polaritons. Our method can be also applied to surface plasmon polaritons and allows to study surface polaritons in subwavelength-scale structures.

We report quantitative, noninvasive and nanoscale-resolved mapping of the free-carrier distribution in InP nanowires with doping modulation along the axial and radial directions, by employing infrared near-field nanoscopy. Owing to the technique's capability of subsurface probing, we provide direct experimental evidence that dopants in interior nanowire shells effectively contribute to the local free-carrier concentration. The high sensitivity of s-SNOM also allows us to directly visualize nanoscale variations in the free-carrier concentration of wires as thin as 20 nm, which we attribute to local growth defects. Our results open interesting avenues for studying local conductivity in complex nanowire heterostructures, which could be further enhanced by near-field infrared nanotomography.

We describe transmission-mode scattering-type near-field optical microscopy (s-SNOM) with interferometric detection. Using this technique, we map the near-field modes of infrared antennas in both amplitude and phase. The use of dielectric probing tips, higher-harmonic demodulation and a complex-valued subtraction of residual background yield accurate near-field images of the antenna modes. We map metallic nanorods, disks and triangles, designed for antenna resonance at mid-infrared frequencies, in good agreement with numerical calculations of the modal field distribution. Furthermore, we show that transmission mode s-SNOM can map the z-component of the antenna near fields. Our results establish a basis for future near-field characterization of complex antenna structures for molecular sensing and spectroscopy. KEYWORDS: nanoantennas, near-field microscopy, infrared spectroscopy, infrared antennas.Resonant metallic particles 1,2 and engineered micro-and nanostructures 3-9 , acting as effective nanoantennas in the optical and infrared range of the spectrum allow for efficient conversion of propagating light into nanoscale confined and strongly enhanced optical fields. Nanoantennas are therefore key elements in the development of highly sensitive optical (bio)sensors 9 and light detectors 10 , optical nano lithography 11,12 or nanoscale optical microscopy 5,13-16 .One of the most relevant fields where antennas have produced a striking impact is molecular spectroscopy. The capacity of nanoantennas to enhance the electromagnetic near field in their vicinity has turned these devices into adequate hosts to locate molecules and perform field-enhanced spectroscopy on them. Among the different spectroscopic options to identify and detect molecular fingerprints, vibrational spectroscopy has proven to be extremely useful for selective detection of molecular groups. Antenna-enhancing effects due to the generation of surface plasmons have been used extensively in the last decade at visible frequencies in Surface-Enhanced Raman Scattering spectroscopy (SERS) 17-19 and more recently, in Surface-Enhanced Infrared Spectroscopy (SEIRS) 20 where molecular vibrations are excited directly by infrared light [21][22][23] . Molecular fluorescence assisted by nanoantennas 16 has also taken advantage of the enhancing capacity of nanoantennas, with additional control on the 3 polarization and directionality of the fluorescence produced. Since the antenna-molecule interaction occurs in the near-field of the antenna, a detailed understanding of the antenna properties and experimental knowledge about the near-field distribution will be essential for optimization of the antenna performance.In addition to the magnitude of the local field enhancement, the near-field phase is an essential parameter in antenna-assisted molecular spectroscopy, as well as in a variety of coherent-control applications 24 . For example, the near-field phase plays a crucial role in antenna-assisted infrared spectroscopy as the interference between molecule and antenna n...

Knowledge about strain at the nanometre scale is essential for tailoring the mechanical and electronic properties of materials. Flaws, cracks and their local strain fields can be detrimental to the structural integrity of many solids. Conversely, the controlled straining of silicon can be used to improve the performance of electronic devices. Here, we demonstrate that infrared near-field microscopy allows direct, non-invasive mapping and a semiquantitative analysis of residual strain fields in polar semiconductor crystals with nanometre-scale resolution. Our experiments with silicon carbide crystals yield optical images of nanoindents showing strain features as small as 50 nm and the evolution of nanocracks. In addition, by imaging nanoindents in doped silicon, we provide experimental evidence for plasmon-assisted near-field imaging of free-carrier properties in nanoscale strain fields. Near-field infrared strain mapping provides possibilities for nanoscale material and device characterization, and could become a tool for nanoscale mapping of the local free-carrier mobility in strain-engineered semiconductors.