The authors demonstrate ultrabroadband time-resolved THz spectroscopy on a single InAs nanowire with 10 nm spatial resolution and sub-100 fs time resolution. Phase-locked ultrashort pulses in the rich terahertz spectral range1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18 have provided key insights into phenomena as diverse as quantum confinement7, first-order phase transitions8,12, high-temperature superconductivity11 and carrier transport in nanomaterials1,6,13,14,15. Ultrabroadband electro-optic sampling of few-cycle field transients1 can even reveal novel dynamics that occur faster than a single oscillation cycle of light4,8,10. However, conventional terahertz spectroscopy is intrinsically restricted to ensemble measurements by the diffraction limit. As a result, it measures dielectric functions averaged over the size, structure, orientation and density of nanoparticles, nanocrystals or nanodomains. Here, we extend ultrabroadband time-resolved terahertz spectroscopy to the sub-nanoparticle scale (10 nm) by combining sub-cycle, field-resolved detection (10 fs) with scattering-type near-field scanning optical microscopy (s-NSOM)16,17,18,19,20,21,22,23,24,25,26. We trace the time-dependent dielectric function at the surface of a single photoexcited InAs nanowire in all three spatial dimensions and reveal the ultrafast (<50 fs) formation of a local carrier depletion layer.

Ultrafast multi-terahertz nano-spectroscopy with sub-cycle temporal resolution

Infrared and optical spectroscopy represents one of the most informative methods in advanced materials research. As an important branch of modern optical techniques that has blossomed in the past decade, scattering-type scanning near-field optical microscopy (s-SNOM) promises deterministic characterization of optical properties over a broad spectral range at the nanoscale. It allows ultrabroadband optical (0.5-3000 µm) nanoimaging, and nanospectroscopy with fine spatial (<10 nm), spectral (<1 cm-1 ), and temporal (<10 fs) resolution. The history of s-SNOM is briefly introduced and recent advances which broaden the horizons of this technique in novel material research are summarized. In particular, this includes the pioneering efforts to study the nanoscale electrodynamic properties of plasmonic metamaterials, strongly correlated quantum materials, and polaritonic systems at room or cryogenic temperatures. Technical details, theoretical modeling, and new experimental methods are also discussed extensively, aiming to identify clear technology trends and unsolved challenges in this exciting field of research.

Modern Scattering-Type Scanning Near-Field Optical Microscopy for Advanced Material Research

This thesis has introduced a novel microscope which combines the outstanding properties of ultrafast multi-terahertz spectroscopy and scattering-type scanning near-field optical microscopy to observe the femtosecond dynamics of low-energy elementary excitations, e.g. phonons, plasmons and excitons, at the surface of solid-state systems. Ultrabroadband phase-locked mid-infrared light pulses are focused onto a nanometer-sized metal tip of the atomic force microscope. When approached to a sample surface, the tip acts as a local optical probe, confining the light pulses at its apex. Detection of the scattered radiation reveals information about the optical properties of the sample with a spatial resolution of 10nm, ultimately limited by the tip radius of curvature. Electro-optic sampling, one of the most notable techniques of terahertz spectroscopy, is employed to detect the scattered oscillating electric near field directly in the time domain with a temporal resolution of 10fs. Together with pump-probe experiments this technique enables the observation of the dynamics of low-energy elementary excitations on sub-cycle timescales, faster than a single oscillation cycle of the multi-terahertz probe pulses. The combination of ultrafast multi-terahertz spectroscopy and near-field microscopy culminates in a unique microscope that achieves an unprecedented combined temporal (10fs) and spatial (10nm) resolution in the mid-infrared wavelength region.%%%%In dieser Dissertation wird ein neuartiges Mikroskop beschrieben, welches die herausragenden Eigenschaften der ultraschnellen multi-Terahertz Spektroskopie und der Nahfeldmikroskopie verbindet. Damit ist es zum ersten Mal moglich die Femtosekunden-Dynamik von niederenergetischen Elementaranregungen, wie zum Beispiel Phononen, Plasmonen oder Exzitonen, direkt an der Oberflache von Festkorpern zu untersuchen. Ultrabreitbandige, phasen-stabile, mittelinfrarote Lichtimpulse werden dazu auf die Spitze eines Rasterkraftmikroskops fokussiert. Angenahert an die Oberflache einer Probe, fungiert diese Spitze als Nanometer grose Sonde, die das Licht an ihrem Ende bundelt. Die Detektion des gestreuten Lichts ermoglicht es dann Informationen uber die lokalen optischen Eigenschaften der Probe direkt unterhalb der Spitze zu erhalten. Die raumliche Auflosung betragt hierbei 10 Nanometer und ist grundsatzlich nur durch den Radius der Spitze limitiert. In diesem Experiment wird das gestreute elektrische Nahfeld direkt mit Hilfe eines Elektro-optischen Detektors gemessen. Diese Methode gewahrleistet eine ultimative Zeitauflosung von 10fs und erlaubt in Kombination mit Anrege-Abtast Experimenten eine direkte Beobachtung der Dynamik von niederenergetischen Elementaranregungen auf einer Sub-Zyklen Zeitskala, schneller als ein einzelner Oszillationszyklus des multi-Terahertz Abtastimpulses. Die Kombination von ultraschneller multi-Terahertz Spektroskopie mit der Nahfeldmikroskopie gipfelt in einem einzigartigen Mikroskop mit einer bisher nicht dagewesenen kombinierten zeitlichen (10fs) und raumlichen…

Ultrafast multi-terahertz nano-spectroscopywith sub-cycle temporal resolution

Scattering-type scanning near-field optical microscopy (s-SNOM) and Fourier transform infrared nanospectroscopy (nano-FTIR) are emerging tools for nanoscale chemical material identification. Here, we push s-SNOM and nano-FTIR one important step further by enabling them to quantitatively measure local dielectric constants and infrared absorption. Our technique is based on an analytical model, which allows for a simple inversion of the near-field scattering problem. It yields the dielectric permittivity and absorption of samples with 2 orders of magnitude improved spatial resolution compared to far-field measurements and is applicable to a large class of samples including polymers and biological matter. We verify the capabilities by determining the local dielectric permittivity of a PMMA film from nano-FTIR measurements, which is in excellent agreement with far-field ellipsometric data. We further obtain local infrared absorption spectra with unprecedented accuracy in peak position and shape, which is the key to quantitative chemometrics on the nanometer scale.

/pdf/quantitative-measurement-of-local-infrared-absorption-and-10uu4xm35r.pdf

Quantitative Measurement of Local Infrared Absorption and Dielectric Function with Tip-Enhanced Near-Field Microscopy.

We report the development of infrared-resonant antenna probes for tip-enhanced optical microscopy. We employ focused-ion-beam machining to fabricate high-aspect ratio gold cones, which replace the standard tip of a commercial Si-based atomic force microscopy cantilever. Calculations show large field enhancements at the tip apex due to geometrical antenna resonances in the cones, which can be precisely tuned throughout a broad spectral range from visible to terahertz frequencies by adjusting the cone length. Spectroscopic analysis of these probes by electron energy loss spectroscopy, Fourier transform infrared spectroscopy, and Fourier transform infrared near-field spectroscopy corroborates their functionality as resonant antennas and verifies the broad tunability. By employing the novel probes in a scattering-type near-field microscope and imaging a single tobacco mosaic virus (TMV), we experimentally demonstrate high-performance mid-infrared nanoimaging of molecular absorption. Our probes offer excellent...

Resonant antenna probes for tip-enhanced infrared near-field microscopy.

Using scattering-type near-field infrared microscopy in combination with a free-electron laser, intersublevel transitions in buried single InAs quantum dots are investigated. The experiments are performed at room temperature on doped self-assembled quantum dots capped with a 70 nm GaAs layer. Clear near-field contrast of single dots is observed when the photon energy of the incident beam matches intersublevel transition energies, namely the p-d and s-d transition of conduction band electrons confined in the dots. The observed room-temperature line width of 5–8 meV of these resonances in the mid-infrared range is significantly below the inhomogeneously broadened spectral lines of quantum dot ensembles. The experiment highlights the strength of near-field microspectroscopy by demonstrating signals from bound-to-bound transitions of single electrons in a probe volume of the order of (100 nm)3.

Intersublevel Spectroscopy on Single InAs-Quantum Dots by Terahertz Near-Field Microscopy

We use a combination of a scattering-type near-field infrared microscope with a free-electron laser as an intense, tunable radiation source to spatially and spectrally resolve buried doped layers in silicon. To this end, boron implanted stripes in silicon are raster scanned at different wavelengths in the range from 10 to 14 µm. An analysis based on a simple Drude model for the dielectric function of the sample yields quantitatively correct values for the concentration of the activated carriers. In a control experiment at the fixed wavelength of 10.6 µm, interferometric near-field signals are recorded. The phase information gained in this experiment is fully consistent with the carrier concentration obtained in the spectrally resolved experiments.

Quantitative determination of the charge carrier concentration of ion implanted silicon by IR-near-field spectroscopy

Scattering scanning near-field optical microscopy (s-SNOM) has been established as an excellent tool to probe domains in ferroelectric crystals at room temperature. Here, we apply the s-SNOM possibilities to quantify low-temperature phase transitions in barium titanate single crystals by both temperature-dependent resonance spectroscopy and domain distribution imaging. The orthorhombic-to-tetragonal structural phase transition at 263 K manifests in a change of the spatial arrangement of ferroelectric domains as probed with a tunable free-electron laser. More intriguingly, the domain distribution unravels non-favored domain configurations upon sample recovery to room temperature as explainable by increased sample disorder. Ferroelectric domains and topographic influences are clearly deconvolved even at low temperatures, since complementing our s-SNOM nano-spectroscopy with piezoresponse force microscopy and topographic imaging using one and the same atomic force microscope and tip.

Near-field resonance shifts of ferroelectric barium titanate domains upon low-temperature phase transition

We present scattering-type scanning near-field optical micro-spectroscopy (s-SNOM) investigations successfully operated in the THz range with a wavelength independent spatial resolution of < 150 nm. As a variable and monochromatic radiation source we use the free-electron laser (FELBE) located at the Forschungszentrum Dresden-Rossendorf (FZD) tunable over the wavelength range from 4–250 μm.

http://ieeexplore.ieee.org/iel5/5604263/5612299/05613022.pdf

Scattering near-field microscopy in the THz region using a free-electron laser

This talk advertises scattering-type scanning near-field. [...]

https://www.mdpi.com/2504-3900/26/1/1/pdf

Hans-Georg von Ribbeck

Papers

Intersublevel Spectroscopy on Single InAs-Quantum Dots by Terahertz Near-Field Microscopy

Quantitative determination of the charge carrier concentration of ion implanted silicon by IR-near-field spectroscopy

Near-field resonance shifts of ferroelectric barium titanate domains upon low-temperature phase transition

Scattering near-field microscopy in the THz region using a free-electron laser

Near-Field THz Nanoscopy with Novel Accelerator-Based Photon Sources