Markus B. Raschke

Optical Near-Field Mapping of Plasmonic Nanoprisms

The optical local-field enhancement on nanometer length scales provides the basis for plasmonic metal nanostructures to serve as molecular sensors and as nanophotonic devices. However, particle morphology and the associated surface plasmon resonance alone do not uniquely reflect the important details of the local field distribution. Here, we use interferometric homodyne tip-scattering near-field microscopy for plasmonic near-field imaging of crystalline triangular silver nanoprisms. Strong spatial field variation on lengths scales as short as 20 nm are observed sensitively depending on structural details and environment. The poles of the dipole and quadrupole plasmon modes, as identified by phase-sensitive probing and calculations performed in the discrete dipole approximation (DDA), reflect the particle symmetry. Together with the observation that the largest enhancement is not necessarily found to be associated with the tips of the nanoprisms, our results provide critical information for the selection of particle geometries as building blocks for plasmonic device applications.

Light on the Tip of a Needle: Plasmonic Nanofocusing for Spectroscopy on the Nanoscale

The efficiency of plasmonic nanostructures as optical antennas to concentrate optical fields to the nanoscale has been limited by intrinsically short dephasing times and small absorption cross sections. We discuss a new optical antenna concept based on surface plasmon polariton (SPP) nanofocusing on conical noble metal tips to achieve efficient far- to near-field transformation of light from the micro- to the nanoscale. The spatial separation of the launching of propagating SPPs from their subsequent apex confinement with high energy concentration enables background-free near-field imaging, tip-enhanced Raman scattering, and nonlinear nanospectroscopy. The broad bandwidth and spectral tunability of the nanofocusing mechanism in combination with frequency domain pulse shaping uniquely allow for the spatial confinement of ultrashort laser pulses and few-femtosecond spatiotemporal optical control on the nanoscale. This technique not only extends powerful nonlinear and ultrafast spectroscopies to the nanoscale but can also generate fields of sufficient intensity for electron emission and higher harmonic generation.

Nano-optical investigations of the metal-insulator phase behavior of individual VO(2) microcrystals.

Despite the relatively simple stoichiometry and structure of VO(2), many questions regarding the nature of its famous metal-insulator transition (MIT) remain unresolved. This is in part due to the prevailing use of polycrystalline film samples and the limited spatial resolution in most studies, hindering access to and control of the complex phase behavior and its inevitable spatial inhomogeneities. Here, we investigate the MIT and associated nanodomain formation in individual VO(2) microcrystals subject to substrate stress. We employ symmetry-selective polarization Raman spectroscopy to identify crystals that are strain-stabilized in either the monoclinic M1 or M2 insulating phase at room-temperature. Raman measurements are further used to characterize the phase dependence on temperature, identifying the appearance of the M2 phase during the MIT. The associated formation and spatial evolution of rutile (R) metallic domains is studied with nanometer-scale spatial resolution using infrared scattering-scanning near-field optical microscopy (s-SNOM). We deduce that even for small crystals of VO(2), the MIT is influenced by the competition between the R, M1, and M2 crystal phases with their different lattice constants subjected to the external substrate-induced stress. The results have important implications for the interpretation of the investigations of conventional polycrystalline thin films where the mutual interaction of constituent crystallites may affect the nature of the MIT in VO(2).

Near-Field Localization in Plasmonic Superfocusing: A Nanoemitter on a Tip

Focusing light to subwavelength dimensions has been a long-standing desire in optics but has remained challenging, even with new strategies based on near-field effects, polaritons, and metamaterials. The adiabatic propagation of surface plasmon polaritons (SPP) on a conical taper as proposed theoretically has recently emerged as particularly promising to obtain a nanoconfined light source at the tip. Employing grating-coupling of SPPs onto gold tips, we demonstrate plasmonic nanofocusing into a localized excitation of approximately 20 nm in size and investigate its near- and far-field behavior. For cone angles of approximately 10-20 degrees , the breakdown of the adiabatic propagation conditions is found to be localized at or near the apex region with approximately 10 nm radius. Despite an asymmetric side-on SPP excitation, the apex far-field emission with axial polarization characteristics representing a radially symmetric SPP mode in the nanofocus confirms that the conical tip acts as an effective mode filter with only the fundamental radially symmetric TM mode (m = 0) propagating to the apex. We demonstrate the use of these tips as a source for nearly background-free scattering-type scanning near-field optical microscopy (s-SNOM).

Apertureless near-field optical microscopy: Tip–sample coupling in elastic light scattering

For linear light scattering in apertureless scanning near-field optical microscopy, we have studied the correlations between the tip radius of the probe, signal strength, spatial resolution, and sample material. Pronounced variations of the near-field distance dependence on tip shape and dielectric function of the sample are observed. For very sharp metal tips, the scattered near-field signal decays on a 5 nm length scale. Despite this highly localized tip–sample coupling, the contrast is found to depend sensitively on the vertical composition of the sample on a length scale given by the penetration depth of the incident light. The resulting implications on the use of the technique as an analytic probe method are discussed.

Ultrabroadband infrared nanospectroscopic imaging.

Significance Through direct measurement of intrinsic vibrational and electronic modes, infrared spectroscopy provides label-free, chemical characterization of molecules and solids. The long micrometer-sized wavelength of infrared light, however, has limited the application of this widely applied spectroscopic technique to ensemble studies, preventing nanoscale spatially resolved spectroscopy of heterogeneous materials. We overcome this limitation by combining scattering-scanning near-field optical microscopy with broadband infrared synchrotron radiation. Using this synchrotron infrared nanospectroscopy (SINS) technique, we achieve spectroscopic imaging over the entire midinfrared with nanometer spatial resolution and high sensitivity. With a spatial resolution 100–1,000 times better than conventional FTIR microscopy, SINS enables the investigation of nanoscale phenomena in soft matter, even under ambient and environmental conditions that are essentially inaccessible by other techniques. Characterizing and ultimately controlling the heterogeneity underlying biomolecular functions, quantum behavior of complex matter, photonic materials, or catalysis requires large-scale spectroscopic imaging with simultaneous specificity to structure, phase, and chemical composition at nanometer spatial resolution. However, as with any ultrahigh spatial resolution microscopy technique, the associated demand for an increase in both spatial and spectral bandwidth often leads to a decrease in desired sensitivity. We overcome this limitation in infrared vibrational scattering-scanning probe near-field optical microscopy using synchrotron midinfrared radiation. Tip-enhanced localized light–matter interaction is induced by low-noise, broadband, and spatially coherent synchrotron light of high spectral irradiance, and the near-field signal is sensitively detected using heterodyne interferometric amplification. We achieve sub-40-nm spatially resolved, molecular, and phonon vibrational spectroscopic imaging, with rapid spectral acquisition, spanning the full midinfrared (700–5,000 cm−1) with few cm−1 spectral resolution. We demonstrate the performance of synchrotron infrared nanospectroscopy on semiconductor, biomineral, and protein nanostructures, providing vibrational chemical imaging with subzeptomole sensitivity.

Near-field imaging of optical antenna modes in the mid-infrared

Optical antennas can enhance the coupling between free-space propagating light and the localized excitation of nanoscopic light emitters or receivers, thus forming the basis of many nanophotonic applications. Their functionality relies on an understanding of the relationship between the geometric parameters and the resulting near-field antenna modes. Using scattering-type scanning near-field optical microscopy (s-SNOM) with interferometric homodyne detection, we investigate the resonances of linear Au wire antennas designed for the mid-IR by probing specific vector near-field components. A simple effective wavelength scaling is observed for single wires with lambda(eff) = lambda /(2.0+/- 0.2), specific to the geometric and material parameters used. The disruption of the coherent current oscillation by introducing a gap gives rise to an effective multipolar mode for the two near-field coupled segments. Using antenna theory and numerical electrodynamics simulations two distinct coupling regimes are considered that scale with gap width or reactive near-field decay length, respectively. The results emphasize the distinct antenna behavior at optical frequencies compared to impedance matched radio frequency (RF) antennas and provide experimental confirmation of theoretically predicted scaling laws at optical frequencies.

Resonant-plasmon field enhancement from asymmetrically illuminated conical metallic-probe tips.

Optical-field enhancement and confinement for an asymmetrically illuminated nanoscopic Au tip suspended over a planar Au substrate is investigated both numerically and experimentally. The spatial field distribution of the tip-sample system was calculated using the full 3D finite-difference time-domain method. The calculation enables investigation of the effects of the substrate-tip placement, angle of incidence, and spectral response. The tip plasmon response leads to a significant (up to ~70 times) local field enhancement between the tip and substrate. The enhancement is found to be extremely sensitive to the tip-sample separation distance. Tip-enhanced Raman scattering experiments were performed and the numerical results provide a consistent description of the observed field localization and enhancement.

Femtosecond Nanofocusing with Full Optical Waveform Control

The simultaneous nanometer spatial confinement and femtosecond temporal control of an optical excitation has been a long-standing challenge in optics. Previous approaches using surface plasmon polariton (SPP) resonant nanostructures or SPP waveguides have suffered from, for example, mode mismatch, or possible dependence on the phase of the driving laser field to achieve spatial localization. Here we take advantage of the intrinsic phase- and amplitude-independent nanofocusing ability of a conical noble metal tip with weak wavelength dependence over a broad bandwidth to achieve a 10 nm spatially and few-femtosecond temporally confined excitation. In combination with spectral pulse shaping and feedback on the second-harmonic response of the tip apex, we demonstrate deterministic arbitrary optical waveform control. In addition, the high efficiency of the nanofocusing tip provided by the continuous micro- to nanoscale mode transformation opens the door for spectroscopy of elementary optical excitations in matter on their natural length and time scales and enables applications from ultrafast nano-opto-electronics to single molecule quantum coherent control.

Thermal Infrared Near-Field Spectroscopy

Despite the seminal contributions of Kirchhoff and Planck describing far-field thermal emission, fundamentally distinct spectral characteristics of the electromagnetic thermal near-field have been predicted. However, due to their evanescent nature their direct experimental characterization has remained elusive. Combining scattering scanning near-field optical microscopy with Fourier-transform spectroscopy using a heated atomic force microscope tip as both a local thermal source and scattering probe, we spectroscopically characterize the thermal near-field in the mid-infrared. We observe the spectrally distinct and orders of magnitude enhanced resonant spectral near-field energy density associated with vibrational, phonon, and phonon-polariton modes. We describe this behavior and the associated distinct on- and off-resonance nanoscale field localization with model calculations of the near-field electromagnetic local density of states. Our results provide a basis for intrinsic and extrinsic resonant manipulation of optical forces, control of nanoscale radiative heat transfer with optical antennas, and use of this new technique of thermal infrared near-field spectroscopy for broadband chemical nanospectroscopy.