arXiv subjects

Richarda Niemann

Publications and source records attributed to Richarda Niemann.

8 recordsLinked to original sources

arXiv2026

In-plane vector-field imaging of propagating surface phonon polaritons

Polariton interferometry through optical near-field microscopy has become a powerful tool in nanophotonics, enabling direct spatial access to the propagation characteristics of strongly confined, evanescent polariton modes. Scattering-type near-field optical microscopy has matured as the prime tool for such studies, yet mostly the out-of-plane components of the optical near fields are probed, owing to the elongated geometry of the nanotip. Here, we demonstrate a complementary far-field nonlinear microscopy approach which allows to selectively probe in-plane polariton field components. Accessing the full vector field is interesting when studying complex mode patterns such as hyperbolic polaritons or skyrmions, where the in-plane field components are typically only inferred from the out-of-plane component but not measured directly. To this end, we use nonlinear infrared-visible wide-field sum-frequency generation microscopy, where the short visible wavelength of the nonlinear signal provides the high spatial resolution to access evanescent modes in the infrared. The symmetry selection rules of the nonlinear process further enable polarization-selective imaging of both in-plane polariton field components through spatial interferometry. The concept is demonstrated experimentally using surface phonon polaritons at the AlN-air interface launched by a gold antenna. A simple, semi-analytical model reproduces the peculiar propagation patterns. Hyperspectral imaging with a tunable narrowband laser further gives access to the polariton dispersion. The wide-field methodology holds high promise for in-depth and high-throughput studies of infrared nanophotonic structures.

physics.optics↗

arXiv2026

Uncovering the properties of homo-epitaxial GaN devices through cross-sectional infrared nanoscopy

Validating material performance in electrical devices is crucial to product development. For Gallium Nitride (GaN) devices, evaluating material factors such as defects, dopant concentration, and overall production quality is essential to ensure their performance in advanced electronic and optoelectronic applications. This work demonstrates that scattering-type scanning near-field optical microscopy (s-SNOM) can meet the demanding performance requirements for characterizing homoepitaxial GaN devices. Specifically, we show that combining s-SNOM results in the mid-IR and terahertz (THz) spectral ranges can disentangle carrier and lattice changes in a GaN p-i-n diode, which is not possible using one spectral range alone. We observe strong, resonant near-field signals near the LO phonon mode of GaN that correlate well with point-dipole models. This data shows great sensitivity to the local carrier density, with changes on the order of 1018 cm-3 easily resolved experimentally. Further, we demonstrate high sensitivity to sub-surface defects, which remain a significant challenge for other non-destructive techniques. To validate the power of s-SNOM imaging, our results are compared to traditional metrologies, including micro-Raman mapping and Kelvin Probe Force Microscopy (KPFM). Our results show that s-SNOM shows superior resolution and sensitivity to perturbations, highlighting the power of this technique in semiconductor device characterization.

cond-mat.mtrl-sci↗

arXiv2026

Tailoring phonon-driven responses in α-MoO3 through isotopic enrichment

The implementation of polaritonic materials into nanoscale devices requires selective tuning of parameters to realize desired spectral or thermal responses. One robust material is α-MoO3, which as an orthorhombic crystal boasts three distinct phonon dispersions, providing three polaritonic dispersions of hyperbolic phonon polaritons (HPhPs) across the mid-infrared (MIR). Here, the tunability of both optical and thermal responses in isotopically enriched α-MoO3 (98MoO3, Mo18O3 and 98Mo18O3) are explored. A uniform ~5 % spectral redshift from 18O enrichment is observed in both Raman- and IR-active TO phonons. Both the in- and out-of-plane thermal conductivities for the isotopic variations are reported. Ab initio calculations both replicate experimental findings and analyze the select-mode three-phonon scattering contributions. The HPhPs from each isotopic variation are probed with s-SNOM and their Q- factors are reported. A Q-factor maxima increase of ~50 % along the [100] in the RB2 and ~100 % along the [001] in the RB3 are reported for HPhPs supported in 98Mo18O3. Observations in both real and Fourier space of higher-order HPhP modes propagating in single slabs of isotopically enriched α-MoO3 without the use of a subdiffractional surface scatterer are presented here. This work illustrates the tunability of α-MoO3 for thermal and nanophotonic applications.

cond-mat.mtrl-sci↗

arXiv2025

Spectral tuning of hyperbolic shear polaritons in monoclinic gallium oxide via isotopic substitution

Hyperbolic phonon polaritons - hybridized modes arising from the ultrastrong coupling of infrared light to strongly anisotropic lattice vibrations in uniaxial or biaxial polar crystals - enable to confine light to the nanoscale with low losses and high directionality. In even lower symmetry materials, such as monoclinic $β$-Ga$_2$O$_3$ (bGO), hyperbolic shear polaritons (HShPs) further enhance the directionality. Yet, HShPs are intrinsically supported only within narrow frequency ranges defined by the phonon frequencies of the host material. Here, we report spectral tuning of HShPs in bGO by isotopic substitution. Employing near-field optical microscopy to image HShPs in $^{18}$O bGO films homo-epitaxially grown on a $^{16}$O bGO substrate, we demonstrate a spectral redshift of $\sim~40~$cm$^{-1}$ for the $^{18}$O bGO, compared to $^{16}$O bGO. The technique allows for direct observation and a model-free estimation of the spectral shift driven by isotopic substitution without the need for knowledge of the dielectric tensor. Complementary far-field measurements and ab initio calculations - in good agreement with the near-field data - confirm the effectiveness of this estimation. This multifaceted study demonstrates a significant isotopic substitution induced spectral tuning of HShPs into a previously inaccessible frequency range, creating new avenues for technological applications of such highly directional polaritons.

physics.optics↗

arXiv2024

Surface Phonon Polariton Ellipsometry

Surface phonon polaritons (SPhPs) have become a key ingredient for infrared nanophotonics, owing to their long lifetimes and the large number of polar dielectric crystals supporting them. While these evanescent modes have been thoroughly characterized by near-field mapping or far-field intensity measurements over the last decade, far-field optical experiments also providing phase information are less common. In this paper, we study surface phonon polaritons at the gallium phosphide (GaP)-air interface in the momentum domain using the Otto-type prism coupling geometry. We combine this method with spectroscopic ellipsometry to obtain both amplitude and phase information of the reflected waves across the entire reststrahlen band of GaP. By adjusting the prism-sample air gap width, we systematically study the dependence of the ellipsometry parameters on the optical coupling efficiency. In particular, we show that the combined observation of both ellipsometry parameters - amplitude and phase - provides a powerful tool for the detection of SPhPs, even in the presence of high optical losses. Finally, we theoretically study how surface phonon polariton ellipsometry can reveal the emergence of vibrational strong coupling through changes in the topology of their complex plane trajectories, opening up a new perspective on light-matter coupling.

physics.optics↗

arXiv2024

Sum-Frequency Generation Spectro-Microscopy in the Reststrahlen Band of Wurtzite-type Aluminum Nitride

Nonlinear-optical microscopy and spectroscopy provide detailed spatial and spectroscopic contrast, specifically sensitive to structural symmetry and order. Ferroics, in particular, have been widely studied using second harmonic generation imaging, which provides detailed information on domain structures but typically lacks spectroscopic detail. In contrast, infrared-visible sum-frequency generation (SFG) spectroscopy reveals details of the atomic structure and bonding via vibrational resonances, but conventionally lacks spatial information. In this work, we combine the benefits of nonlinear optical imaging and SFG spectroscopy by employing SFG spectro-microscopy using an infrared free-electron laser. Specifically, we demonstrate the feasibility of SFG spectro-microscopy for spectroscopy using in-plane anisotropic wurtzite-type aluminum nitride as a model system. We find the experimental spectra to agree well with our theoretical calculations and we show the potential of our microscope to provide spatially resolved spectroscopic information in inhomogeneous systems such as ferroics and their domains in the near future.

physics.optics↗

arXiv2023

Spectroscopic and Interferometric Sum-Frequency Imaging of Strongly Coupled Phonon Polaritons in SiC Metasurfaces

Phonon polaritons enable waveguiding and localization of infrared light with extreme confinement and low losses. The spatial propagation and spectral resonances of such polaritons are usually probed with complementary techniques such as near-field optical microscopy and far-field reflection spectroscopy. Here, we introduce infrared-visible sum-frequency spectro-microscopy as a tool for spectroscopic imaging of phonon polaritons. The technique simultaneously provides sub-wavelength spatial resolution and highly-resolved spectral resonance information. This is implemented by resonantly exciting polaritons using a tunable infrared laser and wide-field microscopic detection of the upconverted light. We employ this technique to image hybridization and strong coupling of localized and propagating surface phonon polaritons in metasurfaces of SiC micropillars. Spectro-microscopy allows us to measure the polariton dispersion simultaneously in momentum space by angle-dependent resonance imaging, and in real space by polariton interferometry. Notably, we directly visualize how strong coupling affects the spatial localization of polaritons, inaccessible with conventional spectroscopic techniques. We further observe the formation of edge states at excitation frequencies where strong coupling prevents polariton propagation into the metasurface. Our approach is applicable to the wide range of polaritonic materials with broken inversion symmetry and can be used as a fast and non-perturbative tool to image polariton hybridization and propagation.

physics.optics↗

arXiv2021

Infrared super-resolution wide-field microscopy using sum-frequency generation

Super-resolution microscopy in the visible is an established powerful tool in several disciplines. In the infrared (IR) spectral range, however, no comparable schemes have been demonstrated to date. In this work, we experimentally demonstrate super-resolution microscopy in the IR range ($λ_{IR}\approx 10-12\,μ$m) using IR-visible sum-frequency generation. We operate our microscope in a wide-field scheme and image localized surface phonon polaritons in 4H-SiC nanostructures as a proof-of-concept. With this technique, we demonstrate an enhanced spatial resolution of $\simλ_{IR}/9$, enabling to resolve the polariton resonances in individual sub-diffractional nanostructures with sub-wavelength spacing. Furthermore we show, that this resolution allows to differentiate between spatial patterns associated with different polariton modes within individual nanostructures.

physics.optics↗