arXiv subjects

Volker Deckert

Publications and source records attributed to Volker Deckert.

9 recordsLinked to original sources

arXiv2026

Optimized near-field optical response via adaptive tip illumination

The performance of tip-enhanced optical microscopy is often limited by inefficient coupling of the excitation field to the plasmonic tip apex, as well as by thermal drift and optical aberrations. Here, we demonstrate that adaptive wavefront shaping based on Zernike mode provides a practical approach to achieving robust near-field optimisation at the tip apex. Using a sequential feedback algorithm, initially using the near-field signal, we narrow the illumination point-spread function and suppress sidelobes. This demonstrates that Zernike-mode control can be used for both aberration correction and field engineering. In tip-enhanced Raman measurements of a Janus MoSSe monolayer, conventional near-field optimisation increases the signal intensity by around 1.4 fold. A second optimisation step based directly on the Raman-band intensity yields a further 5 to 15 fold enhancement, depending on the specific tips used. These results establish a systematic, optics-based strategy for optimising tip fields, providing a transferable framework for improving tip-enhanced and related near-field spectroscopies.

physics.optics↗

arXiv2026

Two-Photon-Induced Direct 3D Printing of Freeform High-Index Phase-Change Sb2S3 Nanostructures

Chalcogenides have recently emerged as an important class of phase-change materials (PCMs) for nanophotonics, owing to their very high refractive index (RI) and low optical loss in the visible to near-infrared range. They exhibit an ultralarge RI change (> 0.7) upon phase transition, which can be triggered by multiple stimuli such as electrical bias, laser illumination or thermal heating. These properties make them highly appealing materials for flat optics and metasurface applications. Current nanophotonic implementations of chalcogenide PCMs mostly rely on two-dimensional (2D) or quasi three-dimensional (3D) thin film patterning based on the coating of chalcogenide materials from a solid-state target. This limits fast prototyping of 3D freeform micro- and nanostructures, thus restricting geometric design freedom and device functionality. Here, we demonstrate a solution-phase direct printing of chalcogenide PCMs into functional structures. The method is based on dip in two photon-induced solidification (DITPS) of a specially synthesized antimony trisulfide (Sb2S3) precursor solution. Direct printing with DITPS is simple, maskless, fast and cost effective, enabling true freeform 3D printing of photonic devices with sub micron resolution. We show direct writing of Sb2S3 helices with different wire cross section profiles on gold and ITO substrates, as well as functional planar Fresnel zone plates (FZPs) and computer generated hologram metasurfaces (CGHMs) in a single printing step. This freeform DITPS approach thus enables rapid 3D prototyping of high index metasurfaces and opens a route to integrating high-index PCMs into existing photonic architectures and device platforms.

physics.optics↗

arXiv2025

Positive-tone Nanolithography of Antimony Trisulfide with Femtosecond Laser Wet-etching

Antimony trisulfide ($Sb_{2}S_{3}$), as an emerging material for integrated photonic devices, has attracted significant attention due to its high index, low loss, and phase-changing property in the optical regime. However, conventional lithography-based fabrication methods involve complex, time-consuming, multistep processes, rendering the photonic application of $Sb_{2}S_{3}$ challenging. Here, we demonstrate that positive-tone fabrication of $Sb_{2}S_{3}$ nanostructures using wet-etch femtosecond laser processing, a straightforward technique for the engraving of micro- and nanoscale structures, can address major fabrication challenges. The patterning mechanism and factors influencing resolution of $Sb_{2}S_{3}$ thin film structures deposited on quartz (transmissive) and gold (reflective) substrates are experimentally investigated and supported by theoretical modelling. Using this approach, the smallest linewidth fabricated is measured at 178 nm. Consequently, multiple test patterns are demonstrated showing versatile functionalities. Functional Fresnel Zone Plates (FZPs) with varying focal length are fabricated and characterized. This study provides a significantly simplified approach for realizing $Sb_{2}S_{3}$ based integrated photonic devices.

physics.optics↗

arXiv2025

Raman Signatures of Single Point Defects in Hexagonal Boron Nitride Quantum Emitters

Point defects in solid-state quantum systems are vital for enabling single-photon emission at specific wavelengths, making their precise identification essential for advancing applications in quantum technologies. However, pinpointing the microscopic origins of these defects remains a challenge. In this work, we propose Raman spectroscopy as a robust strategy for defect identification. Using density functional theory, we systematically characterize the Raman signatures of 100 defects in hexagonal boron nitride (hBN) spanning periodic groups III to VI, encompassing around 30,000 phonon modes. Our findings reveal that the local atomic environment plays a pivotal role in shaping the Raman lineshape, enabling the narrowing of potential defect candidates. Furthermore, we demonstrate that Raman spectroscopy can differentiate defects based on their spin and charge states as well as strain-induced variations, implying the versatility of this approach. Therefore, this study not only provides a comprehensive theoretical database of Raman spectra for hBN defects but also establishes a novel experiment framework for using tip-enhanced Raman spectroscopy to identify point defects. More broadly, our approach offers a universal method for defect identification in any quantum materials.

cond-mat.mtrl-sci↗

arXiv2024

Comparative study of quantum emitter fabrication in wide bandgap materials using localized electron irradiation

Quantum light sources are crucial foundational components for various quantum technology applications. With the rapid development of quantum technology, there has been a growing demand for materials with the capability of hosting quantum emitters. One such material platform uses fluorescent defects in hexagonal boron nitride (hBN) that can host deep sublevels within the bandgap. The localized electron irradiation has shown its effectiveness in generating deep sublevels to induce single emitters in hBN. The question is whether localized (electron beam) irradiation is a reliable tool for creating emitters in other wide bandgap materials and its uniqueness to hBN. Here, we investigate and compare the fabrication of quantum emitters in hBN and exfoliated muscovite mica flakes along with other 3D crystals, such as silicon carbide and gallium nitride, which are known to host quantum emitters. We used our primary fabrication technique of localized electron irradiation using a standard scanning electron microscope. To complement our experimental work, we employed density functional theory simulations to study the atomic structures of defects in mica. While our fabrication technique allows one to create hBN quantum emitters with a high yield and high single photon purity, it is unable to fabricate single emitters in the other solid-state crystals under investigation. This allows us to draw conclusions on the emitter fabrication mechanism in hBN, which could rely on activating pre-existing defects by charge state manipulation. Therefore, we provide an essential step toward the identification of hBN emitters and their formation process.

physics.app-ph↗

arXiv2024

Grayscale Electron Beam Lithography Direct Patterned Antimony Sulfide

The rise of micro/nanooptics and lab-on-chip devices demands the fabrication of three-dimensional structures with decent resolution. Here, we demonstrate the combination of grayscale electron beam lithography and direct forming methodology to fabricate antimony sulfide structures with free form for the first time. The refractive index of the electron beam patterned structure was calculated based on an optimization algorithm that is combined with genetic algorithm and transfer matrix method. By adopting electron irradiation with variable doses, 4-level Fresnel Zone Plates and metalens were produced and characterized. This method can be used for the fabrication of three-dimensional diffractive optical elements and metasurfaces in a single step manner.

physics.optics↗

arXiv2022

A full quantum mechanical approach assessing the chemical and electromagnetic effect in TERS

Tip-enhanced Raman spectroscopy (TERS) is a valuable method for surface analysis with nanometer to angstrom-scale resolution, however, the accurate simulation of particular TERS signals remains a computational challenge. We present a unique approach to this challenge by combining the two main contributors to plasmon-enhanced Raman spectroscopy and to the high resolution in TERS in particular, the electromagnetic and the chemical effect, into one quantum mechanical simulation. The electromagnetic effect describes the sample's interaction with the strong, highly localized and inhomogeneous electric fields associated with the plasmonic tip, and is typically the thematic focus for most mechanistic studies. On the other hand, the chemical effect covers the different responses to the extremely close-range and highly position-sensitive chemical interaction between the apex tip atom(s) and the sample, and, as we could show in previous works, plays an often underestimated role. Starting from a (time-dependent) density functional theory description of the chemical model system, comprised of a tin(II) phthalocyanine (SnPc) sample molecule and a single silver atom as tip, we introduce the electromagnetic effect through a series of point charges that recreate the electric field in the vicinity of the plasmonic Ag nanoparticle. By scanning the tip over the molecule along a 3D grid, we can investigate the system's Raman response on each position for non-resonant and resonant illumination. Simulating both effects on their own already hints at the achievable signal enhancement and resolution, but the combination of both creates even stronger evidence that TERS is capable of resolving sub-molecular features.

physics.chem-ph↗

arXiv2021

The chemical effect goes resonant -- a full quantum mechanical approach on TERS

Lately, experimental evidence of unexpectedly extremely high spatial resolution of tip-enhanced Raman scattering (TERS) has been demonstrated. Theoretically, two different contributions are discussed: an electromagnetic effect, leading to a spatially confined near field due to plasmonic excitations; and the so-called chemical effect originating from the locally modified electronic structure of the molecule due to the close proximity of the plasmonic system. Most of the theoretical efforts have concentrated on the electromagnetic contribution or the chemical effect in case of non-resonant excitation. In this work, we present a fully quantum mechanical description including non-resonant and resonant chemical contributions as well as charge-transfer phenomena of these molecular-plasmonic hybrid system at the density functional and the time-dependent density functional level of theory. We consider a surface-immobilized tin(II) phthalocyanine molecule as the molecular system, which is minutely scanned by a plasmonic tip, modeled by a single silver atom. These different relative positions of the Ag atom to the molecule lead to pronounced alterations of the Raman spectra. These Raman spectra vary substantially, both in peak positions and several orders of magnitude in the intensity patterns under non-resonant and resonant conditions, and also, depending on, which electronic states are addressed. Our computational approach reveals that unique - non-resonant and resonant - chemical interactions among the tip and the molecule significantly alter the TERS spectra and are mainly responsible for the high, possibly sub-Angstrom spatial resolution.

physics.chem-ph↗

arXiv2016

Direct Base-to-Base Transitions in ssDNA Revealed by Tip-Enhanced Raman Scattering

In the present contribution, specifically designed single-stranded DNA (ssDNA) sequences composed of adenine and cytosine were used as nanometric rulers to target the maximum achievable spatial resolution of tip-enhanced Raman spectroscopy (TERS) under ambient conditions. By stepping along a strand with a TERS tip, the obtained spectra allowed for a clear spectral discrimination including conformational information of the nucleobases, and even sharp adenine-cytosine transitions were detected repeatedly with a spatial resolution below 1 nm.

physics.chem-ph↗