arXiv subjects

Martin Hafermann

Publications and source records attributed to Martin Hafermann.

6 recordsLinked to original sources

arXiv2026

Enhancement of Circular Dichroism in Chiral Dielectric Metasurfaces by Ion Beam Irradiation

Resonant chiral dielectric metasurfaces can support circular dichroism exceeding that of natural materials, but their small dissipative losses simultaneously limit the maximization of circular dichroism, which inherently relies on absorption. Importantly, while the condition for optimal circular dichroism in resonant structures can be rigorously formulated based on the concept of critical coupling, controlling the amount of absorption experimentally, and ideally tuning it to the optimal value post-fabrication, remains elusive. Here, we experimentally tailor the dissipative losses of chiral bilayer dielectric metasurfaces post-fabrication using energetic ion beam irradiation. Specifically, we study the transmission characteristics of C4-symmetric chiral metasurface consisting of silicon nanocuboid arrays embedded in silica glass using polarization-resolved spectroscopy. We enhance the circular dichroism from 0.70 in the pristine, unirradiated metasurface to 0.85 after irradiation. Our experimental results are complemented by numerical simulations allowing us to retrieve the refractive index changes induced by the ion beam irradiation in the constituent materials of the metasurface. Our work offers a new approach to globally maximize optical chirality in engineered nanostructures, paving the way towards chiral emission and advanced polarization control applications

physics.optics↗

arXiv2024

Hyper-doping of Silicon for Plasmonics in the Telecommunication Range

We investigate hyper-doping, a promising approach to introduce a high concentration of impurities into silicon beyond its solid solubility limit, for its potential applications in near-infrared plasmonics. We systematically explore the incorporation of dopants into silicon using ion implantation and pulsed laser melting annealing processes. Reflectance spectra analysis shows an achievable plasma wavelength of around 1.5 $\mu$m for dopant concentrations exceeding 4 at.%. Complex refractive index data for the doped silicon samples are extracted, revealing their potential for near-infrared plasmonic applications. Moreover, we propose a fabrication process that allows for the creation of hyper-doped silicon nanoparticles without the need for additional masking steps. Our research paves the way for designing CMOS-compatible plasmonic nanostructures operating in the telecommunication wavelength range. The study's findings offer significant insights into the utilization of hyper-doped silicon for advanced photonic and optoelectronic applications.

physics.optics↗

arXiv2022

Heavily doped zinc oxide with plasma frequencies in the telecommunication wavelength range

We demonstrate heavy and hyper doping of ZnO by a combination of gallium (Ga) ion implantation using a focused ion beam (FIB) system and post-implantation laser annealing. Ion implantation allows for the incorporation of impurities with nearly arbitrary concentrations, and the laser-annealing process enables dopant activation close to or beyond the solid-solubility limit of Ga in ZnO. We achieved heavily doped ZnO:Ga with free-carrier concentrations of ~10^21 cm^(-3), resulting in a plasma wavelength of 1.02 um, which is substantially shorter than the telecommunication wavelength of 1.55 um. Thus, our approach enables the control of the plasma frequency of ZnO from the far infrared down to 1.02 um, providing a promising plasmonic material for applications in this regime.

physics.app-ph↗

arXiv2022

A photonic integrated circuit based erbium-doped amplifier

Erbium-doped fiber amplifiers have revolutionized long-haul optical communications and laser technology. Erbium ions could equally provide a basis for efficient optical amplification in photonic integrated circuits. However, this approach has thus far remained impractical due to insufficient output power. Here, we demonstrate a photonic integrated circuit based erbium amplifier reaching 145 mW output power and more than 30 dB small-signal gain -- on par with commercial fiber amplifiers and beyond state-of-the-art III-V heterogeneously integrated semiconductor amplifiers. We achieve this by applying ion implantation to recently emerged ultralow-loss Si3N4 photonic integrated circuits with meter-scale-length waveguides. We utilize the device to increase by 100-fold the output power of soliton microcombs, required for low-noise photonic microwave generation or as a source for wavelength-division multiplexed optical communications. Endowing Si3N4 photonic integrated circuits with gain enables the miniaturization of a wide range of fiber-based devices such as high-pulse-energy femtosecond mode-locked lasers.

physics.optics↗

arXiv2022

Tuning carrier density and phase transitions in oxide semiconductors using focused ion beams

We demonstrate spatial modification of the optical properties of thin-film metal oxides, zinc oxide and vanadium dioxide as representatives, using a commercial focused ion beam (FIB) system. Using a Ga+ FIB and thermal annealing, we demonstrated variable doping of a band semiconductor, zinc oxide (ZnO), achieving carrier concentrations from 10^18 cm-3 to 10^20 cm-3. Using the same FIB without subsequent thermal annealing, we defect-engineered a correlated semiconductor, vanadium dioxide (VO2), locally modifying its insulator-to-metal transition (IMT) temperature by range of ~25 degrees C. Such area-selective modification of metal oxides by direct writing using a FIB provides a simple, mask-less route to the fabrication of optical structures, especially when multiple or continuous levels of doping or defect density are required.

cond-mat.mtrl-sci↗

arXiv2021

Fast recovery of ion-irradiation-induced defects in Ge2Sb2Te5 thin films at room temperature

Phase-change materials serve a broad field of applications ranging from non-volatile electronic memory to optical data storage by providing reversible, repeatable, and rapid switching between amorphous and crystalline states accompanied by large changes in the electrical and optical properties. Here, we demonstrate how ion irradiation can be used to tailor disorder in initially crystalline Ge2Sb2Te5 (GST) thin films via the intentional creation of lattice defects. We found that continuous Ar ion irradiation at room temperature of GST films causes complete amorphization of GST when exceeding 0.6 (for rock-salt GST) and 3 (for hexagonal GST) displacements per atom (n_dpa). While the transition from rock-salt to amorphous GST is caused by progressive amorphization via the accumulation of lattice defects, several transitions occur in hexagonal GST upon ion irradiation. In hexagonal GST, the creation of point defects and small defect clusters leads to disordering of intrinsic vacancy layers (van der Waals gaps) that drives the electronic metal-insulator transition. Increasing disorder then induces a structural transition from hexagonal to rock-salt and then leads to amorphization. Furthermore, we observed different annealing behavior of defects for rock-salt and hexagonal GST. The higher amorphization threshold in hexagonal GST compared to rock-salt GST is caused by an increased defect-annealing rate, i.e., a higher resistance against ion-beam-induced disorder. Moreover, we observed that the recovery of defects in GST is on the time scale of seconds or less at room temperature.

cond-mat.mtrl-sci↗