arXiv subjects
Publications and source records attributed to Stephan Winnerl.
This work presents the near-field super-scattering and the near-field thermal emission from 2D hyperbolic crystals as general optical phenomena that link near-field nanophotonics to far-field photonics.
Ultrafast magnetism triggered by circularly polarized radiation underpins ultrafast spin control, relevant to future technologies, e.g., opto-spintronics and magnonics. The dynamics are often complicated and intertwined among correlated subsystems, such as electrons, spins, phonons, plasmons, topology, and lattice, due to many-body quantum coupling at ultrafast timescales. Here, we demonstrate light-induced effective magnetic fields generated by selective excitation between non-equidistant Landau quantized states in graphene, a prototypical Dirac material, using circularly polarized pulses. By magnetically tuning the Landau-level transition resonance away from other low-energy excitations, we obtain a clean electrostatically controllable platform and identify the microscopic origin of the light-induced magnetic signals, independent of sublattice coupling. Because different Landau levels carry distinct optical Hall conductivities, direct modification of their occupancies via optical excitations creates transient Faraday rotation signals with dispersive magnetic-field dependence, mirroring the static magneto-optical lineshape. The induced effective magnetic field normalized by the pump electric field exceeds typical reported values for the inverse Faraday effect of electronic origin. Our results establish a clear microscopic picture of the inverse Faraday effect of electronic origin, which can trigger hierarchical dynamics among correlated sublattices once Landau-level transitions are magnetically tuned to coincide with other low-energy excitations in Dirac systems and related materials.
Superconductors carry electrical current without resistance when paired electrons condense into a coherent macroscopic quantum state. In underdoped cuprates, evidence suggests that pairing-related correlations and superconducting fluctuations can survive above the temperature at which global coherence is lost, pointing to phase fluctuations as a key limitation on superconductivity in this regime. Motivated by recent demonstrations of cavity-modified collective states in quantum materials, we investigate whether superconducting coherence can be stabilized by engineering the electromagnetic environment of the superconductor. We study an underdoped YBa$_2$Cu$_3$O$_{7-\delta}$ thin film in a tunable terahertz cavity formed with a semi-transparent gold mirror. From temperature-dependent terahertz transmission measurements, we find that the cavity enhances the superconducting response below the critical temperature, with an increase of the inferred superfluid weight. The effect becomes more pronounced at smaller cavity lengths and is accompanied by an upward shift of the superconducting onset temperature. Calculations based on a cavity-coupled model for phase-fluctuating superconductors capture these trends and support an interpretation in terms of cavity-enhanced phase stiffness. These results showcase the potential of cavity engineering for designing emergent functionalities in correlated systems.
Using infrared (IR) and Raman spectroscopies combined with high-resolution x-ray diffraction, we address several controversial aspects of altermagnetic $\alpha$-MnTe. We show that mechanical stress applied to crystals of this material causes a drastic broadening of Bragg peaks that conceals signatures of additional phases present in the sample. Indeed, spatially resolved Raman spectroscopy reveals that the modes around 175 cm$^{-1}$ often reported in $\alpha$-MnTe are not reproducible across different positions and samples and originate from the secondary phase of MnTe$_2$. By combining spectroscopic probes with ab initio calculations, we establish the IR-active optical phonon of $\alpha$-MnTe around 155 cm$^{-1}$ ($E_{1u}$) and the Raman-active optical phonon around 100 cm$^{-1}$ ($E_{2g}$) at room temperature. Two intense Raman modes around 120 and 140 cm$^{-1}$ are shown to be intrinsic, even though they can not be assigned to $\Gamma$-point optical phonons. These modes couple to magnetic order in $\alpha$-MnTe and also to the transient reflectivity resulting in coherent oscillations. Both 6-fold rotation symmetry and inversion symmetry are preserved in $\alpha$-MnTe within our experimental resolution.
Barium titanate (BaTiO$_3$) is a compelling material for integrated photonics due to its strong electro-optic and second-order nonlinear properties. Crystal Ion Slicing (CIS) presents a scalable and CMOS-compatible route for fabricating thin BaTiO$_3$ films; however, ion implantation during CIS introduces lattice damage that can degrade structural and optical performance. In this study, we demonstrate that post-slicing thermal annealing effectively restores the structural integrity and optical quality of CIS-processed BaTiO$_3$ flakes. Raman spectroscopy confirms the recovery of crystallinity, while second-harmonic generation (SHG) microscopy reveals systematic reorientation of ferroelectric domains and restoration of the associated second-order nonlinear susceptibility tensor $X^{(2)}$. Notably, SHG signals persist even in regions with weak Raman signatures, indicating that long-range ferroelectric order can survive despite partial lattice disruption. Optical measurements show that the linear dispersion of annealed CIS flakes closely matches that of bulk BaTiO$_3$, validating their suitability for photonic integration. Together, these results qualify CIS - combined with thermal annealing - as a viable and scalable manufacturing strategy for high-quality BaTiO$_3$-on-insulator (BTOI) platforms, enabling advanced integrated photonic devices for modulation, frequency conversion, and quantum optics.
We consider the propagation of charge carriers in planar graphene under the combined influence of a constant transversal magnetic field $B$ and an in-plane varying electric potential $\phi(x)$. By suitably designing the potential landscape $\phi(x)$, we may effectively steer charge carriers generated by photo-excitation, for example, in order to achieve an efficient charge separation. These finding may pave the way for transport schemes or photoelectric/photovoltaic applications.
A two-dimensional electron gas (2DEG) forms at the interface of complex oxides like $SrTiO_{3}$ (STO) and $LaTiO_{3}$ (LTO), despite each material having a low native conductivity, as a band and a Mott insulator, respectively. The interface 2DEG hosts charge carriers with moderate charge carrier density and mobility that raised interest as a material system for applications like field-effect transistors or detectors. Of particular interest is the integration of these oxide systems in silicon technology. To this end we study the carrier dynamics in a STO/LTO/STO heterostructure epitaxially grown on Si(001) both experimentally and theoretically. Linear THz spectroscopy was performed to analyze the temperature dependent charge carrier density and mobility, which was found to be in the range of $10^{12}$ $cm^2$ and 1000 $cm^2V^{-1}s^{-1}$, respectively. Pump-probe measurements revealed a very minor optical nonlinearity caused by hot carriers with a relaxation time of several 10 ps, even at low temperature. Density functional theory calculations with a Hubbard U term on ultrathin STO-capped LTO films on STO(001) show an effective mass of 0.64-0.68 $m_{e}$.
We present a novel approach to enhance THz nonlinearity by the resonant excitation of two-dimensional plasmons in grating-gate plasmonic crystals. Using a high-electric-field THz pump-THz probe technique, we investigate the nonlinear interaction of spectrally narrow THz pulses with plasmon oscillations in a two-dimensional electron gas on an AlGaN/GaN interface integrated with metallic grating. Nonlinear effects are observed as ultrafast, pump-induced changes in THz transmission, with relative transparency strongly dependent on plasmonic mode excitation and saturating at pump fluences of about 200 nJ cm-2. The maximal relative transparency, reaching 45 % at 350 nJ cm -2, occurs under resonant excitation of a localized plasmon mode at the strong electrostatic modulation of 2DEG concentration. Transient dynamics reveal ultrafast relaxation times of 15-20 ps, while the effects can be observed at elevated temperatures of up to 150 K. A nonlinear model of plasmonic crystal, based on finite-difference time-domain electrodynamic simulations coupled with viscous hydrodynamic electron transport model, elucidates key nonlinear mechanisms, including near-field effects under metallic gratings, electron heating, plasmon resonance broadening, and redshift. These results demonstrate that even conventional semiconductors such as AlGaN/GaN can achieve nonlinear THz responses comparable to or exceeding those of graphene, showing strong potential for ultrafast THz modulation and nonlinear photonics applications.
The interplay between electronic correlations, density wave orders, and magnetism gives rise to several fascinating phenomena. In recent years, kagome metals have emerged as an excellent platform for investigating these unique properties, which stem from their itinerant carriers arranged in a kagome lattice. Here, we show that electronic structure of the prototypical kagome metal, Fe$_3$Sn$_2$, can be tailored by manipulating the breathing distortion of its kagome lattice with external pressure. The breathing distortion is suppressed around 15 GPa and reversed at higher pressures. These changes lead to a series of Lifshitz transitions that we detect using broadband and transient optical spectroscopy. Remarkably, the strength of the electronic correlations and the tendency to carrier localization are enhanced as the kagome network becomes more regular, suggesting that breathing distortion can be a unique control parameter for the microscopic regime of the kagome metals and their electron dynamics.
On-demand polarization control of electromagnetic waves is the fundamental element of modern optics. Its interest has recently been expanded in the terahertz (THz) range for coherent excitation of collective quasiparticles in matters, triggering a wide variety of non-trivial intriguing physics, e.g., anharmonicity, nonlinear coupling, and metastability. Wavelength tunability in THz polarization control is fundamentally important for the resonant excitation of collective modes. Here, we propose and demonstrate a simple and convenient THz phase retarder based on the Mach-Zehnder interferometer to obtain circular polarization. The efficiency of THz polarization conversion is demonstrated by the achieved high polarization degree of more than 99.9% and a large transmission of ~76%. The simple and compact setup allows us to adapt the phase retarder to existing setups readily and will contribute to further exploration of ultrafast science, e.g., chiral phononics.
The use of the THz frequency domain in future network generations offers an unparalleled level of capacity, which can enhance innovative applications in wireless communication, analytics, and imaging. Communication technologies rely on frequency mixing, enabling signals to be converted from one frequency to another and transmitted from a sender to a receiver. Technically, this process is implemented using nonlinear components such as diodes or transistors. However, the highest operation frequency of this approach is limited to sub-THz bands. Here, we demonstrate the upconversion of a weak sub-THz signal from a photoconductive antenna to multiple THz bands. The key element is a high-mobility HgTe-based heterostructure with electronic band inversion, leading to one of the strongest third-order nonlinearities among all materials in the THz range. Due to the Dirac character of electron dispersion, the highly intense sub-THz radiation is efficiently mixed with the antenna signal, resulting in a THz response at linear combinations of their frequencies. The field conversion efficiency above 2$\%$ is provided by a bare tensile-strained HgTe layer with a thickness below 100 nm at room temperature under ambient conditions. Devices based on Dirac materials allow for high degree of integration, with field-enhancing metamaterial structures, making them very promising for THz communication with unprecedented data transfer rate.
We report high average power THz emission from a GaAs-based large-area photoconductive emitter, excited by a commercial Yb-laser amplifier without any compression schemes, doubled to the green. The LAE is pumped at 11.4 W of green average power (515 nm) and 310 fs pulse duration at 400-kHz repetition rate. We obtain a maximum THz power of 6.7 mW with a spectrum extended up to 3 THz. Using electro-optic sampling (EOS) for detection, we measure a high dynamic range of 107 dB in a measurement time of 70 s. To the best of our knowledge, this is the highest power THz source so far demonstrated by photoconductive emitters, based on a simple and robust, commercially available Yb-laser.
This study investigates the electronic band structure of Chromium Sulfur Bromide (CrSBr) through comprehensive photoluminescence (PL) characterization. We clearly identify low-temperature optical transitions between two closely adjacent conduction-band states and two different valence-band states. The analysis of the PL data robustly unveils energy splittings, bandgaps and excitonic transitions across different thicknesses of CrSBr, ranging from monolayer to bulk. Temperature-dependent PL measurements elucidate the stability of the band splitting below the N\'eel temperature, suggesting that magnons coupled with excitons are responsible for the symmetry breaking and brightening of the transitions from the secondary conduction band minimum (CBM2) to the global valence band maximum (VBM1). Collectively, these results not only reveal band splitting in both the conduction and valence bands, but also point to an intricate interplay between the optical, electronic and magnetic properties of antiferromagnetic two-dimensional van der Waals crystals.
We report THz-pump / mid-infrared probe near-field studies on Si-doped GaAs-InGaAs core-shell nanowires utilizing THz radiation from the free-electron laser FELBE. Upon THz excitation of free carriers, we observe a red shift of the plasma resonance in both amplitude and phase spectra, which we attribute to the heating up of electrons in the conduction band. The simulation of heated electron distributions anticipates a significant electron population in both L- and X-valleys. The two-temperature model is utilized for a quantitative analysis of the dynamics of the electron gas temperature under THz pumping at various power levels.
The layered, air-stable van der Waals antiferromagnetic compound CrSBr exhibits pronounced coupling between its optical, electronic, and magnetic properties. As an example, exciton dynamics can be significantly influenced by lattice vibrations through exciton-phonon coupling. Using low-temperature photoluminescence spectroscopy, we demonstrate the effective coupling between excitons and phonons in nanometer-thick CrSBr. By careful analysis, we identify that the satellite peaks predominantly arise from the interaction between the exciton and an optical phonon with a frequency of 118 cm-1 (~14.6 meV) due to the out-of-plane vibration of Br atoms. Power-dependent and temperature-dependent photoluminescence measurements support exciton-phonon coupling and indicate a coupling between magnetic and optical properties, suggesting the possibility of carrier localization in the material. The presence of strong coupling between the exciton and the lattice may have important implications for the design of light-matter interactions in magnetic semiconductors and provides new insights into the exciton dynamics in CrSBr. This highlights the potential for exploiting exciton-phonon coupling to control the optical properties of layered antiferromagnetic materials.
We study THz-driven condensate dynamics in epitaxial thin films of MgB$_{2}$, a prototype two-band superconductor (SC) with weak interband coupling. The temperature and excitation density dependent dynamics follow the behavior predicted by the phenomenological bottleneck model for the single-gap SC, implying adiabatic coupling between the two condensates on the ps timescale. The amplitude of the THz-driven suppression of condensate density reveals an unexpected decrease in pair-breaking efficiency with increasing temperature - unlike in the case of optical excitation. The reduced pair-breaking efficiency of narrow-band THz pulses, displaying minimum near $\approx0.7$ T$_{c}$, is attributed to THz-driven, long-lived, non-thermal quasiparticle distribution, resulting in Eliashberg-type enhancement of superconductivity, competing with pair-breaking.
Strong circularly polarized excitation opens up the possibility to generate and control effective magnetic fields in solid state systems, e.g., via the optical inverse Faraday effect or the phonon inverse Faraday effect. While these effects rely on material properties that can be tailored only to a limited degree, plasmonic resonances can be fully controlled by choosing proper dimensions and carrier concentrations. Plasmon resonances provide new degrees of freedom that can be used to tune or enhance the light-induced magnetic field in engineered metamaterials. Here we employ graphene disks to demonstrate light-induced transient magnetic fields from a plasmonic circular current with extremely high efficiency. The effective magnetic field at the plasmon resonance frequency of the graphene disks (3.5 THz) is evidenced by a strong (~1{\deg}) ultrafast Faraday rotation (~ 20 ps). In accordance with reference measurements and simulations, we estimated the strength of the induced magnetic field to be on the order of 0.7 T under a moderate pump fluence of about 440 nJ cm-2.
BaFe$_2$As$_2$ is the parent compound for a family of iron-based high-temperature superconductors as well as a prototypical example of the spin-density wave (SDW) system. In this study, we perform an optical pump-probe study of this compound to systematically investigate the SDW order across the pressure-temperature phase diagram. The suppression of the SDW order by pressure manifests itself by the increase of relaxation time together with the decrease of the pump-probe signal and the pump energy necessary for complete vaporization of the SDW condensate. We have found that the pressure-driven suppression of the SDW order at low temperature occurs gradually in contrast to the thermally-induced SDW transition. Our results suggest that the pressure-driven quantum phase transition in BaFe$_2$As$_2$ (and probably other iron pnictides) is continuous and it is caused by the gradual worsening of the Fermi-surface nesting conditions.