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Publications at the University of Jena

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  1. Modeling high-order harmonic generation in quantum dots using a real-space tight-binding approach

    Year of publicationPublished in:The Journal of Chemical Physics : JCP M. Thümmler, A. Croy, U. Peschel, S. Gräfe
    Recently, the size-dependence of high-order harmonic generation (HHG) in quantum dots (QDs) has been investigated experimentally. In particular, for longer driving wavelengths and quantum dots smaller than 3 nm, HHG was strongly suppressed; however, there is no computational model capable of describing the strong-field response of such systems. In this work, we introduce a computationally efficient three-dimensional real-space tight-binding model specifically designed for the simulation of HHG in confined systems. The model parameters are meticulously derived from density functional theory calculations for the semiconductor bulk, followed by a process of Wannierization. Our findings demonstrate that the proposed model accurately captures the observed dependency of the HHG yield on the quantum dot size. In addition, we simulate the HHG yield for elliptically polarized pulses for different QD-sizes and driving wavelengths up to 5 μm. The proposed model fills the theoretical void in simulating HHG within medium-sized nanostructures, which cannot be described by methods applied for periodic solids, or small molecules or atoms.
    University Bibliography Jena:
    fsu_mods_00034842External link

  2. Semiconductor Bloch equations in Wannier gauge with well-behaved dephasing

    Year of publicationPublished in:Computer physics communications: an international journal devoted to computational physics and computer programs in physics M. Thümmler, T. Lettau, A. Croy, U. Peschel, S. Gräfe
    The semiconductor Bloch equations (SBEs) with a dephasing operator for the microscopic polarizations are a well established approach to simulate high-harmonic spectra in solids. We discuss the impact of the dephasing operator on the stability of the numerical integration of the SBEs in the Wannier gauge. It is shown that the commonly used phenomenological approach to apply dephasing is ill-defined in the presence of band crossings and leads to artifacts in the carrier distribution. They are caused by rapid changes of the dephasing operator matrix elements in the Wannier gauge, which render the convergence of the simulation in the stationary basis infeasible. In the comoving basis, also called Houston basis, these rapid changes can be resolved, but only at the cost of a largely increased computation time. As a remedy, we propose a modification of the dephasing operator with reduced magnitude in energetically close subspaces. This approach removes the artifacts in the carrier distribution and significantly speeds up the calculations, while affecting the high-harmonic spectrum only marginally. To foster further development, we provide our parallelized source code.
    University Bibliography Jena:
    fsu_mods_00029532External link

  3. Probing Ultrafast Coherent Bandgap Modulation in Monolayer WSe₂ by Nonlinear Optics

    Year of publicationPublished in:Advanced Optical Materials S. Klimmer, T. Lettau, L. Molina, D. Kartashov, U. Peschel, J. Wilhelm, D. Neshev, G. Soavi
    Light-matter interactions are powerful tools that seamlessly allow both functionalities of sizeable bandgap modulation and non-invasive spectroscopy. While the border between modulation and detection is often assumed to be sharp and well-defined, there are experiments where the boundaries fade. Here, the interplay between bandgap modulation and non-invasive spectroscopy is measured and explained in the case of resonant perturbative nonlinear optics in an atomically thin direct gap semiconductor.A clear deviation from the typical quadratic power scaling of second-harmonic generation near an exciton resonance is reported, and this unusual result is explained based on all-optical modulation driven by the intensity-dependent optical Stark and Bloch–Siegert shifts in the ±K valleys of the Brillouin zone. The experimental results are corroborated by analytical and numerical analysis based on the semiconductor Bloch equations, from which the resonant transition dipole moments and dephasing times of the sample are extracted. These findings redefine the meaning of perturbative nonlinear optics by revealing how coherent light-matter interactions can modify the band structure of a crystal, even in the weak-field regime. Furthermore, the results strengthen the understanding of ultrafast all-optical control of electronic states in 2D materials, with potential applications in valleytronics, Floquet engineering, and light-wave electronics.
    University Bibliography Jena:
    fsu_mods_00029696External link

  5. Integrated high-dimensional time-bin entanglement analyzers for scalable quantum photonics

    Year of publicationPublished in:2025 IEEE Photonics Society Summer Topicals Meeting Series (SUM): proceedings N. Montaut, S. Sciara, M. Chemnitz, M. Monika, U. Peschel, D. Moss, F. Nosrati, Z. Wang, J. Azaña, H. Yu, R. lo Franco, R. Morandotti
    University Bibliography Jena:
    fsu_mods_00027590External link

  6. Progress in integrated and fiber optics for time-bin based quantum information processing

    Year of publicationPublished in:Advanced optical technologies N. Montaut, A. George, M. Monika, F. Nosrati, H. Yu, S. Sciara, B. Crockett, U. Peschel, Z. Wang, R. Lo Franco, M. Chemnitz, W. Munro, D. Moss, J. Azaña, R. Morandotti
    The development of integrated photonic systems, both on-chip and fiber-based, has transformed quantum photonics by replacing bulky, fragile free-space optical setups with compact, efficient, and robust circuits. Photonic platforms incorporating fiber-connected sources of correlated and entangled photon pairs offer practical advantages, such as operation at room temperature, efficient integration with telecom infrastructure, and compatibility with mature and efficient semiconductor fabrication processes for cost-effective and large-scale optical circuits. The stability and scalability of integrated quantum photonics platforms have facilitated the generation and processing of quantum information in the temporal domain within a single spatial mode. Time-bin encoded states, known for their robustness against decoherence and compatibility with existing fiber-optic infrastructure, have shown to be an efficient paradigm for advanced applications like quantum secure communication, information processing, spectroscopy, imaging, and sensing. This review examines recent advancements in fiber- and chip-based platforms for generating non-classical states and their applications as quantum state processors in the time domain. We discuss the generation of pulsed quantum frequency combs using microring resonators and intra-cavity mode-locked laser schemes, enabling co- and cross-polarized quantum photonic states. Additionally, the versatility of these resonator chips for entanglement generation is emphasized, including two- and multi-photon time-bin entangled schemes. We highlight the development of time-bin entanglement analyzers in fiber architectures, featuring ultrahigh stability and post-selection-free capabilities, which enable precise and efficient characterization of two- and higher-dimensional time-bin entanglement. We also review scalable on-chip schemes for quantum key distribution, demonstrating low quantum bit error rates and compatibility with higher-dimensional quantum communication protocols. Further, methods for enhancing temporal resolution in detection schemes, crucial for time-bin encoding, are presented, such as the time-stretch sampling technique using electro-optic modulation. These innovations, relying on readily available, telecom-based fiber-optic components, provide practical, scalable, and cost-effective solutions for advancing quantum photonic technologies. Looking forward, time-bin encoding is expected to play a pivotal role in the advancement of quantum repeaters, distributed quantum networks, and hybrid light-matter systems, advancing the realization of globally scalable quantum technologies.
    University Bibliography Jena:
    fsu_mods_00024634External link

  7. Topologically Tunable Polaritons Based on a Two-Dimensional Crystal in a Photonic Lattice

    Year of publicationPublished in:Physical Review Letters L. Lackner, O. Egorov, A. Ernzerhof, C. Bennenhei, V. Mitryakhin, G. Leibeling, F. Eilenberger, S. Tongay, U. Peschel, M. Esmann, C. Schneider
    Structured optical cavities have advanced as a powerful test bed to study lattice Hamiltonians in general, and topological phenomena in particular. The in situ tuning of topological modes, enabled via substantial modifications of emulated lattice potentials, has remained out of experimental reach due to the commonly utilized monolithic cavity samples. Here, we study the Su-Schrieffer-Heeger (SSH) lattice Hamiltonian, which we emulate in a widely tunable open optical cavity strongly coupled to excitons in an integrated WS_{2} monolayer. The potential landscape comprises a topological domain boundary hosting a topological, exponentially localized mode at the interface between two lattices characterized by different Zak phases. The mode is spectrally tunable over 80 meV. Moreover, we use the unique tilt tunability of our implementation to transform the SSH lattice into a Stark ladder. This transformation couples the topologically protected defect mode to propagating lattice modes and effectively changes the symmetry of the system. Furthermore, it allows us to directly quantify the Zak-phase difference Δ_{Zak}=(1.07±0.11)π between the two topological phases. Our Letter constitutes an important step toward in situ tuning topological lattices to control and guide light on nonlinear chips.
    University Bibliography Jena:
    fsu_mods_00028803External link

  8. Detecting high-dimensional time-bin entanglement in fiber-loop systems

    Year of publicationPublished in:Physical Review A N. Euler, M. Monika, U. Peschel, M. Gärttner
    Many quantum communication protocols rely on the distribution of entanglement between the different participating parties. One example is quantum key distribution (QKD), an application that has matured to commercial use in recent years. However, difficulties remain, especially with noise resilience and channel capacity in long-distance communication. One way to overcome these problems is to use high-dimensional entanglement, which has been shown to be more robust to noise and enables higher secret-key rates. It is therefore important to have access to certifiable high-dimensional entanglement sources to confidently implement these advanced QKD protocols. Here we develop a method for certifying high-dimensional time-bin entanglement in fiber-loop systems. In these systems, entanglement creation and detection can utilize the same physical components, and the number of time bins, and thus the entanglement dimension, can be adapted without making physical changes to the setup. Our certification method builds on previous proposals for the certification of angular-momentum entanglement in photon pairs. In particular, measurements in only two experimentally accessible bases are sufficient to obtain a lower bound on the entanglement dimension for both two- and multiphoton quantum states. Numerical simulations show that the method is robust against typical experimental noise effects and works well even with limited measurement statistics, thus establishing time-bin encoded photons as a promising platform for high-dimensional quantum-communication protocols.
    University Bibliography Jena:
    fsu_mods_00028971External link

  9. Quantum state processing through controllable synthetic temporal photonic lattices

    Year of publicationPublished in:Nature Photonics M. Monika, F. Nosrati, A. George, S. Sciara, R. Fazili, A. Marques Muniz, A. Bisianov, R. Lo Franco, W. Munro, M. Chemnitz, U. Peschel, R. Morandotti
    Quantum walks on photonic platforms represent a physics-rich framework for quantum measurements, simulations and universal computing. Dynamic reconfigurability of photonic circuitry is key to controlling the walk and retrieving its full operation potential. Universal quantum processing schemes based on time-bin encoding in gated fibre loops have been proposed but not demonstrated yet, mainly due to gate inefficiencies. Here we present a scalable quantum processor based on the discrete-time quantum walk of time-bin-entangled photon pairs on synthetic temporal photonic lattices implemented on a coupled fibre-loop system. We utilize this scheme to path-optimize quantum state operations, including the generation of two- and four-level time-bin entanglement and the respective two-photon interference. The design of the programmable temporal photonic lattice enabled us to control the dynamic of the walk, leading to an increase in the coincidence counts and quantum interference measurements without recurring to post-selection. Our results show how temporal synthetic dimensions can pave the way towards efficient quantum information processing, including quantum phase estimation, Boson sampling and the realization of topological phases of matter for high-dimensional quantum systems in a cost-effective, scalable and robust fibre-based setup.
    University Bibliography Jena:
    fsu_mods_00017805External link

  10. Nonlinear Optical Properties of Mono and Multilayer MoWSe₂ Alloys

    Year of publicationPublished in:Advanced Optical Materials M. Hussain, O. Ghaebi, M. Monfared, M. Gruenewald, U. Ahsan, F. Lipilin, J. Luxa, Z. Sofer, U. Peschel, G. Soavi
    Transition metal dichalcogenide (TMD) alloys provide a stable and reliable platform for broadband tuning of excitonic resonances. Here, the nonlinear optical response of Mo₍₁ − ₓ₎WₓSe₂, focusing in particular on second harmonic generation (SHG) and two-photon photoluminescence (TP-PL), is studied. It is found that alloys always display stronger nonlinearities compared to pristine TMDs. In addition, by comparing the resonant energies of SHG and TP-PL, a non-monotonic change of the energy difference between the 1s and 2p states of the A exciton, pointing toward the possibility of tuning the exciton binding energy by alloying and material composition, is found. Finally, layer-dependent SHG and TP-PL, which show an alternate broken/preserved space inversion symmetry for odd/even number of layers and a transition from indirect to direct bandgap when thinning down the layered samples to the monolayer limit, is reported. This work provides useful insights for a better understanding of the optical and electronic properties of TMD alloys, and thus for their use in future photonic and opto-electronic devices.
    University Bibliography Jena:
    fsu_mods_00027243External link

  11. Nonlinear harmonic generation in sub-5 nm plasmonic nanogap metasurfaces

    Year of publicationPublished in:2025 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) S. Beer, J. Gour, P. Paul, A. Alberucci, A. Szeghalmi, T. Siefke, U. Peschel, U. Zeitner, S. Nolte
    University Bibliography Jena:
    fsu_mods_00027672External link

  12. Nonlinear valley selection rules and all-optical probe of broken time-reversal symmetry in monolayer WSe₂

    Year of publicationPublished in:Nature Photonics P. Herrmann, S. Klimmer, T. Lettau, T. Weickhardt, A. Papavasileiou, K. Mosina, Z. Sofer, I. Paradisanos, D. Kartashov, J. Wilhelm, G. Soavi
    In monolayer transition metal dichalcogenides, time-reversal symmetry—combined with broken space-inversion symmetry—defines the spin–valley degree of freedom. As such, the engineering and control of time-reversal symmetry by optical or magnetic fields constitutes the foundation of valleytronics. Here we propose a new approach for the detection of broken time-reversal symmetry and valley imbalance in monolayer WSe₂ based on second-harmonic generation. At room temperature, our method can selectively probe a net valley imbalance generated by ultrafast, coherent and valley-exclusive optical Stark and Bloch–Siegert effects. This work demonstrates the potential and unique capabilities of nonlinear optics as a probe of broken time-reversal symmetry as well as a tool for ultrafast and non-destructive valleytronic operations.
    University Bibliography Jena:
    fsu_mods_00019204External link
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