@article{61523, abstract = {{AbstractMetasurface holography offers a powerful approach for manipulating wavefronts at the nano and micro scale. Extensive research has been conducted to enhance the multiplexing capacity for diverse wavefronts. However, the independence of multiplexed channels is fundamentally restricted in techniques using single‐layer metasurfaces, resulting in unavoidable crosstalk and the need for post‐filtering of the output wavefronts. Here, a universal wavefront multiplexing concept is presented based on non‐injective transformation. By employing joint optimization on two metasurfaces, different channels can be independently designed without any constraints on the output wavefronts. To validate this approach, ultra‐compact orbital angular momentum (OAM) sorters are designed. In these experiments, the output beams from different channels can be independently mapped to 2D positions with high fineness. In another application of wavefront‐multiplexed holography, 10‐channel multiplexing is experimentally achieved with minimal crosstalk and without the need for post‐processing. These results demonstrate the independence between channels enabled by the non‐injective transformation in the method. The precise wavefront control and high multiplexing capacity underscore its potential for scalable wavefront manipulation devices.}}, author = {{Jin, Xiao and Zentgraf, Thomas}}, issn = {{0935-9648}}, journal = {{Advanced Materials}}, publisher = {{Wiley}}, title = {{{Independent Wavefront Multiplexing with Metasurfaces via Non‐Injective Transformation}}}, doi = {{10.1002/adma.202511823}}, volume = {{38}}, year = {{2026}}, } @article{58606, author = {{Mathew, Albert and Aschwanden, Rebecca and Tripathi, Aditya and Jangid, Piyush and Sain, Basudeb and Zentgraf, Thomas and Kruk, Sergey}}, issn = {{1530-6984}}, journal = {{Nano Letters}}, publisher = {{American Chemical Society (ACS)}}, title = {{{Nonreciprocal Metasurfaces with Epsilon-Near-Zero Materials}}}, doi = {{10.1021/acs.nanolett.4c06188}}, year = {{2025}}, } @article{62057, abstract = {{Abstract Time-resolved momentum microscopy is an emerging technique based on photoelectron spectroscopy for characterizing ultrafast electron dynamics and the out-of-equilibrium electronic structure of materials in the entire Brillouin zone with high efficiency. In this article, we introduce a setup for time-resolved momentum microscopy based on an energy-filtered momentum microscope coupled to a custom-made high-harmonic generation photon source driven by a multi-100 kHz commercial Yb-ultrafast laser that delivers fs pulses in the extreme ultraviolet range. The laser setup includes a nonlinear pulse compression stage employing spectral broadening in a Herriott-type bulk-based multi-pass cell. This element allows flexible tuning of the driving pulse duration, providing a versatile time-resolved momentum microscopy setup featuring two operational modes designed to enhance either the energy or time resolution. We show the capabilities of the system by tracing ultrafast electron dynamics in the conduction band valleys of a bulk crystal of the 2D semiconductor WS2. Using uncompressed driving laser pulses, we demonstrate an energy resolution better than (107 ± 2) meV, while compressed pulses lead to a time resolution better than (48.8 ± 17) fs.}}, author = {{Schiller, Karl Jakob and Sternemann, Lasse and Stupar, Matija and Omar, Alan and Hoffmann, Martin and Nitschke, Jonah Elias and Mischke, Valentin and Janas, David Maximilian and Ponzoni, Stefano and Zamborlini, Giovanni and Saraceno, Clara Jody and Cinchetti, Mirko}}, issn = {{2045-2322}}, journal = {{Scientific Reports}}, number = {{1}}, publisher = {{Springer Science and Business Media LLC}}, title = {{{Time-resolved momentum microscopy with fs-XUV photons at high repetition rates with flexible energy and time resolution}}}, doi = {{10.1038/s41598-025-86660-1}}, volume = {{15}}, year = {{2025}}, } @article{62059, author = {{Nitschke, Jonah Elias and Sternemann, Lasse and Gutnikov, Michael and Schiller, Karl and Coronado, Eugenio and Omar, Alan and Zamborlini, Giovanni and Saraceno, Clara and Stupar, Matija and Ruiz, Alberto M. and Esteras, Dorye L. and Baldoví, José J. and Anders, Frithjof and Cinchetti, Mirko}}, issn = {{2950-6360}}, journal = {{Newton}}, number = {{2}}, publisher = {{Elsevier BV}}, title = {{{Tracing the ultrafast buildup and decay of d-d transitions in FePS3}}}, doi = {{10.1016/j.newton.2025.100019}}, volume = {{1}}, year = {{2025}}, } @inproceedings{43051, abstract = {{We demonstrate the numerical and experimental realization of optimized optical traveling-wave antennas made of low-loss dielectric materials. These antennas exhibit highly directive radiation patterns and our studies reveal that this nature comes from two dominant guided TE modes excited in the waveguide-like director of the antenna, in addition to the leaky modes. The optimized antennas possess a broadband nature and have a nearunity radiation efficiency at an operational wavelength of 780 nm. Compared to the previously studied plasmonic antennas for photon emission, our all-dielectric approach demonstrates a new class of highly directional, low-loss, and broadband optical antennas.}}, author = {{Farheen, Henna and Yan, Lok-Yee and Leuteritz, Till and Qiao, Siqi and Spreyer, Florian and Schlickriede, Christian and Quiring, Viktor and Eigner, Christof and Silberhorn, Christine and Zentgraf, Thomas and Linden, Stefan and Myroshnychenko, Viktor and Förstner, Jens}}, booktitle = {{Integrated Optics: Devices, Materials, and Technologies XXVII}}, editor = {{García-Blanco, Sonia M. and Cheben, Pavel}}, keywords = {{tet_topic_opticalantenna}}, pages = {{124241E}}, publisher = {{SPIE}}, title = {{{Tailoring the directive nature of optical waveguide antennas}}}, doi = {{10.1117/12.2658921}}, year = {{2023}}, } @article{25605, abstract = {{The nonlinear process of second harmonic generation (SHG) in monolayer (1L) transition metal dichalcogenides (TMD), like WS2, strongly depends on the polarization state of the excitation light. By combination of plasmonic nanostructures with 1L-WS2 by transferring it onto a plasmonic nanoantenna array, a hybrid metasurface is realized impacting the polarization dependency of its SHG. Here, we investigate how plasmonic dipole resonances affect the process of SHG in plasmonic–TMD hybrid metasurfaces by nonlinear spectroscopy. We show that the polarization dependency is affected by the lattice structure of plasmonic nanoantenna arrays as well as by the relative orientation between the 1L-WS2 and the individual plasmonic nanoantennas. In addition, such hybrid metasurfaces show SHG in polarization states, where SHG is usually forbidden for either 1L-WS2 or plasmonic nanoantennas. By comparing the SHG in these channels with the SHG generated by the hybrid metasurface components, we detect an enhancement of the SHG signal by a factor of more than 40. Meanwhile, an attenuation of the SHG signal in usually allowed polarization states is observed. Our study provides valuable insight into hybrid systems where symmetries strongly affect the SHG and enable tailored SHG in 1L-WS2 for future applications.}}, author = {{Spreyer, Florian and Ruppert, Claudia and Georgi, Philip and Zentgraf, Thomas}}, issn = {{1936-0851}}, journal = {{ACS Nano}}, number = {{10}}, pages = {{16719--16728}}, title = {{{Influence of Plasmon Resonances and Symmetry Effects on Second Harmonic Generation in WS2–Plasmonic Hybrid Metasurfaces}}}, doi = {{10.1021/acsnano.1c06693}}, volume = {{15}}, year = {{2021}}, } @article{22450, abstract = {{We realize and investigate a nonlinear metasurface taking advantage of intersubband transitions in ultranarrow GaN/AlN multi-quantum well heterostructures. Owing to huge band offsets, the structures offer resonant transitions in the telecom window around 1.55 µm. These heterostructures are functionalized with an array of plasmonic antennas featuring cross-polarized resonances at these near-infrared wavelengths and their second harmonic. This kind of nonlinear metasurface allows for substantial second-harmonic generation at normal incidence which is completely absent for an antenna array without the multi-quantum well structure underneath. While the second harmonic is originally radiated only into the plane of the quantum wells, a proper geometrical arrangement of the plasmonic elements permits the redirection of the second-harmonic light to free-space radiation, which is emitted perpendicular to the surface.}}, author = {{Mundry, Jan and Spreyer, Florian and Jmerik, Valentin and Ivanov, Sergey and Zentgraf, Thomas and Betz, Markus}}, issn = {{2159-3930}}, journal = {{Optical Materials Express}}, number = {{7}}, publisher = {{OSA}}, title = {{{Nonlinear metasurface combining telecom-range intersubband transitions in GaN/AlN quantum wells with resonant plasmonic antenna arrays}}}, doi = {{10.1364/ome.426236}}, volume = {{11}}, year = {{2021}}, } @article{26987, abstract = {{Optical metasurfaces are perfect candidates for the phase and amplitude modulation of light, featuring an excellent basis for holographic applications. In this work, we present a dual amplitude holographic scheme based on the photon sieve principle, which is then combined with a phase hologram by utilizing the Pancharatnam–Berry phase. We demonstrate that two types of apertures, rectangular and square shapes in a gold film filled with silicon nanoantennas are sufficient to create two amplitude holograms at two different wavelengths in the visible, multiplexed with an additional phase-only hologram. The nanoantennas are tailored to adjust the spectral transmittance of the apertures, enabling the wavelength sensitivity. The phase-only hologram is implemented by utilizing the anisotropic rectangular structure. Interestingly, such three holograms have quantitative mathematical correlations with each other. Thus, the flexibility of polarization and wavelength channels can be utilized with custom-tailored features to achieve such amplitude and phase holography simultaneously without sacrificing any space-bandwidth product. The present scheme has the potential to store different pieces of information which can be displayed separately by switching the wavelength or the polarization state of the reading light beam.}}, author = {{Frese, Daniel and Sain, Basudeb and Zhou, Hongqiang and Wang, Yongtian and Huang, Lingling and Zentgraf, Thomas}}, issn = {{2192-8614}}, journal = {{Nanophotonics}}, number = {{18}}, pages = {{4543--4550}}, publisher = {{De Gruyter}}, title = {{{A wavelength and polarization selective photon sieve for holographic applications}}}, doi = {{10.1515/nanoph-2021-0440}}, volume = {{10}}, year = {{2021}}, } @article{25227, abstract = {{AbstractQuantum well (QW) heterostructures have been extensively used for the realization of a wide range of optical and electronic devices. Exploiting their potential for further improvement and development requires a fundamental understanding of their electronic structure. So far, the most commonly used experimental techniques for this purpose have been all-optical spectroscopy methods that, however, are generally averaging in momentum space. Additional information can be gained by angle-resolved photoelectron spectroscopy (ARPES), which measures the electronic structure with momentum resolution. Here we report on the use of extremely low-energy ARPES (photon energy ~ 7 eV) to increase depth sensitivity and access buried QW states, located at 3 nm and 6 nm below the surface of cubic-GaN/AlN and GaAs/AlGaAs heterostructures, respectively. We find that the QW states in cubic-GaN/AlN can indeed be observed, but not their energy dispersion, because of the high surface roughness. The GaAs/AlGaAs QW states, on the other hand, are buried too deep to be detected by extremely low-energy ARPES. Since the sample surface is much flatter, the ARPES spectra of the GaAs/AlGaAs show distinct features in momentum space, which can be reconducted to the band structure of the topmost surface layer of the QW structure. Our results provide important information about the samples’ properties required to perform extremely low-energy ARPES experiments on electronic states buried in semiconductor heterostructures.}}, author = {{Hajlaoui, Mahdi and Ponzoni, Stefano and Deppe, Michael and Henksmeier, Tobias and As, Donat Josef and Reuter, Dirk and Zentgraf, Thomas and Springholz, Gunther and Schneider, Claus Michael and Cramm, Stefan and Cinchetti, Mirko}}, issn = {{2045-2322}}, journal = {{Scientific Reports}}, title = {{{Extremely low-energy ARPES of quantum well states in cubic-GaN/AlN and GaAs/AlGaAs heterostructures}}}, doi = {{10.1038/s41598-021-98569-6}}, volume = {{11}}, year = {{2021}}, } @article{21475, author = {{Frese, Daniel and Wei, Qunshuo and Wang, Yongtian and Cinchetti, Mirko and Huang, Lingling and Zentgraf, Thomas}}, issn = {{2330-4022}}, journal = {{ACS Photonics}}, number = {{4}}, pages = {{1013--1019}}, title = {{{Nonlinear Bicolor Holography Using Plasmonic Metasurfaces}}}, doi = {{10.1021/acsphotonics.1c00028}}, volume = {{8}}, year = {{2021}}, } @article{11953, abstract = {{As flexible optical devices that can manipulate the phase and amplitude of light, metasurfaces would clearly benefit from directional optical properties. However, single layer metasurface systems consisting of two-dimensional nanoparticle arrays exhibit only a weak spatial asymmetry perpendicular to the surface and therefore have mostly symmetric transmission features. Here, we present a metasurface design principle for nonreciprocal polarization encryption of holographic images. Our approach is based on a two-layer plasmonic metasurface design that introduces a local asymmetry and generates a bidirectional functionality with full phase and amplitude control of the transmitted light. The encoded hologram is designed to appear in a particular linear cross-polarization channel, while it is disappearing in the reverse propagation direction. Hence, layered metasurface systems can feature asymmetric transmission with full phase and amplitude control and therefore expand the design freedom in nanoscale optical devices toward asymmetric information processing and security features for anticounterfeiting applications.}}, author = {{Frese, Daniel and Wei, Qunshuo and Wang, Yongtian and Huang, Lingling and Zentgraf, Thomas}}, issn = {{1530-6984}}, journal = {{Nano Letters}}, number = {{6}}, pages = {{3976--3980}}, title = {{{Nonreciprocal Asymmetric Polarization Encryption by Layered Plasmonic Metasurfaces}}}, doi = {{10.1021/acs.nanolett.9b01298}}, volume = {{19}}, year = {{2019}}, }