We demonstrate, by theory and experiment, the ultrafast tilting of the dispersion curve of a photonic-crystal waveguide following the absorption of a femtosecond pump pulse. By shaping the pump-beam ...cross section with a nanometric shadow mask, different waveguide eigenmodes acquire different spatial overlap with the perturbing pump, leading to a local flattening of the dispersion by up to 11 %. We find that such partial mode perturbation can be used to adiabatically compress the spectrum of a light pulse traveling through the waveguide.
Transition-metal dichalcogenides (TMDCs) are an aspiring class of materials with unique electronic and optical properties and potential applications in spin-based electronics. Here, we use terahertz ...emission spectroscopy to study spin-to-charge current conversion (S2C) in the TMDC NbSe\(_2\) in ultra-high-vacuum-grown F|NbSe\(_2\) thin-film stacks, where F is a layer of ferromagnetic Fe or Ni. Ultrafast laser excitation triggers an ultrafast spin current that is converted into an in-plane charge current and, thus, a measurable THz electromagnetic pulse. The THz signal amplitude as a function of the NbSe\(_2\) thickness shows that the measured signals are fully consistent with an ultrafast optically driven injection of an in-plane-polarized spin current into NbSe\(_2\). Modeling of the spin-current dynamics reveals that a sizable fraction of the total S2C originates from the bulk of NbSe\(_2\) with the same, negative, sign as the spin Hall angle of pure Nb. By quantitative comparison of the emitted THz radiation from F|NbSe\(_2\) to F|Pt reference samples and the results of ab-initio calculations, we estimate that the spin Hall angle of NbSe\(_2\) for an in-plane polarized spin current lies between -0.2% and -1.1%, while the THz spin-current relaxation length is of the order of a few nanometers.
We introduce the concept of an indirect photonic transition and demonstrate its use in a dynamic delay line to alter the group velocity of an optical pulse. Operating on an ultrafast time scale, we ...show continuously tuneable delays of up to 20 ps, using a slow light photonic crystal waveguide only 300 \(\mu\)m in length. Our approach is flexible, in that individual pulses in a pulse stream can be controlled independently, which we demonstrate by operating on pulses separated by just 30 ps. The two-step indirect transition is demonstrated here with a 30% conversion efficiency.
Coherent radiation with frequencies ranging from 0.3 to 30 THz has recently become accessible by femtosecond laser technology. Terahertz (THz) waves have already found many applications in ...spectroscopy and imaging, and they can be manipulated using static optical elements such as lenses, polarizers, and filters. However, ultrafast modulation of THz radiation is required as well, for instance in short-range wireless communication or for preparing shaped THz transients for coherent control of numerous material excitations. Here, we demonstrate an all-optically created transient metamaterial that permits to manipulate the polarization of THz waveforms with sub-cycle precision. The polarization-modulated pulses are potentially interesting for controlling elementary motions such as vibrations of crystal lattices, rotations of molecules, and the precession of spins.
Topological insulators constitute a new and fascinating class of matter with insulating bulk yet metallic surfaces that host highly mobile charge carriers with spin-momentum locking. Remarkably, the ...direction and magnitude of surface currents can be controlled with tailored light beams, but the underlying mechanisms are not yet well understood. To directly resolve the "birth" of such photocurrents we need to boost the time resolution to the scale of elementary scattering events (\(\sim\) 10 fs). Here, we excite and measure photocurrents in the three-dimensional model topological insulator \(\mathrm{Bi}_2\mathrm{Se}_3\) with a time resolution as short as 20 fs by sampling the concomitantly emitted broadband THz electromagnetic field from 1 to 40 THz. Remarkably, the ultrafast surface current response is dominated by a charge transfer along the Se-Bi bonds. In contrast, photon-helicity-dependent photocurrents are found to have orders of magnitude smaller magnitude than expected from generation scenarios based on asymmetric depopulation of the Dirac cone. Our findings are also of direct relevance for optoelectronic devices based on topological-insulator surface currents.
The ability to actively tune the properties of a nanocavity is crucial for future applications in photonics and quantum information. Two important man-made classes of materials have emerged to mold ...the flow of electromagnetic waves. Firstly, photonic crystals are dielectric nanostructures that can be used to confine and slow down light and control its emission. They act primarily on the electric component of the light field. More recently, a novel class of metallo-dielectric nanostructures has emerged. These so-called metamaterials enable fascinating phenomena, such as negative refraction, super-focusing and cloaking. This second class of materials realizes light control through effective interactions with both electric and magnetic component. In this work, we combine both concepts to gain an active and reversible control of light trapping on subwavelength length scales. By actuating a nanoscale magnetic coil close to a photonic crystal nanocavity, we interact with the rapidly varying magnetic field and accomplish an unprecedented control of the optical properties of the cavity. We achieve a reversible enhancement of the lifetime of photons in the cavity. By successfully combining photonic crystal and metamaterials concepts, our results open the way for new light control strategies based on interactions which include the magnetic component of light.
Automation concept for complex production processes Luhn, G.; Stoschek, B.; Schilling, H. ...
Twenty Fourth IEEE/CPMT International Electronics Manufacturing Technology Symposium (Cat. No.99CH36330),
1999
Conference Proceeding
Automation strategies are confronted with two different trends: increasing equipment integration capability into information and material flow concepts, and production process complexity increasing ...at least at the same speed. In our CIM framework, bottom-up oriented integration of material flow automation is added to traditional top-down oriented production planning, simulating and scheduling scenarios. We introduce a simple mechanism to adjust and control material flow by answering the question of who/which instance is responsible for introducing the basic activity in the production system. There is some evidence (also reported by Hoop and Spearman, 1996) to dedicate this activity to the production equipment (equipment calls for lots). In order to optimize contradictory goals like tool utilization and in-time-delivery, we move from the traditional push principle to a pull principle. At Infineon, a conveyor based transport system running open cassettes is used that covers both interand intra-bay transportation. Its advantages are the (almost) infinite capacity and the immediate start of any transport job. In particular, the concept of direct tool delivery gives evidence that a continuous workflow is a key issue for production optimization. Based on this approach, we introduce an automation concept covering both equipment and transport control by structuring our production system into functional automation layers in order to reduce complexity. Based on this concept, tool and chamber specific equipment set-up as well as on-line parameter calculations (feedforward, feedback) have been implemented fab-wide.
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