Thomas Kaiser

Complete modal decomposition for optical fibers using CGH-based correlation filters

The description of optical fields in terms of their eigenmodes is an intuitive approach for beam characterization. However, there is a lack of unambiguous, pure experimental methods in contrast to numerical phase-retrieval routines, mainly because of the difficulty to characterize the phase structure properly, e.g. if it contains singularities. This paper presents novel results for the complete modal decomposition of optical fields by using computer-generated holographic filters. The suitability of this method is proven by reconstructing various fields emerging from a weakly multi-mode fiber (V approximately 5) with arbitrary mode contents. Advantages of this approach are its mathematical uniqueness and its experimental simplicity. The method constitutes a promising technique for real-time beam characterization, even for singular beam profiles.

Real-time determination of laser beam quality by modal decomposition

We present a real-time method to determine the beam propagation ratio M2 of laser beams. The all-optical measurement of modal amplitudes yields M2 parameters conform to the ISO standard method. The experimental technique is simple and fast, which allows to investigate laser beams under conditions inaccessible to other methods.

Measuring the spatial polarization distribution of multimode beams emerging from passive step-index large-mode-area fibers

We measure the polarization state of each guided transversal mode propagating in step-index large-mode-area fibers (V≈4) using a correlation-filter based measurement technique in combination with a Stokes parameter measurement. The entire emerging beam, expressed in terms of a phase-dependent superposition of linearly polarized modes, demonstrates spatially varying polarization properties. By knowing the information about modal amplitudes and phase differences, full information about the optical field is available.

Fast M 2 measurement for fiber beams based on modal analysis

We report on a fast and experimentally easy technique for measuring the beam propagation ratio M(2) of light guided by optical fibers. A holographic filter enables us to determine amplitudes and phases of the excited fiber eigenmodes. The coherent superposition of modes allows the reconstruction of the optical field. With this information at hand, we are able to simulate the free-space propagation of the beam and to perform a virtual caustic measurement. Associated beam propagation ratios M(2) accurately agree with ISO-standard measurements.

Highly resonant and directional optical nanoantennas

Plasmonic nanoantennas permit many functional components for future generations of nanoscale optical devices. They have been intensively studied and means were devised to engineer their optical response. However, as a metal-based resonator, the low quality factor of a plasmonic antenna hinders its further applications. Here, we propose a novel design to improve the quality factor of a dipolar nanoantenna by combining it with plasmonic Bragg gratings. This specific antenna design can support extraordinary sharp resonances and highly directional emissivity. Therefore, it promises to achieve many novel applications, e.g., in the field of cavity quantum electrodynamics where the strong coupling regime for light and matter comes in reach.

Enhancing resonances of optical nanoantennas by circular gratings.

Optical plasmonic antennas allow for localizing and enhancing light at the nanoscale. To enhance the application opportunities of optical antennas, their quality factor needs to be substantially improved. Here, we numerically and experimentally demonstrate that the resonance of a dipolar metallic disc antenna can be enhanced by a circular grating that obeys the Bragg condition. The supporting grating effectively collects energy from an extended spatial domain and guides it spectrally-selected into the central antenna, leading to a significantly enhanced field intensity at resonance. Accordingly, the quality factor of the antenna is enhanced by at least five times. The approach can be applied to other plasmonic systems, hence constituting an important ingredient to a future plasmonic tool box.

Impedance generalization for plasmonic waveguides beyond the lumped circuit model

We analytically derive a rigorous expression for the relative impedance ratio between two photonic structures based on their electromagnetic interaction. Our approach generalizes the physical meaning of the impedance to a measure for the reciprocity-based overlap of eigenmodes. The consistence with known cases in the radiofrequency and optical domain is shown. The analysis reveals where the applicability of simple circuit parameters ends and how the impedance can be interpreted beyond this point. We illustrate our approach by successfully describing a Bragg reflector that terminates an insulator-metal-insulator plasmonic waveguide in the near-infrared by our mpedance concept.

Characterization of a circular optical nanoantenna by nonlinear photoemission electron microscopy

AbstractWe report on the investigation of an advanced circular plasmonic nanoantenna under ultrafast excitation using nonlinear photoemission electron microscopy (PEEM) under near-normal incidence. The circular nanoantenna is enhanced in its performance by a supporting grating and milled out from a gold film. The considered antenna shows a sophisticated physical resonance behaviour that is ideal to demonstrate the possibilities of PEEM for the experimental investigations of plasmonic effects on the nanoscale. Field profiles of the antenna resonance for both possible linear polarizations of the incident field are measured with high spatial resolution. In addition, outward-propagating Hankel plasmons, which are also excited by the structure, are measured and analysed. We compare our findings to measurements of an isolated plasmonic nanodisc resonator and scanning near-field optical microscopy measurements of both structures. All results are in very good agreement with numerical simulations as well as analytical models that are also discussed in our paper.

Detection of mode conversion effects in passive LMA fibers by means of optical correlation analysis

To qualify passive fibers for (high power) laser beam delivery, different experimental approaches (interferometric, heterodyn, M2, ...for beam characterization at fiber output are under test in the community. Measurement of the individual strength of different components (eigenmodes) contained in the superposition at the fiber output in dependence for example on bending radius seems to be very promising. This can be done by means of optical correlation filters based on DOEs. For a standard telecommunication fiber SMF-28, operated at 633 nm, this could be demonstrated earlier1 by us. Here we present experimental results for quantitative proof of LP modes in LMA fibers as well as in SMF-28 fibers by means of such correlation filters, and demonstrate potential and limitations of this approach.

Metamaterials in waveguide geometries

Most state-of-the-art optical metamaterials are thin layers of functionalized plasmonic nanostructures of certain geometries. To enhance the understanding how such systems can be used in waveguide geometries, we investigate the interaction of a dielectric waveguide with the plasmonic nanostructures. As a model system we choose a double cut-wire on top of a slab waveguide consisting of a 350 nm thick layer of Si3N4 (n=2.0) on a fused silica substrate. Since the waveguide is terminated by air, its refractive index profile is highly asymmetric. Hence, the fundamental TM mode possesses a relatively strong electric field component in propagation direction. This configuration allows to probe plasmonic eigenmodes of the double cut-wire.