Markus Walther - Academia.edu (original) (raw)

Papers by Markus Walther

Applied Physics Letters, 2013

Optical Terahertz Science and Technology, 2007

Journal of Molecular Liquids, 2011

Journal of Applied Physics, 2011

Applied Physics Letters, 2008

Optics Express, 2011

Using terahertz near-field imaging we experimentally investigate the resonant electromagnetic fie... more Using terahertz near-field imaging we experimentally investigate the resonant electromagnetic field distributions behind a split-ring resonator and its complementary structure with sub-wavelength spatial resolution. For the out-of-plane components we experimentally verify ...

Optics express, Jan 20, 2017

As an alternative to metallic resonators, dielectric resonators can increase radiation efficienci... more As an alternative to metallic resonators, dielectric resonators can increase radiation efficiencies of metasurfaces at terahertz frequencies. Such subwavelength resonators made from low-loss dielectric materials operate on the basis of oscillating displacement currents. For full control of electromagnetic waves, it is essential that dielectric resonators operate around their resonant modes. Thus, understanding the nature of these resonances is crucial towards design implementation. To this end, an array of silicon resonators on a quartz substrate is designed to operate in transmission at terahertz frequencies. The resonator dimensions are tailored to observe their low-order modes of resonance at 0.58 THz and 0.61 THz respectively. We employ a terahertz near-field imaging technique to measure the complex near-fields of this dielectric resonator array. This unique method allows direct experimental observation of the first two fundamental resonances.

We have applied a newly developed transient terahertz time- domain spectrometer to study the temp... more We have applied a newly developed transient terahertz time- domain spectrometer to study the temporal development of the dynamics of photogenerated carriers in semiconductor materials. The study presented here include semi-insulating (SI) and low-temperature-grown (LT) GaAs. By measuring the detailed shape of a subpicosecond electrical field pulse (THz pulse) transmitted through the sample at a time T after excitation with

2013 38th International Conference on Infrared Millimeter and Terahertz Waves, 2013

ABSTRACT Optimal focusing of surface plasmon polaritons in the center of a metal disc illuminated... more ABSTRACT Optimal focusing of surface plasmon polaritons in the center of a metal disc illuminated by radially polarized terahertz pulses is demonstrated. Due to the cylinder symmetrical structure surface plasmons can be excited along the entire circumference, which interfere constructively in the center of the disk forming a sharp frequency-depended focal spot. We map the field distribution on the disk by THz near-field microscopy and compare our result to numerical simulations. For comparison, behavior under linearly polarized THz illumination is characterized. Furthermore, first results of semiconducting plasmonic lenses are presented.

In this work we report two designs of subwavelength fibers packaged for practical terahertz wave ... more In this work we report two designs of subwavelength fibers packaged for practical terahertz wave guiding. We describe fabrication, modeling and characterization of microstructured polymer fibers featuring a subwavelength-size core suspended in the middle of a large porous outer cladding. This design allows convenient handling of the subwavelength fibers without distorting their modal profile. Additionally, the air-tight porous cladding serves as a natural enclosure for the fiber core, thus avoiding the need for a bulky external enclosure for humidity-purged atmosphere. Fibers of 5 mm and 3 mm in outer diameters with a 150 \mu m suspended solid core and a 900 \mu m suspended porous core respectively, were obtained by utilizing a combination of drilling and stacking techniques. Characterization of the fiber optical properties and the near-field imaging of the guided modes were performed using a terahertz near-field microscopy setup. Near-field imaging of the modal profiles at the fiber output confirmed the effectively single-mode behavior of such waveguides. The suspended core fibers exhibit broadband transmission from 0.10 THz to 0.27 THz (larger core), and from 0.25 THz to 0.51 THz (smaller core). Due to the large fraction of power that is guided in the holey cladding, fiber propagation losses as low as 0.02 cm-1 are demonstrated. Low-loss guidance combined with the core isolated from environmental perturbations make these all-dielectric fibers suitable for practical terahertz imaging and sensing applications.

CLEO/Europe - EQEC 2009 - European Conference on Lasers and Electro-Optics and the European Quantum Electronics Conference, 2009

ABSTRACT Metamaterial structures are investigated by a novel terahertz near-field imaging techniq... more ABSTRACT Metamaterial structures are investigated by a novel terahertz near-field imaging technique. Our approach maps the inplane electric and out-of-plane magnetic field vectors and thereby provides a detailed investigation of various metamaterial designs and their electromagnetic near-field response.

Metamaterials: Fundamentals and Applications II, 2009

ABSTRACT Experimental investigations of the microscopic electric and in particular the magnetic n... more ABSTRACT Experimental investigations of the microscopic electric and in particular the magnetic near-fields in metamaterials remain highly challenging and current studies rely mostly on numerical simulations to characterize their resonant microscopic behavior. Here, we present a terahertz imaging technique, which allows us to measure the amplitude, phase and polarization of the electric near-fields in the vicinity of the resonant structures in planar metamaterials. By our approach we are able to trace the electric field vectors close to the structures after their excitation on sub-ps time scales with sub-wavelength spatial resolution. From the measured in-plane electric vector fields we are able to reconstruct the out-of-plane magnetic field vectors. As a result we obtain a comprehensive microscopic picture of the electromagnetic response in metamaterials.

Since the first freely propagating THz pulses have been used to conduct spectroscopic investigati... more Since the first freely propagating THz pulses have been used to conduct spectroscopic investigations in the far-IR, THz time-domain spectroscopy (THz-TDS) has evolved into a versatile tool for the study of a variety of different sample materials. However its application to the investigation of biological molecules in this highly interesting spectral regime started only recently, and the work presented in this thesis represents one of the first investigations along this line. We present characteristic THz spectra of individual biomolecular systems in the crystalline and amorphous phase. In particular the correlation between the observed low-frequency modes and molecular structure and association was investigated. Our results show that the mode structure in the THz region represents a unique fingerprint for the molecular structure and conformation, e.g. allowing to distinguish related compounds, to discriminate between different isomers and to even observe structural phase-transitions...

IEEE Transactions on Terahertz Science and Technology, 2013

Workshop on Specialty Optical Fibers and their Applications, 2013

Journal of Molecular Liquids, 2015