Raimund Mueckstein

Imaging of terahertz surface plasmon waves excited on a gold surface by a focused beam

Surface plasmon polariton (SPP) waves formed near a tightly focused THz beam on a metallic surface are detected by an integrated sub-wavelength aperture THz near-field probe. The probe allows mapping the electric field pattern of the SPP wave and tracking the SPP propagation from the center of the focal spot. The SPP nature of the observed wave is confirmed by time-resolved measurements. Analysis of the detected patterns leads to an explanation of how THz SPP waves can be detected by the integrated sub-wavelength aperture probe.

Modelling of surface waves on a THz antenna detected by a near-field probe

We have modelled the experimental system based on the sub-wavelength aperture probe employed in our previous work for terahertz (THz) surface plasmon wave imaging on a bowtie antenna. For the first time we demonstrate the accuracy of the proposed interpretation of the images mapped by the probe. The very good agreement between numerical and experimental results proves that the physical quantity detected by the probe is the spatial derivative of the electric field normal component. The achieved understanding of the near-field probe response allows now a correct interpretation of the images and the distribution of the electric field to be extracted. We have also carried out the first assessment of the probe invasiveness and found that the pattern of the surface plasmon wave on the antenna is not modified significantly by the proximity of the probe. This makes the experimental system an effective tool for near-field imaging of THz antennas and other metallic structures.

Near-Field Analysis of Terahertz Pulse Generation From Photo-Excited Charge Density Gradients

Excitation of photo-current transients at semiconductor surfaces by subpicosecond optical pulses gives rise to emission of electromagnetic pulses of terahertz (THz) frequency radiation. To correlate the THz emission with the photo-excited charge density distribution and the photo-current direction, we mapped near-field and far-field distributions of the generated THz waves from GaAs and Fe-doped InGaAs surfaces. The experimental results show that the charge dynamics in the plane of the surface can radiate substantially stronger THz pulses than the charge dynamics in the direction normal to the surface, which is generally regarded as the dominant origin of the emission.

Mode interference and radiation leakage in a tapered parallel plate waveguide for terahertz waves

To exploit tapered parallel plate waveguides for broadband terahertz (THz) spectroscopy, the impact of the waveguide geometry on transmission of terahertz pulses is investigated experimentally. We find that the approximation of single transverse electro-magnetic mode propagation is insufficient for describing the observed behavior. The TE02 mode plays a particularly important role. The mode composition, however, can be controlled by the gap between the waveguide plates, which affects the main loss mechanism, radiation leakage, and group velocity for the TE02 mode. Balancing the waveguide loss and coupling efficiencies results in an optimal gap for the tapered waveguide.

Near-field probe mapping of the THz electric field distribution on metallic surfaces

We demonstrate for the first time the accuracy of the interpretation of images detected by the sub-wavelength aperture near-field THz probe, which enables mapping the distribution of THz electric field on antennas and metallic surfaces.

Near-field imaging of terahertz plasmon waves with a subwavelength aperture probe

Visualization of terahertz (THz) plasmons with local probes allows studying ultra-fast plasmonic phenomena in the time domain. We demonstrate that the integrated sub-wavelength aperture near-field probe can be used to map THz surface plasmon waves in space and time with high resolution. Using experimental near-field observations of plasmon waves formed on a metallic surface by tightly focused THz pulses and of standing plasmon waves in THz antennas, we show that this probe detects the spatial derivative of the electric field rather than the plasmon field itself. The understanding of the coupling mechanism provides a framework for interpretation of near-field images.

Surface plasmon waves for broadband THz spectroscopy

In this presentation we discuss application of broadband terahertz (THz) time-domain spectroscopy for probing subwavelength (micrometer) size objects. The problem of weak coupling between THz waves and sub-wavelength objects, which limits the use of THz spectroscopy to large samples, is mitigated by employing surface plasmon waves. In one implementation, THz surface waves, excited on a broad-band planar THz bow-tie antenna, are used to enhance the interaction with a small particle placed on the antenna surface. The surface field distribution near the particle is mapped with an integrated sub-wavelength aperture THz near-field probe. We demonstrate that imaging and probing of the subwavelength size dielectric particles (TiO2 and SrTiO3) can be realized using the enhanced THz field between the antenna and the probe. We also discuss THz wave confinement using two sharp metallic needles. We demonstrate that in the near-field region of the needle tips, the electric field of THz pulses is concentrated to a volume smaller than (10 μm)3 without limiting the THz pulse bandwidth. Application of both methods for high spatial resolution imaging and spectroscopy will be discussed.