Development and Applications of Terahertz Near-Field Microscopes for Surface Plasmon Imaging

The confined nature of surface plasmons (SPs) often imposes challenges on their experimental detection and makes specific near-field probes necessary. While various SP detection methods have been developed in the optical domain, only a few examples of SP imaging have been reported in the terahertz range. In this thesis, specific problems of current terahertz near-field detection systems have been addressed which has led to the development of two new SP imaging methods. In the first method, SP imaging is demonstrated using the integrated subwavelength aperture near-field probe. The photoconductive antenna inside the probe is sensitive to the SP electric-field despite the orthogonal spatial orientation between the antenna and the SP polarisation. This enables SP imaging directly on a metallic surface employing a photoconductive antenna. This unexpected sensitivity has been applied to SP imaging in two examples: first, the SP propagation has been imaged on a resonant THz bow-tie antenna and second, the SP excitation by a strongly focused terahertz beam directly on the metallic probe surface has been investigated. The second method presents an electro-optic micro-resonator for SP imaging. A micro-resonator structure has the potential to provide a better sensitivity and spatial resolution, as well as a lower level of invasiveness compared to bulk crystals, which are commonly used in terahertz near-field systems. The micro-resonator design is explained in detail and the impact of the micro-resonator geometry on the probe performance is discussed. This micro-resonator has then been fabricated and embedded into an electro-optic detection system. This detection system has been fully characterised with the focus on two functional units which are essential for its performance: a tapered parallel plate waveguide for broadband terahertz transmission and the balanced detector for noise reduction. The overall performance of the detection system has been evaluated for its use as a terahertz near-field microscope

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

Mapping the distribution of photo-currents responsible for generation of terahertz pulses at semiconductor surfaces

Photo-excited charge carriers at semiconductor surfaces generate pulses of terahertz (THz) radiation. By mapping the spatial distribution of the THz radiation in the near-field and the angular emission pattern in the far-field, we link the THz generation process to the photo-current direction. We find that inplane carrier dynamics play an important role and can even be the dominant source of THz radiation

Physico-chemical foundations underpinning microarray and next-generation sequencing experiments

Hybridization of nucleic acids on solid surfaces is a key process involved in high-throughput technologies such as microarrays and, in some cases, next-generation sequencing (NGS). A physical understanding of the hybridization process helps to determine the accuracy of these technologies. The goal of a widespread research program is to develop reliable transformations between the raw signals reported by the technologies and individual molecular concentrations from an ensemble of nucleic acids. This research has inputs from many areas, from bioinformatics and biostatistics, to theoretical and experimental biochemistry and biophysics, to computer simulations. A group of leading researchers met in Ploen Germany in 2011 to discuss present knowledge and limitations of our physico-chemical understanding of high-throughput nucleic acid technologies. This meeting inspired us to write this summary, which provides an overview of the state-of-the-art approaches based on physico-chemical foundation to modeling of the nucleic acids hybridization process on solid surfaces. In addition, practical application of current knowledge is emphasized

Hybridization thermodynamics of NimbleGen Microarrays

Background While microarrays are the predominant method for gene expression profiling, probe signal variation is still an area of active research. Probe signal is sequence dependent and affected by probe-target binding strength and the competing formation of probe-probe dimers and secondary structures in probes and targets. Results We demonstrate the benefits of an improved model for microarray hybridization and assess the relative contributions of the probe-target binding strength and the different competing structures. Remarkably, specific and unspecific hybridization were apparently driven by different energetic contributions: For unspecific hybridization, the melting temperature Tm was the best predictor of signal variation. For specific hybridization, however, the effective interaction energy that fully considered competing structures was twice as powerful a predictor of probe signal variation. We show that this was largely due to the effects of secondary structures in the probe and target molecules. The predictive power of the strength of these intramolecular structures was already comparable to that of the melting temperature or the free energy of the probe-target duplex. Conclusions This analysis illustrates the importance of considering both the effects of probe-target binding strength and the different competing structures. For specific hybridization, the secondary structures of probe and target molecules turn out to be at least as important as the probe-target binding strength for an understanding of the observed microarray signal intensities. Besides their relevance for the design of new arrays, our results demonstrate the value of improving thermodynamic models for the read-out and interpretation of microarray signals

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

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