Peter Haring Bolivar

Observation of Bloch oscillations in a semiconductor superlattice

We report unambiguous experimental evidence for electron Bloch oscillations: The transient four-wave mixing signal of a biased semiconductor superlattice shows a periodic modulation with a time constant expected from Bloch oscillation theory. The oscillation frequency can be tuned over 400% with the applied electric field. The electron performs up to four oscillation cycles before the coherence is lost.

Label–free THz sensing of genetic sequences: towards ‘THz biochips’

THz–wave–based approaches for the label–free characterization of genetic material are described. Time–resolved THz spectroscopic analysis of genetic sequences (polynucleotides) demonstrate a distinct complex refractive index in the THz frequency range as a function of the binding state of the analysed DNA sequences. By monitoring THz signals, one can thus infer the binding state of oligo– and polynucleotides, enabling the label–free determination of the genetic composition of target polynucleotides by sensing their binding to known probe molecules. Here we review integrated THz sensing array developments exhibiting high sensitivity and single–base mutation detection capabilities. Recent achievements using functionalized biosensing arrays of high–Q resonators are illustrated.

Electrical percolation characteristics of Ge2Sb2Te5 and Sn doped Ge2Sb2Te5 thin films during the amorphous to crystalline phase transition

Percolation effects are studied in phase change materials used for optical and electrical nonvolatile memory applications. Simultaneous measurements of the electrical resistivity and optical reflectivity allow experimental assessment of the percolation behavior associated with the mixed amorphous and crystalline phases in this class of chalcogenide materials. Dynamic analysis of the isothermal amorphous to crystalline phase transition in as-deposited amorphous Ge2Sb2Te5 and Sn doped Ge2Sb2Te5 thin films are performed. A significantly earlier onset and saturation for the change of electrical resistivity in comparison to the optically measured phase transition is observed. This behavior reflects a significant influence of percolation on the electrical properties of this class of materials, leading to a nonlinear relationship between crystallization degree and electrical resistivity. This nonlinearity is associated with the formation of highly conducting crystalline paths in a lower conducting amorphous matr...

Characterization of Ge-Sb-Te thin films deposited using a composition-spread approach

Abstract Ge–Sb–Te Thin films for rewritable phase change optical data storage applications were deposited by magnetron sputtering using a composition-spread approach. The deposition took place in an UHV chamber by cosputtering of three magnetron cathodes equipped with pure Ge-, Sb- and Te-targets in a DC–argon plasma. To investigate the influence of the chemical composition of the phase change material on its optical properties, films with lateral compositional gradients of up to 30 at.% were deposited. The composition, binding states and structure of the films were investigated by EPMA mappings, XPS, AES, RHEED and GIXRD on plain Si wafers, whereas the phase change velocity of Ge–Sb–Te was determined on Si–Al–SiO 2 stacks. The change between amorphous and crystalline phases of the films was induced and characterized with a static tester consisting of an optical microscope with an integrated high power laser diode. The change in reflectivity induced by the laser pulses was measured by a high sensitivity photo detector. Depending on the composition, the duration for the initial crystallization was determined between 220 and 500 ns, while the re-amorphization required between 20 and 120 ns. Structural analyses proved the existence of two crystalline phases with cubic and hexagonal structure in the initialized films.

Pulsed optical THz technology: generation and amplification of coherent THz radiation

State-of-the-art coherent THz radiation sources are reviewed and inversion-less amplification mechanisms are presented, which are applicable to a wide variety of optically impulsively excited THz emitters. The amplification schemes will be experimentally demonstrated and fundamental limitations and prerequisites discussed, stressing analogies and differences to standard amplification by stimulated emission.

Three-dimensional polymer/metal-based resonators for THz-biosensing

In this work, we discuss a new concept for THz-resonators for biosensing. In comparison to planar waveguide approaches, this three-dimensional resonator offers higher Q-factors and improved capabilities for high-throughput measurements. It is based on a metalized periodic groove structure in a parallel-plate configuration.

Integrated THz biomolecular sensors for DNA

A THz wave based approach for the label-free characterization of genetic material is presented. Time-resolved THz spectroscopic analysis of genetic sequences (polynucleotides) demonstrate a clearly distinct complex refractive index in the THz frequency range as a function of the hybridization state (hybridized/denatured) of the analysed DNA sequences. By monitoring THz signals one can thus infer the binding state of oligo- and polynucleotides, enabling the label-free determination of the genetic composition of polynucleotides. Recent advances in the development of high sensitivity integrated THz sensors reaching femtomol detection levels and single base mutation detection capabilities are reviewed.

THz imaging in a Brewster-angle configuration: characterization of thin oxide coatings for fuel cell applications

Time-resolved THz imaging for the incidence-angle dependent 3D tomographic characterization of layered structures is presented. We illustrate the capabilities of the developed system on multi-layer ceramic samples used for solid oxide fuel cells. Diverse methods for determining unknown refractive indices are discussed. The significant influence of the angle of incidence of a THz imaging system on the measured signal is demonstrated, which can be exploited especially in Brewster-angle configurations to enhance the capabilities of any THz tomography system.

Time-resolved detection of far-field THz-radiation patterns: spatially restricted coherence of surface field THz emitters

The measurements of the spatial coherence of a macroscopic ensemble of carriers excited coherently by femtosecond laser pulses is presented. The spatial coherence of the excited ensemble is derived from time- and spatially resolved measurements of the far-field THz-emission pattern. The analysis concentrates on surface field emitters, which are widespread broadband sources of coherent THz-radiation. We find that these emitters are fully spatially coherent for emission frequencies up to 1.6 THz, above which frequency the spatial coherence starts to decrease. For frequencies above 2.5 THz the spatial coherence of the emitter is limited to one THz wavelength.

Photonic engineering of nonlinear-optical properties of hybrid materials for efficient ultrafast optical switching (PHOENIX)

The phoenix project aims to develop all-optical switches based on the combination of inorganic and organic materials in hybrid devices. We present first results in developing low-loss ring resonators fabricated in silicon-on-insulator (SOI) technology, with Q-factors as high as 125.000, and losses of α≈3.5dB/cm in the ring.