We report on a promising approach for the label-free analysis of DNA molecules using direct probing of the binding state of DNA with electromagnetic waves at THz frequencies. Passive THz resonator devices based on planar waveguides are used as sample carriers and transducers for THz transmission analysis. In comparison to a formerly used free-space detection scheme, this method provides a drastically enhanced sensitivity enabling analysis down to femtomol levels. We examine the potential of our approach on biologically relevant DNA samples and demonstrate the detection of single base mutations on DNA molecules.

Integrated THz technology for label-free genetic diagnostics

The valley degree of freedom of electrons in solids has been proposed as a new type of information carrier, beyond the electron charge and spin. The potential of two-dimensional semiconductor transition metal dichalcogenides in valley-based electronic and optoelectronic applications has recently been illustrated through experimental demonstrations of the optical orientation of the valley polarization and of the valley Hall effect in monolayer MoS2. However, the valley Hall conductivity in monolayer MoS2, a non-centrosymmetric crystal, cannot be easily tuned, which presents a challenge for the development of valley-based applications. Here, we show that the valley Hall effect in bilayer MoS2 transistors can be controlled with a gate voltage. The gate applies an electric field perpendicular to the plane of the material, breaking the inversion symmetry present in bilayer MoS2. The valley polarization induced by the longitudinal electrical current was imaged with Kerr rotation microscopy. The polarization was found to be present only near the edges of the device channel with opposite sign for the two edges, and was out-of-plane and strongly dependent on the gate voltage. Our observations are consistent with symmetry-dependent Berry curvature and valley Hall conductivity in bilayer MoS2.

Electrical control of the valley Hall effect in bilayer MoS2 transistors.

Ultrashort terahertz pulses derived from femtosecond table-top sources have become a valuable tool for time-resolved spectroscopy during the last two decades Until recently, the pulse energies and field strengths of these pulses have been generally too low to allow for the use as pump pulses or the study of nonlinear effects in the terahertz range In this review article we will describe methods of generation of intense single cycle terahertz pulses with emphasis on optical rectification using the tilted-pulse-front pumping technique We will also discuss some applications of these intense pulses in the emerging field of nonlinear terahertz spectroscopy

/pdf/intense-ultrashort-terahertz-pulses-generation-and-1ewsfynvhb.pdf

Intense ultrashort terahertz pulses: generation and applications

Plasmonics provides a route to develop ultracompact optical devices on a chip by using extreme light concentration and the ability to perform simultaneous electrical and optical functions. These properties also make plasmonics an ideal candidate for dynamically controlling nonlinear optical interactions at the nanoscale. We demonstrate electrically tunable harmonic generation of light from a plasmonic nanocavity filled with a nonlinear medium. The metals that define the cavity also serve as electrodes that can generate high direct current electric fields across the nonlinear material. A fundamental wave at 1.56 micrometers was frequency doubled and modulated in intensity by applying a moderate external voltage to the electrodes, yielding a voltage-dependent nonlinear generation with a normalized magnitude of ~7% per volt.

https://science.sciencemag.org/content/sci/333/6050/1720.full.pdf

Electrically controlled nonlinear generation of light with plasmonics.

Characterization of semiconductor interfaces by second-harmonic generation

A comprehensive description of a modular optoelectronic measurement system for the characterization of high-frequency microelectronic devices and circuits is presented. Depending on application, specific techniques to generate, synchronize to, and detect high-frequency electric signals are combined covering a frequency range of more than three orders of magnitudes from 2 to 4000 GHz. We discuss on-chip electric-pulse generation by freely positionable photoconductive probes and by direct optical excitation of active devices. Alternatively, for measurements with external, electronically generated signals, the system is laid out to lock onto periodic signals of arbitrary frequency employed as clock signal for the circuit under test. With respect to detection, the following approaches are discussed: sampling with freely positionable electrooptic and photoconductive probe tips, and truly (probe-tip-free) all-optical testing based on the field-dependent optical nonlinearity of the circuit's substrate material. The probes are characterized concerning time resolution, linearity, sensitivity, and invasiveness. We demonstrate with a number of examples that the combination of the various modules allows one to optimize the approach to a specific testing problem. Measurements of the linear and nonlinear behavior of active and passive devices as well as circuits are presented. The electric field, respectively potential, is measured locally (point measurements) or in its spatial distribution (field mapping) both in the near and far field.

Optoelectronic on-chip characterization of ultrafast electric devices: Measurement techniques and applications

Polarization-selective optical second-harmonic generation (SHG) is applied to monitor nonuniform polarization distributions in centrosymmetric media modulated by an external dc electric field. Two different systems are investigated: first, optical SHG from a Au–Si(100) Schottky Barrier is shown to be a direct probe of the electric field polarity near such a metal–semiconductor interface. Second, a poled polymer exhibits a strong anisotropic SHG signal under normal incidence, indicating a nonuniform field enhancement at the edges of the poling electrode. The results demonstrate the sensitivity of SHG to small symmetry perturbations. Consequences for device applications are discussed.

Optical second-harmonic generation as a probe of electric-field-induced perturbation of centrosymmetric media.

Static and high-frequency electric fields in silicon MOS and MS structures probed by optical second-harmonic generation

Electric field‐induced second‐harmonic generation (EFISHG) is shown to be a very powerful method for contactless sampling of high‐speed electrical signals on silicon millimeter‐wave devices. We present picosecond resolution EFISHG measurements of 2 GHz sinusoidal signals on a photoconductive silicon‐on‐sapphire antenna. We further demonstrate the application of the EFISHG technique for spatial mapping of free‐running microwave signals in silicon.

C. Ohlhoff

Papers

Optoelectronic on-chip characterization of ultrafast electric devices: Measurement techniques and applications

Optical second-harmonic generation as a probe of electric-field-induced perturbation of centrosymmetric media.

Static and high-frequency electric fields in silicon MOS and MS structures probed by optical second-harmonic generation

Optical second-harmonic probe for silicon millimeter-wave circuits