Daniel Schoenherr

Extremely broadband characterization of a Schottky diode based THz detector

A Schottky diode based module for direct detection of THz power provides fast measurement capabilities at room temperature. This paper presents the broadband spectral characterization of a Schottky diode in comparison to a Golay cell in the frequency range from 0.1 THz up to 2 THz.

Direct phase detection in continuous-wave photomixing THz systems

A novel system configuration allows the direct measurement of the THz phase in continuous-wave THz systems by electro-optic control of the THz phase. This increases the measurement speed by an order of magnitude as compared to standard phase-sensitive systems.

Characterization of THz emitter based on a high-speed p-i-n photodiode

This paper presents the characterization of a highspeed micro-sized p-i-n photodiode integrated with a circularly toothed log periodic planar antenna employed as a THz emitter. The optical system is based on 1.55 mum wavelength lasers, that allows the use of highly developed components of optical telecommunication.

Time-domain THz spectroscopy using acceptor-doped GaAs photoconductive emitters

We investigate the factors that control the emission of THz radiation by the photoconductive mechanism in Be acceptor-doped GaAs. These factors include the magnitude and direction of the applied bias, the optical pump power, the placement of the pump beam relative to the electrodes, the form of the electrodes and the Be doping level. We address the question as to what constitutes the best THz emitter and quantitatively compare these photoconductive emitters with two other classes of THz emitters, namely, electro-optic and surface-field emitters.

Investigation of p-GaAsSb as a THz Emitter

Our purpose was to determine if GaAsSb might be made to emit THz-frequency electromagnetic radiation. Our result is that, with a suitable electric field imposed and under illumination by ultrashort pulses of near-infrared radiation, GaAsSb indeed emits THz radiation. To the best of our knowledge this is the first report of high-temperature-grown GaAsSb acting as a THz source. THz emission has been observed both by incoherent (pneumatic Golay cell) and by coherent (time-domain spectroscopy using electro-optic detection) experimental configurations. Both simple Ag paint and photolithographically formed Au on Ti antenna structures have been found to produce THz radiation. We compare our results with those from the better-known THz emitter, GaAs. In the case of GaAs, THz radiation is emitted even in the absence of applied bias. This is not the case for GaAsSb. We conclude that the photoconductive mechanism dominates in the emission of THz radiation from epitaxial single-crystal GaAsSb. A further proposal is made that such materials can be used with nanostructured surfaces for special THz-emission characteristics.

Optical mixing in THz Schottky diodes

This paper presents the effect of optical mixing in zero-bias InGaAs Schottky detectors at THz frequencies. The excitation of ultrafast carriers by illumination with two 1.5 mum laser beams is verified. This proves the suitability of the diode for phase sensitive detection in CW THz systems.

Tunable Fabry-Perot THz filter with sub-wavelength grating mirrors

We utilize the reflectivity of one-dimensional metal gratings with sub-wavelength slits to realize mirrors for THz frequencies. Two of them are combined to a Fabry-Perot filter, which features the corresponding transmission bands. By appropriate choice of dimensions, the extraordinary transmission resonance of the sub-wavelength gratings can be superimposed with the Fabry-Perot peak. By varying the resonator length between the grating mirrors, the overlap of both transmission peaks can be controlled. This enables the tuning of the filter bandwidth. The theoretical analysis shows that continuous tuning of the filter bandwidth up to 30% is possible for a two mirror stage. For the performance of comparative measurements, an all-fiber continuous-wave THz system is used. The experimental results are in fairly well agreement with the theoretically predicted tuning properties.

Reliability Investigation of Photoconductive Continuous-Wave Terahertz Emitters

The lifetime of photomixers depends significantly on the operation conditions. High values of bias voltage and/or optical power increase the emitted terahertz power, but considerably decrease the operation lifetime. Interdigitated finger photomixers especially face the problem of thermal destruction due to high current densities. We present an Arrhenius analysis of low-temperature-grown-GaAs photomixers and link the photomixer lifetime to the accessible signal-to-noise ratio (SNR). With a coherent continuous-wave terahertz system designed for 1000 operating hours, we achieve an SNR of 30 dB at 1 THz using an integration time of 500 ms.

Phase stability considerations in coherent CW THz photomixing systems

Wavelength stabilized lasers and the delay stage configuration can significantly influence the phase stability in coherent CW photomixing THz-systems. Depending on the system design, the phase drift can exceed several wavelengths and therefore make the system unsuitable for precise measurements.

Fiber-coupled on-chip THz transceiver

Fast electro-optic modulation of the terahertz phase in continuous-wave photomixing terahertz systems allows the detection of terahertz phase and amplitude with a single sampling point per frequency. Applied to on-chip terahertz transceivers, the same principle reduces the parasitic noise and allows full fiber coupling. This increases stability and therefore the feasibility of optically driven on-chip devices as terahertz spectrometer.