Oleg Cojocari

Resonant-tunnelling-diode oscillators operating at frequencies above 1.1 THz

We present resonant-tunnelling-diode (RTD) oscillators operating at the fundamental frequency of 1111 GHz. We show that our RTDs and RTD oscillators have much room for further improvement of their parameters and for further increase of their operating frequencies. The operating frequencies of several THz should be achievable with RTD oscillators. Our study also shows that operation of RTDs beyond the relaxation-time limit at THz frequencies should be possible. RTD oscillators under study are extremely compact (less than a square millimeter) room-temperature sources of coherent cw THz radiation. Such sources should enable plenty of real-world THz applications.

High-frequency nonlinear characteristics of resonant-tunnelling diodes

The nonlinear response of resonant-tunnelling diodes (RTDs) is analysed theoretically at high frequencies (HFs), which are far above the diode’s tunnel-relaxation-time limit. We show that the HF I-V curve in this regime is substantially different from the static one. The calculated static and oscillation characteristics of a HF RTD oscillator are in good agreement with our measurement results. Our RTD model is applicable to RTDs working at THz frequencies.

Operation of resonant-tunnelling oscillators beyond tunnel lifetime limit

The tunnel lifetime of the electrons in the quantum well of a resonant-tunnelling diode (RTD) is usually assumed to be imposing an inherent fundamental limitation on the operating frequencies of RTD oscillators. Here, we experimentally demonstrate that one can overcome the limitation by heavy doping of the RTD collector. We present RTD oscillators with the fundamental oscillation frequency up to a factor of 3 above the tunnel lifetime limitation. Our results indicate that the inherent frequency limitations of RTDs should lie far above the state-of-the-art frequency of the contemporary RTD oscillators.

A novel system for systematic microwave noise and DC characterization of terahertz Schottky diodes

An automated system is developed to evaluate a large number Schottky diodes for terahertz applications with respect to their dc and noise characteristics using a highly sensitive noise measurement technique for one port devices. An extensive RF switching matrix allows noise characterization of one port devices at selected frequency points over a bandwidth from 2 to 8 GHz. The measurement principle also accounts for the impedance mismatch between the system and the device under test (DUT). Furthermore, the setup includes an automated three-axis nanopositioning system capable of consecutively contacting many Schottky diodes arranged in a honeycomb array. The highly accurate positioning of the DUT allows to create reproducible contacts with the diodes using electrochemically etched whisker tips. The smooth contacting procedure enables several hundred contacts with the same whisker tip. With this system, we evaluate the statistical distribution of dc and noise parameters of Schottky diodes with an anode diameter of 1 /spl mu/m within one honeycomb chip. The system helps in optimizing the production parameters of Schottky diodes for terahertz frequencies.

Operation of resonant-tunnelling-diode oscillators beyond tunnel-lifetime limit at 564 GHz

We present resonant-tunnelling-diode (RTD) oscillators, which are operating at frequencies up to 564 GHz. Due to heavy doping of the collector side of our diodes, the oscillators are operating beyond the tunnel-lifetime (τ) and relaxation-time (τrel) limits of RTDs. At 564 GHz we achieve ωτ≈1.2 and ωτrel≈2.6, the highest previously reported value of ωτ at frequencies >150 GHz was ≈0.6. Our study indicates that operating frequencies of RTD oscillators could be significantly increased and RTDs should be capable of operating at frequencies of several THz.

Micromachined split-block Schottky-diode mixer for 600 GHz

This paper presents simulation results for a 600 GHz micromachined waveguide mixer with integrated horn antenna, a hybrid-integrated planar Schottky diode and IF filter structures on a quartz substrate. In addition to the simulated electrical performance we show approaches for the manufacturing of the split-block mixer by both micro-mechanical milling and silicon etching technology.

600 GHz GaAs Schottky diode mixer in split-block technology

In this paper we present measurement results of a micro-machined split-block waveguide mixer for 600 GHz. The mixer implements a planar GaAs Schottky diode on a quartz substrate. Two different diode designs used for this mixer design are compared. The overall performance of the waveguide mixer is demonstrated by measurements of the mixer in a quasi-optical setup at 600 GHz. In this setup we are able to measure single sideband conversion losses of 9-14 dB at 600 GHz and voltage responsivities of 421-1690 mV/mW.

A Flexible Phase-Insensitive System for Broadband CW-Terahertz Spectroscopy and Imaging

We present a compact, robust, and flexible continuous-wave (CW) terahertz system, ideally suited for both imaging and high-resolution broadband spectroscopy. The setup employs an incoherent detection scheme: A photomixer transmitter is combined with a zero-bias Schottky diode on the receiver side. The useable bandwidth extends to 1500 GHz, with a spectral resolution on the 10 MHz level. In proof-of-principle measurements, we apply our setup to imaging of objects within a paper envelope as well as transmission and reflection-mode spectroscopy, taking advantage of the high spectral resolution of the terahertz source and the broad bandwidth and efficiency of the Schottky receiver.

Highly responsive planar millimeter wave zero-bias schottky detector with impedance matched folded dipole antenna

A compact highly responsive planar zero-bias Schottky detector is proposed for uni-planar and low-cost fabrication. Various zero-bias Schottky diodes are investigated, in particular the optimization of impedance matching by the antenna design itself. The realized folded dipole based detector demonstrates an outstanding system voltage responsivity of 7,079 mV/mW at 86 GHz without lenses or pre-amplification.

THz Autocorrelators for ps Pulse Characterization Based on Schottky Diodes and Rectifying Field-Effect Transistors

When operating Schottky diodes and rectifying field-effect transistors in the saturation regime, where they show a sublinear response to incident THz power, they can be used as fast autorcorrelators yielding information on the pulse envelope. We report on autocorrelation measurements at 3.41 THz of high-power THz pulses for determination of the pulse duration and pulse structure. By fringe-resolved measurements, the THz frequency of the pulse is also obtained. We develop a theoretical model for the rectification process and compare the performance of an antenna-coupled Schottky diode to a large-area field-effect transistor rectifier. While the Schottky diode saturates earlier and can therefore be used for autocorrelation measurements at lower input power, antenna-less large-area field-effect transistors can be used for highest power levels-even at free electron lasers-and turn out to be very robust.