Search

The rapid progress in semiconductor technology has vastly boosted the development of terahertz sources and receivers in terms of compactness, reliability, operation frequency, and output power. In this manuscript, we report on the latest achievements in terahertz sources and receivers and provide a comprehensive overview of their working principles and applications in THz systems.

The demand for THz detectors for beam diagnosis and alignment at THz generating accelerator facilities increases continuously especially for room temperature applications. The Zero-Bias Schottky Diode (ZBSD) and field effect transistor (TeraFET) based Terahertz (THz) detectors are well suited for both, signal power detection at DC as well as Pulse shape diagnostics by down-conversion at intermediate frequencies (IF). The limited signal strength due to the roll-off factor of the low pass filter characteristic of the detectors at higher THz frequencies requires wide-band amplifiers to enhance the IF signal from a few µW to nW well above the noise floor of the subsequent post detection electronics. Using external amplifiers would enhance the signal losses even further due to additional connectors and rf-cable losses and degrade the signal to noise ratio (SNR). In order to maximize the SNR, we propose to have an on-chip amplifier integrated in the detectors intermediate frequency (IF) circuit in the same housing. In this work, we present the design and parametric analysis of components for transition to an IF circuit, which will be integrated in the ZBSD and TeraFET on chip with amplifier. A rigorous design analysis has been done to find the optimal parameters for wide-band operation in order to enhance the detector’s resolution to capture pulses in the pico-second range with the help of fast post detection electronics.

Semiconductor-based broadband room-temperature Terahertz (THz) detectors are well suitable for beam diagnosis and alignment at accelerator facilities due to easy handling, compact size, no requirement of cooling, direct detection and robustness. Zero-Bias Schottky Diode (ZBSD) based THz detectors are highly sensitive and extremely fast, enabling the detection of picosecond scale THz pulses. This contribution gives an overview of direct THz detector technologies and applications. The ZBSD detector developed by our group has undergone several tests with table-top THz sources and also characterized with the free-electron laser (FEL) at HZDR Dresden, Germany up to 5.56 THz. In order to understand the rectification mechanism at higher THz frequencies, detector modelling and optimization is essential for a given application. We show parametric analysis of a antenna-coupled ZBSD detector by using 3D electromagnetic field simulation software (CST). The results will be used for optimization and fabrication of next generation ZBSD detectors, which are planned to be commissioned at THz generating FEL accelerator facilities in near future.

There continues to be a need for an in-situ sensor system to monitor the engine oil of internal combustion engines. Engine oil needs to be monitored for contaminants and depletion of additives. While various sensor systems have been designed and evaluated, there is still a need to develop and evaluate new sensing technologies. This study evaluated Terahertz time-domain spectroscopy (THz-TDS) for the identification and estimation of the glycol contamination of automotive engine oil. Glycol contamination is a result of a gasket or seal leak allowing coolant to enter an engine and mix with the engine oil. An engine oil intended for use in both diesel and gasoline engines was obtained. Fresh engine oil samples were contaminated with four levels of glycol (0 ppm, 150 ppm, 300 ppm, and 500 ppm). The samples were analyzed with THz-TDS and converted to frequency domain parameters of refractive index and absorption coefficient. While both parameters showed potential, the absorption coefficient had the best potential and was able to statistically discriminate among the four contamination levels.

We present a novel technique for the detection terahertz pulses generated by femtosecond lasers. It features a Hz-level resolution and does not require a mechanical delay stage or a second femtosecond laser. It works by opto-electronically downconverting the individual frequency components composing the terahertz pulse. Unlike other techniques, it does not require an external spectrum analyzer or a lock-in amplifier for its implementation, although it can also be implemented using them. We believe this technique is a valuable tool to study the ultimate frequency stability of terahertz pulsed systems and the phase noise of the femtosecond lasers that drives them.

Signals in the terahertz range are mainly wireless, waveguide-bound or on-chip signals. To measure their spectral power, linewidth and harmonics, usually spectrum analyzers are used. Here we present a photonic spectrum analyzer for waveguide-bound signals for the frequency range from 450 GHz to 1.05 THz with the possibility to extend its operation to on-chip signals. Therefore, a photonically generated local oscillator frequency is used to downconvert the signal under test within a photoconductor. A Vivaldi antenna feeds the signal from a dielectric waveguide to the photoconductor. This allows the measurement of any signal that can be transported via a dielectric waveguide. We show details on the transition from a rectangular metallic hollow waveguide to a dielectric waveguide and spectral measurements of a rectangular metallic hollow waveguide source. The same transition can also be used to attach metallic waveguide-coupled ground-source-ground probes in order to characterize on-chip sources.

Terahertz (THz) domain is quickly developing with various applications such as beam diagnostics at particle accelerators, spectroscopy, communications, space science, etc. However, often requiring fast intermediate frequency (IF) electronics. We present the design of a double mirror stub (DMS) based a planar broadband bias tee having an isolation port S 31 with 14.45 GHz bandwidth below −10 dB and S 33 with 13.1 GHz bandwidth above −2 dB. CST simulation and measured results are in very good agreement. The bias tee will be a part of a new generation of on-chip THz detectors based on zero-bias Schottky diode and high electron mobility field effect transistor (HEMT).

We use a high-sampling rate terahertz (THz) homodyne spectroscopy system to estimate thoracic movement from healthy subjects performing breathing at different frequencies. The THz system provides both the amplitude and phase of the THz wave. From the raw phase information, a motion signal is estimated. An electrocardiogram (ECG) signal is recorded with a polar chest strap to obtain ECG-derived respiration information. While the ECG showed sub-optimal performance for the purpose and only provided usable information for some subjects, the signal derived from the THz system showed good agreement with the measurement protocol. Over all the subjects, a root mean square estimation error of 1.40 BPM is obtained.

Field effect transistor (FET) based terahertz rectifiers are promising candidates for sensitive, room-temperature operated high speed (THz) detectors, e.g. in communication, medical, biochemical, security, quality control applications, or beam diagnostic applications at particle accelerators. This paper investigates the equivalent circuit in the intermediate frequency band from 0.1 to 65 GHz with S-parameter measurements in order to enable implementation with high speed post detection electronics. Preliminary results are obtained by de-embedding On-Wafer measurements and compared with theoretical expectations from hall measurements and a simplified equivalent circuit. The knowledge of the channel behavior and impedance is mandatory for impedance matching to IF circuitry in advanced detector designs.

Room temperature terahertz (THz) detectors based on Field effect transistors (FETs) and Zero-bias Schottky diodes (SD) are prominent members for the temporal-spatial characterization of pulses down to the picosecond scale generated at particle accelerators. Comparative study of in house developed THz detectors both at higher and intermediate frequency (IF) is carried out using table top THz systems and commercially available sources. In this paper, we present high frequency and intermediate frequency (IF) response of Gallium Arsenide (GaAs) FET and Zero-bias Schottky diode THz detectors. The IF results obtained are helpful for understanding and designing of optimized IF circuit with broader bandwidth.

Over the last few decades several types of Terahertz (THz) detectors have been developed to maturity, paving the way for various potential applications such as diagnostics of THz generation at particle accelerators. An important class are zero-biased Schottky diode THz detectors that are frequently applied at accelerator facilities for operation at room temperature. Zero-biased Schottky diode THz detectors are having lower noise compared to biased ones due to the absence of shot noise. Here we demonstrate the sensitivity of Schottky detectors using a commercial continuous wave photomixing THz system as source. Both, a commercially available as well as a research-grade compact quasi-optical detector with improved video bandwidth are compared from 0.05 to 1.2 THz in terms of sensitivity. At 1 THz, the research grade quasi optical detector shows 7 dB higher dynamic range than the commercial one.