Spectrum analyzers and spectrometers are essential for designing sources, analyzing material properties, layer structures and fingerprinting substances. We present an ultra-wideband, continuous-wave photonic receiver with kHz-level spectral resolution in the terahertz domain for both heterodyne and homodyne detection. Employed as a spectrum analyzer front end, it records the emitted spectrum of a source under test, assessing spectral purity, spectral shape and undesired frequency components. It outperforms state-of-the-art electronic systems in terms of frequency coverage and system cost with a competitive spectral resolution and noise floor on the few aW/Hz level at room temperature. It covers the important frequencies above 1.5 THz, yet commercially inaccessible, where sources like quantum cascade lasers operate. When combined with a comb-based photonic source, we demonstrate hetero- and homodyne spectroscopy over an unprecedented frequency range from below 100 GHz to 6.5 THz and a very low noise floor. Locking the photonic system to GPS enables tracing back the measured parameters to SI units, being of key importance for metrological applications. The presented setups offer the broadest continuous-wave frequency coverage to date, combined with a sharp spectral resolution, enabling diverse applications ranging from fast non-destructive testing, astronomic high-resolution spectroscopy, to frequency-modulated RADAR.

In this article, we present an alternative sampling approach for continuous-wave terahertz homodyne systems that overcomes limitations regarding the measurement of dispersive samples of currently used techniques. The wavelength-dependent phase-delay mirrors, which were developed for this work, induce a frequency-dependent phase shift of up to pi/2. This technique allows sampling of the THz-field, by tuning the laser sources in such a way that a constant frequency difference is maintained and the center frequency is shifted. In our configuration, the phase shift between transmitter and receiver arms depends only on the center frequency of the lasers. This allows for replacing the movement of a delay stage with variation of the center frequency to capture a THz trace. Consequently, measurements are not constrained by the speed of the delay line anymore. Furthermore, this phase shift is unaffected by differences in path length within the setup and does not require phase modulators. Prior simulations show that these mirrors achieve a phase shift up to pi in the C-band for a difference frequency of 280 GHz, which could be confirmed by our measurements. We successfully demonstrated the first application by measuring sample thickness.

Terahertz wireless communications is an increasingly interesting research topic due to the high demand for un-allocated channels and high data rates. Photonic solutions have shown great potential in this field. However, most photonics assisted THz links so far have employed optoelectronics only on the transmit side. Thus, the full potential of photonic THz communication has not been utilized yet. Here, we introduce optoelectronics also on the receive side by using a photoconductive antenna based heterodyne THz detector. This allows down-conversion of data signals from the W-, D-, and THz-band to the baseband using a laser beat signal as local oscillator. Using electromagnetic modeling, we designed passive radio frequency structures and a receiver package to handle high intermediate frequency output signals. In a homodyne spectroscopic setup, the receiver shows a frequency response superior to state-of-the-art photoconductive antennas due to an improved photoconductive material. In a heterodyne testbed, the receiver exhibits a large intermediate frequency bandwidth of 11 GHz and a conversion gain of -47 dB. This enabled us to employ the receiver in a fully photonic wireless link at sub-terahertz and terahertz frequencies together with a PIN photodiode emitter. We achieved error-free transmission of 4-QAM signals with gross data rates up to 12 Gbit/s at carrier frequencies up to 320 GHz. This work shows the huge potential of optoelectronic receivers for THz wireless communications and enables the exploration of full photonic THz links.

Broadband terahertz spectroscopy is a valuable analytical tool in science and a promising technology for industrial non-destructive, non-contact testing, e.g. thickness measurements of thin dielectric layers. Optoelectronic conversion using photomixers is a widespread approach for coherent terahertz spectroscopy. State-of-the-art spectrometers consist of discrete, fiber-based components, leading to complex and costly setups. In cost-sensitive applications, this prevents the use of these spectrometers. We developed a terahertz spectrometer based on a dedicated photonic integrated circuit and commercial electronic integrated circuits to overcome these limitations. The photonic subsystem can be connected to commercial tunable lasers and provides the optical signal processing to drive the photoconductive antennas. The electronic subsystem includes the required drivers, analog signal processing, and data acquisition. Combined, the system measures $10 \times 16 \times 7.5$ cm3 only. We compare both subsystems individually and as a whole to state-of-the-art lab equipment in terms of spectral performance and measurement speed. Due to the flexibility in measurement modes, the integrated system can be adapted to specific measurement tasks, e.g. 2.8 THz-wide spectra within 0.5 s for high-speed, or 3.6 THz bandwidth with >80 dB dynamic range in less than 3 minutes for high-precision. This is the first realization of a terahertz spectrometer based on photonic and electronic integration rivaling state-of-the-art and non-integrated commercial spectrometers. This approach paves the way for compact and economic terahertz systems, providing access to terahertz technology for cost-sensitive sectors in research and industry.

Optoelectronic continuous-wave THz generation by photomixing is an established tool for table-top spectrometers inside and outside of scientific laboratories. We present newly developed emitters based on ultrafast PIN photodiodes featuring 3 dB overall increased output power. At frequencies beyond 3 THz, the output power is up to >18 dB higher due to the reduced substrate thickness, i.e., internal absorption. As a result, a record bandwidth of 5.5 THz is achieved.

Mining has led to soil contamination with heavy metals in numerous regions. This contamination not only affects large-scale food cultivation but also concerns individuals who grow food in their gardens. Hence, there is a critical requirement to develop efficient and reliable methods for studying and monitoring plant exposure to these harmful substances. Currently, researchers rely on complex and time-consuming laboratory procedures that entail destructive sampling to examine plant exposure to heavy metals. This method is expensive and not suitable for extensive monitoring. In [1], the capability of continuous wave (CW) Terahertz spectroscopy to differentiate between contaminated and uncontaminated leaves of the plant species Arabidopsis halleri has already been demonstrated. In this study, the varying levels of Cadmium and Zinc concentrations in the leaves of this plant are analyzed.

Future mobile networks require phase arrays operating in the millimeter-wave and Terahertz range. RF photonics combines the best of both worlds to address the challenges of broadband beam-steerable RF antenna arrays. However, dielectric rod antennas are required for efficient emission from a multi-element semiconductor array.

We report for the first time to the best of our knowledge experimental results of +/- 20 degrees steering angle of a 210 GHz beam emitter from a 1x4 planar antenna photodiode array coupled through a 1x4 dielectric rod waveguide antenna array.

We experimentally demonstrate for the first time a photonic integrated circuit comprising an optical frequency comb generation unit and an optical injection locking unit, as part of a fully packaged photonic wireless sub-THz receiver module.

Terahertz wireless communications is an increasingly interesting research topic due to the high demand for un-allocated channels and high data rates. Photonic solutions have shown great potential in this field. However, most photonics assisted THz links so far have employed optoelectronics only on the transmit side. Thus, the full potential of photonic THz communication has not been utilized yet. Here, we introduce optoelectronics also on the receive side by using a photoconductive antenna based heterodyne THz detector. This allows down-conversion of data signals from the W-, D-, and THz-band to the baseband using a laser beat signal as local oscillator. Using electromagnetic modeling, we designed passive radio frequency structures and a receiver package to handle high intermediate frequency output signals. In a homodyne spectroscopic setup, the receiver shows a frequency response superior to state-of-the-art photoconductive antennas due to an improved photoconductive material. In a heterodyne testbed, the receiver exhibits a large intermediate frequency bandwidth of 11 GHz and a conversion gain of −47 dB. This enabled us to employ the receiver in a fully photonic wireless link at sub-terahertz and terahertz frequencies together with a PIN photodiode emitter. We achieved error-free transmission of 4-QAM signals with gross data rates up to 12 Gbit/s at carrier frequencies up to 320 GHz. This work shows the huge potential of optoelectronic receivers for THz wireless communications and enables the exploration of full photonic THz links.

Sources for high frequencies in the THz range are of interest in both precision measurements in spectroscopy as well as high carrier frequencies and bandwidth in next-generation communication. Optically generated THz frequencies allow for broad tunability based on telecom technologies. Optical frequency division via an optical frequency comb has the potential to generate ultralow phase-noise THz sources. We present a system that is capable of generating tunable THz frequencies of up to 10 THz at 0.5 THz/s scan rates, phase-locked to a common comb spectrum providing absolute frequency calibration. We show results of combining the photonic source with an electronic receiving system in the waveguide band WR 2.2 (330-500 GHz).

We present a novel waveguide-integrated photoconductive antenna as heterodyne receiver for THz wireless links. The receiver features 20 dB higher conversion gain and 4 GHz higher IF bandwidth compared to the best top-illuminated photoconductive heterodyne receiver demonstrated so far. With this detector, we are able to extend the current data rate records for full photonic THz links by a factor of 5.5 and 4 to 68 Gbit/s and 40 Gbit/s at 120 GHz and 300 GHz, respectively.

As continuous wave (cw) THz spectroscopy advances rapidly, its high potential for sensing and non-destructive testing is becoming increasingly apparent. This is demonstrated in two recent developments: First, I will present our novel terahertz receivers for frequency-domain spectroscopy based on rhodium-doped InGaAs grown by molecular beam epitaxy. These new devices provide a peak dynamic range of 130 dB, an 18 dB improvement over the state of the art. Secondly, I will present a compact terahertz spectroscopy system with a measurement rate of 1 kHz, made possible by a single photonic integrated circuit that acts as the optical driver engine. These new results demonstrate the excellent performance and flexibility of frequency-domain spectroscopy, paving the way for compact and task-specific terahertz systems for science and industry.

Photonic components offer unique advantages for future broadband wireless communication links, especially for carrier frequencies above 100 GHz. We report on the latest developments regarding photonic mixers as transmitters and receivers for use in such wireless links. As an illustration, we present a real-time link at 110 GHz across 30 m free space exploiting both photonic technology from optical C-band and modems from state-of-the-art E-band technology.

In THz homodyne systems, optical delay lines are the key to time-resolved measurements but they come with a high cost and complexity. They also limit the application of the system in industrial environment, due to their sensitivity to vibrations. Another important point is the scanning speed, for which the mechanical delay line sets severe limitations. Frequency scanning-based systems need a change in THz frequency to recover phase information. Furthermore, there is a tradeoff between phase sensitivity and necessary tuning range. This tradeoff is based on the difference in the length of transmitter and receiver arm in the setup. With our approach, we can introduce a controllable phase shift at 280 GHz by frequency tuning of both lasers. For that purpose, chirped mirrors were designed and introduced into a standard continuous wave Terahertz homodyne system, in order to induce a variable phase shift. In our chirped mirror-based configuration, the phase shift between both optical modes depends on the center frequency of the lasers. Thus, moving the delay stage can be replaced by variation of the center frequency in order to record a THz trace. This means that the measurements are no longer limited by the speed of the delay line. This phase shift is independent of the path length difference in the setup and does not need phase modulators. Simulations show, that these mirrors may achieve a phase shift up to pi inside the C-Band for a difference frequency of 280 GHz. To confirm the calculated behavior of the chirped mirrors, initial characterization measurements were performed. We modified an existing delay stage-based THz system to include the chirped mirrors in front of the receiver. This enables the direct comparison while keeping all other parameters constant.

For the first time, we present photoconductive, continuous wave (cw) terahertz (THz) detectors for 1550 nm excitation based on rhodium- (Rh) doped indium gallium arsenide (InGaAs) grown by molecular beam epitaxy. Compared to iron- (Fe) doped material, the Rh-doped InGaAs shows higher carrier mobilities with similar carrier lifetimes. Therefore, these photoconductive antennas outperform InGaAs:Fe-based detectors by a factor of 10 in terms of responsivity and noise-equivalent-power (NEP) while maintaining the same bandwidth. In a homodyne spectrometer configuration, we achieve a record peak dynamic range (DNR) of 132 dB, which constitutes an improvement of 20 dB.

The deployment of fifth-generation (5G) open radio access network (RAN) introduces new opportunities for wireless communication, particularly in the context of x-Haul wireless links. This paper presents a comprehensive demonstration of how photonic-generated sub- THz at 120 GHz was utilized for wireless backhaul within the 5G open RAN platform. The experimental setup highlighted the feasibility and effectiveness of establishing a sub- THz link as the backhaul, marking a significant advancement in this emerging field. Through extensive performance analysis, we evaluated 5G network's throughput, achieving approximately 957 Mbps for downlink with UDP and 269 Mbps with TCP. In the case of uplink, both UDP and TCP exhibited comparable speeds, both exceeding 80 Mbps. This work marks a significant achievement in advancing the development of photonic-generated sub- THz wireless links, holding the potential to enhance wireless transport capabilities, particularly in the context of advanced 5G networks and beyond..

We report on novel continuous wave (cw) photoconductive terahertz (THz) receivers based on rhodium (Rh) doped indium gallium arsenide grown by molecular beam epitaxy. The Rh-doped material exhibits outstanding charge carrier mobility up to 1800 cm2/Vs while maintaining ultrashort carrier lifetimes between 0.26 and 1.5 ps. The photoconductive antennas (PCAs) made from this material exhibit a THz responsivity significantly improved by a factor of 20 to 100, which overcompensates their slightly higher noise levels. In a homodyne coherent THz spectroscopy setup, these novel receivers enable us to measure THz spectra with a record peak dynamic range up to 125 dB and a spectral bandwidth of 4.5 THz. This is an improvement of 13 dB over the previous record with InGaAs:Fe-based PCAs and leads to improved accuracy and measurement speed in cw-THz spectroscopy.

We present novel PIN photodiode (PD) continuous wave (cw) terahertz (THz) emitters with an increased responsivity and reduced substrate thickness compared to the state-of-the-art. Our improved devices feature up to 4 dB higher output power below 500 GHz with maximum power of -0.53 dBm at 115 GHz and strongly reduced THz absorption of the substrate for frequencies above 3 THz. The latter enables us to measure coherent cw THz spectra with a record bandwidth of 5.5 THz, for the first time, which is 1 THz (22%) more than the state-of-the-art.

The pollution of soil and the resulting contamination of plants with heavy metals is a growing concern for, food safety and human health in industrialized countries. Until now, the exposure of plants is studied by means of complex procedures in the laboratory, in which a part of the plant is destroyed. In this study, we demonstrate a non-destructive method using Terahertz spectroscopy to detect heavy metal accumulation in leaves of A. halleri.