We present an optoelectronic mixer for the terahertz (THz) frequency-domain based on an iron-doped InGaAs layer integrated in a plasmonic microcavity. We show that this structure, under 1550-nm-wavelength illumination, allows for more than 70% absorption efficiency in a 220 nm-thin InGaAs absorber and very high Roff/Ron >1000. It leads to THz mixers driven by 1550-nm lasers showing conversion loss as low as ~30 dB at 300 GHz. Therefore, this design is very promising for application as receivers in high-data-rate wireless telecom, in cw-THz spectrometers, or in photonic-senabled THz spectrum analyzers.
Terahertz non-destructive testing offers a highly attractive solution for inline testing of electrode thicknesses in battery production for electric vehicles. Measuring systems with high spectral bandwidths are required to address the thin layers of typically less than 100 μm. In addition, multiple measuring heads are desired at different location on a production line to ensure adequate control at high throughput. We solve this by means of a highly scalable photonic terahertz radar. Its measuring principle is based on frequency-modulated continuous wave technology in conjunction with two-color laser radiation. The number of measuring heads can be easily scaled through the use of laser amplifiers. Another advantage of a photonic continuous wave system is the simple possibility of distributing fiber-coupled measuring heads over long fiber lengths of even more than 100 m. In this article, we show the potential of the system concept by the implementation of an 8-channel system and demonstration of relevant thickness measurements.
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.
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.
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 fiber-coupled terahertz (THz) time-domain spectrometer with a record bandwidth of 10 THz. This significant increase of 3.5 THz in bandwidth compared to state-of-the-art spectrometers is enabled by improved photoconductive antennas (PCAs). In particular, we show that THz absorption in the polar InP:Fe substrate of the standard InGaAs PCAs is the main limiting factor for the bandwidth. To overcome this issue, we have developed a wafer-scale substrate transfer-process for the thin photoconductive InGaAs layer from InP:Fe to low loss, high resistivity float-zone (HRFZ) silicon. The resulting photoconductive membrane (PCM) antennas combine the advantages of ultrafast iron-doped InGaAs photoconductors as efficient THz emitters and receivers with nearly absorption-free and spectrally flat transmission of HRFZ-Si up to 17 THz.
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 π 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.
Photonic integrated circuits (PICs) are one of the key enablers for beyond 5G networks. A novel generation of fully integrated photonic-enabled transceivers operating seamlessly in W- D- and THz-bands is developed within the EU funded project TERAWAY. Photonic integration technology enables key photonic functionalities on a single PIC including photonic up/down conversion. For efficient down-conversion at the photonic integrated receiver, we develop the first waveguide-fed photoconductive antenna for THz communications. Finally, we report on the experimental implementation of a fully photonic-enabled link operating across W- D- and THz-bands.
We demonstrate a fully photonic sub-THz communication link using a PIN photodiode (PD) emitter and an optimized photoconductive antenna (PCA) as a heterodyne receiver. The novel receiver comprises an iron-doped indium gallium (InGaAs) PCA on a silicon lens and passive radio frequency (RF) circuitry, all packaged into a fiber-coupled module. A 3-dB-bandwidth of 11 GHz for the intermediate frequency was measured. We analyzed the capabilities of the receiver in a wireless communication link over a distance of 1 m with a PIN photodiode as the emitter. At a carrier frequency of 120 GHz, we demonstrate error free transmission for net data rates up to 10 Gbit/s with quaternary quadrature amplitude modulation (4-QAM) modulation.
We present THz-TDS with a low-noise single-cavity dual-comb GHz oscillator. This solid-state laser delivers two mutually coherent combs with 70-fs pulses, 1055-nm center wavelength, 110 mW per comb, and a repetition rate difference up to 100 kHz. In a proof of principle THz experiment, we direct the two combs onto two photoconductive antennae to efficiently generate and electrooptically sample the THz waveform. At a repetition rate difference of 37 kHz we achieve 40-dB dynamic range in a 2-s integration time for a spectral resolution of 2 GHz allowing to resolve absorption features up to 3 THz.
Photoconductive emitters and receivers are widely accepted as the best combination for applications requiring broadband and high dynamic range and are nowadays deployed in most commercially available systems. Novel laser sources with higher repetition rate and power levels are a promising route towards further improvements in this area. We present our first steps in this direction by combining state-of-the-art emitters and receivers with an ultra-stable commercial fs laser (MENHIR-1550 SERIES) at 1 GHz repetition rate as the optical source. The output of the laser is amplified and compressed by a commercial fiber amplifier setup. In this experiment, we use 17 mW as the probe beam and 30 mW as the pump beam with a pulse duration of 150 fs, as these are the best operation points for the emitter and receiver available. The emitter is based on iron doped InGaAs in a strip line geometry with an active region of 50 μm x 50 μm while a fiber coupled dipole antenna with a 10 μm gap is used as the receiver. We demonstrate a 1 GHz repetition rate terahertz time-domain spectroscopy (THz-TDS) system with a dynamic range of 73 dB and a bandwidth of 3.5 THz using state-of-the-art THz photoconductive emitter and receiver with a measurement time of 60 s. This result is part of a larger effort to understand the compromises to be realized in terms of repetition rate and average power to take photoconductive emitters and receivers to the next step in dynamic range enhancement.
We present novel photoconductive antennas (PCAs) compatible with 1550 nm excitation for terahertz (THz) time-domain spectroscopy (TDS). Rhodium (Rh) doped InGaAs grown by molecular beam epitaxy is used as the underlying photoconductor. Due to the advantageous combination of sub-picosecond carrier lifetime and excellent electronic properties, InGaAs:Rh based emitters feature an unprecedented emitted THz power of 637 µW. A record peak dynamic range of 110 dB is demonstrated with a THz TDS system using InGaAs:Rh based PCAs only. This sets a new benchmark for THz TDS systems operating at 1550 nm.
A new generation of photoconductive antennas (PCAs) compatible with 1550 nm excitation for terahertz time-domain spectroscopy is presented. Iron (Fe) doped InGaAs grown by molecular beam epitaxy is used as the underlying photoconductor. Due to the advantageous combination of ultrashort carrier lifetime and excellent electronic properties, InGaAs:Fe based PCAs increase the dynamic range for frequencies from 1 THz – 6 THz by more than 10 dB compared to the state-of-the-art.
We present a novel system architecture for coherent cw THz spectrometers. The system features 2.5 THz bandwidth with an acquisition rate of 58 Hz and provides full phase information without active phase modulation. We achieve passive amplitude and phase modulation at a fixed intermediate frequency by using a fast sweeping laser in combination with a static optical fiber delay: By heterodyning the incoming THz signal with the frequency-shifted optical beatnote on a photomixing receiver, we can extract amplitude and phase of the signal with a lock-in detector. To the best of our knowledge, this is the fastest coherent cw THz system demonstrated so far.
Time-domain spectroscopy (TDS) is the most prominent technique for fast acquisition of broadband terahertz (THz) spectra with update rates of several ten Hz up to kHz. However, fast tunable continuous-wave (cw) laser sources enable rapid acquisition of broadband THz signals without the well-known drawbacks of THz-TDS systems: mechanical delay lines and femtosecond pulse lasers. In this work, we make use of a fast tunable laser to demonstrate coherent continuouswave THz spectroscopy with unprecedented speed and bandwidth. The system features three different modes of operation exploiting both broad spectral bandwidth and high frequency resolution. In broadband mode, 2 THz-wide spectra with 800 MHz resolution can be acquired at a continuous update rate (UR) of 24 Hz. To our knowledge, this is the highest update rate of a broadband, phase-sensitive cw THz spectrometer. In high-speed mode, 200 GHz wide spectra are acquired with 800 MHz resolution at an UR of 120 Hz, ideal for high-speed spectroscopy of absorption lines. In high-resolution mode, frequency steps of 20 MHz and a scan range of 200 GHz allow for high-resolution gas spectroscopy. In broadband and high-speed mode, the peak dynamic range exceeds 65 dB for single shot measurements. More than 100 dB peak dynamic range and a 3 THz bandwidth are obtained after 7 min. averaging in the broadband mode. Due to its high update rates, in combination with high bandwidth and flexible operation modes, this system paves the way for industry-scale non-destructive testing based on cw THz technology.
We present terahertz (THz) time-domain spectroscopy (TDS) as a versatile tool for applications in non-destructive testing. Due to fiber-coupled THz systems, which exploit the advantages of mature telecommunication technology, THz- TDS is a promising tool for industrial process control. As an example, we demonstrate thickness measurements on multilayered plastic pipes by combining THz reflection measurements with a transfer matrix method for data evaluation. Furthermore, we show the potential of THz-TDS for time resolved 2D imaging. For this, we combine a photoconductive near-field probe with a commercially available fiber-coupled THz TDS system. Due to the coherent measurement scheme, which provides amplitude and phase information at each sampling point, in combination with an acquisition rate of 40 pulse traces per second, dynamic processes on the picosecond timescale can be monitored with unprecedented resolution. Exemplarily, we visualize the propagation of a THz-wave on the surface a of photoconductive THz emitter with a lateral resolution of 20 μm and sub-picosecond temporal resolution.
Within the last decade, photoconductive terahertz (THz) systems have become well-established tools in scientific laboratories and industrial R&D departments. In particular, the exploitation of telecommunication technology around 1.5 μm wavelength enabled this development. Continuous wave (cw) THz systems benefit especially from telecom technology since the required optical components are already available. As no femtosecond fiber-laser is needed, photonic integration may lead to extremely compact cw THz devices. We present a fully fiber-coupled cw THz system in combination with optimized InGaAs-based emitter and detector antennas and an optical phase modulator. This system can be employed as both, a highly precise spectroscopic tool and a high-speed measurement system for non-destructive testing. In addition, we present recent results on heterodyne cw THz spectroscopy. This is a prerequisite for future broadband, wireless telecommunication systems using THz carrier frequencies. The fiber-coupled heterodyne receiver is able to detect THz signals up to 1 THz with an intermediate frequency of 2.2 GHz. These are the highest values reported for photoconductive heterodyne receivers so far.
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