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Photonic Terahertz Signal Analyzers

Periodic Reporting for period 4 - Pho-T-Lyze (Photonic Terahertz Signal Analyzers)

Reporting period: 2021-12-01 to 2022-11-30

For a long time, the Terahertz range (100 GHz-10 THz), situated at the junction between microwave electronics and infrared optics, has been considered as the “Terahertz gap” as there were only inefficient ways of signal generation and detection. Within the past two decades, the THz range has developed into a vivid research field with many -so far mostly academic- applications. Still, the lack of adequate and affordable component- and spectral analysis techniques hinders technological progress in this very attractive frequency range with large societal impact. Key applications of Terahertz components lie, e.g. in future communications, particularly 6 G and future standards, non-destructive testing, imaging and security as well as environment and medicine in form of spectroscopic identification of agents.. Spectrum analysis and vector network analysis are enabling technologies for component development throughout the microwave band. Spectrum analyzers measure the output power of a source or generator vs. frequency, allowing for metrological characterization. Vector network analyzers launch an electromagnetic wave with tunable frequency into a device under test and measure the transmitted and the reflected fraction. With an appropriate device model, individual component contributions can be analyzed and the measured transmission and reflection data be compared to simulations in order to determine design flaws. Vector network analyzers (VNAs) have to use frequency-extenders to reach into the THz frequency band. About seven different extender setups are required to cover the range of 100 GHz- 1.1 THz. Extenders become increasingly expensive the higher the THz frequency and are, therefore, only used by a handful of groups per country. Electronic spectrum analyzers face similar problems as VNAs.
The systems developed within this proposal will boost component development in the Terahertz range and foster Terahertz technology. The objectives are the development of inexpensive and powerful photonic THz characterization tools with extreme frequency coverage based on telecom-wavelength compatible photomixing technology as an alternative to conventional electronic concepts, namely:
1.) Photonic vector network analyzers with extreme frequency coverage, and
2.) Photonic Terahertz spectrum analyzers (PSAs)
Photonic Terahertz sources and receivers (WP1):
We tailored the photonic sources and receivers to the requirements of the project. In terms of sources, a continuous-wave (CW) photoconductor mixes a pair of lasers with a difference frequency of, say, 1 THz to a THz current by absorbing the beat note. An attached antenna converts the current into THz light. The detection process works inversely, i.e. a THz signal is mixed with a beat note of two lasers, resulting in a down-conversion to DC. The following work has been performed:
• The absorption within the source n-i-pn-i-p superlattice photomixer was increased by a factor of 2.7 by integrating them with an on-chip passive optical waveguide.
• We have employed in-house developed ErAs:In(Al)GaAs photoconductors in both continuous-wave and pulsed (time domain spectroscopy) systems.
• For the pulsed PVNA, we have also realized a transceiver, where source and detector are realized on the same chip.
• For CW receivers, we have developed Vivaldi endfire-coupled photoconductors that receive the signal right from a dielectric waveguide.
• For a free space version, we have developed fiber-pigtailed packages with an integrated silicon lens.
Photonic spectrum analyzer (WP2+WP6):
We have developed a concept based on a photoconductor where a laser beat note acts as local oscillator:
• We have demonstrated a free space system that covers <50 GHz to 1.15 THz with potential extension to several THz.
• A noise floor of -90 dBm/Hz has been achieved at 1 THz.
• We have developed systems with two different resolutions: a system with ~2 MHz resolution based on a standard THz photomixing system and a Hz-level resolution based on an electro-optic continuous-wave comb.
• Several demonstration examples have been carried out, e.g. analyzing the stability of several lines of a mode-locked THz pulse on the few Hz level, characterization of the emitted spectrum of several sources (backward wave oscillator, photomixer, WR-coupled electronic VNA head)
• On-chip transition via a WR hollow metal waveguide enables characterization of integrated circuits.
Pulsed Photonic Vector Network Analyzer (WP3):
• Demonstration of pulsed 1.5 port and two port photonic VNAs.
• The dynamic range of 35-47 dB at 10 Hz equivalent noise bandwidth in the 1.1-1.5 THz band exceeds that of state of the art two port electronic VNAs.
• Calibration techniques developed.
• Data analysis techniques developed for synchronous evaluation of transmission and reflection in order to improve the data quality of extracted material and device parameters.
• Several application examples demonstrated and published: a THz isolator, low loss and lossy dielectric plates, a Bragg mirror.
On-Chip PVNA and planar dielectric waveguide circuitry (WP4+WP5):
• On-chip dielectric waveguides with a frequency coverage from 0.45-1.5 THz developed
• Various components developed: splitters/combiners, bends, and coupling structures
• Transition to hollow metal WR waveguides demonstrated in order to enable on-chip measurements
On-Chip PVNA calibration and applications (WP7):
• Calibration techniques developed
• Data analysis and noise rejection techniques developed.
• Several waveguide-integrated devices were characterized: Whispering gallery mode resonator, on-chip Fabry-Pérot resonator, THz fiber Bragg grating
Progress beyond the state of the art achieved:
• Operational two port pulsed vector network analyzer
• On-Chip continuous-wave photonic vector network analyzer realized
• Operational photonic spectrum analyzer
• On-chip transition for photonic spectrum analyzer realized
• Developed data evaluation routines and noise reduction techniques show superior performance of the systems beyond the state of the art. Refractive index, absorption coefficient and sample thickness can be determined accurately without any mechanical measurement for both lossy and transparent materials.
• Several application examples of the newly developed systems demonstrated

Several actions for exploitation and dissemination were taken. The most important ones were:
• Demonstrating the effectiveness of the noise rejection techniques at the exhibition of the 47th conference on infrared, millimeter and Terahertz waves (2022, Delft).
• Nature communications publication: Versatility of the data evaluation routines demonstrated at the example of height difference measurements of only 50 nm using 0.6-0.8 THz bandwidth.
• Non-scientific publication (Scientia) of some results in order to reach a broader audience
• Two keynote presentations at international conference (44th and 47th international conference on infrared, millimeter and Terahertz waves)
• Successful application for an ERC Proof of Concept (GA 101057162, PhoSTer THz) in order to develop a pre-commercial photonic spectrum analyzer prototype
• 10 peer-reviewed journal publications (one further submitted, one in progress), 15 conference contributions, 2 published dissertations (two further ones submitted) and 2 granted patents (another one pending), ~10 invited talks by the PI or project members (unpublished)
Dielectric THz waveguides
1550 nm waveguide-coupled fully ballistic p-i-n photomixer with antenna
Logo of the ERC Starting Grant "Pho-T-Lyze", grant No. 713780
Image of the pulsed PVNA setup
Waveguide-coupled photoconductive receiver within a photonic VNA port
Vivaldi endfire antenna-coupled photoconductor for efficient coupling to a dielectric waveguide