Dominik Walter Vogt

Multi-core fibers are pivotal for increasing core density and enabling space-division multiplexing to meet the growing demand for high data transmission rates beyond the limits of single-mode fibers. In the terahertz regime, bundled polymer fibers offer a high-capacity alternative for chip-to-chip links. This work introduces a gap-controlled approach to modulating coupling in solid multi-core terahertz fibers, supported by analytical models and simulations. Experimental validation using 3D-printed cyclic olefin copolymer 1 × 3 fibers demonstrates precise control from −3 dB power splitting to inter-core isolation exceeding −35 dB—sufficient for high-order QAM formats. Rectangular core geometry enables polarization multiplexing, effectively doubling channel capacity, while near-field scanning provides what is believed to be the first spatially resolved visualization of evanescent coupling dynamics. The monolithic design improves integration density by 280% compared to discrete fiber bundles, establishing a scalable framework for high-density terahertz photonic systems.

The growing demand for compact, high-performance terahertz (THz) photonic systems calls for waveguide designs that balance bandwidth, loss, and integration flexibility. Hybrid photonic crystal (HPC) waveguides, formed by silicon pillar lattices between parallel metallic plates, enable single-mode, wide-bandwidth (>100 GHz) propagation with strong confinement and dispersion control for terahertz integrated systems. This work systematically compares square, hexagonal, and isosceles triangular lattice geometries under identical periodicity to optimize HPC waveguides for single-mode and low-loss propagation.Notably, the widest photonic bandgap does not necessarily translate into the broadest fundamental mode bandwidth. Both square and hexagonal lattices achieve wide low-loss bandwidths of approximately 140 GHz with propagation loss below 0.05 dB/mm, while the triangular lattice operates at higher frequencies with a narrower ∼ 70 GHz bandwidth. Near-zero dispersion spans ∼ 100 GHz for square and hexagonal lattices, compared to ∼ 40 GHz for the triangular design. Considering current fabrication limitations of hybrid structures, the square lattice emerges as the preferred option for fabrication. Overall, the proposed methodology provides a framework for co-design of bandwidth, attenuation, and dispersion under fabrication constraints, while demonstrating the flexibility of HPC structures for THz photonic systems.

Sub-wavelength thin disc terahertz microresonators have been proven to exhibit unprecedented quality factors, exceptionally low losses and extensive evanescent fields. We investigate the microresonators' potential for ultra-thin film sensing using poly(methyl methacrylate) (PMMA) films for a compelling proof-of-concept. Highly sub- wavelength thin films ($\lambda$/10 000) with thicknesses of tens of nanometers were deposited atop the thin disc microresonator, each causing a pronounced frequency shift and a marked reduction in the intrinsic quality factor; these differences were measurable using a standard continuous-wave terahertz spectrometer with an added terahertz microresonator frequency reference. This demonstration benchmarks the exceptional sensitivity of thin disc terahertz microresonators, and, to our knowledge, marks the first recorded use of a whispering-gallery mode resonator for ultra-thin film sensing in the THz domain. Our findings create exciting possibilities for future applications, such as detecting ultra-thin films of sophisticated analytes.

Second-order nonlinear optical processes allow the conversion of electromagnetic signals to a higher frequency domain. Such so-called "upconversion" processes have been of interest for the optical detection of terahertz signals for decades. To enhance the nonlinear interaction, many investigations have focused on using high quality resonators, which improve the upconversion efficiency but limit the bandwidth. Here, we present a hybrid scheme that takes advantage of the high optical quality of a crystalline resonator while allowing the upconversion of signals at multiple frequency bands. We observed upconversion of six THz channels with MHz bandwidth, centered around 0.157 THz, and separated by about 10 GHz. The highest photon conversion efficiency at 0.157 THz was observed to be 3 × 10⁻⁶ per milliWatt of optical pump power. By numerically analyzing the landscape of nonlinear coupling in such resonators, we establish the conditions under which the demonstrated scheme can serve as an efficient multi-channel upconverter.

We demonstrate the capabilities of a novel frequency-domain terahertz spectrometer based on a comb-locked frequency synthesizer, which provides absolute frequency calibration. The inherent stability and repeatability of the scans allow for the combination of fast data acquisition with an average time-limited signal-to-noise ratio. We demonstrate kilohertz level frequency resolution in terahertz precision spectroscopy of ultra-high quality whispering-gallery-mode resonators. Spectra covering multiple free spectral ranges (>36 GHz) with sub-20 kHz resolution are acquired in 5 s. We analyze the coupling behavior and temperature tuning of single resonances and, for the first time, observe minute red and blue shifts of different mode families. The experimental results are supported with finite element simulations.

As the field of terahertz (THz) photonics advances, we present a monolithic gallium arsenide (GaAs) disk-shaped whispering gallery mode resonator that has potential as a component in THz nonlinear optics. GaAs is a material with significant optical nonlinearity which can be enhanced when the crystal is shaped into a microdisk resonator. A 4-mm-disk-resonator was fabricated using single-point diamond turning and was characterized to obtain a quality (Q) factor of 2.21k at ∼150 GHz and 1.41k at ∼300 GHz. We also demonstrated the blue-shifting of up to ∼0.3 GHz of the THz modes using a block of metal. This post-fabrication degree of freedom could be useful for phase-matching requirements for nonlinear optical processes, such as detection based on optical up-conversion of THz radiation. This proof-of-concept demonstration can pave the way for the implementation of a compact, tunable and efficient device which could be integrated into nonlinear photonic platforms for THz generation, manipulation and detection.

Terahertz (THz) technology is rapidly evolving, and the advancement of data and information processing devices is essential. Silicon THz microresonators provide perfect platforms to develop compact and integrated devices that could transform THz technology. Here we present a systematic study on the key figure of merit of silicon THz disc microresonators - the quality factor (Q-factor) - in dependence on the substrate’s resistivity. Our results show that the Q-factor depends linearly on the resistivity and a variation in resistivity from 10 kΩcm to 15 kΩcm changes the Q-factor from 50k to 76k at 0.6 THz. Moreover, we experimentally determine that the silicon material absorption is inversely proportional to the substrate’s resistivity. In general, the presented methodology is ideally suited to precisely measure the material absorption of low-loss materials in the THz domain, which is challenging using conventional THz spectroscopy.

Highly sensitive terahertz (THz) sensors for a myriad of applications are rapidly evolving. A widespread sensor concept is based on the detection of minute resonance frequency shifts due to a targeted specimen in the sensors environment. Therefore, high resolution continuous wave (CW) THz spectrometers provide powerful tools to investigate the sensors' performances. However, unpredictable yet non negligible frequency drifts common to widely used optoelectronic CW THz spectrometers based on photomixers limit the sensors' accuracy for sensing and metrology. Here, we overcome this deficiency by introducing an ultra-high quality (Q) THz microresonator frequency reference. Measuring the sensor's frequency shift relative to a well-defined frequency reference eliminates the unwanted frequency drift, and fully exploits the capabilities of modern optoelectronic CW THz spectrometers as well as THz sensors. In a proof-of-concept experiment, we demonstrate the accurate and repeated detection of minute resonance frequency shifts of less than 5MHz at 0.6THz of a THz microresonator sensor.

Artificial structures that exhibit narrow resonance features are key to a myriad of scientific advances and technologies. In particular, exploration of the terahertz (THz) spectrum—the final frontier of the electromagnetic spectrum—would greatly benefit from high-quality resonant structures. Here we present a new paradigm of terahertz silicon disc microresonators with subwavelength thickness. Experimental results utilizing continuous-wave THz spectroscopy establish quality factors in excess of 120,000 at 0.6 THz. Reduction of the disc thickness to a fraction of the wavelength reduces the losses from the silicon substrate and paves the way to unparalleled possibilities for light–matter interaction in the THz frequency range.

The terahertz spectrum provides tremendous opportunities for broadband gas-phase spectroscopy, as numerous molecules exhibit strong fundamental resonances in the THz frequency range. However, cutting-edge THz gas-phase spectrometer require cumbersome multi-pass gas cells to reach sufficient sensitivity for trace level gas detection. Here, we report on the first demonstration of a THz gas-phase spectrometer using a sub-wavelength thick ultrahigh-Q THz disc microresonator. Leveraging the microresonator’s ultrahigh quality factor in excess of 120,000 as well as the intrinsically large evanescent field, allows for the implementation of a very compact spectrometer without the need for complex multi-pass gas cells. Water vapour concentrations as low as 4 parts per million at atmospheric conditions have been readily detected in proof-of-concept experiments.