This work presents an electrically-small lens that has been redesigned towards a flat interface. This way, the lens is easier to integrated, compared to an earlier introduced spherical core-shell lens concept. The lens is created from a single dielectric host material by conformally machining holes into the material. In this process, two artificial dielectric layers are created; The first layer is used for anti-reflection purposes, whereas the second is used to convert the spherical interface to a flat interface. The two layers enable the use of holes with lower aspect ratio drilling, compared to classical gradient-index lenses. The lens is designed to operate in the 140-170 GHz bandwidth, and a prototype with height of only 2.2 mm and diameter of 6.6 mm was fabricated and characterize. The prototype is small enough to fit in many integrated circuit packages. The flat lens was compared to a non-flat core lens in terms of pattern quality, return loss and dielectric loss, with only negligible performance degradation.

The thermal energy radiated by silicon wafers with different conductivities is characterized experimentally in the mm and sub-mm wave ranges. These samples are heated up, and the energy that they radiate thermally is captured by different horn antennas covering the frequency band between 75 and 500 GHz. The energy radiated (in the order of pW) and collected by the horn antennas is subsequently detected by zero bias Schottky diodes. The measured thermal radiated power agrees with the prediction from Planck's law for the highly doped wafers, corresponding to high conductivities. However, for low conductivities, the measurements show a descending pattern as a function of the frequency, which is not in line with expectations from Planck's law. Parallelly, we have developed a theoretical model providing a classical explanation of these results [1].

The presence of quasiparticles typically degrades the performance of superconducting microwave circuits. The readout signal can generate nonequilibrium quasiparticles, which lead to excess microwave loss and decoherence. To understand this effect quantitatively, we measure quasiparticle fluctuations and extract the quasiparticle density across different temperatures, readout powers, and resonator volumes. We find that microwave power generates a higher quasiparticle density as the active resonator volume is reduced and show that this effect sets a sensitivity limit on kinetic inductance detectors. We compare our results with theoretical models of direct microwave photon absorption by quasiparticles and conclude that an unknown, indirect mechanism plays a dominant role in quasiparticle generation. These results provide a route to mitigate quasiparticle generation due to readout power in superconducting devices.

Photoconductive antennas (PCAs) are promising candidates for sensing and imaging applications. In recent years, our group has investigated their properties under pulsed laser illumination in transmission using a time-domain (TD) Norton equivalent circuit. Here, we extend this analysis to the link between a photoconductive source and a receiver introducing for the latter a second TD Norton equivalent circuit. We also evaluate the transfer function of a dispersive quasi-optical (QO) link. Specifically, a field correlation approach based on the high-frequency techniques is used to evaluate the spectral transfer function between two bow-tie-based PCAs, including the QO link. The detected currents in the receiving circuit are reconstructed using stroboscopic sampling of the modeled THz pulses, equivalent to what is actually performed by THz TD systems. Both the amplitude and the waveforms of these currents are evaluated. The QO link is then experimentally characterized to validate the proposed methodology. The comparison between the simulations and the measurements is excellent.

Drude's description of the response of low-temperature gallium arsenide to optical pulse excitation is used to evaluate the components of a time-domain Norton equivalent circuit of a photoconductive antenna (PCA) source. The saturation of the terahertz (THz) radiated power occurring at large optical excitation levels was previously associated by the scientific community to radiation and charge screening of the bias. With the present circuit, we are able to model accurately the measured saturation as only due to the EM feedback from the antenna to the bias. The predicted THz radiated power is shown to match very accurately the measurements when the circuit is combined with an accurate description of the experimental conditions and the modeling of the THz quasi-optical (QO) channel.

State-of-the-art THz pulsed commercial systems operating over large bandwidth suffer from high dispersion or low radiation efficiency due to the poor coupling between the transmitter and receiver photoconductive antennas (PCAs). In this work, we present the fabrication and characterization of a leaky-lens PCA that has the potential to solve this problem. The presented PCA is based on a low-temperature grown gallium arsenide (LT-GaAs) membrane with a 1:15 bandwidth coverage (0.1-1.5 THz), where the frequency response is constant. In order to fabricate the PCA on an LT-GaAs membrane, a novel fabrication process is developed. This process is dramatically faster than previously used processes (∼1.5 h instead of ∼20 h). Furthermore, an experimental validation of the radiated power together with the comparison to a standard bow-tie-based PCA fabricated on the same LT-GaAs wafer is shown in this article. We show that the PCA source on the LT-GaAs membrane is more efficient due to the enhanced leaky wave radiation. The leaky-lens PCA stands out as a great candidate to improve the coupling efficiency in THz pulsed commercial systems, where the maximum laser power that can be used is limited by the dispersion in the optic fiber.

We present the design, fabrication and characterization of a broadband lossless matching layer for silicon lens arrays. The proposed matching layer is based on silicon frusta (truncated pyramids) on top of the lens array fabricated by means of laser ablation. This matching layer is advantageous over quarter-wavelength dielectric matching layers since it covers more than an octave of bandwidth. We compare the performance of this matching layer with the commonly-used parylene-C matching layer at the center of the targetted band (500 GHz) in a lens-antenna integrated system. We measure a 1.6 dB higher transmission of the proposed silicon frusta matching compared to the parylene-C matching layer.