## Tuesday, June 30, 2020

### Abstract-Collective modes and terahertz near-field response of superconductors

##### Zhiyuan Sun, M. M. Fogler, D. N. Basov, and Andrew J. Millis

We theoretically study the low-energy electromagnetic response of Bardeen-Cooper-Schrieffer–type superconductors focusing on propagating collective modes that are observable with terahertz near-field optics. The interesting frequency and momentum range is ω<2Δ and q<1/ξ, where Δ is the gap and ξ is the coherence length. We show that it is possible to observe the superfluid plasmons, amplitude (Higgs) modes, Bardasis-Schrieffer modes, and Carlson-Goldman modes using the terahertz near-field technique, although none of these modes couple linearly to far-field radiation. Coupling of terahertz near-field radiation to the amplitude mode requires particle-hole symmetry breaking, while coupling to the Bardasis-Schrieffer mode does not and is typically stronger. For parameters appropriate to layered superconductors of current interest, the Carlson-Goldman mode appears in the near-field reflection coefficient as a weak feature in the subterahertz frequency range. In a system of two superconducting layers with nanometer-scale separation, an acoustic phase mode appears as the antisymmetric density fluctuation mode of the system. This mode produces well-defined resonance peaks in the near-field terahertz response and has strong anticrossings with the Bardasis-Schrieffer and amplitude modes, enhancing their response. In a slab consisting of many layers of quasi-two-dimensional superconductors, realized for example in samples of high-Tc cuprate compounds, many branches of propagating Josephson plasmon modes are found to couple to the terahertz near-field radiation.

### First Dutch detector array ready for Gusto mission

Jessica Vermeer

The first detector array for NASA’s Gusto mission has passed pre-shipment review, reports the Netherlands Institute for Space Research (SRON). The array is being shipped to the University of Arizona, where it will be integrated into the balloon observatory. The Gusto mission is to measure emissions from cosmic material between stars. SRON and Delft University of Technology are developing three 8-pixel arrays for 4.7, 1.9 and 1.4 THz. The team has now delivered its first array — for the 4.7 THz channel.
NASA’s Galactic/extragalactic ULDB Spectroscopic Terahertz Observatory (Gusto) is a balloon observatory that will drift in Earth’s atmosphere for over 75 days, at the edge of space at 36 km altitude. The launch is scheduled for December 2021 from Antarctica. The observatory consists of a telescope of one meter in diameter and three observation instruments carried by an ultra-long duration balloon (ULDB). It contains three array receivers for electromagnetic radiation of 1.4, 1.9 and 4.7 THz.
Delivering the first array is the result of an international collaboration involving SRON, TU Delft, the University of Arizona and NASA. The 4.7 THz channel is the most challenging to realize because it requires the highest sensitivity and most precise pointing of the lens-antenna beam. The design, manufacturing, assembly and testing of the array were carried out at SRON, while the superconducting detectors were developed at TU Delft.

Gusto has three channels to map respectively ionized nitrogen (NI), carbon (CII), and oxygen (OI) emission lines in the spectrum of the interstellar medium — the material floating in between stars. This helps scientists to determine the life cycle of interstellar gas in our Milky Way, witness the formation and destruction of star-forming clouds and understand the dynamics and gas flow in the vicinity of the center of our Galaxy.

## Monday, June 29, 2020

### Abstract-A theoretical investigation on reciprocity-inspired wide-angle spectrally-selective THz absorbers augmented by anisotropic metamaterials

Mansoureh Mohammadi, Hamid Rajabalipanah,  Ali Abdolali,

https://www.nature.com/articles/s41598-020-67399-3

In this paper, a theoretical framework relying on the reciprocity theorem is proposed to accurately design a spectrally-selective THz superstrate-loaded metamaterial absorber (SLMA) exhibiting wide-angle feature. By leveraging high-order Floquet harmonics in a generalized transmission line model characterizing the conventional metamaterial absorbers (MAs), it is demonstrated that MAs suffer from impedance mismatch, especially at near grazing angles. From an impedance matching viewpoint, this major challenge is tackled in this paper via two different designs, exploiting a magneto-electric anisotropic Huygens' metamaterial and a multilayer dielectric structure at a certain distance over the MA plane. The numerical results corroborate well the theoretical predictions, elucidating that the proposed SLMA significantly broadens the angular performance of the MA up to near grazing angles (about 80°), where high absorptivity is still achieved in both principal planes. The deteriorating effect of diffraction modes has been comprehensively analyzed. In comparison to the previous wide-angle MA reports based on intricate particle geometries and brute-force optimizations, the proposed design features a straightforward semi-analytical algorithm, which can also be re-developed for microwave, mid-infrared, and optical frequency bands and for any type of MA element. The proposed SLMA would be very promising for various wavelength-selective applications such as sensors and imaging.

### Abstract-Backward terahertz difference frequency generation via modal phase-matching in a planar LiNbO3 waveguide

B. N. Carnio and A. Y. Elezzabi

https://www.osapublishing.org/ol/abstract.cfm?uri=ol-45-13-3657

The backward difference frequency generation process is used to produce narrowband terahertz radiation via modal phase-matching in a ${\mathrm{S}\mathrm{i}\mathrm{O}}_{2}\text{-}{\mathrm{L}\mathrm{i}\mathrm{N}\mathrm{b}\mathrm{O}}_{3}$-air planar waveguide. The ${\mathrm{T}\mathrm{M}}_{0}$ pump mode, ${\mathrm{T}\mathrm{E}}_{0}$ signal mode, and ${\mathrm{T}\mathrm{E}}_{0}$ or ${\mathrm{T}\mathrm{E}}_{2}$ idler modes are selected to satisfy the backward difference frequency generation phase-matching condition, thus allowing narrowband (i.e., $<100\phantom{\rule{thickmathspace}{0ex}}\mathrm{G}\mathrm{H}\mathrm{z}$ linewidth) terahertz radiation generation in the spectral range of 2.4–3.2 THz. To date, this is the first investigation of terahertz radiation generation in a waveguide via the modal phase-matched backward difference frequency generation process.
© 2020 Optical Society of America

## Sunday, June 28, 2020

### Abstract-Terahertz and Photonics Seamless Networks

Tetsuya Kawanishi

https://ieeexplore.ieee.org/document/9024641

Radio links using terahertz (THz) waves can provide high-speed wireless transmission whose bitrate is higher than 100Gb/s. Congestion of radio spectrum in THz bands (0.1-10 THz) is not so significant for the time being. However, multi- level modulation formats would be required in in THz bands as well as in conventional millimeter- wave or microwave bands, to increase spectral efficiency. We provide overviews on spectral efficiency and transmission capacity of high-speed radio links using THz bands. Reduction power consumption in radio equipment is also very important to reduce operation cost of networks. We describe a survey result on power consumption of short-distance wireless systems. The result implies that THz high-speed radio links would be useful to reduce power consumption per bit in transmission. However, transmission distances in THz systems would be shorter than a few kilometers due to attenuation in the air. Thus, we should rely on seamless networks consisting of THz radio- links and optical fibers, where devices developed for optical fiber transmission can be used to THz wave generation and detection.

### Abstract-Multi-foci metalens for terahertz polarization detection

Ruoxing Wang, Jin Han, Jianlong Liu, Hao Tian, Weimin Sun, Li Li, and Xianzhong Chen

 Schematic of the reflective THz multi-foci metalens for polarization detection. The ellipticity angle 𝜒$\chi$ and the direction of the major axis 𝛽$\beta$ of the incident and reflected polarization states are indicated in polarization ellipses.
https://www.osapublishing.org/ol/abstract.cfm?URI=ol-45-13-3506

We propose a reflective terahertz (THz) metalens with four focal points for polarization detection of THz beams. The metalens is composed of $Z$-shaped resonators with spatially variant orientations, a reflective gold layer, and a dielectric spacer between them. The polarization states of the focal points include left circular polarization, right circular polarization, an incident polarization state, and a polarization state whose major axis is rotated $\pi /4$ in comparison with that of the incident polarization. The handedness, ellipticity, and major axis of the polarization state can be determined based on the light intensities of the focal points. The uniqueness of the designed device renders this technique very attractive for applications in compact THz polarization detection and information processing.
Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.