Jee-Ho Kim

Abstract:

Millimeter and sub-millimeter wavelength astronomical heterodyne receivers operating in the simplest doublesideband (DSB) mode down-convert both the upper and lower sidebands (USB/LSB) near the local oscillator (LO) frequency to the same intermediate-frequency (IF) band, causing spectral confusion. The down-converted USB and LSB signals can be separated using a sideband-separating (2SB) scheme, which requires passive components such as hybrid couplers, in-phase power splitters, and directional couplers, in addition to a pair of near-identical DSB mixers. Traditionally, these passive components are implemented using waveguide technology, making them bulky and increasingly difficult to fabricate with high precision at higher frequencies.

In this manuscript, we present a compact 2SB receiver concept. By rearranging the pair of DSB mixers and replacing these waveguide-based passive components with superconducting planar-circuit counterparts, we aim to substantially reduce the receiver size while potentially minimizing transmission losses. We demonstrate this approach by presenting the design of a compact 2SB receiver that continuously covers the Atacama Large Mm/sub-mm Array (ALMA) Band 6 and Band 7 frequency ranges, intended as a second-generation instrument for the Africa Millimetre Telescope (AMT).

Abstract:

ABSTRACT We analyse the stellar abundances of massive galaxies (log M*/M⊙ > 10.5) at redshift, z = 2, in the IllustrisTNG simulation with the goal of guiding the interpretation of current and future observations, particularly from JWST. We find that the effective size, Re, of galaxies strongly affects the abundance measurements: both [Mg/H] and [Fe/H] are anticorrelated with Re, while the relative abundance [Mg/Fe] slightly increases with Re. The α enhancement as tracked by [Mg/Fe] traces the formation time-scale of a galaxy weakly, and mostly depends on Re. Aperture effects are important: measuring the stellar abundances within 1 kpc instead of within Re can make a large difference. These results are all due to a nearly universal, steeply declining stellar abundance profile that does not scale with galaxy size – Small galaxies appear metal-rich because their stars live in the inner part of the profile where abundances are high. The slope of this profile is mostly set by the gas-phase abundance profile and not substantially modified by stellar age gradients. The gas-phase abundance profile, in turn, is determined by the strong radial dependence of the gas fraction and star-formation efficiency. We develop a simple model to describe the chemical enrichment, in which each radial bin of a galaxy is treated as an independent closed-box system. This model reproduces the gas-phase abundance profile of simulated galaxies, but not the detailed distribution of their stellar abundances, for which gas and/or metal transport are likely needed.

Abstract:

We present the design and performance analysis of waveguide-based superconductor–insulator–superconductor (SIS) mixers employing niobium titanium nitride (NbTiN) transmission lines and tunnel junctions comprising niobium/aluminium oxide (Nb/AlO_x/Nb) or niobium/aluminium nitride (Nb/AlN/Nb) topologies. These superconducting quantum devices are developed to enable wideband heterodyne operation for next-generation far-infrared (FIR) space missions in the sub-terahertz regime. The mixer circuits are optimised for simultaneous broadband radio-frequency (RF) and intermediate-frequency (IF) operation using microstrip architectures that incorporate end-stub, end-loaded, and twin-junction tuning networks. Threedimensional electromagnetic simulations, combined with quantum-mixing analyses, predict receiver noise temperatures approaching four times the quantum limit across 0.50–0.79 THz for the AlO_x-based design. The AlN-based variant further extends the RF coverage to 0.39–0.82 THz, satisfying the requirements of several proposed space missions while overlapping the Atacama Large Millimetre/sub-millimetre Array (ALMA) Bands 8 and extended Band 9. These simulated results demonstrate that NbTiN-based microstrip SIS mixers have the potential to deliver high sensitivity and wide instantaneous bandwidth at sub-terahertz frequencies, establishing them as strong candidates for next-generation space-borne and ground-based FIR observatories.

Abstract:

The Line Emission Terahertz Observatory (LETO) is a candidate European Space Agency (ESA) M8 mission designed to investigate the interstellar medium, star formation, and the evolution of galaxies through velocity-resolved spectroscopy of key far-infrared emission lines. To meet the demanding sensitivity, spectral-resolution, and mapping-speed requirements of the mission, LETO will employ multi-pixel heterodyne receiver arrays operating across four frequency bands between 0.45 and 5.5 THz.

This paper presents proposed concepts for the LETO Band 1 array receiver, which targets the frequency range below 1 THz using superconductor-insulator-superconductor (SIS) mixer technology. A wideband SIS mixer design covering the baseline 450–650 GHz frequency range is described. The design builds upon proven receiver technologies developed for astronomical instruments, thereby reducing technical risk while maintaining near quantum-limited sensitivity.

Two complementary array architectures are investigated. The first employs a conventional single-polarisation mixer configuration that can be reconfigured to support either single- or dual-polarisation observations. The second is based on a fourprobe dual-polarisation mixer concept that eliminates mechanically reconfigurable optical components while providing flexible observing modes. Several operational configurations are presented, demonstrating how the proposed architecture can balance sensitivity, mapping speed, and spacecraft resource constraints. These concepts form the basis for future development of the LETO Band 1 receiver should the mission progress to subsequent study phases.

Abstract:

We present the design, simulation, fabrication, and initial characterization efforts toward a broadband superconductor-insulator-superconductor (SIS) mixer covering ALMA Bands 6 and 7 (211–373 GHz) on a single chip. The mixer employs a twin-junction SIS architecture integrated on a 10 µm-thick silicon-on-insulator (SOI) membrane substrate and is intended as a building block for future sideband-separating and dual-polarization receivers. To support the fabrication of these devices, front-side SIS circuit processing, SOI membrane release techniques, and gold beam-lead integration have been developed and refined. Initial device-release trials have successfully produced mixer chips with intact membranes and beam leads, while experimental setup for cryogenic mixer characterization has also been completed.