On-Chip Solutions for Future THz Imaging Spectrometers

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The mysteries of the early Universe are largely enshrouded in dust, product of the violent process of star formation. Due to the vast distances of our Universe, infrared light emitted by the heated dust back in those early stages can still be observed today, which has been observed to contribute to about half of the total cosmic background radiation. Gases fueling star-formation also radiate, but in the form of emission lines, which leave distinct spectral signatures that allow the study of the underlying physical processes. Given the expansion of the Universe, the evolutionary information is encoded in the cosmological redshift observed, making the far-infrared or terahertz (THz) regime specially suited for probing star-formation. Superconducting on-chip broadband THz imaging spectrometers with moderate spectral resolution coupled to large telescopes will allow the investigation the early Universe processes over large cosmological volumes. In this dissertation we propose two enabling technologies toward the advancement of this on-chip superconducting instruments: a broadband and moderate spectral resolution channelizing filter-bank, and a broadband phased array antenna as a reflector feed with beam-steering capabilities.

Octave-band THz channelizing filter-banks with moderate spectral resolution of the order R=500 are investigated in this work. These systems allow for a size reduction of several orders of magnitude compared to conventional spectrometers with similar spectral resolution. The proposed filters are half-wavelength resonators, which naturally provide a free-spectral range of an octave. The performance of those filters, both when in isolation and when embedded in a filter-bank, is analyzed using a newly-developed circuit model. This tool also provides design insights such as the required filter ordering and separation within the filter-bank to enable an efficient circuit. The actual implementation of the superconducting filter-bank on a chip is investigated for two of the main on-chip technologies: co-planar waveguide (CPW) and microstrip. Despite the easier manufacturing of co-planar circuitry, that technology is not suited for channelizing THz filter-banks as it suffers from radiation issues. Instead microstrip technology is non-radiative and, although it suffers from the moderate dissipation in deposited dielectrics such as a-Si, it provides a very reliable platform to build THz filter-banks. Half-wavelength I-shaped resonators are proposed as suitable filtering structures with which frequency-sparse filter-banks have been built to test their performance in semi-isolation. The measurements were based on both a frequency response characterization of the filters as well as their optical efficiency, showing good agreement between the two. The measured performance of these filters showed pass-bands with an average peak coupling efficiency of 27% and a spectral resolution R≈940. The coupling is significantly better than earlier results based upon planar technology.

The coupling between the quasi-optical reflector system of a telescope and the on-chip filter-bank requires of a broadband antenna. Currently, broadband integrated anti-reflection-coated lenses are being developed for this purpose, but their manufacturing is specially complicated for cryogenics and require mechanical actuators to perform beam scanning in the case of a multi-object spectrometer. In this dissertation, we propose a broadband phased-array antenna concept with electronic beam-steering that exploits two key properties of superconductors in its feeding network: the negligible conductor loss and the tunable kinetic inductance with a bias current. The focused connected array antenna concept proposed is based on the broadband impedance matching enabled by the connected arrays and the largely frequency-independent far fields of near-field focused apertures. To demonstrate this concept we designed, fabricated and tested two low frequency (3-6 GHz) prototypes in PCB technology: one pointing broadside and another one scanning. The measured fields met the predictions to a large degree and provided with a reflector aperture efficiency in excess of 60% over an octave of bandwidth and allowing to scan one half-power beamwidth at the lowest frequency with a frequency-averaged scan loss of 0.2 dB. Both the directivity and the gain were measured, allowing to report the losses, which chiefly originated from the tin-finished copper lines in the PCB. As a result, we can expect a highly-efficient reflector feed at THz frequencies with beam-steering capabilities in the near future.

The beam-steering concept proposed for the phased-array antenna relies on the current-dependent kinetic inductance of superconducting lines. With this effect, the phase velocity of biased superconducting lines may be modified, allowing thereby an electronic tuning of the phase-shift introduced. Prior to the integration of such phase-shifters with the phased-array antenna, we devised an on-chip platform based a tunable Fabry-Pérot resonator to quantify the phase-shifting capabilities at THz frequencies. In this concept, the dc bias currents are injected in the proximity of the edges of the resonator through 9th order Chebyshev stepped-impedance low-pass filters, whose high rejection mitigates any possible disturbance to the THz resonances. Using a circuit model including the resonator and the low-pass filters, as well as the simulated properties of the superconducting buried microstrip lines used in the designs, we anticipate an expected maximum tuning of dφ/φ=-df/f≈2%. With such tuning range millimeter-long tunable delay lines will be required for THz superconducting phased-array.