Design of a KID-based on-chip FTS for astrophysical applications

Imaging spectroscopy at millimeter wavelengths is a key observational tool for astrophysics and cosmology, enabling the separation and characterization of multi-component diffuse emission, compact extragalactic sources, high-redshift galaxies, Cosmic Microwave Background spectral distortions, and line-intensity mapping signals. However, conventional Fourier transform spectrometers are often bulky, mechanically complex, and difficult to integrate in cryogenic focal planes or space-compatible payloads. In this work, we present the design status of a compact W-band (75−110 GHz) on-chip Fourier transform spectrometer integrated with a Kinetic Inductance Detector on a silicon substrate. The baseline architecture comprises three main subsystems: a planar antenna, an integrated FTS (power splitter, phase shifter and combiner), and the KID. Electromagnetic design and optimization of the individual subsystems indicate low reflection, suitable transmission balance, and stable isolation across the operating band, while the phase-shifter concept offers a route to spectral-resolution tuning through controlled modification of the propagation phase. In parallel, NbTiN thin films with different thicknesses have been fabricated and characterized to extract critical temperature, sheet resistance, and kinetic-inductance parameters relevant to the superconducting transmission-line and phase-shifter design. These results support the feasibility of a cryogenically compatible on-chip FTS architecture for future compact spectroscopic instruments in astrophysical and cosmological applications.