Relationship Between Loaded Quality Factor and Responsivity for NbN-Based MKIDs Using Dual-Function Spiral Strip

We investigated the relationship between loaded quality factor (QL) and responsivity of microwave kinetic inductance detectors (MKIDs) using a dual-function spiral strip (DFSS). Three types of DFSSs with different QL values were designed, and the S₂₁ properties were simulated when kinetic inductance of a superconductor was changed by using a 2.5-dimentional simulator. The simulated results showed that a very high Q_L value was not necessarily required. The resonator frequencies of the 25-MKID array using a better type of DFSS (Q_L ~ 3500) were adjusted to be separated by 5 MHz around 2.0 GHz. The 25-MKID array were fabricated using 20-nm thick niobium nitride films deposited on r-plane sapphire substrates. We clearly observed 25 half-wavelength resonances around 2.0 GHz at 2.245 K. The optical responses were also confirmed using hot and cold radiation. The dependence of the responsivity on the Q_L indicated that the Q_L value of approximately 2000 could obtain higher responsivity.