Design and Electrical Performance of the Kinetic Inductance Detectors of the OLIMPO Experiment

We are going to describe the design and the electrical performance of the horn-coupled lumped element kinetic inductance detectors (LEKIDs) for the OLIMPO experiment. OLIMPO is a balloon-borne mission, devoted to the study of the largest structures in the Universe, by detecting the Sunyaev-Zel'dovich effect of the Cosmic Microwave Background (CMB) photons crossing clusters of galaxies. The multi-band focal planes (centered at 150, 200, 350 and 480 GHz), large aperture telescope (2.6 m), photometric and spectrometer configuration (OLIMPO is equipped with a plug-in differential Fourier transform spectrometer) make OLIMPO able to characaterise all the components of the Sunyaev-Zel'dovich effect with unprecedented precision. The design of the LEKIDs has been optimised to cope with a wide range of optical loadings (corresponding to both photometric and spectrometric configurations). The electrical characterisation of the LEKID arrays was performed in the OLIMPO cryostat, at a temperature around 300 mK and under a constant optical load lower than 500 fW. The readout electronics is a customised ROACH-2 coupled to a MUSIC DAC/ADC board. The averaged responsivities for the arrays, measured from the phase readout, are 1.18x10¹¹ rad/W for the 150 GHz array, 4.10x10¹⁰ rad/W for the 200 GHz array, 1.09x10¹² rad/W for the 350 GHz array, and 4.41x10¹¹ rad/W for the 480 GHz array. Therefore, measuring the spectral noise density for each pixels of the arrays, whose average values are 2.5x10^-5 rad/Hz^(1/2) for the 150 GHz array, 3.6x10^-5 rad/Hz^(1/2) for the 200 GHz array, 5.1x10^-5 rad/Hz^(1/2) for the 350 GHz array, and 1.9x10^-5 rad/Hz^(1/2) for the 480 GHz array, we obtained the values of the averaged electrical noise equivalent power (NEP) over all the arrays: 2x10^-16 W/Hz^(1/2), 9x10^-16 W/Hz^(1/2), 5x10^-17 W/Hz^(1/2), and 4x10^-17 W/Hz^(1/2) respectively for the four OLIMPO arrays.