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The next generation of cosmic microwave background (CMB) experiments, such as CMB-S4, will require large arrays of multi-chroic, polarisation-sensitive pixels. Arrays of lumped-element kinetic inductance detectors (LEKIDs) optically coupled through an antenna and transmission line structure are a promising candidate for such experiments. Through initial investigations of small prototype arrays, we have shown this compact device architecture can produce intrinsic quality factors < 10^5, allowing for MUX ratios to exceed 10^3. Moreover, we have demonstrated that additional noise from two-level systems can be reduced to an acceptable level by removing the dielectric from over the capacitive region of the KID, while retaining the microstrip coupling into the inductor. To maximise the efficiency of future focal planes, it is desirable to observe multiple frequencies simultaneously within each pixel. Therefore, we utilise the proven transmission line coupling scheme to introduce band-defining structures to our pixel architecture. Initially targeting the peak of the CMB at 150-GHz, we present a preliminary study of these narrow-band filters in terms of their spectral bandwidth and out of band rejection. By incorporating simple in-line filters we consider the overall impact of adding such structures to our pixel by investigating detector performance in terms of noise and quality factor. Based on these initial results, we present preliminary designs of an optimised mm-wave diplexer that is used to split-up the 150 GHz atmospheric window into multiple sub-bands, before reaching the absorbing length of the LEKID. We present measurements from a set of prototype filter-coupled detectors as the first demonstration towards construction of large-format, multi-chroic, antenna-coupled LEKIDs with the sensitivity required for future CMB experiments.
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