Beam Pattern Measurements of Millimeter-Wave Kinetic Inductance Detector Camera With Direct Machined Silicon Lens Array

We have developed 220 and 440-GHz cameras using microwave kinetic inductance detectors (MKIDs) for astronomical observations. The optical system of the MKID camera is based on double-slot antennas and extended hemispherical silicon lens arrays. The lens diameter is three times the target wavelength. The 220-GHz camera and the 440-GHz camera have 9 pixels and 102 pixels, respectively. The silicon lens array has been directly machined using a high-speed spindle on an ultra-precision machine. The shape fabrication error and the surface roughness of the top of the lens were typically less than 10 μm (peak-to-valley) and about 0.7 μm (rms), respectively. The beam patterns of the MKID camera were measured and are in good agreement with the calculations.