Proof-of-Concept Demonstration of Vector Beam Pattern Measurements of Kinetic Inductance Detectors

We present results from the first vector beam pattern measurement of microwave kinetic inductance detectors (MKIDs). Vector beam patterns require sampling of the E-field of the receiver in both amplitude and phase. MKIDs are inherently direct detectors and have no phase response to incoming radiation. We map the amplitude and phase patterns of the detector beam profile by adapting a two-source heterodyne technique. Our testing strategy recovers the phase information by creating a reference signal to trigger data acquisition. The reference is generated by mixing the slightly offset low-frequency signals from the output of the two synthesizers used to drive the submillimeter sources. The key requirement is that the time-series record always begins at the same set phase of the reference signal. As the source probe is scanned within the receiver beam, the wavefront propagation phase of the receiver changes and causes a phase offset between the detector output and reference signals. We demonstrated this technique on the central pixel of a test array operating at 350 GHz. This methodology will enable vector beam pattern measurements to be performed on direct detectors, which have distinct advantages reducing systematic sources of error, allowing beam propagation, and removing the far-field measurement requirement such that complicated optical systems can be measured at a point that is easily accessible, including the near field.