Surface Wave Control for Large Arrays of Microwave Kinetic Inductance Detectors

Abstract

Large ultra-sensitive detector arrays are needed for present and future observatories for far infra-red, submillimeter wave (THz), and millimeter wave astronomy. With increasing array size, it is increasingly important to control stray radiation inside the detector chips themselves, the surface wave. We demonstrate this effect with focal plane arrays of 880 lens-antenna coupled Microwave Kinetic Inductance Detectors (MKIDs). Presented here are near field measurements of the MKID optical response versus the position on the array of a reimaged optical source. We demonstrate that the optical response of a detector in these arrays saturates off-pixel at the ∼ −30 dB level compared to the peak pixel response. The result is that the power detected from a point source at the pixel position is almost identical to the stray response integrated over the chip area. With such a contribution, it would be impossible to measure extended sources, while the point source sensitivity is degraded due to an increase of the stray loading. However, we show that by incorporating an on-chip stray light absorber, the surface wave contribution is reduced by a factor >10. With the on-chip stray light absorber the point source response is close to simulations down to the ∼ −35 dB level, the simulation based on an ideal Gaussian illumination of the optics. In addition, as a crosscheck we show that the extended source response of a single pixel in the array with the absorbing grid is in agreement with the integral of the point source measurements.

Wideband on-chip terahertz spectrometer based on a superconducting filterbank

“…Furthermore, there are four 'dark MKIDs' that are placed away from the terahertz line as a reference for signal power coupled to the MKIDs by way of stray light or surface waves. 25 The chip is equipped with a mesh of 40 nm-thick β-phase Ta on its backside to reduce the propagation of stray radiation inside the chip. The terahertz signal line is terminated by a CPW with an aluminium center line, which absorbs the remaining terahertz signal after the filterbank, to prevent reflection of power that is not drawn out by the filter channels.…”

Complex Beam Mapping and Fourier Optics Analysis of a Wide-Field Microwave Kinetic Inductance Detector Camera

“…The field of view is ∼ 6 × 6 cm at the array, similar to the array size. The array used has 880 pixels lens-antenna coupled for 350 GHz with a stray light absorber on chip to kill in chip stray light (or the "surface wave") 16 . Each pixel is a hybrid quarter wavelength MKID made of NbTiN with an active portion for radiation detection of 40 nm thick Al.…”

“…The entire array is on a single readout line from 4-8 GHz. For more details on the design, fabrication and optical verification see Yates et al 16 . The pixels have a separation of 2 mm, or 1.2 f#λ so the beams are designed to overlap to improve mapping speed 17 .…”

An Ultrawideband Leaky Lens Antenna for Broadband Spectroscopic Imaging Applications

“…The measured side lobe levels are around 2 to 5 dB higher than simulated, which we tentatively attribute to multiple reflections inside the lens increasing the effective detected power at large incident angles. An additional possible contribution might be the detection of residual stray-light via surface waves in the chip [13].…”