Characterization of wide-field optics in the Terahertz regime imposes new and demanding requirements for testing systems. Basic optical parameters can be determined from scalar planar characterization, obtained using monochromatic or thermal sources located in the instrument focal plane. In contrast, important features, such as the spillover efficiency, wave front error, or aperture efficiency cannot be easily measured by such approaches. Moreover, when instruments have a curved focal plane, designed to match the hosting telescope, even basic parameters are difficult to extract from scalar planar measurements. In such cases, the use of phase and amplitude information is mandatory. From a complex planar measurement, the complete information of the optical system can be obtained, allowing the estimation of all relevant optical parameters. In this work, we present and demonstrate experimentally a technique to perform such measurement based on the use of continuous wave photonic terahertz sources. Here, we present our results at 350 GHz and 850 GHz, demonstrating the feasibility of performing measurements at different submillimeter frequencies using a single experimental setup. The proposed system was implemented to fully characterize a wide-field submillimeter camera based on kinetic inductance detectors designed to be deployed at the APEX Telescope in Chile.

We present the design, fabrication, and characterization of a broadband leaky lens antenna for broadband, spectroscopic imaging applications. The antenna is designed for operation in the 300-900 GHz band. We integrate the antenna directly into an Al-NbTiN hybrid microwave kinetic inductance detector (MKID) to measure the beam pattern and absolute coupling efficiency at three frequency bands centered around 350, 650, and 850 GHz, covering the full antenna band. We find an aperture efficiency \eta _{ap} \approx 0.4 over the whole frequency band, limited by lens reflections. We find a good match with simulations for both the patterns and efficiency, demonstrating a 1:3 bandwidth in the submillimeter wavelength range for future on-chip spectrometers.

For astronomical instruments, accurate knowledge of the optical pointing and coupling is essential to characterize the alignment and performance of (sub-)systems prior to integration and deployment. Ideally, this requires the phase response of the optical system, which for direct (phase insensitive) detectors was not previously accessible. Here, we show development of the phase-sensitive complex beam pattern technique using a dual optical source heterodyne technique for a large-field-of-view microwave kinetic inductance detector camera at 350 GHz. We show here how you can analyze the measured data with Fourier optics, which allows integration into a telescope model to calculate the on-sky beam pattern and telescope aperture efficiency prior to deployment at a telescope.

Complex field mapping is a powerful tool to characterize the optical performance of astronomical instruments, and has become the standard for characterizing heterodyne array cameras. Recently, an adaptation of the heterodyne beam mapping technique was demonstrated on a single pixel of a direct detector instrument. We present a novel measurement apparatus and data acquisition techniques to efficiently reconstruct the complex field pattern of individual pixels across a direct detector focal plane array. These techniques are scalable to high pixel counts as the technology maturation and scientific requirements push to larger arrays. For this demonstration, we used an engineering model of the low-frequency band of the APEX microwave kinetic inductance detector camera with a center frequency of ν = 350 GHz. Amplitude and phase radiation patterns were measured from all 880 pixels of the test array in two orthogonal polarizations. We also discuss an updated postprocessing pipeline using the complex field data to characterize the optical performance of the array. Using the measured complex field pattern, we extract the co- and cross-polarization patterns and Gaussian beam parameters, and propagate the beam from the measurement plane to additional planes of interest across all pixels in the test array. Complex field measurements of direct detectors allow more precise characterization of beam parameters when compared to thermal measurements, particularly for individualized fitting in postprocessing not reliant on the accuracy of the probe system alignment. These techniques enable high-precision characterization of individualized beam parameters as well as the overall optical system to very large format arrays with modest computational processing power. These results demonstrate the diagnostic power of the presented measurement and analysis techniques.

The development of astrophysics kilo-pixel imaging systems requires a dedicated cryogenics and optics setup to measure the performance of the detector arrays in terms of sensitivity, crosstalk, dynamic range, and spatial response. We have developed such testbed for the characterization of large format arrays of microwave kinetic inductance detectors (MKIDs), capable of measuring detector chips of 6 cm × 6 cm, operating in the 350 and 850 GHz band. The testbed is a wide field camera that produces an aberration free image of the chip outside of the cryostat. The cryostat is based upon a commercial pulse-tube cooled 3 K system with an He3-He3-He4 sorption cooler that reaches a base temperature below 250 mK. We will describe the thermomechanical solutions implemented in our system to minimize the thermal loading on the cold stage, needed to reach the low base temperature. We will also give the optical design including straylight control.

With increasing array size, it is increasingly important to control stray radiation inside the detector chips themselves. We demonstrate this effect with focal plane arrays of absorber coupled Lumped Element microwave Kinetic Inductance Detectors (LEKIDs) and lens-antenna coupled distributed quarter wavelength Microwave Kinetic Inductance Detectors (MKIDs). In these arrays the response from a point source at the pixel position is at a similar level to the stray response integrated over the entire chip area. For the antenna coupled arrays, we show that this effect can be suppressed by incorporating an on-chip stray light absorber. A similar method should be possible with the LEKID array, especially when they are lens coupled.

Next generation sub-mm imaging instruments require kilo-pixel focal plane arrays filled with background limited detectors. Microwave kinetic inductance detectors (MKIDs) are a state-of-the-art detector for future instruments due to their inherent multiplexing capabilities. An MKID consists of a superconducting resonator coupled to a feed-line that is used for readout. In the device presented here radiation coupling is achieved by coupling the MKID directly to a planar antenna. The antenna is placed in the focus of an extended hemispherical lens to increase the filling factor and to match efficiently to fore optics. In this paper, we present the design and the optical performance of MKIDs optimized for operation in a 100-GHz band around 850 GHz. We have measured the coupling efficiency, frequency response, and beam patterns and compare those results to simulated performance. We obtain an excellent agreement between the measured and simulated beam pattern, frequency response, and absolute coupling efficiency between a thermal calibration source and the power absorbed in the detector. Additionally, we demonstrate that antenna coupled MKIDs offer background limited radiation detection down to ∼100 aW of power absorbed in the detector.

Large ultrasensitive detector arrays are needed for present and future observatories for far infrared, 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 at a similar level 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.

This contribution presents the design and sub-mm wave measurements of a wideband dual polarized leaky lens antenna coupled to kinetic inductance detector (KIDs) to be specifically used for tightly spaced focal plane arrays. The antenna is planar and composed by two crossed slots, fed by two orthogonal coplanar waveguide (CPW) lines. In transmission, the crossed CPW lines are fed differentially in order to couple the radiation into the slots. The slot antenna feeds a dielectric lens to achieve directive patterns. The main goal of this work is to show the measurement results of the patterns and efficiency, and compare this antenna with its singly polarized version. The measured received power from an incoherent source is increased by a factor of 2 compared to a single-polarized version of the antenna.

We present the design, fabrication, and full characterisation (sensitivity, beam pattern, and frequency response) of a background limited broadband antenna coupled kinetic inductance detector covering the frequency range from 1.4 to 2.8 THz. This device shows photon noise limited performance with a noise equivalent power of 2.5 × 10^-19W/Hz^1/2 at 1.55 THz and can be easily scaled to a kilo-pixel array. The measured optical efficiency, beam pattern, and antenna frequency response match very well the simulations.

Microwave Kinetic Inductance Detectors (MKIDs) are becoming a very promising candidate for next generation imaging instruments for the far infrared. A MKID consists of a superconducting resonator coupled to a feed-line used for the readout. In the devices presented here radiation coupling is achieved by coupling the MKID directly to planar antenna. The antenna is placed in the focus of an elliptical lens to increase the filling factor and to match efficiently to fore-optics. In this paper we present the design and the optical performance of MKIDs optimized for operation at 350 GHz. We have measured a device consisting of 14 pixels, characterized the coupling efficiency, antenna-lens frequency response and beam pattern and compared these to theoretical simulations. The optical efficiency has been measured by means of a black body radiator mounted in an ADR cryostat, through the variation of the black body temperature a variable illumination of each pixel (from 0.1 fW to 2 pW) is achieved. The frequency response and beam pattern have been directly measured in a He3 cryostat directly via the cryostat window and without the use of intermediate optics.