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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.

A superconducting microstrip half-wavelength resonator is proposed as a suitable band-pass filter for broadband moderate spectral resolution spectroscopy for terahertz (THz) astronomy. The proposed filter geometry has a free spectral range of an octave of bandwidth without introducing spurious resonances, reaches a high coupling efficiency in the pass-band and shows very high rejection in the stop-band to minimize reflections and cross-talk with other filters. A spectrally sparse prototype filter-bank in the band 300–400 GHz has been developed employing these filters as well as an equivalent circuit model to anticipate systematic errors. The fabricated chip has been characterized in terms of frequency response, reporting an average peak coupling efficiency of 27% with an average spectral resolution of 940.

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 $\nu = {\text{350}}$ 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.

A pulsed photoconductive terahertz (THz) source is presented that is able to radiate milliwatt (mW) level average power over a large bandwidth, by exploiting both the optical and electrical properties of photoconductive sources and the ultrawideband properties of connected antenna arrays. An optical system composed of a microlenses array splits the laser beam into $\boldsymbol{N \times N}$ spots that host the active excitation of the antenna arrays. An “ad hoc” network is introduced to bias the array active spots in order to implement a connected antenna array configuration. The array feeds a silicon lens to increase the directivity of the radiated THz beam. A dipole and a slot array are designed. Prototypes have been fabricated and measured. Power and spectrum measurements of the prototypes are in excellent agreement with the expected results. The proposed solutions achieve excellent power radiation levels by exploiting accurate electromagnetic design. Thus, they can offer enhancements to any active system relying on pulsed photoconductive antennas.

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.

There is an increasing demand for large format detector arrays with large bandwidths and high antenna efficiencies for future THz astronomical radiometric applications. For direct detection instruments, it is also desired to have antennas with dual polarization reception in order to increase the received power from incoherent sources, thereby improving the observing speed of the instrument. The main goal of this work is the validation of the incoherent detection of two orthogonal polarizations by a leaky lens antenna, coupled to a single microwave kinetic inductance detector (MKID). The resonant frequency of the MKID changes depending on the absorbed power over a distributed transmission line. The proposed antenna is composed of two crossed leaky wave slots feeding a silicon extended hemispherical lens. The slots are coupled to four aluminum (Al) coplanar waveguide (CPW) lines that incoherently absorb the incoming THz radiation. The antenna and the power absorbing CPW lines are embedded inside the MKID, allowing efficient radiation detection at THz frequencies where no lossless superconductors are available. The proposed dual-polarized device absorbs power incrementally over four different CPWs incoherently and is therefore simulated in reception (deriving a plane-wave response) similarly to what is done in distributed absorbers. We compare numerically and experimentally the proposed dual-polarized leaky lens coupled MKID and its single polarization counterpart and show that the dual polarized device receives twice as much power as the single-polarized one. Eventually, the dual-polarized device, when used with air-bridges, provides the same angular selectivity and twice the throughput of the single-polarized one.

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, itis 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 offpixel 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.

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.

This paper presents the first demonstration of the applicability of the leaky lens antenna concept at THz frequencies. The antenna is integrated with a Kinetic Inductance Detector, so that the two of them function as an ultra sensitive detector over a bandwidth ranging from 0.15 to 1.5 THz. The system has been manufactured and characterized in terms radiation pattern properties and frequency response. We find that the measurements agree very well with the calculations. This demonstrates the manufacturability of the Leaky Lens for use at THz frequency, opening the possibilities for novel broad-band detection concepts.

This paper describes the combined Dutch efforts toward the development of large wideband focal plane array receivers based on kinetic inductance detectors (KIDs). Taking into account strict electromagnetic and detector sensitivity requirements for future ground and space based observatories, this work has led to the identification of well-suited coupling strategies based on the use of lens antenna and the demonstration of their feasibility. Moreover, some specific antenna design difficulties that characterize KIDs-based designs have been investigated, and innovative feeds for the focal plane array which could allow the receivers to be sensitive over a decade of Bandwidth have been proposed.