Design of an Integrated Filterbank for DESHIMA: On-Chip Submillimeter Imaging Spectrograph Based on Superconducting Resonators

A. Endo 0 1 P. van der Werf 0 1 R.M.J. Janssen 0 1 P.J. de Visser 0 1 T.M. Klapwijk 0 1 J.J.A. Baselmans 0 1 L. Ferrari 0 1 A.M. Baryshev 0 1 S.J.C. Yates 0 1 0 P. van der Werf Leiden Observatory, Leiden University , P.O. Box 9513, 2300 RA Leiden, The Netherlands 1 A.M. Baryshev Kapteyn Astronomical Institute, University of Groningen , P.O. Box 800, 9700 AV Groningen, The Netherlands An integrated filterbank (IFB) in combination with microwave kinetic inductance detectors (MKIDs), both based on superconducting resonators, could be used to make broadband submillimeter imaging spectrographs that are compact and flexible. In order to investigate the possibility of adopting an IFB configuration for DESHIMA (Delft SRON High-redshift Mapper), we study the basic properties of an IFB circuit using electromagnetic simulation. In an idealized situation, one could integrate onto a 4-inch wafer a 9 pixel 920 color 3 dimensional imaging device, which instantaneously covers multiple submillimeter telluric windows with a resolving power of f /df = 1000. We also simulate a slightly more realistic situation, in which the dimensions of the filters differ from their designed values, to see how fabrication errors introduce scattering in the transmission of the channels. 1 Introduction Submillimeter galaxies (SMGs) are massive star-forming galaxies found in the early Universe. The distribution of SMGs across a wide range of redshifts z is of great importance for studies of the cosmic history of star- and galaxy-formation, and the evolution of the cosmic large scale structure [1]. While SMGs are expected to be found in vast quantities in the coming years, by virtue of large format 2D submillimeter imaging cameras, the measurement of the third dimensionredshift z containing the historywould most likely become the rate-limiting step. An ideal instrument for quickly measuring the z of SMGs would be a submillimeter direct detection spectrometer with a resolving power of f/df 1000, to match the typical width of the emission lines from SMGs [2] with rotation velocities of 300 km s1. Although the first generation of these so-called z-machines has successfully detected lines from high redshift sources [3, 4], the instantaneous bandwidth and number of pixels are limited by both the number of detectors and the optics. Moreover, the spatial sampling has been limited to 0 or 1 dimensions. DESHIMA (Delft SRON High-redshift Mapper) is a project to build a submillimeter imaging spectrograph using the advantage of multiplexability that microwave kinetic inductance detectors (MKIDs) have. The number of detectors of MKID cameras [5] are quickly exceeding 10,000. If these detectors are to be used for an imaging spectrograph, it is enough for ten or more pixels to instantaneously cover multiple submillimeter telluric windows with sufficient frequency resolution. Such an instrument would be ideal for submillimeter telescopes, such as the Atacama Pathfinder EXperiment (APEX), the Atacama Submillimeter Telescope Experiment (ASTE), and the Cerro Chajnantor Atacama Telescope (CCAT). However, it is challenging to build such a spectrometer using grating optics, from the perspective of size, complexity, and also to achieve a convenient spatial sampling. One of the alternative frequency-selecting elements is a filterbank. Filterbanks have been commonly used at microwave frequencies, and it is known [6] that a pair of coupled coaxial cable resonators can be used as a band pass filter with an attractive resolving power of 1000. At submillimeter wavelengths, such a filterbank will scale down in size to where the entire filterbank will fit onto a single chip, together with the MKIDs. Such an integrated filterbank (IFB) could take advantage of the compactness, flexibility and mass-production capability that superconducting thin-film technology offers also to MKIDs. Independently from us, a similar concept was proposed by Kovacs and Zmudizinas (private communication). In this study, we use numerical simulation to investigate the possibility of adopting an IFB design for DESHIMA. We will begin with a single filter, and then analyze multiple filters in a chain. 2 Numerical Simulation of the Integrated Filterbank The integrated filterbank design considered in this study is depicted in Fig. 1(a, b). The circuit can consist of either coplanar waveguides (CPWs) or microstrip lines (MSLs), or a combination of both. CPWs have an advantage that it is easier to Fig. 1 (Color online) (a) Conceptual drawing of the integrated filterbank. The length scales are modified to enhance visibility. Each detection channel consists of a filter, made of a pair of low-loss resonators which are resonant at the signal wavelength (100 m), and an MKID, made of a lossy resonator which is resonant at the wavelength of the readout tone (5 mm). The length of the resonators is made slightly different from channel to channel. The low-loss section is made fully out of NbTiN (white), whereas the lossy section uses Al (black). The gray (blue in color) section indicates the region where there is no metal. At the end of the signal line is a matched absorber. (b) Example of a layout of DESHIMA on a 4-inch diameter wafer. There are 9 spiral antennas placed in the center with a nearest-neighbor pixel distance of 2 mm. From each antenna stretches a transmission line towards the integrated filterbank. The filterbank is designed to cover 320475 GHz and 600950 GHz with a resolving power of 1000 using 920 channels per pixel. The readout signal can access the MKIDs from the bonding pads located near the edge of the wafer. The inset is a close-up view of the filters of some channels. The longer filters (red) for the low-frequency band are spaced /2 away from each other, whereas the shorter filters (blue) for the higher frequency band are spaced away from each other make shorts to the ground plane. Moreover, lens-antenna coupled, NbTiN/Al hybrid MKIDs based on CPWs have demonstrated [7] photon noise limited sensitivity under loading powers as low as 100 fW: The single-channel loading power expected for a typical observation using DESHIMA. On the other hand, the ground plane of an MSL could offer better stray light shielding, compared to a CPW which can couple to stray light through the parasitic slotline mode. The signal shining on the antenna will propagate along a transmission line (signal line, hereafter) to which multiple filters are coupled. Each channel of the filterbank consists of two sections; the first is a pair of low-loss resonators which act as a narrow bandpass filter, and the second is a lossy resonator which functions as an MKID. The MKID is a microwave resonator, coupled to the filter on one end and to the readout transmission line on the other end. At the end of the signal line is a long, lossy transmission line that absorbs the uncaptured signal to suppress standing waves. The low-loss section can be made of superconductors with gap frequencie (...truncated)