Kenichi Karatsu

LiteBIRD: a small satellite for the study of B-mode polarization and inflation from cosmic background radiation detection

LiteBIRD [Lite (Light) satellite for the studies of B-mode polarization and Inflation from cosmic background Radiation Detection] is a small satellite to map the polarization of the cosmic microwave background (CMB) radiation over the full sky at large angular scales with unprecedented precision. Cosmological inflation, which is the leading hypothesis to resolve the problems in the Big Bang theory, predicts that primordial gravitational waves were created during the inflationary era. Measurements of polarization of the CMB radiation are known as the best probe to detect the primordial gravitational waves. The LiteBIRD working group is authorized by the Japanese Steering Committee for Space Science (SCSS) and is supported by JAXA. It has more than 50 members from Japan, USA and Canada. The scientific objective of LiteBIRD is to test all the representative inflation models that satisfy single-field slow-roll conditions and lie in the large-field regime. To this end, the requirement on the precision of the tensor-to-scalar ratio, r, at LiteBIRD is equal to or less than 0.001. Our baseline design adopts an array of multi-chroic superconducting polarimeters that are read out with high multiplexing factors in the frequency domain for a compact focal plane. The required sensitivity of 1.8μKarcmin is achieved with 2000 TES bolometers at 100mK. The cryogenic system is based on the Stirling/JT technology developed for SPICA, and the continuous ADR system shares the design with future X-ray satellites.

Beam Pattern Measurements of Millimeter-Wave Kinetic Inductance Detector Camera With Direct Machined Silicon Lens Array

We have developed 220 and 440-GHz cameras using microwave kinetic inductance detectors (MKIDs) for astronomical observations. The optical system of the MKID camera is based on double-slot antennas and extended hemispherical silicon lens arrays. The lens diameter is three times the target wavelength. The 220-GHz camera and the 440-GHz camera have 9 pixels and 102 pixels, respectively. The silicon lens array has been directly machined using a high-speed spindle on an ultra-precision machine. The shape fabrication error and the surface roughness of the top of the lens were typically less than 10 μm (peak-to-valley) and about 0.7 μm (rms), respectively. The beam patterns of the MKID camera were measured and are in good agreement with the calculations.

Optical Efficiencies of Lens-Antenna Coupled Kinetic Inductance Detectors at 220 GHz

We have been developing a terahertz camera based on antenna-coupled superconducting resonators, the so-called microwave kinetic inductance detectors (MKIDs), and a silicon lens array. The MKID consists of a coplanar waveguide coupled to a double slot antenna and is patterned on a high-quality aluminum film grown by molecular beam epitaxy. The camera is sensitive at frequencies of 200-240 GHz. Its bandwidth is limited by the impedance properties of the double slot antenna. The design, fabrication, and optical evaluations of the planar antennas and silicon lens arrays are presented in this paper. The MKID camera has been evaluated both in dark conditions and under optical radiation in a 0.1-K dilution refrigerator. The electrical noise equivalent power was around 5×10-18 W/√(Hz) in dark conditions and 4×10-16 W/√(Hz), which is much lower than the photon noise level, with the optical load. The optical efficiency of the camera was estimated by three independent methods, and the results were consistent with each other and equal to 20%-25% without an anti-reflection coating on the lens surface.

Developments of wide field submillimeter optics and lens antenna-coupled MKID cameras

Wide field cryogenic optics and millimeter-wave Microwave Kinetic Inductance Detector (MKID) cameras with Si lens array have been developed. MKID is a Cooper-pair breaking photon detector and consists of supercon- ducting resonators which enable microwave (~GHz) frequency multiplexing. Antenna-coupled Aluminum CPW resonators are put in a line on a Si substrate to be read by a pair of coaxial cables. A 220 GHz - 600 pixels MKID camera with anti-reflection (AR) coated Si lens has been demonstrated in an 0.1 K cryostat. A compact cryogenic system with high refractive index materials has been developed for the MKID camera.

LiteBIRD: mission overview and design tradeoffs

We present the mission design of LiteBIRD, a next generation satellite for the study of B-mode polarization and inflation from cosmic microwave background radiation (CMB) detection. The science goal of LiteBIRD is to measure the CMB polarization with the sensitivity of δr = 0:001, and this allows testing the major single-field slow-roll inflation models experimentally. The LiteBIRD instrumental design is purely driven to achieve this goal. At the earlier stage of the mission design, several key instrumental specifications, e.g. observing band, optical system, scan strategy, and orbit, need to be defined in order to process the rest of the detailed design. We have gone through the feasibility studies for these items in order to understand the tradeoffs between the requirements from the science goal and the compatibilities with a satellite bus system. We describe the overview of LiteBIRD and discuss the tradeoffs among the choices of scientific instrumental specifications and strategies. The first round of feasibility studies will be completed by the end of year 2014 to be ready for the mission definition review and the target launch date is in early 2020s.

Fabrication of 721-pixel silicon lens array of a microwave kinetic inductance detector camera

Abstract. We have been developed a lens-integrated superconducting camera for millimeter and submillimeter astronomy. High-purity silicon (Si) is suitable for the lens array of the microwave kinetic inductance detector camera due to its high refractive index and low dielectric loss at low temperatures. The camera is an antenna-coupled Al coplanar waveguide on a Si substrate. Thus the lens and the device are made of the same material. We report a fabrication method of a 721-pixel Si lens array with an antireflection (AR) coating. The Si lens array was fabricated with an ultraprecision cutting machine. It uses TiAlN-coated carbide end mills attached with a high-speed spindle. The shape accuracy was less than 50  μm peak-to-valley and the surface roughness was arithmetic average roughness (Ra) of 1.8  μm. The mixed epoxy was used as an AR coating to adjust the refractive index. It was shaved to yield a thickness of 185  μm for 220 GHz. Narrow grooves were made between the lenses to prevent cracking due to the different thermal expansion coefficients of Si and the epoxy. The surface roughness of the AR coating was Ra of 2.4 to 4.2  μm.

Development of a Compact Cold Optics for Millimeter and Submillimeter Wave Observations

We have developed an optics for 220 GHz observations, which is a compact cold re-imaging one from a telescope focal plane, with F/# = 6 to a detector plane with F/# = 1 at 100 mK. It employs two high refractive lenses, high purity alumina (n=3.1) and silicon (n=3.4). To reduce the incident stray light into the detector, a cold nested baffle composed of four reflectors with the same spherical shape has been developed. The stray light power is simulated to be 0.2 μW which corresponds a quarter of that of a without-baffles case. The total transmittance of three kinds of IR blocking filters is 0.78 at the observation frequency, and less than 10-10 above 6 THz. Thermal flow power into the detector, including the stray light power, is about 0.7 μW. The cold optics with an 600 pixels MKID camera has been cooled down to 100 mK.

Development of 1000 arrays MKID camera for the CMB observation

A precise measurement of the Cosmic Microwave Background (CMB) provides us a wealth of information about early universe. LiteBIRD is a future satellite mission lead by High Energy Accelerator Research Organization (KEK) and its scientific target is detection of the B-mode polarization of the CMB, which is a footprint of primordial gravitational waves generated during inflation era, but has not been successfully observed so far due to lack of sensitivity. Microwave Kinetic Inductance Detector (MKID) is one candidate of sensitive millimeterwave camera which will be able to detect the B-mode polarization. We have been developing MKID at National Astronomical Observatory of Japan (NAOJ) in cooperation with KEK and RIKEN for the focal plane detector of the LiteBIRD. The developed technologies are: fabrication process of MKIDs with epitaxially-formed aluminum (Al) on silicon (Si) wafer; optical system of the camera consisting of double-slot antenna with Si lens array; and readout circuit utilizing Fast Fourier Transform Spectrometer (FFTS). With these technologies, we designed a prototype MKIDs camera for the LiteBIRD.

A Frequency Sweeping Readout System for Kinetic Inductance Detectors Based on Submillimeter Radio Astronomy

We have been developing a superconductive detector array for terahertz astronomical observation using Microwave Kinetic Inductance Detector (MKID) technology. MKID is accomplished by making a strip of superconductor part of a microwave resonant circuit, and monitoring the phase and amplitude of a probe signal transmitted through the resonator. Frequency-domain multiplexing will allow up to thousands of resonators to be read out through a single line. We developed a 600-pixel 220 GHz detector array and a 102-pixel 440-GHz detector array. We have developed a new scheme readout system for the detector arrays, and which uses a frequency-sweeping probe signal instead of a fixed-frequency probe signal. This scheme enables us a direct measurement of the changed resonance frequency after optical loading.

Study of Superconducting Bilayer for Microwave Kinetic Inductance Detectors for Astrophysics

Due to their multiplexing capability and their good sensitivity to radiation from submillimeter to X-ray wavelengths, microwave kinetic inductance detectors (MKIDs) are increasingly used in the field of astrophysics. The Advanced Technology Center of the National Astronomical Observatory of Japan is developing MKIDs for astronomical observations such as CMB B-mode search with LiteBIRD. MKIDs are made of superconductors whose energy gap determines the detector frequency range. The energy gap depends on Tc, the critical temperature of the superconductor. It is thus important to be able to adjust Tc in order to choose the suitable frequency range. When using a single-layer MKID, the Tc is fixed by the superconducting gap energy of the unique component and cannot be changed. One possibility is to make a bilayer MKID using the proximity effect to adjust its critical temperature. This paper presents our new study on MKIDs made of superconductor/metal bilayers. We investigated niobium and copper bilayers (Nb/Cu) and fabricated different bilayers in our clean room. The critical temperature of each of them has been measured. We show that the Tc depends on the ratio between Nb and Cu thicknesses and that we are able to control it. Then, we characterized one of these Nb/Cu bilayers (Nb = 8 nm and Cu = 22 nm) once integrated in a MKID. We measured the temperature dependence of the resonant frequency, and we achieved quality factors as high as 2 × 104. The measurement of the noise spectrum provided a lower limit equal to -85 dBc/Hz, and the calculation of the noise equivalent power has shown that the sensitivity of the Nb/Cu bilayer MKID is not very far from that of an Al monolayer MKID.