Tom Nitta

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.

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.

Broadband Corrugated Horn Array with Direct Machined Fabrication

We have developed a broadband corrugated horn in the 120-270 GHz and a horn array in the 80-180 GHz bands. The geometry of corrugations is so simple that the horn array can be directly machined from a bulk of aluminum with an end mill. The cross polarization and near sidelobe levels are less than -20 and -30 dB, respectively. The return loss is less than -15 dB in most design frequency bands, and the beam pattern is symmetric. The beam pattern and the return loss are measured in the 120-170 GHz range at room temperature. They are in good agreement with the simulation. It is possible to reduce reflection at the aperture surface and to reduce the weight by carving the unnecessary part. This design provides an octave bandwidth of the corrugated horn array at reasonable machining time.

Development of octave-band planar ortho-mode transducer with kinetic inductance detector for LiteBIRD

We demonstrate a design of octave-band circular waveguide coupled planar ortho-mode transducer (OMT) with Microwave Kinetic Inductance Detector (MKID) for LiteBIRD mission, a small-size satellite for cosmic microwave background (CMB) polarization signal full-sky mapping. In our 4-pixel prototype design, each single pixel is sensitive to two frequency bands (90 GHz and 150 GHz) corresponding to atmospheric window. Silicon on insulator (SOI) has been selected for OMT structure and a broadband coplanar waveguide (CPW) 180-degree hybrid is designed to cancel higher modes of a circular waveguide and add two signals from the fundamental mode together. After a microstrip bandpass diplexer, a microstrip line to coplanar waveguide transition structure couples signal to MKID. MKIDs are designed with Nb ground plane and Al/Ti bilayer center strip line to achieve low frequency response and high sensitivity. A 4-pixel module is under test and we plan to deploy these multi- chroic polarimeters on Nobeyama 45m telescope.

Design of corrugated-horn-coupled MKID focal plane for CMB B-mode polarization

A focal plane based on MKID has been designed for cosmic microwave background (CMB) B-mode polarization experiments. We are designing and developing a focal plane with broadband corrugated horn array, planar OMT, 180 degree hybrid, bandpass filters, and MKIDs. The focal plane consists of 3 octave bands (55 - 108 GHz, 80 - 160 GHz, 160 - 320 GHz), 10 hexagonal modules. Broadband corrugated horn-array has been directly machined from an Al block and measured to have a good beam shape which is consistent with electromagnetic field simulations in octave bands. The horn array is designed to be low standing-wave, light weight, and electromagnetic shield. The broadband 4 probes ortho-mode transducer (OMT) is fabricated on Si membrane of an SOI wafer. A broadband 180 degree hybrid made with coplanar waveguide (CPW) is used to reduce higher modes of the circular waveguide. Two bandpass filters of each polarization are patterned with Nb microstrip. A prototype of the broadband corrugated horn coupled MKIDs has been fabricated and tested.

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.

Design of wide-field Nasmyth optical system for a submillimeter camera

Abstract. A wide-field Nasmyth optical system that connects a planned 10-m Ritchey-Chrétien telescope to a submillimeter camera is reported. This diffraction-limited system has a 1-deg field of view at 850 GHz, filled with a more than 20,000-pixel camera. The system enables us to carry out large field surveys of distant galaxies within reasonable time scales. The size of the Nasmyth optics is reasonably compact and its cryogenic part including the vacuum window, cryogenic lens, and IR block filters can be built using existing technologies at a reasonable cost. This type of optical system can be applied for the optical design of millimeter, terahertz, and other submillimeter instruments.

Design of wide-field Nasmyth optics for a submillimeter camera

We designed wide FoV (1 degree) Nasmyth optics which transformed the f/6 Nasmyth focus to f/1 at a 850GHz superconducting camera for a planning 10-m Ritchey-Chrétien telescope. This optical system consists of reflecting mirrors at room temperature and a refractive lens at 4K. It enables us to carry out wide FoV imaging observations at the diffraction limit (Strehl ratio < 0.89) with a more than 100,000 pixel camera equipped in a 10-m telescope. The size of this system is reasonably compact (whole size:1.6 mx3.3 mx2.6 m, cryogenic part:0.7 mx0.7 mx1.0 m). The cryogenic part of this system such as vacuum window, cryogenic lens and IR block filters can be made with existing technologies at reasonable cost. The optical system can extend to the millimeter wave and the terahertz domain.