LiteBIRD is a candidate for JAXA’s strategic large mission to observe the cosmic microwave background (CMB) polarization over the full sky at large angular scales. It is planned to be launched in the 2020s with an H3 launch vehicle for three years of observations at a Sun-Earth Lagrangian point (L2). The concept design has been studied by researchers from Japan, U.S., Canada and Europe during the ISAS Phase-A1. Large scale measurements of the CMB B-mode polarization are known as the best probe to detect primordial gravitational waves. The goal of LiteBIRD is to measure the tensor-to-scalar ratio (r) with precision of r < 0:001. A 3-year full sky survey will be carried out with a low frequency (34 - 161 GHz) telescope (LFT) and a high frequency (89 - 448 GHz) telescope (HFT), which achieve a sensitivity of 2.5 μK-arcmin with an angular resolution 30 arcminutes around 100 GHz. The concept design of LiteBIRD system, payload module (PLM), cryo-structure, LFT and verification plan is described in this paper.
We present our design and development of a polarization modulator unit (PMU) for LiteBIRD space mission. LiteBIRD is a next generation cosmic microwave background (CMB) polarization satellite to measure the primordial B-mode. The science goal of LiteBIRD is to measure the tensor-to-scalar ratio with the sensitivity of δr < 10-3. The baseline design of LiteBIRD is to employ the PMU based on a continuous rotating half-wave plate (HWP) at a telescope aperture with a diameter of 400 mm. It is an essential for LiteBIRD to achieve the science goal because it significantly reduces detector noise and systematic uncertainties. The LiteBIRD PMU consists of a multi-layered sapphire as a broadband achromatic HWP and a mechanism to continuously rotate it at 88 rpm. The whole system is maintained at below 10K to minimize the thermal emission from the HWP. In this paper, we discuss the current development status of the broadband achromatic HWP and the cryogenic rotation mechanism.
Laser direct writing of optical devices and circuits is attracted attention because of its ability of three-dimensional fabrication without any mask. Recently, Yb-fiber or solid-state laser has been commonly used for fabrication in addition to traditional Ti:S laser. However, it is reported that waveguide cannot be fabricated in fused silica by using the fundamental light from Yb-based femtosecond laser. Some groups reported on waveguide fabrication by using second-harmonic beam of such lasers, but wavelength conversion using nonlinear process has drawbacks such as destabilization of laser power and beam deformation by walk off.
In this study, we investigated fabrication of low-loss waveguide in fused silica by using the fundamental beam (1030nm) from an Yb solid-state femtosecond laser with a pulse duration of 250 fs. The NA of focusing objective lens was 0.42. The fabricated waveguide was made to have a circular cross-section by shaping laser beam with a slit. We fixed repetition rate to 150 kHz, and identified appropriate scan speed and pulse energy for fabrication of low loss waveguide. Waveguide fabricated with appropriate condition had a propagation loss of 0.2 dB/cm, and this is the first report on optical waveguides in a fused silica fabricated by femto-second laser pulses at a wavelength of 1030nm.
When a linearly polarized light wave propagates in a chiral medium, the polarization plane azimuth rotates
clockwise or counter-clockwise depending on the handedness of the material. This effect is called optical activity.
It can be observed in a number of crystals and organic liquids, however the rotatory power of chiral materials
available in nature is useally very small. That is why chiral planar micro- or nano-structures, which possess a
much stronger rotatory power than natural chrial media, have attracted a considerable attention in recent years.
We demonstrate large optical activity of chiral subwavelength gratings having no in-plane mirror symmetry and
fabricated with metal thin films. For zeroth-order transmitted light, the chirality of these gratings manifests itself
in the non-coplanarity of the electric field vectors at the air- and substrate-sides of the metal layer and can be
interpreted in terms of the surface pllasmon enhanced non-local
light-matter interaction. We demonstrate also
that in all-dielectric subwavelength chiral gratings, the optical activity can be enhanced even stronger by using
waveguide resonance. In the terahertz (THz) region, we obtain rotation of the polarization zimuth of a linearly
polarized THz wave by using double-layered metal chiral structure with complimentary patterns.
Planar chirality can lead to interesting polarization effects whose interpretation has invoked possible violation of reciprocity and time reversality. We show that a quasi-two-dimensional array consisting of gold nanoparticles with no symmetry plane and having sub-wavelength periodicity and thickness exhibits giant specific rotation (~104 °/mm) at normal incidence. The rotation is the same for light incident on the front and back sides of the sample. Such reciprocity manifests three-dimensionality of the structure arising from the asymmetry of light-plasmon coupling at the air-metal and substrate-metal interfaces of the structure. The structures thus enable nanoscale polarization control but violate no symmetry principle.