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Register a new userA Millimeter-Wave Achromatic Half Wave Plate
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We have constructed an achromatic half wave plate (AHWP) suitable for the millimeter wavelength band. The AHWP was made from a stack of three sapphire a-cut birefringent plates with the optical axes of the middle plate rotated by 50.5 degrees with respect to the aligned axes of the other plates. The measured modulation efficiency of the AHWP at 110 GHz was $96 pm 1.5$%. In contrast, the modulation efficiency of a single sapphire plate of the same thickness was $43 pm 4$%. Both results are in close agreement with theoretical predictions. The modulation efficiency of the AHWP was constant as a function of incidence angles between 0 and 15 degrees. We discuss design parameters of an AHWP in the context of astrophysical broad band polarimetry at the millimeter wavelength band.
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Pancharatnam-based achromatic half-wave plates (AHWP) achieve high polarization efficiency over a broad waveband. These AWHPs generally contain a property whereby the optic axis is dependent on the electromagnetic frequency of the incident radiation. When the AHWP is used to measure incident polarized radiation with a finite detection bandwidth, this frequency dependence causes an uncertainty in the determination of the polarization angle due to the limited knowledge of the shape of the source spectrum and detection band. To mitigate this problem, we propose new designs of the AHWP which eliminate the frequency dependence of the optic axis over the bandwidth whilst maintaining high modulation efficiency. We carried out this optimization by tuning the relative angles among the individual half-wave plates of the five and nine layer AHWPs. The optimized set of relative angles achieves a frequency-independent optic axis over the fractional bandwidth, a bandwidth over which polarization efficiency is greater than 0.9, of 1.3 and 1.5 for the five and nine layer AHWPs, respectively. We also study the susceptibility of the alignment accuracy on the polarization efficiency and the frequency dependence of the optic axis, which provides a design guidance for each application.
A cryogenic achromatic half-wave plate (HWP) for submillimetre astronomical polarimetry has been designed, manufactured, tested, and deployed in the Balloon-borne Large-Aperture Submillimeter Telescope for Polarimetry (BLASTPol). The design is based on the five-slab Pancharatnam recipe and it works in the wavelength range 200-600 micron, making it the broadest-band HWP built to date at (sub)millimetre wavelengths. The frequency behaviour of the HWP has been fully characterised at room and cryogenic temperatures with incoherent radiation from a polarising Fourier transform spectrometer. We develop a novel empirical model, complementary to the physical and analytical ones available in the literature, that allows us to recover the HWP Mueller matrix and phase shift as a function of frequency and extrapolated to 4K. We show that most of the HWP non-idealities can be modelled by quantifying one wavelength-dependent parameter, the position of the HWP equivalent axes, which is then readily implemented in a map-making algorithm. We derive this parameter for a range of spectral signatures of input astronomical sources relevant to BLASTPol, and provide a benchmark example of how our method can yield improved accuracy on measurements of the polarisation angle on the sky at submillimetre wavelengths.
Metasurfaces incorporating graphene hold great promise for dynamic manipulation of terahertz waves. However, it remains challenging to design a broadband graphene-based terahertz metasurface with switchable functionality of half-wave plate (HWP) and quarter-wave plate (QWP). Here, we propose a graphene-metal hybrid metasurface for achieving broadband switchable HWP/QWP in the terahertz regime. Simulation results show that, by varying the Fermi energy of graphene from 0 eV to 1 eV, the function of the reflective metasurface can be switched from an HWP with polarization conversion ratio exceeding 97% over a wide band ranging from 0.7 THz to 1.3 THz, to a QWP with ellipticity above 0.92 over 0.78-1.33 THz. The sharing bandwidth reaches up to 0.52 THz and the relative bandwidth is as high as 50%. We expect this broadband and dynamically switchable terahertz HWP/QWP will find applications in terahertz sensing, imaging, and telecommunications.
We have generated more than 100 watts of radial polarized beam from a Yb fiber laser using a photonics crystal segmented half-wave-plate. We demonstrated the high power handling capability of such a photonics crystal segmented half-wave-plate and show that it is a promising external radial polarization converter for high power Yb fiber laser used in laser cutting industry.
Spider is a balloon-borne array of six telescopes that will observe the Cosmic Microwave Background. The 2624 antenna-coupled bolometers in the instrument will make a polarization map of the CMB with approximately one-half degree resolution at 145 GHz. Polarization modulation is achieved via a cryogenic sapphire half-wave plate (HWP) skyward of the primary optic. We have measured millimeter-wave transmission spectra of the sapphire at room and cryogenic temperatures. The spectra are consistent with our physical optics model, and the data gives excellent measurements of the indices of A-cut sapphire. We have also taken preliminary spectra of the integrated HWP, optical system, and detectors in the prototype Spider receiver. We calculate the variation in response of the HWP between observing the CMB and foreground spectra, and estimate that it should not limit the Spider constraints on inflation.
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