Development of large terahertz heterodyne receiver arrays for future space observations

If one wants to investigate atoms, molecules and dusts in the interstellar medium, one should not make observations using ‘normal’ light, but instead peer them using a very specific type of infrared light, which is the terahertz radiation. The radiation at this specific wavelength contains the fingerprints of various substances and can travel in a long distance to us, but they can only be observed from space because they are blocked by our atmosphere. To increase the observation speed within the limited lifetime of a space telescope, a large terahertz array receiver is favorable. In this PhD thesis, I studied a few key technologies of large terahertz array receivers. To resolve a spectral line signal from space, sensitive terahertz detectors use the signal from a local source to mix with the signal from space. Therefore, special optical components are necessary for coupling the signal from the local source into the detectors. Three chapters in my thesis are the development of the optics for local sources. Furthermore, I also explored a new type of detector, so called hot electron bolometer mixer using MgB2 material, which is different from those commonly used at 4 K low temperature. It can work at a higher temperature (20K), and make the use of a “tiny” cooler, which is considered revolutionary for future space observatories. In the end, I also reported a detailed concept and design study of a 1000-pixel terahertz array receiver, which can act as a reference for next generation of large arrays

Heterodyne performance and characteristics of terahertz MgB2hot electron bolometers

We have studied THz heterodyne detection in sub-micrometer MgB2 hot electron bolometer (HEB) mixers based on superconducting MgB2 films of ∼5nm (HEB-A), corresponding to a critical temperature (Tc) of 33.9 K, and ∼7nm (HEB-B), corresponding to a \u1d447\u1d450 of 38.4 K. We have measured a double sideband (DSB) receiver noise temperature of 2590 K for HEB-A and 2160 K for HEB-B at 1.6 THz and 5 K. By correcting for optical losses, both HEBs show receiver noise temperatures of ∼1600 K referenced to the front of anti-reflection (AR)-coated Si lenses. An intermediate frequency (IF) noise bandwidth of 11 GHz has been measured for both devices. The required local oscillator (LO) power is about 13 μW for both HEBs. We have also measured a DSB receiver noise temperature of 3290 K at 2.5 THz and 5 K but with an AR-coated lens optimized for 1.6 THz. Besides, we have observed a step-like structure in current voltage (IV) curves, which becomes weaker when the LO power increases and observable only in their differential resistance. Such a correlated structure appears also in the receiver output power as a function of voltage, which is likely due to electronic inhomogeneities intrinsic to the variations in the thickness of the MgB2 films. Different behavior in the IV curves around the low bias voltages, pumped with the same LO power at 1.6 and 5.3 THz, was observed for HEB-B, suggesting the presence of a high-energy σ-gap in the MgB2 film

39 THz spatial filter based on a back-to-back Si-lens system

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Supplementary document for Numerical Study of the Plasmonic Slab Lens for Improving Direct-write Nano Lithography - 6807882.pdf

optimization of the film thickness, and comparison of lithography for the bowknot aperture probe with different PSL

39 THz spatial filter based on a back-to-back Si-lens system

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement. We present a terahertz spatial filter consisting of two back-to-back (B2B) mounted elliptical silicon lenses and an opening aperture defined on a thin gold layer between the lenses. The beam filtering efficiency of the B2B lens system is investigated by simulation and experiment. Using a unidirectional antenna coupled 3rd-order distributed feedback (DFB) quantum cascade laser (QCL) at 3.86 THz as the source, the B2B lens system shows 72% transmissivity experimentally with a fundamental Gaussian mode as the input, in reasonably good agreement with the simulated value of 80%. With a proper aperture size, the B2B lens system is capable of filtering the non-Gaussian beam from the QCL to a nearly fundamental Gaussian beam, where Gaussicity increases from 74% to 99%, and achieves a transmissivity larger than 30%. Thus, this approach is proven to be an effective beam shaping technique for QCLs, making them to be suitable local oscillators in the terahertz range with a Gaussian beam. Besides, the B2B lens system is applicable to a wide frequency range if the wavelength dependent part is properly scaled

81 supra-THz beams generated by a Fourier grating and a quantum cascade laser

Large heterodyne receiver arrays (~100 pixel) allow astronomical instrumentations to map more area within limited space mission lifetime. One challenge is to generate multiple local oscillator (LO) beams. Here, we succeeded in generating 81 beams at 3.86 THz by combining a reflective, metallic Fourier grating with an unidirectional antenna coupled 3rd-order distributed feedback (DFB) quantum cascade laser (QCL). We have measured the diffracted 81 beams by scanning a single pyroelectric detector at a plane, which is in the far field for the diffraction beams. The measured output beam pattern agrees well with a simulated result from COMSOL Multiphysics, with respect to the angular distribution and power distribution among the 81 beams. We also derived the diffraction efficiency to be 94 ± 3%, which is very close to what was simulated for a manufactured Fourier grating (97%). For an array of equal superconducting hot electron bolometer mixers, 64 out of 81 beams can pump the HEB mixers with similar power, resulting in receiver sensitivities within 10%. Such a combination of a Fourier grating and a QCL can create an LO with 100 beams or more, enabling a new generation of large heterodyne arrays for astronomical instrumentation.ImPhys/OpticsQN/Gao La