Aqueous blackbody calibration source for millimeter-wave/terahertz metrology

Area of Science:

• Electromagnetics and Applied Physics
• Radiometry and Calibration
• Terahertz Science and Technology

Background:

• Accurate calibration sources are crucial for precise measurements in the millimeter-wave (MMW) and terahertz (THz) frequency regimes.
• Existing calibration sources often face limitations in broad bandwidth, temperature stability, or emissivity.
• Liquid water's high absorption properties make it a promising candidate for a broadband emitter based on the principle of reciprocity.

Purpose of the Study:

• To describe a novel calibrated broadband emitter for MMW through THz frequencies.
• To detail the design and performance of an aqueous blackbody calibration source utilizing liquid water.
• To quantify the radiometric temperature uncertainty of the developed calibration source.

Main Methods:

• Utilized liquid water as the primary emitter material due to its high absorptivity.
• Employed an optical trap geometry within an expanded polystyrene (EPS) container to shape the water and minimize interface reflectance.
• Maintained a uniform, selected temperature for the water emitter.
• Analyzed the effect of s- and p-polarized reflections at 45 degrees on effective aperture emissivity.

Main Results:

• Achieved a theoretical effective aperture emissivity greater than 98.8% for water reflectance of 40% at 45 degrees in W-band.
• Demonstrated a maximum radiometric temperature uncertainty of +/-0.40 K from W-band to 450 GHz, independent of water temperature.
• Observed an increased maximum uncertainty of -3 K at 1 THz due to EPS scattering and absorption effects.

Conclusions:

• The aqueous blackbody calibration source provides a highly emissive and stable radiometric standard for MMW and THz applications.
• The optical trap geometry effectively minimizes undesirable reflectance, enhancing measurement accuracy.
• The source exhibits low temperature uncertainty across a wide frequency range, with predictable increases at higher THz frequencies.