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Dielectric resonators are key components for many microwave and millimetrewave applications, including high-Q filters and frequency-determining elementsfor precision frequency synthesis. These often depend on the quality of thedielectric material. The commonly used material for building the best cryogenicmicrowave oscillators is sapphire. However sapphire is becoming a limitingfactor for higher frequencies design. It is then important to find newcandidates that can fulfil the requirements for millimetre wave low noiseoscillators at room and cryogenic temperatures. These clocks are used as areference in many fields, like modern telecommunication systems, radioastronomy (VLBI), and precision measurements at the quantum limit.High-resolution measurements were made of the temperature-dependence of theelectromagnetic properties of a polycrystalline diamond disk at temperaturesbetween 35 K and 330 K at microwave to sub-millimetre wave frequencies. Thecryogenic measurements were made using a TE01{\delta} dielectric mode resonatorplaced inside a vacuum chamber connected to a single-stage pulse-tubecryocooler. The high frequency characterization was performed at roomtemperature using a combination of a quasi-optical two-lens transmission setup,a Fabry-Perot cavity and a whispering gallery mode resonator excited withwaveguides. Our CVD diamond sample exhibits a decreasing loss tangent withincreasing frequencies. We compare the results with well known crystals. Thiscomparison makes clear that polycrystalline diamond could be an importantmaterial to generate stable frequencies at millimetre waves.

Electromagnetic properties of polycrystalline diamond from 35K to room temperature and microwave to terahertz frequencies