Novel detectors for traceable THz power measurements

Abstract

Several novel types of detectors for the measurement of electromagnetic radiation in the THz spectral range are described. Firstly, detectors based on pyroelectric foil coated with different absorbers have been developed focusing on the following features: high accuracy due to well-characterized absorption, high sensitivity, large area absorbers and frequency and polarization independence. A three-dimensional design with five absorptions gave an overall absorption of more than 98 %. Secondly, detectors based on pyroelectric foils with thin metal layers were realized. An absorption of 50 % can be obtained if the thickness of the layers is carefully adjusted. According to electromagnetic theory this degree of absorption is independent of the polarization and frequency of the radiation in a wide range from at least 20 GHz to 5 THz. The third type of detector is based on a new type of volume absorber with a polished front surface and a gold-coated back side. It is the absorber of choice of the standard power detector for disseminating the spectral power responsivity scale. This standard detector allows the application of a physical model to calculate its spectral responsivity in the range from 1 THz to 5 THz if the detector has been calibrated at one single frequency. Finally, a THz detector calibration facility was set up and is now in operation at PTB to calibrate detectors from customers with an uncertainty as low as 1.7 %.

Terahertz power calibration using absolute reference calorimeter

“…In this calorimeter, we used an optically polished neutral-density filter glass (Schott, NG1) used as the absorber. The reflectance at 1 THz was reported to be 0.174, with a standard uncertainty of 4% [12,16]. From Eq.…”

“…For a source-based approach, several techniques based on blackbody radiation have been reported [3][4][5][6]; however, these approaches suffer from the presence of unwanted radiation, such as infrared radiation. A second approach that utilizes a detector-based standard has the advantage of precisely measuring absolute power; therefore, several national metrology institutes around the world have focused on the development of the detector-based standard [7][8][9][10][11][12][13]. In this regard, pulse THz power measurements have also been demonstrated with the use of certain types of calibrated detectors [14,15].…”

Characterization of a Large-Area Pyroelectric Detector from 300 GHz to 30 THz

“…The detector and its underpinning technology are described in detail in [6]. It consists of a pyroelectric polyvinylidene fluoride (PVDF) foil coated on both sides with a thin layer of a metal oxide.…”

Selective Pyroelectric Detection of Millimetre Waves Using Ultra-Thin Metasurface Absorbers

“…Pyroelectric detectors are proven to be one of the most effective, compact and relatively inexpensive types of uncooled thermal sensors widely used for infrared (IR) detection 1 . Nowadays, following a fast progress in terahertz (THz) and millimetre-wave (MMW) technologies 2 3 4 5 , a lot of efforts are being made to improve the sensitivity of pyroelectric detection at longer wavelengths λ far beyond the IR range 5 6 7 8 9 10 . In particular, the short MMW and sub-MMW bands (λ ≈ 0.5–3 mm) remain highly attractive for various applications, including security imaging and surveillance, nondestructive evaluation and quality control of materials etc., due to an opportunity to combine the options of relativity high penetrability of such waves through atmosphere and different non-metallic objects versus high-frequency THz, IR and optical radiation, as well as attainability of spatial resolution of the order of several millimetres acceptable for imagining of concealed targets 2 3 4 11 .…”

“…This problem appears due to rapidly diminishing absorption of long-wave radiation in a thin pyroelectric film when the wavelength increases. Currently, noticeable technological efforts are being undertaken to create a sensitive broadband pyro-detector applicable to spectroscopy tasks that is achieved via integration of absorptive metallic, oxide or carbon nanotubes coatings with pyroelectric films 9 10 . On the other hand, in spectrally selective detection the detector’s sensitivity can be maximized if the radiation is absorbed by a thin metamaterial structure whose heating is further sensed with a pyroelectric.…”