Thermal detectors are a cornerstone of infrared and terahertz technology due to their broad spectral range. These detectors call for efficient absorbers with a broad spectral response and minimal thermal mass. A common approach is based on impedance-matching the sheet resistance of a thin metallic film to half the free-space impedance. Thereby, one can achieve a wavelength-independent absorptivity of up to 50%. However, existing absorber films typically require a thickness of the order of tens of nanometers, which can significantly deteriorate the response of a thermal transducer. Here, we present the application of ultrathin gold (2 nm) on top of a surfactant layer of oxidized copper as an effective infrared absorber. An almost wavelength-independent and long-time stable absorptivity of 47(3)%, ranging from 2 μm to 20 μm, can be obtained. The presented absorber allows for a significant improvement of infrared/terahertz technologies in general and thermal detectors in particular.

shift at internal reflection, since the intensity of the portion of the field in question is not found to be reduced appreciably when a quarter-wave plate is inserted in various azimuths and the analyzer is rotated. It is observed, as would be expected, that the reversal of the fields is more distinct when the film is birefringent than when it is optically isotropic; but qualitatively there is little difference between films of weak and of stronger birefringence.The polarizing method is of considerable practical advantage in a laboratory where refractive index measurements are made on a large number of thin films and the time element is important.A Polaroid microscope cap analyzer, such as the one listed by the Rausch & Lomb Optical Company and which fits over eyepieces up to 27 mm. in diameter, has been found to be suitable for the purpose described. Such an analyzer is, of course, a necessity in identifying the double boundaries observed when birefringent films or crystals are examined on an Abbe or other total reflection refractometer (4). Bellingham and Stanley (London) are now offering an Abbe refractometer with polarizing eyepiece (2). The polarizing screen for the illuminator may be obtained from the Polaroid Corporation or other dealers. The combination of polarizing screen and cap analyzer may be used in a simple polariscope for examination of specimens preliminary to their measurement.

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High quality factor (Q) nanomechanical resonators have received a lot of attention for sensor applications with unprecedented sensitivity. Despite the large interest, few investigations into the frequency stability of high-Q resonators have been reported. Such resonators are characterized by a linewidth significantly smaller than typically employed measurement bandwidths, which is the opposite regime to what is normally considered for sensors. Here, the frequency stability of high-Q silicon nitride string resonators is investigated both in open-loop and closed-loop configurations. The stability is here characterized using the Allan deviation. For open-loop tracking, it is found that the Allan deviation gets separated into two regimes, one limited by the thermomechanical noise of the resonator and the other by the detection noise of the optical transduction system. The point of transition between the two regimes is the resonator response time, which can be shown to have a linear dependence on Q. Laser power fluctuations from the optical readout are found to present a fundamental limit to the frequency stability. Finally, for closed-loop measurements, the response time is shown to no longer be intrinsically limited but instead given by the bandwidth of the closed-loop tracking system. Computed Allan deviations based on theory are given as well and found to agree well with the measurements. These results are of importance for the understanding of fundamental limitations of high-Q resonators and their application as high performance sensors.