The blackbody radiation limit has traditionally been set forth as the ultimate performance limit of thermal detectors.
However, this fundamental limit assumes that the detector absorbs uniformly throughout the thermal spectrum. In much
the same way as photon detectors can achieve very high D* because they do not absorb photon energies below their
bandgap, so too can thermal detectors except that thermal detectors are not limited to cryogenic operation. In both cases,
the enhanced theoretical D* is achieved because the radiation noise is reduced in a device that does not absorb at a
uniform high level throughout the thermal emission band. There are multiple ways to achieve high D* in thermal
detectors. One is to use materials that absorb only in a certain spectral range, just as in photon detectors. For example a
detector made from PbSe, with proper optical coupling, absorbs only photons with wavelengths shorter than 4.9μm. The
radiation limited detectivity of such a device can theoretically exceed 9 x 1010cmHz1/2/W in the MWIR. Even with
Johnson and 1/f noise estimates included, it can still exceed 2.5x1010cmHz1/2/W in the MWIR. Another technique,
applicable for narrowband thermal detectors, is probably even more powerful. Consider a thermal detector that is almost
completely transparent. Here, the radiation noise has been reduced but the signal has been reduced even more. However,
if the device is now placed inside an optical cavity, then at one wavelength and in one direction, the nearly transparent
detector couples to the cavity resonance to absorb at 100%. Radiation from all other wavelengths and directions are
rejected by the cavity or are absorbed only weakly by the detector. It is shown that theoretically, the D* of these devices
are roughly proportional to the inverse square root of the spectral resonant width under certain conditions. It is also
shown that even including Johnson noise and 1/f noise, the practically achievable D* approaches or exceeds 1011
cmHz1/2/W.
In this paper, we propose a resonator structure with a high Q factor, which has the potential to replace the Quartz crystal. In order to achieve a higher Q, complete decoupling of the resonating structure from the supporting ends is desired. One of the ways of achieving this is to distribute the stress uniformly throughout the support beam rather than their being concentrated at the fixed ends. This suggests a resonating mass supported by torsion wires. Here, three resonating structures are connected in a serial fashion. The outer two resonators act as interfaces to the external oscillator circuit, while the centre one is fully decoupled. The losses at the end of torsion wires are thus kept to lowest value. The other losses, namely damping and internal friction losses, are kept to a minimum by providing vacuum and using high quality material for the torsion wire. Modal analysis and static stress analysis were done on the structures and results clearly show the fundamental torsion mode of vibration and extremely small stresses at the fixed ends of the torsion wire.
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