We have developed fiber-based optical thermocouples (OTCs) for fast temperature sensing in extreme environments. Our OTCs consist of a thin film of dysprosium-doped yttrium aluminum garnet (Dy:YAG)—a well-known two-color thermometry phosphor—deposited on the end of a sapphire fiber using pulsed laser deposition. Temperature sensing is achieved by comparing the relative intensities of photoluminescence arising from two closely spaced Dy3 + excited states. Using a combination of time-gated detection and blackbody background subtraction, we are able to measure Dy:YAG’s photoluminescence up to 2033 K, which is one of the highest temperatures obtained in literature. However, we are only able to use the photoluminescence spectra for temperature sensing up to 1773 K due to poor signal-to-noise ratio for higher temperatures. These results suggest the possibility of measuring higher temperatures with time-gated detectors designed for low-light levels. After characterizing the fiber-based OTCs’ temperature response, we next demonstrate their functionality using subsecond pulsed CO2 laser heating using both intensified charge-coupled device detection and a photodiode-based software time-gating technique. In the lab, we have utilized this technique to measure temperatures at rates up to 80 kHz. In addition, we comment on the applicability of OTCs to fast temperature sensing in turbulent flows and estimate rise times on the order of several hundred microseconds for a 1-μm OTC film.
Electrical conduction in materials used in microbolometer technology, such as vanadium oxide (VOx) and amorphous silicon (a-Si), is via carrier hopping between localized states. The hopping conduction parameters determine the temperature coefficient of resistance (TCR), its temperature dependence, and its relationship to resistivity. The electrical noise has a 1/f component that is also associated to the hopping parameters and thus correlated to TCR. Current research on conduction in cross linked metal nanoparticles organized in an insulating matrix shows that TCR and noise can be controlled independently, potentially allowing for precise tailoring of the detector response for differing applications.
Authentication/tamper-indication is required in a wide range of applications, including nuclear materials management and product counterfeit detection. State-of-the-art techniques include reflective particle tags, laser speckle authentication, and birefringent seals. Each of these passive techniques has its own advantages and disadvantages, including the need for complex image comparisons, limited flexibility, sensitivity to environmental conditions, limited functionality, etc. We have developed a new active approach to address some of these short-comings. The use of an active characterization technique adds more flexibility and additional layers of security over current techniques. Our approach uses randomly-distributed nanoparticles embedded in a polymer matrix (tag/seal) which is attached to the item to be secured. A spatial light modulator is used to adjust the wavefront of a laser which interacts with the tag/seal, and a detector is used to monitor this interaction. The interaction can occur in various ways, including transmittance, reflectance, fluorescence, random lasing, etc. For example, at the time of origination, the wavefront-shaped reflectance from a tag/seal can be adjusted to result in a specific pattern (symbol, words, etc.) Any tampering with the tag/seal would results in a disturbance of the random orientation of the nanoparticles and thus distort the reflectance pattern. A holographic waveplate could be inserted into the laser beam for verification. The absence/distortion of the original pattern would then indicate that tampering has occurred. We have tested the tag/seal’s and authentication method’s tamper-indicating ability using various attack methods, including mechanical, thermal, and chemical attacks, and have verified our material/method’s robust tamper-indicating ability.