Low-weight, passive, thermal-adaptive radiation technologies are needed to maintain an operable temperature for spacecraft while they experience various energy fluxes. Vanadium dioxide (VO2) is a commonly used dynamic response material that can transition from a low emissivity (insulating) state to a high emissivity (radiating) state near room temperature. In this study, we used a thin-film coating with the Fabry-Perot (FP) effect to enhance emissivity contrast (Δε) between the VO2 phase-change states. This coating utilizes a novel hybrid material architecture that combines VO2 with the mid- and long-wave infrared transparent chalcogenide, zinc sulfide (ZnS) as a cavity spacer layer. We simulated the design parameter space to obtain a theoretical maximum Δε of 0.63 and grew a prototype device. Using x-ray diffraction, Raman spectroscopy, and Fourier Transform Infrared (FTIR) Spectroscopy, we determined that an intermediate buffer layer of TiO2 is necessary to execute the polycrystalline growth of VO2 on ZnS. We optically characterized the pulsed laser deposition grown VO2 and ZnS using IR-spectroscopic ellipsometry. Through measuring the temperature-dependent FTIR spectroscopy, our prototype sample demonstrated FP-cavity enhanced adaptive thermal emittance.
As a result of their ability to amplify input light, ultra-high quality factor (Q) whispering gallery mode optical resonators fabricated from silica have demonstrated extremely low threshold nonlinear behaviors (eg FWM, Raman). However, while the cavity Q may reduce the threshold, it is not able to improve the efficiency. By coating optical resonators with gold nanorods functionalized with small molecule coatings or magnetic nanoparticles, we are able to increase the nonlinearity of the material system and demonstrate an efficient frequency comb generator in the near-IR. Additional nonlinear behaviors, e.g. Anti-Stokes/Stokes generation, are also observed with low thresholds.
Azobenzene is capable of reversibly switching its conformation upon the UV/Visible optical exposure due to its reversible trans/cis photoisomerization. By merging this organic material with conventional photonic devices, new architectures can be developed. In our study, we developed hybrid organic/inorganic whispering gallery mode microcavities consisting of a self-assembled 4-(4-diethylaminophenylazo)pyridine (Aazo) monolayer anchored on an integrated SiO2 optical microtoroid. As the Aazo monolayer changed conformations, the resonant wavelength was tuned. The surface density of Aazo was modified by introducing CH3 spacer molecules providing control over the magnitude of the shift. Owing to the uniformity of Aazo monolayer, cavity quality factors reached above 1 million in the near-IR range. Two optical lasers were simultaneously coupled into the Aazo-coated devices with a single waveguide. The 1300 nm laser is used to excite and monitor a single resonant wavelength of the cavity, and the 410 nm laser triggers the thermodynamically stable trans-Aazo to photoswitch to the thermodynamically unfavored cis-Aazo. When the Aazo photoswitches, the cavity resonant wavelength at near-IR wavelength shifts due to a change of refractive index in the Aazo layer. To revert the molecule back to trans-Aazo, a CO2 laser is used to heat the device system. Even after storage in air, the switching behavior is unchanged. Theoretical analyses are conducted based on density functional theory of the Aazo isomers combined with finite element method simulations of the optical mode. The theoretical results agree with the experimental findings.
To make transformative leaps in human health and wellness, our approach to healthcare must be reimagined. Researchers are pursuing multiple avenues, such as integrating health monitoring systems into wearable devices and leveraging machine learning methods to analyze health data. However, many of these approaches are pursued in isolation, and the resulting data is not cross correlated. By working directly with clinicians and clinician-researchers as well as bioinformaticians, we are identifying and addressing their critical needs. This presentation will discuss a recently developed instrument for measuring the elasticity of living tissue. Inspired by conventional mechanical compression testing, the portable instrument replaces the conventional pressure sensor with an array of optical fiber polarimetric sensors to improve both the resolution and sensitivity. These improvements allow the mechanical properties of unprocessed, living, resected tissue to be analyzed. To date, animal tissue samples (several organ systems and cartilage) have been measured. As a result of the improvement in resolution, micron-scale mechanical deformation behavior has been detected, in agreement with the tissue architecture. More complex investigations into the biomechanical properties of tumors (patient derived xenographs) are ongoing as well as improvements in the system design to accelerate data collection and analysis.
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