Optical direct detection of thermal vibrations of ultralow stiffness micro-nano structures.

Nanomechanics;thermal vibration;optical detection;bead-on-a-string(BOS) fibers;quadrant photo detectors (QPD);young's modulus

Sri Sukanta Chowdhury

University:University of Louisville

Department:Electrical and Computer Engineering

关键词: Nanomechanics;    thermal vibration;    optical detection;    bead-on-a-string(BOS) fibers;    quadrant photo detectors (QPD);    young's modulus;   

Others  :  https://ir.library.louisville.edu/cgi/viewcontent.cgi?article=4503&context=etd

美国|英语

来源: The Universite of Louisville's Institutional Repository

【 摘 要 】

A direct detection optical vibrometer is constructed around an 850 nm laser and a quadrant photodetector (QPD). The limit of detection is 0.2 fW which corresponds to a minimum amplitude of 0.1 Å. The vibrometer is used to measure the thermal vibration spectra of low stiffness micromechanical structures have nanometer features. One structure measured is a cantilevered 30 μm diameter glass fiber. Vibration amplitudes as low as 1.1 Å are measured. The thermal vibration spectra show fundamental resonances at 80-250 Hz and a signal to noise ratio (SNR) of 23-55 dB. Young’s modulus of glass in the cantilevers, estimated from the spectra, agree to within 3 % of the manufacturer’s value, which is somewhat more accurate than force-elongation measurements made of 50-100 mm long fibers which differ by 5 %. Mass changes due to adhering small drops of liquids to the tip of the fiber cantilevers shifts the resonant frequency with a sensitivity of 120 ng. The mass detection limit would decrease by 2-3 orders by increasing the length of the time series data. The intended purpose of the vibrometer development is the measurement of the thermal vibration of polymer bead-on-string (BOS) fibers with enough sensitivity to detect time-varying changes in the spectra that relate to molecular-level and temperature dependent changes, such as evaporation, solidification, crystallization and strain-dependent chain reorganizations of the polymer material. Time dependent variations in the BOS spectra are observed in vibrometer measurements that, if attributable to material properties, would represent 2.5-5.2 % change in elastic modulus, 20-40 % loss in water mass due to evaporation, with the minimum detectable change in these properties being 0.06 % for the measured spectra. The vibrometer provides an important tool for the real-time study of changing properties of BOS fibers, as well as other low stiffness microstructures, especially those composed of polymers and other soft mater.