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28 Jan 2002

Volume 80, Issue 4, pp. 535-701

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Photoacoustic characterization of the mechanical properties of thin films

Carmen M. Hernandez, Todd W. Murray, and Sridhar Krishnaswamy

Appl. Phys. Lett. 80, 691 (2002); http://dx.doi.org/10.1063/1.1434303 (3 pages) | Cited 24 times

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Narrow band photoacoustics (laser ultrasonics) are used to characterize the properties of free-standing nanometer-sized thin films. Photoacoustic generation is achieved by use of a microchip laser which deposits pulsed laser energy in the form of a spatially periodic source on the structure. The resulting narrow band ultrasonic modes are monitored using a Michelson interferometer. By varying the geometry of the spatially periodic source, a wide range of acoustic wave numbers is probed. Results are presented for two-layer thin film aluminum/silicon-nitride (Al/Si3N4) membranes. For such thin films, only the two lowest order guided modes are generated and these in turn can be related to sheet and flexural modes in plates. The mechanical properties and residual stress in the thin films are evaluated from measured acoustic dispersion curves for these two lowest order modes. © 2002 American Institute of Physics.
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68.60.Bs Mechanical and acoustical properties
43.58.-e Acoustical measurements and instrumentation
62.65.+k Acoustical properties of solids
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.D- Elasticity

Acoustic communications in an enclosure using single-channel time-reversal acoustics

M. G. Heinemann, A. Larraza, and K. B. Smith

Appl. Phys. Lett. 80, 694 (2002); http://dx.doi.org/10.1063/1.1445479 (3 pages) | Cited 4 times

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This letter presents experimental results of a scheme for encrypted acoustic communication in the reverberant environment of an enclosure using single-channel time-reversal acoustics. Because time-reversal acoustics in an enclosure can focus a signal within a half wavelength of sound, it possesses natural encryption for points other than the intended receiver. In this way, distinct multiple messages can be sent simultaneously from one source to different locations in the enclosure. The communication scheme is based on multiple broadband signals with identical bandwidth and windowed source spectra.
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84.40.Ua Telecommunications: signal transmission and processing; communication satellites
43.25.Cb Macrosonic propagation, finite amplitude sound; shock waves

Photoluminescence degradation in organic light-emitting devices

Lin Ke, Peng Chen, and Soo Jin Chua

Appl. Phys. Lett. 80, 697 (2002); http://dx.doi.org/10.1063/1.1446209 (3 pages) | Cited 6 times

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Polymer degradation under electrical stress was studied using the device structure indium tin oxide/poly(p-phenylene vinylene)/Ca/Ag. The devices that were used in this study do not show any significant decrease in electroluminescent intensity for up to several thousand hours of operation. However for devices that were electrically stressed and then exposed to normal ambient, there was a significant decrease in the photoluminescence. Those that were not electrically stressed but exposed to normal ambient or were electrically stressed but not exposed to normal ambient, there was no significant degradation in the photoluminescence. These results showed that both conditions of electrical stress and photo-oxidation are necessary for a significant decrease of the photoluminescence intensity. Degradation is accompanied by an increase in the x-ray crystallization peak. We interpret that the photoluminescence intensity degradation is due to the change in polymer crystallinity caused by electrical stress followed by photo-oxidation. © 2002 American Institute of Physics.
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85.60.Jb Light-emitting devices
78.66.Qn Polymers; organic compounds
78.55.Kz Solid organic materials
78.60.Fi Electroluminescence
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