Impulsive Stimulated Thermal Scattering

Abstract

Impulsive stimulated thermal scattering (ISTS) is a purely optical, non‐contacting method for characterizing the acoustic behavior of surfaces, thin membranes, coatings, and multilayer assemblies, as well as bulk materials . The method has emerged as a useful tool for materials research in part because: (1) it enables accurate, fast, and nondestructive measurement of important acoustic (direct) and elastic (derived) properties that can be difficult or impossible to evaluate in thin films using other techniques; (2) it can be applied to a wide range of materials that occur in microelectronics, biotechnology, optics, and other areas of technology; and (3) it does not require specialized test structures or impedance‐matching fluids which are commonly needed for conventional mechanical and acoustic tests. Further, recent advances in experimental design have simplified the ISTS measurement dramatically, resulting in straightforward, low‐cost setups, and even in the development of a commercial ISTS photoacoustic instrument that requires no user adjustments of lasers or optics. With this tool, automated single‐point measurements, as well as scanning‐mode acquisition of images of acoustic and other physical properties, are routine. ISTS, which is based on transient grating (TG) methods, is a spectroscopic technique that measures the acoustic properties of thin films over a range of acoustic wavelengths. This article, focuses only on ISTS measurement of acoustic motions in thin films. It begins with an overview of other related measurement techniques. It then describes the ISTS acoustic data and demonstrates how it can be used to determine: (1) the stress and flexural rigidity in thin membranes; (2) the elastic constants of membranes and supported films; and (3) the thicknesses of single or multiple films in multilayer stacks.

Pressure, temperature, and orientation dependent thermal conductivity of $α$-1,3,5-trinitro-1,3,5-triazinane ($α$-RDX)

“…Two time domain signals are present in the recorded measurement: the 10's nanosecond fast damped oscillatory portion and the 20 microsecond slow decay. The damped oscillatory portion can be used to calculate the sound speed based on the frequency of the damped oscillations and the wavelength of the probe beam 47 . Rather than use a power spectrum fit 47,49,66 to find the frequency of the oscillatory portion, a nonlinear fit based on an analysis of variance was used,…”

“…ISTS is a non-contact optical technique for measuring thermal conductivity and sound speed in-situ. [42][43][44][45][46][47][48][49][50] A pair of 28 ps pump pulses (500 Hz repetition rate) at 1064 nm and continuous wave 532 probe beams were generated using the ±1 order of a diffractive optical element. A spatial mask removed extraneous orders for both beams.…”

“…Since RDX has weak absorption at the pump wavelength, the diffracted signal was small enough that a heterodyne technique was employed, wherein a spatially coincident local oscillator (LO) was mixed with the diffracted signal to simultaneously increase weak diffraction signals and reduce parasitic oscillator contributions. [46][47][48] A calcite phase plate was used to control the relative phase between the signal and LO beams. In phase and out of phase were defined to be 0 • and 180 • , respectively, relative between the LO and signal beam phases.…”