Characterization of Mechanical and Thermal Properties Using Ultrafast Optical Metrology

Abstract

Ultrafast lasers have long been used to study the dynamics of fast optical, electronic, and chemical processes in materials. These tools can also be used in a variety of optical pump and probe spectroscopies to generate and detect acoustic signals with frequencies on the order of 100 GHz, and to generate and detect thermal waves with penetration depths on the scale of nanometers. The short wavelengths of these probes make them ideal for the study of the mechanical and thermal properties of thin films, their interfaces, and nanostructures. We describe the picosecond-laser acoustics technique and demonstrate how it can be used to extract the elastic constants and the adhesion of thin films and probe the normal modes of vibration of nanostructures. The thermal properties of thin films are also accessible through time-domain thermoreflectance. Since the mechanical and thermal properties can be obtained quickly on micrometer-scale regions of a sample, spatial mapping of the properties is also possible.

Coherent Control of GHz Resonant Modes by an Integrated Acoustic Etalon

“…In conclusion, we have shown how a compliant supporting layer can allow multiple harmonics to persist simultaneously in a supported membrane resonator, the dynamics of which differ fundamentally from the echo behavior typically observed when metal transducers are placed on noncompliant substrates [8,16,19]. Because the compliant layer forms part of the acoustic cavity, …”

“…The acoustic dynamics of supported thin films under ultrafast laser excitation have been the subject of several recent studies [8,[16][17][18][19][20]. The absorption of laser pulses by a metal transducer in contact with a relatively hard material such as silicon or sapphire is known to lead to a train of acoustic echoes (rather than excited vibrational modes of the film) if the metal thickness is much larger than the optical penetration depth [8,16]. If the transducer thickness is reduced to the order of the optical penetration depth, direct coupling of energy from the laser pulse to the first resonant vibrational mode of the transducer leads to strong excitation of this mode.…”

Design of Transition-Metal Nitride Thin Films for Thermoelectrics

“…There are a few techniques that were developed to solve this issue such as the hot wire method, 3-ω method and the timedomain thermoreflectance. 290 TDTR is used to determine thermal conductivity of our thin film samples. 290 This technique uses the idea of an acoustic strain wave that is generated when the materials relax from thermal stress by the thermal expansion due to a local heating from an optical pulse.…”

“…290 TDTR is used to determine thermal conductivity of our thin film samples. 290 This technique uses the idea of an acoustic strain wave that is generated when the materials relax from thermal stress by the thermal expansion due to a local heating from an optical pulse. This acoustic strain wave will later propagate in …”

“…So, we can obtain the amount of temperature change from ∆T (z) = (1 − R) Q C(ξA) e −z/ξ , (6.18) where R is the sample reflectivity, Q is the optical beam energy, C is the specific heat per unit volume, A is the optical spot area, ξ is the optical absorption length, and z is the distance into sample. TDTR uses the piezo-optic effect 290 to obtain thermal conductivity of thin film samples by fitting the change of reflectivity of a thin metal layer, typically ∼100 nm of Al that was put on top of the thin film sample II Here the experiment is done by following: a short and low energy pump pulsed laser beam (typically ∼100 ns and ∼1nJ) is focused on the metal film to heat up for a few Kelvin. After absorption of a pump laser pulse for a few nanoseconds, the metal film will cool down due to heat propagation passing through interface into film sample.…”