Ultrafast laser-induced strain waves in thin ruthenium layers.

Area of Science:

• Materials Science
• Acoustics
• Optics

Background:

• Laser-induced acoustic waves are crucial for material characterization.
• Understanding material properties at the nanoscale requires advanced probing techniques.
• Ruthenium thin films have applications in electronics and catalysis.

Purpose of the Study:

• To investigate time-dependent optical diffraction from laser-induced acoustic waves in ruthenium thin films.
• To analyze the thermo-optic and strain-optic effects in ruthenium layers.
• To study the generation and detection of ultra-high frequency acoustic waves (130-750 GHz) in ultra-thin films (1.2-20 nm).

Main Methods:

• Time-dependent optical diffraction measurements.
• Generation and detection of longitudinal acoustic waves using pulsed lasers.
• Analysis of acoustic wave propagation in ruthenium thin films on glass substrates.

Main Results:

• Thermo-optic and strain-optic effects were identified as dominant optical responses.
• A strong dependency of the speed of sound on layer thickness (and thus frequency) was observed.
• A faster signal decay at higher frequencies due to acoustic impedance mismatch was measured.
• Acoustic wave frequency and phase remained constant for >2 ps, with an extrapolated initial phase of -π/2.

Conclusions:

• The study demonstrates the feasibility of generating and detecting THz acoustic waves in nm-thick ruthenium films.
• Thickness-dependent sound speed and acoustic impedance mismatch influence wave propagation and signal decay.
• The observed initial phase suggests potential dynamic phase/frequency changes within the first 2 ps post-excitation.