Enhancement of ultrahigh-frequency vibration of an Au/Si₃N₄ composite resonator with picosecond ultrasound

Abstract

We found that the second through-thickness vibration mode of a gold/silicon-nitride composite thin film is selectively excited by picosecond ultrasound with high efficiency by controlling their thickness ratio. Its mechanical vibration frequency exceeds 50 GHz, being promising for ultrahigh-sensitive mass detector. We developed a theoretical model based on the strain-energy ratio between gold and silicon-nitride layers, and successfully explained the enhancement of the second vibration mode. The sensor chip was applied to detect immobilized proteins through the change in resonance frequency.An ultrafast light pulse excites subterahertz-frequency acoustic waves in nanoscale materials through impulsive thermal expansion, and they are detected by another ultrafast light pulse through modification of surface dielectric constant (photoelastic effect). This optical pumpprobe technique is called picosecond ultrasonic (PSU) method, which was first established by Thomsen and coworkers. 1) Several researchers evolve this method for studying behaviors of ultrahigh-frequency phonons in nanoscale materials. Their achievements include highfrequency surface-wave imaging, 2) evaluation of ultrahigh-frequency acoustic damping, 3,4) thermal excitation of shear waves, 5) discovery of anomalous elasticity of nanomaterials. [6][7][8] Recently, PSU method is intensively studied for ultrahigh-frequency oscillators: A resonator can be a mass-sensitive sensor because its resonance frequencies are changed by the mass-loading effect, and the mass sensitivity becomes significantly higher as the resonator mass decreases, corresponding to higher-frequency resonator. Because the PSU method allows excitation of ultrahigh-frequency vibrations of nanoscale materials, it is promising for ultrahigh-sensitive sensors. Surface acoustic wave resonances were detected by fabricating nanowires [9][10][11] and nanodots 12) on substrates, achieving ∼10 GHz resonators. Ultrafast vibrations (∼10 GHz) of GaAs nanopillars 13) and Au nanorings 14) were also successfully realized.In these methods, however, there is a frequency limitation caused by the lithography fineness. Furthermore, they require multistep fabrication procedures costly. The simplest resonator is a flat thin film, whose resonant frequencies can be easily increased by thinning the deposited film.The thin-film resonator were actually utilized for biosensors for detecting target proteins. 15) The disadvantage of the deposited thin-film resonator is significant vibrational-energy leakage 1/??

Elastic constant of dielectric nano-thin films using three-layer resonance studied by picosecond ultrasonics

“…In this study, we made Pt/NiO/Pt films and determined the elastic constants of NiO from the multilayer resonance. We use a one-dimensional free-resonance model, [31][32][33] and calculate resonance frequencies f and corresponding distributions of strain ε(z) and strain energy U(z) by solving the frequency equation, where z denotes the coordinate along the thickness direction. Here, we briefly describe the procedure. )…”

GHz-range resonant ultrasound spectroscopy for a free-standing nano film studied by picosecond ultrasonics

“…We analytically calculate resonance frequencies of a free-standing multilayer using a one-dimensional continuous model. 47,48) We used mass densities of AlN, Ru, and Cr of their bulk values of 3260, 12360, and 7200 kg m −3 , respectively. The forward calculation with reported elastic constants [49][50][51][52][53][54] allows mode identification up to the 17th mode of 63.8 GHz.…”