Pythagoras was the first to realize that at some frequencies, the vibrations of strings are greatly amplified. Our tone scheme is based on this discovery. No matter how big or small an object is, these natural vibrations always exist in it, and they are frequently used to determine its species, components, and appearance.
For example, molecular vibrations at a terahertz rate have become the most common fingerprints for the identification of chemicals and the structural analysis of large biomolecules.
Since this category comprises a large variety of functional particles as well as the majority of biological cells and viruses, natural vibrations of particles at the mesoscopic scale have recently attracted growing interest. However, current technologies have not been able to access the natural vibrations of these microscopic particles.
These particles with sizes ranging from 100 nm to 100 μm are expected to vibrate faintly at megahertz to gigahertz rates. However, high Rayleigh-wing scattering prevents current Raman and Brillouin spectroscopies from resolving this frequency range, and piezoelectric techniques, which are frequently used in macroscopic systems, dramatically degrade at frequencies over a few megahertz.
This vibrational spectroscopy enables the interrogation of structures and mechanical properties of particles in a non-destructive way. Specifically, the important biomechanical properties of cells, which are related to their species and living states, can be inferred from the vibrational spectra.
Professor Dr. Xiao Yunfeng
In a new paper published in Nature Photonics titled “Single-particle photoacoustic vibrational spectroscopy using optical microresonators,” a team led by Professor Xiao Yunfeng at Peking University has demonstrated the real-time measurement of natural vibrations of single mesoscopic particles using optical microresonators, extending the reach of vibrational spectroscopy to a new spectral window.
The working principle of microresonator-based vibrational spectroscopy is summarized according to Dr. Tang Shuijing as follows: “Mesoscopic particles are expected to vibrate at MHz to GHz rates, and their vibrational amplitudes are usually too subtle to be detected by traditional techniques. To address this issue, a new vibrational spectroscopy has been proposed. It involves using a short laser pulse to heat the particle and induce its vibrations.”
“By directly placing the particle onto a high-Q optical microresonator, the vibrations of the particle generate acoustic waves within the microresonator, ultimately perturbating its optical mode,” says Shuijing, a research associate professor at Peking University.
The scientists placed mesoscopic particles on a silicon microspherical resonator with a quality factor of around 106 and a radius of about 30 μm during the vibrational spectroscopy tests. They then used a pulsed laser (with a wavelength of 532 nm and a duration of 200 ps) to irradiate particles and stimulate their vibrations, with the incident energy density of approximately 2 pJ μm−2.
The microresonator was linked with a continuous-wave probe laser to excite its optical mode, and by observing the power of the transmitted laser, the vibrations of the particles were picked up in real-time. The vibrational spectra of the particles were produced by applying Fourier transformation to the temporal responses.
Mesoscopic particles with various compositions, sizes, and internal architectures were used to successfully validate vibrational spectroscopy. The results showed an unprecedented signal-to-noise ratio of 50 dB and a detection bandwidth over 1 GHz.
The researchers went on to show biomechanical fingerprinting of the species and life states of microorganisms at the single-cell level using this ground-breaking method. They discovered that due to the well-defined and consistent morphology of some biological species, the natural frequencies of microbial cells of the same species are bunched, generating distinctive fingerprints.
“This vibrational spectroscopy enables the interrogation of structures and mechanical properties of particles in a non-destructive way. Specifically, the important biomechanical properties of cells, which are related to their species and living states, can be inferred from the vibrational spectra,” said Dr. Xiao Yunfeng, who is a Boya Professor at Peking University.
“This technology allows vibrational spectroscopy of a wide range of mesoscopic particles, which could revolutionarily advance our understanding of the mesoscopic world with unprecedented precision,” he added.
Living cells are intricate biosystems, and cellular development, health, and illness are significantly influenced by their mechanical properties. This work provides a new fingerprinting technology to study biological systems at the single-cell level, and would lead to new insights and discoveries in various scientific fields.
