Single-Particle Photoacoustic Vibrational Spectroscopy using Optical Microresonators

The utilization of optical microresonators for single-particle photoacoustic vibrational spectroscopy.

The phenomenon of amplified vibrations in strings at specific frequencies was initially observed by Pythagoras. The aforementioned discovery serves as the foundation for our tone system. Natural vibrations are present in objects of various sizes and are commonly employed to determine their species, components, and shape. An illustration of the prevalent use of molecular vibrations at a terahertz frequency is observed in their widespread application as distinctive characteristics for chemical identification and the examination of complex biomolecular structures.

In recent times, there has been an increasing focus on the natural oscillations of particles at the mesoscopic level. This area of study has gained attention due to its encompassment of several functional particles, as well as the majority of biological cells and viruses. Nevertheless, the inherent oscillations of these mesoscopic particles have eluded detection by current technological methods.

It is anticipated that particles within the size range of 100 nm to 100 μm will exhibit subtle vibrations at frequencies spanning from megahertz to gigahertz. The existing Raman and Brillouin spectroscopies are unable to resolve this frequency regime due to the presence of substantial Rayleigh-wing scattering. Additionally, the performance of piezoelectric techniques, commonly utilized in macroscopic systems, experiences a significant degradation when operating at frequencies exceeding a few megahertz.

The recent publication titled "Single-phase tickets photoacoustic vibrational spectroscopy using optical microresonators" in the prestigious journal Nature Photonics presents the findings of a research team led by Professor Xiao Yunfeng from Peking University. The study showcases the successful utilization of optical microresonators to capture real-time measurements of the inherent vibrations of individual mesoscopic particles. This breakthrough expands the capabilities of vibrational spectroscopy by introducing a novel spectral window.

Dr. Tang Shuijing provides a summary of the operational mechanism of microresonator-based vibrational spectroscopy as follows: It is anticipated that mesoscopic particles will exhibit vibrations at frequencies ranging from megahertz to gigahertz, with vibrational amplitudes typically too small to be discerned by conventional methodologies. In order to tackle this matter, a novel vibrational spectroscopy technique has been put forth. The process entails the utilization of a brief laser pulse to elevate the temperature of the particle and stimulate its oscillations. According to Shuijing, a research associate professor at Peking University, when the particle is positioned directly onto a high-Q optical microresonator, its oscillations produce acoustic waves within the microresonator, thereby perturbing its optical mode.

In the course of the vibrational spectroscopy tests, the scientists applied mesoscopic particles onto a silicon microspherical resonator possessing a radius of roughly 30 μm and a quality factor of approximately 106. Subsequently, a pulsed laser was employed, possessing a wavelength of 532 nm and a period of 200 ps, to subject particles to irradiation and induce their vibrational activity. The incident energy density was estimated to be around 2 pJ μm−2.

The microresonator was subjected to optical excitation of its mode by the use of a continuous-wave probe laser. The real-time monitoring of the transmitted laser's strength allowed for the detection of the particles' vibrations. The vibrational spectra of particles were generated by employing Fourier transformation on the temporal responses.

The efficacy of vibrational spectroscopy was effectively validated through the utilization of mesoscopic particles that possess varying constituents, sizes, and interior structures. The findings demonstrated a signal-to-noise ratio of 50 dB, which is unique, and a detection bandwidth exceeding 1 GHz.

The researchers showcased the ability to do biomechanical fingerprinting of microbes at the individual cell level, effectively identifying their species and life conditions using this groundbreaking approach. It has been observed that microbial cells of the same species exhibit clustered natural frequencies, resulting in distinct fingerprints. This phenomenon can be attributed to the well-defined and stable morphology exhibited by specific biological species.

Vibrational spectroscopy facilitates the examination of the structures and mechanical characteristics of particles in a non-invasive manner. According to Dr. Xiao Yunfeng, a Boya Professor at Peking University, the vibrational spectra can provide insights into the biomechanical features of cells, namely those that are associated with their species and living conditions.

The individual stated that this technique enables the application of vibrational spectroscopy to a diverse array of mesoscopic particles. This has the potential to significantly enhance our comprehension of the mesoscopic realm by providing unparalleled precision.

Living cells are intricate biological systems, and their mechanical characteristics are of significant importance in relation to cellular functionality, growth, and pathological conditions. This paper presents a novel fingerprinting technique for investigating biological systems at the individual cell level, which has the potential to generate novel findings and enhance understanding across several scientific disciplines.