J. Eur. Opt. Society-Rapid Publ. 2025, 21, 26 Ó The Author( s), published by EDP Sciences, 2025 https:// doi. org / 10.1051 / jeos / 2025014 Available online at: https:// jeos. edpsciences. org
Journal of the European Optical Society-Rapid Publications REVIEW ARTICLE
Light and sound interplay in whispering gallery mode resonators
Gabriele Frigenti, Daniele Farnesi, Simone Berneschi, Stefano Pelli, Giancarlo C. Righini, Gualtiero Nunzi Conti, and Silvia Soria Huguet *
1 CNR-IFAC, Institute of Applied Physics“ N. Carrara”, Via Madonna del Piano 10, 50019 Sesto Fiorentino, Italy
Received 25 November 2024 / Accepted 25 March 2025
Abstract. Whispering Gallery Mode Resonators( WGMRs) are robust and compact structures that confine resonant photons and phonons for extended time. This extraordinary confinement greatly enhances light and sound interactions and allows a plethora of fundamental phenomena to happen, ranging from nonlinear optics with continuous wave lasers to exceptional point going through mode shifting, splitting and broadening. These WGMR are highly versatile since their design can be tailored to the application by modifying either their size or the material used for their fabrication. We will focus on three dimensional WGMR, we will describe the physical working principles, their fabrication and their applications as sensors.
Keywords: Whispering gallery mode, Resonators, Optomechanics, Sensors.
1 Introduction
The first studies on the generation of sound waves through the light-matter interaction are dated back to the 1870 – 1880 decade, and are especially associated with the activity of Alexander Graham Bell on telecommunications. Bell designed an ingenious device called the phonophone, using the photoconductivity properties of selenium foils discovered by Willoughby Smith in 1873. The phonophone allowed to translate speech into light modulation and then light modulation again into speech, achieving the first wireless phone call in history [ 1 ]. Obviously, this result sparked the interest of the scientific community but quantitative studies were limited by the technology available at the time( mostly due to the lack of sensitive microphones and bright light sources). Such technological barrier was overcome in the 20th century, with the advent of capacitive microphones and of laser sources, which brought new life to the subject and allowed the implementation of the photoacoustic( PA) effect in several fields [ 1 ]. The PA effect is the emission of an acoustic shockwave from an optical absorber after its excitation with a short laser pulse. The shockwave is the final step of a complex chain of thermo-elastic processes involving the absorber and the host material surrounding it. Thus, the PA effect is an effective tool for the investigation of the absorption spectrum of the absorber, its elastic properties and its non-radiative de-excitation, since it combines high optical selectivity and mechanical relaxation.
Recently, PA has been implemented with success in the biomedical field for its optical specificity towards natural
* Correspodning author: s. soria1s. soria @ ifac. cnr. it
( melanin, hemoglobin, lipids, collagen, etc.) or artificial( metal nanoparticle, dyes, quantum dots, etc.) optical absorbers and unhindered propagation of acoustic waves into biological tissues( skin, blood, etc.). Most PA detectors are piezoelectric ultrasound transducers and are available on the market in various designs, allowing to cover various possibilities in terms of detection( e. g. bandwidth, acceptance angle, sensitivity). However, they are difficult to miniaturize because this process leads to strong degradation in performances. Optical detection of the PA wave represents a solution to these problems, since it proved effective to combine miniaturisation with high-performances [ 2 ].
The PA effect excites the vibrational modes of particles regardless of their size. These natural vibrations were studied first by the Greek philosopher Pythagoras who noticed that string vibrations were greatly enhanced at given frequencies. Vibrational spectroscopy is a well-known non-destructive technique used for identifying the spectral fingerprints of several objects, such as stellar oscillations( millihertz range), electronics elements( kilohertz range) and large biomolecules( terahertz). However, the current optical and piezoelectric spectroscopy fails to detect faint vibrations of mesoscopic objects occuring in the megahertz-gigahertz frequency band [ 3 ].
The first studies combining photons and phonons( either optical or acoustic ones) can be traced back to Chandrasekhara Venkata Raman and Léon Brillouin. Both Stimulated Raman Scattering( SRS) and Stimulated Brillouin Scattering( SBS) are pure gain processes and, consequently, naturally phase-matched nonlinear phenomena. Both phenomena are widely used for vibrational spectroscopy, but they cannot resolve vibrations of mesoscopic
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