Coupling light and sound: giant nonlinearities from oscillating bubbles and droplets

Abstract

Nonlinear optical processes are vital for fields including telecommunications, signal processing, data storage, spectroscopy, sensing, and imaging. As an independent research area, nonlinear optics began with the invention of the laser, because practical sources of intense light needed to generate optical nonlinearities were not previously available. However the high power requirements of many nonlinear optical systems limit their use, especially in portable or medical applications, and so there is a push to develop new materials and resonant structures capable of producing nonlinear optical phenomena with low-power light emitted by inexpensive and compact sources. Acoustic nonlinearities, especially giant acoustic nonlinear phenomena in gas bubbles and liquid droplets, are much stronger than their optical counterparts. Here, we suggest employing acoustic nonlinearities to generate new optical frequencies, thereby effectively reproducing nonlinear optical processes without the need for laser light. We critically survey the current literature dedicated to the interaction of light with nonlinear acoustic waves and highly-nonlinear oscillations of gas bubbles and liquid droplets. We show that the conversion of acoustic nonlinearities into optical signals is possible with low-cost incoherent light sources such as lightemitting diodes, which would usher new classes of low-power photonic devices that are more affordable for remote communities and developing nations, or where there are demanding requirements on size, weight and power. . 1: (a) Generation of new optical frequencies via a nonlinear optical interaction. High-power monochromatic laser light interacts with the nonlinear optical medium, e.g. a dielectric microresonator, to generate new optical frequencies [9]. (b, c) The nonlinear properties of sound can be used to generate optical nonlinearities without the need for high-power laser light. Low-power light couples to a sound wave via a deformable fluid, e.g. gas bubble or liquidmetal nanoparticle, exploiting strong nonlinear acoustic effects. This converts acoustic frequencies to the optical domain, effectively reproducing the result of the nonlinear optical interaction in (a). Images from www.freeimages.com and www.dreamstime.com. FIG

Acoustic frequency combs using gas bubble cluster oscillations in liquids: a proof of concept

“…1 a). Unlike in the scenario of an optical frequency comb generation using a high-power laser light and exploiting fundamentally weak nonlinear-optical effects 16 , we show that the application of low-pressure harmonic signals can trigger a strong nonlinear response of the cluster resulting in the generation of multiple ultraharmonic frequency peaks. The interaction with stochastic noise-induced bubble cluster oscillations at its natural frequency, which is typically much lower than that of a driving ultrasound, results in the amplitude modulation of the the bubble cluster response and the appearance of sidebands around the main peaks.…”

“…Whereas frequency combs with a Hz-range spacing can find certain applications 17 , in the gas bubble system investigated in the present work we use the high-kHz range that can potentially be extended to the high-MHz range 24 . This opens up opportunities for using acoustic combs instead of optical ones or in addition to them in a number of practical situations where operation at higher frequencies may be required 16 .…”

Harmonic and subharmonic waves on the surface of a vibrated liquid drop

“…Our results can also find applications in the emergent field of acoustic frequency comb generation [55] and their application in underwater distance measurements [56]. Similar to an optical frequency comb, an acoustic frequency comb is a spectrum consisting of a series of discrete, equally spaced elements that have a well-defined phase relationship between each other (for a review see, e.g., [40]). Optical frequency combs have typically been produced by mode-locked lasers or exploiting nonlinear optical effects in optical fibres and nonlinear photonic microresonators.…”

“…Our results should find practical applications in the emergent research directions of liquid optomechanics [36][37][38][39][40][41] and hybrid metamaterial structures [42][43][44]. Thus far, optomechanical structures have mostly been implemented by using the solid-state technology [45] because modern electronic, photonic and phononic devices and circuits are based on solid-state platforms.…”

Precise Underwater Distance Measurement by Dual Acoustic Frequency Combs

“…Acoustics—the science of sound—has expanded in a wealth of directions and plays a significant role in many arenas ranging from basic science to engineering. This new type of user‐friendly acoustic comb can enrich the comb family, and could open up new opportunities in all aspects of acoustics and enable revolutionary progress in fields such as precise dimensional measurement, communication, and underwater time/frequency transfer in the near future.…”