## 4 Conclusions and Open Questions
Thirdly, in the field of nanophotonics it is well established that luminescent quantum emitters have relatively long emission lifetimes of several tens of nanoseconds and non-directional emission patterns. These intrinsic optical properties are not suitable for applications in many nanophotonic devices such as single-photon sources [109], where fast radiative rates are required for operation at high frequencies, but directionality is needed to achieve a high efficiency of light collection [110, 111]. One of the approaches to the resolution of this problems exploits the Purcell effect--a modification of the spontaneous emission rate of an emitter of light in the presence of a resonant cavity [110, 111, 112]. The Purcell effect has also been observed in systems consisting of plasmonic nanoparticles--nanoantennas [61, 110, 111, 112]--which can modify the photonic environment of fluorescent sources of light by enhancing the spontaneous emission rate. Given that the physics of sonoluminescence can also be explained in terms of quantum radiation [113, 114], it is conceivable that the properties of sonoluminescence-based light sources could also be controlled using the Purcell effect originating from the plasmonic properties of metal nanoparticles. Moreover, such tests could be conducted in opto-microfluidic channels used in biomedical and pharmaceutical research, where, for example, sonoluminescence of bubbles nucleated in pits formed on a silicon substrate has been reported in [86]. Due to the decent optical properties of silicon, the pits formed in this material possess the properties of optical micro-cavities, where a strong Purcell effect has been observed [110, 111, 112]. An enhancement of the sonoluminescence light intensity could also be investigated in an acousto-optical device [115], where a microscopically patterned metallic film simultaneously plays the roles of a bubble-generating substrate and of an optical resonator [109]. Furthermore, the Purcell effect could be used to inhibit the emission of a source of light [110]. Subsequently, from the point of view of the fundamental physics, one might be interested in studying the potential effects of light emission inhibition under otherwise favourable for the production of sonoluminescence conditions. Yet, since the existence of acoustic and elastic counterparts of the optical Purcell has been demonstrated [116, 117] and the possibility of combining the optical and acoustic Purcell effects has been pointed out [118], one might control sonochemiluminescence light intensity using both optical and acoustic techniques. It is also worth noting that sonoluminescence processes may be of interest to the burgeoning field of metamaterials [119, 120]. In general, a metamaterial is any artificial material engineered to have a property that is not found in naturally occurring media. For instance, an electromagnetic metamaterial affects electromagnetic waves that interact with its structural features. Since the typical size of the features of a metamaterial is smaller than the wavelength of the incident wave, photonic metamaterials that operate at optical frequencies are usually structured at the nanometre scale [119]. Similarly, the features of acoustic metamaterials designed to control, direct, and manipulate acoustic waves in fluids and solids are typically smaller than the wavelength of the incident acoustic wave [121]. Among different kinds of metamaterials, hyperbolic metamaterials have attracted special attention since their hyperbolic dispersion relationship gives rise to useful effects, including enhancement of spontaneous emission, negative refraction, and the ability to overcome the optical diffraction limit [120]. The concept of hyperbolic metamaterials has been extended to waves other than electromagnetic waves and light, where, in particular, it has been suggested that acoustic waves existing in hyperbolic metamaterials can be regarded as analogues of gravitational waves [122].