Radioplasmonics: Plasmonic Transducers in the Radiofrequency Regime for Resonant Thermo-acoustic Imaging in Deep Tissues

Abstract

In this work, we introduce a radically new approach for achieving doubly resonant light-to-sound conversion with radiofrequency waves, namely, electromagnetic waves in the range of 1–100 MHz. By taking the profit from recently published metamaterials exhibiting plasma-like responses in the radio range, we introduce the concept of “radioplasmonics” that deals with localized surface plasmons in the radio regime. In analogy with conventional plasmonics, radioplasmonics can be exploited to design microtransducers that effectively convert radio-waves into heat through resonant electromagnetic absorption. Then, by tuning the Young’s modulus of the transducers, we can achieve resonant acoustic vibrations in the same range of frequencies as the plasmonic resonances. In this way, plasmonic heating is converted into resonant thermo-acoustic expansion and its consequent generation of pressure waves. The latter can then be used for ultrasound imaging. We show that, in this double resonance framework, the intensity of the generated acoustic waves is above the current detection level under realistic conditions. The importance of using the radio range is related to its ability to deeply penetrate water and biological tissues. Hence, the proposed approach paves the way to the first total-body thermo-acoustic imaging able to reach a single-cell resolution.

Ultrasound generation in water via quasi-periodically snapping polymeric core–shell micro-bead excited with radiowaves

“…On the other hand, the polarizability of the sphere is defined as is the quasi-static polarizability, is the wavenumber of the surrounding medium and is its refractive index. Now, in order to obtain a Surface Plasmon resonance, the materials’ permittivity must obey the condition at the sought spectral position 1 , 21 . This causes a resonant behavior for the particle’s polarizability and therefore a resonant absorption in the corresponding spectrum that corresponds to the excitation of the dipolar SP of the particle.…”

“…This effect is not observed in the absence of such optical resonance, as shown later on. At this frequency, the amplitude of the pressure signals generated by photoacoustic effect is maximized by a laser pulse duration of 1.5 ns 1 . For these optimized parameters, the (acoustic) resonant dynamic of the polymeric micro-bead couples sinergetically with the generated heat (via a snap-through-buckling mechanism) forming a stochastically resonating bistable system.…”

“…In a previous work, we showed the possibility of creating a new class of micro-transducers enabling total body thermo-acoustic imaging in the radio range. To this aim, we introduced two new concepts that act in a synergic manner: Radio-Plasmonics and Resonant Thermoacoustic Expansion 1 . Radio-Plasmonics consists in the possibility of exciting surface plasmons at megahertz frequency to promote resonant optical absorption and then enhanced heat generation.…”

“…These numbers suggest the plausibility of a new generation of ultrasound imaging techniques based on bistable micro transducers and plasmonic metamaterials in the radio range. In this respect, in light of the theory introduced in 1 , we foresee three options to modulate the occurrence of resonant optical absorption in the MICS band: (1) Modulate the radius (and therefore the volume) of the micro-bead. (2) Modify the permittivity of the CNT doping material by adjusting their type and/or percentage weight.…”

“…In light of this last relation, we foresee two options to modulate the occurrence of acoustic resonances in a selected frequency band: (1) Modulate the radius (and therefore the volume) of the micro-bead and (2) modulate the elastic modulus of its constituents and their distributions (inertia moments). However, as recalled in 1 , the effectiveness of the proposed working mechanism relies on the concept of a doubly resonant system . In this respect, to effectively excite the acoustic resonance it is necessary a perturbation frequency close to its value i.e.…”