Acoustic Far-Field Hypersonic Surface Wave Detection with Single Plasmonic Nanoantennas

Abstract

2The ability of metallic nanostructures to confine light at sub-diffraction volumes in their near-field allows the local enhancement of inherently weak phenomena such as Raman scattering [1], infrared absorption [2] and higher harmonic generation [3]. The decay of these optical excitations, given the large absorption cross-sections and fast electronic relaxation processes, make nanostructured conducting materials efficient local transducers of far-field electromagnetic radiation into mechanical energy. In addition, the strong optical modulation provided by the launched coherent acoustic modes in these nanostructures allow their exploitation as exquisitely sensitive mechanical probes of their near environment. The efficient generation of acoustic waves by nanostructured transducers and the strong self-modulation provided by the launched coherent phononic modes have enabled their application as, for instance, light-source modulators [4], photoacoustic amplifiers [5] and mass sensors [6] [7].The spectrally narrow acoustic modes obtained in these nanostructures are defined by the resonator's constitution and multiple boundary conditions such as size, shape, composition, their substrate and embedding media. This parameter space has been systematically explored where resonances were tuned by changes in adhesion layer thickness [8], mechanical constraints [9], by positioning resonators over trenches [10] and even by mode-interference from a delayed two-pump excitation scheme [11]. Equally importantly, the damping of acoustic vibrations, which defines the spectral linewidth of modes, is a prominent aspect in the application of these systems, and has also received considerable attention, being successfully modelled for infinite isotropic environments [10,[12][13][14][15][16]. However the multiple decay mechanisms in environments without spherical or cylindrical symmetry, such as when particles lie on a substrate, remain poorly understood [17]. In these cases, to determine the damping and quality factors of resonances current practice, [18], is to use empirical fitting of the decaying modulated time-domain signals.Among different contributions to the measured effective damping of a particular phononic mode, the radiation of acoustic waves to the embedding matrix or the substrate, is qualitatively assessed via the mismatch in acoustic impedance (Z) between the vibrating object and its environment. In the longitudinal plane wave limit the impedance is given by Z = ρ j c Lj (being ρ j the j medium density and c Lj the corresponding longitudinal wave speed). However, such a qualitative analysis may be misleading as Z is a mode-dependent parameter, and for which low-damping can be obtained even for perfect impedance match [16]. Accordingly, theoretical calculations have systematically predicted shorter acoustic damping times for particles in solid matrixes and longer damping times for liquid environments when compared to experimental values [18]. Nevertheless, the damping through the coupling of nanostructures to the substrate ...

Mesoporous Thin Films for Acoustic Devices in the Gigahertz Range

“…Ultrasound imaging in the kHz-MHz has revolutionized medical applications; a similar impact can be predicted for nanoacoustic waves for non-destructive testing at the nanoscale. [13][14][15] The design and engineering of acoustic interference devices have been traditionally based on almost atomically-flat interfaces presenting roughness only well below the phonon wavelength. The individual nanometric layer thicknesses are usually very well defined.…”

Phonon engineering with superlattices: Generalized nanomechanical potentials

“…Applications include fast modulators in semiconductor lasers [6], novel approaches for the generation of THz radiation [7] and the nanomechanical characterization of biological tissue [8,9]. Optical tools such as ultrafast pumpprobe spectroscopy and inelastic Brillouin scattering have enabled the study of phononic spectra, temporal dynamics and coherence properties on the nanoscale [10][11][12][13][14][15][16][17]. This paved the way to establish nanoacoustics also as a platform for the simulation of wave dynamics [18].…”

Acoustic Coupling between Plasmonic Nanoantennas: Detection and Directionality of Surface Acoustic Waves

“…Furthermore, they are interesting platforms to study optomechanical interactions and control the propagation of sound and heat at the nanoscale . Nanofabrication techniques have made it possible to tune the mechanical response of plasmonic nanostructures, allowing the efficient generation and detection of hypersound in a localized and controlled manner. − Triggering of SAWs using ultrafast laser pulses and their characterization have been implemented in different configurations and systems, including ferroelectric materials, micro- and nanostructured crystals, − multilayer cavities, , metallic films, , and periodic structures of plasmonic nanoantennas. − …”

“…Absorption of the light pulse generates an excited electron population in the confined geometry, which subsequently decays via different electron–electron and electron–phonon coupling mechanisms . This carrier relaxation produces a sudden thermal expansion of the ionic lattice that leads to the emission of coherent acoustic phonons. , The photoinduced impulsive thermal strain in these nanostructures coupled to a substrate generates a field of SAWs that propagate over distances orders of magnitude longer than the characteristic subnanometric displacements at the source . Detection of these traveling SAWs in the far field requires high sensitivity, and only a few systems are vulnerable to these vibrations because of their small amplitude.…”

“…Detection of these traveling SAWs in the far field requires high sensitivity, and only a few systems are vulnerable to these vibrations because of their small amplitude. Metallic nanoantennas at their plasmonic resonances have proven to be effective in detecting this type of disturbance . However, one of the main aspects still under study regarding the manipulation of SAWs is the control and characterization of their directionality. , In this work, we generate SAWs by coupling coherent phonons optically excited in a single V-shaped gold nanoantenna to the underlying amorphous glass substrate.…”