Electro‐optic modulators have been identified as the key drivers for optical communication and signal processing. With an ongoing miniaturization of photonic circuitries, an outstanding aim is to demonstrate an on‐chip, ultra‐compact, electro‐optic modulator without sacrificing bandwidth and modulation strength. While silicon‐based electro‐optic modulators have been demonstrated, they require large device footprints of the order of millimeters as a result of weak non‐linear electro‐optical properties. The modulation strength can be increased by deploying a high‐Q resonator, however with the trade‐off of significantly sacrificing bandwidth. Furthermore, design challenges and temperature tuning limit the deployment of such resonance‐based modulators. Recently, novel materials like graphene have been investigated for electro‐optic modulation applications with a 0.1 dB per micrometer modulation strength, while showing an improvement over pure silicon devices, this design still requires device lengths of tens of micrometers due to the inefficient overlap between the thin graphene layer, and the optical mode of the silicon waveguide. Here we experimentally demonstrate an ultra‐compact, silicon‐based, electro‐optic modulator with a record‐high 1 dB per micrometer extinction ratio over a wide bandwidth range of 1 μm in ambient conditions. The device is based on a plasmonic metal‐oxide‐semiconductor (MOS) waveguide, which efficiently concentrates the optical modes’ electric field into a nanometer thin region comprised of an absorption coefficient‐tuneable indium‐tin‐oxide (ITO) layer. The modulation mechanism originates from electrically changing the free carrier concentration of the ITO layer which dramatically increases the loss of this MOS mode. The seamless integration of such a strong optical beam modulation into an existing silicon‐on‐insulator platform bears significant potential towards broadband, compact and efficient communication links and circuits.

We demonstrated that ultraviolet Raman spectroscopy is an effective technique to measure the transition temperature ( T _c ) in ferroelectric ultrathin films and superlattices. We showed that one-unit-cell-thick BaTiO ₃ layers in BaTiO ₃ /SrTiO ₃ superlattices are not only ferroelectric (with T _c as high as 250 kelvin) but also polarize the quantum paraelectric SrTiO ₃ layers adjacent to them. T _c was tuned by ∼500 kelvin by varying the thicknesses of the BaTiO ₃ and SrTiO ₃ layers, revealing the essential roles of electrical and mechanical boundary conditions for nanoscale ferroelectricity.

We show that distributed Bragg reflector GaAs/AlAs vertical cavities designed to confine photons are automatically optimal to confine phonons of the same wavelength, strongly enhancing their interaction. We study the impulsive generation of intense coherent and monochromatic acoustic phonons by following the time evolution of the elastic strain in picosecond-laser experiments. Efficient optical detection is assured by the strong phonon backaction on the high-$Q$ optical cavity mode. Large optomechanical factors are reported ($\sim \mathrm{THz}/\mathrm{nm}$ range). Pillar cavities based in these structures are predicted to display picogram effective masses, almost perfect sound extraction, and threshold powers for the stimulated emission of phonons in the range $\mu \mathrm{W}–\mathrm{mW}$, opening the way for the demonstration of phonon “lasing” by parametric instability in these devices.

Coherent acoustic phonons modulate optical, electronic and mechanical properties at ultrahigh frequencies and can be exploited for applications such as ultratrace chemical detection, ultrafast lasers and transducers. Owing to their large absorption cross-sections and high sensitivities, nanoplasmonic resonators are used to generate coherent phonons up to terahertz frequencies. Generating, detecting and controlling such ultrahigh frequency phonons has been a topic of intense research. Here we report that by designing plasmonic nanostructures exhibiting multimodal phonon interference, we can detect the spatial properties of complex phonon modes below the optical wavelength through the interplay between plasmons and phonons. This allows detection of complex nanomechanical dynamics by polarization-resolved transient absorption spectroscopy. Moreover, we demonstrate that the multiple vibrational states in nanostructures can be tailored by manipulating the geometry and dynamically selected by acousto-plasmonic coherent control. This allows enhancement, detection and coherent generation of tunable strains using surface plasmons.

We report a direct determination of the dynamic behavior of confined acoustic phonons in nanocavities by picosecond acoustics. We provide the broadband, high resolution transmission amplitude curve in the subterahertz range, and we give evidence of resonant transmission peaks in three successive stop bands, in quantitative agreement with acoustic simulations. We furthermore demonstrate transit times in the nanosecond range at the cavity peaks reflecting the strong confinement of resonant phonons within the cavity layer. On the other hand, picosecond transit times are measured in the stop band, shorter than in any of the constituting materials, a tunneling effect well known both in photonic crystals and in macroscopic phononic systems.

Acoustic nanocavities display localized acoustic states similar to the confined electronic levels in atoms and quantum wells. Exploiting this parallel and concepts used in photonics, we describe novel acoustic devices operative in the THz range (nm wavelength) based on cavities as the basic building block. These include phonon molecules, acoustic minibands, structures displaying acoustic Wannier-Stark ladders and Bloch oscillations, and devices with generalized engineered phonon potentials and confined energy states. We demonstrate that both the spectra and the dynamics of acoustic waves can be tailored with great versatility in these devices.