In the past decade, there has been a surge in research at the boundary between photonics and phononics 1 . Most efforts centered on coupling light to motion in a high-quality optical cavity 2 , typically geared towards observing the quantum state of a mechanical oscillator 3 . It was recently predicted that the strength of the light-sound interaction would increase drastically in nanoscale silicon photonic wires 4 . Here we demonstrate, for the first time, such a giant overlap between nearinfrared light and gigahertz sound co-localized in a small-core silicon wire. The wire is supported by a tiny pillar to block the path for external phonon leakage, trapping 10 GHz phonons in an area below 0.1 µm 2 . Since our geometry can be coiled up to form a ring cavity, it paves the way for complete fusion between the worlds of cavity optomechanics and Brillouin scattering. The result bodes well for the realization of low-footprint optically-pumped lasers/sasers 5 and delay lines 6 on a densely integrated silicon chip.The diffraction of light by sound was first studied by Léon Brillouin in the early 1920s. Therefore such inelastic scattering has long been called Brillouin scattering 7 . On the quantum level, the process annihilates pump photons while creating acoustic phonons and redshifted Stokes photons. The effect is known as stimulated Brillouin scattering (SBS) when the sound is generated by a strong modulated light field. This sets the stage for a self-sustaining feedback loop: the beat note between two optical waves (called the pump and the Stokes) generates sound that reinforces the initial beat note.In a seminal experimental study 8 , Brillouin scattering was viewed as a source of intense coherent sound. Later, the effect became better known as a noise source in quantum optics 9 and for applications such as spectrally pure lasing 10-12 , microwave signal processing 13,14 , slow light 15 , information storage 6 and phononic band structure mapping 16 .Traditionally 5-20 , the photon-phonon interaction was mediated by the material nonlinearity. Electrostriction drove the phonon creation, and phonon-induced permittivity changes lead to photon scattering. This conventional image of SBS as a bulk effect, without reference to geometry, breaks down in nanoscale waveguides. The impressive progress in engineering radiation pressure in micro-and nanoscale systems [21][22][23][24][25] recently inspired the * raphael.vanlaer@intec.ugent.be theoretical prediction of enormously enhanced photonphonon coupling 4,26-28 in silicon nanowires. In such waveguides, boundary effects can no longer be neglected. Thus both electrostriction and radiation pressure create phonons. Equivalently, the new theory takes into account not only bulk permittivity changes but also the shifting material boundaries. The strong photon confinement offered by these waveguides boosts both types of optical forces. However, destructive interference between the two contributions may still completely cancel the photon-phonon coupling. The giant light-sound overlap...

Abstract-The high index contrast silicon-on-insulator platform is the dominant CMOS 1 compatible platform for photonic integration. The successful use of silicon photonic chips in optical communication applications has now paved the way for new areas where photonic chips can be applied. It is already emerging as a competing technology for sensing and spectroscopic applications. This increasing range of applications for silicon photonics instigates an interest in exploring new materials, as silicon-oninsulator has some drawbacks for these emerging applications, e.g. silicon is not transparent in the visible wavelength range. Silicon nitride is an alternate material platform. It has moderately high index contrast, and like silicon-on-insulator, it uses CMOS processes to manufacture photonic integrated circuits. In this paper, the advantages and challenges associated with these two material platforms are discussed. The case of dispersive spectrometers, which are widely used in various silicon photonic applications, is presented for these two material platforms.

Laser frequency combs, sources with a spectrum consisting of hundred thousands evenly spaced narrow lines, have an exhilarating potential for new approaches to molecular spectroscopy and sensing in the mid-infrared region. The generation of such broadband coherent sources is presently under active exploration. Technical challenges have slowed down such developments. Identifying a versatile highly nonlinear medium for significantly broadening a mid-infrared comb spectrum remains challenging. Here we take a different approach to spectral broadening of mid-infrared frequency combs and investigate CMOS-compatible highly nonlinear dispersion-engineered silicon nanophotonic waveguides on a silicon-on-insulator chip. We record octave-spanning (1,500–3,300 nm) spectra with a coupled input pulse energy as low as 16 pJ. We demonstrate phase-coherent comb spectra broadened on a room-temperature-operating CMOS-compatible chip.

There is a rapidly growing demand to use silicon and silicon nitride (Si 3 N 4 ) integrated photonics for sensing applications, ranging from refractive index to spectroscopic sensing. By making use of advanced CMOS technology, complex miniaturized circuits can be easily realized on a large scale and at a low cost covering visible to mid-IR wavelengths. In this paper we present our recent work on the development of silicon and Si 3 N 4 -based photonic integrated circuits for various spectroscopic sensing applications. We report our findings on waveguide-based absorption, and Raman and surface enhanced Raman spectroscopy. Finally we report on-chip spectrometers and on-chip broadband light sources covering very near-IR to mid-IR wavelengths to realize fully integrated spectroscopic systems on a chip.

We present a degenerate four-wave mixing experiment on a silicon nitride (SiN) waveguide covered with gated graphene. We observe strong dependencies on signal-pump detuning and Fermi energy, i.e. the optical nonlinearity is demonstrated to be electrically tunable. In the vicinity of the interband absorption edge (2|EF | ≈ ω) a peak value of the waveguide nonlinear parameter of ≈ 6400 m −1 W −1 , corresponding to a graphene nonlinear sheet conductivity |σ