Project Summary

TAILPHOX project addresses the design and implementation of silicon phoXonic crystal
structures that allow a simultaneous control of both photonic and phononic waves. The final goal is to push the performance of optical devices well beyond the state of the art by this radically new approach. By merging both fields (nanophotonics and nanophononics) within a same platform, novel unprecedented control of light and sound in very small regions will be achieved. The project will cover from the development of theoretical and numerical tools to deal simultaneously with light and sound to the application to three high-impact scenarios in the field of ICT: i) phonon-assisted light emission in silicon, ii) control of photon speed (delay and storage) by stimulated Brillouin scattering (SBS) in silicon photonic chips, and iii) realization of highly-sensitive dual phoXonic sensors.

Concept and objectives

Interaction between photons (light waves) and phonons (sound waves) inherently takes place in a large number of optical structures and devices. Sometimes, this interaction is "positive" in the sense that it can be employed to implement devices such as acousto-optic modulators or Raman/Brillouin spectrometers and sensors. On the other hand, a "negative" interaction provides undesired effects, such as an upper limit in the optical power carried by an optical fibre as a consequence of stimulated Brillouin scattering (SBS). In addition, light emissionin silicon, the key material for microelectronics, is quite an improbable phenomenon since this process is phonon-mediated, making the silicon laser one of the “holy grails” in optics. In this sense, it can be thought that by properly engineering the background propagating medium so that the propagation features of both photons and phonons are
tailored, it will become feasible to improve the performance of the "positive" applications, remove the limits imposed by the "negative" interactions or even create and confine tailored phonons which can assist photon emission in the case of light sources. To achieve these challenging goals, the creation of a properly designed periodic pattern in the medium, which allows simultaneously controlling photons and phonons as well as enhancing their interaction, is a radically new approach.Propagation of photons or phonons can be controlled separately by introducing periodicity in the medium, giving rise to the so-called photonic and phononic crystals, respectively. The use of periodic structures permits the observation of interesting phenomena, such as localization in ultrasmall
volumes or energy-velocity reduction by orders of magnitude (so-called slow-waves). In addition, periodic structures also permit tailoring the dispersion for both photons and phonons, giving rise to quite surprising effects such as, for instance, negative refraction or superprism. Thus, it becomes clear that if a material is engineered to be periodic so that both photons and phonons are simultaneously affected (and controlled), the previously mentioned effects and phenomena can be managed in an extremely novel way. Moreover, the performance and efficiency of a device can be radically improved, the most suitable structure to achieve this goal being a phoXonic crystal. This can be defined as a periodic structure that displays band gap properties and can be used to forbid photon and/or phonon propagation, create high-Q phoXonic cavities (for light and sound), and create bands with special propagation properties, e.g., slow photons/phonons, not achievable simultaneously in conventional homogeneous materials for both kinds of waves. This new concept of phoXonic crystal constitutes the basis upon which the TAILPHOX approach is built. With regard to the material platform to be used in the implementation of future ICT devices, there exists a broad agreement that silicon will become the future material for nanophotonics, as it has been so far for microelectronics. The use of mature fabrication tools and processes of Complementary Metal Oxide Semiconductor (CMOS) foundries can give rise to high-volume and low-cost manufacturing of nanophotonic devices fully built on Silicon-On-Insulator (SOI) wafers. However, there still exists a hard work to do in order to create reliable SOI photonic devices that can become an alternative to their microelectronic counterparts. This work necessarily involves a better understanding
of the photon propagation in silicon as well as its interaction with phonons.

Proposed application scenarios

1. Light emission in silicon: a room-temperature, electrically-pumped laser working at optical communications wavelengths (1.2-1.6 μm) fully built using silicon is perhaps the most pursued goal within photonics. It is well known that silicon is intrinsically a poor light emitter. Some recent approaches to produce light emission in Si include: an all-optical silicon Raman laser (by Intel, but which needs a laser pump), doping with silicon nanocrystals and erbium on a silicon or silica matrix (that has very low efficiency), or wafer bonding of III-V lasers onto SOI (with its need for other noncompatible technologies). The TAILPHOX approach would permit a phonon-assisted silicon emitter fully compatible with CMOS fabrication lines. The objective is to demonstrate that the use of tailored phonons can augment the efficiency of the light emission processes in silicon as the proof-of-concept of a phonon-assisted silicon light source.

2. Compact and efficient devices based on SBS in phoXonic waveguides. Currently, there is a strong interest in controlling the speed of light in order to realize optical delay lines and memories. Recently, a powerful method based on SBS in optical fibres has been proposed. Delays of tens of nanoseconds in lengths of about 1 km of single-mode fibre have been demonstrated. These functionalities could be implemented in a more compact way in silicon integrated circuits by using slow-waveguides in 2D phoXonic crystals. The tight confinement of light and sound in such a small region, as well as the slow group velocities of light and sound, will enhance strongly the acousto-optic interaction, so that such nanosecond delays could be realized in distances on the order of millimetres. The TAILPHOX objective here is to achieve similar performance (nanoseconds delay in a millimetre waveguide) but in a silicon chip. The wavelength region of interest is that of optical communications, mainly in the 1.55 μm window. This approach could also lead to Brillouin lasing (making use of the
Brillouin gain) by introducing a cavity.

3. PhoXonic sensors. Bio- and chemical sensors based on photonic crystal cavities or
waveguides have been demonstrated. Surely the same can be done with phonons, although this has not yet been done. The TAILPHOX approach would allow the analysis of complex molecules while determining two independent properties in parallel, refractive index and elastic modulus or acoustic impedance. The data would be obtained from (bio)molecules within the bulk of the phoXonic cavity and those adsorbed on its surface. The required test volume would be below that of comparable methods such as Surface Plasmon Resonance (SPR) or acoustic microsensors. Furthermore, the concept also includes the new route for a phoXonic sensor, probably based on target material dependent Brillouin/Raman scattering highly amplified in a cavity or a waveguide, thereby taking advantage of photon-phonon interaction and reducing system requirements to optical source and detector. Here the objective is the proof-of-concept of a dual phoXonic sensor with enhanced sensing
performance for both photons and phonons.

TailPhox approved by the EC on 02/02/2009 under Grant agreement no. 233833.