The realization of miniaturized optofluidic platforms opens up novel potentialities for the achievement of devices with enhanced functionality and compactness. Such integrated systems bring fluid and light together and exploit their micro-scale interaction for a large variety of applications. The high sensitivity of compact microphotonic devices can generate effective microfluidic sensors, with integration capabilities. By turning the technology around, the exploitation of fluid properties holds the promise of highly flexible, tunable or reconfigurable microphotonic devices. We overview some of the exciting developments to date.

Slow light has attracted significant interest recently as a potential solution for optical delay lines and time-domain optical signal processing 1,2. Perhaps even more significant is the possibility of dramatically enhancing nonlinear optical effects 3,4 due to the spatial compression of optical energy 5,6,7. Two-dimensional (2D) silicon photonic crystal (PhC) waveguides have proven to be a powerful platform for realizing slow light, being compatible with on-chip integration and offering wide-bandwidth and dispersion-free propagation 2. Here, we report the slow light enhancement of a nonlinear optical process in a 2D silicon PhC waveguide. We observe visible third-harmonic generation (THG) at a wavelength of 520nm with only a few watts of peak power, and demonstrate strong THG enhancement due to the reduced group velocity of the near-infrared pump signal. This demonstrates yet another unexpected nonlinear function realized in a CMOS-compatible silicon waveguide. Main text Although silicon has been the material of choice for the CMOS industry and more recently for integrated photonics, its optical properties-e.g light emission-still provide major challenges. In addition to an indirect band-gap and inversion symmetry,

superior performance with respect to speed and operation bandwidth than electronic based devices. [3-5] As a fundamental χ (3) process, FWM has found a wide range of applications in wavelength conversion, [6,7] optical frequency comb generation, [8,9] optical sampling, [10,11] quantum entanglement, [12,13] and many others. [14,15] Implementing nonlinear photonic devices in integrated form offers the greatest dividend in terms of compact footprint, high stability, high scalability, and mass-producibility. [1,2,16] Although silicon has been a leading platform for integrated photonic devices for many reasons, [1] including the fact that it leverages the well-developed complementary metal-oxide-semiconductor (CMOS) fabrication technologies, [17] its strong twophoton absorption (TPA) at near-infrared telecommunications wavelengths poses a fundamental limitation for devices operating in this wavelength region. Other CMOS compatible platforms such as silicon nitride (SiN) and doped silica [2,18] have a much lower TPA, although they still suffer from intrinsic limitation arising from a much lower Kerr nonlinearity. The increasing demand for high performing nonlinear integrated photonic devices has motivated the search for highly