Design and Integration
Nanophotonics is the ideal approach for mass market optical components. The integration of multiple optical components in a single chip leads to mechanically stable, photonic integrated circuits (PICs) that can be fabricated in large volumes and at low cost. The reduction of devices to extremely small footprints and optical mode volumes results in low energy consumption and has allowed nanophotonic devices to span applications as diverse as commercial optical transceivers, single photon sources and quantum computation circuits. CAPPA has extensive experience in the design and realisation of photonic integrated circuits and devices, including CMOS-compatible paradigms such as Si photonics, SiN systems and Ge-on-Si devices, and the Centre is involved in several major national and EU projects related to photonics integration.
Many of the most useful mechanisms available for manipulating light rely on nonlinear optical effects, i.e. where the response of certain properties is nonlinear with respect to the optical power. For example, four-wave mixing and self-phase modulation are used for wavelength conversion and signal regeneration in all-optical communication devices, parametric down-conversion and four-wave mixing are used for photon pair generation in quantum optics, and frequency conversion processes are commonly used to increase the number of sources available for otherwise hard to access wavelength regions, e.g. optical parametric oscillators for the infrared regime or second harmonic generation for green laser pointers. However, many systems suffer from an underlying physical limitation – in general, the nonlinear response of a material is many orders of magnitude weaker than the linear response, requiring high optical powers and resulting in low efficiency.
Nanophotonics can provide solutions; reducing the volume of devices allows achieving very localised high optical powers without excessive wallplug energy demands, and structures such as photonic crystals can be used to enhance a material’s nonlinear response. One example is that of so-called slow light, where the group velocity of light is greatly reduced within a carefully designed nanophotonic device. Slow light has potential for achieving fast and efficient optical modulators for telecoms, and for optical memory applications. Material slow light is generally associated with an absorption resonance and intrinsically linked to optical losses. Structural slow light is associated with a structural resonance and can, in theory, be loss free – for example PhC waveguides have no intrinsic optical losses and only suffer from defect scattering induced optical losses. Additionally, structural slow light is a passive process and offers significant technological advantages. It is therefore the primary approach of implementing slow light in an on-chip platform. Over the last decade tremendous efforts have been made in improving slow light waveguides, with PhC waveguides particularly taking the lead.