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.
In addition to the general optical design capabilities described in the industry section, CAPPA uses software such as mode solvers (e.g. Photon Design FIMMWAVE/FIMMPROP) and Lumerical to design and optimise advanced nanophotonic devices, waveguides and circuits, with a particular focus on Photonic Crystals. Photonic crystal devices provide superior light confinement and wavelength selectivity relative to waveguide-based devices. A photonic crystal (PhC) is an artificial material consisting of a periodic variation of the refractive index, with the effect that the propagation of photons is modified in a manner similar to that of electrons propagating in a semiconductor crystal. Light that scatters in each period may interfere destructively or constructively, and can make the crystal transparent or opaque. For certain geometries and sufficiently high refractive index contrasts, the propagation of light may be completely forbidden, giving what is known as the photonic band gap (PBG). One dimensional and two dimensional photonic crystals can be used to realise mirrors with 99.99% reflectivities, which can be used to confine light in the wavelength scale and realise resonators with Q-factors of many millions.
For PhC lasers, the large Q-factors of such mirrors is a key benefit, resulting in the selection of a single longitudinal mode, even though the mirror size is only a few 10s of microns long. This offers advantages over DBR designs, as the effective length of the mirror region is shorter, reducing the length of the overall laser cavity and increasing the free spectral range.
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.
Photonic integration, the consolidation and miniaturisation of photonic functions (laser sources, waveguides, modulators, switches, transmitters/receivers, etc.) onto a single chip, is driving technological advances across a wide range of application areas. Not only does it underpin the multi-billion euro industries of telecommunications and data centre interconnects, it is also becoming increasingly prevalent in areas such as biomedical imaging and medical devices, gas sensing and environmental monitoring, and pharmaceutical and industrial process monitoring. CAPPA is strongly active in the area, possessing not only the expertise to design and characterise Photonic Integrated Circuits (PICs), but also to fabricate PICs and related nanophotonic devices. CAPPA has full access to the state-of-the-art cleanroom facilities of the Tyndall National Institute (via a Memorandum of Understanding) and has users trained in the use of a number of key systems; e.g. they are principal users of the electron beam lithography suite, used to accurately define nanometre-scale features such as for photonic crystals. CAPPA are a partner in the Irish Photonic Integration Centre (IPIC), a key Science Foundation Ireland-funded Research Centre as well as major PIC-related EU H2020 projects such as REDFINCH and COSMICC.
A particular interest is in Silicon-based mid-infrared PICs, which are important for gas sensing and medical applications. The silicon material systems represent the state-of-the-art integrated mid-infrared spectroscopy systems. The high refractive index contrast allows the realisation of sub-wavelength structures such as ring resonators and photonic crystals, and the leveraging of a wealth of designs optimised at the telecoms wavelengths transferred into the mid-IR. Several works have demonstrated implementations of mid-IR silicon-based platforms at different wavelength ranges using Si-on-sapphire (SOS), Si-on-silicon nitride (SON) or undercut silicon, Ge-on-Si or SiGe-on-Si. SiGe-on-Si is particularly interesting since it allows fine control of the material properties such as the bandgap or the refractive index by balancing the Ge concentration in the alloy, while extending the operation range up to at least 8 µm.