We demonstrate Anderson localisation of visible light on a chip and report quality factors exceeding highly engineered two-dimensional cavities. Our results reverse the trend, observed so far, of the quality of disorder-induced light confinement being orders of magnitude lower than engineered devices. Furthermore, by implementing a sensitive imaging technique, we directly visualise the localised modes, determine their position on the device and measure their spatial extension.Our findings prove the potential of disorder-induced localised light for scalable, room temperature, optical devices, operating in the visible range of wavelengths. * Electronic address: l.sapienza@soton.ac.uk 1 arXiv:1605.08614v2 [physics.optics]

We report on the increased extraction of light emitted by solid-state sources embedded within high refractive index materials. This is achieved by making use of a local lensing effect by sub-micron metallic rings deposited on the sample surface and centered around single emitters. We show enhancements in the intensity of the light emitted by InAs/GaAs single quantum dot lines into free space as high as a factor 20. Such a device is intrinsically broadband and therefore compatible with any kind of solid-state light source. We foresee the fabrication of metallic rings via scalable techniques, like nano-imprint, and their implementation to improve the emission of classical and quantum light from solid-state sources. Furthermore, while increasing the brightness of the devices, the metallic rings can also act as top contacts for the local application of electric fields for carrier injection or wavelength tuning.PACS numbers: 42.82. Bq, 78.55.Cr, 78.60.Lc, 78.67.Hc a) Electronic mail: o.trojak@soton.ac.uk b) Electronic mail: l.sapienza@soton.ac.uk; www.quantum.soton.ac.uk 1 Extracting light into free space is one of the challenges to face when dealing with solidstate emitters embedded within high-index materials. At the air interface total internal reflection can trap most of the light within the higher index material, thus preventing efficient light extraction, that can be as low as a few percent. Such an issue needs to be faced when dealing with emitters like light-emitting diodes 1 and lasers 2 based, for instance, on quantum wells or quantum dots. In the same way as for classical light emitters, extraction efficiency has been the focus of intensive research when dealing with intrinsically dimmer sources like single-photon emitters for fundamental science 3,4 and quantum information technology applications 5 . Amongst solid-state quantum light sources, molecular beam epitaxial quantum dots (QDs) are of particular interest as they are directly grown on a semiconductor chip (thus allowing easy integration within optical circuits), they can have lifetime-limited emission lines 6 and can emit pure and indistinguishable single photons 7 .Several approaches have been followed to increase the extraction efficiency of light emitted by QDs into free space. For instance, optical cavities embedding single emitters have been fabricated to channel the emitted light into specific optical modes. Examples are micropillars, based on distributed Bragg reflectors 7 , nanowires 8 and circular grating cavities on suspended membranes 9,10 . Such optical cavities require the coupling of the emission from a source into a specific optical mode that the emitter needs to be resonant with. Fabrication processes can require multilayer growth (for micropillars) and deep etching (for micropillars and nanowires) or a calibrated etch (for circular grating cavities). High aspect ratio devices or suspended membranes also require non-trivial fabrication processes if one wants to include electrical tuning or injection to improve the device performance...

Integrated quantum devices are at the basis of the realisation of scalable, high-performance quantum technology, including quantum computers and quantum communication schemes, where single photons are emitted, guided, manipulated and detected on a chip. Engineered nano-devices enable the efficient confinement of light and, ultimately, the control of the spontaneous emission dynamics of single emitters, which is crucial for cavity quantum electrodynamics experiments and for the de-

Enhancing light-matter interactions on a chip is of paramount importance for classical and quantum photonics, sensing and energy harvesting applications. Several photonic geometries have been developed, allowing high extraction efficiencies, enhanced light-matter interactions and control over the spontaneous emission dynamics of solid-state quantum light sources. To this end, a device geometry resilient to nanofabrication imperfections, providing high-quality light confinement and control over the emitted light properties, would be desirable. We demonstrate that aperiodic arrangements, whose geometry is inspired by natural systems where scattering elements are arranged following Fibonacci series, represent a platform for enhancing the light-matter interaction in on-chip nano-photonic devices, allowing to achieve efficient visible light confinement. We use optically-active defect centers in silicon nitride as internal light sources to image and characterize, by means of micro-photoluminescence spectroscopy, the individual optical modes confined by photonic membranes with Vogel-spiral geometry. By studying the statistics of the measured optical resonances, in partnership with rigorous multiple scattering theory, we observe log-normal distributions and report quality factors with values as high as 2201±443. Our findings improve the understanding of the fundamental physical properties of light-emitting Vogel-spiral systems and show their application to active nano-photonic devices. These results set the basis for further development of quantum devices that leverage the unique properties of aperiodic Vogel spiral order on a chip, including angular momentum states, thus producing mode structures for information processing and communications on the chip.

We show that fabrication imperfections in silicon nitride photonic crystal waveguides can be used as a resource to efficiently confine light in the Anderson-localised regime and add functionalities to photonic devices. Our results prove that disorder-induced localisation of light can be utilised to realise an alternative class of high-quality optical sensors operating at room temperature. We measure wavelength shifts of optical resonances as large as 15.2 nm, more than 100 times the spectral linewidth of 0.15 nm, for a refractive index change of about 0.38. By studying the temperature dependence of the optical properties of the system, we report wavelength shifts of up to about 2 nm and increases of more than a factor 2 in the quality factor of the cavity resonances, when going from room to cryogenic temperatures. Such a device can allow simultaneous sensing of both local contaminants and temperature variations, monitored by tens of optical resonances spontaneously appearing along a single photonic crystal waveguide. Our findings demonstrate the potential of Anderson-localised light in photonic crystals for scalable and efficient optical sensors operating in the visible and near-infrared range of wavelengths.