Subwavelength engineered metamaterial waveguides and devices are considered as fundamental building blocks for the next generation of integrated photonic circuits. Here we present an overview of recent advances in this surging field.

Subwavelength metamaterial structures in silicon waveguides open new degrees of freedom to control on-chip light propagation. They have been applied to many silicon photonic devices such as fiber-chip couplers, waveguide crossings, microspectrometers, ultra-fast optical switches, athermal waveguides, evanescent field sensors, polarization rotators and colorless interference couplers [1]. Here we report a demonstration of a metamaterial coupler and an InAs-on-InP quantum dash buried heterostructure laser optimized for mutual integration by direct facet-to-facet coupling with -1.2 dB coupling efficiency, coupled laser relative intensity noise (RIN) of -150 dB/Hz and 152 kHz linewidth. We further demonstrate the design of multi-line spectral notch filters based on narrow linewidth metamaterial Bragg filters synthesized using the layer-peeling method.

Subwavelength grating (metamaterial) waveguide devices are becoming established as key building blocks for advanced integrated photonic circuits. In this talk we will present an overview of our recent advances in this surging field.

1Universidad de Málaga, ETSI Telecomunicación, Campus de Teatinos s/n, 29071 Málaga, Spain 2National Research Council of Canada, Ottawa, Ontario K1A0R6, Canada 3 Optoelectronics Research Centre, University of Southampton, SO17 1BJ, United Kingdom Centre de Nanosciences et de Nanotechnologies, CNRS, Univ. Paris-Sud, Université Paris-Saclay, 91405 Orsay cedex, France 5 Institute of Photonics and Electronics AS CR, v.v.i., Chaberska 57, 18251 Prague 8, Czech Republic * aom@ic.uma.es

We report our advances in development of subwavelength engineered structures for integrated photonics, specifically high-efficiency fiber-chip couplers, broadband surface grating couplers and ultra-broadband nanophotonic beam splitters.

Non-classical light sources that can produce streams of correlated on-demand photons are a central building block for optics based quantum information technologies. There are numerous possible approaches for producing such a light source. One of the most promising is the solid-state single photon source based on a single quantum dot in III-V semiconductors. Utilizing a single InAs quantum dot in an InP nanowire, we previously demonstrated a bright and efficient source for single photons [1] and entangled photon pairs [2] that emits around 950 nm. In order to interface with telecom systems, single photon sources emitting at longer wavelengths are required. A few works have extended the emission to the telecom band using a single InAs/InP quantum dot in a micro-cavity [3-4]. However, improving the source brightness and the extraction efficiency remains a challenging task. In this contribution, by modifying the growth conditions and the pre-growth pattern for an InAs dot in an InP nanowire, we demonstrate a bright light source that emits in the telecom O-band, an important step towards the demonstration of a single photon source.

We report our advances in development of subwavelength engineered metamaterial structures in silicon waveguides, specifically high-efficiency fiber-chip couplers, ultra-broadband surface grating couplers and nanophotonic beam splitters, evanescent field waveguide sensors and on-chip Fourier-transform spectrometers.

We report our recent advances in development of subwavelength engineered dielectric metamaterial structures for integrated photonics. By locally engineering the refractive index of silicon by forming a pattern of holes at the subwavelength scale it is possible to manipulate the flow of light in silicon nanophotonic waveguides [1], [2], [3], [4], [5], [6]. Specifically, we present a broadband fibre-chip edge coupler with a coupling efficiency exceeding 90% for direct coupling between cleaved SMF-28 optical fibres and silicon nanophotonic waveguides [7], as well as the first implementation of a subwavelength nanostructure for laser-to-SOI chip coupling experiments with an InGaAsP/InP buried heterostructure laser. Furthermore, we report on dispersion engineered colourless multimode interference coupler operating over an ultra-broad wavelength range 1260 nm – 1675 nm, exceeding the O, E, S, C, L and U optical communication bands. We also present an integrated polarization controller with largely relaxed fabrication tolerances and tuneable over the complete C-band, with a measured polarization extinction range of 40 dB [8]. Implementations of subwavelength grating engineered structures for evanescent field sensing and mid-infrared silicon photonic circuits [9] with waveguide propagation losses < 0.9 dB/cm at 3.88 µm will also be discussed.

One of the key challenges in developing quantum networks is to generate single photons with high brightness, purity, and long temporal coherence. Semiconductor quantum dots potentially satisfy these requirements; however, due to imperfections in the surrounding material, the coherence generally degrades with increasing excitation power yielding a broader emission spectrum. Here we overcome this power broadening regime and demonstrate the longest coherence at exciton saturation where the detection count rates are highest. We detect single-photon count rates of 460,000 counts per second under pulsed laser excitation while maintaining a single-photon purity of greater than 99 Importantly, the enhanced coherence is attained with quantum dots in ultraclean wurtzite InP nanowires, where the surrounding charge traps are filled by exciting above the wurtzite InP nanowire bandgap. By raising the excitation intensity, the number of possible charge configurations in the quantum dot environment is reduced, resulting in a narrower emission spectrum. Via Monte Carlo simulations we explain the observed narrowing of the emission spectrum with increasing power. Cooling down the sample to 300 mK, we further enhance the single-photon coherence two-fold as compared to operation at 4.5 K, resulting in a homogeneous coherence time, T2, of 1.2 ns.

We present experimental results of efficient subwavelength index engineered surface grating couplers, developed in a standard 220-nm SOI substrate. rating couplers are fabricated using a single full etch or a dual etch process (full etch and 0nm shallow etch). The measured efficiency of -2.2 dB, -2.5 dB, and -0. dB are reported for single-etch couplers without and with metal reflector in the near-IR wavelengths. We demonstrate a peak coupling efficiency of -1.3 dB for a novel and flexible dual-etch grating coupler with interleaved deep and shallow etched trenches. Introduction: rating couplers are an effective approach to couple light to or from integrated circuits, enabling flexible placing of the interface on the chip surface and automated wafer scale testing [1-4]. In these structures, the directionality, i.e. the amount of power coupled towards an optical fibre, is an important parameter that determines the overall coupling efficiency. rating directionality strongly depends on the thickness of the silicon and the buried oxide (BOX) layers. Thicker Si waveguides yield higher directionality [2], while the 220 nm Si thickness is usually used in silicon photonics foundry offerings [3]. The directionality can also be improved by specialized gratings, at the expense of a more complex fabrication [1-5]. Fig. 18. (a) Measured coupling efficiency of single-etch subwavelength engineered grating couplers near 1.55 μm and 1.3 μm wavelengths. SEM images of fabricated devices: (b) twostep and (c) continuously apodized grating couplers. Single-etch grating couplers: We recently designed and fabricated a series of efficient single-etch grating couplers for a standard 220-nm-thick Si layer over 3-μm and 2-μm thick BOX layers [6-8]. We found that the directionality, can be improved by optimizing the radiation angle. Specifically, we demonstrated that the thin film interference from the bottom oxide layer is maximized for a coupling angles of 2° (3-μm BOX, near 1.55 ECIO|o-13

Producing advanced quantum states of light is a priority in quantum information technologies. While remarkable progress has been made on single photons and photon pairs, multipartite correlated photon states are usually produced in purely optical systems by post-selection or cascading, with extremely low efficiency and exponentially poor scaling. Multipartite states enable improved tests of the foundations of quantum mechanics as well as implementations of complex quantum optical networks and protocols. It would be favorable to directly generate these states using solid state systems, for better scaling, simpler handling, and the promise of reversible transfer of quantum information between stationary and flying qubits. Here we use the ground states of two optically active coupled quantum dots to directly produce photon triplets. The wavefunctions of photogenerated excitons localized in these ground states are correlated via molecular hybridization and Coulomb interactions. The formation of a triexciton leads to a triple cascade recombination and sequential emission of three photons with strong correlations. The quantum dot molecule is embedded in an epitaxially grown nanowire engineered for single-mode waveguiding and improved extraction efficiency at the emission wavelength. We record 65.62 photon triplets per minute, surpassing rates of all earlier reported sources, in spite of the moderate efficiency of our detectors. Our structure and data represent a breakthrough towards implementing multipartite photon entanglement and multi-qubit readout schemes in solid state devices, suitable for integrated quantum information processing.

Subwavelength grating structures with a pitch smaller than the Bragg resonance length can be used for engineering the refractive index in silicon photonic waveguides. We discuss the principles, design, fabrication and applications of subwavelength nanostructures in silicon photonics and review our recent results on fiber-chip coupling structures, surface grating couplers, waveguide crossings, and athermal waveguides with a polymersilicon hybrid core.

The silicon photonic wire evanescent field (PWEF) sensor platform offers the advantages of small sensor size, high levels of function integration, and low cost manufacturing that comes with the use of established semiconductor fabrication processes. The technology should be fully compatible with existing infrastructure in molecular analysis and research and the manufactured sensor array chip cost should be low enough that the chips can be considered disposable. Furthermore, since many applications require simultaneous monitoring of many different simultaneous binding reactions, the possibility of integrating tens or even hundreds of independent molecular sensors on a single disposable sensor chip is very compelling. We present an overview of our multiplexed photonic wire sensor chip and a reader instrument that allows up to 128 independent binding reactions to be monitored simultaneously [1]. A complete photonic wire molecular biosensor microarray chip architecture and supporting instrumentation is discussed. This microarray system is used to demonstrate a multiplexed assay for serotyping E. coli bacteria based on polyclonal antibody probe molecules. A coherent detection scheme that enables direct read-out of the optical phase and an order of magnitude enhancement of sensitivity compared to conventional detection is also discussed [2]. Finally, we present advances in Fourier-transform interferometer arrays for spectroscopic sensing. A planar waveguide Fourier-transform spectrometer with densely arrayed Mach-Zehnder interferometers is presented. Subwavelength gratings are used to produce an optical path difference without waveguide bends. The fabricated device comprises of an array of 32 Mach-Zehnder interferometers, which produce a spatial interferogram without any moving parts, yielding a spectral resolution of 50 pm and a free-spectral range of 0.78 nm. As a result of similar propagation losses in subwavelength grating waveguides and conventional strip waveguides- loss imbalance is minimized and high interferometric extinction ratio of -25 to -30 dB is obtained. Furthermore, phase and amplitude errors arising from normal fabrication variation are compensated by the spectral retrieval process using calibration measurements [3].

Multimode-Interference (MMI) devices are fundamental building blocks in photonic integrated circuits, where they are used for power splitting and combining, optical switches and modulators, Mach-Zehnder interferometers and 90o hybrids for coherent optical receivers. MMIs are based on the self-image principle, by which the guided modes of the multimode region interfere to form replicas of the input field with specific amplitude and phase relations. These relations are known to depend on i) the core/cladding refractive indexes (n1/n2), ii) the core width (W) and length (L) of the multimode region and iii) the number, width and position of the access ports. In this work, we show that by using sub-wavelength structures within an MMI, the self-imaging properties can be significantly altered, leading to ultra-short or ultra-broadband devices.

A complete photonic wire molecular biosensor microarray chip architecture and supporting instrumentation is described. Chip layouts with 16 and 128 independent sensors have been fabricated and tested, where each sensor can provide an independent molecular binding curve. Each sensor is 50 μm in diameter, and consists of a millimeter long silicon photonic wire waveguide folded into a spiral ring resonator. An array of 128 sensors occupies a 2 × 2 mm2 area on a 6 × 9 mm2 chip. Microfluidic sample delivery channels are fabricated monolithically on the chip. The size and layout of the sensor array is fully compatible with commercial spotting tools designed to independently functionalize fluorescence based biochips. The sensor chips are interrogated using an instrument that delivers sample fluid to the chip and is capable of acquiring up to 128 optical sensor outputs simultaneously and in real time. Coupling light from the sensor chip is accomplished through arrays of sub-wavelength surface grating couplers, and the signals are collected by a fixed two-dimensional detector array. The chip and instrument are designed so that connection of the fluid delivery system and optical alignment are automated, and can be completed in a few seconds with no active user input. This microarray system is used to demonstrate a multiplexed assay for serotyping E. coli bacteria using serospecific polyclonal antibody probe molecules.

We report on the experimental demonstration of ultrafast all-optical switching and wavelength down-conversion based on a novel nonlinear Mach-Zehnder interferometer with subwavelength grating and wire waveguides. Unlike other periodic waveguides such as line-defects in a 2D photonic crystal lattice, a subwavelength grating waveguide confines the light as a conventional index-guided structure and does not exhibit optically resonant behaviour. Since the device had no dedicated port to input optical signal to control switching a new approach was also implemented for all-optical switching control.

Recently developed subwavelength structures in silicon waveguides are presented. Subwavelength grating waveguides are discussed along with practical components that exploit the principle of subwavelength refractive index engineering, including fiber-chip grating couplers (<1dB coupling loss), waveguide crossings (−0.02dB loss), a polarization converter (−16 dB extinction ratio), a waveguide multiplexer (crosstalk -35 dB), and a Terahertz optical switch.