Design of a Slit-Groove Coupler for Unidirectional Excitation of the Guided Surface Plasmon Polaritons Through a Plasmonic Slot Waveguide

In this paper, a plasmonic-photonic nanostructure has been introduced for efficient unidirectional coupling of free-space radiation to surface plasmon polariton (SPP) waves under normal illumination on a subwavelength slit. The structure consists of a conventional metallic slit-groove nanostructure integrated with a plasmonic waveguide to support SPP waves along the desired direction with a remarkable lateral confinement. The unidirectional coupling is achieved by using an integrated plasmonic distributed reflector designed under Bragg condition. This reflector basically distributes part of the light coupled through the slit into the SPP modes of the waveguide. Numerical simulations show that up to 26 % of the normally incident light couples to the transversely localized field of the surface plasmon. In addition, the ratio of mode current density of the surface plasmon, launched in the desired direction, to that in the opposite direction can reach about 23 times. This structure shows a 2.5-fold improvement in coupling efficiency relative to a standard slit-groove structure. Also, the transmission distance for the new nanostructure is shown to be more than 8 times greater than that of the standard nanostructure.

Dispersion Properties of Photonic Crystals and Silicon Nanostructures Investigated by Fourier-Space Imaging

Jana Jágerská

State-of-the-art nanophotonic devices based on semiconductor technology use total internal reflection or the photonic bandgap effect to reduce the waveguide core dimensions down to hundreds of nanometers, ensuring strong optical confinement within the scale of the wavelength. Within the framework of this thesis, we investigate the light propagation in such devices by direct experimental reconstruction of their dispersion relation ω (k), where ω is the optical frequency and k the wave vector of the supported modes. Knowledge of the dispersion relation provides us with comprehensive information about the guided field, including the number of supported modes, their phase and group velocity as well as the higher order dispersion. As a principal characterization tool, an original experimental technique referred as Fourier-space imaging is used. It is based on far-field analysis of optical signal radiated out of the plane of the structure, which makes it possible to retrieve accurately, non-invasively and in one step the complex dispersion of both the leaky and the truly guided optical modes. The latter is feasible provided that the device is equipped with vanishingly weak grating probes that scatter a small part of the guided light into the light cone. The Fourier-space imaging technique was applied to study the optical properties of a large number of nanophotonic devices, ranging from simple nanowire waveguides to complex photonic crystal structures. In the first part of the work, silicon-on-insulator slot waveguides, coupled ridge waveguides and nanowire waveguide arrays are addressed. Besides the phase and group index dispersion, we investigate the phenomenon of mode splitting in coupled systems, being able to probe the coupling lengths with an accuracy of ±50 nm. In the case of waveguide arrays, beam steering using both thermo-optic effect and wavelength tuning was demonstrated. Concerning the photonic crystal devices, we primarily focus on the phenomenon of slow light propagation in line-defect and coupled-cavity photonic crystal waveguides. The latter represent a special type of a waveguide, which allows for substantial optical signal retardation by evanescent coupling along a chain of photonic crystal cavities. The main motivation was to accurately measure the group index of the slow light modes and recognize the main factors limiting its maximum achievable value. Among others, experimental observation of dispersion curve renormalization, enhanced out-of-plane and back-scattering as well as light localization due to residual disorder were reported. Finally, a detailed experimental study of hollow-core photonic crystal structures intended for optical sensing applications is presented.

EPFL2011

Tunable and Broadband Nanostructured Photonic Devices

Pierre-Yves Baroni

The main topic of this thesis is the fabrication and characterization of structures smaller than the wavelength of light, for operation in the visible or near infrared spectral range. In the first part of this thesis, the fabrication of periodic structures in one and two dimensions with an interference photolithography technique is described in detail. The structure periodicities that have been fabricated with this process ranges between 270 nm and a few microns on areas that can be as large as 100 cm2. Typical structure thicknesses are approximately equal to the period. In particular, a square lattice of pillars with a period of 270 nm has been created on the (non-flat) surface of a quartz microlens array and exhibits anti-reflective properties. Experimental results show a 15 % attenuation of the reflectivity and a 3 % enhancement of the transmissivity over the visible spectral range. In the second part of the thesis the structures are fabricated by e-beam lithography to ensure very precise devices shapes. The experimental results are obtained in the infrared range with two different structures, called photonic crystals. The first structure, a superprism, is a triangular lattice of pillars infiltrated by liquid crystals. A displacement of the output light spot is measured to be 20.5 µm for a wavelength variation of 27 nm. The structure length is 70 µm. The device, based on standard silicon technology, should allow integration of the device as a multiplexer/demultiplexer system into optical micro-circuits. The second structure, a tunable resonant cavity, is a wavelength filter working in the near infrared spectrum. The active area is composed of a photonic waveguide with a triangular lattice made of sub-micrometer holes. Additional nanostructuring in the light waveguide acts as a resonant cavity. The tunability of the device is obtained due to the liquid crystals which are infiltrated into the nanostructure. A 32 nm shift of the transmitted light peak inside the photonic band gap is measured by changing the temperature from room temperature to 45 °C.

EPFL2010

Numerical study and optimization of photonic crystals

Momchil Minkov

Photonic crystals (PhCs) are engineered nanostructures that enable an extraordinary control over the flow of light. These structures can be fabricated out of common semiconductors, are compatible with existing industrial fabrication technologies, and are expected to play a major role in future devices integrating photonic circuits - e.g. for telecommunications or in future quantum technologies. In this thesis, we explore a wide range of properties of the most common class of PhCs, formed by a lattice of circular holes in a semiconductor slab. To compute the electromagnetic eigenmodes of a given structure, we use fast mode-expansion methods, which are presented in detail here. The first application consists in a detailed analysis of the effects of fabrication disorder on the PhC structures. It is by now well-known that disorder is in many cases the limiting factor in device performance. Here, we shed more light on its effects, by statistically comparing various designs for PhC cavities with a high quality factor, and by analyzing the effect of irregular hole shapes on a PhC waveguide. The second application presented here stems from the fact that PhCs are in fact tremendously flexible, and their features are determined by a large number of controllable parameters. This is on one hand a great advantage, but on the other a great challenge when it comes to finding the optimal device for a given application. To face this challenge, we have developed an automated optimization procedure, using a global optimization algorithm for the exploration of an insightfully selected parameter space. This was applied to various devices of interest, and inevitably resulted in a vast improvement of their qualities. Specifically, we demonstrate various high-Q cavity designs, and a slow-light coupled-cavity waveguide with extraordinary features. We also present several experimental confirmations of the validity of our designs. Finally, we discuss two domains in which PhCs (and our optimization procedure) can be expected to play a major role. The first one is integrating quantum dots with the goal of long-range, photon-assisted dot-dot coupling, with implications for quantum information processing. We develop a semi-classical formalism, and analyze the magnitude and attenuation length of this coupling in large PhC cavities, as well as in a waveguide. The second outlook is in the field of topological photonics. We describe an array of resonators, in which an effective gauge field for photons can be induced through an appropriate time-periodic modulation of the resonant frequencies. This results in a Quantum Hall effect for light, and, in a finite system, one-directional edge states immune to fabrication disorder are predicted. We discuss the possibilities for a practical implementation, for which a PhC slab is among the most promising platforms.

EPFL2016