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

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

Integrated nanophotonic waveguide-based devices for IR and Raman gas spectroscopy

On-chip devices for absorption spectroscopy and Raman spectroscopy have been developing rapidly in the last few years, triggered by the growing availability of compact and affordable tunable lasers, detectors, and on-chip spectrometers. Material processing that is compatible with mass production has been proven to be capable of long low-loss waveguides of sophisticated designs, which are indispensable for high-light–analyte interactions. Sensitivity and selectivity have been further improved by the development of sorbent cladding. In this review, we discuss the latest advances and challenges in the field of waveguide-enhanced Raman spectroscopy (WERS) and waveguide infrared absorption spectroscopy (WIRAS). The development of integrated light sources and detectors toward miniaturization will be presented, together with the recent advances on waveguides and cladding to improve sensitivity. The latest reports on gas-sensing applications and main configurations for WERS and WIRAS will be described, and the most relevant figures of merit and limitations of different sensor realizations summarized

Long, stitch-free slot waveguide with s-bend tapered couplers for IR-sensing applications using electron beam lithography

We use the fixed beam moving stage (FBMS) electron beam lithography technique to pattern a 10 mm long slot waveguide with s-bend tapered double-tip couplers. The fabrication method solves two major limitations of the FBMS mode, namely, the requirement for fixed-width structures and the incidence of stage placement drift for patterns involving elements of different widths. This has been achieved by fracturing the outline of the structure into fixed-width elements of gradually increasing width and creating intermediate overlap areas between the elements to mitigate the stage placement drifts

Er-Yb waveguide amplifiers in novel silicate glasses

A set of novel silicate glasses containing ZnO and co-doped with Er3+ and Yb3+ was designed as substrates for optical waveguide amplifiers. Characterized by exceptionally low up-conversion, minimum Er concentration quenching and high mechanical as well as chemical stability, the reported glasses can compete with phosphate-based materials typically used in the state-of-art active devices. Straight channel waveguides with propagation losses as low as 0.18 dB/cm were fabricated in these substrates using Ag+ double left right arrow Na+ and K+ double left right arrow Na+ thermal ion exchange. Net on-chip gain values of 6.7 dB at 1537 nm were measured and a net fiber to-fiber gain of 5 dB, was achieved when pumped at 976 nm. A six-level spatially resolved numerical model of an Er-Yb co-doped active waveguide was developed to analyze and optimize the amplifier performance. Modification of the rare-earth dopant concentration and the channel waveguide geometry was proposed to increase the gain figure and improve the overall amplifier efficiency

Characterisation of photonic crystal and nanophotonics devices with Fourier optics

We report on the experimental investigation of the Floquet-Bloch modes propagation inside photonic crystal (PhC) structures as well as their emission from the structures with far-field optics. We demonstrate that far-field optical experiments can be used to retrieve fundamental optical properties of PhC structures, Such as the light path, the dispersion Curves above and below the light cone, and the equi-frequency Surfaces both in one and two dimensions, in a single step and without numerical post-processing

Suspended tantalum pentoxide rib waveguides for laser absorption spectroscopy

Development of miniature photonic sensors using optical waveguides currently represents an attractive research direction. Owing to more than two decades of research on quantum/interband cascade lasers, laser absorption spectroscopy in the mid-infrared (MIR) domain can be brought to photonic chips. Importantly, the so-called molecular fingerprints, unique molecular absorption spectra, can be addressed in MIR. But waveguides need to adapt for sensing scenarios and one desired feature is a large overlap between the light and the analyte

Long, stitch-free slot waveguide with s-bend tapered couplers for IR-sensing applications using electron beam lithography

We use the fixed beam moving stage (FBMS) electron beam lithography technique to pattern a 10 mm long slot waveguide with s-bend tapered double-tip couplers. The fabrication method solves two major limitations of the FBMS mode, namely, the requirement for fixed-width structures and the incidence of stage placement drift for patterns involving elements of different widths. This has been achieved by fracturing the outline of the structure into fixed-width elements of gradually increasing width and creating intermediate overlap areas between the elements to mitigate the stage placement drifts.acceptedVersio