Efficient light coupling into a photonic crystal waveguide with flatband slow mode

We design an efficient coupler to transmit light from a strip waveguide into the flatband slow mode of a photonic crystal waveguide with ring-shaped holes. The coupler is a section of a photonic crystal waveguide with a higher group velocity, obtained by different ring dimensions. We demonstrate coupling efficiency in excess of 95% over the 8 nm wavelength range where the photonic crystal waveguide exhibits a quasi constant group velocity vg = c/37. An analysis based on the small Fabry-P\'erot resonances in the simulated transmission spectra is introduced and used for studying the effect of the coupler length and for evaluating the coupling efficiency in different parts of the coupler. The mode conversion efficiency within the coupler is more than 99.7% over the wavelength range of interest. The parasitic reflectance in the coupler, which depends on the propagation constant mismatch between the slow mode and the coupler mode, is lower than 0.6% within this wavelength range.Comment: 11 pages, 7 figures, submitted to Photonics and Nanostructures - Fundamentals and Application

Photonic crystal waveguides for silicon integrated optics

This thesis reports experimental and theoretical studies on photonic crystal waveguides in the silicon-on-insulator platform. The work presents a new variant of Fabry-Pérot method for waveguide characterization. In this method, the reflectivity at the ends of the waveguide under study is enhanced by lithographically patterned mirrors. The thesis also studies a new type of photonic crystal, where the planar photonic crystal lattice is defined with ring-shaped holes (RPhC). By choosing a suitable ring parameter, the RPhC waveguide exhibits low and quasi constant group velocity over a wavelength range of several nanometers. The effect of the modefield width on the dispersion properties of the waveguide is discussed. A short and efficient coupler between the slow mode in an RPhC waveguide and the mode in a conventional silicon waveguide is designed. A relationship between coupling efficiency and the phase match between the coupler mode and the slow mode is observed. These results and observations are important in designing slow-light devices for all-optical signal processing and communication systems. Use of RPhC waveguides in other applications, particularly in biosensing, is also studied. An electron beam writing method that minimizes the writing time of the RPhC lattice is presented. The experimental results on an RPhC waveguide are the first reported for such structure and they show slowdown factors of up to 22 for the group velocity, compared to the group velocity in vacuum

Annotationes Epigraphicae X. Zu einigen Inschriften aus den römischen Provinzen Germania inferior und Germania superior

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Effect of atomic layer deposition on the quality factor of silicon nanobeam cavities

In this work we study the effect of thin-film deposition on the quality factor (Q) of silicon nanobeam cavities. We observe an average increase in the Q of 38±31% in one sample and investigate the dependence of this increase on the initial nanobeam hole sizes. We note that this process can be used to modify cavities that have larger than optimal hole sizes following fabrication. Additionally, the technique allows the tuning of the cavity mode wavelength and the incorporation of new materials, without significantly degrading Q

Nanolaminate structures fabricated by ALD for reducing propagation losses and enhancing the third-order optical nonlinearities

We demonstrate a novel atomic layer deposition (ALD) process to make high quality nanocrystalline titanium dioxide (TiO2) and zinc oxide (ZnO) with intermediate Al2O3 layers to limit the crystal size. The waveguide losses of TiO2/Al2O3 nanolaminates measured using the prism coupling method for both 633 nm and 1551 nm wavelengths are as low as 0.2 ± 0.1 dB/mm with the smallest crystal size. We also show that the third-order optical nonlinearity in ZnO/Al2O3 nanolaminates can be enhanced by nanoscale engineering of the thin film structure.Peer reviewe

Coupling slot-waveguide cavities for large-scale quantum optical devices

By offering effective modal volumes significantly less than a cubic wavelength, slot-waveguide cavities offer a new in-road into strong atom-photon coupling in the visible regime. Here we explore two-dimensional arrays of coupled slot cavities which underpin designs for novel quantum emulators and polaritonic quantum phase transition devices. Specifically, we investigate the lateral coupling characteristics of diamond-air and GaP-air slot waveguides using numerically-assisted coupled-mode theory, and the longitudinal coupling properties via distributed Bragg reflectors using mode-propagation simulations. We find that slot-waveguide cavities in the Fabry-Perot arrangement can be coupled and effectively treated with a tight-binding description, and are a suitable platform for realizing Jaynes-Cummings-Hubbard physics.Comment: 11 pages, 7 figures, submitte

Optimizing SOI Slot Waveguide Fabrication Tolerances and Strip-Slot Coupling for Very Efficient Optical Sensing

Slot waveguides are becoming more and more attractive optical components, especially for chemical and bio-chemical sensing. In this paper an accurate analysis of slot waveguide fabrication tolerances is carried out, in order to find optimum design criteria for either homogeneous or absorption sensing mechanisms, in cases of low and high aspect ratio slot waveguides. In particular, we have focused on Silicon On Insulator (SOI) technology, representing the most popular technology for this kind of devices, simultaneously achieving high integration capabilities, small dimensions and low cost. An accurate analysis of single mode behavior for high aspect ratio slot waveguide has been also performed, in order to provide geometric limits for waveguide design purposes. Finally, the problem of coupling into a slot waveguide is addressed and a very compact and efficient slot coupler is proposed, whose geometry has been optimized to give a strip-slot-strip coupling efficiency close to 100%

Reduced propagation loss in silicon strip and slot waveguides coated by atomic layer deposition

When silicon strip and slot waveguides are coated with a 50nm amorphous titanium dioxide (TiO2) film, measured losses at a wavelength of 1.55μm can be as low as (2±1)dB/cm and (7±2)dB/cm, respectively. We use atomic layer deposition (ALD), estimate the effect of ALD growth on the surface roughness, and discuss the effect on the scattering losses. Because the gap between the rails of a slot waveguide narrows by the TiO2 deposition, the effective slot width can be back-end controlled. This is useful for precise adjustment if the slot is to be filled with, e. g., a nonlinear organic material or with a sensitizer for sensors applications