Design and fabrication of ridge waveguide-based nanobeam cavities for on-chip single-photon sources

We report on the design of nanohole/nanobeam cavities in ridge waveguides for on-chip, quantum-dot-based single-photon generation. Our design overcomes limitations of a low-refractive-index-contrast material platform in terms of emitter-mode coupling efficiency and yields an outcoupling efficiency of 0.73 to the output ridge waveguide. Importantly, this high coupling efficiency is combined with broadband operation of 9 nm full-width half-maximum. We provide an explicit design procedure for identifying the optimum geometrical parameters according to the developed design. Besides, we fabricate and optically characterize a proof-of-concept waveguide structure. The results of the microphotoluminescence measurements provide evidence for cavity-enhanced spontaneous emission from the quantum dot, thus supporting the potential of our design for on-chip single-photon sources applications

Temperature Effects Explain Continental Scale Distribution of Cyanobacterial Toxins

Insight into how environmental change determines the production and distribution of cyanobacterial toxins is necessary for risk assessment. Management guidelines currently focus on hepatotoxins (microcystins). Increasing attention is given to other classes, such as neurotoxins (e.g., anatoxin-a) and cytotoxins (e.g., cylindrospermopsin) due to their potency. Most studies examine the relationship between individual toxin variants and environmental factors, such as nutrients, temperature and light. In summer 2015, we collected samples across Europe to investigate the effect of nutrient and temperature gradients on the variability of toxin production at a continental scale. Direct and indirect effects of temperature were the main drivers of the spatial distribution in the toxins produced by the cyanobacterial community, the toxin concentrations and toxin quota. Generalized linear models showed that a Toxin Diversity Index (TDI) increased with latitude, while it decreased with water stability. Increases in TDI were explained through a significant increase in toxin variants such as MC-YR, anatoxin and cylindrospermopsin, accompanied by a decreasing presence of MC-LR. While global warming continues, the direct and indirect effects of increased lake temperatures will drive changes in the distribution of cyanobacterial toxins in Europe, potentially promoting selection of a few highly toxic species or strains.Peer reviewe

Yeni potansiyel-bazlı formulasyonların çok seviyeli hızlı çokkutup yöntemiyle çözümleri.

In this thesis, recently introduced potential-based formulations that are based on direct usage of magnetic vector and electric scalar potentials, instead of the equivalent field-based formulations, are investigated. These new potential-based formulations can alleviate the well-known low-frequency breakdowns. Therefore, these formulations can be useful in providing the solution of a plethora of problems in future and emerging technologies that are difficult to analyze via standard solvers. The aim of this thesis is to combine potential formulations with special low-frequency implementations of the multilevel fast multiple algorithm (MLFMA) to tackle with finely discretized problems. Thesis also includes the explanation of low-frequency breakdown mechanisms. In addition to the known breakdown of the electric-field integral equation, a hidden breakdown of the potential integral equations (PIEs) is shown. A remedy with the cost of an additional integral equation is proposed. All explanations for the low-frequency breakdown are supported with numerical results. Among low-frequency stable implementations of MLFMA, two methods are implemented for PIEs. One of them is MLFMA based on multipoles without diagonalization. In this method, classical aggregation, translation, and disaggregation procedures in MLFMA are realized without plane-wave expansion. The other one is recently proposed MLFMA implementation with approximate diagonalization. In this method, diagonalization is realized approximately with scaled spherical and plane waves. Accuracy and efficiency of the implementations are shown with numerical results.M.S. - Master of Scienc

Verimli Enerji Hasatı ve Hassas Algılama Uygulamaları İçin Nanoanten Geometrilerinin Benzetimleri ve Optimizasyonları

Nanometre ölçeğinde imal edilen ve enerji hasatı, biyo-algılama ve optik görüntüleme gibi çeşitli uygulamalarda kullanılabilen nanoantenler, son yıllarda nanoteknoloji alanındaki gelişmelere paralel olarak bilimsel ve teknik literatürde artarak önemkazanmaktadır. Ancak literatürde, nanoanten geometrilerinin incelenmesi üzerine tam ve kapsamlı bir çalışma bulunmamaktadır. Gerekli benzetim sayılarının çok yüksek olmasına ek olarak, nanoanten benzetimlerindeki modelleme yetersizlikleri (mükemmel iletken veya diyelektrik modellerin kullanılamaması, çok yoğun ayrıklaştırmalara ihtiyaç duyulması, kötü koşullu matris denklemlerinin iteratif çözümlerinin gerekmesi, vs.) kapsamlı bir çalışma yapılmasını zorlaştırmaktadır. Bu doğrultuda, bu proje kapsamında (1) hızlı ve hassas plazmonik modellemelere olanak sağlayacak etkin bir benzetimortamının geliştirilmesi, (2) geliştirilen benzetim ortamında çok sayıda ve farklı nanoanten geometrilerinin kapsamlı analizleri, (3) benzetim ortamının nanoanten optimizasyonlarına olanak sağlayacak şekilde düzenlenmesi ve optimizasyon algoritmalarıyla birleştirilmesi ve (4) elde edilen sonuçlar ışığında hedeflenen frekans bölgelerindenanoanten optimizasyonlarının gerçekleştirilmesi ve optimal geometrilerin bulunması hedeflenmektedir

Low-frequency Breakdown of the Potential Integral Equations and Its Remedy

Potential integral equations (PIEs) introduced recently also have a low-frequency breakdown that becomes visible when considering the electric charge density and near-zone electric field intensity values. As opposed to the well-known low-frequency breakdown of the electric-field integral equation (EFIE), which shows itself as both inaccuracy and ill-conditioning, the low-frequency breakdown of PIEs is not related to the conditioning properties of the related matrix equations. Therefore, we clearly distinguish the low-frequency breakdowns of EFIE and PIEs by categorizing the underlying mechanisms as field imbalance and source imbalance. Numerical examples are presented to provide a complete picture of these integral equations at low-frequencies, besides an effective post-processing method to mitigate the inaccuracies in PIEs