Experimental review of graphene

This review examines the properties of graphene from an experimental perspective. The intent is to review the most important experimental results at a level of detail appropriate for new graduate students who are interested in a general overview of the fascinating properties of graphene. While some introductory theoretical concepts are provided, including a discussion of the electronic band structure and phonon dispersion, the main emphasis is on describing relevant experiments and important results as well as some of the novel applications of graphene. In particular, this review covers graphene synthesis and characterization, field-effect behavior, electronic transport properties, magneto-transport, integer and fractional quantum Hall effects, mechanical properties, transistors, optoelectronics, graphene-based sensors, and biosensors. This approach attempts to highlight both the means by which the current understanding of graphene has come about and some tools for future contributions.Comment: Equal contributions from all author

Constant-loss taper and low-index waveguide for silicon photonics applications

The field of Silicon Photonics (SPs) has been growing in recent years due to the potential of the technology to produce cost-effective photonics components and systems. One of the biggest practical challenge facing SPs is achieving efficient, broadband light coupling from a fiber to an integrated waveguide on a silicon-on-insulator (SOI) platform. This problem arises because SOI is a large index contrast platform (Si/SiO2, ~3.5/1.5) enabling light confinement in a very small cross-sectional area (~0.1 um2); an area hundreds of times smaller than a standard SMF-28 fiber. Given the large mismatch in modal area between the fiber and integrated waveguide, only very little fiber-to-chip light coupling can be achieved without mode conversion. To this end, edge couplers using inverted tapers are currently used in order to achieve both efficient and broadband light coupling.In this thesis, an in-depth analysis of the mode conversion process through various inverted tapers is presented. Using coupled-mode theory and finite-difference time-domain (FDTD) simulations in 2D and 3D, the losses generated by linear and parabolic tapers are tracked within the tapers. In this way, we reveal that the losses produced by these tapers are localized at the tapers' tips, where the tapers seemingly expand too abruptly. To circumvent this problem, we developed a novel constant-loss design framework that we apply in order to design a novel constant-loss taper (CLT). The CLT evenly distributes the mode conversion loss through the taper, a strategy that maximizes the taper's efficiency according to basic concepts from the calculus of variations. Through simulations we show that the CLT is always more efficient than a linear or parabolic taper, regardless of the taper's length; the CLT is also shown to be more tolerant to fabrication errors than its linear/parabolic counterparts. As a proof of concept, 15 um long tapers (linear, parabolic and CLT) are fabricated on SOI and a novel facet preparation technique is developed. The efficiency of the CLT's fiber-to-SOI coupling is measured at just 0.95 dB with great agreement to simulated results; this represents the highest efficiency per length ratio ever reported.Finally, this thesis presents a novel low-index waveguide. This waveguide's geometry is similar to the slot-waveguide but differs in that it maximizes the modal gain of the waveguide; a key metric necessary for potential on-chip light amplification. The low-index waveguide does not limit the thickness of the core to sub 50 nm dimensions, we show that a thicker slot can increase the modal gain of the waveguide to 70%, a strong enhancement from an otherwise 20-25% with a slot-waveguide.L'industrie de la photonique sur silicium connaît une forte croissance ces dernières années en partie grâce au potentiel de la technologie servant à produire économiquement des composantes et systèmes optiques. Toutefois, l'un des plus grands problèmes pratiques auquel fait face cette industrie est le couplage efficace et large bande de lumière d'une fibre aux guides d'ondes intégrés sur une plateforme de silicium sur isolant (SOI). Ce problème est causé par le grand contraste entre les indices de réfraction (Si/SiO2 ~3.5/1.5); ce qui permet le confinement de la lumière dans des modes optiques possédant des aires transversales extrêmement petites (0.1 um2), soit des dimensions des centaines de fois plus petites que celles d'une fibre typique. Étant donné cette forte différence d'aires, seulement très peu de lumière peut être couplée d'une fibre à un guide d'ondes sur SOI sans l'utilisation d'un convertisseur de mode. À cet effet, les coupleurs utilisant des entonnoirs inversés ont démontré qu'ils pouvaient permettre un couplage efficace et large bande.Dans cet ouvrage, nous présentons une analyse détaillée du processus de conversion du mode optique dans des entonnoirs inversés. En utilisant des simulations par différences finies dans le domaine temporel en 2D et 3D, les pertes optiques générées par différents types d'entonnoirs inversés sont analysées à l'intérieur même des entonnoirs. De cette façon, nous révélons que les pertes générées dans des entonnoirs linéaires ou paraboliques génèrent la majorité de leurs pertes près de leurs extrémités, suggérant que ces entonnoirs croissent de façon prématurée. Afin de pallier ce problème, nous proposons un nouveau cadre de design à distribution régulière de pertes afin de créer un nouvel entonnoir à pertes uniformément distribuées (constant-loss taper, CLT). Le CLT maximise l'efficacité de conversion optique du cône selon des principes de calculs de variations. Au moyen de simulations, nous démontrons que le CLT est plus efficace qu'un entonnoir linéaire ou parabolique, peu importe la longueur de ceux-ci. Par ailleurs, le CLT se révèle comme étant plus résistant aux erreurs de fabrication. Afin de démontrer le concept de conception du CLT, des entonnoirs (linéaire, parabolique et CLT) d'une longueur de 15 um sont fabriqués sur une plateforme SOI. L'efficacité du couplage d'une fibre à un guide d'ondes SOI utilisant un CLT est mesurée à 0.95 dB, soit une efficacité semblable à celle simulée. Cette efficacité représente le plus grand ratio efficacité/longueur d'entonnoir jamais reporté dans le domaine. Finalement, cette thèse présente également un nouveau guide d'ondes à faible index. Ce guide d'ondes utilise une géométrie similaire à celle d'un guide d'ondes à fente, mais diffère parce qu'elle maximise le gain modal du guide d'ondes; une caractéristique clé pour une potentielle amplification de lumière sur puce. Le guide d'ondes à faible index ne limite pas l'épaisseur de la fente à des dimensions inférieures à 50 nm. Nous démontrons qu'avec un coeur plus épais il est possible d'augmenter le gain modal à 70%; une grande amélioration comparativement aux 20-25% typiques des guides d'ondes à fente

Ideal, constant-loss nanophotonic mode converter using a Lagrangian approach

Coupling light between an optical fiber and a silicon nanophotonic waveguide is a challenge facing the field of silicon photonics to which various mode converters have been proposed. Inverted tapers stand out as a practical solution enabling efficient and broadband mode conversion. Current design approaches often use linearly-shaped tapers and two dimensional approximations; however, these approaches have not been rigorously verified and there is not an overarching design framework to guide the design process. Here, using a Lagrangian formulation, we propose an original, constant-loss framework for designing shape-controlled photonic devices and apply this formalism to derive an ideal constant-loss taper (CLT). We specifically report on the experimental demonstration of a fabrication-tolerant, 15-\ub5m-long CLT coupler, that produces 0.56 dB fiber-chip coupling efficiency, the highest efficiency-per-length ratio ever reported.Peer reviewed: YesNRC publication: Ye

Ideal, constant-loss nanophotonic mode converter using a Lagrangian approach

Coupling light between an optical fiber and a silicon nanophotonic waveguide is a challenge facing the field of silicon photonics to which various mode converters have been proposed. Inverted tapers stand out as a practical solution enabling efficient and broadband mode conversion. Current design approaches often use linearly-shaped tapers and two dimensional approximations; however, these approaches have not been rigorously verified and there is not an overarching design framework to guide the design process. Here, using a Lagrangian formulation, we propose an original, constant-loss framework for designing shape-controlled photonic devices and apply this formalism to derive an ideal constant-loss taper (CLT). We specifically report on the experimental demonstration of a fabrication-tolerant, 15-\ub5m-long CLT coupler, that produces 0.56 dB fiber-chip coupling efficiency, the highest efficiency-per-length ratio ever reported.Peer reviewed: YesNRC publication: Ye