Deposits of clastic carbonate-dominated (calciclastic) sedimentary slope systems in the rock record have been identified mostly as linearly-consistent carbonate apron deposits, even though most ancient clastic carbonate slope deposits fit the submarine fan systems better. Calciclastic submarine fans are consequently rarely described and are poorly understood. Subsequently, very little is known especially in mud-dominated calciclastic submarine fan systems. Presented in this study are a sedimentological core and petrographic characterisation of samples from eleven boreholes from the Lower Carboniferous of Bowland Basin (Northwest England) that reveals a >250 m thick calciturbidite complex deposited in a calciclastic submarine fan setting. Seven facies are recognised from core and thin section characterisation and are grouped into three carbonate turbidite sequences. They include: 1) Calciturbidites, comprising mostly of highto low-density, wavy-laminated bioclast-rich facies; 2) low-density densite mudstones which are characterised by planar laminated and unlaminated muddominated facies; and 3) Calcidebrites which are muddy or hyper-concentrated debrisflow deposits occurring as poorly-sorted, chaotic, mud-supported floatstones. These

Phd by thesis

Control of spontaneously emitted light lies at the heart of quantum optics. It is essential for diverse applications ranging from miniature lasers and light-emitting diodes, to single-photon sources for quantum information, and to solar energy harvesting. To explore such new quantum optics applications, a suitably tailored dielectric environment is required in which the vacuum fluctuations that control spontaneous emission can be manipulated. Photonic crystals provide such an environment: they strongly modify the vacuum fluctuations, causing the decay of emitted light to be accelerated or slowed down, to reveal unusual statistics, or to be completely inhibited in the ideal case of a photonic bandgap. Here we study spontaneous emission from semiconductor quantum dots embedded in inverse opal photonic crystals. We show that the spectral distribution and time-dependent decay of light emitted from excitons confined in the quantum dots are controlled by the host photonic crystal. Modified emission is observed over large frequency bandwidths of 10%, orders of magnitude larger than reported for resonant optical microcavities. Both inhibited and enhanced decay rates are observed depending on the optical emission frequency, and they are controlled by the crystals’ lattice parameter. Our experimental results provide a basis for all-solid-state dynamic control
of optical quantum systems.

Controlling the dynamics of spontaneous emission from quantum dots by photonic crystals

Over the past decade, the field of two-dimensional (2D) layered materials has surged, promising a new platform for studying diverse physical phenomena that are scientifically intriguing and technologically relevant. Contacts are the communication links between these 2D materials and the three-dimensional world for probing and harnessing their exquisite electronic properties. However, fundamental challenges related to contacts often limit the ultimate performance and potential of 2D materials and devices. This article provides a comprehensive overview of the basic understanding and importance of contacts to 2D materials and various strategies for engineering and improving them. In particular, we elucidate the phenomenon of Fermi level pinning at the metal/2D contact interface, the Schottky versus Ohmic nature of the contacts and various contact engineering approaches including interlayer contacts, phase engineered contacts, and basal versus edge plane contacts, among others. Finally, we also discuss some of the relatively under-addressed and unresolved issues, such as contact scaling, and conclude with a future outlook.

https://par.nsf.gov/servlets/purl/10066926

Contact engineering for 2D materials and devices

Modern data centers increasingly rely on interconnects for delivering critical communications connectivity among numerous servers, memory, and computation resources. Data center interconnects turned to optical communications almost a decade ago, and the recent acceleration in data center requirements is expected to further drive photonic interconnect technologies deeper into the systems architecture. This review paper analyzes optical technologies that will enable next-generation data center optical interconnects. Recent progress addressing the challenges of terabit/s links and networks at the laser, modulator, photodiode, and switch levels is reported and summarized.

Recent advances in optical technologies for data centers: a review

(IEEE Transactions on Electron Devices,36(12):2816-2820)Field-Drifting Resonant Tunneling Through a-Si:H/a-Sil-xCx:H Quantum Wells at Different Locations of the i-Layer of a p-i-n Structure

Impact of minority carrier lifetime on the performance of strained germanium light sources

Vibration from railway tunnels has a significant environmental impact on inhabitants of buildings near underground railway lines. The problem is of increasing importance because of the introduction of new underground lines in urban areas, increasing public sensitivity to noise and vibration, and the need to conform to increasingly stringent legislation. There is a great need for railway-design engineers to model vibration from railway tunnels. A model is required to predict absolute levels of vibration from running trains and to assess the performance of vibration countermeasures before these are implemented. The model should be accurate and should not require substantial computational resources. This paper presents a new model for calculating vibration from railway tunnels, which combines both accuracy and computational efficiency. The model is based on the assumption that the near-field displacement of the tunnel is not influenced by the existence of a free surface. Displacements at the tunnel-soil interface are calculated using a model of a tunnel wall in a full-space, known as the Pipe-in-Pipe (PiP) model. Greens functions for a two-anda-half-dimensional elasto-dynamic full-space are then used to calculate the internal source in a full-space that would produce the same displacements at the tunnel-soil interface as calculated by the PiP model. The internal source is then used to calculate the far-field displacements based on Greens functions for a two-and-a-half-dimensional half-space. The results and computation time of this model are compared with those of an alternative coupled Finite-Element-Boundary-Element (FE-BE) model that accounts for a tunnel wall in a half-space. M.F.M Hussein, S. Gupta, H.E.M. Hunt, G. Degrande, and J.P. Talbot INTRODUCTION Significant vibration in buildings near underground tunnels is attributed to moving trains. Vibration is generated at the wheel-rail interface due to irregularities of the tracks and wheels. The characteristics of the transmission path, i.e. the tunnel and its surrounding soil, determine the amount of vibration that reaches a building. Vibration can travel long distances, e.g. more than 200m in ground with soft clay and silt [6], causing annoyance to people and malfunctioning of sensitive equipment. Vibration from underground railways can be isolated by reducing the railpad stiffness, using floating-slab tracks and/or supporting buildings on springs. There are other well known techniques which are described in the literature, see ref. [4] for example. Design engineers need an accurate and computationally efficient model to predict vibration from railway tunnels and assess the performance of these countermeasures. The model should be accurate due to the high financial cost of vibration countermeasures and the difficulty of retrospective replacement. It should also be computationally efficient as time is always a crucial factor when taking engineering decisions. There are two popular approaches for calculating vibration from railway tunnels. The first is the Pipe-in-Pipe (PiP) model [3]. The tunnel wall and its surrounding infinite soil are modelled as two concentric pipes. The inner pipe represents the tunnel wall and is modelled using thin shell theory. The outer pipe, with its outer radius being set to infinity, represents an infinite soil with a cylindrical cavity and is modelled using elastic continuum theory. The PiP model is computationally efficient on account of the uniformity along and around the tunnel. The disadvantage of this model is that it does not account for a free surface or layering of the ground, and therefore it is more applicable to deep-bored tunnels. The second approach is the coupled Finite-Element-Boundary-Element (FEBE) model [1,2]. The tunnel wall is modelled using the Finite-Element method while the surrounding soil is modelled using the Boundary-Element method. This model is accurate, provided careful consideration is given to mesh discretization. The advantage of this method is the possibility of modelling a free surface and/or a multilayered ground. It can also account for tunnel walls with non-circular geometries. The running time of this model has been greatly reduced in the last few years by taking advantage of periodicity in the longitudinal direction. The Floquet transform is incorporated by dividing the infinite model into repeating-units, and only one of these units is discretized. Even with this recent development, the model takes a long time to run and requires significant computational resources. It is useful for research purposes but still computationally expensive as a design tool. This paper presents a variant to the PiP model which accounts for the free surface, thereby increasing accuracy without sacrificing computational efficiency. The model is based on the assumption that the near-field displacement of the tunnel is not influenced by the existence of the free surface. Displacements at the tunnel-soil interface are first calculated using the PiP model. The internal source in a full-space that would produce the same displacements at the tunnel-soil interface as calculated by the PiP model is then computed using Greens functions for a two-and-a-halfICSV13, July 2-6, 2006, Vienna, Austria dimensional full-space. The internal source and Greens functions for a two-and-ahalf-dimensional half-space are finally used to calculate the far-field displacements. The new model is described further in the following sections. An outline of the model is first presented, followed by a comparison of the results and computation time with those of the equivalent coupled FE-BE model. Figure 1: layout of the model showing the force-displacement lines in (a) three dimensions and (b) in the section at 0 z = . OUTLINE OF THE MODEL In this paper, the vertical displacement at the free surface due to a load applied on the tunnel invert is calculated. The load takes the form ) ( ~ t z i e F F ω ξ + = and the displacement takes the form ) ( ~ t z i e w w ω ξ + = , as shown in Figure 1. These forms are the basis of the analysis in the wavenumber-frequency domain, which can be used to calculate the displacement for any sort of loading such as a concentrated harmonic load, see [5] for more details. The displacement amplitude w~ can be calculated as shown in the following subsections by: (1) calculating the displacements at the tunnel-soil interface using the PiP model; (2) using the Greens functions for a two-and-a-half-dimensional elastodynamic full-space to calculate the equivalent internal source in a full-space; and (3) using the Greens functions for a two-and-a-half-dimensional elasto-dynamic half(a) (b) x z y S x0 ) ( ~ t z i e w w ω ξ + = ) ( ~ t z i e F ω ξ + ) ( ~ t z i e w ω ξ +

An efficient model for calculating vibration from a railway tunnel buried in a half-space

A complementary metal-oxide semiconductor compatible on-chip light source is the holy grail of silicon photonics and has the potential to alleviate the key scaling issues arising due to electrical interconnects. Despite several theoretical predictions, a sustainable, room temperature laser from a group-IV material is yet to be demonstrated. In this work, we show that a particular loss mechanism, inter-valence-band absorption (IVBA), has been inadequately modeled until now and capturing its effect accurately as a function of strain is crucial to understanding light emission processes from uniaxially strained germanium (Ge). We present a detailed model of light emission in Ge that accurately models IVBA in the presence of strain and other factors such as polarization, doping, and carrier injection, thereby revising the road map toward a room temperature Ge laser. Strikingly, a special resonance between gain and loss mechanisms at 4%--5% $\ensuremath{\langle}100\ensuremath{\rangle}$ uniaxial strain is found resulting in a high net gain of more than $400\phantom{\rule{0.28em}{0ex}}{\text{cm}}^{\ensuremath{-}1}$ at room temperature. It is shown that achieving this resonance should be the goal of experimental work rather than pursuing a direct band gap Ge.

https://dr.ntu.edu.sg/bitstream/10356/80546/1/Room%20temperature%20lasing%20unraveled%20by%20a%20strong%20resonance%20between%20gain%20and%20parasitic%20absorption%20in%20uniaxially%20strained%20germanium.pdf

Room temperature lasing unraveled by a strong resonance between gain and parasitic absorption in uniaxially strained germanium

Photonic system component counts are increasing rapidly, particularly in CMOS-compatible silicon photonics processes. Large numbers of cascaded active photonic devices are difficult to implement when accounting for constraints on area, power dissipation, and response time. Plasma dispersion and the thermo-optic effect, both available in CMOS-compatible silicon processes, address a subset of these criteria. With the addition of a few back-end-of-line etch processing steps, silicon photonics platforms can support nano-opto-electro-mechanical (NOEM) phase shifters. Realizing NOEM phase shifters that operate at CMOS-compatible voltages (≤ 1.2 V) and with low insertion loss remains a challenge. Here, we introduce a novel NOEM phase shifter fabricated alongside 90 nanometer transistors that imparts 5.63 radians phase shift at 1.08 volts bias over an actuation length of 25μm with an insertion loss of less than 0.04 dB and 3 dB bandwidth of 0.26 MHz.

Dual slot-mode NOEM phase shifter.

The two stage a.k.a. four energy level NBTI model has been comprehensively re-evaluated by investigating its predictive capabilities beyond the ultra-short stress duration for which it was originally validated. It is found that the model, with its default parameters, can indeed reproduce short time (∼1s) stress and subsequent recovery with good accuracy. The default model however does not anticipate well-known experimental results for longer stress duration. Other combination of parameters may be used to improve prediction of long time stress, but at the cost of inaccurate recovery estimation. The capability of this model to predict AC NBTI is also explored. We conclude that the model predictions are highly sensitive to input parameters, and we find no combination of parameters that reproduces the observed DC, AC, and duty-cycle dependent NBTI phenomena.

Shashank Gupta

Papers

Impact of minority carrier lifetime on the performance of strained germanium light sources

An efficient model for calculating vibration from a railway tunnel buried in a half-space

Room temperature lasing unraveled by a strong resonance between gain and parasitic absorption in uniaxially strained germanium

Dual slot-mode NOEM phase shifter.

A comprehensive and critical re-assessment of 2-stage energy level NBTI model