Do single photons tunnel faster than light?

Experiments done in the early 1990's produced a surprising result: that single photons pass through a photonic tunnel barrier with a group velocity faster than the vacuum speed of light. Subsequent experiments with classical pulses have also revealed apparent superluminal group velocities as well as tunneling times that saturate with barrier length, a phenomenon known as the Hartman effect. In this paper we show that the measured delays are in fact cavity lifetimes as opposed to transit times. This interpretation resolves the Hartman paradox and shows that tunneling is not superluminal as widely believed.Comment: Invited Paper presented at the SPIE Conference "The Nature of Light: What are Photons?", San Diego, CA, August 26, 200

Improved design of a DFB Raman fibre laser

A Raman fibre laser based on phase shifted DFB structures is modelled for the first time. Using parameters of realistic devices, the model predicts low-threshold and highly-efficient laser output. The change of position and width of the phase shift were found to have a substantial impact on laser performanc

High strength steel in fire

High-performance materials are necessary to meet the future demands of the construction industry, which is strongly influenced by a growing population and depletion of natural resources. Sustainable development is central to research and development into innovative structural materials, and requires solutions to be economically viable whilst equally providing a positive contribution towards environmental and social factors. High strength steels (HSS) have the potential to contribute towards such demands by reducing the weight of structures when employed in appropriate applications. Lighter structures require smaller foundations, shorter transportation and construction times and also lower CO2 emissions. A particular challenge related to the use of HSS in structures include increased likelihood of stability issues resulting from the reduction in section thickness, and limiting deflection and vibration criteria are also more likely to be critical. Nevertheless, when used appropriately, they can provide a sustainable solution. Their use in structural applications is further hindered by a lack of performance data and design guidance under fire conditions. This paper compares the mechanical properties, particularly strength and stiffness of HSS (yield strengths between 460-700 MPa) and mild steel (yields between 235-460 MPa) at elevated temperatures, through a critical review of published literature. Various alloying and processing routes used to achieve high yield strength are assessed. At the same time, the review considers available information on the strengthening mechanisms that can be utilised to retain the strength and/or stiffness of the material in the event of a fire. Using the information gathered, an extensive testing programme is developed which will enable design guidance for the fire design of HSS structures to be proposed.Engineering and Physical Sciences Research Council, TW

The Zero-n Gap Soliton

Periodic structures consisting of alternating layers of positive index and negative index materials possess a novel band gap at the frequency at which the average refractive index is zero. We show that in the presence of a Kerr nonlinearity, this zero-n gap can switch from low transmission to a perfectly transmitting state, forming a nonlinear resonance or gap soliton in the process. This zero-n gap soliton is omnidirectional in contrast to the usual Bragg gap soliton of positive index periodic structure

Bioethics as a Dynamic Issue: Holistic Approaches to Understanding and Applying Ethics to Study Design

Anthropologists have challenged bioethicists to incorporate more holistic approaches to applying ethics in ‘real world’ situations. Where bioethicists tend to use systematic philosophical approaches to moral dilemmas, anthropologists apply malleable approaches designed to be responsive to variable cultural contexts. For example, anthropologists emphasize the importance of community and the effects of social issues, political economy, and cultural tradition in decision-making. This difference in approach contributes to the contentious relationship between anthropologists and bioethicists. Despite nuanced perspectives, anthropologists have not enjoyed a durable role in shaping contemporary bioethics. The lack of nuance becomes problematic when researchers attempt to reconcile ethical issues against a standard of morality rather than what Patricia Marshalls defines as a “culturally constituted and continually evolving” process. Reflecting on the IRB review of my work with African descendants in the United States and Nigeria, I will discuss the importance of conceptualizing bioethics as a dynamic issue, especially when working with communities abroad. Comparing these review processes is instructive about the organizational structures, influenced by culture and society, that impact decision-making in respective communities. In this presentation, I suggest that a focus on the way that IRB reviews are done can also offer insights and contextualization into community decision-making. Within a cross-cultural setting, considerations of the IRB review processes can lead to more informed conversations on bioethics and can aid researchers in applying more holistic approaches to study design

Hartman effect in presence of Aharanov Bohm flux

The Hartman effect for the tunneling particle implies the independence of group delay time on the opaque barrier width, with superluminal velocities as a consequence. This effect is further examined on a quantum ring geometry in the presence of Aharonov-Bohm flux. We show that while tunneling through an opaque barrier the group delay time for given incident energy becomes independent of the barrier thickness as well as the magnitude of the flux. The Hartman effect is thereby extended beyond one dimension and in the presence of Aharonov-Bohm flux.Comment: 4 pages, 4 figure

Comment on "Macroscopic violation of special relativity" by Nimtz and Stahlhofen [arXiv:0708.0681v1]

A recent paper by G. Nimtz and A. A. Stahlhofen [arXiv:0708.0681v1] makes the following claims: (1) that the authors have observed a macroscopic violation of special relativity, (2) that they have demonstrated quantum mechanical behavior of evanescent modes on a meter-length scale, and (3) that barriers are crossed in zero time, implying superluminal (faster than light), and indeed, infinite tunneling velocity. Here I suggest that all these claims are erroneous and are based on a misinterpretation of a purely classical measurement accurately described by Maxwell's equations.Comment: This is a comment on a paper posted on the arXiv on August 5, 2007 and reported in the New Scientist on August 18, 200