Excitation of surface plasmons at a SiO2/Ag interface by silicon quantum dots: Experiment and theory

The excitation of surface plasmons (SPs) by optically excited silicon quantum dots (QDs) located near a Ag interface is studied both experimentally and theoretically for different QD-interface separations. The Si QDs are formed in the near-surface region of an SiO2 substrate by Si ion implantation and thermal annealing. Photoluminescence decay-rate distributions, as derived from an inverse Laplace transform of the measured decay trace, are determined for samples with and without a Ag cover layer. For the smallest, investigated Si-QDs-to-interface distance of 44 nm the average decay rate at lambda=750 nm is enhanced by 80% due to the proximity of the Ag-glass interface, with respect to an air-glass interface. Calculations based on a classical dipole oscillator model show that the observed decay rate enhancement is mainly due to the excitation of surface plasmons that are on the SiO2/Ag interface. By comparing the model calculations to the experimental data, it is determined that Si QDs have a very high internal emission quantum efficiency of (77±17)%. At this distance they can excite surface plasmons at a rate of (1.1±0.2)×104 s¿1. From the model it is also predicted that by using thin metal films the excitation of surface plasmons by Si QDs can be further enhanced. Si QDs are found to preferentially excite symmetric thin-film surface plasmons

Phase mapping of ultrashort pulses in bimodal photonic structures: A window on local group velocity dispersion

The amplitude and phase evolution of ultrashort pulses in a bimodal waveguide structure has been studied with a time-resolved photon scanning tunneling microscope (PSTM). When waveguide modes overlap in time intriguing phase patterns are observed. Phase singularities, arising from interference between different modes, are normally expected at equidistant intervals determined by the difference in effective index for the two modes. However, in the pulsed experiments the distance between individual singularities is found to change not only within one measurement frame, but even depends strongly on the reference time. To understand this observation it is necessary to take into account that the actual pulses generating the interference signal change shape upon propagation through a dispersive medium. This implies that the spatial distribution of phase singularities contains direct information on local dispersion characteristics. At the same time also the mode profiles, wave vectors, pulse lengths, and group velocities of all excited modes in the waveguide are directly measured. The combination of these parameters with an analytical model for the time-resolved PSTM measurements shows that the unique spatial phase information indeed gives a direct measure for the group velocity dispersion of individual modes. As a result interesting and useful effects, such as pulse compression, pulse spreading, and pulse reshaping become accessible in a local measuremen

Fiscal Consequences of Electoral Institutions

There are more than 500,000 elected officials in the United States, 96 percent of whom serve in local governments. Electoral density - the number of elected officials per capita or per governmental unit - varies greatly from place to place. The most electorally dense county has more than 20 times the average number of elected officials per capita. In this paper, we offer the first systematic investigation of the link between electoral density and fiscal policy. Drawing on principal-agent theories of representation, we argue that electoral density presents a tradeoff between accountability and monitoring costs. Increasing the number of specialized elected offices promotes issue unbundling, reducing slack between citizen preferences and government policy; but the costs of monitoring a larger number of officials may offset these benefits, producing greater latitude for politicians to pursue their own goals at the expense of citizen interests. Thus, we predict diminishing returns to electoral density, suggesting a U-shaped relationship between the number of local officials and government fidelity to citizen preferences. Using a county-level dataset of all elected officials in the United States, we evaluate this theory along with competing theories from the existing literature. Empirically, we find evidence that public sector size decreases with electoral density up to a point, beyond which budgets grow as more officials are added within a community

Hiding in Plain Sight: Timing and Transparency in the Administrative State

Anecdotal evidence of agencies burying bad news is rife in law and politics. The bureaucracy regularly is accused of announcing controversial policies on holidays and weekends when public attention is elsewhere. We show that this conventional wisdom is wrong, or at least significantly incomplete. The conventional wisdom is riddled with theoretical holes, and there is little systematic empirical evidence to support it. After critiquing the conventional account of agencies hiding bad news, we articulate and defend a revised theory of strategic timing in administrative law. We argue that timing decisions rarely affect the visibility of decisions but can drive up the costs of monitoring and responding for interest groups and legislative coalitions. Agency discretion to choose when to announce policy decisions can even allow agencies to influence which interest groups monitor the regulatory process and therefore whose preferences must be taken into account. We evaluate both the conventional wisdom and our revised theory using twenty-five years of empirical evidence. We then develop the implications for administrative law doctrine and institutional design of the bureaucracy

Pulse tracking in complex photonic structures

Time-resolved near-field microscopy allows the propagation of ultrafast pulses to be visualized en route while they travel through complex photonic structures. These measurements enable the unambiguous determination of both local phase and group velocities. We illustrate this powerful technique by tracking an ultrashort wavepacket as it completes several round trips in a ring resonator

Unhealthy weight control behaviours in adolescent girls: a process model based on self-determination theory

This study used self-determination theory (Deci, E.L., & Ryan, R.M. (2000). The 'what' and 'why' of goal pursuits: Human needs and the self-determination of behavior. Psychological Inquiry, 11, 227-268.) to examine predictors of body image concerns and unhealthy weight control behaviours in a sample of 350 Greek adolescent girls. A process model was tested which proposed that perceptions of parental autonomy support and two life goals (health and image) would predict adolescents' degree of satisfaction of their basic psychological needs. In turn, psychological need satisfaction was hypothesised to negatively predict body image concerns (i.e. drive for thinness and body dissatisfaction) and, indirectly, unhealthy weight control behaviours. The predictions of the model were largely supported indicating that parental autonomy support and adaptive life goals can indirectly impact upon the extent to which female adolescents engage in unhealthy weight control behaviours via facilitating the latter's psychological need satisfaction