Micromachined array tip for multifocus fiber-based optical coherence tomography

High-resolution optical coherence tomography demands a large detector bandwidth and a high numerical aperture for real-time imaging, which is difficult to achieve over a large imaging depth. To resolve these conflicting requirements we propose a novel multifocus fiber-based optical coherence tomography system with a micromachined array tip. We demonstrate the fabrication of a prototype four-channel tip that maintains a 9-14-microm spot diameter with more than 500 microm of imaging depth. Images of a resolution target and a human tooth were obtained with this tip by use of a four-channel cascaded Michelson fiber-optic interferometer, scanned simultaneously at 8 kHz with geometric power distribution across the four channels.     

Electron spin lifetimes in Hg0.78Cd0.22 Te and InSb

We have made direct pump–probe measurements of spin lifetimes in long wavelength narrow-gap semiconductors at wavelengths between 4 and 10 μm and from 4 to 300 K. In particular, we measure remarkably long spin lifetimes, τ_s300 ps, even at 300 K for epilayers of degenerate n-type InSb. In this material the mobility is approximately constant between 77 and 300 K, and we find that τ_s is approximately constant in this temperature range. In order to determine the dominant spin relaxation mechanism we have investigated the temperature dependence of τ_s in non-degenerate lightly n-type Hg_0.78Cd_0.22Te of approximately the same band gap as InSb, and find that τ_s varies from 356 ps at 150 K to 24 ps at 300 K. Our results, both in magnitude and temperature dependence of τ_s, imply that the Elliott–Yafet model dominates in these materials.     

Dual-axes confocal microscopy with post-objective scanning and low-coherence heterodyne detection

We present a dual-axes confocal microscope that employs postobjective scanning and low-coherence heterodyne detection to collect vertical cross-sectional images from biological tissue with high axial resolution, reduced noise from scattered light, deep tissue penetration, and a large dynamic range. This architecture can be scaled down to millimeter dimensions with microelectromechanical systems technology for performance of in vivo optical biopsy.     

Parametric processes in microstructured and highly nonlinear optical fibres

Optical parametric amplification is an important nonlinear process in photonic crystal fibres driven by the small effective area and the special dispersion characteristics of these fibres. Aside from its role in supercontinuum generation, parametric amplification can lead to efficient wavelength conversion, limited primarily by the uniformity of the diameter of currently available fibres. The related Bragg scattering process can be used for both wavelength conversion and optical switching, again limited by the physical characteristics of the fibres used.     

Landau–Pomeranchuk–Migdal effect in a quark–gluon plasma and the Boltzmann equation

We show how the Landau–Pomeranchuk–Migdal effect on photon production rates in a quark–gluon plasma can be derived via the usual Boltzmann equation. To do this, we first derive the electromagnetic polarization tensor using linear response theory, and then formulate the Boltzmann equation including the collisions mediated by soft gluon exchanges. We then identify the resulting expression for the production rate with that obtained by the field-theoretic formalism recently proposed by Arnold, Moore and Yaffe. To illustrate the LPM effect we solve the Boltzmann equation in the diffusion approximation.     

Consequence for Wavefunction Collapse Model of the Sudbury Neutrino Observatory Experiment

It is shown that data on the dissociation rate of deuterium obtained in an experiment at the Sudbury Neutrino Observatory provides evidence that the Continuous Spontaneous Localization wavefunction collapse model should have mass–proportional coupling to be viable.     

Extracting the wavefunction of the LSP at the LHC

We consider associated production of squarks and gluinos with the lightest supersymmetric particle (LSP), or states nearly degenerate in mass with it. Though sub-dominant to pair production of color SU(3)-charged superpartners, these processes are directly sensitive to the wavefunction composition of the lightest neutralinos. Exploiting event-shape variables - including some introduced here for the first time - we are able to identify the composition of the LSP by selecting events involving a single high-pT jet recoiling against missing transverse energy. We illustrate the proposed technique on a set of benchmark cases and propose methods for applying these results in more realistic experimental environments.     

Crime and punishment: Does it pay to punish?

Crime is the result of a rational distinctive balance between the benefits and costs of an illegal act. This idea was proposed by Becker more than forty years ago (Becker (1968) [1]). In this paper, we simulate a simple artificial society, in which agents earn fixed wages and can augment (or lose) wealth as a result of a successful (or not) act of crime. The probability of apprehension depends on the gravity of the crime, and the punishment takes the form of imprisonment and fines. We study the costs of the law enforcement system required for keeping crime within acceptable limits, and compare it with the harm produced by crime. A sharp phase transition is observed as a function of the probability of punishment, and this transition exhibits a clear hysteresis effect, suggesting that the cost of reversing a deteriorated situation might be much higher than that of maintaining a relatively low level of delinquency. Besides, we analyze economic consequences that arise from crimes under different scenarios of criminal activity and probabilities of apprehension.