RESUMEN

We demonstrate that Aharonov-Albert-Vaidman weak values have a direct relationship with the response function of a system, and have a much wider range of applicability in both the classical and quantum domains than previously thought. Using this idea, we have built an optical system, based on a birefringent photonic crystal, with an infinite number of weak values. In this system, the propagation speed of a polarized light pulse displays both superluminal and slow light behavior with a sharp transition between the two regimes. We show that this system's response possesses two-dimensional, vortex-antivortex phase singularities. Important consequences for optical signal processing are discussed.

RESUMEN

We present an experimental and theoretical study of a simple, passive system consisting of a birefringent, two-dimensional photonic crystal and a polarizer in series, and show that superluminal dispersive effects can arise even though no incident radiation is absorbed or reflected. We demonstrate that a vector formulation of the Kramers-Kronig dispersion relations facilitates an understanding of these counterintuitive effects.

RESUMEN

The causality principle does not forbid negative group delays of analytic signals in electronic circuits; in particular, the peak of a pulse can leave the exit port of a circuit before it enters the input port. Furthermore, pulse distortion for these "superluminal" analytic signals can be negligible in both the optical and electronic domains. Here we suggest a possible extension of these ideas to microelectronics. The underlying principle is that negative feedback can be used to produce negative group delays. Such negative group delays can be used to cancel out the positive group delays introduced by transistor latency, as well as the propagation delays due to the interconnections between transistors. Using this principle, it may be possible to speed up computer systems.

RESUMEN

We consider pulse propagation in a linear anomalously dispersive medium where the group velocity exceeds the speed of light in vacuum ( c) or even becomes negative. A signal velocity is defined operationally based on the optical signal-to-noise ratio, and is computed for cases appropriate to the recent experiment where such a negative group velocity was observed. It is found that quantum fluctuations limit the signal velocity to values less than c.

RESUMEN

We describe a classical nonlinear optical system that displays superfluidity and its breakdown. The system consists of a self-defocusing refractive medium inside a Fabry-Pérot cavity with a cylindrical obstacle. We have numerically solved for the transmitted beam when an incident plane wave strikes the cavity at an oblique angle. The presence of the incident beam pins the steady-state phase of the output, preventing the formation of vortices or time-dependent flow. When the incident beam is switched off, a transient wake of moving optical vortices is produced. This is analogous to the breakdown of superfluidity above a critical velocity.

RESUMEN

The subharmonic emission from insonified contrast microbubbles was used to create a new imaging modality called Subharmonic Imaging. The subharmonic response of contrast microbubbles to ultrasound pulses was first investigated for determining adequate acoustic transmit parameters. Subharmonic A-lines and gray scale images were then obtained using a laboratory pulse-echo system in vitro and a modified ultrasound scanner in vivo. Excellent suppression of all backscattered signals other than from contrast microbubbles was achieved for subharmonic A-lines in vitro while further optimization is required for in vivo gray scale subharmonic images.

Asunto(s)

Medios de Contraste/administración & dosificación , Riñón/diagnóstico por imagen , Animales , Velocidad del Flujo Sanguíneo , Perros , Procesamiento de Imagen Asistido por Computador , Inyecciones Intravenosas , Riñón/irrigación sanguínea , Fantasmas de Imagen , Arteria Renal/diagnóstico por imagen , Venas Renales/diagnóstico por imagen , Ultrasonografía

RESUMEN

We report what is believed to be the first observation of a nonlinear mode in a cylindrical nonlinear Fabry-Perot cavity. The field enhancement from cavity buildup, as well as the large chi((3)) optical nonlinearity that is due to resonantly excited (85)Rb vapor, allows the nonlinear mode to form at low incident optical powers of less than 1 mW. The mode is observed to occur for both self-focusing and self-defocusing nonlinearity.

RESUMEN

The scanning tomographic acoustic microscope (STAM) was proposed in 1982 as a method of improving the resolution capability of the scanning laser acoustic microscope (SLAM) based on the principles of tomography. By modifying the SLAM with a quadrature detector, tomographic projections that contain both the amplitude and phase information of the scattered wavefield can be acquired. Subsequently, multiple projections acquired with different incident waves are combined using the "back-and-forth" propagation algorithm to form the tomographic reconstruction. The first STAM reconstructions have been obtained to experimentally demonstrate the superior resolution capability of the STAM over the SLAM. In this paper, the implementation of the STAM is described, and experimental reconstructions of multiple-layer specimens are demonstrated.

RESUMEN

Using correlated photons from spontaneous parametric downconversion, we have measured both the absolute quantum efficiencies and the time responses of four single-photon detectors. Efficiencies as high as (76.4 ± 2.3)% (at 702 nm) were seen, which to our knowledge are the highest reported single-photon detection efficiencies. An auxiliary retroreflection mirror was found to increase the net detection efficiency by as much as a factor of 1.19. The narrowest time profile for coincidences between two detectors displays a peak with 300 ps FWHM. We also investigated the presence of afterpulses and the effects of saturation and varying device parameters.