Cathodoluminescence imaging spectroscopy is used to determine the propagation distance of surface plasmon polaritons near the surface plasmon resonance on both silver and gold films. Surface plasmon polaritons are generated by a focused ͑diameter of 5 nm͒ electron beam spot in the metal and coupled out through a grating. By gradually varying the distance between the excitation spot and the grating the damping is probed. Propagation lengths as small as several hundred nanometers are probed, and an increase in propagation length is observed if the wavelength is increased above resonance. The measured data are compared with the calculated propagation lengths taking into account both absorption in the film and leakage radiation, and it is found that other loss mechanisms appear to be significant as well. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2208556͔ Surface plasmon polaritons ͑SPPs͒ are electromagnetic waves that propagate at a metal-dielectric interface.1 Due to the strong dispersion of the metal's dielectric constant, the propagation length of SPPs is strongly frequency dependent. 2In the visible and ͑near͒ infrared, propagation distances as long as 10-100 m have been measured for silver and gold films.3 Near the surface plasmon resonance frequency, typically in the UV/visible, the propagation lengths are much smaller, and it becomes increasingly difficult to excite and measure the propagation of SPPs with conventional optical techniques. On the other hand, it is this region just below the surface plasmon resonance frequency that holds great potential for applications in photonics, because the wavelength of the SPPs in this region is much smaller than that of its photonic counterpart, the SPP group velocity is reduced, and the SPP field is highly concentrated. The near-resonance SPPs therefore have the potential to enable the scaling down of optoelectronic devices, to control the propagation of optical pulses, and to concentrate light. In view of these applications it is important to have knowledge of the propagation length of near-resonance SPPs.In this letter we will show that a combination of scanning electron microscopy ͑SEM͒ and cathodoluminescence ͑CL͒ imaging spectroscopy can be used to generate SPPs on a nanometer scale and to measure the propagation distance of SPPs on thin metal films at length scales as small as several hundred nanometers. We excite SPPs by electron irradiation of a metal film 4,5 at a well known distance from a nanoscale grating, that is made in the metal. The grating couples the SPP to the free space photon modes detected in the far field, and by varying the distance between excitation spot and grating the SPP damping can be probed ͓see schematic in Figs. 1͑a͒ and 1͑b͔͒. Experiments were performed on both silver and gold films, at frequencies ranging from close to resonance to the near infrared. The experimentally found propagation lengths were compared with the values expected on perfectly smooth films, calculated using the optical constants measured on the samples u...

An important challenge in medical diagnostics is to design all-in-one contrast agents that can be detected with multiple techniques such as magnetic resonance imaging (MRI), X-ray computed tomography (CT), positron emission tomography (PET), single photon emission tomography (SPECT) or fluorescence imaging (FI). Although many dual labeled agents have been proposed, mainly for combined MRI/FI, constructs for three imaging modalities are scarce. Here gold/silica nanoparticles with a poly(ethylene glycol), paramagnetic and fluorescent lipid coating were synthesized, characterized and applied as trimodal contrast agents to allow for nanoparticle-enhanced imaging of macrophage cells in vitro via MRI, CT and FI, and mice livers in vivo via MRI and CT. This agent can be a useful tool in a multitude of applications, including cell tracking and target-specific molecular imaging, and is a step in the direction of truly multi-modal imaging.

Downconversion of one visible photon to two near-infrared photons may increase the efficiency of c-Si solar cells by 30%. The lanthanide ion couple Er 3+-Yb 3+ is well known for efficient upconversion but for the reverse process, downconversion, fast multiphonon relaxation from the 4 F 7/2 level has been shown to compete with downconversion. Here we report efficient downconversion for the Er-Yb couple in Cs 3 Y 2 Br 9. The low phonon energy in this bromide host suppresses multiphonon relaxation and efficient two step energy transfer from the 4 F 7/2 level of Er 3+ is observed and results in strong 1000 nm emission from Yb 3+. Based on emission spectra and luminescence life time measurements an intrinsic downconversion efficiency close to 200% is determined.

Emission spectra are presented for Au 38 clusters in the temperature regime between 4 and 300 K. At room temperature, a broad emission band around 1.35 eV is observed. Upon cooling, fine structure appears that reveals the presence of at least four emission bands. The lowest energy emission band is resonant with the highest occupied molecular orbital-lowest unoccupied molecular orbital energy gap for Au 38 clusters. We propose that the higher emission energy bands result from emission from higher excited states, which competes with nonradiative relaxation. The high oscillator strengths of the transitions allows for competition between radiative and nonradiative decay from the various excited states although nonradiative decay dominates, giving rise to a low quantum yield and a fast decay (<1 ns).