The excitation of surface plasmons in individual silver nanowires and gold nanorods is investigated by means of high-resolution electron energy loss spectroscopy in a transmission electron microscope. The transverse and longitudinal modes of these nanostructures are resolved, and the size variation of the plasmon peaks is studied. The effect of electromagnetic coupling between closely spaced nanoparticles is also observed. Finally, the relation between energy-loss measurements and optical spectroscopy of nanoparticle plasmon modes is discussed.

Electron energy-loss spectroscopy and energy-filtered transmission electron-microscope imaging are used to characterize the energy distribution of the surface plasmon of isolated and coupled gold nanorods. Local-field enhancement and spectral shift of the plasmon modes are observed for two interacting nanoparticles. The spatial modes measured by energy loss are shown to share qualitative similarities with the electromagnetic field distribution around gold nanorods induced by optical excitation as simulated using the discrete dipoleapproximation method.Knowledge of spatial characteristics of surface-plasmon ͑SP͒ modes in noble-metal nanoparticles and nanostructures is essential for a direct control of electric field confinement in the near field. Experimental methodologies such as nearfield optics and electron microscopy serve as powerful basic tools for investigation of plasmonic interactions. SP are electromagnetic excitations propagating at the interface between a dielectric and a conductor. They arise via the coupling of optical fields to oscillations of the conductor's electron plasma. Physically, these oscillations can be induced by either the driving electric field of an incident resonant optical field or impulsively by the transient field associated with a fast electron passing near a nanoparticle.SP are used for a variety of applications such as guiding electromagnetic energy in subwavelength-sized optoelectronic devices, 1 enhanced fluorescence spectroscopy, 2 Raman spectroscopy, 3,4 near-field imaging, 5 and biosensing. 6 Electron energy-loss spectroscopy ͑EELS͒ in a transmission electron microscope ͑TEM͒ can be used to obtain the SP spectrum by analyzing the energy of initially quasimonoenergetic electrons after they have interacted with a sample. 7 State of the art electron microscopy has achieved spatial resolution as low as 0.1 nm and energy resolution as low as ϳ0.2 eV. 8 For this reason, the optical plasmon modes of metallic nanoparticles have recently become accessible using EELS.Recent experiments have shown that the dielectric function of metallic nanostructures and the mode structure required for understanding or engineering optical plasmons of a single nanoparticle can be determined via EELS. 9-11 Of course, engineered nanostructures are getting more complex, involving the coupling of simple elements to enable new functionality such as negative index materials. 12 For this reason, a number of optical spectroscopy techniques 13,14 and theoretical analyses have been used to study the distance dependence of near-field interactions between coupled systems, using both near and far-field detection. Hao et al. 15 theoretically considered metallic nanostructures of different shapes and sizes. Experimental studies have investigated coupled gold ͑Au͒ nanodisks in an array 16 and singlenanoparticle pairs. 17,18 To date few attempts using EELS have been made to study coupled nanoparticles and particularly their effect on the surface-plasmon modes and their ability to produce high-field enhancements.Here,...

A simple imaging system, together with complex semidefinite programming, is used to generate the transmission matrix (TM) of a multimode fiber. Once the TM is acquired, we can modulate the phase of the input signal to induce strong mode interference at the fiber output. The optical design does not contain a reference arm, no internal reference signal is used, and no interferometric measurements are required. We use a phase-only spatial light modulator to shape the profile of the propagating modes, and the output intensity patterns are collected. The semidefinite program uses a convex optimization algorithm to generate the TM of the optical system using intensity only measurements. This simple, yet powerful, method can be used to compensate for modal dispersion in multimode fiber communication systems. It also yields great promise for the next generation biomedical imaging, quantum communication, and cryptography.