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The fluorescence of molecules on silver-island films exhibits nonexponential decay and is several orders of magnitude more rapid than on a silica surface, while the total emission intensity is slightly increased. This behavior results from the electromagnetic coupling between the fluorescing molecules and the plasmon resonances of the silver islands. We obtain good agreement with a model that uses the same theory for the electromagnetic coupling as has been used to account for surface-enhanced Raman scattering.

Both the spectral dependence and the magnitude of the excitation spectra of surface-enhanced Raman scattering are found to be quantitatively related to the absorption spectra for silver-island films. This relationship is maintained even when the local electromagnetic enhancement at the islands is varied by coating the film with a layer of absorbing dye molecules. The results illustrate a simple experimental way to determine independently the role of the electronic plasma resonances in the enhancement process on these films and shed new insights into the details of the electromagnetic interactions responsible for surface-enhanced Raman scattering.

Anomalous, low-frequency Raman scattering from localized acoustic vibrations of roughness features on a metal surface are observed. The observed mode frequency shifts with excitation frequency and with the index of refraction of the surrounding medium. The shifts arise from resonant laser excitation of localized dipolar plasmons. This resonant optical coupling is a key to the phenomenon of enhanced Raman scattering from molecules adsorbed on rough metal surfaces.PACS numbers: 68.25,+ j, 78.30.Er It has recently been established that an essential condition for the observation 1 of an enormous enhancement of the Ram an-scattering (RS) cross section of surface-adsorbed molecules is the presence of surface roughness. 2 " 4 In this Letter, we study enhanced RS from an extremely lowfrequency vibrational mode that is much lower in frequency than any molecular mode. It is characteristic of the rough metal surface itself and is attributed to a localized acoustic vibration of the roughness features themselves. Thus we are able to examine critically the necessity for and the scale of the required surface roughness. The low-frequency RS has two unusual properties. We observe a dramatic variation in the frequency shift, f p , of the peak RS intensity, when the laser excitation frequency, co ex , is changed and also when the index of refraction of the surrounding medium, n s , is changed. To our knowledge, this is the first observation of such frequency shifts in RS where there is no wave-vector dependence. We ascribe these effects to the resonant optical excitation of a dipolar plasmon which selectively enhances RS from acoustic vibrations localized on some roughness features on the metal surface. We obtain good agreement with a simple, physical model, yielding information on the scale of the surface roughness. Our results also help explain the key role of roughness in surface-enhanced RS (SERS).Most of our experiments are performed in an electrochemical cell, 4 using reagent grade chemicals and triply distilled and deionized water. The electrode surface was prepared using a mild anodization procedure. 1 We used aback-scattering optical geometry in most cases and dispersed © 1980 The American Physical Society 355 the scattered light with a Spex 1401 double monochromator followed by a third monochromator that was scanned in tandem. All the gratings were holographic, and the slit widths were adjusted for a band pass of -0.2 cm" 1 , giving excellent resolution and good rejection of elastic scattering and allowing the observation of RS down to ~2 cm" 1 from the laser frequency. Figure 1(a) shows a typical scan of both the anti-Stokes (negative frequency shift) and Stokes side of the low-frequency spectrum for a silver electrode in an electrolyte of 0.1M KI and 0.05M pyridine in water, with X ex = 514.5 nm. This peak is the most intense feature of the SERS spectrum, and is -20 times as intense as the pyridine peak at 1008 cm" 1 . A study of the temperature dependence down to 2 °K indicates that the ratio of Stokes to anti-Stokes ...

Electron drag between two two-dimensional electron gases in magnetic fields has been observed with a polarity opposite that for zero field. This negative drag requires that the electrons have a hole-like dispersion. Density dependence measurements in the integer quantum Hall regime show that drag is negative only when the upper Landau level of one layer is more than half filled while the other is less than half filled, indicating that hole-like dispersion is present in a half of each Landau level. Negative drag is argued to be a consequence of disorder. 73.40.Hm, 71.61.Ey, 73.20.Dx

We report nanoscale scanned probe ferromagnetic resonance force microscopy (FMRFM) imaging of individual ferromagnetic microstructures. This reveals the mechanism for high spatial resolution in FMRFM imaging: the strongly inhomogeneous local magnetic field of the cantilever mounted micromagnetic probe magnet used in FMRFM enables selective, local excitation of ferromagnetic resonance (FMR). This approach, demonstrated here in individual permalloy disks, is straightforwardly extended to excitation of localized FMR modes, and hence imaging in extended films.