Mohamad G. Banaee

Double-Resonance Plasmon Substrates for Surface-Enhanced Raman Scattering with Enhancement at Excitation and Stokes Frequencies

We report a surface-enhanced Raman scattering (SERS) substrate with plasmon resonances at both excitation and Stokes frequencies. This multilayer structure combines localized surface plasmons on the nanoparticles with surface plasmon polaritons excited on a gold film. The largest SERS enhancement factor for a gold device is measured to be 7.2 x 10(7), which is more than 2 orders of magnitude larger than that measured on a gold nanoparticle array on a glass substrate. The largest SERS enhancement for a silver device is measured to be 8.4 x 10(8).

Lithographically Fabricated Optical Antennas with Gaps Well Below 10 nm

Metal nanostructures that effi ciently capture or radiate electromagnetic waves at optical frequencies offer a means to concentrate electromagnetic energy into deep subwavelength regions. Wessel noted that these structures can therefore be considered antennas. [ 1 ] Recent work has focused on more effi cient designs, termed ‘optical antennas’, which employ small gaps or very sharp tips. [ 2‐4 ] Optical antennas present opportunities for ultrasensitive spectroscopy, near-fi eld scanning optical microscopy, and compact subwavelength light sources. [ 5‐7 ] However, the achievable feature sizes are usually determined by fabrication, being approximately given by the gap size or tip sharpness. Here, we report a top-down fabrication procedure to fabricate pairs of nanoparticles separated by a controllable gap size that can be as small as 3 nm. As an application, we show that the enhancement factors of surfaceenhanced Raman scattering (SERS) increase signifi cantly for smaller gap sizes, indicating greatly enhanced electromagnetic fi elds within the gaps. We anticipate that the fabrication method we introduce here for nanoparticle pairs with nanoscale gaps would be useful not only for SERS, where it could potentially enable single-molecule sensitivity, but also for other applications in plasmonics. Raman spectroscopy is a powerful analytical method, enabling molecules to be identifi ed through their characteristic vibrational spectra. Through SERS, the Raman cross-section of molecules adsorbed to nanostructures can be increased by orders of magnitude. SERS has attracted renewed attention since the fi rst demonstrations of singlemolecule sensitivity. [ 8 , 9 ] Increasing the adoption of the SERS technique further, however, requires fabrication methods capable of routinely delivering reproducible substrates with high enhancement factors. Dimer structures, consisting of two metallic nanoparticles closely placed together, are the simplest optical antenna structures that are confi rmed to be single-molecule SERS active. [ 10 , 11 ] It is believed that substantial electromagnetic fi elds generated in dimer gaps are one of the main enhancement mechanisms in single-molecule SERS. [ 12 , 13 ] This motivates the development of a reproducible

Gold nanorings as substrates for surface-enhanced Raman scattering

Surface-enhanced Raman scattering using gold nanoring substrates is studied. The measured enhancement factors of arrays of single nanorings and nanoring dimers are compared with that of an array of nanodisk dimers. The measured average enhancement factor for the single nanorings is 4.2 x 10(6). The experimental enhancement factors are compared with the electromagnetic enhancement factors predicted by simulations.

Mixed dimer double-resonance substrates for surface-enhanced Raman spectroscopy.

Surface enhanced Raman spectroscopy is performed on mixed dimers, consisting of pairs of gold nanoparticles with different shapes and plasmon frequencies. These are termed double resonance substrates. The results are compared to double dimer geometry.

Mixed dimer double resonance substrates for surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy is performed on pairs of gold nanoparticles, for which the nanoparticles in each pair have different shapes. The dimers therefore exhibit two plasmon resonances. These structures, termed mixed dimer double-resonance substrates, enable strongfield enhancement at pump and Stokesfrequenciesinsurface-enhancedRamanspectroscopy.Theextinctionspectraofmixeddimersaremeasured and simulated to identify their plasmon resonances. The experimentally determined enhancement factors of double-resonance structures are compared to those of single-resonance substrates.

Lithographically fabricated optical antennas with sub-10nm gaps formed by a sacrifical layer

We lithographically fabricate arrays of optical antennas with ∼6nm gaps. The enhancement factor from surface-enhanced Raman scattering measurement is ∼5 times larger than the same structure with ∼18nm gaps.

Surface-Enhanced Raman Scattering from a Double-Resonance Plasmon Structure

We report surface-enhanced Raman scattering measurements of a benzenethiol monolayer on a double resonance surface plasmon structure. The device enhances excitation and Raman scattered light simultaneously. The largest enhancement factor is measured to be 1.1×108.

Surface-Enhanced Raman Spectroscopy with Gold Nanoring Dimers

Surface-enhanced Raman scattering of benzenethiol on gold nanoring dimers with 20 nm gaps was studied. The localized surface plasmon resonance wavelength was determined using reflection spectroscopy. A SERS enhancement factor of 2.0x10 6 was obtained.