Yizhuo Chu

Experimental observation of narrow surface plasmon resonances in gold nanoparticle arrays

We demonstrate that coupling between grating diffraction and localized surface plasmons in two-dimensional gold nanoparticle arrays in water leads to narrow near-infrared resonance peaks in measured far field extinction spectra. Good agreement is obtained between finite difference time domain (FDTD) calculations and experimental extinction spectra. The FDTD calculations predict that the gold nanoparticle arrays exhibit near-field electric field intensity (E2) enhancements approximately one order of magnitude greater than those of single isolated gold nanoparticles.

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).

Experimental study of the interaction between localized and propagating surface plasmons

The interaction between localized and propagating surface plasmons is investigated in a structure consisting of a two-dimensional periodic gold nanoparticle array, an SiO2 spacer, and a gold film. The resonance wavelengths of the two types of surface plasmons supported by the structure are tailored by changing the gold nanoparticle size and the array period. An anticrossing of the resonance positions is observed in the reflection spectra, demonstrating the strong coupling between localized and propagating surface plasmons.

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

Double resonance surface enhanced Raman scattering substrates: an intuitive coupled oscillator model

The strong coupling between localized surface plasmons and surface plasmon polaritons in a double resonance surface enhanced Raman scattering (SERS) substrate is described by a classical coupled oscillator model. The effects of the particle density, the particle size and the SiO2 spacer thickness on the coupling strength are experimentally investigated. We demonstrate that by tuning the geometrical parameters of the double resonance substrate, we can readily control the resonance frequencies and tailor the SERS enhancement spectrum.

High Directivity Optical Antenna Substrates for Surface Enhanced Raman Scattering

A two-dimensional array of gold optical antennas integrated with a one-dimensional array of gold strips and mirrors is introduced and fabricated. The experimental results show that this design achieves average surface-enhanced Raman scattering (SERS) enhancement factors as high as 1.2 × 10(10) , which is more than two orders of magnitude larger than optical antennas without the gold strips and gold mirror.

Single molecule surface enhanced Raman spectroscopy with an optical antenna chip

We propose and fabricate a chip containing optical antennas for single molecule Surface-Enhanced Raman Spectroscopy (SMSERS). We verify that SMSERS is achieved using experiments with Rhodamine 6G (R6G) isotopologues.

Collimation of Raman scattering with plasmonic structures

We demonstrate the collimation of Raman scattering by a SERS substrate consisting of optical antennas, a metallic reflector and a 1D grating of metal strips. A ~6.1° FWHM angle perpendicular to the strips is measured.

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.

Strong coupling between localized and propagating surface plasmons: Experimental observation

We report on the experimental observation of strong coupling between localized and propagating surface plasmons when a gold nanoparticle array is in the vicinity of a gold film. The resonances exhibit an anticrossing behavior.