The Raman cross-section from a molecule is believed to enhance by more than 10 orders of magnitude when it is adsorbed on a cluster of silver nanoparticles. These large enhancements are attributed to the resonant excitation of the surface plasmon modes of the cluster those have very large localized electric field near its surface. The resonant position and the electric field of these modes are very sensitive to the structure of metal particles and the size and shape of the cluster. Using multiple scattering in the wave-vector space between the individual particles in the cluster we have calculated the resonant position of these modes and their enhanced electric field for clusters of different shape formed from two, three, and four nanospheres and nanoshells. We find the maximum enhancement in the cross-section can reach up to 10 orders of magnitude for silver particle clusters. We also find important new results for the chain like clusters of three or more particles where there is a dramatic increase in the enhancement due to very sharp resonant features of the modes. These features may be helpful in identifying the cluster shape and size in the surface enhanced Raman scattering experiments.
Surface enhanced Raman spectroscopy has become a viable technique for the detection of single molecules. This highly sensitive technique is due to the very large (up to 14 orders in magnitude) enhancement in the Raman cross section when the molecule is adsorbed on a metal nanoparticle cluster. We report here SERS (Surface Enhanced Raman Spectroscopy) experiments performed by adsorbing analyte molecules on nanoscale silver particle clusters within the gelatin layer of commercially available holographic plates which have been developed and fixed. The Ag particles range in size between 5 - 30 nanometers (nm). Sample preparation was performed by immersing the prepared holographic plate in an analyte solution for a few minutes. We report here the production of SERS signals from Rhodamine 6G (R6G) molecules of nanomolar concentration. These measurements demonstrate a fast, low cost, reproducible technique of producing SERS substrates in a matter of minutes compared to the conventional procedure of preparing Ag clusters from colloidal solutions. SERS active colloidal solutions require up to a full day to prepare. In addition, the preparations of colloidal aggregates are not consistent in shape, contain additional interfering chemicals, and do not generate consistent SERS enhancement. Colloidal solutions require the addition of KCl or NaCl to increase the ionic strength to allow aggregation and cluster formation. We find no need to add KCl or NaCl to create SERS active clusters in the holographic gelatin matrix. These holographic plates, prepared using simple, conventional procedures, can be stored in an inert environment and preserve SERS activity after several weeks subsequent to preparation.
Very large enhancement up to 14 orders of magnitude in the Raman cross section from a molecule adsorbed on a single cluster of a few nano metal particles has been reported recently. The enhancement is believed mainly due to the enhanced electromagnetic (em) field because of the excitation of the localized surface em mode. Using scattering t-matrix approach, we have developed a Green's function theory in the Fourier (wave vector) space to solve the Maxwell equations for the enhanced electric field near the spherical metal particle cluster. The large enhancement in the field is due to the multiple scattering of the localized modes of the individual metal particles that has been included exactly. The advantage of working in the wave vector space is that one does not need the use of complicated translational addition theorem required in the real space as used in earlier calculations. Therefore, our approach can be easily extended to different shape or size of the metal particle cluster. We find the enhancement in the Raman cross section can reach up to 10 orders of magnitude for silver particle cluster. The enhancement is in a broad frequency range and is below the Mie resonance of the single metal sphere. The results for gold particle cluster are also presented.
The San Jose State University Physics Department, located in Silicon Valley, provides students with a high quality education in optics and provides local high-tech industry and government laboratories with a partner for optics- related research and development projects. There are approximately 50 undergraduate majors and 20 graduate (M.S.) students in the Department. Core courses leading to the B.S. in Physics are offered with upper division courses in Modern Optics, Lasers and Applications, Advanced Optics Lab, Advanced Instrumentation Lab, and Individual Studies as well as graduate courses in Electro-optics, Graduate Optics, Optical Metrology, and Laser Spectroscopy. Graduates are well prepared to enter the lasers and optics industry or go onto graduate school. A 4000 square-foot lab in the Science Building houses the Institute for Modern Optics, an organized research unit in the College of Science. One of the major goals of the Institute is to facilitate collaborative research between the local optics industry and the faculty and students at SJSU. The Department is presently developing a new biophotonics lab for single molecule studies with a dual beam optical tweezers already operational. A National Science Foundation Research Experience for Undergraduates Program grant provides research support for undergraduates.
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