Correlating Second Harmonic Optical Responses of Single Ag Nanoparticles with Morphology

Abstract

The linear and nonlinear optical properties of nanoparticles are of significant importance for fundamental science and technological applications. 1,2 The second harmonic (SH) response of nanoparticles is particularly interesting due to the potential dependence on particle shape and deviations from high symmetry (e.g., spherical). However, ensemble measurements 3 (e.g. SH from colloidal solutions) obscure deeper insights due to the inevitable inhomogeneity of particle shape and size. Thus, measurements on a single-particle level are necessary to gain a more complete understanding of the optical properties of nanocrystal materials. 4 Undertaking such measurements is, however, impeded by the lack of a methodology allowing precise correlation of optical measurements and highresolution imaging of the particle morphology by transmission electron microscopy (TEM).This communication reports the results of an approach that permits correlation of SH activity with nanoparticle morphology imaged by TEM, which achieves significantly higher resolution than atomic force microscopy and scanning electron microscopy. The essence of our approach is to create position markers on an optical and electron transparent substrate (Si 3 N 4 thin film, ∼100 nm in thickness) that allows both optical measurements and TEM imaging of the identical nanoparticles. 5 The SH activities of single Ag nanoparticles (spheres versus rods) and cluster structures (e.g., dimers and trimers) are compared. The direct determination of the structure and the resultant structure-spectroscopic correlation allows gaining a new insight into the mechanism for the nonlinear (i.e., resonant second harmonic) response from each specific nanoparticle or cluster.The position bar-markers were fabricated by electron-beam lithography on a specially designed Si 3 N 4 window supported on a Si wafer. Briefly, the Si 3 N 4 substrate was first coated with a thin layer (8 nm) of silicon monoxide via thermal evaporation. This changes the hydrophobic Si 3 N 4 surface to hydrophilic, which provides better adhesion and distribution of aqueous Ag nanoparticles. The substrates were then spin-coated with electron resist, followed by electron-beam patterning. The patterned substrate was further deposited with a thin layer (20 nm) of chromium, which acts as the positional marker material, followed by liftoff of electron resist. The substrate with position markers was further coated with a positively charged polymer (polydimethyldidodecylammonia chloride, PDDA), which captures negatively charged Ag nanoparticles synthesized by citrate reduction of AgNO 3 . As shown in Scheme S1 (Supporting Information), SH maps were acquired with a home-built nonlinear optical microscopy system including a tunable Ti:sapphire femtosecond laser (Spectra Physics). The detection optics allowed detection of wavelengths from 400 to 500 nm, while the excitation was tunable from 800 to 1000 nm with ∼100-fs pulse (see Supporting Information).We first investigated the SH activity of single Ag nanoparticles and cl...

Nonlinear plasmonic imaging techniques and their biological applications

“…For example, SHG signal, in transmission mode, from thin layers of ellipsoidal silver nanoparticles embedded in silica glass was detected [117]. Following that, the second harmonic optical response from single silver nanoparticle was reported ( Figure 3A1) [124]. Gold nanospheres embedded in dielectric matrix such as gelatin were also reported to generate polarization-dependent SHG signal ( Figure 3A2) [32].…”

A common-path interferometer for time-resolved and shot-noise-limited detection of single nanoparticles

“…This sensitivity to size and shape, however, makes studies of ensembles of nanoparticles particularly vulnerable to distributions in sizes, shapes, crystal defects, etc.. Isolating a single particle once and for all eliminates inhomogeneous broadening and any implicit averaging inherent to even the most carefully selected ensembles. Only single-particle experiments permit to study a particle's elastic interaction with its specific close environment [5], to correlate optical and structural properties [6], or to obtain new insight in their linear and nonlinear optical properties [7,8,9].…”

Superluminality phenomenon at a femtosecond laser pulse propagation in a medium containing nanorods

“…(16)] gives E Q -T A R G E T ; t e m p : i n t r a l i n k -; e 0 1 7 ; 1 1 6 ; 6 5 3 dt dz ¼ 2DΩ; (17) which is the equation for characteristics. Remember that a characteristic is a curve in the plane ðz; tÞ along which the temporal integral of the pulse intensity Pðz; tÞ and instantaneous frequency Qðz; tÞ remain constant or change according to ordinary differential equations with respect to the z-coordinate.…”

“…[1][2][3][4][5][6] A large number of investigations deal with the optical response of a large ensemble of metal nanoparticles embedded into a medium and focus their attention on nonlinear refractive index changing and the third-order susceptibility changing in dependence of a nanoparticles concentration. [7][8][9][10][11][12] Obviously, a clarification of the physical mechanism for nanoparticle optical response dependence on the nanorods aspect ratio and nanorods orientation, and local enviroment, [13][14][15][16][17][18] as well as photothermal reshaping of nanorods or wires at the temperatures less than the bulk melting temperature [19][20][21][22] are very important questions. Various mathematical and physical models for an explanation of photothermal reshaping of nanosized nanoparticles are proposed up to now.…”