- Plasmonic Nanostructures Studied Using Nonlinear Optical Spectroscopy at the Limits of Spatial Precision and Temporal Resolution.
Jarrett, Jeremy White, Knappenberger, Kenneth L., Cao, Jianming, Steinbock, Oliver, Roper, Michael Gabriel, Florida State University, College of Arts and Sciences, Department of...
Show moreJarrett, Jeremy White, Knappenberger, Kenneth L., Cao, Jianming, Steinbock, Oliver, Roper, Michael Gabriel, Florida State University, College of Arts and Sciences, Department of Chemistry and Biochemistry
Advances in nonlinear optical spectroscopy have been employed to investigate interactions between nanostructures and electromagnetic energy. These efforts are directed toward the larger goal of optimizing the structure of photonic nanoparticle assemblies for the use and control of energy at the nanoscale. Statistical localization methods are combined with nonlinear optical microscopy to form the basis of Nonlinear Optical Localization using Electromagnetic Signals (NOLES) imaging, which has...
Show moreAdvances in nonlinear optical spectroscopy have been employed to investigate interactions between nanostructures and electromagnetic energy. These efforts are directed toward the larger goal of optimizing the structure of photonic nanoparticle assemblies for the use and control of energy at the nanoscale. Statistical localization methods are combined with nonlinear optical microscopy to form the basis of Nonlinear Optical Localization using Electromagnetic Signals (NOLES) imaging, which has been used to locate nonlinear signal hot spots within a plasmonic network with nanometer spatial accuracy. Initial studies aimed to utilize plasmon amplification in gold nanoparticle dimers to understand how different experimental parameters affect the achievable localization accuracy. It was found that energy matching between the laser excitation source and the localized surface plasmon resonance (LSPR) in conjunction with incidence polarization angle of the source was critical for achieving the high localization accuracy. Phase-dependent NOLES imaging is an extension of NOLES imaging in which different polarization states of light are used as the excitation source. In this way, it is possible to study chiro-optical properties of structures using circular dichroism measurements. Asymmetric gold nanobowtie structures were first examined with this method. It was found that these chiral structures generated a circular dichroism response. Further experiments employed continuous polarization variation measurements to determine the relative magnetic dipolar and electric dipolar contributions to the observed nonlinear optical (NLO) signal. The results demonstrated that both chiral image contrast, which resulted from left and right circularly polarized excitation, and the corresponding localization precision were dependent upon the relative magnetic dipole/electric dipole ratio (G/F). For example, left-to-right image enhancements of over 400% were obtained for bowties with the highest G/F ratio. These different polarization states of light were created using a pulse replica generator (PRG). This optical device, composed of wedges of birefringent materials, generates phase-locked, orthogonally polarized pulse replicas with a computer-controlled inter-pulse time delay. Sub-cycle temporal delays were introduced to reliably and easily create left-handed and right-handed circularly polarized light for second harmonic generation (SHG)-detected circular dichroism measurements. Using the PRG, the chiro-optical properties of deterministically constructed nanolens assemblies—homodimers, heterodimers, and heterotrimers (three nanoparticles arranged with cascading size)—were investigated. Experiments on these different structures revealed a systematic increase in the observed circular dichroism response going from homodimer to heterodimer to heterotrimer. Additionally, the nonlinear hot spot in the heterotrimer was determined, with NOLES imaging, to have a polarization-dependent location, which indicates that the large nanosphere in the assembly acts as an antenna to efficiently collect and direct energy toward the smaller nanoparticle constituents. In addition to studying SHG from these nanoassemblies, two-photon photoluminescence (TPPL) was examined in order to determine the origins of the circular dichroism response in the materials. Through a combination of dark-field spectroscopy and linearly polarized TPPL measurements, it was determined that the photoluminescence was mediated by the plasmon. TPPL-detected circular dichroism measurements were performed, and the results agreed well with the circular dichroism response observed with SHG-detection. These results, in combination with one-photon photoluminescence measurements, suggest that interference between higher-order, quadrupolar plasmon modes in the large nanoparticle constituents and dipolar plasmon modes in the smaller nanoparticle constituents gave rise to the circular dichroism response. Not only can the pulse replica generator be used to perform phase-dependent NOLES imaging, but it can also be used to perform time-resolved microscopy. It has been used to investigate plasmon dephasing in a series of gold nanorods with different aspect ratios. Dephasing times for resonant plasmon modes were extracted from analysis of interferometric TPPL and SHG data. These results showed that the dephasing time increased from 22 ± 4 fs to 31 ± 9 fs as the LSPR resonance energy decreased from 1.76 eV to 1.53 eV, as a result of less efficient plasmon damping due to the interband scattering for lower energy resonances. Femtosecond two-dimensional electronic spectroscopy (2DES) was used to determine superatom state-resolved dynamics of the Au25(SC8H9)18-monolayer-protected cluster. By examining time-dependent cross-peak amplitudes for specific excitation and detection photon energy combinations, electronic relaxation dynamics, mediated by specific superatom states, were distinguished. Using 2DES, hot electron relaxation (200 ± 15 fs) within the superatom D manifold of lowest-unoccupied molecular orbitals was differentiated from hot hole relaxation (290 ± 20 fs) in the superatom P states. Quantification of the time-dependent amplitudes and energy positions of cross peaks in the 2D spectra revealed the underlying electronic relaxation mechanism that gave rise to an unusual time-dependent blue shift of the transient bleach signal.
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