Single Nanostructure Nonliear Behavior of Plasmonic Metal Nanoparticles Studied Using Ultrafast Correlated Light and Electron Microscopy
Open Access
- Author:
- Zhao, Tian
- Graduate Program:
- Chemistry
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- August 09, 2019
- Committee Members:
- Kenneth Luther Knappenberger, Jr., Dissertation Advisor/Co-Advisor
John B Asbury, Committee Member
Kenneth Luther Knappenberger, Jr., Committee Chair/Co-Chair
Venkatraman Gopalan, Outside Member
Dr. Tae-Hee Lee, Committee Member
Philip C Bevilacqua, Program Head/Chair - Keywords:
- Nanostructure
Plasmonic
3D imaging
electronic dynamic
Ultrafast Microscopy
Super-resolution - Abstract:
- Single molecule interferometric nonlinear optical(INLO) microscopy has been employed to provide structure-specific descriptions of plasmon-mediated nonlinear optics and electron dynamics in metal nanostructures. At the start, INLO measurements were performed to study polarization-dependent and time-resolved photoluminescence (PL) properties of gold nanorods (AuNRs). AuNRs corresponding to three different length-to-diameter aspect ratios (AR)1.86, 2.91, and 3.90were examined using single-nanorod spectroscopy and imaging; the nanorod volume was approximately constant over the three sample types. Aspect-ratio-dependent longitudinal surface plasmon resonances (LSPRs) were observed at 2.08 ± 0.19 eV, 1.76 ± 0.12 eV, and 1.53 ± 0.15 eV for the 1.86-AR, 2.91-AR, and 3.90-AR samples, respectively. The resonant LSPR mode frequencies of the nanorods were determined from interferometrically detected TPPL signals. Dephasing times (T2) for resonant plasmon modes were extracted from the analysis of interferometric TPPL and second harmonic generation data. These results showed that the dephasing time increased from 22 ± 4 to 31 ± 9 fs as the LSPR resonance energy decreased from 1.76 to 1.53 eV, as a result of less efficient plasmon dephasing due to interband scattering for lower energy resonances. From INLO study of single AuNRs with different aspect ratios, the influence of interband effect on plasmon dephasing mechanism in gold nanoparticles was addressed. Having demonstrated the capability of interferometric nonlinear optical imaging microscopy on understanding the interband scattering effect, the influence of interfacial scattering effect on AuNRs with silica shells have been studied. The samples include cetyltrimethylammonium bromide (CTAB)-passivated nanorods, as well as ones encapsulated by five-nanometer and twenty nanometer-thick silica shells. The Q-factor results showed a trend of increasing quality factor, rising by 46% from 54±8 to 79±9, in going from CTAB- and 20-nm silica-coated AuNRs. The straightforward method of INLO enables the analysis of plasmon responses to environmental influences, such as analyte binding and solvent effects, as well as quantification of structure-specific plasmon coherence dynamics. Next, the influence of interparticle coupling on electron dynamics in AuNRs assemblies was studied through INLO microscopy. A specific interference is Fano resonance which results from the coupling between a broad, spectrally bright, super radiant mode and a narrower, spectrally dark, sub radiant mode. A dolmen structure, which contains a dimer formed from parallel nanorods, capped by a single orthogonal rod, provided an outstanding model for studying this type of resonant inter-particle coupling. The orientation of the capping rod, with respect to the dimer base, influences the inter-particle coupling strength and plasmon coherence times. The finite difference time domain (FDTD) method provided electric-magnetic field projections for different structures. The studies revealed that when the capping rod was tilted to 30º with respect to the long axis of the dimer base, constructive interference between the capping rod and the dimer base was maximal. This inner-particle interference also can be tuned by changing the incident polarization of the excitation field. When the incident polarization was matched the longitudinal axis of the capping rod, the interference reached the maximum. Conversely, the interference was suppressed if the incident polarization was gradually changed to the axis orthogonal to the capping rod. UCLEM was carried out to examine whether Fano interference could affect electron relaxation in the coupled systems. The INLO data revealed that Fano interference dramatically influenced the electronic relaxation rate. When the incident polarization matched the longitudinal axis of the capping rod in the asymmetric dolmen, the longest coherence time was obtained, 16fs. The coherence time can decrease to 8fs when the incident polarization was misaligned with respect to the longitudinal axis of the capping rod. This behavior can be explained by the radiative suppression of the plasmon resonance when Fano interactions were optimal. These results are particularly important because they illustrate how radiative damping channels can be suppressed by Fano interference in complex nanoparticle assemblies. The other coupled structure studied in this thesis is Fano switch, an octamer structure where a large, central, hemicircular disk is surrounded by an evenly spaced circular ring of seven smaller nanodisks. A series of Fano switches with different dimensions, resulting in Fano interferences that spanned 1.82eV to 1.55eV, were used to understand nonlinear optical response in this type of coupled structure. Polarization-resolved dark-field scattering and INLO measurements at single-nanoparticle sensitivity were used to characterize the interparticle coupling affected plasmon coherence and chirality of Fano switches. Time-resolved TPPL-detected interferometric nonlinear optical autocorrelation measurements were used to quantify the polarization-dependent coherence time and understand the super-sub radiant modes coupling in different Fano switches. When the Fano interference was resonant with the fundamental, the polarization-dependent Fourier analysis shows that the coherence time increased 40%, from 16fs to 27f, which corresponds to a concomitant Fano interference strength increase. The Fano induced longer coherence time was attributed to the suppressed radiative damping channel due to the generation of the sub-radiant mode in Fano switches. Correspondingly, a longer coherence time also resulted in an enhanced chiral field in resonant Fano switches. 2~ stronger chiral field was detected in resonant Fano switches than in off resonant samples. Power-dependent behaviors of Fano switches also resolved from TPPL spectra power studies. Based on results from spectrally resolved power studies, a third-order nonlinear behavior in the high energy side of TPPL spectra was observed in resonant Fano switches with the incident polarization at 0. This higher photon order can be attributed to the strong local field enhancement due to the modes coupling in resonant Fano switches. Because plasmonic structures with greater plasmon mediated local electromagnetic fields, affected by plasmon coherence time, are desired by most plasmonic-based photonic applications, UCLEM with single-particle sensitivity described here can provide new understandings of structure-specific coherence relaxation of coupled plasmonic materials. The final portion of this thesis focuses on understanding three-dimensional localization information of plasmonic gold nanoparticles. Two-photon photoluminescence and four-wave mixing nonlinear optical signals from plasmonic gold nanorods (AuNRs), imaged at the single-particle level, were used to demonstrate the capability of our home-built 3-D nonlinear optical microscopy combined with variable displacement-change point detection (VD-CPD) method. Astigmatisms were generated using axial sample-position displacements spanning the range from ±10 nm to ± 90 nm with a minimum step-size resolution of ± 3 nm. Based on the current data, 20-nm point source localization was achieved in the axial dimension using a single imaging objective. The influence of plasmon enhancement on achievable axial localization was also quantified. Two AuNR systems with different length-to-diameter aspect ratios (AR, where AR = 1.86 and 3.90) were selected for this purpose; the AR = 1.86 and AR = 3.90 had non-resonant and resonant longitudinal surface plasmon resonances (LSPR) energies, respectively, with the laser fundamental. Matching the fundamental wave LSPR energies resulted in increased axial localizations. Power-dependent analysis of the LSPR-mediated NLO images revealed that resonantly excited AuNRs resulted in third-order signals. The axial localization provided by VD-CPD exceeds what could be obtained using astigmatic imaging alone by factor 2.5. This advance will facilitate the in-depth study of photonic materials and complex biological environments that can benefit from increased axial position determinations.