Owing to the intense electromagnetic fields generated by electronic excitation of metal nanostructures, plasmon-supporting materials have great potential for increased performance in many photonic applications. In this research area, we are examining the nanoscopic details of how the arrangement of these particles within a network influences several of the functional aspects of these materials, including plasmon resonator frequency, selective interaction with specific polarization states of light, the efficiency of nanoscale energy localization, and plasmon coherence. Increased understanding of all of these properties are critical to the effective use of electromagnetic energy.
Structurally Precise Nanoclusters
Until recently, nanoscale structure-function assignments for metals were largely restricted to substrate-supported samples that could be optically addressed at the single-structure level and correlated to electron microscope images. Monolayer-protected nanoclusters are an emerging class of photonic nanomaterials that allow for solution-phase structure correlations with high precision. This is possible because these nanoclusters can often be isolated with atomic structural and compositional precision in a colloidal suspension. As a result, monolayer-protected nanoclusters are being used a model systems for understanding heterogeneous photocatalysis and catalysis at large scale.
Femtosecond Nonlinear Optical Spectroscopy and Imaging Techniques
Femtosecond Nonlinear Optical Microscopy
In order to understand how nanoscale structure influences the “quality factor” of plasmon resonances, it is important to experimentally determine their electronic coherence times. For metal nanoparticles, these coherence times are typically on the time scale of 10s of femtoseconds (10-15 seconds). Therefore, “ultrafast” time-resolved measurements are needed. In addition to the requirement of fast time resolution, examinations must be made with single-nanoparticle sensitivity. The Knappenberger group has pioneered interferometric nonlinear optical measurements capable of resolving plasmon coherence dynamics of single nanoparticles using sequences of collinear, phase-stabilized femtosecond laser pulses. The Knappenberger group members actively employ these measurements to quantify plasmon mode quality factors for networks of plasmonic nanostructures.
Nanoparticles can be used to selectively amplify specific light polarization states. The Knappenberger group has recently developed interferometric nonlinear optical techniques that use an orthogonal pair of temporally delayed, phase-locked laser pulses. By scanning the time delay with approximately 10 attosecond resolution, it is possible to conduct complete-polarization-variation analysis of nonlinear optical signals. These methods can be applied to generate optical image contrast based on circular dichroism and can be used to quantify the relative electric and magnetic dipolar contributions to nanoparticle nonlinear optical responses.
Super-Resolution Nonlinear Optical Microscopy and Spectroscopy
Nanoparticle networks can be used to transfer and confine energy to nanoscale volumes. However, tracking and pinpointing these regions of energy confinement require spatial resolution that exceeds the diffraction limit of most optical microscopy measurements. The Knappenberger group takes advantage of plasmon amplification of nonlinear optical signals to pinpoint these regions with approximately 1-nm spatial accuracy. To date, we have applied these methods to map plasmon-mode-specific energy localization using second harmonic signals generated from plasmonic nanolens trimers. These measurements provide insight into the spatial mode density of complex plasmon resonances. Recently, we have combined nonlinear optical interferometry with “super-spatial-resolution” imaging to carry ultrafast spectroscopy measurements beyond the diffraction limit of light. Knappenberger group members have also demonstrated that specific plasmon modes can be “fingerprinted” using this super-resolution interferometric approach by determining the extinction spectra for various sub-diffraction domains within the nanoparticle network.
2-D Electronic Spectroscopy
The Knappenberger group uses two-dimensional femtosecond visible and near infrared spectroscopy to examine state-to-state electronic energy relaxation dynamics of structurally precise metal nanoclusters. A significant advantage of 2-D methods over one-dimensional transient spectroscopy techniques is the ability to achieve high temporal resolution while also obtaining spectral information content for both sample excitation and signal detection. In this way, the flow of electronic energy through photonic material systems can be mapped. The Knappenberger group is interested in understanding how nanoscale structure influences energy flow through photonic nanoclusters.