Structural Photonics Research in the Knappenberger Group
Photonic nanomaterials offer great potential for using and controlling light. These capabilities, which depend critically on nanomaterial structure, can be leveraged to yield improvements in applications such as energy conversion, quantum information, telecommunications, and medical diagnostics and therapeutics, among others. Members of the Knappenberger Research Group actively contribute to the understanding of Structural Photonics, which describes the structure-function interplay of light-matter interactions. To make advances on these materials systems, our group innovates femtosecond optical spectroscopy and imaging tools capable of revealing new aspects of how nanoscale structure determines these interactions. Specific areas include:
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 intra-network arrangement of these particles influences several functional aspects of these materials, including plasmon resonator frequency, selective interaction with specific polarization states of light, efficiency of nanoscale energy localization, and plasmon coherence, which are critical to the effective use of electromagnetic energy.
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 clusters (MPCs) 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, MPCs are being used as model systems for heterogeneous photocatalysis and catalysis at large scale.
Super-resolution Optical Microscopy
The Knappenberger group develops methods capable of pinpointing the source of optical signals with 1-nm spatial accuracy, which exceeds the diffraction limit by approximately 160x. Recently, we have combined nonlinear optical interferometry with this “super-spatial-resolution” imaging approach to carry out ultrafast spectroscopy measurements beyond the diffraction limit of light. This capability allows us to visualize energy transfer and confinement on the nanoscale. Knappenberger group members have also demonstrated that specific photonic modes can be “fingerprinted” using this super-resolution interferometric approach by determining the extinction spectra for various sub-diffraction domains within a network.
Ultrafast Microscopy. In order to understand how nanoscale structure influences the “quality factor” of photonic resonances, it is important to determine experimentally their electronic coherence times. For nanomaterials, 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. We actively employ these measurements to quantify quality factors for a variety of nanostructures.
Polarization-Selective and Magnetic Resonance Imaging. Nanoparticles can be used for selective amplification of specific light polarization states. We developed 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 magnetic resonance.
Ultrafast Multidimensional Spectroscopy.
The Knappenberger group uses two-dimensional femtosecond visible and near infrared spectroscopy to examine state-to-state electronic energy relaxation dynamics of quantum MPCs. 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.