Kenneth Knappenberger Jr.

Honors and Awards

Fellow of the American Association for the Advancement of Science (AAAS), 2020

Keynote Lecture, RACI National Centenary Conference, Royal Australain Chemical Institue, 2017

Plenary Lecture, Photonics 2016, Optical Society Meeting, 2016

Keynote Lecture, International Symposium on Molecular Spectroscopy, 2016

Harold Kohn Endowed Alumni Lecturer, The Pennsylvania State University, 2016

Coblentz Award, The Coblentz Society, 2016

Visiting Lecturer, Katholieke Universiteit, Leuven, 2015

Developing Scholar Award, Florida State University, 2015

Joseph Wang Award in Nanoscience, 2015

Visiting Professor, Politecnico di Milano, 2015

Keynote Lecture, National Science Foundation, Fourteenth Southern School on Computational Chemistry and Materials Science, 2014

Journal of Chemical Physics, Editor's Pick of the Year Publication, American Institute of Physics, 2013

Young Investigator Award, Inter-American Photochemical Society, 2012

CAREER Award, National Science Foundation, 2011

Young Investigator Award, Department of Defense, Air Force Office of Scientific Research, 2010

NRC/NIST Postdoctoral Research Fellowship, National Research Council (declined), 2005-2007

Research Excellence Award, Spectra-Physics Lasers, 2005

Dalalian Research Fellowship, The Pennsylvania State University, 2004

Education

B.S., Lock Haven University, 2000

Ph.D., The Pennsylvania State University, 2005

Postdoctoral Fellow, University of California–Berkeley, 2005-2008

Research

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:

Plasmonic Nanostructures

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.

Quantum Photonics

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.