Dr. Wonmi Ahn Profile

Dr. Wonmi Ahn

at US Naval Research Lab

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Publications (3)

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PROCEEDINGS ARTICLE | September 21, 2017

Control of vibration-cavity polaritons in the frequency domain (Conference Presentation)

Proc. SPIE. 10343, Metamaterials, Metadevices, and Metasystems 2017

KEYWORDS: Molecules, Chemistry, Vibration control, Optical resonators, Infrared radiation, Polaritons, Molecular interactions, Photochemistry, Light-matter interactions

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We will focus on approaches which make use of light-matter interactions to alter the chemical behavior of a target molecular species. This is done through cavity coupling to a molecular vibration. Coupling vibrational transitions to resonant optical modes creates vibrational polaritons shifted from the uncoupled molecular resonances and provides a convenient way to modify the energetics of molecular vibrations. This approach is a viable method to explore controlling chemical reactivity and energy relaxation. Here, we demonstrate frequency domain results for vibrational bands strongly coupled to optical cavities. We experimentally and numerically describe strong coupling between a Fabry-Pérot cavity and several molecular species (e.g., poly-methylmethacrylate, thiocyanate, hexamethyl diisocyanate). We investigate strong and weak coupling regimes through examination of cavities loaded with varying concentrations of a urethane monomer. Rabi splittings are in excellent agreement with an analytical description using no fitting parameters. We show that coupling strength is a function of molecule/cavity mode overlap by systematically altering the position of a molecular slab throughout a first order cavity with results agreeing well with analytical and transfer matrix predictions. Further, remote molecule-molecule interaction will be explored by placing discrete and separated molecular layers throughout a cavity. In addition to establishing that coupling to an optical cavity modifies the energy levels accessible to the coupled molecules, this work points out the possibility of systematic and predictive modification of the excited-state kinetics of vibration-cavity polariton systems. Opening the field of polaritonic coupling to vibrational species promises to be a rich arena amenable to a wide variety of infrared-active bonds that can be studied in steady state and dynamically.

PROCEEDINGS ARTICLE | March 13, 2015

Template-guided self-assembly strategies for discrete and extended optoplasmonic materials

Proc. SPIE. 9374, Advanced Fabrication Technologies for Micro/Nano Optics and Photonics VIII

KEYWORDS: Gold, Plasmonics, Electron beam lithography, Titanium dioxide, Resonators, Nanoparticles, Metals, Dielectrics, Photonics, Electromagnetism

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Optoplasmonic materials that contain both metallic (plasmonic) and dielectric building blocks can sustain synergistic electromagnetic interactions between photonic and plasmonic resonances and, thus, pave the way to overcoming the limitations of the respective building blocks. A significant challenge in realizing the full potential of these unique electromagnetic materials is the integration of building blocks with different chemical compositions and sizes into defined morphologies. We demonstrate in this paper that template guided self-assembly strategies show great promise in realizing intricate discrete and extended optoplasmonic materials. Selected examples of optoplasmonic materials and the underlying fabrication methods are discussed. The first example combines dielectric microspheres as whispering gallery mode resonators with plasmonic antennas. The latter are located at defined locations in close vicinity of (but not attached to) the dielectric microsphere. The interactions of WGMs with plasmonic resonators located in their evanescent field are analyzed. The second example describes two-dimensional interdigitated arrays of 250 nm diameter TiO2 NPs and clusters of electromagnetically strongly coupled 60 nm gold nanoparticles. It is demonstrated that delocalized photonic– plasmonic modes in the arrays achieve a cascaded E-field enhancement in the gap junctions of the gold NP clusters.

PROCEEDINGS ARTICLE | August 27, 2014

Electron optics of nanoplasmonic metamaterials in bio/opto theranostics

Proc. SPIE. 9166, Biosensing and Nanomedicine VII

KEYWORDS: Gold, Metamaterials, Near infrared, Plasmonics, Refractive index, Scattering, Metals, Nanoplasmonics, Polarizability, Lawrencium

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Opto-electronic coupling of plasmonic nano-antennas in the near infrared water window in vitro and in vivo is of growing interest for imaging contrast agents, spectroscopic labels and rulers, biosensing, drug-delivery, and optoplasmonic ablation. Metamaterials composed of nanoplasmonic meta-atoms offer improved figures of merit in many applications across a broader spectral window. Discrete and coupled dipole approximations effectively describe localized and coupled resonance modes in nanoplasmonic metamaterials. From numeric and experimental results have emerged four design principles to guide fabrication and implementation of metamaterials in bio-related devices and systems. Resonance intensity and sensitivity are enhanced by surface-to-mass of meta-atoms and lattice constant. Fano resonant coupling is dependent on meta-atom polarizability and lattice geometry. Internal reflection in plasmonic metaatom- containing polymer films enhances dissipation rate. Dimensions of self-assembled meta-atoms depend on balancing electrochemical and surface forces. Examples of these principles from our lab compare computation with images and spectra from ordered metal-ceramic and polymeric nanocomposite metamaterials for bio/opto theranostic applications. These principles speed design and description of new architectures for nanoplasmonic metamaterials that show promise for bioapplications.

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