Impact of strong coupling on chemical properties and nonlinear response of organic semiconductors
Restricted (Penn State Only)
- Author:
- Krainova, Nina
- Graduate Program:
- Electrical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- October 03, 2023
- Committee Members:
- Tom Jackson, Major Field Member
Kenneth Knappenberger, Outside Unit & Field Member
Xingjie Ni, Major Field Member
Noel Giebink, Chair & Dissertation Advisor
Madhavan Swaminathan, Program Head/Chair - Keywords:
- strong coupling
exciton-polaritons
polaron-polaritons
polariton chemistry
electroabsoption
photoluminescence
SHG
photocurrent - Abstract:
- Polariton chemistry is an emerging and intriguing field that seeks to address a fundamental question: Can we modify and control molecular properties by shaping the optical environment? By hybridizing light and matter and creating quasi-states that possess the properties of both, known as polaritons, can molecules harness photonic characteristics? We focus on intermolecular interactions in a strong light-matter coupling regime. More specifically, our research centers on the investigation of charge transfer events, with photocurrent or photoluminescence serving as our observables. For our photocurrent study, we looked into the photoexcitation of charged molecules, which transform into polaron-polaritons in a strong coupling regime. Fascinatingly, we have discovered evidence of an accelerated charge-transfer process when charged molecules are strongly coupled to light, as opposed to when they are not. The key finding lies in the distinct bias dependence observed between weakly and strongly coupled charged molecules within the same photonic structure. Weaker bias dependence of strongly coupled molecules, according to Onsager theory, indicates that a photoexcited polaron hops further away from its counter anion compare to weakly coupled case. Thus, we conclude that strong coupling regime enhances the intermolecular charge transfer rate. We explain the effect by considering the delocalized nature of polaritons and their ability to simultaneously sample all strongly coupled molecules, benefiting from molecules with fast charge-transfer rates. In our photoluminescence study, we sought a means to harness a donor exciton in donor-acceptor blends that feature high donor concentrations through the application of strong light-matter coupling. Our motivation was to utilize the delocalized nature of photons to promote the transmission of excitation from a distant donor to a donor in close proximity to an acceptor, thereby facilitating the formation of a charge-transfer state. We investigated two distinct systems in which exciton-polaritons originate from the first and second excited states of singlet excitons. Regrettably, we did not observe any systematic trends in the emission spectrum between excitons in strong and weak coupling regimes. We argue that temperature plays a critical role, and only at cryo temperatures where the coherent lifetime is much longer can the effect be observed. Unfortunately, unlike our photocurrent study, our investigation of the photoluminescence was conducted at room temperature. In the latter part of our research, we attempt to understand the effect of strong coupling on the nonlinear optical response of molecules. Specifically, we study electroabsorption (EA) which is responsible for the imaginary part of chi3 nonlinearity. We predict that the molecule with a degenerate excited state will have both linear and quadratic field dependencies of its EA response. Remarkably, we observe that the functional dependence of the EA response on the bias for the lower polariton (LP) strongly varies along the LP dispersion curve. When lower polariton approaches the energy of the uncoupled transition, its bias dependence becomes more quadratic; alternatively, when the energy of LP father away from the transition, the bias dependence is more linear. Furthermore, for both the uncoupled transition and middle polariton we observe the same strongly quadratic bias dependence, distinctly different from that for the lower polariton. Lastly, we have established the groundwork for an exploration of chi2 nonlinearity in an anisotropic organic crystal. We have verified the strong coupling regime for a primal crystal axis by altering the energy of an optical mode. We do not observe a strong coupling signature for light with polarization perpendicular to the primal crystal axis. We witness the strongest second harmonic signal when the incident light (fundamental) has twice the wavelength of the lower polariton, however, at the same time similar magnitude enhancement of SHG occurs at DBR sidebands. This preliminary finding undermines the notion that an exciton-polariton possesses strong hyperpolarizability. Additional measurements are suggested in the last chapter to solidify our findings.