Efficient Upper-Excited State Fluorescence in an Organic Hyperbolic Metamaterial

Area of Science:

• Photonic materials
• Quantum optics
• Molecular spectroscopy

Background:

• Upper-excited state emission in molecules is typically inefficient due to rapid nonradiative relaxation.
• Enhancing radiative decay rates is crucial for observing emission from higher energy states.
• Metamaterials offer unique photonic environments to control light-matter interactions.

Purpose of the Study:

• To investigate the potential of hyperbolic metamaterials (HMMs) for enhancing upper-excited state emission.
• To demonstrate spectrally tunable fluorescence from the second singlet excited state (S2) of zinc tetraphenylporphyrin (ZnTPP).
• To achieve ultrafast fluorescence decay via radiative decay engineering.

Main Methods:

• Embedding zinc tetraphenylporphyrin (ZnTPP) molecules within a hyperbolic metamaterial (HMM) structure.
• Systematically varying the HMM stack's periodicity to tune the photonic density of states.
• Spectroscopic analysis of fluorescence intensity and decay lifetimes.
• Transfer matrix modeling to predict Purcell enhancement of radiative rates.

Main Results:

• An approximately 18-fold increase in fluorescence intensity from the S2 state of ZnTPP compared to the S1 state.
• Tunable fluorescence emission from red (S1-dominated) to blue (S2-dominated) by adjusting the HMM periodicity.
• Observed instrument-limited decay lifetimes below 10 picoseconds for S2 emission.
• Experimental results consistent with broadband Purcell enhancement of radiative rates.

Conclusions:

• Hyperbolic metamaterials can significantly enhance radiative decay rates, enabling efficient upper-excited state emission.
• This approach allows for spectrally tunable and ultrafast fluorescence via radiative decay engineering.
• The findings open new avenues for harnessing molecular upper-excited states for advanced optical applications.