Organic molecules are attractive to both physicists and chemists because molecules could have high quantum efficiencies in light emission and be chemically synthesized to have transitions at desired wavelengths. Moreover, single molecules, as isolated individual quantum systems, are actually versatile sources of single photons since a single two-level system cannot emit two photons simultaneously, as each excitation and emission cycle require a finite time. Compared to various other solid-state single photon emitters such as quantum dots, color center in diamond, defects in two-dimensional materials, single molecules embedded in crystalline organic matrix, possess several unique properties including small size of about one nanometer (suitable for high-density doping), flexibility in the synthesis, and strong and stable Fourier-limited zero-phonon lines at low temperature. In particular, dibenzoterrylene (DBT) molecules embedded in anthracene (AC) crystal have been actively studied as definitely stable single-photon emitters with non-blinking emission and lifetime-limited linewidth, the integration of single DBT molecules with planar photonic circuits has been also explored.
However, the quantum efficiency of single DBT molecules in anthracene matrix, as a critical piece of information, has not been experimentally measured. The quantum efficiency of an emitter indicates the ability to emit a photon once an excitation photon is absorbed. While the theoretical definition of quantum efficiency is crystal clear, its experimental measurement is highly nontrivial. In the past two decades, there have been several experiments reporting the measurements of absolute quantum efficiency of single emitters. The existing applied methods can be classified into two types, i.e., (i) study the emitter's decay rate with nano-controlled variation of the optical environment and (ii) measure the saturation of the emission of the emitter with pre-characterized total detection efficiency of the system. Both methods are difficult to implement for DBT molecules which possess an in-plane dipole orientation in the AC crystal and thus their quantum efficiencies have not been measured at the single-molecule level.
The research group led by Professor Xue-Wen Chen from Huazhong University of Science and Technology proposed a simple method to measure and confirm the near 100% intrinsic quantum efficiency of single DBT molecules embedded in AC microcrystal by monitoring the fluorescence lifetime change during the process of natural sublimation of the microcrystal. The intrinsic quantum efficiency could be extracted from the variation of the Purcell factor with the thickness. The research results are published in Chinese Optics Letters , Volume 20, Issue 7, 2022 (P. Ren, et al, Probing fluorescence quantum efficiency of single molecules in an organic matrix by monitoring lifetime change during sublimation).
The AC microcrystal exposed in air will slowly sublimate, the decrease of the thickness of the microcrystal due to sublimation induces the change of the optical environment of the molecules, and consequently, the change of the Purcell factor or the local density of optical states (LDOS), which manifests through the modification of the fluorescence lifetime. With the help of the home-built confocal microscope combined with an atomic force microscope (AFM) shown in Figure 1 (a), researchers observed the change of fluorescence lifetime on the same molecule during the sublimation and accurately recorded the lifetimes and the corresponding crystal thicknesses at different time. By identifying the orientation of the molecule emission dipole from the radiation pattern through back focal plane (BFP) imaging which is shown in Figure 1(b), researchers established a Purcell factor distribution with a function of crystal thickness and molecule position to describe the sublimation induced lifetime change and analyze the quantum efficiency, corresponding results are shown in Figure 1(c). They finally deduced the average intrinsic quantum efficiency of the single DBT molecules embedded in AC microcrystal is 95%, which is agrees with the reported near-unity values for DBT molecules at ensemble level in low temperature.
Figure 1 Probing fluorescence quantum efficiency of single DBT molecules in anthracene (AC) microcrystal. (a) Sketch of the experimental setup. (b) Measured back-focal plane (BFP) image of the emission from a single DBT molecule in AC microcrystal. (c) The quantum efficiency analysis of different DBT molecules
This work innovatively utilizes the natural sublimation of AC microcrystal which induces optical environment change for embedded DBT molecules and experimentally probes fluorescence quantum efficiency of single DBT molecules by monitoring the fluorescence lifetime change due to the optical environment variation. Such simple approach also can be applied on other organic molecules sharing in-plane orientation in organic matrix host.