Organic molecules as single-photon sources

TL;DR

This paper surveys organic single-photon sources based on single molecules, focusing on PAHs as emitters embedded in crystalline or amorphous hosts. It covers molecular electronic structure, vibronic coupling, host-matrix effects, preparation techniques, and photon-collection strategies, highlighting Purcell-enhanced architectures and the current state of performance metrics such as purity, indistinguishability, and collection efficiency. The work identifies key challenges—namely achieving near-unity 00ZPL fraction and high collection efficiency across scalable platforms—and discusses strategies like deuteration, Stark and optical tuning, and cavity/QED engineering to push toward ideal two-level emitters. It also outlines future opportunities in on-chip integration, electrical excitation, and advanced quantum-state generation using coupled emitters or spin degrees of freedom, underscoring the practical potential of molecular SPSs in quantum technologies.

Abstract

The development of single-photon sources has been nothing but rapid in recent years, with quantum emitter-based systems showing especially impressive progress. In this article, we give an overview of the developments in single-photon sources based on single molecules. We will introduce polycyclic hydrocarbons as the most commonly used emitter systems for the realization of an organic solid-state single-photon source. At cryogenic temperatures this special class of fluorescent molecules demonstrates remarkable optical properties such as negligible dephasing, indefinite photostability, and high photon rates, which make them attractive as fundamental building blocks in emerging quantum technologies. To better understand the general properties and limitations of these molecules, we discuss sample preparation, light collection strategies and relevant emitter parameters such as absorption and emission spectra, lifetime, and dephasing. We will also give an overview of light extraction strategies as a crucial part of a single-photon source. Finally, we conclude with a look into the future, displaying current challenges and possible solutions.

Figures (3)

• Figure 1: Basic properties of molecular quantum emitters. (a) Molecular structures of the most common guest and host species. The inset next to the structure of dibenzoterrylene (DBT) illustrates the electronic density associated with its $S_0\rightarrow S_1$ transitionSadeq2018. (b) Emission spectrum of DBT in pDCB highlighting its 0-0 zero phonon line (00ZPL; green), phonon wing of the zero phonon line (yellow) and vibronic transitions (red). The inset shows a Jablonski diagram of a single molecule. (c) Illustrations of the insertions site of DBT in pDCBZirkelbach2022 from two different direction. (d) 00ZPL excitation spectrum of a single DBT molecule in pDCB under weak excitation. The indicated linewidth $\gamma$ (full width at half maximum) is extracted by fitting a Lorentzian expression $F(\omega)=F_0\frac{(\gamma/2)^2}{(\omega-\omega_0)^2+(\gamma/2)^2}$, where $F_0$ is the maximal fluorescence signal and $\omega-\omega_0$ is the excitation laser detuning. (e) Second order autocorrelation function of the red-shifted emission from the same molecule under resonant excitation. (f) Excitation spectrum for DBT molecules in pDCB over a range of about 1THz showing several hundreds of individual molecule resonances. Images in (c) are reproduced from Zirkelbach et al.,Zirkelbach2022 Journal of Chemical Physics 156, 104301 (2022). Copyright 2022 Author(s), licensed under a Creative Commons Attribution (CC BY 4.0) license.
• Figure 2: Relevant emitter parameters. (a) Typical 00ZPL wavelengths spanning from 580nm to 780nm for different molecules and host matrices. (b) Simulated distribution of molecule frequencies for two different IHB widths $\Delta f$. (c) Simulated photon emission rate as a function of time for a pulsed photon source; solid lines illustrate 50ns repetition period, while dashed lines correspond to a 25ns period. (d) Jablonski diagram which includes internal conversion (IC), a non-radiative decay process with rate $\gamma_\mathrm{nr}$, which competes with the direct radiative decay at a rate $\gamma_\mathrm{r}$. (e) Jablonski diagram of different fluorescence emission processes: 00ZPL (resonant emission) in green, phonon wing in yellow, vibronic transitions in red. Below is an illustration of a corresponding emission spectrum. (f) Simulation of a single molecule excitation spectrum for two different homogeneous linewidths. The broader linewidth, corresponding to extra dephasing, produces distinguishable photons and requires higher excitation powers to reach the same emission level. (g) Simulations of repeated scans over two emitters with different magnitudes of spectral diffusion resulting in different amounts of sweep-to-sweep resonance frequency variation. (h) Simplified Jablonski diagram illustrating the transition to the long-lived triplet state $T_1$ via intersystem crossing (ISC). Here $k_{23}$ and $k_{31}$ are average rates into and out of the triplet state, respectively. Below is a simulated photon detection trace, where shaded time intervals correspond to a "bright" emitter in $S_0$-$S_1$ states, blank intervals denote a "dark" emitter in $T_1$ state, and black lines signify individual photon emissions. (i) Illustration of a single molecule resonance shifting under a controlled perturbation, e.g., an electric field (Stark tuning), pressure (strain tuning), or prolonged laser exposure (optical tuning).
• Figure 3: Most common photon collection methods used with molecular single photon emitters. (a) Aspheric lens or cryogenic objective directly collecting emission from a molecule embedded into a host material. (b) Solid immersion lens (SIL) used to increase the collection angle Wrigge2008Trebbia2009Siyushev2014Rezai2018Colautti2020bZirkelbach2022. (c) Additional layers can be used for reflecting and directing emission or engineering planar antenna structuresLee2011Checcucci2016Chu2017aWei2020Lombardi2020Murtaza2023. (d) Free-standing optical waveguides (top) or on-chip dielectric photonic structures such as sub-wavelength waveguides (middle) or resonators (bottom) used to route emissionTurschmann2017Lombardi2018Rattenbacher2019Boissier2021Shkarin2021Colautti2020bRen2022Rattenbacher2023. (e) Plasmonic gap (top) or v-groove (middle) waveguides and plasmonic gold nanoparticles fabricated on a glass surface (bottom)Grandi2019Kumar2020Zirkelbach2020. (f) Fabry-Perot cavity funneling emission into the cavity modeWang2017Wang2019.