Physics and chemistry of single-photon emitters
Quantum technologies leverage quantum mechanics to enable novel function, or improve upon existing technologies in ways that cannot be achieved using classical physics, resulting in 'quantum advantage'. Quantum photonic technologies make use of single-photons as quantum particles. Photons are exceptional carriers of quantum information: they can be transmitted over long distances while retaining coherence and entanglement, they are easily manipulated with both free-space and integrated photonics, and they are transmitted at light-speed. Quantum computational advantage has recently been demonstrated using photons, and they lend themselves naturally to quantum communication (e.g. quantum key distribution) and quantum sensing (e.g. imaging or spectroscopy). Quantum photonic technologies require low-less generation, manipulation, and detection of single-photons. While technologies for manipulating and detecting single-photons have greatly matured, the reliable generation of indistinguishable single-photons remains a challenge.
Fig. 1 (left) Schematic of a colloidal InP/ZnSe/ZnS quantum dot (QD), (right) schematic of a Photon Correlation Fourier Spectroscopy (PCFS) optical setup. Ref: A. H. Proppe,† D. B. Berkinsky† et al., Nat. Nanotechnology, 2023 and Y. Li, K. W. Nat. Nano news & views
This area of my research focuses on the physics, chemistry, and engineering of solid-state single-photon emitters (SPEs). While spontaneous parametric down conversion and four-wave mixing are commonly used sources of single photons, they are probabilistic and inefficient. Solid-state SPEs, such as self-assembled semiconductor quantum dots (QDs), offer deterministic quantum light generation with near-unity indistinguishability and single-photon purity. While colloidal QDs synthesized in solution have been successful in solution-processed optoelectronic devices, they have been less explored for quantum photonic applications. They exhibit longer radiative lifetimes (T1) and shorter optical coherence times (T2) due to dark lowest energy states and broader linewidths (due to spectral diffusion). However, recent discoveries of fast radiative recombination and high two-photon interference visibilities in CsPbBr3 perovskite QDs have sparked interest in colloidal QDs as solid-state quantum emitters. My research combines expertise in colloidal QD photophysics, surface chemistry, and single-molecule spectroscopy to characterize these novel quantum emitters and unlock their potential applications. Below are a few summaries of my research contributions to this area.
Stable and pure single-photon emission with 200+ ps optical coherence times in InP/ZnSe/ZnS QDs. Recent work by Samsung introduced core/shell/shell InP/ZnSe/ZnS QDs with high quantum yield, which were used to fabricate efficient QD light-emitting diodes. In collaboration with Samsung, we demonstrated spectrally stable single-photon emission from InP/ZnSe/ZnS colloidal QDs. Using interferometric photon correlation spectroscopy - known as photon correlation Fourier spectroscopy (PCFS, Fig. 1), we found that these dots exhibited narrow linewidths (~5 µeV). This corresponds to optical coherence times up to 250 ps (Fig. 2). These dots showed minimal spectral diffusion on timescales up to 50 ms, significantly surpassing other colloidal systems. Moreover, these InP/ZnSe/ZnS dots displayed single-photon purities of 0.077-0.086 without spectral filtering.
These results showcase the potential of InP/ZnSe/ZnS colloidal QDs as solid-state SPEs, offering narrower linewidths and higher single-photon purities than other colloidal QD systems and quantum defect emitters. However, they remain far from practical application: to fully leverage their quantum optical properties, these InP/ZnSe/ZnS QDs must be integrated into cavity structures to greatly reduce their radiative lifetimes, further reduce linewidths, and enhance the fraction of photons emitted into the zero-phonon line. Implementing similar strategies could pave the way for InP/ZnSe/ZnS QDs as a scalable material for coherent and stable single-photon generation.
Fig.2. PCFS interferograms for two different InP/ZnSe/ZnS QDs at 4K. Interferograms are shown for 3 different inter-photon arrival times, demonstrating minimal deterioration of T2 between 0.1 and 50 ms. Ref: A. H. Proppe,† D. B. Berkinsky† et al., Nat. Nanotechnology, 2023
Fig. 3. Emission spectrum of a single InP/ZnSe/ZnS QD at 15K. We developed an independent Boson model to deconvolve the homogeneous lineshape into contributions from dephasing of the zero-phonon line (ZPL), fine-structure splitting into bright and dark states, and phonon sidebands. Ref: D. B. Berkinsky,† A. H. Proppe† et al., ACS Nano, 2023
Narrow Intrinsic Linewidths and Electron-Phonon Coupling of InP Colloidal Quantum Dots. InP QDs are widely used in display applications and exhibit high efficiency and color purity in QD light-emitting diodes. To optimize color purity, understanding the mechanisms of spectral broadening is crucial: while synthetic tuning can minimize ensemble-level broadening by achieving monodisperse QD sizes, single QD linewidths are influenced by exciton-phonon scattering and fine-structure splitting. We used photon-correlation Fourier spectroscopy to extract average single QD linewidths of 50 meV at 293K for red-emitting InP/ZnSe/ZnS QDs, which are among the narrowest linewidths observed in colloidal QDs. We measured the emission lineshapes of single single QDs between 4K and 293K, and developed a modified independent boson model to fit the spectra (Fig. 3). Our analysis reveals that at low temperatures, inelastic acoustic phonon scattering and fine-structure splitting are the primary broadening mechanisms, while at elevated temperatures, pure dephasing from elastic acoustic phonon scattering dominates, and optical phonon scattering plays a minimal role across all temperatures.
In contrast, for CdSe/CdS/ZnS QDs, we find that optical phonon scattering contributes more significantly to the linewidth broadening at elevated temperatures, resulting in inherently broader single-dot linewidths compared to InP/ZnSe/ZnS QDs. Our modelling enables the parameterization of linewidth broadening for different material classes. These insights suggest that red-emitting InP/ZnSe/ZnS QDs possess narrower linewidths than commonly synthesized CdSe QDs, indicating their potential for achieving QDLEDs with exceptional color purity.