Abstract Photon upconversion through lanthanide-doped nanoparticles is of great significance for various applications. However, the current development of upconversion nanoparticles is hindered by the low quantum efficiency and long radiative lifetimes of lanthanide ions, restricting their applications in time-dependent nanophotonics. Herein, we report ultrafast upconversion superfluorescence with a lifetime of sub-2.5 ns in lanthanide-doped nanoparticles at room temperature. Upon excitation with an 800-nm fs-pulsed laser, we achieve a large number ( N = 912) of correlated dipoles in Nd 3+ -concentrated nanoparticles, resulting in collective coherent emission with two orders of magnitude amplification in intensity and more than three orders of magnitude improvement in the radiative decay rate. Furthermore, we demonstrate that the control of excitation power and emitting sample length enables the lifetime manipulation of upconversion emission in a wide range from μs to sub-ns, accompanied by the typical superfluorescence signature of Burnham-Chiao ringing. These findings may benefit applications in many advanced technologies such as quantum counting and high-speed super-resolution bioimaging.
Abstract Photon upconversion through lanthanide-doped nanoparticles is of great significance for various applications. However, the current development of upconversion nanoparticles is hindered by the low quantum efficiency and long radiative lifetimes of lanthanide ions, restricting their applications in time-dependent nanophotonics. Herein, we report ultrafast upconversion superfluorescence with a lifetime of sub-2.5 ns in lanthanide-doped nanoparticles at room temperature. Upon excitation with an 800-nm fs-pulsed laser, we achieve a large number ( N = 912) of correlated dipoles in Nd 3+ -concentrated nanoparticles, resulting in collective coherent emission with two orders of magnitude amplification in intensity and more than three orders of magnitude improvement in the radiative decay rate. Furthermore, we demonstrate that the control of excitation power and emitting sample length enables the lifetime manipulation of upconversion emission in a wide range from μs to sub-ns, accompanied by the typical superfluorescence signature of Burnham-Chiao ringing. These findings may benefit applications in many advanced technologies such as quantum counting and high-speed super-resolution bioimaging.
Superfluorescence (SF) in lanthanide doped upconversion nanoparticles (UCNPs) is a room-temperature quantum phenomenon, first discovered in 2022. In a SF process, the many emissive lanthanide ions within a single UCNP are coherently coupled by an ultra-short (ns or fs) high-power excitation laser pulse. This leads to a superposition of excited emissive states which decrease the emissive lifetime of the UCNP by a factor proportional to the square of the number of lanthanide ions which are coherently coupled. This results in a dramatic decrease in UCNP emission lifetime from the μs regime to the ns regime. Thus SF offers a tantalizing prospect to achieving superior upconversion photon flux in upconversion materials, with potential applications such as imaging and sensing. This perspective article contextualizes how SF-UCNPs can be regarded as a second generation quantum technology, and notes several challenges, opportunities, and open questions for the development of SF-UCNPs.
Superfluorescence (SF) is a unique quantum mechanical behavior arising from the self-organization between emitters, thus forming a cooperatively coupled assembly. In contrast to isotropic spontaneous emission or normal fluorescence, SF produces a short but intense burst of light, which makes it ideal for a wide variety of applications in photonics, electronics, and optical computing. Due to the prerequisite of cooperative emitter coupling, SF has been conventionally observed under cryogenic conditions in limited systems, such as atomic gases, and a few bulk material systems. Here we show lanthanide-doped upconversion nanoparticles (UCNPs) as a medium to achieve antiStokes shift SF at room temperature. We observe such room temperature upconverted SF in a few nanoparticles assembly, and in a single nanoparticle, the latter of which is the smallest-ever SF media. In particular, we found that under near-infrared light (800 nm) excitation, each lanthanide ion in a single UCNP nanocrystal can be considered as an individual emitter that interact with each other to establish coherence and to enable anti-Stokes shift SF emission. More importantly, when compared to the microsecond scale slow lifetime of typical upconversion luminescence, the upconverted SF has a 10,000-fold accelerated lifetime (τ = 46 ns of SF v.s. τ = 455.8 μs of normal upconversion luminescence). When taken together, the observed ultrafast upconverted SF in both UCNP assembly and single nanocrystals under NIR light excitations, is uniquely well-positioned for applications in on-chip optical computing, and biophotonics, especially in deep tissue ultra-fast dynamic sensing.
Superfluorescence (SF) in lanthanide doped upconversion nanoparticles (UCNPs) is a room-temperature quantum phenomenon, first discovered in 2022. In a SF process, the many emissive lanthanide ions within a single UCNP are coherently coupled by an ultra-short (ns or fs) high-power excitation laser pulse. This leads to a superposition of excited emissive states which decrease the emissive lifetime of the UCNP by a factor proportional to the square of the number of lanthanide ions which are coherently coupled. This results in a dramatic decrease in UCNP emission lifetime from the microsecond regime to the nanosecond regime. Thus SF offers a tantalizing prospect to achieving superior upconversion photon flux in upconversion materials, with potential applications such as imaging and sensing. This perspective article contextualizes how SF-UCNPs can be regarded as a second generation quantum technology, and notes several challenges, opportunities, and open questions for the development of SF-UCNPs.
Abstract Superfluorescence (SF), a collective quantum phenomenon originating from correlated emitters, has historically required cryogenic temperatures or magnetic fields and been limited to narrowband Stokes-shifted emission. Although room-temperature SF has recently been realized in lanthanide-doped systems, active control over its spectral properties remains significant challenges. Here, we propose a state-entropy framework to reveal the coherence evolution and macroscopic dipole correlation in upconversion systems, providing a theoretical foundation for manipulating superradiant processes across a broad spectral range. Based on this framework, we designed an excitation strategy to decouple and regulate the distinct energy-population pathways, specifically excited-state absorption and energy transfer upconversion/cross-relaxation, in ~23 nm NaNdF₄ nanocrystals at room temperature. This approach enables multi-stage upconversion superfluorescence with tunable oscillations and nanosecond to sub-microsecond lifetimes. Most importantly, it substantially extends the accessible spectral window of room-temperature UC-SF from the previously reported two orange-red bands to six distinct emission bands covering the ultraviolet to the red (384-659 nm), accompanied by a wide-gamut wavelength-synthesized color evolution. Furthermore, cascaded excitation enables real-time emission enhancement by restoring the macroscopic dipoles correlation during the SF decay phase. This work demonstrates principles that may inform future designs of on-chip ultrafast optical computing based on spectrally tunable SF with encodable photonic emission.
Upconversion luminescence (UCL) of lanthanide-doped nanomaterials is usually a low-efficiency nonlinear process, involving multi-step, multi-channel transitions in a multi-level system. Here, we demonstrate quantum control and enhancement of multi-color (e.g., red and green) UCLs of NaYF4:Yb3+/Er3+ nanoparticles with metallic Fabry-Perot micro-cavities. Besides realization of controlled single-color UCLs, their internal quantum efficiencies are strongly enhanced, up to 3–4 orders of times. Experimental results indicate that the controlled single-color UCLs and enhancements are caused not only by modifications of the spontaneous radiation rates for the red- and green-color transitions but also by influencing the intermediate transitions to result in modified distributions of electrons in each of the multiple Er3+ levels, facilitating emission of either red- or green-color light. This work suggests a way to control photon emissions in systems with multi-channel transitions and/or multi-step excitations.
Fundamental studies of light-matter interactions are important for basic knowledge and in applications. Thanks to advances in experimental and theoretical methods, nowadays it is possible to perform such studies in a broad dynamic range, covering timescales from that of elementary interactions to real time. In the present work, we perform an experimental-theoretical study of light intensity-dependent femtosecond and CW-laser induced frequency upconversion in hierarchically structured core-multishell nanoparticles co-doped with NdIII, YbIII, and ErIII. Upconversion spectra recorded with CW and femtosecond excitation are qualitatively similar whereas the intensity dependence of upconversion depends on excitation mode (CW or femtosecond). To further assess the observed intensity dependence, we perform light-matter interaction simulations in the dynamic range from 100 fs to 3 ms for 18-level system describing the UCNPs, including 9 NdIII levels, 2 YbIII, and 7 ErIII levels and a classical model for the excitation source. The calculated time- and intensity-dependent energy level population are compared with measured spectra to understand CW vs femtosecond laser-induced upconversion. To further discuss the differences between CW and femtosecond laser-induced light-matter interactions for the systems studied here, we perform semi-classical pulse propagation simulations and ultrafast pump-probe measurements to study how the light source bandwidth, relative to the absorption linewidth, influence light absorption and transmission and further connect these results with the intensity dependence. Overall, we report our progress toward mechanistic studies of light-matter interaction and photophysical pathways following femtosecond excitation and UCNPs.