Spatial Heterogeneity of Biexcitons in Two-Dimensional Ruddlesden-Popper Lead Iodide Perovskites

Relaxation Channels of Two Types of Hot Carriers in Gold Nanostructures

A fundamental understanding of hot carrier relaxation in metal nanostructures is essential for realizing their application potential in energy conversion and photocatalysis. Despite previous investigations of the relaxation of hot carriers generated by surface plasmon resonance (SPR) excitation and interband transitions (IBTs), the hot carrier relaxation lifetimes and their associated mechanisms remain unclear. Herein, we demonstrate two distinct hot carrier relaxation channels in gold plasmonic nanostructures. The experimental observations reveal that the hot carrier relaxation is faster following SPR excitation than that from IBTs in gold nanoparticles and nanorods. The experimental results and theoretical calculations indicate that the numerous plasmon-induced hot carriers undergo surface-mediated carrier-carrier scattering in large gold nanostructures, whereas almost all IBT-induced hot carriers experience bulk carrier-carrier scattering. These findings advance our understanding of hot carrier relaxation and contribute to a clearer microscopic description of scattering channels in plasmonic nanostructures.

Optically accessible long-lived electronic biexcitons at room temperature in strongly coupled H- aggregates

Photon absorption is the first process in light harvesting. Upon absorption, the photon redistributes electrons in the materials to create a Coulombically bound electron-hole pair called an exciton. The exciton subsequently separates into free charges to conclude light harvesting. When two excitons are in each other's proximity, they can interact and undergo a two-particle process called exciton-exciton annihilation. In this process, one electron-hole pair spontaneously recombines: its energy is lost and cannot be harnessed for applications. In this work, we demonstrate the creation of two long-lived excitons on the same chromophore site (biexcitons) at room temperature in a strongly coupled H-aggregated zinc phthalocyanine material. We show that exciton-exciton annihilation is suppressed in these H- aggregated chromophores at fluences many orders of magnitudes higher than solar light. When we chemically connect the same aggregated chromophores to allow exciton diffusion, we observe that exciton-exciton annihilation is switched on. Our findings demonstrate a chemical strategy, to toggle on and off the exciton-exciton annihilation process that limits the dynamic range of photovoltaic devices.

Extracting double-quantum coherence in two-dimensional electronic spectroscopy under pump-probe geometry

Two-dimensional electronic spectroscopy (2DES) can be implemented with different geometries, e.g., BOXCARS, collinear, and pump-probe geometries. The pump-probe geometry has the advantage of overlapping only two beams and reducing phase cycling steps. However, its applications are typically limited to observing the dynamics with single-quantum coherence and population, leaving the challenge to measure the dynamics of the double-quantum (2Q) coherence, which reflects the many-body interactions. We demonstrate an experimental technique in 2DES under pump-probe geometry with a designed pulse sequence and the signal processing method to extract 2Q coherence. In the designed pulse sequence, with the probe pulse arriving earlier than the pump pulses, our measured signal includes the 2Q signal as well as the zero-quantum signal. With phase cycling and data processing using causality enforcement, we extract the 2Q signal. The proposal is demonstrated with rubidium atoms. We observe the collective resonances of two-body dipole-dipole interactions in both the D1 and D2 lines.