Biexcitons in Ruddlesden-Popper Metal Halides Probed by Nonlinear Coherent Spectroscopy

Abstract

Excitons and their correlated complexes underpin the rich photophysics of quantum-confined semiconductors. Among these, biexcitons -- bound states of two electrons and two holes -- provide a sensitive probe of Coulomb correlations, exciton-exciton interactions, and the role of the dielectric environment. In Ruddlesden-Popper metal halide materials (RPMHs), strong quantum and dielectric confinement stabilize excitons with binding energies of hundreds of meV, creating an ideal platform for multi-exciton phenomena. Conventional linear spectroscopies, such as photoluminescence and transient absorption, reveal biexciton signatures but suffer from spectral congestion and reabsorption artifacts. Two-dimensional coherent spectroscopies, particularly two-quantum (2Q) multidimensional techniques, uniquely access multi-exciton coherences and provide unambiguous estimates of biexciton binding energies. This minireview surveys the spectroscopic evidence for biexcitons in RPMHs, highlights the advantages of nonlinear multidimensional approaches, and situates biexciton physics within the broader context of excitonic materials, including GaAs quantum wells, quantum dots, and transition-metal dichalcogenides. By emphasizing the interplay of exciton-exciton annihilation, excitation-induced dephasing, and biexciton formation, we argue that multidimensional coherent spectroscopy offers the most reliable pathway to disentangle many-body interactions in quantum-well derivatives of metal-halide perovskites.

Figures (5)

• Figure 1: Crystal structure of a prototypical 2D HOIP: phenylethylammonium lead iodide ((PEA)2PbI4). (Bottom) Linear absorption spectrum of (PEA)2PbI4 taken at $T= 5$ K; $\Delta \sim 35\pm 5$ meV represents the energy spacing within the excitonic fine-structure and $E_B \sim 250$ meV is the exciton binding energy associated with the main exciton peak. Figure extracted from Ref. srimath2020exciton.
• Figure 2: (a) Absorption (solid) and photoluminescence (dashed) spectra, of (F-PEA)2PbI4 measured at 15 K. (b) Measured photoluminescence spectra (dots) of (F-PEA)2PbI4 taken at 15 K, fit to a double Gaussian function (lines) to determine peak positions and calculate the biexciton binding energy. (c) Temperature dependent photoluminesence, measured with a pump fluence of 389 $\mu$J/cm$^2$. Figure reproduced from Ref. koch2025spectroscopic.
• Figure 3: (a) The geometry of the excitation pulse-train beam pattern (blue-green) and the resonant four-wave mixing signal (SFWM, yellow-green), detected by interference with a local oscillator (LO). We use the BoxCARS beam geometry, in which three pulse trains (A$^{\star}$, B, C) propagating along the corners of a square are focused onto the sample with a common lens, defining incident wave vectors $\vec{k}_A$, $\vec{k}_B$, and $\vec{k}_C$. The LO beam, on the fourth apex of the incident beam geometry, co-propagates with $S_{\mathrm{FWM}}$ with the wave vector imposed by the chosen phase-matching conditions. (b) Schematic representation of the effect of light-matter interactions in double-barreled Feynman diagrams. The arrows represent the light-matter interaction, either on the bra (left-hand side) or ket (right-hand side) of the density matrix. Arrows that point to the diagram represent excitation along the ladder of states, while arrows pointing away represent de-excitation. By changing the time-ordering of the pulse sequence in the fixed phase-matching geometry displayed in part (a), we measure two distinct nonlinear responses: (c) 1Q rephasing signal and (d) 2Q non-rephasing signal.
• Figure 4: Absolute one-quantum rephasing 2D coherent spectrum of (a) (F-PEA)2PbI4 measured at 7K (Data reproduced from Ref. koch2025spectroscopic), (b) (PEA)2PbI4 measured at 5 K, (Data reproduced from Ref. thouin2019enhanced) and (c) (PEA)2SnI4 measured at 5 K (Data reproduced from Ref. rojas2023many).
• Figure 5: Absolute two-quantum non-rephasing 2D coherent spectrum of (a) (F-PEA)2PbI4 measured at 7K, (b) (PEA)2PbI4 measured at 5 K, and (c) (PEA)2SnI4 measured at 5 K. Figure (b) and (c) extracted and modified from Ref. thouin2018stable and rojas2023many.