Self-Hybridized Exciton-Polariton Photodetectors From Layered Metal-Organic Chalcogenolates

Exciton-polaritons (EPs) arising from strong light-matter coupling offer new pathways for controlling optoelectronic properties. While typically requiring closed optical cavities for strong coupling, we demonstrate that 2D metal-organic chalcogenolates (MOCs), mithrene (AgSePh), with a high refractive index (~2.5) and strong excitons enable self-hybridized polaritons photodetectors (PDs) without top mirrors, simplifying device architecture. Through thickness-tuned multimode polariton engineering, we achieve photodetection of sub-bandgap photons via lower polariton states, validated through reflectance, photoluminescence (PL), and photocurrent spectroscopy with quantitative theoretical agreement. Trap-assisted two-photon absorption enables sustained strong coupling even under sub-bandgap excitation. The polariton dispersion yields ultrafast group velocities (~65 μm/ps), extending exciton diffusion lengths from hundreds of nanometers to several micrometers. Strong-coupling devices demonstrate a 2.38-fold enhancement in photo-to-dark current ratio compared to weak-coupling counterparts, establishing a practical route to polariton-enhanced photodetection and light harvesting.

💡 Research Summary

This paper introduces a novel route to strong light‑matter coupling in photodetectors by exploiting the intrinsic high refractive index (~2.5) and strong excitonic resonances of a two‑dimensional metal‑organic chalcogenolate (MOC), specifically AgSePh (named mithrene). Unlike conventional polariton devices that require external mirrors (DBRs or metallic reflectors) to form a closed optical cavity, the authors demonstrate that sufficiently thick mithrene layers act as self‑contained Fabry‑Pérot cavities. When the thickness exceeds ~200 nm, multiple cavity modes coexist and hybridize with the exciton, producing a series of upper and lower polariton branches (UEPs and LEPs) with Rabi splittings larger than 650 meV.

The authors combine transfer‑matrix simulations with angle‑resolved reflectance, photoluminescence (PL), and photoluminescence excitation (PLE) measurements to map the polariton dispersion. The polariton peaks shift to higher energies with increasing incidence angle, confirming the dispersive nature of the hybrid states. Importantly, the lower polariton (LEP) branches extend well below the intrinsic bandgap (2.65 eV), creating substantial sub‑bandgap absorption.

A key discovery is that sub‑bandgap photons (e.g., 633 nm, 1.96 eV) can still generate strong coupling. This occurs via trap‑assisted two‑photon absorption (TPA): shallow trap states around 725 nm absorb one photon, promoting carriers to an intermediate level; a second photon then excites them into the excitonic manifold, allowing exciton formation despite the photon energy being below the bandgap. The authors verify this mechanism by showing that excitation at 785 nm (1.58 eV) fails to produce LEP emission, whereas 405 nm and 633 nm excitations do, and by demonstrating super‑linear power dependence characteristic of TPA.

To translate these optical phenomena into device performance, two types of two‑terminal photodetectors are fabricated: (i) “weak‑coupling” devices on sapphire with ~30 nm mithrene, which cannot support cavity modes and thus behave as conventional PDs; and (ii) “strong‑coupling” devices on a high‑reflectivity DBR substrate with >300 nm mithrene, which exhibit self‑hybridized polaritons. The strong‑coupling devices show photocurrent generation from photons up to ~0.55 eV below the bandgap, directly linked to the LEP absorption. The polariton dispersion yields an estimated group velocity of ~65 μm ps⁻¹, implying an effective exciton diffusion length of several micrometers—an order of magnitude larger than typical exciton diffusion in organic or 2D semiconductors.

Electrically, the strong‑coupling photodetectors achieve a 2.38‑fold increase in the photo‑to‑dark current ratio compared with their weak‑coupling counterparts, demonstrating that the polariton‑mediated absorption not only broadens spectral response but also improves carrier collection efficiency.

Overall, the study provides compelling evidence that (1) high‑index 2D materials can serve as self‑cavities for robust strong coupling without external mirrors; (2) trap‑mediated two‑photon processes enable sub‑bandgap polariton formation; and (3) the resulting polariton dispersion dramatically enhances both optical absorption and charge transport. These insights open a practical pathway toward scalable, mirror‑free polaritonic optoelectronic devices for broadband photodetection, solar energy harvesting, and potentially low‑threshold polariton lasers.