Strong Exciton-Exciton Scattering of Exfoliated van der Waals InSe toward Efficient Continuous-Wave Near-Infrared P-Band Emission

Etchless InSe Cavities Based on Bound States in the Continuum for Enhanced Exciton-Mediated Emission

Recently, fervent research interest is sparked to indium selenide (γ-InSe) due to its dazzling optical and electronic properties. The direct bandgap in the near-infrared (NIR) range ensures efficient carrier recombination in InSe, promoting impressive competency for lavish NIR applications. Nevertheless, the photoluminescence (PL) efficiency of InSe is significantly limited by out-of-plane (OP) excitons, adverse to practical devices. Herein, a facile and effective solution is proposed by introducing photonic bound-states-in-the-continuum (BIC) modes to enhance excitons in InSe through strengthened exciton-photon coupling. This cavity is constructed simply by patterning a polymer grating onto the InSe flake without an etching process, achieving an impressive PL enhancement of over 200 times. By adjusting the cavity resonance wavelength, it can selectively amplify the exciton emission or the exciton-exciton scattering process, which is not observable off-cavity at room temperature. Additionally, the second harmonic generation (SHG) process in InSe can also be largely enhanced by over 400 times on the cavity. Notably, the etchless cavity design can be further extended to other nanostructures beyond grating. This research presents a feasible and efficient approach to enhancing the optical performance of OP excitons, paving a prospective avenue for advanced linear and nonlinear photonic devices.

Emerging 2D Materials and Van der Waals Heterostructures for Advanced NIR, SWIR, and MWIR Emitters

Infrared (IR) emitters have drawn considerable attention for applications in deep-tissue imaging, optical communication, and thermal sensing. While III-V and II-VI semiconductors are traditionally used in these emitters, their reliance on complex epitaxial growth to overcome lattice mismatch and thermal expansion challenges leads to intricate device structures and limits their integrability. In contrast, 2D materials provide a more flexible solution, offering diverse optical bandgaps and the ability to be vertically restacked in arbitrary crystal orientations to form complex van der Waals (vdW) heterostructures, which can be further integrated onto diverse device platforms. This review highlights recent advancements in 2D-based IR emitters, focusing on the NIR, SWIR, and MWIR regions. It discusses the photoluminescence properties of 2D materials and innovative vdW engineering techniques used to develop IR light-emitting diodes (LEDs). The review also explores how external stimuli, such as electric fields and strain, can enable tunable emission wavelengths and examines the integration of 2D-based emitters with photonic structures, like cavities and waveguides, to create hybrid photonic devices. Finally, the review addresses the challenges and prospects of 2D-based IR technologies, highlighting their potential to transform IR light sources across various applications.

Strong Linearly Polarized Light Emission by Coupling Out-of-Plane Exciton to Anisotropic Gap Plasmon Nanocavity

With exceptional quantum confinement, 2D monolayer semiconductors support a strong excitonic effect, making them an ideal platform for exploring light-matter interactions and as building blocks for novel optoelectronic devices. Different from the well-known in-plane excitons in transition metal dichalcogenides (TMD), the out-of-plane excitons in indium selenide (InSe) usually show weak emission, which limits their applications as light sources. Here, by embedding InSe in an anisotropic gap plasmon nanocavity, we have realized plasmon-enhanced linearly polarized photoluminescence with an anisotropic ratio up to ∼140, corresponding to degree of polarization (DoP) of ∼98.6%. Such polarization selectivity, originating from the polarization-dependent plasmonic enhancement supported by the "nanowire-on-mirror" nanocavity, can be well tuned by the InSe thickness. Moreover, we have also realized an InSe-based light-emitting diode with polarized electroluminescence. Our research highlights the role of excitonic dipole orientation in designing nanophotonic devices and paves the way for developing InSe-based optoelectronic devices with polarization control.