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

Abstract

P-band emission is a superlinear low-coherence emission through exciton–exciton (X–X) scattering into photon-like states. It occurs without the prerequisites of population inversion or macroscopical coherence, rendering lower power consumption than the widely explored superlinear low-coherence emissions including superfluorescence, amplified spontaneous emission, and random lasing, and holds great potential for speckle-free imaging and interferometric sensing. However, competition processes including exciton dissociation and annihilation undermine its operation at room temperature and/or low excitation conditions. Here we report room-temperature P-band emission from InSe microflakes with excitation density of 1010 cm–2, offering 2-orders-of-magnitude lower operation density compared to the state-of-the-art superlinear low-coherence emissions. The efficient P-band emission is attributed to a large X–X scattering strength of 0.25 μeV μm2 due to enhanced spatial confinement along with intrinsic material metrics of 3D/2D exciton complex and asymmetric electron/hole mass. These findings open an avenue toward strong low-coherence near-infrared light sources based on van der Waals semiconductors.

Understanding the Origin and Implication of the Indirect-to-Direct Bandgap Transition in Multilayer InSe

“…The P-band emission from InSe is a result of an exciton–exciton scattering process, in which one exciton is scattered to higher energy states (near the conduction band edge), while the other is scattered to lower energy, where it recombines to produce light. The P-band emission energy is related to the spacing between Rydberg-like exciton energy levels, and it can be used to measure the exciton binding energy, denoted as E b , given by ,, where n = 1,2,3··· is the quantum number of the scattered exciton energy state, δ is a constant less than 1, k B is the Boltzmann constant, and T eff is an effective exciton temperature. Noting that the free exciton–exciton scattering only occurs when n = ∞ and using fitted values for T = 0 K, we obtain E b ≈ E A (0) – E P (0) = 10 meV from eq , which is consistent with the commonly reported value of 15 meV. , Recent work demonstrated that the exciton binding energy can reach up to 20 meV, indicating a significant variability in the experimental determination of E b .…”

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

“…In contrast to group VI TMDs, InSe undergoes a crossover from indirect to direct band gap transition as the layer thickness increases . As a result, two-dimensional InSe flakes with a layer number above 7 all support direct exciton emission with out-of-plane dipole orientation as well as a thickness controllable emission wavelength, making it a promising candidate for developing nanophotonic and optoelectronic devices for on-chip applications. − However, compared to the TMD monolayer, the excitonic emission in InSe is still relatively weak. Thus, so far, there is still a lack of reports on the InSe-based light-emitting diode.…”