Valence-band-driven multilevel exciton emission from single GaSb quantum dots in the telecom S-band

Hank Hammer

Abstract: III-V semiconductor quantum dots (QDs) are excellent sources of indistinguishable single photons, usually described by a few levels built from the lowest confined electron and hole orbitals. Droplet-etched GaSb QDs, emitting in the telecom S-band, depart from this picture. Under quasiresonant excitation a single QD exhibits a dense multiplet of sharp transitions, and it has remained unclear whether these arise from independent charge complexes or from a genuinely multilevel exciton system. Here, we establish the latter. Eight-band k.p and configuration-interaction calculations reveal strongly mixed heavy- and light-hole states crowded within a few millielectronvolts, yielding several bright transitions from excitons that share the electron ground state but differ in hole orbital character. Photon cross-correlation measurements between pairs of transitions show an antibunching signature on a microsecond timescale, far beyond the usual nanosecond antibunching dip. This slow antibunching is a hallmark of population exchange between neutral excitons mediated by a shared charge reservoir. A rate-equation model fitted globally across excitation power identifies this reservoir as a photoinduced hole and the corresponding positive trion. A single GaSb QD is thus a valence-band-driven multilevel emitter whose bright excitons are linked through its charge state.

Shear exciton-polaritons in a low symmetry van der Waals crystal

Ramachandra Bangari

Abstract: Polaritons, quasiparticles arising from strong light-matter coupling between photons and various excitations in materials, such as excitons, plasmons, and phonons, have garnered significant interest in the field of nanophotonics. Recent studies have demonstrated that low-symmetry crystals such as monoclinic β-Ga₂O₃ support hyperbolic shear phonon-polaritons with anisotropic and asymmetric in-plane propagation, due to nonzero off-diagonal components in the permittivity tensor even after diagonalizing its real part (1). In this talk, we will discuss shear exciton-polaritons with a model permittivity tensor and its manifestation in rhenium diselenide (ReSe₂), a triclinic van der Waals (vdW) crystal with reduced in-plane symmetry. Unlike the more commonly studied transition metal dichalcogenides, ReSe₂ exhibits distinct anisotropic optical properties. Previous studies have reported polarization-resolved photoluminescence and transmission arising from the anisotropic excitons in ReSe₂ (2), as well as anisotropic exciton polaritons in related rhenium disulfide (3). Our work employs cryogenic polarized Fourier transform infrared microscopy to probe the optical anisotropy of ReSe₂. We observe rich self-hybridized anisotropic exciton-polariton branches and significant polarization conversion from polarized-to-unpolarized reflectance and co- and cross-polarized reflectance spectra of ReSe₂ flakes with different thicknesses on gold. We model the polarized reflectance using the transfer-matrix method (4), considering linearly polarized excitons and an anisotropic band edge, enabling extraction of the in-plane complex permittivity tensor of ReSe₂ in the near-infrared. Our results show that the two prominent excitons are non-orthogonal and result in a permittivity tensor whose real and imaginary parts cannot be diagonalized simultaneously, establishing a platform for shear exciton-polaritons. Our work provides a framework for characterizing and studying low-symmetry excitonic vdW crystals, such as ReSe₂, and positions them as promising candidates for applications in directional light propagation, polarization and phase control of light, and polarization-sensitive photodetectors.

References:
Passler, N. C., Ni, X., Hu, G., Matson, J. R., Carini, G., Wolf, M., Schubert, M., Alù, A., Caldwell, J. D., Folland, T. G., & Paarmann, A. (2022). Hyperbolic shear polaritons in low-symmetry crystals. Nature, 602(7898), 595–600.
Arora, A., Noky, J., Drüppel, M., Jariwala, B., Deilmann, T., Schneider, R., Schmidt, R., Del Pozo-Zamudio, O., Stiehm, T., Bhattacharya, A., Krüger, P., Michaelis de Vasconcellos, S., Rohlfing, M., & Bratschitsch, R. (2017). Highly Anisotropic in-Plane Excitons in Atomically Thin and Bulklike 1T′‑ReSe2. Nano Letters, 17(5), 3202–3207.
Chakrabarty, D., Dhara, A., Das, P., Ghosh, K., Roy Chaudhuri, A., & Dhara, S. (2026). Anisotropic Exciton-Polaritons Reveal Non-Hermitian Topology in van der Waals Materials. Nano Letters, 26(12), 4089–4095.
Passler, N. C., Ni, X., Carini, G., Chigrin, D. N., Alù, A., & Paarmann, A. (2023). Layer-resolved resonance intensity of evanescent polariton modes in anisotropic multilayers. Physical Review B, 107(23), 235426.