Light-Matter Interaction in III-Nitride Waveguides: Propagating Polaritons and Optical Gain

III-nitride waveguides featuring AlInN claddings and GaN/AlGaN quantum wells (QWs) offer promising perspectives for applications in many fields of short-wavelength photonics. Thanks to their nearly lattice-matched nature, these structures exhibit an excellent material quality, leading, e.g., to strong light-matter interaction in such QWs, and several promising phenomena.

In the low carrier density regime, the strong coupling between QW excitons and waveguide photons results in propagating hybrid light-matter particles, called (exciton-)polaritons, which combine photon-like propagation and exciton-like interactions. These interactions lead to a strong optical nonlinearity, which could be useful for integrated all-optical devices. Due to their strong exciton binding energy (~40 meV in the present structures), III-nitride devices have the potential to maintain these nonlinearities up to room temperature.

In the high carrier density regime, a GaN/AlGaN QW electron-hole plasma can provide gain to an optical field in the UV, which can be useful for realizing near-UV laser diodes and semiconductor optical amplifiers. The performance of current UV devices featuring AlGaN claddings is limited by poor material quality. The improved structural quality of waveguides with lattice-matched AlInN claddings could therefore circumvent these issues.

This study aims at an in-depth investigation of the optical properties of III-nitride waveguides with AlInN claddings and GaN/AlGaN QWs grown on freestanding GaN substrates. In a sample with an active region that was optimized for strong exciton-photon coupling, we observe propagating polaritons in the low-density regime. A sample with an active region that was optimized for homogeneous near-resonant excitation with a 355 nm laser shows elevated optical gain in the high-density regime. The nearly lattice-matched nature of the entire structure leads to a high structural and optical quality. We found inhomogeneous broadening values between 8 and 11 meV, and a standard deviation in the QW emission energy well below 1 meV over a 50 × 50 µm$^2$ area. We calculated the band structure and transition energies of the QWs using self-consistent Schrödinger-Poisson k ·p calculations and found an excellent agreement with experiments.

The waveguide polaritons feature a normal mode splitting as large as 60 meV at low temperature, thanks to the large overlap between the optical mode and the active region, a polariton decay length up to 100 µm for photon-like polaritons and a lifetime of 1-2 ps. These decay lengths and lifetimes are limited by residual absorption occurring in the waveguide. The large normal mode splitting and elevated in-plane homogeneity are important assets for the realization of polaritonic integrated circuits.

We also demonstrate optically-pumped waveguides exhibiting narrow bandwidth (3.8 nm) optical gain around 370 nm. Due to the high refractive index contrast between the cladding layers and the active regio