Soliton formation in a bound state in the continuum GaN waveguide polariton laser

TL;DR

The study demonstrates symmetry-protected bound states in the continuum (BIC) in a GaN polariton waveguide, evidenced by suppressed far-field emission and a surrounding polarization vortex that confirms topological protection. As pump power increases, polaritons condense on the BIC, producing lasing from blueshifted BIC states with a measurable energy shift on the order of $\sim$1 meV and increased k-space broadening due to interactions. Crucially, repulsive polariton-polariton interactions combined with the negative effective mass of the BIC lead to the formation of a bright soliton localized in real space, whose width in reciprocal space $\delta k_x$ grows with the soliton energy, in agreement with a modified Gross-Pitaevskii model that includes reservoir effects. The results highlight how interaction-driven soliton formation on a BIC can modify the emission angular distribution and extraction efficiency, offering new avenues for BIC-based polariton lasers and photonic information processing.

Abstract

We study polaritonic bound states in the continuum (BIC) created in GaN waveguides. The existence of symmetry-protected BICs is confirmed by the suppression of light emission and the observation of a polarization vortex in momentum space. Upon increasing the pumping, polariton population accumulates at the BIC and we observe polariton lasing from the blueshifted BIC states. The assessment of the polariton BIC emission energy and of its momentum broadening as a function of pumping power, i.e. of polariton density, indicates the formation of a bright soliton above the lasing threshold. Soliton formation at the BIC is induced by the combination of negative mass BIC and of repulsive polariton-polariton interactions.

Figures (9)

• Figure 1: Bound state in continuum in a GaN polariton waveguide. (a) Scheme of the experimental structure. (b,c) Dispersion relation of the polaritonic guided modes measured along the in-plane direction perpendicular to the grating and exhibiting a BIC (b - experiment, c - theory (taking exciton lifetime 2 ps). (d) Dispersion relation of the polaritonic guided modes along the transverse direction, i.e. parallel to the grating, for a nonzero $k_x$, $k_x=0.5\mu m^{-1}$, showing parabolic behaviour; (e,f) 3D dispersion relations, $E(k_x,k_y)$ (e - experiment, f - theory) around $k_x=k_y=0$. The red arrow indicates the wave vector value, which corresponds to the panel (d).
• Figure 2: The BIC properties. (a) Intensity of emission as a function of wave vector and its quadratic fit; (b,c) Linear polarization orientation (b - experiment, c - theory) demonstrating a vortex. In experiment, the signal from the central part is too weak. The elliptic curve is a guide for the eyes allowing to follow the tangential orientation of the linear polarization.
• Figure 3: BIC lasing. (a) Experimental spectra at $k_x=$0.15 $\mu$m$^{-1}$ for different pumping powers relatively to the lasing threshold, demonstrating a build-up of emission from the BIC. At high power a ghost is detected at 3.45 eV because of the intense BIC peak; (b) BIC emission intensity as a function of the pumping power, exhibiting a typical threshold curve.
• Figure 4: BIC solitons. Experimental (a,b,c) and theoretical (e,f,g) dispersions along the in-plane direction demonstrating the condensation at a BIC state and illustrating the blueshift of the BIC above threshold and its broadening in k-space due to the interactions. (d) Energy of the BIC as a function of normalized pumping power. (h) Size of the soliton in k-space as a function of the soliton blueshift energy (dots - experiment, blue curve - theory).
• Figure S5: Image of the 1D lattice obtained with an scanning electronic microscope.