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Strong gravitational lensing is a powerful tool for astrophysics and cosmology. I will present the Euclid Collaboration's current lens-finding efforts, including the "lens-finding engine" in Euclid's Quick Release 1, and the project I lead on double-source-plane lenses (DSPLs). DSPLs—systems with two Einstein rings at different source redshifts—are uniquely valuable for both cosmological distance-ratio measurements and for constraining galaxy mass profiles. They help break the mass-sheet transformation, a key degeneracy that otherwise limits inferences from single-plane lenses, by jointly modelling both rings.

As a case study, I will discuss the "Jackpot" lens, modelled with a composite mass distribution of stars plus dark matter. We assume the stellar mass traces the lens light with a radial mass-to-light gradient, while the dark matter halo follows a generalised NFW (gNFW) profile. Using lensing-only constraints, we measure a dark-matter inner slope of γ = 1.22 ± 0.09. Together, Euclid's systematic lens discovery and targeted DSPL analyses illustrate how forthcoming samples will deliver robust constraints on both cosmology and the internal structure of massive galaxies.

BIO

Tian Li is currently a PhD student studying cosmology and strong gravitational lensing at the University of Portsmouth. He completed his undergraduate degree at Beijing Normal University and master's degree at Leiden University. He is a member of the Euclid Collaboration, the LSST Dark Energy Science Collaboration, and the Strong Lensing Science Collaboration. He is now leading the discovery and modelling of double-source-plane lenses in Euclid and the strong lensing cosmology forecast project in DESC.