Our group in collaboration with Tomoki Ozawa from the Università di Trento in Italy, has recently realized a topological laser. Such a laser implements a resonating mode that is intrinsically protected, thanks to the topological properties of its architecture, against external perturbations and fabrication defects.

The discovery of topological phase transitions in condensed matter, which was awarded the 2016 Noble prize in physics, has profoundly influenced the field of solid-state physics. The hallmark of these phase transitions is the emergence of localized states whose properties are unaffected by the presence of defects, dislocations and other kinds of disorder. For example, topological insulators, which are one of the most notable manifestations of these exotic phases, present an electrical conductivity along their edge that is perfectly insensible to the presence of irregularities. Recently the extension of this topological physics to the realm of photonics has triggered considerable efforts to engineer novel generations of optical devices (e.g. waveguides, optical isolators, diodes, and, of course, lasers) whose performances are unaffected by perturbations of their environment.

In our group we have realised a topologically robust laser by taking profit of the physics of cavity polaritons, a half-light /half-matter quasi-particle confined in cylindrically-shaped optical cavities (see the figure on the right). By coupling these micro-cylinders together, we have fabricated a uni-dimensional lattice that presents well-defined topological properties leading to the emergence of localized photonic modes at the edges of this lattice. Then, by using the gain associated to the matter part of polaritons, we triggered laser emission from one of these modes, thus realizing a topological laser.

###### Scanning electron microscope image of the polariton SSH chained employed for the observation of 1D topological lasing.

Our realisation is the first 1D topological laser ever reported, and it profits from the topological properties of the Su-Schrieffer-Heeger Hamiltonian. Almost simultaneously, the group of Boubacar Kante at UCSD demonstrated a 2D topological laser with chiral edge states, by breaking the time reversal symmetry in a 2D photonic crystal. These two discoveries push the field of topological photonics to the area of integrated lasers, opening the way to new architectures intrinsically robust to imperfections.

The polariton 1D topological laser:

*Lasing in topological edge states of a 1D lattice,* P. St-Jean, V. Goblot, E. Galopin, A. Lemaître, T. Ozawa, L. Le Gratiet, I.Sagnes, J. Bloch and A. Amo, **Nat. Photon.** 11, 651 (2017).

The 2D topological laser from the group of Boubacar Kante:

*Nonreciprocal lasing in topological cavities of arbitrary geometries*, B. Bahari, A. Ndao, F. Vallini, A. El Amili, Y. Fainman, and B. Kanté, **Science** -online publication.