Experimental demonstration of Ultrametricity in photonic spin glass

For the first time we report the ultrametric structure of the replica space with clustered domains in random lasers, as predicted in the Parisi Ansatz. 

In the ultrametric space all triangles are either equilateral or isosceles with the third side shorter than the two equal ones. This is the topology of hierarchic structures and it is intrinsic of spin glasses.

We give the experimental prove of this universal behaviour in photonics by showing that the number of the states forming isosceles triangles in the metric space increases through the laser threshold giving rise to the construction of a genealogical tree.

Moreover, from the hierarchical topology of the states we obtain a direct observation of the complex energy landscape with evident breaking of ergodicity in the glassy regime.

Microlasers as highly sensitive biosensors 

We successfully use polymeric whispering gallery microlasers as biosensors for detecting small amounts of proteins, down to 400 pg. They have the advantage of working in free space without any need for waveguiding for input excitation or output signal detection. The photonic microsensors can be easily patterned on microscope slides and operate in air and solution. We estimate the limit of detection up to 148 nm/RIU for three different protein dispersions. In addition, the sensing ability of passive spherical resonators in the presence of dielectric nanoparticles that mimic proteins is described by massive ab initio numerical simulations.

News & Views on Nature Physics

We highlight the impressing experiments by Tivedi et al. on thermally reconfigurable random lasers. Janus micro-colloids act as heat reservoirs and attract titania particles due to the Soret effect. Thanks to their accumulation around patterns of "hot colloids" and under pumping programmable random laser action is nicely demonstated.