In iron based superconductors the s- and d-wave pairing are in close competition. In most of the studies the leading pair instability occurs in s-wave channel with the d-wave being close second. Such a competition is probed by an inelastic photon (Raman) scattering. The Cooper pair excitons of a sub-dominant d-wave channel are excited in a two-photon process with the incoming and outgoing photon polarizations crossed.

In the two recent experiments the two separate peaks appear in this configuration. We attribute them to the Cooper pair and particle-hole excitons. Unlike in the BCS these two peaks do not merge in the presence of a sign changing order parameter.

In the second part of the talk I will present the Raman studies of the normal state of pnictides revealing the strong signal in the wide range of temperatures with critical temperature behavior. We analyze this feature in terms of the Ising nematic fluctuations of a d-wave symmetry. The transmutation into a sharp and strong in-gap feature is discussed in both channels based on the notion of critical fluctuations breaking the discrete symmetry.