molecules. Simple hydrogen-like states can be studied using a plain Schrodinger<br>
equation. However, when an EC is coupled to a medium with a frequency- or<br>
momentum-dependent dielectric function, "(!; q), the quantum-mechanical de-<br>
scription of such system becomes less trivial. The _rst part of this talk describes<br>
a treatment of many-body states of charged particles coupled to a frequency-<br>
dependent medium. I will show that the problem of dynamically screened ECs<br>
can be formally reduced to a simple problem of an EC in an e_ectively static<br>
medium. The environment's e_ective dielectric constant `perceived' by ECs de-<br>
pends on the symmetries of EC wavefunctions and their binding energies. This<br>
approach is applicable to individual electrons as well as to large many-body com-<br>
plexes. Predictions of environmental e_ects on EC binding were experimentally<br>
veri_ed using neutral, charged and defect-bound excitons in monolayer WS2<br>
screened by liquid, metallic and semiconducting environments.<br>
In the second part of the talk I will show that coupling of excitons to some<br>
metamaterials with momentum-dependent dielectric functions, "(~k), allows cre-<br>
ation of ultrarelativistic excitons with high momenta. As a result of the Lorentz<br>
contraction, one can observe transient one-dimensional relativistic excitons in<br>
two-dimensional graphene.<br>