Dzyaloshinskii-Moriya interaction in LiCu$_2$O$_2$*

The far-infrared optical spectrum of the spin 1/2 helimagnet LiCu$_2$O$_2$ was investigated in magnetic fields up to 14Tesla. In the paramagnetic state a spin resonance line was observed corresponding to a $g$-factor of about 2. The resonance broadens and practically disappears around the phase transition temperature of T=25K, but it is recovered at lower temperatures at a new position. The 2.5K field dependence of the spin susceptibility and resonance frequency is characteristic of a magnetically ordered system, where the Dzyaloshinskii-Moriya interaction causes a finite gap of 1.5meV in the spin wave spectrum at q=0.