Subcycle control of terahertz waveform polarization using all-optically induced transient metamaterials

Abstract

Coherent radiation with frequencies ranging from 0.3 to 30 THz has recently become accessible using femtosecond laser technology. These terahertz (THz) waves have already been applied in spectroscopy and imaging and can be manipulated using static optical elements such as lenses, polarizers and filters. However, ultrafast modulation of THz radiation is required as well, for instance, in short-range wireless communication or for preparing shaped THz transients for the coherent control of numerous material excitations. Here, we demonstrate an all-optically created transient metamaterial that allows us to manipulate the polarization of THz waveforms with subcycle switch-on times. The polarization-modulated pulses are potentially interesting for controlling elementary motions such as the vibration of crystal lattices, the rotation of molecules and the precession of spins. N. Kamaraju and Tobias Kampfrath from the Fritz Haber Institute and international collaborators have developed a fast metamaterial switch for terahertz radiation. Terahertz waves are important for imaging and sensing applications, but are difficult to manipulate in a dynamic manner. Metamaterials made from on-chip metallic resonators are able to control terahertz waves through a variation in the effective refractive index of the device, but also lack the necessary intrinsic flexibility. To achieve flexible and ultrafast switching, the researchers used an optical excitation pulse that writes spatially varying patterns of excited charge carriers into a homogeneous semiconductor slab. Because the refractive index of the material is different between areas that are illuminated and those that are not, this approach creates a metamaterial capable of controlling terahertz waves and whose properties can be dynamically varied by changing the spatial structure of the optical excitation pulse.