Femtosecond coherent control of terahertz radiation by transient nanophotonic structures

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Coherent radiation with frequencies ranging from 0.3 to 30 THz has recently become accessible by femtosecond laser technology. Terahertz (THz) waves have many applications, in particular for spectroscopy of low-frequency excitations (such as molecular vibrational/rotational modes, phonons and spin waves in solids), for imaging as well as short-range wireless communication at THz bit rates. THz light can be manipulated by static elements such as lenses, polarizers, and filters. However, ultrafast modulation of THz radiation is required as well, for instance in ultrafast optical switches or frequency shifters, whose development is just at the beginning. In this proposal, we will realize an extremely flexible approach towards the ultrafast manipulation of THz radiation by transient nanophotonic structures. For this purpose, we irradiate a semiconducting slab with a visible femtosecond laser beam whose spatial profile can be shaped arbitrarily by a light modulator. Through light absorption in the semiconductor, the beam pattern is translated into a density pattern of an electron-hole plasma in the slab. Irradiated areas become metallic whereas dark areas remain dielectric. Since features much smaller than the THz wavelength (roughly 300 micrometers) can be realized with optical radiation, we can switch on a tailored nanooptical structure for THz radiation on a femtosecond timescale.Using this approach, we will realize and study transient periodic structures, namely metamaterials and photonic crystals with three objectives:(1) We will realize transient polarizers and waveplates by generating metamaterials with a highly anisotropic unit cell, which brings about the required birefringence. We will also realize transient photonic crystals and investigate the extent to which we can tune their dispersion relation to realize large band gaps and slow light. Ohmic loss of THz radiation by free charge carriers will also be addressed.(2) By varying the charge-carrier density in the transient structures, we will test the validity of basic optical concepts. For example, at which carrier density does a structure become metallic at THz frequencies? How metallic must an array of transient disks be such that it exhibits an appreciable coupling to the magnetic-field component of the THz wave?(3) Finally, we will coherently manipulate THz radiation on-the-fly, that is, we will ultrafast modify an optical nanostructure whilst a THz pulse propagates within. An important application of such ultrafast nanostructures is frequency shifting of THz pulses, which will allow us to access the underexplored frequency window from 5 to 8THz. In addition, we plan to adiabatically compress the spectrum of a THz pulse (spectral lensing).