THZ generation by ultra-short laser pulses propagating in nonuniform plasma chanels

Summary form only given. Electromagnetic THz radiation spans the range of wave lengths from millimeters to infrared. It has many potential applications in biological imaging, spectroscopy of solids and liquids, and remote sensing. Coherent pulses of THz are especially of interest for time domain spectroscopy. Conventional sources of THz using short pulse lasers rely on pulse generation in a solid and are generally limited to mJ/pulse. Higher energies per pulse can be generated at accelerator facilities with intense bunched electron beams via synchrotron or transition radiation. Recently, intense THz pulses with energies in excess of 100 mJ/pulse have been generated as transition radiation by a laser generated and accelerated electron beam passing from plasma to vacuum. We propose a scheme for THz generation that involves the creation of miniature corrugated plasma channels (period ~40 mm) that act as slow wave structures. THz can be generated in these channels by pulses of current driven by a laser pulse or injected from an accelerator. We consider the excitation of THz electromagnetic waves in a plasma by the ponderomotive force of an ultra-short laser pulse. For a uniform plasma such excitation is weak because electromagnetic waves have no density perturbation and do not couple to the ponderomotively driven plasma current. EM waves in a nonuniform plasma channel can be excited. Further, if the channel is axially modulated the EM waves can be slowed down and phase matched to the ponderomotive wave. We calculate the excitation of these waves by both fixed shape laser pulses and by parametric decay. Our calculations show that in the case of a laser driver, a large fraction of laser pulse energy can be down converted to THz. Thus, this scheme offers the possibility of producing 100 mJ pulses of duration of hundreds of femtoseconds. Experimental techniques for generating modulated channels are also explored