Tailoring terahertz radiation by controlling tunnel photoionization events in gases

http://iopscience.iop.org/1367-2630/13/12/123029

Paper

Various applications ranging from nonlinear terahertz (THz) spectroscopy to remote sensing require broadband and intense THz radiation, which can be generated by focusing two-color laser pulses into a gas. In this setup, THz radiation originates from the buildup of electron density in sharp steps of attosecond duration due to tunnel ionization, and the subsequent acceleration of free electrons in the laser field. We show that the spectral shape of the THz pulses generated by this mechanism is determined by the superposition of contributions from individual ionization events. This provides a straightforward analogy to linear diffraction theory, where the ionization events play the role of slits in a grating. This analogy offers simple explanations for recent experimental observations and opens new avenues for THz pulse shaping based on temporal control of the ionization events. We illustrate this novel technique by tailoring the spectral width and position of the resulting radiation using multi-color pump pulses.

GENERAL SCIENTIFIC SUMMARY  Introduction and background. Generation of THz radiation currently attracts much interest because of its exceptionally wide range of applications. One of these applications is employment of ionizing short laser pulses in gases, which allows the production of single-cycle THz pulses at relatively high energies. In this scheme, a short femtosecond pulse, typically consisting of fundamental and second harmonic, is focused into a gas. THz emission results from free electron creation due to tunnel ionization and subsequent acceleration in the laser field.

Main results. Tunnel ionization occurs in the vicinity of extrema of the pump electric field. Therefore, free electrons are born in a sequence of 'discrete ionization events'. Here we show that the resulting THz spectrum is determined by spectral interference of extremely broad contributions from these ionization events. Through this approach we can explain the basic features of the emitted THz radiation.

Wider implications. The interference forming the THz spectrum can be seen in close analogy to a linear diffraction grating, where the ionization events play the role of the grating slits. This analogy provides a direct tool to engineer spectral properties in a broad spectral range, from THz to near infrared, by controlling positions and amplitudes of the extrema of the pump electric field. We believe that this novel strategy can be exploited to build an ultra-broadband THz pulse shaper.