Tailoring terahertz radiation by controlling tunnel photoionization events in gases

Abstract

Applications ranging from nonlinear terahertz 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 the electron density in sharp steps of attosecond duration due to tunnel ionization, and 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 superposition of contributions from individual ionization events. This provides a straightforward analogy with 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.

Boosting Terahertz Generation in Laser-Field Ionized Gases Using a Sawtooth Wave Shape

“…Recently, large-sized organic crystals were used to deliver THz pulses with GV=m electric field strength, and a conversion efficiency of about 1% was demonstrated [10]. The second method is based on focusing a femtosecond pulse together with its second harmonic into a gas cell and create a plasma [11][12][13][14][15][16][17][18][19][20][21][22][23][24]. In such two-color pump setup, free electrons produced by tunnel ionization acquire a nonzero drift velocity and generate a quasi-dc current which is responsible for THz emission [19].…”

“…In such two-color pump setup, free electrons produced by tunnel ionization acquire a nonzero drift velocity and generate a quasi-dc current which is responsible for THz emission [19]. The mechanism underlying THz generation in gases is intrinsically related to the optically induced stepwise increase of the free electron density near the extrema of the ionizing optical field [15,16,22]. An easy explanation is that the Fourier transform of a steplike function in time has its maximum near the zero frequency.…”

“…Boosting the THz yield is confirmed through comprehensive 3D computations that take all propagation effects into account. Selecting the first four Fourier harmonics of the sawtooth waveform already guarantees an impressive increase of the THz energy up to 5 μJ from a 300 μJ pump pulse.We start with the local current (LC) approximation [15], which neglects propagation effects. The free electron density ρðtÞ and current JðtÞ are governed by ∂ρðtÞ ∂t ¼ W½EðtÞ ½ρ 0 − ρðtÞ ;…”

“…We start with the local current (LC) approximation [15], which neglects propagation effects. The free electron density ρðtÞ and current JðtÞ are governed by ∂ρðtÞ ∂t ¼ W½EðtÞ ½ρ 0 − ρðtÞ ; ð1Þ…”

“…1(a)]. Assuming a small size of the plasma spot, E THz ðtÞ is evaluated by the low-frequency filtering of g∂ t JðtÞ, where g is a geometrical factor [15]. Ionization mostly happens near the extrema of EðtÞ.…”