Frequency mixing the fundamental-and second-harmonic fields of an ultrafast laser in any one of a number of materials can generate radiation at terahertz frequencies. A better understanding of this process leads to a brighter source of light at these very useful wavelengths.

Coherent control of terahertz supercontinuum generation in ultrafast laser–gas interactions

Our study of THz wave generation in the pulsed laser induced air plasma with individually controlled phase, polarization, and amplitude of the optical fundamental wave (omega) and its second harmonic (2omega) indicates that the third-order nonlinear optical process mixing the omega and 2omega beams in the ionized plasma is the main mechanism of the efficient THz wave generation. The polarity and the strength of the emitted THz field are completely controlled by the relative phase between the omega and 2omega waves. The measured THz field amplitude is proportional to the pulse energy of the fundamental beam and to the square root of the pulse energy of the second-harmonic beam once the total optical pulse energy exceeds the plasma formation threshold. The optimal THz field is achieved when all waves (omega, 2omega, and THz waves) are at the same polarization in the four-wave-mixing process.

Coherent Control of THz Wave Generation in Ambient Air

A transient photocurrent model is developed to explain coherent terahertz emission from air irradiated by a symmetry-broken laser field composed of the fundamental and its second harmonic laser pulses. When the total laser field is asymmetric across individual optical cycles, a nonvanishing electron current surge can arise during optical field ionization of air, emitting a terahertz electromagnetic pulse. Terahertz power scalability is also investigated, and with optical pump energy of tens of millijoules per pulse, peak terahertz field strengths in excess of 150 kV/cm are routinely produced.

Terahertz emission from ultrafast ionizing air in symmetry-broken laser fields

We attribute a strong forward directed THz emission from femtosecond laser filaments in air to a transition-Cherenkov emission from the plasma space charge moving behind the ionization front at light velocity. Distant targets can be easily irradiated by this new source of THz radiation.

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Conical Forward THz Emission from Femtosecond-Laser-Beam Filamentation in Air

We review the generation of broadband THz radiation from femtosecond photo-induced gas plasmas, with an emphasis on the highly efficient "AC-bias" case where the plasma is generated and driven by a superposition of fundamental and second-harmonic optical fields. The dependence on experimental parameters such as pulse energy, air pressure, polarization and focusing are presented, and compared to the predictions from semi-quantitative models for the THz generation process, namely (i) a microscopic photocurrent model and (ii) a four. wave mixing model. We also employ these models to the case of few-cycle pulses, where the observed THz emission is related directly to the carrier-envelope phase of the pulses, and hence provides a mechanism with which to measure this phase.

Broadband THz emission from gas plasmas induced by femtosecond optical pulses: From fundamentals to applications

Intense radiation in the terahertz (THz) frequency range can be generated by focusing of an ultrashort laser pulse composed of both a fundamental wave and its second-harmonic field into air, as reported previously by Cook
	 [Opt. Lett.25, 1210 (2000)]. We identify a threshold for THz generation that proves that generation of a plasma is required and that the nonlinearity of air is insufficient to explain our measurements. An additional THz field component generated in the type I β-barium borate crystal used for second-harmonic generation has to be considered if one is to avoid misinterpretation of this kind of experiment. We conclude with a comparison that shows that the plasma emitter is competitive with other state-of-the-art THz emitters.

Markus Kress

Papers

Terahertz-pulse generation by photoionization of air with laser pulses composed of both fundamental and second-harmonic waves.