Free-space electro-optic sampling is an alternative method for the characterization of freely propagating terahertz beams with subpicosecond temporal resolution. In contrast to resonant photoconductive dipole antennas, free-space electro-optic sampling via the linear electro-optic effect (Pockels effect) offers a flat frequency response over an ultrawide bandwidth and the potential for a simple cross-correlation signal of the terahertz and optical pulses.

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 ($2\omega$) indicates that the third-order nonlinear optical process mixing the $\omega$ and $2\omega$ 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 $2\omega$ 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 ($\omega$, $2\omega$, and THz waves) are at the same polarization in the four-wave-mixing process.

Terahertz (THz) radiation, which occupies a relatively unexplored portion of the electromagnetic spectrum between the mid-infrared and microwave bands, offers innovative sensing and imaging technologies that can provide information unavailable through conventional methods such as microwave and X-ray techniques. With the advancement of THz technologies, THz sensing and imaging will impact a broad range of interdisciplinary fields, including chemical and biological detections and identifications. In particular, THz radiation offers the opportunity for transformational advances in defense and security. Recent work shows that THz technologies are promising for the standoff detection and identification of explosive targets.

We have generated electromagnetic beams from a variety of semiconductors. When a bare semiconductor wafer was illuminated by femtosecond optical pulses, electromagnetic waves radiate from the surface and form collinear diffraction-limited electromagnetic beams in the inward and outward directions. The amplitude and phase of the radiated field depend on carrier mobility, the strength and polarity of the static internal field at the semiconductor surface.