Coherently additive THz diamond emitters and their applications

Institute for Laser Science
Associated Professor Hitoki YONEDA
yoneda@ils.uec.ac.jp
http://www.ils.uec.ac.jp/~yoneda

The aim of this research is to develop a high power Terahertz (THz) radiation source using a diamond photoconductive device. The objectives is to open a new field of nonlinear optics in THz frequency range. THz radiation covers the frequency range between 100 GHz and 10 THz (i.e. a wavelength between 3 mm and 30 μm), which spans the spectral interval between the microwave- and the infrared regions of the electromagnetic spectrum.

Emission from a photoconductive antenna driven by an ultra-short-pulse laser is one of most powerful sources in this region (Fig.1). The emission density on the emitter is limited by the shielding of the applied electric field due to the generated THz electric field. The near-field THz electric field E_THz is given by

, whereE_b is the applied electric field η₀ is the free-space impedance, ε_r is the relative dielectric constant of the semiconductor substrate, and σ_s is surface photoconductivity. This equation implies that the emitted radiation should saturate at

. Therefore, the application of a higher electric field on the gap is the only way to generate higher power radiation. That is reason why we use a diamond as a photoconductive material.

Fig.1

To date, a 2-kV DC voltage can be applied to the 10-μm PCD gap with an over-coated layer for keeping sufficiently high dark-current resistivity. This electric field strength is two orders of magnitude larger than that of conventional GaAs emitters. We produced high energy density (10μJ/cm²) THz emission by inducing coherently-additive emission from more than 2000 photoconductive emitters using an ultra-short pulse Kr*F laser (Fig.2).

Fig.2

To produce higher power THz radiation, we are developing a system having angular multiplexing for the pumping laser pulses and electrical pulse stacking for the applied electric field on the emitters. In this system, intense coherently-additive THz emission will be obtained with incoherent laser driving pulses.