Besides visible light optics, many strides are being done to improve our capability to harness information using other wavelengths of light. One such wavelength range is in the sub-millimeter range, or in terms of frequency, the terahertz (THz) domain. Recently, terahertz radiation has been used for medical imaging, since THz it is non-ionizing and is also able to discern differences in water content and tissue density. In addition, astronomers have also been utilizing terahertz frequencies for observational astronomy as well as detecting traces of water and other molecules in space. Chemists and biochemists also use THz radiation to provide new insights in spectroscopy. With its significant use in science and industry, being able to generate THz radiation is of importance.
In this paper, the researchers look into generating THz radiation with the use of a one-dimensional metal array having submillimeter-sized structures, as opposed to nanofabricated ones. The metal array was deposited on the back of a semi-insulating gallium arsenide (SI-GaAs) substrate, a common material used in THz time-domain spectroscopy (THz-TDS) applications. Using a titanium-sapphire femtosecond pulsed laser in a standard THz-TDS setup, they were able to observe an increase in intensity of the emitted THz radiation. The cause of the increase is under investigation but for now is attributed to extraordinary optical transmission.
A one-order-of-magnitude terahertz (THz) emission enhancement in the transmission geometry, over a 0.7-THz broadband range, was observed in semi-insulating gallium arsenide (SI-GaAs) integrated with a subwavelength one-dimensional metal line array (1DMLA). THz emission of the 1DMLA samples showed an intensity increase and exhibited a full-width-at-half-maximum broadening relative to the emission of the bare substrate. Improved index matching could not account for the observed phenomenon. A nonlinear dependence of the integrated THz emission intensity on the number of illuminated lines and on the pump power was observed. The actual origin of the increased THz emission is still under investigation. At present, it is attributed to extraordinary optical transmission.
Maria Angela B. Faustino, Lorenzo P. Lopez, Jessica Pauline Afalla, Joselito Muldera, Nathaniel Hermosa, Arnel A. Salvador, Armando S. Somintac, and Elmer S. Estacio, “Terahertz emission enhancement in semi-insulating gallium arsenide integrated with subwavelength one-dimensional metal line array,” Opt. Lett. 41, 4515-4517 (2016)