Terahertz-Pulse Emission Through Laser Excitation of Surface Plasmons in a Metal Grating

The second-order processes of optical-rectification and photoconduction are well known and widely used to produce ultrafast electromagnetic pulses in the terahertz frequency domain. We present a new form of rectification that relies on the excitation of surface plasmons in metal films deposited on a shallow grating. Multiphoton ionization and ponderomotive acceleration of electrons in the enhanced evanescent field of the surface plasmons results in a femtosecond current surge and emission of terahertz electromagnetic radiation. Using gold, this rectification process is third or higher-order in the incident field.

Figure 1

(top) Schematic of the experimental setup. BS, Beam Splitter; OC, Optical Chopper; G, Grating on rotation mount; DC, 1-mm thick ZnTe Detection Crystal with black card to block pump light; WP, Wave Plate; WPrism, Wollaston Prism. (bottom), Grating profile obtained by AFM; $a=340\mathrm{nm}$, $b=160\mathrm{nm}$, $c=500\mathrm{nm}$, and $d=40\mathrm{nm}$ etch depth.

Figure 2

The circles (diamonds) show the normalized terahertz-pulse fluence vs. the angle of incidence of the pump laser obtained by rectification on a gold-covered grating excited by $p$-polarized ($s$-polarized) radiation. The fluence incident on the grating at 800 nm was $3.0\mathrm{mJ}/{\mathrm{cm}}^2$. The squares show the absorption of the 800-nm $p$-polarized pump beam. The inset shows the terahertz fluence for excitation with $s$-polarized radiation multiplied 150 times.

Figure 3

Comparison of terahertz electric field generated in a 0.5-mm ZnTe crystal with that generated in the grating for an incident 800-nm laser intensity of $3.0\mathrm{mJ}/{\mathrm{cm}}^2$. (inset) The absolute value of the Fourier transform of the time-domain traces.

Figure 4

Input laser-power dependence of the terahertz-pulse fluence for $⟨110⟩$ ZnTe (squares) and the grating structure (circles). The solid lines are least-squares fits to power laws with the upper solid line corresponding to a power of 1.8, the lower solid line a power of 3.5, and the dashed line 2.0.