Optical control over transmission of terahertz radiation through arrays of subwavelength holes of varying size

We modulate the transmission of terahertz (THz) radiation through periodic arrays of subwavelength holes in a metallic film by using pulses of visible-wavelength light to photoexcite the semiconducting substrate of the hole arrays. By varying the photodoping level of the semiconductor we are able to switch off the resonant transmission of THz radiation through the array. By varying the size of the holes, we demonstrate the crucial role that surface modes play in the resonant transmission and ultimately in the photomodulation behavior of these structures. We demonstrate that the surface-wave transmission mechanism can allow for very efficient optical modulation of radiation transmission.

Figure 1

(Color online) (a) Diagram of the experimental measurement. A visible pump pulse with center wavelength 400 nm illuminates an array of holes in a gold film on substrate of silicon. 40 ps after the arrival of the pump pulse, a probe pulse in the THz range is transmitted through the hole array and subsequently detected in the far field. (b) Side profile of the photoexcited structure, indicating the $100\mu \text{m}$ array pitch and hole size $a$.

Figure 2

(Color online) THz transmission spectra through arrays of $45\mu \text{m}$ holes with various fluences of 400 nm pump beam. Photoexcitation quenches the EOT resonances, indicated as the $⟨1,0⟩$ and $⟨1,1⟩$ peaks.

Figure 3

(Color online) (a) Photomodulation versus fluence of 400 nm pump pulses for three sizes of hole and a reference surface of silicon. (b) Experimentally measured photomodulation vs hole size for various fluence of 400 nm pump pulses. For the $25\mu \text{m}$ holes it was not possible to measure photomodulation for the lowest fluences due to the low level of transmitted THz signal. (c) Numerically modeled photomodulation of the THz transmission at resonance as a function of hole size at a 400 nm pump fluence of $0.197\text{J}/{\text{m}}^2$, with and without loss in the silicon substrate. Inset: Modeled transmission spectra through $55\mu \text{m}$ holes in a lossless substrate, with and without photoexcitation.