Ultrafast optical switching of the THz transmission through metallic subwavelength hole arrays

We demonstrate ultrafast optical switching of the transmission of terahertz radiation through a metal grating with subwavelength holes. By fabricating the grating on a semiconductor silicon substrate, we are able to control the grating transmission intensity by varying the photodoping level of the silicon and thereby the resonant coupling to the metal grating. As such, we are able to switch the transmission on picosecond time scales with low visible light intensities, observing a factor of 2–5 improvement in photomodulation efficiency at resonance wavelengths over a bare silicon surface.

Figure 1

(Color online) (a) Optical microscope image of the sample, showing gold and silicon (black) regions. (b) The transmission spectrum for plane-wave transmission. The arrows indicate peak wavelengths predicted by Eq. (1). Inset: The time domain measurements of the transmitted and reference electric fields.

Figure 2

(Color online) (a) Transmission spectra measured $10\mathrm{ps}$ after photoexcitation of the back interface of the grating using $3.1\mathrm{eV}$ photon pulses. (b) Transmission after excitation of the front interface. (c) Dots indicate the absolute transmission at the (1,0) peak at $\lambda =346\mu \mathrm{m}$ after photoexciting the front (filled) and back (nonfilled) interfaces. The dotted and solid lines are for photoexcitation of the hole array and bare silicon surfaces, respectively. (d) Comparison between excitation of the front interface with $1.5\mathrm{eV}$ photons and $3.1\mathrm{eV}$ photons.

Figure 3

(Color online) (a) Results of finite element method calculations showing the electric-field strength in the diffracted orders of the (1,0) peak with and without photoexcitation by $0.1\mathrm{J}∕{\mathrm{m}}^2$, $3.1\mathrm{eV}$ pulses. The excitation region is treated as a homogeneous layer below the holes, $125\mathrm{nm}$ deep (dark region in the inset). (b) The change in diffracted intensity on photoexcitation is predominantly caused by an increase in reflection from the grating.

Figure 4

(Color online) (a) Transmission spectrum measured as a function of time after excitation using $1\mathrm{J}∕{\mathrm{m}}^2$, $1.5\mathrm{eV}$ photon pulses. (b) Comparison of switching dynamics: peak transmission (at $250\mu \mathrm{m}$) as a function of time for bulk silicon (gray solid line) and hole array (blue/dark gray dotted line) after excitation by $1.5\mathrm{eV}$ photons and hole array (red/light gray dashed line) after excitation by $3.1\mathrm{eV}$ photons. Note that the data have been scaled and offset to allow direct comparison of the dynamics. Inset: recovery time for $0.1\mathrm{J}∕{\mathrm{m}}^2$ at $20\mathrm{K}$.