High efficiency photomodulators for millimeter wave and THz radiation

Abstract

Photomodulators for mm-wave and THz radiation are an essential component for many imaging and signal processing applications. While a myriad of schemes have been devised to enhance photomodulation by enhancing the light-matter interaction, there has been less focus on the photoconductive materials themselves, which are often the limiting factor. Here, we present an approach to increase the photomodulation efficiency of silicon by orders of magnitude, using post treatment of off-the-shelf silicon wafers. The increase in efficiency removes the need for bulky and costly amplified laser sources, and creates the potential for compact and cost-effective modulators for real-world applications. By passivating the surfaces of long bulk-lifetime silicon wafers with Al2O3, the recombination of the photoexcited carriers at the surfaces is mostly eliminated. This results in vastly longer excess carrier lifetimes (up to ~50 ms), with corresponding increases in photoconductivity. The resulting modulators are highly efficient, with the transmission through them being reduced from ~90% to <10% over a narrow frequency band with a continuous wave excitation intensity of just 10 Wm−2, whilst modulation factors of greater than 80% can be achieved over a broad band with similar intensities. We also discuss the limitations of such long-lifetime modulators for applications where the switching speed or spatial resolution of a modulator may be critical.

Terahertz Single-Pixel Imaging Improved by Using Silicon Wafer with SiO2 Passivation

“…However, for samples S1 and S9, both of which have almost the same optical reflectivity, as shown by Figure 3, the former has much smaller terahertz MF than the latter. This is because the dielectric layer added on top of the silicon surface terminates dangling bonds that arise due to the lattice termination, resulting in a reduction in trap density [18]. Therefore, we show that the SiO 2 not only plays a role of chemical passivation, but also acts as an anti-reflection film that enhances the use of light energy [26].…”

“…While various approaches have been adopted to boost photomodulation [15][16][17], there has been less focus on the semiconducting photoconductive materials themselves. Quite recently, Hooper et al [18] demonstrated that a passivated silicon wafer can greatly improve the efficiency of THz photomodulation. However, THz single-pixel imaging based on such a THz photomodulator has not been reported to date.…”

High‐Performance Photo‐Induced Spatial Terahertz Modulator Based on Micropyramid Silicon Array

“…It is worth noting that another Si modulator that exhibits high MD greater than 80% is previously realized at similar short λ of 623 nm. [ 31 ] In that case by passivating the Si surface the lifetime of the excess carrier in the silicon surface is greatly increased, leading to a large surface photoconductive and notably enhanced modulation efficiency. Both modulators work well under low power density, thus eliminate the need for intense laser sources.…”

“…However, these film‐coated Si modulators generally cannot suppress the light reflection or maintain the same high modulation efficiency at a very short illumination light wavelength. One interesting modulator is the surface‐passivated Si modulator, as we mentioned before, [ 31 ] which can provide a very high MD (≈80–100%) at 623 nm by increasing the carrier lifetime from the intrinsic 10 µs to ≈50 ms. However, the long carrier lifetime decreases the modulation speed to tens of Hz while increases the lateral diffusion lengths of the carrier to 7 mm.…”

“…However, the long carrier lifetime decreases the modulation speed to tens of Hz while increases the lateral diffusion lengths of the carrier to 7 mm. [ 31 ] As to the Si‐MPA proposed here, the surface carrier lifetime is the same as the intrinsic one of the used bare silicon wafer (≈10 µs). The carrier diffusion length, besides carrier lifetime, is also includes illumination wavelength, power density and impinging area dependent according to Kannegulla et al [ 30 ] The spatial distribution of THz transmission through Si substrates under illumination of 638 nm and 1.2 W cm −2 continuous light waves is calculated to be about 1 mm.…”