# The Opto-Electronic Functional Devices Based on Three-Dimensional Lead Halide Perovskites
## 3 The LHP Based Functional Devices
### Modulators  
Modulators and switches are common optical components found in integrated optical circuits. These days, LHP have been utilized to construct modulators or switches working in THz range, for which the corresponding working principle is presented in Figure 8. The light modulation or switching by LHP hybrid devices are based on the THz absorption by charge carriers. By combining LHP with a proper material (for instance, silicon has been frequently used), a heterostructure is produced, of which the electronic structure bends at the interface. When pump light is turned on, photogenerated carriers are to be excited and tend to drift to the interface and accumulate there. When the THz waves are incident at the devices, the wave will be attenuated by the carriers, through mechanisms including electron-hole scattering, electron-phonon scattering, and electron-impurity scattering [112]. The actual absorption coefficient of the device changes in correspondence to pump laser. Consequently, the intensity of the incident THz waves is modulated. In Table 3, the performance from some of the recent LHP THz modulators is presented. These days, both modulators with planar structure and metamaterials have been reported, as shown in Figure 9. [PERSON] et al. [23] reported the use of MAPbI3/Silicon structure to modulate THz wave in 0.2-2.6 THz range, by using a 450 nm cw laser as the pump light. The MAPbI3 thin film was conveniently spin-coated onto a silicon substrate to form heterostructures. The silicon layer could strongly absorb the 450 nm laser and generate charge carriers which then drift to the LHP-silicon interface, resulting in an enhanced carrier density. It is found that the device transmittance and reflection could be efficiently modulated, and a modulation depth (MD) of nearly 100% could be achieved. As compared to silicon-based modulator, the MD is greatly enhanced in the hybrid device and the saturation intensity of the device (\(\sim\)0.42 W/cm\({}^{2}\)) is much lower than that in graphene/silicon based devices [115]. [PERSON] et al. reported the optical modulation of an intenser THz wave using 532 nm laser [22]. Since the absorption at 532 nm for silicon is considerably lower than that at 450 nm, the THz absorption enhancement at 532 nm is less. At a 1.5 W/cm\({}^{2}\) laser illumination, a reduced MD (68%) has been achieved. [PERSON] et al. demonstrated a silicon-PEDOT:PSS-LHP THz wave modulator driven by a 980 nm continuous wave (cw) laser [21], in contrast to previous reports using visible wavelength. In order to obtain an even larger MD, the authors introduced both PCBM and PEDOT:PSS layers to further elongate the carrier recombination time. At a moderate NIR intensity of 2.18 W/cm\({}^{2}\), a MD of \(-\)45 dB is achieved. Figure 8: (**a**) Operation mechanism of the all-optical THz modulator under laser irradiation Reproduced with permission from [21]; published by Wiley, 2020. (**b**) Schematic of the THz all-optical modulation measurement; (**c**) THz transmittance under different laser irradiance and (**d**) the corresponding modulation depth (MD). Reproduced with permission from [22]; published by Springer Nature, 2016. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Materials** & **Pump (nm)** & **MD** & **Saturation Intensity** & **Range (THz)** \\ \hline MAPbI3/silicon [23] & 450 & \(\approx\)100\% & 0.42 W/cm\({}^{2}\) & 0.2–2.6 \\ MAPbI3/silicon [22] & 532 & 68\% & 0.5 W/cm\({}^{2}\) & 0.2–2 \\ MAPbBr3/silicon [113] & 450 & 80\% & 3 W/cm\({}^{2}\) & 0.2–2.6 \\ PCBM*/MAPbI3/PEDOT:PSS/silicon [21] & 980 & 99.5\% & 2.18 W/cm\({}^{2}\) & 0.3–0.5 \\ CsPbBr3/silicon [114] & 450 & 45.5\% & 2 W/cm\({}^{2}\) & 0.23–0.35 \\ \hline \hline \end{tabular} PCBM is short for [6,6]-phenyl-C61-butyric acid methyl ester.  
\end{table} 
Table 3: Device parameters for the LHP-based THz modulators. In addition to the 3D LHP films, low-dimensional LHP has also been used in THz modulators. [PERSON] et al.