Pump‐Color Selective Control of Ultrafast All‐Optical Switching Dynamics in Metaphotonic Devices

Abstract Incorporating active materials into metamaterials is expected to yield exciting advancements in the unprecedented versatility of dynamically controlling optical properties, which sheds new light on the future optoelectronics. The exploration of emerging semiconductors into terahertz (THz) meta‐atoms potentially allows achieving ultrafast nanodevices driven by various applications, such as biomedical sensing/imaging, ultrawide‐band communications and security scanners. However, ultrafast optical switching of THz radiation is currently limited to a single level of tuning speed, which is a main hurdle to achieve multifunctionalities. Here, a hybrid metadevice which can realize the pump‐wavelength controlled ultrafast switching response by the functionalization of double photoactive layers is demonstrated experimentally. A whole cycle of electromagnetically induced transparency switching with a half‐recovery state changes from 0.78 ns to 8.8 ps as pump wavelength varies from near infrared to near ultraviolet regions. The observed pump‐color selective switching speed changing from nanosecond scale to picosecond scale is ascribed to the wavelength‐dependent penetration length of Ge and the contrasting defect states between noncrystalline Ge and epitaxial Si layers. It is believed that the schemes regarding pump‐color controllable ultrafast switching behavior introduced here can inspire more innovations across the field of ultrafast photonics and can boost the reconfigurable metamaterial applications.

. Numerically simulated transmission and group delay spectra by changing the photoconductivity of PALs lying below the EIT meta-atoms. The photoconductivity generated in the silicon layer corresponds to the NIR pumping case, while the change in the germanium layer represents the NUV pumping case. Values of the photoconductivity proportional to the pumping fluence are optimized to match the curves shown in Figure 1 with the same color.     We establish a semiempirical model to obtain the ratio It should be noted that the input pump fluence at any wavelength may be changed in order to gain sufficient modulation depth. Once this ratio is fixed, we then try to adjust the conductivity to get modulation depth just up to 100% of the suppression of EIT resonance, leading to the determination of initial effective conductivity in Ge and Si layers.
The effective photoconductivity of Si layer is considered to be decayed exponentially as in Figure 3a, since the THz pulse profile (several picoseconds) is much shorter than the relaxation time of carriers in Si (more than a nanosecond). However, the decay constant of free carriers in Ge film is less than the width of THz pulse profile, as clearly shown in Figure   S3. In our experiment, a pump pulse is used to excite the free carriers in Ge film, leading to the photoconductivity in the Ge layer existing only about 2 picoseconds. The overlap between Ge photoconductivity profile and THz profile results in the modulation of EIT effect, and its  Figure S4, S5, S6, S7, S8, S9, and S10.

Supplementary Note S2: Switching dynamics influenced by the pump wavelength
Since the newly developed combination of double photoactive layers with metasurfaces in this work is to verify that pump-color selective switching dynamic changed from nanoseconds to picoseconds, it is also necessary to compare the switching dynamics at pump wavelengths covering the absorption band of Ge and Si material. The numerical data covering the wavelength from 400 nm to 1500 nm is vividly illustrated in Figure S4, S5, S6, S7, S8 for 400 nm, 600 nm, 800 nm, 1100 nm, 1500 nm, respectively. As expected from the relaxation dynamics in Ge and Si, the switching dynamic can be classified as two processes: fast process within 4 ps and slow process within 2 ns. Consistent with our experimental expectation pump by NVU-beam (i.e. 400 nm), the switching dynamic depends solely on the Ge film with ultrafast speed on the time scale several picoseconds (shown in Figure S4), then followed by no evolution process since photons absorbed in Si are negligible. When pumped by the NIRbeam (i.e. 800 nm, 1100 nm, 1500 nm in Figure S6, S7, S8, respectively), the fast switching process exists but is rather small so that its switching effect can be ignorable. Thus, the switching-on dynamics in these cases are almost dependent on the relaxation of free carriers in Si. The mechanism behind is that the penetration length of Ge is longer than the corresponding thickness so that sufficient number of photons is illuminated into the Si layer.
The photoconductivity generated in Si layer almost leads to the saturable switching-on state of EIT effect, and thereby the additional small photoconductivity in Ge layer causes little influence on the annihilation of EIT effect. An intriguing phenomenon observed in Figure S5 is that both fast and slow switching-on processes are obvious as pumped by visible light (i.e. switching process by varying the pump wavelength in the visible light region, offering a degree of freedom to control ultrafast dynamics for all-optical switching metadevices.

Supplementary Note S3: Switching dynamics influenced by the PA layer thickness
Since the dynamic of proposed metadevice mainly relies on the number of photons penetrating through the Ge layer, we herein systematically investigate the switching dynamics affected by the thickness of Ge layer, as shown in Figure S4, S9, S10. When the pump wavelength is selected as 400 nm, we purposely change the thickness of Ge layer from 120 nm to 40 nm to observe the distinction of switching dynamics. As anticipated, the fast switching process is less dominant as the thickness of Ge layer decreases. It is noticeable that no fast switching process occurs even pumped by NUV beam in the case for the thickness of Ge layer down to 40 nm. Therefore, no fast switching process can be achieved by varying the pump wavelength if the Ge layer is too thin. On the other hand, no slow switching process takes place if the Ge layer is too thick to prevent the penetration of photons through the Ge layer. To conclude, the thickness of Ge layer intimately plays an important role in the appropriateness of realizing pump-color selective ultrafast dynamics in the proposed metadevice.