2, University of Southampton, Southampton, , United Kingdom
As the direction of technological advancement pushes towards miniaturization of devices with high operating speed and efficiency, germanium (Ge) as compared to silicon (Si), is more superior in terms of performance. Ge has higher carrier mobilities and large intrinsic carrier concentrations which makes it highly suitable for photonic devices. The fundamental energy band gap of Ge can be narrowed down through strain engineering so that it becomes a direct band gap semiconductor, which has enabled the realization of electrically pumped Ge-based lasers. Several low-loss waveguides and modulators have also been demonstrated on Ge integrated Si-based photonic systems. In addition, Ge has the added advantage of CMOS compatibility in microelectronics. The integration of semiconductors as active media into metamaterials offers vast opportunities for a wide range of innovative technologies enabled by strong light-matter interactions within the semiconductors.
Despite its wide applications in microelectronic and optoelectronic devices, there does not exist any demonstration of ultrafast flexible Ge thin-film based metaphotonic devices. In the previous demonstrations of photoswitching on GaAs and other semiconductors (Si on Sapphire), the recombination time of the carriers is >1 ns, which indicates a slow switching time. In order to achieve an ultrafast photoswitching time, superlattices were implemented but lattice matching is crucial to achieving short carrier lifetimes. The fabrication of superlattices requires the use of MBE which is a complicated and precise process as many growth factors must be considered.
In our work, we have designed a Ge-based metaphotonic device by evaporating 310 nm thickness of Ge thin film onto the terahertz metamaterial arrays. The terahertz metamaterial arrays were fabricated onto a flexible Kapton film substrate via photolithography. Optical-pump Terahertz-probe spectroscopy was used to study the relaxation dynamics of Ge and to optically pump and modulate the strength of the resonances.
From our results, we achieved a transmission modulation of ~ 90 % with a switching speed at ultrafast picosecond timescale of ~ 17 ps. A sub-picosecond decay time constant of ~670 fs is obtained from theoretical fitting of our relaxation dynamics which we attribute to the defect states present in the evaporated germanium thin film.
This is the first demonstration of Ge-based ultrafast flexible photoswitch. Our fabrication is simple, cost-effective, and involves thermal evaporation of a thin-film single element semiconductor material (Germanium) that shows such an ultrafast photoswitching of Fano resonant metamaterial. The simplicity of our concept suggests that it is universally applicable to the current state-of-the-art photonic devices. Our device could function as an ultrafast modulator or active filters. It could also pave the path for the realization of flexible electronic and photonic devices based on Ge.