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Nanophotonics

Controllable all‐optical modulation speed in hybrid silicon‐germanium devices utilizing the electromagnetically induced transparency effect

Junhu Zhou, Chenxi Zhang, Qirui Liu, Jie You, Xin Zheng, Xiang’ai Cheng, Tian Jiang

Published Feb 1, 2020• 37 citations

Abstract

Abstract Incorporating auxiliary all‐optical modulation speeds as optional response modes into a single metamaterial is a promising research route towards advanced terahertz (THz) applications ranging from spectroscopy and sensing to communications. Particularly, a plethora of dynamically tunable optical functionalities are determined by the resonant light‐matter interactions. Here, an electromagnetically induced transparency (EIT) resonator stacked with two traditional semiconductor films, namely silicon (Si) and germanium (Ge), is experimentally demonstrated. A giant switching feature of the EIT window with a peak at 0.65 THz occurs when the Si or Ge film is excited by ultrafast optical pulses, allowing for an optically tunable group delay of the THz wave packet. The recovery time for the slow and fast on‐off‐on switching cycles is 1.7 ns and 11 ps, respectively, which are mapped as the pump delay time of Si and Ge. Two optional response modes are integrated on the same device, where the modulation speed varies by three orders of magnitude, endowing the modulator more compact. This work provides new prospects for the design and construction of novel chip‐scale THz devices based on EIT and their applications in areas of sophisticated optical buffering and active filtering.

DOI: 10.1515/nanoph-2020-0017

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We demonstrate and analyze the use of metamaterials featuring an analogue of electromagnetically induced transparency (EIT) in slow light technology. For most metamaterials, EIT-like effects suffer from intrinsic ohmic loss, and the metamaterial-based slow-light effect can only be tuned passively, which limits their application in slow light devices. We propose a hybrid metamaterial with a unit cell composed of a ring resonator formed from photoactive silicon (Si) and a rectangular bar formed from metallic silver (Ag). Based on an analogue of EIT in the designed hybrid metamaterial, we theoretically demonstrate an all-optical tunable slow-light effect in the telecommunication window. We successfully demonstrate the possibility of designing novel all-optical tunable chip-scale slow-light devices that could be used in optical buffering.

Abstract Active control of electromagnetically induced transparency (EIT) in metamaterials promises tremendous potential applications. However, previously reported strategies most require sensitive materials as well as additional experimental apparatus, which are inconvenient for on-chip operation and restrain their implementations in fully integrated design. We propose a low-loss all-dielectric metamaterial with orthogonal double bright modes, which shows a narrowband transparency window at 950.05 nm with a high Q -factor of 2111. Just by varying the incident light polarization angle, an on-to-off in-situ EIT modulation without changing Q -factor is perfectly achieved, and the modulation depth can be up to 99.7%. Additionally, optical control of group delay with up to 36.22 ps is also demonstrated. This work provides a simple route to design chip-scale devices with optically dynamic controllability and finds fascinating applications in low-loss active slow light devices, optical switch and sensitive bio-sensing.

We propose a scheme to design a new type of optical metamaterial that can mimic the functionality of four-state atomic systems of N-type energy-level configuration with electromagnetically induced transparency (EIT). We show that in such metamaterial a transition from a single EIT to a double EIT of terahertz radiation may be easily achieved by actively tuning the intensity of the infrared pump field or passively tuning the geometrical parameters of resonator structures. In addition, the group velocity of the terahertz radiation can be varied from subluminal to superluminal by changing the pump field intensity. The scheme suggested here may be used to construct chip-scale slow and fast light devices and to realize rapidly responded switching of terahertz radiation at room temperature.

ABSTRACT Electromagnetically induced transparency (EIT) in terahertz (THz) metamaterials relies on the coherent coupling between a radiative (bright) mode and a sub‐radiant (dark) mode. Understanding the dynamic interplay between the bright and dark modes is key to manipulating their mutual interference and hence the transparency. Here, we use nonlinear 2D‐THz spectroscopy to scrutinize the dynamics arising from the nonlinearities of the EIT‐like phenomenon in a metamaterial platform comprising two coupled resonators. From the temporal profiles of the nonlinear pump‐probe and photon‐echo signals, we found that the bright mode relaxation time is almost twice the time for the coherent exchange of energy between the two coupled resonators. The multi‐peak nature of the photon‐echo signal and the corresponding temporal signatures further provide a direct visualization of the interference between the dressed states that drives the transparency window in our THz metamaterial. A time‐resolved density‐matrix model accurately describes the observed features, including the cross‐peak behavior and the temporal dynamics, establishing the coherent mode coupling as the origin of the transparency window.