Zehan Yao

Coupling Tai Chi Chiral Metamaterials with Strong Optical Activity in Terahertz Region

We demonstrated Tai Chi chiral metamaterials of mirror-symmetric and complementary structures in terahertz (THz) region. We investigated the properties of these structures by calculating the transmissions under different circularly polarization waves and analyzing the surface current distributions. The chiral mirror-symmetric structure has circular dichroism and strong optical activity due to electromagnetic field coupling between layers with sub-wavelength thickness. Moreover, the structures with multiple metallic layers or twist angle between layers can realize chirality tunability, which can be used for THz polarized devices. Besides, the complementary structure has potential applications for polarization-insensitive devices in THz region.

Manipulating Magnetoinductive Coupling with Graphene-Based Plasmonic Metamaterials in THz Region

Coupling between physical modes can trigger some new physical phenomena such as frequency shift, new mode, and Rabi splitting. Resonant modes in graphene-based metamaterials provide a new platform for the research of coupling. In this work, we demonstrate that the plasmonic coupling of split ring resonator (SRR) dimer in graphene-based metamaterials can be easily manipulated. This magnetoinductive coupling can switch on/off the dark modes easily, which is usually done by symmetry-breaking structure previously. Furthermore, the dark mode can also be activated by Fermi energy as well as carrier concentration changing with either physical or chemical methods conveniently. In addition, different graphene-based SRR dimer configurations present different coupling strengths, which benefits the designing and optimizing of graphene-based metamaterials. The demonstration could enhance the versatility of both coupling studies in terahertz (THz) region and graphene-based metamaterials for THz devices.

Interface Properties Probed by Active THz Surface Emission in Graphene/SiO2/Si Heterostructures

Graphene/semiconductor heterostructures demonstrate an improvement of traditional electronic and optoelectronic devices because of their outstanding charge transport properties inside and at the interfaces. However, very limited information has been accessed from the interfacial properties by traditional measurement. Herein, we present an active THz surface emission spectroscopy for the interface build-in potential and charge detrapping time constant evaluation from the interface of graphene on SiO2/Si (Gr/SiO2/Si). The active THz generation presents an intuitive insight into the depletion case, weak inversion case, and strong inversion case at the interface in the heterostructure. By analyzing the interface electric-field-induced optical rectification (EFIOR) in a strong inversion case, the intrinsic build-in potential is identified as -0.15 V at Gr/SiO2/Si interface. The interface depletion layer presents 44% positive THz intrinsic modulation by the reverse gate voltage and 70% negative THz intrinsic modulation by the forward gate voltage. Moreover, a time-dependent THz generation measurement has been used to deduce the charge detrapping decay time constant. The investigation will not only highlight the THz surface emission spectroscopy for the graphene-based interface analysis but also demonstrate the potential for the efficient THz intrinsic modulation as well as the enhancement of THz emission by the heterostructures.

Giant angular dependence of electromagnetic induced transparency in THz metamaterials

The giant electromagnetic induced transparency (EIT) phenomenon is observed in symmetrical metamaterials with angular dependence in the THz region. This is due to the asymmetrical electromagnetic field distribution on the surface of the metamaterials, which induces asymmetric current distribution. Blueshift with the increase of the unit cell period has been observed, which is due to the unusual electromagnetic interaction between units at oblique incidence. This EIT demonstrates an angular dependent high Q-factor, which is sensitive to the dielectric environment. The angle-induced EIT effect could pave the way for future tunable sensing applications in the THz region.