Broadband modulation of terahertz waves through electrically driven hybrid bowtie antenna-VO2 devices

Chunrui Han (1) , Edward Parrott (1) , Georges Humbert (2) , Aurelian Crunteanu (3) , Emma Pickwell-Macpherson (1)

1 CUHK - The Chinese University of Hong Kong [Hong Kong]
2 XLIM-PHOT - Photonique Fibre et Sources Cohérentes
3 XLIM-RFEI - RF-ELITE : RF-Electronique Imprimée pour les Télécommunications et l'Energie

Broadband modulation of terahertz (THz) light is experimentally realized through the electrically driven metal-insulator phase transition of vanadium dioxide (VO 2) in hybrid metal antenna-VO 2 devices. The devices consist of VO 2 active layers and bowtie antenna arrays, such that the electrically driven phase transition can be realized by applying an external voltage between adjacent metal wires extended to a large area array. The modulation depth of the terahertz light can be initially enhanced by the metal wires on top of VO 2 and then improved through the addition of specific bowties in between the wires. As a result, a terahertz wave with a large beam size (~10 mm) can be modulated within the measurable spectral range (0.3–2.5 THz) with a frequency independent modulation depth as high as 0.9, and the minimum amplitude transmission down to 0.06. Moreover, the electrical switch on/off phase transition depends very much on the size of the VO 2 area, indicating that smaller VO 2 regions lead to higher modulation speeds and lower phase transition voltages. With the capabilities in actively tuning the beam size, modulation depth, modulation bandwidth as well as the modulation speed of THz waves, our study paves the way in implementing multifunctional components for terahertz applications. The terahertz (THz) frequency regime attracts attention from both the scientific research and industry sectors due to its technological potential in biomedicine 1,2 , security 3 , imaging 4 and material characterizations 5. The ter-ahertz components, such as modulators 6,7 , switches 8,9 , lenses 10 , waveplates 11 , and filters are essential in efficiently manipulating the THz waves for specific applications. The fast growth of the THz technology asks for not only construction but also daily improvement of the functional components. One significant terahertz device, the optoelectronic compatible THz modulator, plays a key role in THz imaging and wireless communication systems. The establishment of active THz modulators require active materials working at THz frequencies. It is well known that the semiconducting heterostructures are THz active due to their intersubband transitions taking place on a meV energy scale 12,13. The unique scattering behaviors, such as strong absorption and reflection, induced by the coupling of THz pulses to the intersubband transitions enable implementation of an active THz attenuator, filter or emitter. Additionally, the charge density on the semiconductor surface can be tuned through either electrostatic gating 14 or photo-excited interband transitions 15 , leading to real time control of THz waves. Similarly, graphene has been recognized as a THz active material since graphene electrons have strong intraband transitions at terahertz frequencies 16,17. The transition rate can be changed by shifting the Fermi energy of graphene optically or electrically such that the absorption/reflection of THz waves during/ after the transition can be modulated in an active way 18–20. Both semiconductors and graphene are desirable in controlling the charge density and hence the electrical conductivity of the active material, but the tunable range is restricted by either the ultrathin two dimensional electron gas interface or the atomic thin graphene single layer, hence the experimental modulation depth is limited to ~0.5. Metamaterials with scalable geometry are excellent in amplitude, frequency and polarization manipulation of electromagnetic waves 21–23 but lose the dynamic real-time tunability. Lots of efforts are underway through effective combination of metamaterials with semiconductors 24–26 and graphene 27,28. The achievements are obvious but most of them suffer from narrow bandwidth because the electric transition and the subsequent coupling with the metamaterial resonance are always frequency dependent. To achieve ulrabroadband THz modulation without compromise of the modulation depth, speed, beam size as well as real time control, new active materials as well as precise metamaterial designs are required.

Domaines: Electromagnétisme

Chunrui Han, Edward Parrott, Georges Humbert, Aurelian Crunteanu, Emma Pickwell-Macpherson. Broadband modulation of terahertz waves through electrically driven hybrid bowtie antenna-VO2 devices. Scientific Reports, 2017, 7 (1), ⟨10.1038/s41598-017-13085-w⟩. ⟨hal-01611516⟩