David Jessop, Christian W. Sol, Long Xiao, Stephen Kindness, Philipp Braeuninger-Weimer, Hungyen Lin, Jonathan Griffiths, Yuan Ren, Varun Kamboj, Stephan Hofmann, J. Axel Zeitler, Harvey Beere, David Ritchie, Riccardo Degl'Innocenti
The growing interest in terahertz (THz) technologies in recent years has seen a wide range of demonstrated applications, spanning from security screening, non-destructive testing, gas sensing, to biomedical imaging and communication. Communication with THz radiation offers the advantage of much higher bandwidths than currently available, in an unallocated spectrum. For this to be realized, optoelectronic components capable of manipulating THz radiation at high speeds and high signal-to-noise ratios must be developed. In this work we demonstrate a room temperature frequency dependent optoelectronic amplitude modulator working at around 2 THz, which incorporates graphene as the tuning medium. The architecture of the modulator is an array of plasmonic dipole antennas surrounded by graphene. By electrostatically doping the graphene via a back gate electrode, the reflection characteristics of the modulator are modified. The modulator is electrically characterized to determine the graphene conductivity and optically characterization, by THz time-domain spectroscopy and a single-mode 2 THz quantum cascade laser, to determine the optical modulation depth and cut-off frequency. A maximum optical modulation depth of ~ 30% is estimated and is found to be most (least) sensitive when the electrical modulation is centered at the point of maximum (minimum) differential resistivity of the graphene. A 3 dB cut-off frequency > 5 MHz, limited only by the area of graphene on the device, is reported. The results agree well with theoretical calculations and numerical simulations, and demonstrate the first steps towards ultra-fast, graphene based THz optoelectronic devices.
Varun Kamboj, Philipp Braeuninger-Weimer, Piran Kidambi, David Jessop, Angadjit Singh, Juraj Sibik, Yuan Ren, Stephan Hofmann, J. Axel Zeitler, Harvey Beere, David Ritchie
We report the characterization of centimeter sized graphene field-effect transistors with ionic gating which enables active frequency and amplitude modulation of terahertz (THz) radiation. Chemical vapour deposited graphene with different grain sizes were studied using THz time-domain spectroscopy. We demonstrate that the plasmonic resonances intrinsic to graphene can be tuned over a wide range of THz frequencies by engineering the grain size of the graphene. Further frequency tuning of the resonance, up to ~65 GHz, is achieved by electrostatic doping via ionic gating. These results present the first demonstration of tuning the intrinsic plasmonic resonances in graphene.
A terahertz quantum cascade laser has been realized from an isotropic disordered hyperuniform design. Such a system
presents a photonic band-gap although it is characterized by an efficient depletion of the long range order. Hyperuniform
patterns allow greater versatility in engineering band gaps in comparison to standard photonic-crystal materials.
Bidimensional hyperuniform patterns were simulated for hexagonal tiles composed of high refractive index disks merged
in a low dielectric constant polymeric matrix. Based on this design, quantum cascade lasers were fabricated by standard
photolithography, metal evaporation, lift-off and dry-etching techniques in a half-stack bound to continuum active region
emitting around 2.9 THz.
The integration of quantum cascade lasers with devices capable of efficiently manipulating terahertz light represents a fundamental step for many different applications. Split-ring resonators, subwavelength metamaterial elements exhibiting broad resonances that are easily tuned lithographically, represent the ideal route to achieve such optical control of the incident light. We have realized a design based on the interplay between metallic split rings and the electronic properties of a graphene monolayer integrated into a single device. By acting on the doping level of graphene, an active modulation of the optical intensity was achieved in the frequency range between 2.2 and 3.1 THz, with a maximum modulation depth of 18%.
The integration of quantum cascade lasers with devices capable of efficiently manipulating terahertz light, represents a fundamental step for many different applications. Split-ring resonators, sub-wavelength metamaterial elements exhibiting broad resonances that are easily tuned lithographically, represent the ideal route to achieve such optical control of the incident light. We have realized a design based on the interplay between metallic split rings and the electronic properties of a graphene monolayer integrated into a single device. By acting on the doping level of graphene, an active modulation of the optical intensity was achieved in the frequency range between 2.2 THz and 3.1 THz, with a maximum modulation depth of 18%.
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