Graphene Integrated Metamaterial Devices for Terahertz Modulation

Change log
Kindness, Stephen 

Terahertz (THz) research has experienced impressive progress in recent decades, with unique applications emerging in fields such as spectroscopy, communications, and imaging, all of which require fast and accurate control of the THz radiation properties. To unlock the full potential of THz radiation, it is essential to develop a catalogue of high speed, electrically controllable modular THz devices, which can be implemented with standardised sources to build versatile THz systems. Due to THz frequencies lying outside of the typical operation range for the mature microwave and photonic technologies, alternative approaches are required to solve the unique engineering problems associated with operating in this frequency regime. This thesis will look to design and develop novel device architectures using metamaterial arrays to strongly couple with THz radiation, whilst implementing electrically tunable graphene to modify the strength and nature of this interaction, leading to active control of the amplitude, phase, polarisation and frequency of THz radiation.

Chapter 1 discusses two THz sources: THz time-domain spectroscopy (THz-TDS) and THz quantum cascade lasers (THz-QCLs). A range of THz modulator architectures will then be discussed, focusing on the methods which involve converting passive metamaterial arrays into actively tunable devices, including the implementation of microelectromechanical systems (MEMS), photoactive semiconductors, and electrostatically tunable graphene.

Chapter 2 outlines the basic device design principles for the graphene integrated metamaterial devices shown in this thesis. The steps to build a basic split ring resonator (SRR) and graphene amplitude modulator device are discussed, starting with the design process using finite element electromagnetic simulation, before outlining the fabrication steps required to build the device, and finally performing the experimental procedure to test the device performance in a THz-TDS system. The SRR/graphene device discussed in this chapter demonstrates amplitude modulation depths in the region of 12 %, achieved by electrostatically tuning the graphene conductivity using a voltage range of 30 V.

Chapter 3 investigates more sophisticated graphene integrated metamaterial devices which involve lithographically selecting targeted areas of the metamaterial structure to be actively tuned by graphene. More interesting modulation effects, such as resonant frequency tuning, are achieved by integrating a coupled resonator metamaterial array with targeted graphene damping. A continuous tuning of the resonant frequency over a 60 GHz range, and binary tuning of over 200 GHz are achieved. Due to the highly dispersive nature of the coupled resonator array, dramatic phase and group delay modulation effects are also demonstrated.

Chapter 4 builds on the work from chapter 3, converting the coupled resonator and graphene devices into polarisation modulators by adding an intrinsic chirality into the metamaterial design. Two different devices are designed and fabricated, both achieving electrical control over the polarisation angle and ellipticity of the transmitted radiation. Polarisation tuning of up to 30 degrees is experimentally confirmed, with linear radiation successfully converted into perfectly circular radiation, achieving an ellipticity tuning range from 0.6 to 1.0.

Chapter 5 discusses the implementation of the active devices described in the previous chapters with quantum cascade lasers, for the conversion of a standard THz source into a highly versatile amplitude, polarisation and frequency controllable modular THz system. MHz modulation speeds of QCLs are achieved by directing a QCL output through the polarisation devices, with the polarisation angle actively tuned by up to 9 degrees. Dramatic modulation effects are achieved by externally coupling radiation back into a QCL using the devices as electrically tunable mirrors in an external cavity configuration. 100 % amplitude modulation of the QCL is achieved using this method, and the frequency of THz output is also successfully manipulated, with 20 GHz binary tuning of the laser output achieved.

Chapter 6 discusses future methods which could be used to enhance the modulation depths of the devices described in this thesis. A modified reflection modulation scheme is proposed and theoretically described which could greatly enhance the tuning range of the modulators. Preliminary TDS experiments are performed showing near 100 % amplitude modulation depths by employing this modulation scheme. Further to this, a continuous π/2 phase tuning range is achieved, corresponding to a 10 times improvement compared to the standard transmission and reflection modulation schemes. A modified QCL feedback scheme is also described which could utilize this enhanced phase modulation to achieve continuous tuning of the QCL output over 10s of GHz. A similar scheme is simulated to produce greatly enhanced polarization modulation depths with nearly 90 degrees polarization angle tuning, and near linear to perfectly circular polarisation modulation predicted. A potential modular TDS set-up is also proposed which involves implementing the polarisation modulators into the standard set-up, for fast material birefringence characterisation.

Ritchie, David
Terahertz, Graphene, Metamaterials, Modulation, Optoelectronic Devices
Doctor of Philosophy (PhD)
Awarding Institution
University of Cambridge
EPSRC (1635658)