Development of Active Terahertz Modulators for use with Quantum Cascade Lasers and Near-Field Imaging of their Individual Resonators
This thesis describes the development of active terahertz modulators based on arrays of coupled resonators loaded with monolayer graphene. The structures affect the amplitude, phase, and polarisation of the transmitted and reflected radiation. These devices were simulated using finite element modelling (FEM) software, fabricated using standard cleanroom lithographic techniques and characterised using terahertz (THz) time-domain spectroscopy (TDS). Devices designed to modulate the phase and frequency of the transmitted and reflected THz radiation were fabricated and characterised operating at ∼ 1.5 THz and 3 THz. Both the 3 THz and 1.5 THz devices were coupled with THz quantum cascade lasers (QCLs). The phase modulators were integrated with a partially suppressed QCL to form an external cavity. The devices acted as optoelectronic mirrors. By changing the bias on the device, the reflected phase changed the effective length of the cavity and affected the mode competition within the laser. The same experiment was also conducted with a gold mirror on a micrometre stage, and the position was changed to vary the external cavity length. They produced commensurate results. These results were compared with simulations of the external cavity and QCL set-up using reduced rate equations, which showed good agreement. Different devices that use coupled bright and dark resonators with capacitative coupling and some that use a double layer of resonators and magnetic coupling were simulated and fabricated. They were designed to change the polarisation of a linear THz beam, changing either the angle of rotation of the linear polarisation or converting from linear to elliptical polarisation, depending on the frequency. These devices were then coupled with THz QCLs to manipulate the linear polarisation output. For further improvement and optimisation of active metasurface devices, we need to understand the behaviour of the individual elements in response to THz incident illumination and their interaction with each other, which is not possible with FEM software. In response, metasurface devices were investigated with room temperature aperture and scattering scanning near-field optical microscopes (SNOM), which can provide subwavelength resolution. Furthermore, they showed the bonding and anti-bonding modes of the sub-wavelength features. For future direct integration of metasurfaces with the QCL facet inside the cryostat, the development of a scattering SNOM (s-SNOM) operating at cryogenic temperatures (∼10 K) was started. However, this proved to have multiple unforeseen difficulties, and much work was undertaken to reduce the vibration so that topography could be taken and so that it functioned as a cryogenic atomic force microscope (AFM).
AFM, graphene, Modulators, Nanoscopy, QCL, SNOM, Terahertz
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