Research in the terahertz (THz) band, which is broadly defined as 0.1-10 THz, is an active area of research driven by applications in sixth generation (6G) and beyond for communications, spectroscopy, imaging, and sensing. In order to exploit the full potential of all these applications, fast integrated circuitry is required. This work revolves around removing this bottleneck. Achievement of efficient dynamic modulation requires the implementation of active material. Amongst many different approaches to achieve active modulation, metamaterials/graphene-based technology is establishing itself as a benchmark for THz operation due to its versatility, power efficiency, small footprint, and integration capabilities. Our devices have been modulated all-electronically, as described in Chapters 4 and 6, and all-optically as reported in Chapter 5.
The fabrication of the novel design based on metamaterial (MM) and graphene
for amplitude, phase, and polarization modulations is reported in Chapter 3.
The optoelectronic behaviour of this modulator is tested in a THz time-domain
spectroscopy (THz-TDS) setup as demonstrated in Chapter 4. By choosing the
appropriate THz-TDS setup configuration, a spectral amplitude extinction ratio of >10 dB (>93%) at the resonant frequency of 0.8 THz is demonstrated. The spectral phase of THz radiations is actively tuned by >27o at 0.62 THz frequency. Linear to circular polarization conversion with nearly 100% of conversion efficiency is reported demonstrating almost an independent control of circular dichroism (CD) and optical activity (OA) as mentioned explicitly in Chapter 6. Dynamic changes of ellipticity are reported to exceed 0.3 in ratio at resonance. The OA of transmitted THz radiations is continuously rotated by >21.5o at 0.71 THz by varying the gate. These values are in line with acquainted literature with graphene-based or 2-dimensional electron gas modulators but with higher reconfiguration speed. The helicity, either right or left circular polarization states, of elliptical waves can be controlled. These results are of
great importance for fundamental research of polarization spectroscopy, polarization imaging, or THz applications in the pharmaceutical and biomedical fields.
An all-electronic controlled metamaterial-based THz modulator is demonstrated
to achieve a recorded operating speed >3 GHz which is limited by the available
instrumentation as illustrated in Section 7.1. The achievements in the modulation speed (in GHz range), amplitude extinction ratio (>10 dB), phase shift tuning (27o), and nearly decoupled control of OA and CD of THz waves are the key values of this device, which is undoubtedly meaningful for communication applications and has a certain impact on the THz modulator technology.
The achieved GHz modulation speed of this hybrid MMs/graphene device is
within very good agreement with previous literature reported on pristine graphene. This result provides an upper intrinsic limit of the maximum reconfiguration speed of these devices to 100s of GHz and, at the same time, reinforces the use of metamaterial/graphene optoelectronic devices for ultrafast modulation of terahertz waves. This overall remarkable performance of an optoelectronic modulator based on metamaterial/graphene resonators is capable of efficiently modulating THz radiation all-electronically with GHz-reconfiguration speed. It is worth highlighting that this exceptionally high reconfiguration speed, the highest reported so far to the best of our
knowledge for a graphene-based integrated device, was not achieved at the expense of the other performances, e.g. amplitude and polarization modulation depths. These results represent great progress for several terahertz research and ultrafast photonic applications, such as the realization of fast deep, and efficient THz circuitry for the investigation of exotic quantum phenomena, wireless communications, and laser diodes stabilization in quantum electronics.
