Exploring Graphene Physics for Optical Sensing
Since the discovery of graphene in 2004, it has drawn significant attention in many different research or application fields due to its extraordinary electrical and optical properties. As a two-dimensional material, both graphene's electrical and optical properties can be dramatically modified due to small perturbations from the surrounding environment. Therefore, graphene has great potential as active medium for versatile and sensitive sensors. Early studies have demonstrated excellent sensitivity of graphene field-effect transistor for gas molecule sensing and action potential detection. As will be shown in this dissertation, graphene optical sensor can also be designed to have good sensitivity and enables new possibilities of optical spectroscopic measurement as well as opto-electronic studies. With the versatility of graphene, we show that graphene provides a great platform for electrochemical optical sensing as well as opto-electronic sensor in bio-electric detection.
There are three major topics in this dissertation. The first one is an extensive study of
the intrinsic optical phenomena of Dirac electrons in monolayer graphene. The chemical potential of graphene can be tuned efficiently by electric field due to the two-dimensional nature and the unusual band structure of graphene. We study gate-dependence of both interband and intraband excitations with THz, infrared to visible optical spectroscopy. This work provides a first glance at the fundamental linear optical properties of graphene and forms the basis of application of graphene optical sensing.
In the second part, a tunable mid-infrared laser for graphene optical sensor application was developed. Lasers are powerful tools in sensing techniques since due to coherent behavior as well as the ability of performing imaging and spectroscopy. We develope our own mid-infrared laser source through parametric nonlinear down-conversion by two different methods, namely ,difference frequency generation and synchronized pumped optical parametric oscillator. Both methods generate strong, coherent, ultrafast mid-infrared light source from 2.4 to 4.7μm covering most of the molecular vibrational resonances. These light generation setups are ideal for infrared spectroscopy of molecules and will be employed to perform graphene-enhanced spectroscopy in the later chapters.
At last, the possibility of graphene optical sensing in both electrochemical and bio-electrical contexts are explored. As mentioned earlier, graphene's versatility makes it not only a remarkable transparent electrode, but also an interesting electro-chemical platform to study molecular dynamics during a chemical reaction. Here we demonstrate that, with micro-fabrication of graphene, we create an attractive platform for vibrational spectroscopy, which has interface specificity, sub-monolayer detection sensitivity and imaging capability, at the electrolyte/electrode interfaces. In another direction, we study the potential application of graphene in bio-electric detection and imaging. Research into the development of bio-electric imaging has been quite extensive in the past decade since brain research starts to allow us decoding the basic signaling through these imaging techniques. We show that, by integrating graphene with specially designed waveguides, we achieve sensitive local charge detectors and could have potential application in neuro-science for wide-field and highly-parallel action potential sensing.