Resonant Cavity-Enhanced Grphene Surface Plasmons For Infrared Spectroscopy

Surface-enhanced infrared absorption?SEIRA?spectroscopy allows the in-situ real-time,non-destructive,and label-free detection of specific molecules of interest,which shows great potential applications in various fields,such as food safety,environmental monitoring and biomedical sciences.This technology employs surface plasmons to break through the optical diffraction limit and thus enhance the interaction between the incident light and the molecule at the sub-wavelength scales,enabling it to accurately recognize the vibrational modes of molecules in the mid-infrared region.Compared to the metal surface plasmons,graphene surface plasmons possess the advantages of stronger field confinement and lower propagation loss at infrared frequencies.Particularly,such plasmonic responses can be actively tuned by an external gating voltage,a feature that is not available in traditional metal surface plasmons.This provides an effective strategy to realize the selective enhancements of molecular infrared vibrational fingerprints,significantly pushing the development of SEIRA spectroscopy.Despite this progress,the recent experiments show that the enhancement factor of SEIRA spectroscopy based on graphene surface plasmons is relatively small.Such an unsatisfied result is ascribed to the week interaction between graphene plasmonic mode and molecular vibrational modes,which is partly caused by the low infrared absorptions?1⁵%?of graphene plasmons.Therefore,improving the excitation efficiency of graphene surface plasmons to realize the high-sensitivity SEIRA spectroscopy technology is of great scientific significance and application value.To address the low enhancement factor of the current SEIRA spectroscopy based on graphene surface plasmons,this thesis proposed a new strategy to improve the enhancement factor of SEIRA spectroscopy based on resonant cavity-enhanced graphene surface plasmons.The thesis would be carried out from the aspects of theoretical model,numerical simulation and experimental research.The theoretical model of the resonant cavity-enhanced graphene surface plasmons was first established.Then the graphene surface plasmonic device with high absorption were designed and fabricated.Finally,the devices were used to detect the infrared spectra of the molecular film.The revealed results would lay the foundation for the realization of high-sensitivive SEIRA spectroscopy.The achieved results are listed as follows:?1?The enhancement mechanism of graphene surface plasmonic infrared spectroscopy was studied.By analyzing the interaction between graphene surface plasmons and the incident light,the quantitative expression of the mode energy of graphene surface plasmons was derived,which was then verified by the numerical simulation.Based on such expression,the relationship between the enhancement factor and the local electric field was further derived,which allows us to understand the small enhancement factor of SEIRA spectroscopy based on graphene surface plasmons.It is found that the enhancement factor exhibits a linear relationship with the absorption of the device and the electron relaxation time of graphene.The results could provide strategies to achieve plasmonic devices with large enhancement factor.?2?The quantitative expression of the graphene surface plasmonic absorption was derived in the"two-port"and"single-port"resonant systems according to the temporal coupled mode theory.Then a strategy was proposed to improve the infrared absorption of the device based on the resonant cavity-enhanced graphene surface plasmons.A physical model of graphene surface plasmonic device with high absorption was established.By numerically simulating the spectral characteristics of the device,the quantitative relationship between the coupling ratio,loss rate of the resonant system and the structural,material parameters of the device was revealed.The obtained results could provide theoretical guidance for the followed experiments.?3?Graphene surface plasmonic devices were prepared on SiO₂/Si substrate based on the micro-nano processing of graphene field effect transistor and the nano-patterning of graphene.The top-gate tuning mechanism of graphene Fermi energy was investigated using the semiconductor characteristic analysis system.It is observed that the transfer characteristic curve of graphene exhibits the unique feature of bipolar.The quantitative relationship between external gate-voltage and graphene Fermi energy was further derived.Then an electrically tuning infrared spectroscopy detection system was constructed and used to measure the absorption spectra of the devices.It is found that the graphene surface plasmons can be resonantly excited and the graphene plasmonic infrared absorption spectra of devices can be dynamically tuned by appling the gate-voltage.By optimizing the structural parameters of the substrate,the absorption of the device could be improved from 3%to 14%.?4?A resonant cavity-enhanced graphene surface plasmonic device was designed and fabricated to achieve high absorption of graphene plasmons,which enabled us to realize the high sensitivity detection of infrared spectrum of molecular film.The experimental results showed that by optimizing the length of the cavity and the number of graphene layers,the localized surface plasmonic absorption of 92%can be achieved in graphene nanoribbons for the first time,which is improved by a factor of 30 with respect to the reported graphene surface plasmonic devices.Such device with high absorption was further employed to detect the infrared spectrum signal of 8 nm thick polyethylene oxide?PEO?molecular film.It is demonstrated that the enhancement factor up to 162 can be achieved,which is improved by one order of magnitude compared to the reported graphene surface plasmonic devices.?5?A shared substrate was designed and fabricated to simultaneously realize surface-enhanced Raman spectroscopy?SERS?and surface-enhanced infrared absorption?SEIRA?spectroscopy of probed molecules with high sensitivity.This was done by combining the resonant cavity-enhanced graphene surface plasmonic device with metallic nanoparticles.It is demonstrated that Raman signal of Rhodamine 6G?R6G?solution is significantly enhanced with the detection limit as low as 10^-9 M and SERS enhancement factor up to 10⁵.Moreover,the infrared spectrum of PEO molecular film is enhanced by a factor of up to 170.Our study provides an effective solution to address the small SEIRA enhancement factor of the current shared substrates.