Far-Field Analysis of Plasmon Temperature-Dependence in Graphene Nanoresonators

Doped graphene behaves like a two-dimensional metallic sheet by exhibiting a robust plasmonic response from THz to mid-infrared frequencies. The extremely confined graphene plasmons (GP) that are supported by these graphene sheets have effective wavelengths that are up to two orders of magnitude shorter than the free-space wavelength. Despite the rising popularity of graphene plasmonics, the majority of GP research has been done at room temperature, and the temperature dependence of GP characteristics has not yet received much attention.<br/>We report on the far-field analysis of the temperature-dependent mid-IR plasmonic response of graphene nanoresonators. As the temperature decreases from 300 to 100K, we see that the intensities of the resonance peak in the extinction spectra increase by 10-76%. The observation has two main factors: First, temperature-induced hole doping of graphene strengthens its oscillator properties. Second, the enhanced quality factor of the resonances provides additional evidence that reduced plasmon damping caused by the suppression of temperature-dependent scattering mechanisms lengthens the graphene plasmon lifetime.<br/>We analyze the DC electrical characteristics of graphene while changing its temperature from 300 to 100K. The charge neutrality point (CNP) of graphene modifies from <i>V</i>_CNP = 62V at 300K to 76.5V at 100K. The additional hole doping, Δ<i>p </i>= 1.04×10¹²cm⁻², is what causes the shift of Δ<i>V</i>_CNP = 14.5V. The cooling of graphene alters the doping concentration in addition to lowering the carrier scattering rate, which increases carrier mobility. Additionally, the hole mobility exhibits a monotonic rise with lower temperatures.<br/>By analyzing the temperature-dependent normalized extinction spectra, we comprehensively study the temperature-dependent plasmonic characteristics of the graphene nanoresonators. Reduced temperature causes all extinction peaks to be more intense and causes a slight blueshift. This tendency can be attributed to the increased carrier density and mobility in graphene. Similar to the electrical measurement, the additional hole doping Δ<i>p </i>= 1.37×10¹²cm^–2 associated with cooling from 300 to 100K is present.<br/>By calculating the intensities of GP resonance for observed extinction spectra, it was possible to examine the temperature-dependent electrodynamic response of the nanoresonators. Most of the time, peak intensities rose as temperature decreased. The increase in the peak intensities ranged from 10 to 76% as the temperature was cooled from 300 to 100K. We attribute the rising peak intensities to the increased doping due to cooling.<br/>When the fermi energy decreases, the temperature dependence of the peak intensities gets stronger. This is most likely due to the fact that at lower carrier densities, the temperature-induced doping becomes more important than the electrostatic doping level. At the maximum gate bias, we also evaluate how the peak intensity varies with temperature for different resonator widths. The peak intensity in this regime is predicted to be mostly dependent on the plasmon scattering rate. Narrower resonators are predicted to contribute more to the temperature-independent extrinsic scattering rate. Indeed, compared to a wider one, the normalized extinction peaks in the narrower resonators are substantially less temperature sensitive.<br/>We have studied the far-field temperature-dependent plasmonic response of graphene nanoresonators. The graphene plasmon resonance intensifies when the temperature decreases from 300 to 100K. The increased <i>p</i> doping brought on by cooling and the inhibition of the phonon-mediated scattering mechanism are both responsible for the increase in GP peak intensity. Our findings pave the way for enhanced graphene plasmonic devices that operate at cryogenic temperatures by illuminating significant but previously unrecognized temperature effects on tunable plasmons in large-area graphene-SiO₂ systems.