Ultra-confined acoustic THz graphene plasmons revealed by photocurrent nanoscopy (1601.05753v4)

Abstract: Terahertz (THz) fields are widely applied for sensing, communication and quality control. In future applications, they could be efficiently confined, enhanced and manipulated - well below the classical diffraction limit - through the excitation of graphene plasmons (GPs). These possibilities emerge from the strongly reduced GP wavelength, lp, compared to the photon wavelength, l0, which can be controlled by modulating the carrier density of graphene via electrical gating. Recently, GPs in a graphene-insulator-metal configuration have been predicted to exhibit a linear dispersion (thus called acoustic plasmons) and a further reduced wavelength, implying an improved field confinement, analogous to plasmons in two-dimensional electron gases (2DEGs) near conductive substrates. While infrared GPs have been visualised by scattering-type scanning near-field optical microscopy (s-SNOM), the real-space imaging of strongly confined THz plasmons in graphene and 2DEGs has been elusive so far - only GPs with nearly free-space wavelength have been observed. Here we demonstrate real-space imaging of acoustic THz plasmons in a graphene photodetector with split-gate architecture. To that end, we introduce nanoscale-resolved THz photocurrent near-field microscopy, where near-field excited GPs are detected thermoelectrically rather than optically. The on-chip GP detection simplifies GP imaging, as sophisticated s-SNOM detection schemes can be avoided. The photocurrent images reveal strongly reduced GP wavelengths (lp = l0/66), a linear dispersion resulting from the coupling of GPs with the metal gate below the graphene, and that plasmon damping at positive carrier densities is dominated by Coulomb impurity scattering. Acoustic GPs could thus strongly benefit the development of deep subwavelength-scale THz devices.

• The paper demonstrates a nanoscale-resolved THz photocurrent method to image acoustic graphene plasmons with extreme field confinement.
• It utilizes a split-gate h-BN encapsulated graphene photodetector that enables independent control of carrier densities to form pn junctions.
• Numerical simulations confirm a 70-fold reduction in plasmon wavelengths and validate the linear dispersion predicted by analytical models.

Analyzing Acoustic THz Graphene Plasmons via Photocurrent Nanoscopy

The paper "Acoustic THz graphene plasmons revealed by photocurrent nanoscopy" presents a comprehensive paper on acoustic terahertz (THz) graphene plasmons through the innovative use of photocurrent nanoscopy. The work pivots on a demonstration of the real-space imaging of acoustic THz plasmons in a graphene photodetector with a split-gate architecture, a significant leap forward in observing these phenomena with reduced reliance on traditional optics-based methods like scattering-type scanning near-field optical microscopy (s-SNOM).

Key Findings and Methodology

The authors employ a sophisticated nanoscale-resolved THz photocurrent near-field microscopy method, where graphene plasmons (GPs) are excited and subsequently detected thermoelectrically. This approach circumvents conventional optical detection, streamlining the imaging process of GPs and demonstrating its applicability in mapping acoustic plasmon modes. The paper reveals substantially diminished GP wavelengths (approximately λp ≈ λ0/66) and offers insights into the linear dispersion that arises from interactions between the GPs and the underlying metal gate. Interestingly, the research identifies Coulomb impurity scattering as a primary damping mechanism at positive carrier densities.

The innovative split-gate photodetector design utilizes a h-BN-encapsulated graphene sheet situated above paired AuPd gates. By varying the gate voltages individually, researchers achieved control over carrier concentrations on either side of a small inter-gate gap, effectively generating pn-junctions conducive to plasmon mapping activities. This strategic experimental setup facilitated the discovery and detailed analysis of the thermoelectric and physical responses of GPs in graphene.

Analytical and Numerical Approaches

The findings substantiate the linear dispersion of THz acoustic plasmons via analytical models that incorporate the heterostructure’s distinct layer compositions and reflect the experimental acoustic plasmon dispersion. The correlation between experimental data and theoretical calculations highlights the role of the heterostructure in GP confinement and suggests a 70-fold reduction in GP wavelengths compared to free-standing graphene scenarios.

Numerical simulations further examine electromagnetic field distributions and confinement properties of acoustic THz plasmons. These simulations reveal significant field localization within the h-BN layer, characterizing an extreme plasmonic field confinement that surpasses the capabilities of conventional dielectric loading. The paper quantitatively verifies field enhancement ratios and establishes the subwavelength vertical field confinement within the heterostructure—a pivotal advancement with potential applications in developing nanoscale THz devices.

Implications for Future Research and Applications

The identification and characterization of acoustic THz GPs emphasize their potential for developing THz-frequency devices, including sensors, modulators, and waveguides. Furthermore, these plasmons’ properties—strongly confined electromagnetic fields and a linear dispersion relation—pose opportunities for novel explorations in light-matter interactions at the nanoscale.

The practical implications of this research are far-reaching. With the capability for purely electrical GP detection, on-chip graphene plasmonic functionalities could be significantly enhanced. This improvement paves the way for more efficient and integrated THz technologies—the on-chip functionalities fostered by such developments could redefine device architectures, enhancing operation within the coveted THz frequency range. Additionally, the introduced imaging technique could catalyze the exploration of local THz photocurrents in various 2D materials, broadening the horizons of semiconductor research and applications.

In conclusion, this paper substantially enriches the understanding of acoustic THz GPs in graphene, proposing groundbreaking methods of their detection and characterization, and setting a foundation for furthering both theoretical research and practical device innovation in the field of nanophotonics and THz technologies.