Optical nano-imaging of gate-tuneable graphene plasmons (1202.4996v2)

Abstract: The ability to manipulate optical fields and the energy flow of light is central to modern information and communication technologies, as well as quantum information processing schemes. However, as photons do not possess charge, controlling them efficiently by electrical means has so far proved elusive. A promising way to achieve electric control of light could be through plasmon polaritons - coupled excitations of photons and charge carriers - in graphene. In this two-dimensional sheet of carbon atoms, it is expected that plasmon polaritons and their associated optical fields can be readily tuned electrically by varying the graphene carrier density. While optical graphene plasmon resonances have recently been investigated spectroscopically, no experiments so far have directly resolved propagating plasmons in real space. Here, we launch and detect propagating optical plasmons in tapered graphene nanostructures using near-field scattering microscopy with infrared excitation light. We provide real-space images of plasmonic field profiles and find that the extracted plasmon wavelength is remarkably short - over 40 times smaller than the wavelength of illumination. We exploit this strong optical field confinement to turn a graphene nanostructure into a tunable resonant plasmonic cavity with extremely small mode volume. The cavity resonance is controlled in-situ by gating the graphene, and in particular, complete switching on and off of the plasmon modes is demonstrated, thus paving the way towards graphene-based optical transistors. This successful alliance between nanoelectronics and nano-optics enables the development of unprecedented active subwavelength-scale optics and a plethora of novel nano-optoelectronic devices and functionalities, such as tunable metamaterials, nanoscale optical processing and strongly enhanced light-matter interactions for quantum devices and (bio)sensors.

• The paper experimentally images propagating and localized graphene plasmons using s-SNOM, achieving real-space resolution with over 40× wavelength confinement.
• The paper demonstrates electrical tuning of plasmon wavelengths via carrier density modulation, with experimental results aligning with theoretical predictions.
• The paper introduces a design paradigm for opto-electronic devices by leveraging substrate permittivity and nanostructure engineering in graphene.

Optical Nano-Imaging of Gate-Tuneable Graphene Plasmons: An Overview

This paper presents the experimental realization and imaging of gate-tuneable graphene plasmons using scattering-type scanning near-field optical microscopy (s-SNOM) at infrared frequencies. The research advances the understanding of plasmonic properties in two-dimensional (2D) materials, specifically graphene, by providing real-space images of propagating and localized plasmon modes with unprecedented resolution.

Graphene plasmons exhibit a high degree of optical field confinement and tunability due to the material's unique electronic properties. The authors utilized tapered graphene nanostructures to probe and visualize these plasmons. Through s-SNOM, they launched and detected plasmon modes, revealing a plasmon wavelength significantly shorter—over 40 times—than the illumination wavelength. This level of confinement is attributed to the large wave vector mismatch between graphene plasmons and free-space photons, which is a fundamental challenge addressed by the paper.

The paper's key achievements include:

• Real-space imaging of propagating graphene plasmons and their interference patterns in tapered graphene ribbons.
• Demonstration of carrier-density-dependent tuning of plasmon wavelengths via electrical gating.
• Introduction of a new paradigm for designing opto-electronic devices with graphene, emphasizing its role in subwavelength-scale optics.

The research systematically investigates the relationship between plasmon wavelength and substrate permittivity. By employing different substrate dielectric constants, the authors achieved significant variation in the plasmon wavelength, which aligns well with theoretical predictions based on the fine-structure constant and substrate permittivity. The paper further demonstrated localized plasmonic modes within nanostructures, offering resonant conditions that enhance near-field signals and pave the way for applications such as nanoscale optical transistors, tunable metamaterials, and potential quantum sensing devices.

The authors leverage a numerical model to simulate local density of optical states (LDOS) calculations, aligning closely with experimental images. These simulations offer insights into plasmon propagation distances and the factors affecting plasmon losses, such as substrate interactions and intrinsic material properties.

The implications of these findings are significant in the fields of nano-optics and optoelectronics. Graphene-based plasmonics could enable a new generation of optical devices capable of dynamic control and tuning through electrical means. The work encourages further exploration of higher mobility graphene to enhance plasmon propagation distances, thus broadening practical applications in telecommunications and information processing.

The capability of directly observing and manipulating graphene plasmons holds promise for furthering graphene's use in practical device scenarios. The advancement positions graphene as a versatile material for developing optically active, tunable nanophotonic devices, thereby fostering potential developments in optical computing and nanoscale circuits. Future research may focus on improving graphene quality and exploring novel device architectures to leverage the unique plasmonic properties of graphene for real-world applications.