Highly confined low-loss plasmons in graphene-boron nitride heterostructures (1409.5674v1)

Abstract: Graphene plasmons were predicted to possess ultra-strong field confinement and very low damping at the same time, enabling new classes of devices for deep subwavelength metamaterials, single-photon nonlinearities, extraordinarily strong light-matter interactions and nano-optoelectronic switches. While all of these great prospects require low damping, thus far strong plasmon damping was observed, with both impurity scattering and many-body effects in graphene proposed as possible explanations. With the advent of van der Waals heterostructures, new methods have been developed to integrate graphene with other atomically flat materials. In this letter we exploit near-field microscopy to image propagating plasmons in high quality graphene encapsulated between two films of hexagonal boron nitride (h-BN). We determine dispersion and particularly plasmon damping in real space. We find unprecedented low plasmon damping combined with strong field confinement, and identify the main damping channels as intrinsic thermal phonons in the graphene and dielectric losses in the h-BN. The observation and in-depth understanding of low plasmon damping is the key for the development of graphene nano-photonic and nano-optoelectronic devices.

• The paper demonstrates unprecedented low plasmon damping and strong field confinement in graphene-hBN heterostructures.
• It reveals subwavelength plasmon modes with wavelengths as short as 70 nm, achieving confinement 150 times smaller than free-space light.
• Experimental findings align with theoretical models that attribute damping primarily to intrinsic thermal phonons and dielectric losses.

Highly Confined Low-Loss Plasmons in Graphene--Boron Nitride Heterostructures

The paper "Highly confined low-loss plasmons in graphene--boron nitride heterostructures" explores the integration of high-quality graphene with hexagonal boron nitride (h-BN) films using van der Waals heterostructures. This paper leverages scattering-type scanning near-field optical microscopy (s-SNOM) to characterize and understand plasmons with exceptional field confinement and low propagation loss in these heterostructures. The key achievement of this work is the demonstration of unprecedented low damping of graphene plasmons, essential for the advancement of graphene-based nano-photonic and nano-optoelectronic devices.

Summary of Key Findings

1. Plasmon Confinement and Damping: The research reveals that graphene encapsulated in h-BN exhibits low plasmon damping alongside strong field confinement, surpassing previous graphene-based systems. The intrinsic thermal phonons in graphene, combined with dielectric losses in h-BN, are identified as the primary damping channels.
2. Subwavelength Confinement: The paper reports a plasmon wavelength as low as 70 nm, achieving a field confinement that is 150 times smaller than the free-space light wavelength. This results in a volume confinement of propagating optical fields by a factor of approximately $10^7$, a substantial metric in plasmonics.
3. Damping Mechanisms: An analysis of the damping mechanisms reveals that the impurity scattering is insignificant as a damping source. Instead, the combination of intrinsic thermal phonon interactions in graphene and dielectric losses from the h-BN environment is consistent with the observed low plasmon damping measured across varying carrier densities and excitation frequencies.
4. Graphene Carrier Density and Purity: The heterostructure, achieved through a polymer-free van der Waals assembly technique, maintains high graphene mobility by reducing disorder. This cleanliness allows an exploration of the optical response across a wide range of carrier densities, confirming that intrinsic thermal phonons limit the carrier transport mobility.
5. Experimental and Theoretical Agreement: The paper achieves an excellent match between experimental data and theoretical models that incorporate complex factors like thin-film effects and graphene's nonlocal conductivity. These findings point to the theoretical model's robustness in predicting graphene plasmon behavior in such dielectric environments.

Implications and Future Directions

The encapsulation of graphene in h-BN substrates provides a distinct route for designing next-generation plasmonic devices. The minimized damping and high field confinement evident in this paper are pivotal for developing highly efficient graphene-based photonic components such as nano-optoelectronic switches and single-photon emitters. Moreover, these findings highlight the potential for improved interaction between graphene plasmons and h-BN phonon polaritons, paving the way for advanced tunable metamaterials.

The paper suggests that further reductions in plasmon damping towards the ultimate intrinsic limit may be achievable through mitigating dielectric losses in the h-BN. This could involve further refinement of the encapsulation process or alternative environmental modifications.

In summary, the integration of graphene with h-BN harnesses unique plasmonic properties beneficial for numerous applications, making it a key focus area in nano-photonics research. This work establishes a foundational understanding and showcases significant strides towards practical applications of graphene plasmons. Future research could extend these findings by exploring different substrates and processing techniques to optimize device performance further.