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.

Highly confined low-loss plasmons in graphene-boron nitride heterostructures

The excitation of surface-phonon-polariton (SPhP) modes in polar dielectric crystals and the associ- ated new developments in the field of SPhPs are reviewed. The emphasis of this work is on providing an understand- ing of the general phenomenon, including the origin of the Reststrahlen band, the role that optical phonons in polar dielectric lattices play in supporting sub-diffrac- tion-limited modes and how the relatively long opti- cal phonon lifetimes can lead to the low optical losses observed within these materials. Based on this overview, the achievements attained to date and the potential tech- nological advantages of these materials are discussed for localized modes in nanostructures, propagating modes on surfaces and in waveguides and novel metamaterial designs, with the goal of realizing low-loss nanophoton- ics and metamaterials in the mid-infrared to terahertz spectral ranges.

Low-loss, infrared and terahertz nanophotonics using surface phonon polaritons

With the increasing requirements for microelectromechanical systems (MEMS) regarding stability, miniaturization and integration, novel materials such as wide band gap semiconductors are attracting more attention. Polycrystalline SiC has first been implemented into Si micromachining techniques, mainly as etch stop and protective layers. However, the outstanding properties of wide band gap semiconductors offer many more possibilities for the implementation of new functionalities. Now, a variety of technologies for SiC and group III nitrides exist to fabricate fully wide band gap semiconductor based MEMS. In this paper we first review the basic technology (deposition and etching) for group III nitrides and SiC with a special focus on the fabrication of three-dimensional microstructures relevant for MEMS. The basic operation principle for MEMS with wide band gap semiconductors is described. Finally, the first applications of SiC based MEMS are demonstrated, and innovative MEMS and NEMS devices are reviewed.

http://iopscience.iop.org/article/10.1088/0022-3727/40/20/S19/pdf

Group III nitride and SiC based MEMS and NEMS: materials properties, technology and applications

For several decades, thermoelectric advancements have largely relied on the reduction of lattice thermal conductivity (κL ). According to the Boltzmann transport theory of phonons, κL mainly depends on the specific heat, the velocity, and the scattering of phonons. Intensifying the scattering rate of phonons is the focus for reducing the lattice thermal conductivity. Effective scattering sources include 0D point defects, 1D dislocations, and 2D interfaces, each of which has a particular range of frequencies where phonon scattering is most effective. Because acoustic phonons are generally the main contributors to κL due to their much higher velocities compared to optical phonons, many low-κL thermoelectrics rely on crystal structure complexity leading to a small fraction of acoustic phonons and/or weak chemical bonds enabling an overall low phonon propagation velocity. While these thermal strategies are successful for advancing thermoelectrics, the principles used can be integrated with approaches such as band engineering to improve the electronic properties, which can promote this energy technology from niche applications into the mainstream.

Manipulation of Phonon Transport in Thermoelectrics.

Thermo-mechanical behavior of low-dimensional systems: The local bond average approach

In this paper we present a methodology to affect the stability of polytypes formation during heteroepitaxial growth of SiC on Si. This methodology is based on the investigation of growth related parameters. These parameters involve substrate temperature, effect of impurities on surface diffusion, strain, and super-saturation conditions as solved by using SSMBE growth (Solid Source molecular beam epitaxy).

SiC Polytype Stability Influenced by Ge Impurities

In this work, a trilayer graphene is used as a thin non dielectric spacer with a high index of refraction, between Au film
and Au NPs. Encouraged by the sharpness of the localized surface plasmon resonance LSPR induced by this system, we
performed sensitivity measurements to refractive index change in the surrounding medium of the sensor. The presence of
graphene led to both higher sensitivity and sharper full width at half maximum FWHM and thus higher figure of merit
FOM (2.8) compared to the system without graphene (2.1).

Enhanced gold film-coupled graphene-based plasmonic nanosensor

SiC sputtered and e-beam evaporated layers have been deposited on 4H-SiC substrates. Recrystallization has been tried by high temperature annealing with two plateaus at 1400°C and 1700°C. The crystallinity was systematically investigated by non destructive near-field microscopy techniques, particularly adapted to the thin thickness of our layers. Good recrystallization is confirmed only on the e-beam evaporated layers and after the higher annealing plateau by preserving 4H substrate polytype.

SiC structural characterization by non destructive near-field microscopy techniques

Surface phonon-polariton, surface plasmon-polariton, and surface phonon–plasmon-polariton are evanescent electromagnetic waves confined to the surfaces of different classes of materials, which gives each of them particular characteristics suitable for diverse applications. Natural or forced injection of free carriers in a dielectric may change the surface phonon-polariton into a surface phonon–plasmon-polariton. Understanding this effect provides an insight into the fundamental physics of surface electromagnetic waves on dielectrics and offers tools that can be used to develop new technologies. In this contribution, we experimentally study the transition from surface phonon-polariton to surface phonon–plasmon-polariton on a yttrium-doped aluminum nitride polycrystalline substrate by thermal injection of free carriers. We perform this study using reflectivity measurements in the far- and mid-infrared spectral range and at a variable temperature, taking the necessary precautions to eliminate any errors that may arise from measurement artifacts and inaccurate analysis of the spectra. We demonstrate that thermal injection of a significant free carrier density can tune the surface phonon-polariton into a much shorter mean free path surface phonon–plasmon-polariton.

Transition from surface phonon-polariton to surface phonon–plasmon-polariton by thermal injection of free carriers

Mid-IR nanoscopy has proved its ability to recognize material at nanometer scale through spectroscopic or even single wavelength imaging [1,2]. A high optical contrast can indeed be obtained between different materials as the permittivity of solid materials exhibit strong variations near phononic resonances, usually below 1000cm−1. One limitation of the technique is that a weak contrast is obtained if the permittivities of two materials are too close, which can be the case for example far from phonon resonances. As another example, doping imaging in silicon is difficult still difficult at 10.6µm, although the free electrons or holes can modify notably the permittivities as shown in figure 1. It has been shown previously that even longer wavelengths seem to be necessary to image unambiguously different doped zones [3,4] at least with common near-field probes. However, in a recent paper we have calculated that the phonon confinement occurring in decananometric scale objects such as near-field probes can lead to a drastic modification of the probe's permittivity [5]. This effect can be very useful as for a range of probe permittivity, much stronger contrast can be expected. To experimentally demonstrate these theoretical expectations, we have built a mid-IR near field microscope and imaged doped Si gratings with a period of 2 µm (fabricated in CEA-LETI) with carefully prepared tungsten tip stuck on a tuning fork. The detection has been done using tips with different radius (30 nm to 100 nm) and a very strong contrast between P doped and P+ doped can be observed for a variety of small-radius probes. An example of highly contrasted profile obtained on the doped silicon grating is shown in figure2.

Michel Kazan

Papers

SiC Polytype Stability Influenced by Ge Impurities

Enhanced gold film-coupled graphene-based plasmonic nanosensor

SiC structural characterization by non destructive near-field microscopy techniques

Transition from surface phonon-polariton to surface phonon–plasmon-polariton by thermal injection of free carriers

Mid-IR imaging of doped silicon gratings at a decananometer scale