Joshua D. Caldwell

Bilayer graphene grown on 4H-SiC (0001) step-free mesas.

We demonstrate the first successful growth of large-area (200 × 200 μm(2)) bilayer, Bernal stacked, epitaxial graphene (EG) on atomically flat, 4H-SiC (0001) step-free mesas (SFMs) . The use of SFMs for the growth of graphene resulted in the complete elimination of surface step-bunching typically found after EG growth on conventional nominally on-axis SiC (0001) substrates. As a result heights of EG surface features are reduced by at least a factor of 50 from the heights found on conventional substrates. Evaluation of the EG across the SFM using the Raman 2D mode indicates Bernal stacking with low and uniform compressive lattice strain of only 0.05%. The uniformity of this strain is significantly improved, which is about 13-fold decrease of strain found for EG grown on conventional nominally on-axis substrates. The magnitude of the strain approaches values for stress-free exfoliated graphene flakes. Hall transport measurements on large area bilayer samples taken as a function of temperature from 4.3 to 300 K revealed an n-type carrier mobility that increased from 1170 to 1730 cm(2) V(-1) s(-1), and a corresponding sheet carrier density that decreased from 5.0 × 10(12) cm(-2) to 3.26 × 10(12) cm(-2). The transport is believed to occur predominantly through the top EG layer with the bottom layer screening the top layer from the substrate. These results demonstrate that EG synthesized on large area, perfectly flat on-axis mesa surfaces can be used to produce Bernal-stacked bilayer EG having excellent uniformity and reduced strain and provides the perfect opportunity for significant advancement of epitaxial graphene electronics technology.

Perfect interferenceless absorption at infrared frequencies by a van der Waals crystal

Citation: Baranov, D. G., Edgar, J. H., Hoffman, T., Bassim, N., & Caldwell, J. D. (2015). Perfect interferenceless absorption at infrared frequencies by a van der Waals crystal. Physical Review B, 92(20), 6. doi:10.1103/PhysRevB.92.201405

Nanoscale Mapping and Spectroscopy of Nonradiative Hyperbolic Modes in Hexagonal Boron Nitride Nanostructures

The inherent crystal anisotropy of hexagonal boron nitride (hBN) provides the ability to support hyperbolic phonon polaritons, that is, polaritons that can propagate with very large wave vectors within the material volume, thereby enabling optical confinement to exceedingly small dimensions. Indeed, previous research has shown that nanometer-scale truncated nanocone hBN cavities, with deep subdiffractional dimensions, support three-dimensionally confined optical modes in the mid-infrared. Because of optical selection rules, only a few of the many theoretically predicted modes have been observed experimentally via far-field reflection and scattering-type scanning near-field optical microscopy (s-SNOM). The photothermal induced resonance (PTIR) technique probes optical and vibrational resonances overcoming weak far-field emission by leveraging an atomic force microscope (AFM) probe to transduce local sample expansion caused by light absorption. Here we show that PTIR enables the direct observation of previously unobserved, dark hyperbolic modes of hBN nanostructures. Leveraging these optical modes and their wide range of angular and radial momenta could provide a new degree of control over the electromagnetic near-field concentration, polarization in nanophotonic applications.

High-Contrast Infrared Absorption Spectroscopy via Mass-Produced Coaxial Zero-Mode Resonators with Sub-10 nm Gaps

We present a wafer-scale array of resonant coaxial nanoapertures as a practical platform for surface-enhanced infrared absorption spectroscopy (SEIRA). Coaxial nanoapertures with sub-10 nm gaps are fabricated via photolithography, atomic layer deposition of a sacrificial Al2O3 layer to define the nanogaps, and planarization via glancing-angle ion milling. At the zeroth-order Fabry-Pérot resonance condition, our coaxial apertures act as a zero-mode resonator (ZMR), efficiently funneling as much as 34% of incident infrared (IR) light along 10 nm annular gaps. After removing Al2O3 in the gaps and inserting silk protein, we can couple the intense optical fields of the annular nanogap into the vibrational modes of protein molecules. From 7 nm gap ZMR devices coated with a 5 nm thick silk protein film, we observe high-contrast IR absorbance signals drastically suppressing 58% of the transmitted light and infer a strong IR absorption enhancement factor of 104∼105. These single nanometer gap ZMR devices can be mass-produced via batch processing and offer promising routes for broad applications of SEIRA.

Active tuning of surface phonon polariton resonances via carrier photoinjection

Surface phonon polaritons (SPhPs) are attractive alternatives to infrared plasmonics for subdiffractional confinement of infrared light. Localized SPhP resonances in semiconductor nanoresonators are narrow, but that linewidth and the limited extent of the Reststrahlen band limit spectral coverage. To address this limitation, we report active tuning of SPhP resonances in InP and 4H-SiC by photoinjecting free carriers into nanoresonators, taking advantage of the coupling between the carrier plasma and optic phonons to blueshift SPhP resonances. We demonstrate state-of-the-art tuning figures of merit upon continuous-wave excitation (in InP) or pulsed excitation (in 4H-SiC). Lifetime effects cause the tuning to saturate in InP, and carrier redistribution leads to rapid (<50u2009ps) recovery of the resonance in 4H-SiC. This work demonstrates the potential for this method and opens a path towards actively tuned nanophotonic devices, such as modulators and beacons, in the infrared, and identifies important implications of coupling between electronic and phononic excitations.Infrared surface phonon polariton tuning is achieved by photoinjecting free carriers into resonators.

Strong Coupling of Epsilon-Near-Zero Phonon Polaritons in Polar Dielectric Heterostructures.

We report the first observation of epsilon-near-zero (ENZ) phonon polaritons in an ultrathin AlN film fully hybridized with surface phonon polaritons (SPhP) supported by the adjacent SiC substrate. Employing a strong coupling model for the analysis of the dispersion and electric field distribution in these hybridized modes, we show that they share the most prominent features of the two precursor modes. The novel ENZ-SPhP coupled polaritons with a highly propagative character and deeply subwavelength light confinement can be utilized as building blocks for future infrared and terahertz nanophotonic integration and communication devices.

Fabrication of phonon-based metamaterial structures using focused ion beam patterning

The focused ion beam (FIB) is a powerful tool for rapid prototyping and machining of functional nanodevices. It is employed regularly to fabricate test metamaterial structures but, to date, has been unsuccessful in fabricating metamaterial structures with features at the nanoscale that rely on surface phonons as opposed to surface plasmons because of the crystalline damage that occurs with the collision cascade associated with ion sputtering. In this study, we employ a simple technique of protecting the crystalline substrate in single-crystal 4H-SiC to design surface phonon polariton-based optical resonance structures. By coating the material surface with a thin film of chromium, we have placed a material of high sputter resistance on the surface, which essentially absorbs the energy in the beam tails. When the beam ultimately punches through the Cr film, the hard walls in the film have the effect of channeling the beam to create smooth sidewalls. This demonstration opens the possibility of further rapid-...

On the Driving Force of Shockley Stacking Fault Motion in 4H-SiC

Shockley stacking faults (SSFs) are extended, planar defects in silicon carbide SiC that are the cause of the observed drift in the forward voltage that occurs during bipolar injection within both bipolar or unipolar SiC devices. Efforts to understand the primary driving force for SSF nucleation and expansion have been put forth in an effort to eradicate the incorporation of the basal plane dislocations from which the SSFs nucleate and/or to minimize the expansion of the SSFs themselves. Up until recently, the reported driving force models were all based on the hypothesis that SSFs were thermodynamically stable with respect to the 4H-SiC host lattice. Therefore, these prior models focused on explaining the reason for the improved stability of the material in the faulted state. However, annealing 4 and high temperature device operation 5 experiments illustrated that SSFs could be contracted and the Vf drift recovered. The results of these experiments therefore clearly showed that SSFs are not the preferred state of 4H-SiC at thermal equilibrium and T>30C. Here, we introduce and discuss a possible mechanism describing the primary driving force governing SSF expansion and contraction that is consistent with the previously reported experimental observations. Further, we also will present further experimental and simulation results that strengthen support for this model. Finally, we report simulation results that imply SSF-induced degradation of the Vf is due to a reduction in the carrier lifetime within the 3C-SiC material of the SSFs, which act as electron traps within the host lattice material. The model we present builds upon the mechanisms reported by Lambrecht and Miao and Galeckas et al., where the relative energy associated with the filling of the two-dimensional density of states of the SSFs under

Chapter 12 Semiconductor Nanophotonics Using Surface Polaritons

The properties of crystalline materials are dictated by the physical arrangement and behaviour of their constituent atoms. The behaviour of electrons and phonons (lattice vibrations), generally dominate the optoelectronic properties of a material. Electrons typically occupy either bound states, where they are unable to move and contribute to charge transfer, or in a delocalised state, where they can propagate through the lattice and carry electric current. These states form energy bands, called valence and conduction bands (see Fig. 12.1a), and the energies of these bands are one of the key ways of classifying materials. In a metal, these bands of energies overlap, and as a result, electrons are always able to freely propagate throughout the material. On the other hand, in an insulator these bands are separated by an energy difference (the band gap) and there is a small electronic density of states near the valence and conduction band edges. Semiconductors exist in-between these two extremes. They possess a band gap, but charge carriers can be moved between bands through either external stimuli or during the growth process via doping. This means that whilst semiconductors intrinsically behave as insulators, perturbations induced by thermal energy, light, dopants, or an electric field can switch them into acting as a conductor. Furthermore, in polar semiconductors the charge separation between the ionic lattice sites allow for crystalline vibrations (phonons) to couple with infrared to terahertz light. This broad range of interactions is what has made semiconductors an integral part of electronics, light emitting diodes, lasers, detectors and photovoltaics.

Reconfigurable infrared hyperbolic metasurfaces using phase change materials

Metasurfaces control light propagation at the nanoscale for applications in both free-space and surface-confined geometries. However, dynamically changing the properties of metasurfaces can be a major challenge. Here we demonstrate a reconfigurable hyperbolic metasurface comprised ofxa0a heterostructure of isotopically enriched hexagonal boron nitride (hBN) in direct contact with the phase-change material (PCM) single-crystal vanadium dioxide (VO2). Metallic and dielectric domains in VO2 provide spatially localized changes in the local dielectric environment, enabling launching, reflection, and transmission of hyperbolic phonon polaritons (HPhPs) at the PCM domain boundaries, and tuning the wavelength of HPhPs propagating in hBN over these domains by a factor of 1.6. We show that this system supports in-plane HPhP refraction, thus providing a prototype for a class of planar refractive optics. This approach offers reconfigurable control of in-plane HPhP propagation and exemplifies a generalizable framework based on combining hyperbolic media and PCMs to design optical functionality.Here, the authors demonstrate that the dispersion of hyperbolic phonon polaritons can be controlled using the permittivity changes inherent in the different phases of phase change materials. This enables direct launching, reflection, transmission and refraction at the phase change material domain boundaries.