Conventional optical components are limited to size scales much larger than the wavelength of light, as changes to the amplitude, phase and polarization of the electromagnetic fields are accrued ...gradually along an optical path. However, advances in nanophotonics have produced ultrathin, so-called 'flat' optical components that beget abrupt changes in these properties over distances significantly shorter than the free-space wavelength. Although high optical losses still plague many approaches, phonon polariton (PhP) materials have demonstrated long lifetimes for sub-diffractional modes in comparison to plasmon-polariton-based nanophotonics. We experimentally observe a threefold improvement in polariton lifetime through isotopic enrichment of hexagonal boron nitride (hBN). Commensurate increases in the polariton propagation length are demonstrated via direct imaging of polaritonic standing waves by means of infrared nano-optics. Our results provide the foundation for a materials-growth-directed approach aimed at realizing the loss control necessary for the development of PhP-based nanophotonic devices.
The excitation of surface-phonon-polariton (SPhP) modes in polar dielectric crystals and the associated new developments in the field of SPhPs are reviewed. The emphasis of this work is on providing ...an understanding of the general phenomenon, including the origin of the Reststrahlen band, the role that optical phonons in polar dielectric lattices play in supporting sub-diffraction-limited modes and how the relatively long optical 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 technological 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 nanophotonics and metamaterials in the mid-infrared to terahertz spectral ranges.
Resonant optical Stark effect in monolayer WS2 Cunningham, Paul D.; Hanbicki, Aubrey T.; Reinecke, Thomas L. ...
Nature communications,
12/2019, Letnik:
10, Številka:
1
Journal Article
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Abstract
Breaking the valley degeneracy in monolayer transition metal dichalcogenides through the valley-selective optical Stark effect (OSE) can be exploited for classical and quantum valleytronic ...operations such as coherent manipulation of valley superposition states. The strong light-matter interactions responsible for the OSE have historically been described by a two-level dressed-atom model, which assumes noninteracting particles. Here we experimentally show that this model, which works well in semiconductors far from resonance, does not apply for excitation near the exciton resonance in monolayer WS
2
. Instead, we show that an excitonic model of the OSE, which includes many-body Coulomb interactions, is required. We confirm the prediction from this theory that many-body effects between virtual excitons produce a dominant blue-shift for photoexcitation detuned from resonance by less than the exciton binding energy. As such, we suggest that our findings are general to low-dimensional semiconductors that support bound excitons and other many-body Coulomb interactions.
The presence of lone-pair electrons (LPE) is known to lead to large anharmonicities in materials and thus to low lattice thermal conductivity (κ). Materials with LPE typically have κ values that are ...lower than the κ values of those with similar compositions but without LPE. In this study, we investigate thermal transport properties in Cs–Sb–Se ternary compounds by first-principles methods and show that LPE do not necessarily lead to the lowest κ. We find that the calculated κ in LPE free Cs3SbSe4 is smaller (∼0.1 W/mK) than the κ in CsSbSe2 (∼0.3 W/mK), which has LPE. This is unusual and contradictory to the traditional concept in which LPE lead to low κ values. We explain this difference in terms of the local structural instabilities. The small ionic radius of Sb5+ brings the Se2– closer. This leads to a Coulomb repulsion between them making the SbSe43– polyhedra in Cs3SbSe4 less stable. In addition, the bonding topology disconnects these units as compared to CsSbSe2 where there are shared Se atoms. This results in soft phonons and large thermal displacements for the Sb and Se for this material. This gives larger anharmonicity and higher scattering rates in Cs3SbSe4 than those from LPE-driven anharmonicity of CsSbSe2. Higher scattering rates and softer modes due to weaker interactions result in the κ of Cs3SbSe4 being lower than that of CsSbSe2.
Identifying materials with good electron transport and poor thermal transport properties for thermoelectric applications has been challenging. Here we report a series of new materials including ...Tl3TaSe4 and Tl3VS4 with promising thermoelectric properties giving thermoelectric figure of merit, zT ≈ 0.8 at room temperature using first-principles calculations. This high zT stems from the high electrical conductivity and ultralow thermal conductivity (κ). We calculate κ ≈ 0.1–0.2 W/m·K from a phonon Boltzmann’s transport equation and κ ≈ 0.3–0.4 W/m·K from the two-channel model. Low phonon group velocities due to weakly bonded Tl atoms and strong anharmonicity associated with s2 lone electrons pair give rise to such a low κ in these systems.
A key ingredient for a quantum network is an interface between stationary quantum bits and photons, which act as flying qubits for interactions and communication. Photonic crystal architectures are ...promising platforms for enhancing the coupling of light to solid-state qubits. Quantum dots can be integrated into a photonic crystal, with optical transitions coupling to photons and spin states forming a long-lived quantum memory. Many researchers have now succeeded in coupling these emitters to photonic crystal cavities, but there have been no demonstrations of a functional spin qubit and quantum gates in this environment. Here, we have developed a coupled cavity-quantum dot system in which the dot is controllably charged with a single electron. We perform the initialization, rotation and measurement of a single electron spin qubit using laser pulses, and find that the cavity can significantly improve these processes.
Acoustic cavities in 2D heterostructures Zalalutdinov, Maxim K; Robinson, Jeremy T; Fonseca, Jose J ...
Nature communications,
06/2021, Letnik:
12, Številka:
1
Journal Article
Recenzirano
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Two-dimensional (2D) materials offer unique opportunities in engineering the ultrafast spatiotemporal response of composite nanomechanical structures. In this work, we report on high frequency, high ...quality factor (Q) 2D acoustic cavities operating in the 50-600 GHz frequency (f) range with f × Q up to 1 × 10
. Monolayer steps and material interfaces expand cavity functionality, as demonstrated by building adjacent cavities that are isolated or strongly-coupled, as well as a frequency comb generator in MoS
/h-BN systems. Energy dissipation measurements in 2D cavities are compared with attenuation derived from phonon-phonon scattering rates calculated using a fully microscopic ab initio approach. Phonon lifetime calculations extended to low frequencies (<1 THz) and combined with sound propagation analysis in ultrathin plates provide a framework for designing acoustic cavities that approach their fundamental performance limit. These results provide a pathway for developing platforms employing phonon-based signal processing and for exploring the quantum nature of phonons.
Properties of Fluorinated Graphene Films Robinson, Jeremy T; Burgess, James S; Junkermeier, Chad E ...
Nano letters,
08/2010, Letnik:
10, Številka:
8
Journal Article
Recenzirano
Graphene films grown on Cu foils have been fluorinated with xenon difluoride (XeF2) gas on one or both sides. When exposed on one side the F coverage saturates at 25% (C4F), which is optically ...transparent, over 6 orders of magnitude more resistive than graphene, and readily patterned. Density functional calculations for varying coverages indicate that a C4F configuration is lowest in energy and that the calculated band gap increases with increasing coverage, becoming 2.93 eV for one C4F configuration. During defluorination, we find hydrazine treatment effectively removes fluorine while retaining graphene’s carbon skeleton. The same films may be fluorinated on both sides by transferring graphene to a silicon-on-insulator substrate enabling XeF2 gas to etch the Si underlayer and fluorinate the backside of the graphene film to form perfluorographane (CF) for which calculated the band gap is 3.07 eV. Our results indicate single-side fluorination provides the necessary electronic and optical changes to be practical for graphene device applications.
We explore the electronic response of single-walled carbon nanotubes (SWNT) to trace levels of chemical vapors. We find adsorption at defect sites produces a large electronic response that dominates ...the SWNT capacitance and conductance sensitivity. This large response results from increased adsorbate binding energy and charge transfer at defect sites. Finally, we demonstrate controlled introduction of oxidation defects can be used to enhance sensitivity of a SWNT network sensor to a variety of chemical vapors.
A sharp tip of atomic force microscope is employed to probe van der Waals forces of a silicon oxide substrate with adhered graphene. Experimental results obtained in the range of distances from 3 to ...20 nm indicate that single-, double-, and triple-layer graphenes screen the van der Waals forces of the substrate. Fluorination of graphene, which makes it electrically insulating, lifts the screening in the single-layer graphene. The van der Waals force from graphene determined per layer decreases with the number of layers. In addition, increased hole doping of graphene increases the force. Finally, we also demonstrate screening of the van der Waals forces of the silicon oxide substrate by single- and double-layer molybdenum disulfide.
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