Improving the quality factors of plasmonic silver cavities for strong coupling with quantum emitters.

Plasmonic cavities (PCs) made of metallic nanostructures can concentrate electromagnetic radiation into an ultrasmall volume, where it might strongly interact with quantum emitters. In recent years, there has been much interest in studying such a strong coupling in the limit of single emitters. However, the lossy nature of PCs, reflected in their broad spectra, limits their quality factors and hence their performance as cavities. Here, we study the effect of the adhesion layer used in the fabrication of metal nanostructures on the spectral linewidths of bowtie-structured PCs. Using dark-field microspectroscopy, as well as electron energy loss spectroscopy, it is found that a reduction in the thickness of the chromium adhesion layer we use from 3 nm to 0.1 nm decreases the linewidths of both bright and dark plasmonic modes. We further show that it is possible to fabricate bowtie PCs without any adhesion layer, in which case the linewidth may be narrowed by as much as a factor of 2. Linewidth reduction increases the quality factor of these PCs accordingly, and it is shown to facilitate reaching the strong-coupling regime with semiconductor quantum dots.

Plasmon-induced efficient hot carrier generation in graphene on gold ultrathin film with periodic array of holes: Ultrafast pump-probe spectroscopy.

Using ultrafast pump-probe reflectivity with a 3.1 eV pump and coherent white light probe (1.1-2.6 eV), we show that graphene on gold nanostructures exhibits a strong coupling to the plasmonic resonances of the ordered lattice hole array, thus injecting a high density of hot carriers in graphene through plasmons. The system being studied is single-layer graphene on an ultrathin film of gold with periodic arrangements of holes showing anomalous transmission. A comparison is made with gold film with and without hole array. By selectively probing transient carrier dynamics in the spectral regions corresponding to plasmonic resonances, we show efficient plasmon induced hot carrier generation in graphene. We also show that due to high electromagnetic field intensities at the edge of the submicron holes, fast decay time (10-100 fs), and short decay length (1 nm) of plasmons, a highly confined density of hot carriers (very close to the edge of the holes) is generated by Landau damping of plasmons within the holey gold film. A contribution to transient decay dynamics due to the diffusion of the initial nonuniform distribution of hot carriers away from the hole edges is observed. Our results are important for future applications of novel hot carrier device concepts where hot carriers with tunable energy can be generated in different graphene regions connected seamlessly.

Complex plasmon-exciton dynamics revealed through quantum dot light emission in a nanocavity.

Plasmonic cavities can confine electromagnetic radiation to deep sub-wavelength regimes. This facilitates strong coupling phenomena to be observed at the limit of individual quantum emitters. Here, we report an extensive set of measurements of plasmonic cavities hosting one to a few semiconductor quantum dots. Scattering spectra show Rabi splitting, demonstrating that these devices are close to the strong coupling regime. Using Hanbury Brown and Twiss interferometry, we observe non-classical emission, allowing us to directly determine the number of emitters in each device. Surprising features in photoluminescence spectra point to the contribution of multiple excited states. Using model simulations based on an extended Jaynes-Cummings Hamiltonian, we find that the involvement of a dark state of the quantum dots explains the experimental findings. The coupling of quantum emitters to plasmonic cavities thus exposes complex relaxation pathways and emerges as an unconventional means to control dynamics of quantum states.

Pressure-induced isostructural electronic topological transitions in 2H-MoTe2: x-ray diffraction and first-principles study.

Synchrotron x-ray diffraction measurements on powder 2H-MoTe2 (P63/mmc) up to ∼46 GPa have been performed along with first-principles based density functional theoretical analysis to probe the isostructural transition in low pressure regime and two electronic topological transitions (ETT) of Lifshitz-type in high pressure regime. The low pressure isostructural transition at ∼7 GPa is associated with the lattice parameter ratio c/a anomaly and the change in the compressibility of individual layers. The pressure dependence of the volume by linearizing the Birch-Murnaghan equation of state as a function of Eulerian strain shows a clear change of the bulk modulus at the ETT pressure of ∼20 GPa. The minimum of c/a ratio around 32 GPa is associated with the change in topology of electron pockets marked as second ETT of Lifshitz-type. We do not observe any structural transition up to the maximum applied pressure of ∼46 GPa under quasi-hydrostatic condition.

Vacuum Rabi splitting of a dark plasmonic cavity mode revealed by fast electrons.

Recent years have seen a growing interest in strong coupling between plasmons and excitons, as a way to generate new quantum optical testbeds and influence chemical dynamics and reactivity. Strong coupling to bright plasmonic modes has been achieved even with single quantum emitters. Dark plasmonic modes fare better in some applications due to longer lifetimes, but are difficult to probe as they are subradiant. Here, we apply electron energy loss (EEL) spectroscopy to demonstrate that a dark mode of an individual plasmonic bowtie can interact with a small number of quantum emitters, as evidenced by Rabi-split spectra. Coupling strengths of up to 85 meV place the bowtie-emitter devices at the onset of the strong coupling regime. Remarkably, the coupling occurs at the periphery of the bowtie gaps, even while the electron beam probes their center. Our findings pave the way for using EEL spectroscopy to study exciton-plasmon interactions involving non-emissive photonic modes.

A new low temperature solid modification in 1-isothiocyanato-4-(trans­4-propylcyclohexyl)benzene (3CHBT) probed by Raman spectroscopy and quantum chemical calculations.

Raman spectra of 1-isothiocyanato-4-(trans 4-propylcyclohexyl)benzene (3CHBT) were studied in the region, 1450-2300cm-1 at twenty two different temperatures in the range, 83-293K in cooling and heating cycles. All bands in this region were critically evaluated in term of linewidth, peak position and relative intensity. Raman bands at ~2085cm-1 and ~2120cm-1 shows clear evidence of a solid modification through anomaly in temperature dependence peak positions and linewidths variation in the temperature range 173-203K. A detailed analysis of the variation of the linewidth and peak position of the two component bands leads to the conclusion that the molecular/dimer arrangement in crystalline packing changed between 173K and 203K. This solid modification was also analyzed at the molecular level. The ~2085cm-1 and ~2120cm-1 bands were corresponded as parallel and anti-parallel dimers of 3CHBT, which are identified as dimer I (D1) and dimer II (D2), respectively. The structures of both the dimers have been optimized by quantum chemical calculations employing density functional theoretic (DFT) methods.

Pressure-induced Lifshitz and structural transitions in NbAs and TaAs: experiments and theory.

High pressure Raman, resistivity and synchrotron x-ray diffraction studies on Weyl semimetals NbAs and TaAs have been carried out along with density functional theoretical (DFT) analysis to explain pressure induced structural and electronic topological phase transitions. The frequencies of first order Raman modes harden with increasing pressure, exhibiting a slope change at [Formula: see text] GPa for NbAs and [Formula: see text] GPa for TaAs. The resistivities of NbAs and TaAs exhibit a minimum at pressures close to these transition pressures and also a change in the bulk modulus is observed. Our first-principles calculations reveal that the transition is associated with an electronic Lifshitz transition at [Formula: see text] for NbAs while it is a structural phase transition from body centered tetragonal to hexagonal phase at [Formula: see text] for TaAs. Further, our DFT calculations show a structural phase transition at 24 GPa from body centered tetragonal phase to hexagonal phase.

Graphene oxide-MnFe2O4 magnetic nanohybrids for efficient removal of lead and arsenic from water.

We show that the hybrids of single-layer graphene oxide with manganese ferrite magnetic nanoparticles have the best adsorption properties for efficient removal of Pb(II), As(III), and As(V) from contaminated water. The nanohybrids prepared by coprecipitation technique were characterized using atomic force and scanning electron microscopies, Fourier transformed infrared spectroscopy, Raman spectroscopy, X-ray diffraction, and surface area measurements. Magnetic character of the nanohybrids was ascertained by a vibrating sample magnetometer. Batch experiments were carried out to quantify the adsorption kinetics and adsorption capacities of the nanohybrids and compared with the bare nanoparticles of MnFe2O4. The adsorption data from our experiments fit the Langmuir isotherm, yielding the maximum adsorption capacity higher than the reported values so far. Temperature-dependent adsorption studies have been done to estimate the free energy and enthalpy of adsorption. Reusability, ease of magnetic separation, high removal efficiency, high surface area, and fast kinetics make these nanohybrids very attractive candidates for low-cost adsorbents for the effective coremoval of heavy metals from contaminated water.