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Register a new userDependence of the Energy Transfer to Graphene on the Excitation Energy
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Fluorescence studies of natural photosynthetic complexes on a graphene layer demonstrate pronounced influence of the excitation wavelength on the energy transfer efficiency to graphene. Ultraviolet light yields much faster decay of fluorescence, with average efficiencies of the energy transfer equal to 87% and 65% for excitation at 405 nm and 640 nm, respectively. This implies that focused light changes locally the properties of graphene affecting the energy transfer dynamics, in an analogous way as in the case of metallic nanostructures. Demonstrating optical control of the energy transfer is important for exploiting unique properties of graphene in photonic and sensing architectures.
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The near-field interaction between fluorescent emitters and graphene exhibits rich physics associated with local dipole-induced electromagnetic fields that are strongly enhanced due to the unique properties of graphene. Here, we measure emitter lifetimes as a function of emitter-graphene distance d, and find agreement with a universal scaling law, governed by the fine-structure constant. The observed energy transfer- rate is in agreement with a 1/d^4 dependence that is characteristic of 2D lossy media. The emitter decay rate is enhanced 90 times (transfer efficiency of ~99%) with respect to the decay in vacuum at distances d ~ 5 nm. This high energy-transfer rate is mainly due to the two-dimensionality and gapless character of the monoatomic carbon layer. Graphene is thus shown to be an extraordinary energy sink, holding great potential for photodetection, energy harvesting, and nanophotonics.
Raman spectra of few-layer WS2 have been measured with up to seven excitation energies, and peculiar resonance effects are observed. The two-phonon acoustic phonon scattering signal close to the main E2g1 peak is stronger than the main peaks for excitations near the A or B exciton states. The low-frequency Raman spectra show a series of shear and layer-breathing modes that are useful for determining the number of layers. In addition, hitherto unidentified peaks (X1 and X2), which do not seem to depend on the layer thickness, are observed near resonances with exciton states. The polarization dependences of the two peaks are different: X1 vanishes in cross polarization, but X2 does not. At the resonance with the A exciton state, the Raman-forbidden, lowest-frequency shear mode for odd number of layers appears as strong as that for the allowed case of even number of layers. This mode also exhibits a strong Breit-Wigner-Fano line shape and an anomalous polarization behavior at this resonance.
We investigate the resonance energy transfer (RET) rate between two quantum emitters near a suspended graphene sheet in vacuum under the influence of an external magnetic field. We perform the analysis for low and room temperatures and show that, due to the extraordinary magneto-optical response of graphene, it allows for an active control and tunability of the RET even in the case of room temperature. We also demonstrate that the RET rate is extremely sensitive to small variations of the applied magnetic field, and can be tuned up to a striking six orders of magnitude for quite realistic values of magnetic field. Moreover, we evidence the fundamental role played by the magnetoplasmon polaritons supported by the graphene monolayer as the dominant channel for the RET within a certain distance range. Our results suggest that magneto-optical media may take the manipulation of energy transfer between quantum emitters to a whole new level, and broaden even more its great spectrum of applications.
Excitons, composite electron-hole quasiparticles, are known to play an important role in optoelectronic phenomena in many semiconducting materials. Recent experiments and theory indicate that the band-gap optics of the newly discovered monolayer transition-metal dichalcogenides (TMDs) is dominated by tightly bound valley excitons. The strong interaction of excitons with long-range electromagnetic fields in these 2D systems can significantly affect their intrinsic properties. Here, we develop a semi-classical framework for intrinsic exciton-polaritons in monolayer TMDs that treats their dispersion and radiative decay on the same footing and can incorporate effects of the dielectric environment. It is demonstrated how both inter- and intra-valley long-range interactions influence the dispersion and decay of the polaritonic eigenstates. We also show that exciton-polaritons can be efficiently excited via resonance energy transfer from quantum emitters such as quantum dots, which may be useful for various applications.
The total kinetic energy release in the neutron induced fission of $^{235}$U was measured (using white spectrum neutrons from LANSCE) for neutron energies from E$_{n}$ = 3.2 to 50 MeV. In this energy range the average post-neutron total kinetic energy release drops from 167.4 $pm$ 0.7 to 162.1 $pm$ 0.8 MeV, exhibiting a local dip near the second chance fission threshold. The values and the slope of the TKE vs. E$_{n}$ agree with previous measurements but do disagree (in magnitude) with systematics. The variances of the TKE distributions are larger than expected and apart from structure near the second chance fission threshold, are invariant for the neutron energy range from 11 to 50 MeV. We also report the dependence of the total excitation energy in fission, TXE, on neutron energy.
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