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Using ultrafast resonant pump-probe spectroscopy in an unconventional experimental setup we investigate the spectral shape and dynamics of absorption features related to the A exciton in an hexagonal boron nitride (hBN)/MoSe2/hBN van der Waals heterostructure. While in a pure two-level system a pump-probe experiment measures the occupation or the polarization dynamics, depending on the time ordering of the pulse pair, in the transition metal dichalcogenide (TMD) system both quantities get thoroughly mixed by strong exciton–exciton interaction. We find that for short positive delays the spectral lines experience pronounced changes in their shape and energy and they relax to the original situation on a picosecond time scale. For negative delays, distinctive spectral oscillations appear indicating the first-time observation of perturbed free induction decay for a TMD system. The comparison between co-circular and cross-circular excitation schemes further allows us to investigate the rapid inter-valley scattering. By considering a three-level system as a minimal model including the local field effect, excitation-induced dephasing (EID), and scattering between the excited states we explain all phenomena observed in the experiment with excellent consistency. Our handy model can be even further reduced to two levels in the case of a co-circular excitation, for which we derive analytic expressions to describe the detected signals. This allows us to trace back the spectral shapes and shifts to the impact of local field effect and EID thus fully reproducing the complex behavior of the observed effects.

Using ultrafast resonant pump-probe spectroscopy in an unconventional experimental setup we investigate the spectral shape and dynamics of absorption features related to the A exciton in an hexagonal boron nitride (hBN)/MoSe2/hBN van der Waals heterostructure. While in a pure two-level system a pump-probe experiment measures the occupation or the polarization dynamics, depending on the time ordering of the pulse pair, in the transition metal dichalcogenide (TMD) system both quantities get thoroughly mixed by strong exciton–exciton interaction. We find that for short positive delays the spectral lines experience pronounced changes in their shape and energy and they relax to the original situation on a picosecond time scale. For negative delays, distinctive spectral oscillations appear indicating the first-time observation of perturbed free induction decay for a TMD system. The comparison between co-circular and cross-circular excitation schemes further allows us to investigate the rapid inter-valley scattering. By considering a three-level system as a minimal model including the local field effect, excitation-induced dephasing (EID), and scattering between the excited states we explain all phenomena observed in the experiment with excellent consistency. Our handy model can be even further reduced to two levels in the case of a co-circular excitation, for which we derive analytic expressions to describe the detected signals. This allows us to trace back the spectral shapes and shifts to the impact of local field effect and EID thus fully reproducing the complex behavior of the observed effects. [ABSTRACT FROM AUTHOR]

The local field effect involves the modification of the spontaneous emission rate of an emitting center in a nanoparticle (NP) due to its environment which modifies the photon density of states and the electric field acting within the NP. We are unaware of reports of this effect for organic molecules so that we encapsulated a green dye and a porphyrin in a silica shell and dispersed these in media of different refractive index. The emission decays were monoexponential so that the calculated lifetimes could be fitted by a model recently employed for the local field effect. It was found that a good fit was achieved for the smaller NPs employed, indicating the importance of the effect, but the effect was not important for the larger NPs where different behavior was observed due to agglomeration.

Abstract In this work, whole visible transparent chalcogenide glass ceramics (ChGCs) were prepared by heat treating a chalcohalide glass (with a molar composition of 20GeS 2 -35Ga 2 S 3 -45CsCl, GGC) for different time, and the formation of nano-crystals (NCs) belonging to the Ga 2 S 3 phase was identified. The femtosecond Z-scan technique was employed to observe, variations in the third-order optical nonlinearity (TONL) properties, especially the nonlinear refractive behaviour, due to the appearance of the Ga 2 S 3 NCs in the GGC glass network. The experimental results show that both sign and magnitude of the nonlinear refractive index (n 2 at 800 nm) changed as a result of the local field effect generated by the size-confined Ga 2 S 3 NCs. The maximum n 2 value obtained from the Ga 2 S 3 NCs was over 4 times that of the base glass and over 230 times to that of silica glass. [ABSTRACT FROM AUTHOR]

In recent years the physics of two-dimensional semiconductors was revived by the discovery of the class of transition metal dichalcogenides. In these systems excitons dominate the optical response in the visible range and open many perspectives for nonlinear spectroscopy. To describe the coherence and polarization dynamics of excitons after ultrafast excitation in these systems, we employ the Bloch equation model of a two-level system extended by a local field describing the exciton–exciton interaction. We calculate four-wave mixing (FWM) signals and analyze the dependence of the temporal and spectral signals as a function of the delay between the exciting pulses. Exact analytical results obtained for the case of ultrafast ( δ -shaped) pulses are compared to numerical solutions obtained for finite pulse durations. If two pulses are used to generate the nonlinear signal, characteristic spectral line splittings are restricted to short delays. When considering a three-pulse excitation the line splittings, induced by the local field effect, persist for long delays. All of the found features are instructively explained within the Bloch vector picture and we show how the exciton occupation dynamics govern the different FWM signals.

A series of erbium doped SGS antimony-germanate glass embedding silver (Ag 0 ) nanoparticles have been synthesized by a one-step melt-quench thermochemical reduction technique. The effect of NPs concentration and annealing time on the structural and photoluminescent (PL) properties were investigated. The Raman spectra as a function of temperature measured in-situ allow to determine the structural changes in vicinity of Ag + ions and confirmed thermochemical reduction of Ag + ions by Sb 3+ ions. The surface plasmon resonance absorption band was evidenced near 450 nm. The impact of local field effect generated by Ag 0 nanoparticles (NPs) and energy transfer from surface of silver NPs to trivalent erbium ions on near-infrared and up-conversion luminescence was described in terms of enhancement and quench phenomena. [ABSTRACT FROM AUTHOR]

This paper reports the detailed Judd-Ofelt (J-O) analysis of Er 3+ -doped antimony phosphate glass system incorporated with silver nanoparticles (Ag NPs). Such glass system of composition (59.5-x)Sb 2 O 3 –35P 2 O 5 –5MgO–0.5Er 2 O 3 –xAgCl (0 < x < 0.9 mol.%) are synthesized via melt-quenching method. Samples are characterized to determine the influence of Ag NPs concentration dependent modifications in the spectroscopic properties. TEM image manifested the nucleation of Ag NPs of average diameter 4.6 nm and their homogeneous dispersion in the glass matrix. Three surface plasmon resonance (SPR) absorption bands of Ag NPs are probed at 544, 570 and 628 nm. UV–Vis–NIR spectra of glass samples exhibited ten significant absorption peaks of Er 3+ ion. Bonding of Er 3+ ions with ligands is changed from ionic to covalent character. J-O analysis demonstrated significant increase of Ω 2 with increasing the concentration of Ag NPs which indicated an improvement in the covalency and asymmetry of Er 3+ ions environment. Down-conversion photoluminescent (PL) spectra displayed eleven emission bands of Er 3+ ions in which the intensities are enhanced significantly (as much as 17.33-fold for sample containing 0.5 mol% of Ag NPs) due to the incorporation of Ag NPs. This intensity enlargement is mainly attributed to the SPR mediated local field effect induced by Ag NPs. The achievement of high stimulated cross-section of this glass system makes them prospective for solid state lasers. [ABSTRACT FROM AUTHOR]

We experimentally demonstrate the enhanced absorptive nonlinear response from a hybrid system developed through the noncovalent interaction of C-2-phenylethenilcalix [4]resorcinarene (CPECR) with the silver nanoparticles (Ag NPs). The modified absorption spectrum and significantly reduced emission spectrum confirm the hybrid system formation. The characteristic shift in the Raman peaks ensures the charge transfer between CPECR and Ag. The absorptive nonlinearity of the hybrid system at the surface plasmon resonance (SPR) band of Ag NPs was studied by the z-scan method using a Q-switched Nd: YAG laser with nanosecond pulsed beams having a repetition rate of 10 Hz at 532 nm. The dominance of the hybrid system formation over the pure system is the emergence of impressive nonlinear response at low input intensity resulting from the synergistic effect of effective two-photon absorption, the photo-induced charge transfer between CPECR and Ag and the local field effect produced by the SPR of Ag NPs. [Display omitted] • The third-order nonlinear signature of C-2-phenylethenilcalix [4]resorcinarene(CPECR) is identified by the z-scan method with nanosecond pulsed laser beams having a repetition rate of 10 Hz at 532 nm. • We developed a new hybrid system (CPECR-Ag) through the non-covalent interaction between CPECR and silver nanoparticles (Ag NPs). • The highlight of the hybrid system formation is that its optical limiting threshold is ten times lower than that of CPECR, and its effective two-photon absorption coefficient is three times that of Ag NPs, while a nonlinear signature is absent in CPECR at the same intensity. [ABSTRACT FROM AUTHOR]

The plasmon-induced local electric field enhancement of Au-Ag nanoshell could be fine tuned by dielectric coating. Theoretical calculation results indicate that the anti-bonding plasmon-induced local field peak at a shorter wavelength always fades down as the dielectric coating thickness is increased. For bonding plasmon-induced local field peak at a longer wavelength, the dielectric coating thickness-dependent monotonic intensity decrease only takes place when the Au-Ag nanoshell has a very small or very large gold core. Because of the competition between the distance-dependent field reduction and dielectric polarization-induced field enhancement, nonmonotonic intensity change could be observed for bonding plasmon-induced local field effect. By choosing an appropriate value of the gold core radius, the local field could be enhanced firstly and then fades down, and the maximum local field enhancement could be further enhanced by increasing the dielectric constant of the coating layer. The obtained results present a potential application in the molecule binding participant surface-enhanced fluorescence and surface-enhanced Raman scattering (SERS). [ABSTRACT FROM AUTHOR]

In this work, we extend the discrete interaction model/quantum mechanics (DIM/QM) method to calculate the frequency-dependent hyperpolarisabilities of molecules near metal surfaces. The DIM/QM method is a polarisable quantum mechanics/molecular mechanics method, which represents the metal surface atomistically and thus allows for explicitly modelling the influence of the local environment on the optical properties of a molecule. The interactions between the metal surface and the molecules include both the image field and local field effects. The image field effects arise from the response induced in the metal surface due to the molecule’s electronic charge distributions whereas the local field effects arise from interactions between the metal surface and the external light. The frequency-dependent first-hyperpolarisability is obtained in an efficient way based on time-dependent density functional theory and the (2n+1) rule. The method was tested for calculating the first-hyperpolarisability responsible for the second-harmonic generation of a fumaramide[2]rotaxane interacting with a sliver surface. The first-hyperpolarisability of the fumaramide[2]rotaxane is very small in the gas phase due to near inversion symmetry. We find that the breaking of the symmetry due to interactions with the metal surface leads to a significant induced first-hyperpolarisability. The image field effects are found to be modest and short-range. In contrast, we find that the local field effects are large and rather long-range, illustrating the importance of including these effects directly in the simulations. Comparison with experimental results shows good qualitative agreement. [ABSTRACT FROM AUTHOR]

We report the excitonic Rabi oscillations in self assembled quantum dots observed by photon echo spectroscopy. The amplitude of the observed Rabi oscillations increases with the pulse area, which is different from the results of previous reports. A numerical calculation of the Rabi oscillations including the local field effect in a quantum dot well reproduces the experimental oscillations, indicating that the local field effect is one of the important origins of the modification of the Rabi oscillations in quantum dots (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Pure α-NaYF4: Nd3+ nanocrystals of various sizes from 6 nm to 18 nm were successfully synthesized by a solvothermal method. The monodispersed oleate-capped nanocrystals were dispersed in several nonpolar solvents to obtain stable colloidal. Under the excitation by an 803 nm laser, luminescent decay curves of the colloidal were measured. The relationship between emission lifetimes of samples and effective refractive indices of solvents was studied for the local field correction factor. In the 18 nm sample, the real cavity model was preferable model. As the particle size decreased to 6 nm, the preferred model altered from the real cavity model to the virtual cavity model which might be derived from changing of polarizability density. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Abstract: The effect of colloid gold nanoparticles (AuNPs) on the fluorescence excitation spectrum of α-fetoprotein (AFP) has been investigated experimentally. The excitation spectral peaks of AFP with low concentration from 0.01ngml−1 to 12ngml−1 increase monotonically with increasing of AFP concentration. When some gold colloids were added to the AFP solution, the excitation peak at 285nm decreases distinctly. By comparing the excitation peak intensity of AFP solution with gold colloids and without gold colloids at different AFP concentrations, the quenching effect from gold nanoparticle was more effective at lower AFP concentration. So the range of concentration from 0.01ngml−1 to 0.09ngml−1 will be the potential range of applications because of the higher sensitivity. The physical origin based on local field effect was investigated to illuminate this local environment dependent fluorescence quenching. The changing extent of quenching with different AFP concentrations can be attributed to the nonlinear decreasing of the local field factor of gold nanoparticles as a function of environmental dielectric constant. [ABSTRACT FROM AUTHOR]

Abstract: α-FeOOH nanorods (NRs) with diameters varying from 12nm to 25nm were prepared by colloid chemistry method, and their third-order nonlinear susceptibilities (χ (3)) were investigated by Z-scan technique. Both the real part and imaginary part of χ (3) increase with increasing the NRs’ size in the near resonant and transparency region. χ (3) of α-FeOOH NRs with the diameter of 25nm can reach 7.7×10−11 esu in the near resonant region, which is almost 16 times of that of the corresponding bulk. The quantum confinement effect and local field effect make the dominant contribution to the nonlinear enhancement. [Copyright &y& Elsevier]

Abstract: The local field effect on the spontaneous emission rates of emitters embedded in a crystal dielectric is studied by microscopic numerical simulations. The ratio of the local radiation field to the macroscopic radiation field is calculated microscopically for the cases of doping a single luminescent ion and the embedding of a nanoparticle with a luminescent ion in the crystal. The results are compared with the two well-known macroscopic models, the real-cavity and virtual-cavity models. [Copyright &y& Elsevier]

The scaling law of voltage contrast in the scanning electron microscpe is discussed. The concerned parameters are the size of a specimen electrode or an interconnecting line, the extracting electric field intensity for secondary electrons, the peak value and variation rate of applied specimen voltage. Three parameters are independent of each other. Both local field effect and transit time effect are taken into account. [ABSTRACT FROM PUBLISHER]