Dipolar vibrational modes in spherical semiconductor quantum dots

Spatially quantized dipolar phonon modes in a spherical quantum dot ~QD! made of a polar isotropic material are considered in the framework of a continnum model. Different mechanical boundary conditions are analyzed, which are shown to strongly influence the spectrum of the dipole-active modes. The phonon-related polarizability of a single QD and an average dielectric function of a composite containing QDs are calculated. Numerical results are presented for CdSe and InP dots. A strongly dipole-active gap mode is predicted for InP QDs embedded in a matrix with a defined range of dielectric constant. The effect of increasing QD concentration in ensembles is discussed in terms of the dipole–dipole interaction between the dots, which can result in their bulk-like FIR absorption spectra with a peak at the transverse optical ~TO! phonon frequency instead of the Fro¨hlich frequency. It is suggested that similar effects might occur in individual microcrystals, which can explain their absorbtion of FIR radiation at the TO phonon frequency, despite having a size much smaller than the radiation wavelength.Fundação para a Ciência e a Tecnologia(FCT

Effective dielectric response of semiconductor composites

Using a previously proposed diagrammatic approach for the calculation of the renormalized polarizability of spherical inclusions in a homogeneous matrix we obtain an effective dielectric response of a composite of nonpercolating inclusions taking into account a continuous distribution of sizes of the spheres. We apply this theory to semiconductor-doped glasses ~SDG! calculating both the electron and phonon responses in the far-infrared and optical spectral regions, respectively, and also to a strongly inhomogeneous semiconductor alloy. For SDG, we compare our calculated results with available experimental data on interband optical absorption and obtain good agreement by choosing the appropriate distribution function of sizes of the spheres. In the case of the CdxHg12xTe alloy, we find an interesting interplay of the composite effects and phononplasmon coupling resulting in a rich structure for the reflectivity spectra. We compare these results to those calculated using another approach, which is widely used for describing the dielectric properties of composites, the self-consistent approximation, and discuss the relation between the two approaches

Tamm polaritons and cavity modes in the FIR range

Tamm polaritons (TPs) are formed at the interface between two semi-infinite periodic dielectric structures (Bragg mirrors) or other reflectors. Contrary to usual surface polaritons, TPs exist inside the "light cone", even though their amplitude also decreases exponentially with the distance from the interface as it is characteristic of evanescent waves. They couple to elementary excitations in the materials or structures that form the interface, such as metal plasmons or semiconductor excitons. Here we discuss the formation of TPs in the far-infrared (FIR) spectral range, in the optical-phonon reststrahlen band of a polar semiconductor such as GaAs, with a Bragg reflector (BR) as the second mirror. Their dispersion relation and the frequency window for the TP existence are discussed for a GaAs-BR interface and also structures containing a gap between the two reflectors.Financial support from the Portuguese Foundation for Science and Technology (FCT) through Project PEst-C/FIS/UI0607/2013 is acknowledged.info:eu-repo/semantics/publishedVersio

Polaron relaxation in a quantum dot due to anharmonic coupling within a mean-field approach

We study the electronic relaxation in a quantum dot within the polaron approach by focusing on the reversible anharmonic decay of longitudinal optical LO phonons forming the polaron into longitudinalacoustic LA phonons. The coherent coupling between the LO and LA phonons is treated within a mean-field approach.We derive a temperature-dependent interlevel coupling parameter, related to the Grüneisen parameter and the thermal-expansion coefficient, which characterizes an effective decay channel for the electronic or excitonic states. Within this theory, we obtain a characteristic anharmonic decay time of 1 ns, 2–3 orders of magnitude longer than previous predictions based on the Fermi’s Golden Rule. We suggest that coherent relaxation due to carrier-carrier interaction is an efficient alternative to the (too slow) polaron decay.Fundação para a Ciência e a Tecnologia (FCT) - PTDC/FIS/64404/2006; PTDC/FIS/72843/200

Is polaron effect important for resonant Raman scattering in self-assembled quantum dots?

While the diagonal (or intra-level) interaction of a confined exciton with optical phonons in selfassembled quantum dots (SAQD’s) is rather weak, the non-diagonal one can lead to a considerable change of the exciton spectrum and the formation of a polaron. An impact of this effect on resonant inelastic light scattering is studied theoretically. The polaron spectrum is obtained by numerical diagonalisation of the exciton–phonon interaction Hamiltonian in a truncated Hilbert space of the non-interacting excitons and phonons. Based on this spectrum, the probability of the multi-phonon Raman scattering is calculated, which is compared to that obtained within the standard perturbation theory approach (where phonon emission and absorption are irreversible). It is shown that there are two major effects of the polaron formation: (i) the intensity of the two-phonon (2 LO) peak, relative to that of the fundamental 1 LO one is strongly increased and (ii) the resonant behaviour of the 1 LO peak differs considerably from the perturbation theory predictions. With the correct theoretical interpretation, resonant Raman scattering in SAQD’s opens the possibility of accessing the (renormalised) exciton spectrum and exciton–phonon coupling constantsFundação para a Ciência e a Tecnologia (FCT

Modeling of a plasmonic biosensor based on a graphene nanoribbon superlattice

We present a semi-analytical theoretical model, which describes the operation of a selective molecular sensor [1] employing a double resonance between a dipole-active molecular vibration mode, tunable surface plasmons in a periodic structure of graphene nanoribbons (NRs), and the incident light, in the THz-to-IR range, used for testing. The model is based on the solution of Maxwell’s equa tions for the NR structure deposited on a dielectric substrate, using the electromagnetic Green’s function, and is extended to the case of an additional (buffer) layer present between the NRs and the substrate. Both the graphene NRs and the layer of adsorbed molecules are considered as two-dimensional, since their thicknesses are very small in comparison with the wavelength of the incident light. The model is applied to different molecular systems, the protein studied in Ref. [1], for which an excellent agreement with experimental data is obtained, and an organometallic molecule Cd(CH3)2. Two different assumptions concerning the way of sticking of the analyte molecules to the sensor’s surface are considered and the limitations of this sensing principles are discussed.Funding from the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Financing UID/FIS/04650/2019. Authors also acknowledge FEDER and the Portuguese Foundation for Science and Technology (FCT) for support through projects POCI-01-0145-FEDER-028114 and PTDC/FIS-MAC/28887/2017. MIV also acknowledges support from the European Commission through the project “Graphene-Driven Revolutions in ICT and Beyond”- Core 3 (Ref. No. 881603)

Phonon modes and raman scattering in SixGe1-x nanocrystals: microscopic modelling

Si1-xGex nanocrystals (NCs) of different composition and size were generated using the Molecular Dynamics (MD) method by minimizing NC’s total energy calculated using Tersoff’s empirical potential and applying rigid boundary conditions. The dynamical matrix of the relaxed NC was constructed and the NC phonon modes were calculated. The localisation of the principal (Si-Si, Si-Ge and Ge-Ge) modes is investigated by analysing their inverse participation ratio. The dependence of the corresponding Raman spectra, obtained by employing the bond polarisability model, upon x and the NC size is presented and compared to previous calculated results and available experimental data.Fundação para a Ciência e a Tecnologia (FCT

Resonant excitation of confined excitons in nanocrystal quantum dots using surface plasmon-polaritons

Surface plasmon-polaritons (SPPs) in a multilayer structure consisting of a metallic film and one or more layers of nanocrystal (NC) quantum dots (QDs) are studied theoretically. It is shown that there is a resonance coupling between the plasmonpolaritons propagating along the metal/NC-layer interface and excitons confined in the dots, which produces a considerable effect on the optical properties of the structure unless the dispersion of the QD size is too large. Using a transfer matrix formalism, multilayer structures consisting of NC composite and metallic films are considered and it is demonstrated that the coupling extends over several layers constituting the structure. It can be explored in order to selectively excite QDs of different size by making a layer-by layer assembled NC planar structure and using an attenuated total reflection (ATR) configuration for the SPPenhanced excitation of the dots. In particular, it opens the possibility to control the relative intensity of light of different color, emitted by the QDs of different size.Fundação para a Ciência e a Tecnologia (FCT) - PTDC/FIS/113199/200

Exciton-photon interactions in semiconductor nanocrystals: Radiative rransitions, non-radiative processes and environment effects

In this review, we discuss several fundamental processes taking place in semiconductor nanocrystals (quantum dots (QDs)) when their electron subsystem interacts with electromagnetic (EM) radiation. The physical phenomena of light emission and EM energy transfer from a QD exciton to other electronic systems such as neighbouring nanocrystals and polarisable 3D (semi-infinite dielectric or metal) and 2D (graphene) materials are considered. In particular, emission decay and FRET rates near a plane interface between two dielectrics or a dielectric and a metal are discussed and their dependence upon relevant parameters is demonstrated. The cases of direct (II–VI) and indirect (silicon) band gap semiconductors are compared. We cover the relevant non-radiative mechanisms such as the Auger process, electron capture on dangling bonds and interaction with phonons. Some further effects, such as multiple exciton generation, are also discussed. The emphasis is on explaining the underlying physics and illustrating it with calculated and experimental results in a comprehensive, tutorial manner.Funding from the Ministry of Science and Higher Education of the Russian Federation (State Assignment No 0729-2020-0058), the European Commission within the project "GrapheneDriven Revolutions in ICT and Beyond" (Ref. No. 696656), from the Portuguese Foundation for Science and Technology (FCT) in the framework of the PTDC/NAN-OPT/29265/2017 "Towards high speed optical devices by exploiting the unique electronic properties of engineered 2D materials" project and the Strategic Funding UID/FIS/04650/2019 is gratefully acknowledged

Cascade up-conversion of photoluminescence in quantum dot ensembles

We present a theoretical model and Monte Carlo simulation results that naturally explain all the features of the thermally activated photoluminescence upconversion effect also known as anti-Stokes photoluminescence ASPL observed in ensembles of colloidal semiconductor nanocrystal quantum dots QDs . The proposed ASPL mechanism includes the following principal ingredients: i optical-phonon-assisted absorption of an incident photon in a relatively large dot in the ensemble, ii emission of a higher-energy photon from the zero-phonon exciton-polaron state, with an upconversion equal to one optical-phonon energy, and iii cascade reabsorption and re-emission processes involving QDs of successively smaller sizes within the sample, rendering the experimentally observed large anti-Stokes shift of the energy of the photon that finally leaves the sample. The results obtained by the Monte Carlo modeling based on the proposed mechanism reproduce all the experimentally observed ASPL trends in colloidal QD solutions.Fundação para a Ciência e a Tecnologia (FCT