Raman Scattering Efficiency Research Articles

A spectroscopic sensing platform for MARCKS protein monolayers

We developed a highly sensitive silicon platform, suitable to assess the molecular organization of protein samples. Prototype platforms were obtained using different electrochemical protocols for the electrodeposition of Ag-nanoparticles onto the hydrogenated silicon surface. A platform with high Surface Enhanced Raman Scattering efficiency was selected based on the surface coverage and the number density of particles size distribution. The performance of the platform was determined by studying the interaction of Myristoylated Alanine-Rich C Kinase Substrate (MARCKS) protein with the substrate according to its molecular organization. The chemical and structural characteristics of MARCKS molecules were examined under two configurations: i) a disordered distribution given by a MARCKS solution drop deposited onto the platform and, ii) a compact monolayer transferred to the platform by the Langmuir-Blodgett method. Raman spectra show vibrational bands of Phenylalanine and Lysine residues specific for the protein effector domain, and evidence the presence of alpha helix structure in both configurations. Moreover, we distinguished the supramolecular order between the compact monolayer and random molecular distribution. The platforms containing Ag-nanoparticles are suitable for studies of protein structure and interactions, advancing a methodological strategy for our long term goal, which is to explore the interaction of proteins with model membranes.

The artificial control of enhanced optical processes in fluorescent molecules on high-emittance metasurfaces.

Plasmon-enhanced optical processes in molecules have been extensively but individually explored for Raman scattering, fluorescence, and infrared light absorption. In contrast to recent progress in the interfacial control of hot electrons in plasmon-semiconductor hybrid systems, plasmon-molecule hybrid systems have remained to be a conventional scheme, mainly assuming electric-field enhancement. This was because it was difficult to control the plasmon-molecule interface in a well-controlled manner. We here experimentally substantiate an obvious change in artificially enhanced optical processes of fluorescence/Raman scattering in fluorescent molecules on high-emittance plasmo-photonic metasurfaces with/without a self-assembled monolayer of sub-nm thickness. These results indicate that the enhanced optical processes were successfully selected under artificial configurations without any additional chemical treatment that modifies the molecules themselves. Although Raman-scattering efficiency is generally weak in high-fluorescence-yield molecules, it was found that Raman scattering becomes prominent around the molecular fingerprint range on the metasurfaces, being enhanced by more than 2000 fold at the maximum for reference signals. In addition, the highly and uniformly enhancing metasurfaces are able to serve as two-way functional, reproducible, and wavelength-tunable platforms to detect molecules at very low densities, being distinct from other platforms reported so far. The change in the enhanced signals suggests that energy diagrams in fluorescent molecules are changed in the configuration that includes the metal-molecule interface, meaning that plasmon-molecule hybrid systems are rich in the phenomena beyond the conventional scheme.

Modelling of the near‐field of metallic nanoparticle gratings: localized surface plasmon resonance and SERS applications

We numerically study the influence of the shape of gold nanostructures on the spectral position of their localized surface plasmon resonance and on Surface Enhanced Raman Scattering efficiency. Calculations are performed using a Finite-Difference Time-Domain (FDTD) method, whose accuracy for dispersive media is enhanced through the use of a Drude-Lorentz model. The relevance of this method is then pointed out by comparing the calculations with experimental data for both the plasmon resonance and the Raman signal enhancement. The influence of a thin layer of water is also investigated.

The absolute efficiency of resonant Raman scattering by longitudinal optical phonons in ZnTe near the E0 gap

The absolute efficiency of resonant Raman scattering by longitudinal optical phonons is determined for an orientated ZnTe bulk crystal and single-crystalline ZnTe epilayers with incident photon energies near the E0 band gap at a temperature of 2 K. The ZnTe layers were grown by metal-organic vapour-phase epitaxy (MOVPE) on (001) GaAs and (001) GaSb, respectively, with a thickness of about 2 mu m. Absolute values of the Raman scattering efficiency (RSE) have been determined in z(x,x)z and z(y,x)z backscattering configuration using a sample-substitution method and correcting the measured intensities with respect to absorption, reflection, and refraction. Measurements of the absorption coefficient and the reflectivity were performed on the same samples and at the same temperature. Well resolved maxima of the RSE appear at the m=1 and m=2 discrete energy levels of the band-gap exciton for incoming resonance and at the m=1 level for outgoing resonance. Valence-band splitting due to residual biaxial strain in the ZnTe epilayers results in a splitting of the m=1 resonance maximum for both incoming and outgoing resonance.

Raman scattering in quantum wells in a high magnetic field: Fröhlich interaction.

An explicit expression for the Raman-scattering efficiency in quantum wells (QW's) under high magnetic fields is given for the allowed Fr\"ohlich electron-phonon interaction mechanism. The basic features of the theory are studied with a parabolic band approximation, which allows us to understand the physics involved in the selection rules and the double resonance conditions. Furthermore, a Luttinger Hamiltonian is used to describe the heavy-hole--light-hole admixture in III-V compounds. Selection rules are derived for backscattering configuration and circular polarizations. Phonons can couple via Fr\"ohlich interaction with either light or heavy components of a QW subband, and both intrasubband and intersubband scattering become possible, the selection rule being \ensuremath{\Delta}N=0 for the Landau quantum number. The phonon confinement is studied in thin QW's and a comparison among the different phonon modes is presented: only even phonon modes couple via Fr\"ohlich interaction for ${\mathit{q}}_{\mathrm{\ensuremath{\perp}}}$=0. We have calculated the Raman polarizability of a 100-A\r{} GaAs/AlAs multiple quantum well and compared it with recent experimental results.

One-LO-phonon resonant Raman scattering in wide-gap diluted magnetic semiconductors.

The efficiency of resonant Raman scattering by LO phonons in wide-gap diluted magnetic semiconductors is investigated near the ${\mathit{E}}_{0}$ gap in external magnetic fields with ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Te as an example. A theoretical model for the absolute Raman-scattering efficiency (RSE) in zinc-blende-type semiconductors, taking into account Wannier-Mott excitons as intermediate electronic states, is given for the special case of Zeeman-split conduction and valence bands. Neglecting Landau quantization this Zeeman splitting arises from the exchange interaction between band electrons and localized magnetic moments of ${\mathrm{Mn}}^{2+}$ ions. Deformation-potential (DP) and Fr\"ohlich (F) interactions for the exciton--one-phonon coupling are both taken into account. The Zeeman splitting and exciton states are considered in the framework of the envelope-function approximation using a parabolic model for the conduction and valence bands. Explicit expressions for the RSE as a function of the laser frequency ${\mathrm{\ensuremath{\omega}}}_{\mathit{L}}$ and of the external magnetic field B are given. Conditions for double resonance are discussed and selection rules are deduced for different scattering configurations. Absolute Raman intensities for scattering by LO phonons via DP and F exciton-phonon interaction in ${\mathrm{Cd}}_{0.8}$${\mathrm{Mn}}_{0.2}$Te have been measured in the Faraday configuration with external magnetic fields B=0--4 T. Quantitative comparison between experiment and theory shows that (a) an inhomogeneous broadening of the excitonic states due to the random distribution of Cd and Mn atoms has to be taken into account and (b) the measured RSE in the DP scattering configurations is strongly influenced by depolarization of the scattered light near resonance.

Local vibrational modes of Ge-rich c-Si1-xGex alloys.

The local vibrational modes of Ge-rich c-${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ alloys (x\ensuremath{\ge}0.75) are calculated using a parameter-free, mass-defect Green's-function approach. A microscopic expression of the Raman-scattering efficiency is then evaluated for inelastic light scattering from these modes. Particular attention is paid to the modes which are responsible for the fine structure in the phonon spectra which is observed in addition to the strong Ge-Ge, Si-Si, and Si-Ge modes. The corrections to the extended Ge-Ge modes due to alloy disorder are calculated with use of the coherent-potential approximation and also included in the Raman susceptibility.

Resonant Raman scattering in quantum wells in high magnetic fields: Deformation-potential interaction.

A theoretical study of one-phonon resonant Raman scattering in a quantum well (QW) in high magnetic fields has been performed. The Raman profiles are calculated as a function of magnetic field, quantum-well thickness, and laser frequency. The basic theory is first developed assuming parabolic masses in the plane perpendicular to the growth direction of the QW. Selection rules for deformation-potential-allowed scattering are given and a compact analytical expression for the Raman-scattering efficiency is obtained for infinite barriers. The double-resonance conditions are derived as a function of the magnetic field or well thickness. In a second part of the work, the heavy-hole--light-hole valence-band admixture has been taken into account through a 4\ifmmode\times\else\texttimes\fi{}4 Luttinger Hamiltonian. It is shown that phonons couple the heavy component of a band with the light component of the other via a deformation potential, giving rise to the selection rule \ensuremath{\Delta}n=\ifmmode\pm\else\textpm\fi{}2 (\ensuremath{\Delta}N=0 for the Landau quantum number). The Raman polarizability has been calculated for a 100-\AA{} GaAs/AlAs multiple QW for different magnetic fields as a function of laser energy and magnetic field. Comparison with recent experimental results allows us to classify the leading double resonance as due to transitions between the excitons formed with the first and second heavy-hole subbands, and the first electron subband with Landau quantum number N=1.

Impurity-induced resonant Raman scattering.

An explicit expression for impurity-induced one-LO-phonon forbidden resonant Raman scattering in diamond and zinc-blende-type semiconductors, which includes excitonic effects, is presented. It is derived by fourth-order perturbation theory and can be applied in a photon energy range below and above the exciton energy. We have considered both neutral and ionized impurities in the exciton-impurity coupling. Discrete and continuous exciton states have been taken as virtual intermediate states in the process, and the matrix elements corresponding to different excitonic transitions have been calculated analytically. The different contributions to the squared Raman polarizability are compared; the most important ones are found to be due to discrete-discrete-discrete transitions. An analysis of the dependence of the Raman-scattering efficiency on impurity concentration, screening factor, and lifetime broadening is presented and quantitative differences with the forbidden Fr\"olich Raman scattering by LO phonons are discussed. These results are used to calculate the absolute value of the Raman efficiency around the ${\mathit{E}}_{0}$ gap of AlSb and the ${\mathit{E}}_{0}$+${\mathrm{\ensuremath{\Delta}}}_{0}$ gap of GaAs. The actual role of the impurity-induced scattering in terms of the impurity concentration is clarified.

Microscopic theory of second-order Raman scattering in silicon under uniaxial stress.

A microscopic calculation of the second-order Raman-scattering efficiency of silicon, with special emphasis on the effects of uniaxial stress, is presented. Employing diagrammatic perturbation theory, the scattering efficiency is obtained for incident- and scattered-photon energies either on or off resonance. A numerical evaluation employs realistic models for the electronic band structure and phonon spectra, and sums over all two-, three-, and four-band scattering channels. The scattering efficiency is normalized by the integrated first-order efficiency, and the results of the calculation are quantitatively compared with experimental data for second-order Raman scattering in crystalline silicon under [001] stress, taken with an incident laser energy of 2.34 eV.

Resonant first- and second-order Raman scattering in AlSb.

We have studied the resonant Raman scattering by one and two LO phonons in the vicinity of the ${\mathit{E}}_{0}$ gap of AlSb at 20, 100, and 300 K. The first-order allowed and forbidden scattering, as well as their interferences, are greatly enhanced by excitonic effects. A theory that takes into account excitons as virtual intermediate states has been used for the interpretation of the measured absolute scattering efficiencies. Good quantitative agreement is obtained for allowed and forbidden scattering and a reasonable qualitative agreement for their interferences. In the case of forbidden scattering, the efficiencies measured at high temperature are stronger than the low-temperature ones, a fact that can be explained if we assume an extrinsic impurity-induced Fr\"ohlich mechanism. Second-order scattering is compared with predictions based on uncorrelated electron-hole pairs. In order to reproduce the absolute values of the Raman-scattering efficiency the theoretical intensities must be multiplied by a factor of 6, while in the first-order spectrum a factor of a thousand is needed if uncorrelated electrons are assumed. This means that excitonic effects are less important in second-order resonant Raman scattering. This can be attributed to the fact that the incoming photons resonate mainly with the continuum.

Raman scattering of polaritons in a KTiOPO4 crystal

The spontaneous Raman scattering of polaritons in the biaxial orthorhombic KTiOPO4(KTP) crystal was investigated with a cw Ar+ laser. The dispersion branches of polaritons associated with A1-phonons have been calculated from spectral data for different wave vector directions from near-forward scattering experiments. The angular dependence of Raman scattering efficiency (RSE) in 60-300cm-1 and 500-800cm-1 regions are shown schematically. The collective interaction of multi-mode lattice vibrations and an "overall" effect are discussed phenomenally.

First- and second-order resonant Raman scattering in graphite.

We report a determination of the Raman-scattering efficiency for first- and second-order phonon lines in graphite. These measurements are of interest for the analysis of the Raman spectra of amorphous carbon films, materials of increasing technological interest.

Resonant Raman scattering by optical phonons in ZnSe and Zn 0.97Mn 0.03Se near the E0 gap

The efficiency of resonant one-LO-phonon Raman scattering in oriented cubic ZnSe and Zn 0.97Mn 0.03Se single crystals is studied near the E 0 gap at 115 and 80 K, respectively, and compared with theoretical calculations. Absolute values of the Raman scattering efficiency (RSE) were determined for scattering by the deformation-potential and Fröhlich electron-phonon interaction. A quantitative comparison of the experimental data with existing model calculations for the RSE, where bound and continuum exciton states are considered as intermediate electronic states in the scattering process, shows good agreement. No significant differences between constructive and destructive interference of the two scattering mechanisms were found. Near resonance strong Raman intensities were observed in backscattering configurations, where scattering via the deformation-potential and Frölich interaction is forbidden according to calculated Raman selection rules.

Triply resonant second-order Raman scattering at the E0 and E0+ Delta 0 gap of GaP under uniaxial stress.

Using compressive uniaxial stress we have tuned the energy splitting between the spin-orbit-split and the light-hole valence band in GaP to be equal to the energy of two phonons (LO or TO). Thus, triply resonant second-order Raman scattering, where all three denominators involved in the Raman-scattering efficiency resonate simultaneously, becomes possible. We have observed the two-LO-, LO + TO-, and two-TO-phonon lines in triple-resonance conditions. We discuss the results in terms of Fr\"ohlich and deformation-potential interaction.

Electronic interband Raman scattering in direct III-V semiconductors. non-local effects

The interband Raman Scattering efficiency is evaluated within the effective mass approximation. The Hamiltonian describing the electronic states contains non-local potentials and therefore the interaction with the radiation field has to be modified in order to preserve the gauge independence of physical quantities. The results are compared to previous calculations, showing the importance of non-local corrections.

Raman scattering in antiferromagnetic FePS3 and FePSe3 crystals.

Raman spectra of iron phosphorus trichalcogenide crystals (FeP${X}_{3}$,X=S,Se) are measured, and a compilation of the peak frequencies is given. The spectral features due to first-order phonon Raman scattering are analyzed and assigned. At low temperatures in iron phosphorus trisulfide the vibrational Raman-scattering efficiency displays an anomalous enhancement and new structures appear. The temperature dependence of the integrated intensity, bandwidth, and frequency of the Raman peaks is studied and correlated with the antiferromagnetic ordering. It is found that the spin superstructure produces a folding of the phonon dispersion curves into the center of the Brillouin zone and activates new phonon modes in the Raman spectrum.

Effect of Pressure on the Raman Scattering Efficiency of GaP

AbstractThe effect of pressures up to 10 GPa on the efficiencies for Raman scattering by TO and LO phonons is measured in GaP for the 1.92 and 2.18 eV laser lines. These scattering efficiencies show a logarithmic decrease with pressure. From the measured efficiencies, the lattice and electro‐optic contributions to the Raman polarizability are obtained as functions of pressure. The decrease of STO with increasing pressure is interpreted as a preresonant process induced by the increase of the E0‐gap with pressure. The second order susceptibility is also discussed on the bases of a simple dispersion model.

Nonlinear Optical Coefficients and the Raman Scattering Efficiency of LO and TO Phonons in Acentric Insulating Crystals

The relations between nonlinear optical coefficients and Raman scattering efficiencies of LO and TO phonons in crystals of arbitrary (acentric) structure are derived, under the adiabatic, electrostatic, and harmonic approximations usual to lattice dynamical theory. Expressions for the total electro-optic coefficients and the nonlinear optical coefficients are presented in terms of Raman efficiencies and the optic-mode frequencies. The use of the macroscopic field in computing the "electro-optic" contribution to LO scattering efficiencies is shown to be justified. The practical conditions under which these expressions may be solved are examined using scattering data for LiNb${\mathrm{O}}_{3}$, LiTa${\mathrm{O}}_{3}$, GaAs, BeO, ZnS, ZnO, and CdS. Theory and experiment are found to be in good agreement. It appears that for many materials nonlinear coefficients and electro-optic coefficients may be determined from Raman data with accuracy comparable to direct measurement in the traditional manner.

Quantitative Determination of Sources of the Electro-Optic Effect in LiNbO3and LiTaO3

The electro-optic effect in crystals can be separated into two types of microscopic interaction: an electron-lattice contribution in which the applied field produces a lattice displacement, which in turn modifies the electronic polarizability (or refractive index), and a direct electron-field contribution in which the applied field modifies the electronic polarizability in the absence of lattice displacements. The latter contribution in LiNb${\mathrm{O}}_{3}$ and LiTa${\mathrm{O}}_{3}$ can be estimated from second-harmonic-generation experiments by Miller and Savage, and accounts for less than 10% of the refractive-index change. Each polar-lattice optic mode in LiNb${\mathrm{O}}_{3}$ and LiTa${\mathrm{O}}_{3}$ ($4{A}_{1}+9E$) contributes separately to the electro-optic effect an amount proportional to the product of its Raman-scattering efficiency and infrared oscillator strength. We have measured the absolute scattering efficiencies for LiNb${\mathrm{O}}_{3}$ and LiTa${\mathrm{O}}_{3}$. The oscillator strengths for LiNb${\mathrm{O}}_{3}$ have been measured by Barker and Loudon. We find that the dominant contribution to the electro-optic coefficients ${r}_{33}$ and ${r}_{13}$ comes from the lowest-frequency ${A}_{1}$ mode; and to ${r}_{42}$ and ${r}_{22}$ from the next lowest $E$ mode. These same modes dominate the low-frequency dielectric constant. The absolute values of ${r}_{13}$, ${r}_{33}$, ${r}_{42}$, and ${r}_{22}$ calculated from the combined Raman, infrared, and second-harmonic-generation data are in excellent agreement with the electro-optic coefficients measured directly by Turner. In addition to the absolute scattering efficiencies for all the transverse and longitudinal modes in LiTa${\mathrm{O}}_{3}$ and LiNb${\mathrm{O}}_{3}$, we have also determined the mode frequencies and linewidths, which are important in calculating Raman gain.