Raman Frequencies Research Articles

Splitting of charge-density and spin-density excitations of a bipolar plasma in a frequency shifter of mixed type-I and type-II quantum wells

The Raman frequency of the intersubband charge-density excitation plasmon in type-I and type-II quantum wells is known to be very sensitively controlled via a low-power optical pump signal. We find that, above a threshold electron density of approximately $4\ifmmode\pm\else\textpm\fi{}2\ifmmode\times\else\texttimes\fi{}{10}^{10}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}2}$, the charge density plasmon mode splits first into two, and then with increasing density into three closely spaced frequencies. A similar splitting occurs in the spin-density wave plasmon associated with the same intersubband transition. We analyze the results including the coupling to the longitudinal optical phonon and hypothesize that the splittings arise from the special situation of a structure of spatially separated bipolar plasmons.

Ultrafast Raman Loss Spectroscopy Unravels the Dynamics in Entangled Singlet and Triplet States in Thioxanthone.

Thioxanthone (TX), an aromatic ketone, exhibits significant solvent-dependent photophysical properties. Herein, we employed time-resolved ultrafast Raman loss spectroscopy (URLS) to decipher the solvent-dependent structural dynamics in entangled singlet and triplet states of photoexcited TX. The evolution of the vibrational spectrum reveals structural changes that occur during the intersystem-crossing (ISC) process and the subsequent energy dissipation to the surrounding solvent. The C═O stretch (∼1320 cm-1) of TX in the excited state acts as the marker band as it undergoes a red shift with time constants of ∼45 and ∼5 ps in acetonitrile and methanol, respectively. Such a red shift is an indicator of the softening of the bond due to the change in the electronic spin states. We also observed a blue shift in Raman frequencies corresponding to the C═C stretch and the C═O stretching modes of TX in acetonitrile and methanol, indicating vibrational cooling in the excited singlet and triplet states. In the case of TX in cyclohexane, vibrational modes at 190 and 415 cm-1 exhibit a blue shift with a time constant of ∼700 fs, which represents the structural distortion during internal conversion (S2 → S1) process. The kinetics of amplitudes of these modes follows biexponential growth with time constants of ∼3 and ∼14 ps representing the time scales for the ISC process and the planarization process in the triplet state, respectively. The URLS study therefore provides a direct measure of the various stages of the solvent-dependent structural dynamics in the excited state of TX.

Picosecond supercontinuum generation in large mode area photonic crystal fibers for coherent anti-Stokes Raman scattering microspectroscopy

We perform a detailed theoretical and experimental investigation of supercontinuum generation in large-mode-area photonic crystal fibers pumped by a high-energy, high-repetition rate picosecond Nd:YVO4 laser, with the goal of using it as the Stokes beam in coherent anti-Stokes Raman scattering setup. We analyze the influence of fiber structure and length on the supercontinuum power, spectral shape, and group delay dispersion. We identify the experimental conditions for stable supercontinuum generation, with microjoule-level pulse energy and the spectrum extending beyond 1600 nm, which allows excitation of Raman frequencies up to 3000 cm−1 and beyond. We demonstrate reliable and efficient operation of a coherent anti-Stokes Raman spectroscopy and microscopy setup using this supercontinuum source.

Scattering of Ginzburg–Frank and Cherenkov Types Under Self-Focusing of Nanosecond Laser Pulses in Liquids

We study the dynamics of nonlinear optical processes such as self-focusing, self-phase modulation, and stimulated Raman scattering in Kerr-liquids under the nanosecond laser pulse excitation. The results prove the existence of the transition Ginzburg–Frank-type effect, which promotes the appearance of new spectral components of the laser radiation at the medium boundary. The generation of extended anti-Stokes frequency-angular bands of stimulated Raman scattering is explained. When the velocity of a self-focusing focal spot matches the phase velocity of the non-linear polarization at the anti-Stokes Raman frequency and the phase velocity of the scattered axial radiation, the most intense frequency-angular bands appear. They are described by the equations typical of the Cherenkov radiation.

Mechanochemistry in [6]Cycloparaphenylene: A Combined Raman Spectroscopy and Density Functional Theory Study.

Raman spectroscopy under high pressures up to 10 GPa and density functional computations up to 30 GPa are combined to obtain insights into the behavior of a prototypical nanohoop conjugated molecule, [6]cycloparaphenylene ([6]CPP). Upon increasing pressure, the nanohoop undergoes deformations, first reversible ovalization and then at even higher pressures aggregates are formed. This irreversible aggregation is caused by the formation of new intermolecular σ-bonds. Frequencies and derivatives of the Raman frequency shifts as a function of pressure are well reproduced by the computations. The frequency behavior is tied to changes in aromatic/quinonoid character of the nanohoop. The modeling at moderate high pressures reveals the deformation of the [6]CPP molecules into oval-like and peanut-like shapes. Surprisingly the pressure derivatives of the observed Raman mode shifts undergo a sudden change around a pressure value that is common to all Raman modes, indicating an underlying geometrical change extended over the whole molecule that is interpreted by the computational modeling. Simulations predict that under even larger deformations caused by higher pressures, oligomerization reactions would be triggered. Our simulations demonstrate that these transformations would occur regardless of the solvent, however pressures at which they happen are influenced by solvent molecules encapsulated in the interior of the [6]CPP.

Detail investigations of SmFeO3 under extreme condition

Temperature and pressure dependent Raman spectroscopic studies are presented which showed signatures of spin-reorientation transition in SmFeO3. The x-ray diffraction study as a function of pressure is also presented that shows the variation in structural parameters like inter-atomic distances, bond angles and degree of octahedral tilt. The electron density plots obtained from Rietveld refinement of the high pressure x-ray diffraction provides a direct visualization of the deformation at the Fe-site, while there is more symmetric structure at the Sm site. The variation in the FeO6 octahedral tilt at high pressures results in large variations in the phonon spectra at the respective pressures indicating modification in local symmetry leading to changes in magnetic structure. The observations in Raman spectroscopy studies are supplemented by density functional theoretical studies. The Raman frequencies calculated from the DFT matched well with the observed values and both showed a subtle modulation between 8 and 12 GPa.

DFT study on Raman frequencies of molten lead (II) chloride

ABSTRACTThe study concerns ab inito calculations of the essential Raman frequencies of molten lead (II) chloride near the melting point. Modelling of topologically disordered lead (II) chloride was carried out within the framework of the density functional theory using the Perdew–Burke-Ernzerhof (PBE) functional and optimised basis sets. Calculations were performed for a cluster containing 24 formula units. The optimum geometry of the cluster was determined and the local structure parameters were found. Nano-size effect leads to the picture of damped oscillations on radial atomic density distributions typical of molten salts. Distorted octahedra of PbCl6 were detected inside the cluster. Ab initio calculation of the Raman spectrum of the octahedral complexes inside the cluster structure was implemented. It was shown that the spectrum has a peak at 192 cm−1, which agrees well with the experimental Raman spectra of lead (II) chloride melt near the melting point.

Solitons and Josephson-type oscillations in Bose-Einstein condensates with spin-orbit coupling and time-varying Raman frequency

The existence and dynamics of solitons in quasi-one-dimensional Bose-Einstein condensates (BEC) with spin-orbit coupling (SOC) and attractive two-body interactions are described for two coupled atomic pseudo-spin components with slowly and rapidly varying time-dependent Raman frequency. By varying the Raman frequency linearly in time, it was shown that ordinary nonlinear Schr\"odinger-type bright solitons can be converted to striped bright solitons and vice versa. The internal Josephson oscillations between atom-number of the coupled soliton components, and the corresponding center-of-mass motion, are studied for different parameter configurations. In this case, a mechanism to control the soliton parameters is proposed by considering parametric resonances, which can emerge when using time-varying Raman frequencies. Full numerical simulations confirm variational analysis predictions when applied to the region where regular solitons are expected. In the limit of high frequencies, the system is described by a time-averaged Gross-Pitaevskii formalism with renormalized nonlinear and SOC parameters and modified phase-dependent nonlinearities. By comparing full-numerical simulations with averaged results, we have also studied the lower limits for the frequency of Raman oscillations in order to obtain stable soliton solutions.

High-Pressure Elastic, Vibrational and Structural Study of Monazite-Type GdPO4 from Ab Initio Simulations

The GdPO4 monazite-type has been studied under high pressure by first principles calculations in the framework of density functional theory. This study focuses on the structural, dynamical, and elastic properties of this material. Information about the structure and its evolution under pressure, the equation of state, and its compressibility are reported. The evolution of the Raman and Infrared frequencies, as well as their pressure coefficients are also presented. Finally, the study of the elastic constants provides information related with the elastic and mechanical properties of this compound. From our results, we conclude that monazite-type GdPO4 becomes mechanically unstable at 54 GPa; no evidence of soft phonons has been found up to this pressure at the zone center.

Raman Spectral Band Oscillations in Large Graphene Bubbles.

Raman spectra of large graphene bubbles showed size-dependent oscillations in spectral intensity and frequency, which originate from optical standing waves formed in the vicinity of the graphene surface. At a high laser power, local heating can lead to oscillations in the Raman frequency and also create a temperature gradient in the bubble. Based on Raman data, the temperature distribution within the graphene bubble was calculated, and it is shown that the heating effect of the laser is reduced when moving from the center of a bubble to its edge. By studying graphene bubbles, both the thermal conductivity and chemical reactivity of graphene were assessed. When exposed to hydrogen plasma, areas with bubbles are found to be more reactive than flat graphene.

Determining Eigenfrequencies and Homogeneous Widths of Lines of Intermolecular Vibrations in Water and in Aqueous Solutions of Hydrogen Peroxide Using Raman Spectroscopy

We have examined low-frequency Raman spectra of intermolecular vibrations of weak aqueous solutions of hydrogen peroxide and water. The differences between the observed Raman frequencies and the data from the literature on the IR-absorption frequencies of the same vibrations, as well as the interrelations between the frequencies and the widths of Lorentzian contours that approximate the vibrational spectra of the dynamic susceptibility, have been discussed. Based on a model of free damped vibrations of a classical oscillator with an inhomogeneous broadening, we have explained these effects for the first time. The homogeneous line widths and the damping times of the observed intermolecular vibrations have been determined. The eigenfrequencies of these vibrations have been calculated, and they have been shown to agree well with the data from the literature on the frequencies of IR absorption of water. We have shown that these parameters of intermolecular vibrations for water and for aqueous solutions of hydrogen peroxide differ from each other.

Filamentation of mid-IR pulses in ambient air in the vicinity of molecular resonances

Properties of filaments ignited by multi-millijoule, 90fs mid-infrared pulses centered at 3.9μm are examined experimentally by monitoring plasma density, losses, spectral dynamics and beam profile evolution at different focusing strengths. By changing from strong (f=0.25 m) to loose (f=7 m) focusing, we observe a shift from plasma-assisted filamentation to filaments with low plasma density. In the latter case, filamentation manifests itself by beam self-symmetrization and spatial self-channeling. Spectral dynamics in the case of loose focusing is dominated by the nonlinear Raman frequency downshift, which leads to the overlap with the CO2 resonance in the vicinity of 4.2μm. The dynamic CO2 absorption in the case of 3.9μm filaments with their low plasma content is the main mechanism of energy losses and, either alone or together with other nonlinear processes, contributes to the arrest of intensity.

1164.4 nm and 1174.7 nm dual-wavelength Nd : GdVO4/Cr4+ : YAG/YVO4 passively Q-switched Raman microchip laser.

A synchronous pulsed, dual-wavelength Raman laser at 1164.4nm and 1174.7nm has been demonstrated in a Nd:GdVO4/Cr4+:YAG/YVO4 passively Q-switched Raman microchip laser (PQSRML). The 1164.4nm and 1174.7nm dual-wavelength first-order Stokes laser oscillation is attributed to the conversion of the 1063.2nm and 1063.43nm two-longitudinal-mode fundamental lasers with Raman frequency shifts of 816 cm-1 and 890 cm-1, respectively. Stable dual-wavelength Raman laser pulses with nearly equal spectral intensities have been achieved independent of the pump power. A pulse repetition rate as high as 139.4kHz has been achieved with T0=85%, and the pulse width has been shortened to 825ps with T0=70%. A dual-wavelength Raman laser with sub-nanosecond pulse width and peak power of over 1kW has been achieved in the Nd:GdVO4/Cr4+:YAG/YVO4 PQSRML.

(Invited) Avalanche Photoemission in Suspended Carbon Nanotubes: Light without Heat

We report bright electroluminescence from suspended carbon nanotube (CNT) field effect transistors (FETs) under extremely low applied electrical powers (∼nW). Here, light emission occurs under positive applied gate voltages, with the FET in its “off” state. This enables us to apply high bias voltages (4 V) without heating the CNT. Under theseconditions, we observe light emission at currents as small as 1 nA and corresponding electrical powers of 4nW, which is 3 orders of magnitude lower than previous studies. Thermal emission is ruled out by monitoring the G band Raman frequency, which shows no evidence of heating under these small electrical currents. The mechanism of light emission isunderstood on the basis of steep band bending that occurs in the conduction and valence band profiles at the contacts, which produces a peak electric field of 500 kV/cm, enabling the acceleration of carriers beyond the threshold of exciton emission. The exciton-generated electrons and holes are then accelerated in this field and emit excitons in an avalanche process. This is evidenced by an extremely sharp increase in the current with bias voltage (45 mV/dec). We also observe light emission at negative applied gate voltages when the FET is in its “on” state at comparable electrical powers to those reported previously (∼5 μW). However, substantial Joule heating (T > 1000 K) is also observed under these conditions, and it is difficult to separate the mechanisms of thermal emission from hot carrier photoemission in this regime. Bo Wang, Fatemeh Rezaeifar, Jihan Chen, Sisi Yang, Rehan Kapadia and Stephen B. Cronin, ACS Photonics , DOI:10.1021/acsphotonics.7b00455 (2017)

High-pressure yield strength of rocksalt structures using quartz Raman piezometry

The Raman frequencies of quartz are used to evaluate deviatoric stresses in rocksalt-structure media in diamond-anvil cell experiments to pressures up to 20GPa. The piezospectroscopic effect in quartz is modeled by first-principles calculations. Non-hydrostatic stresses measured in halogen salts give yield strength estimates of 3GPa in the B1 structure (NaCl), and 4.5GPa for the B2 structure (KCl and KBr). Raman measurements in MgO show that the yield strength is reached at around 6±1GPa. Measurements on quartz alone indicate similar yield strength. The estimated yield strength in MgO is thus likely a lower bound, in consistency with former radial X-ray diffraction measurements that gave a yield strength of 8±1GPa, and lower-pressure large-volume press experiments indicating a yield strength of 6GPa at 5GPa. Former values of the yield strength below 2GPa determined by pressure gradient measurements were underestimated due to unverified assumptions in boundary conditions. The yield strength increases with increasing coordination of ionic solids, likely contributing to increase viscosity at phase transitions near the upper–lower mantle boundary.

Calculation of the Raman frequency and the damping constant (linewidth) of the stretching modes for the metal-organic compound DMMg close to the paraelectric-ferroelectric transitions

ABSTRACTWe calculate the Raman frequencies of the two stretching modes of as a function of temperature close to the paraelectric-ferroelectric transition (Tc = 270 K) in (CH3)2NH2Mg(HCOO)3 referred as DMMg. By assuming that the Raman frequencies of those two modes which exhibit anomalous behaviour near Tc due to the ordering of the dimethlyammonium cations (DMA+) as observed experimentally, are associated with the spontaneous polarization (order parameter), their Raman frequencies and damping constants are predicted at various temperatures. The two models (pseudospin-phonon coupled and energy-fluctuation models) are used for the prediction of the damping constant which also explain the critical behaviour of the observed FWHM data for the stretching modes close to the paraelectric-ferroelectric transition in DMMg.

Structure and optical properties of 2D layered MoS2 crystals implemented with novel friction induced crystal growth

We used X-ray diffraction, and Raman and photoluminescence (PL) spectroscopies to examine the structure and optical properties of molybdenum disulfide (MoS2) crystals grown by friction at the interface between two materials. MoS2 is produced chemically from molybdenum dithiocarbamates (MoDTC) in synthetic oil under sliding friction conditions. The X-ray diffraction (XRD) patterns indicate that the structure of the MoS2 is layered with the c-axis perpendicular to the surface. The MoS2 layer was formed on stainless steel and germanium by friction at the interface between these materials and high carbon chromium bearing steel. The number of layers is estimated to be N (N > 6) from the distance between the Raman frequencies of the E12g and A1g modes. For MoS2 grown on stainless steel, exciton peak is observed in the PL spectrum at room temperature. These results show that this friction induced crystal growth method is viable for synthesizing atomic layers of MoS2 at solid surfaces.

Calculation of the Raman and IR frequencies from the volume data at high pressures in N2

Raman and IR frequencies of N2 in the molecular state are calculated as a function of pressure up to 160 GPa by using the volume data from the literature through the mode Grüneisen parameter. By determining the Grüneisen parameters for the lattice modes and vibrons which decrease mostly with increasing pressure from the observed frequency (Raman, IR) and volume data, the Raman and IR frequencies of those modes are calculated at various pressure at room and low temperatures. We find that the Raman and IR frequencies increase as the pressure increases in N2 as observed experimentally. This method of calculating the Raman and IR frequencies from the volume data can be applied to the some other molecular crystals.

Long-wave infrared picosecond parametric amplifier based on Raman shifter technology.

A new method for a long-wave infrared (LWIR), picosecond difference frequency generation (DFG) source using one near-infrared laser and a Raman shifter is experimentally tested and characterized. The signal seed for DFG is a Stokes pulse generated via transient stimulated Raman scattering in a nonlinear medium with a Raman frequency in the 2-20 µm range. A study of the dynamics of the transient Raman regime in liquid C6D6 has shown that the efficiency of Stokes production can be increased and the central wavelength can be controlled by chirping the pump pulse in order to compensate for chirping caused by self-phase modulation. High energy, ≥3 µJ, picosecond pulses at 10.6 µm have been generated in a GaSe crystal pumped by 1 mJ pulses of 1060 nm light from a Nd:glass laser.

Spectroscopic and DFT Characterization of a Highly Reactive Nonheme FeV–Oxo Intermediate

The reaction of [(PyNMe3)FeII(CF3SO3)2], 1, with excess peracetic acid at -40 °C generates a highly reactive intermediate, 2b(PAA), that has the fastest rate to date for oxidizing cyclohexane by a nonheme iron species. It exhibits an intense 490 nm chromophore associated with an S = 1/2 EPR signal having g-values at 2.07, 2.01, and 1.94. This species was shown to be in a fast equilibrium with a second S = 1/2 species, 2a(PAA), assigned to a low-spin acylperoxoiron(III) center. Unfortunately, contaminants accompanying the 2(PAA) samples prevented determination of the iron oxidation state by Mössbauer spectroscopy. Use of MeO-PyNMe3 (an electron-enriched version of PyNMe3) and cyclohexyl peroxycarboxylic acid as oxidant affords intermediate 3b(CPCA) with a Mössbauer isomer shift δ = -0.08 mm/s that indicates an iron(V) oxidation state. Analysis of the Mössbauer and EPR spectra, combined with DFT studies, demonstrates that the electronic ground state of 3b(CPCA) is best described as a quantum mechanical mixture of [(MeO-PyNMe3)FeV(O)(OC(O)R)]2+ (∼75%) with some FeIV(O)(•OC(O)R) and FeIII(OOC(O)R) character. DFT studies of 3b(CPCA) reveal that the unbound oxygen of the carboxylate ligand, O2, is only 2.04 Å away from the oxo group, O1, corresponding to a Wiberg bond order for the O1-O2 bond of 0.35. This unusual geometry facilitates reversible O1-O2 bond formation and cleavage and accounts for the high reactivity of the intermediate when compared to the rates of hydrogen atom transfer and oxygen atom transfer reactions of FeIII(OC(O)R) ferric acyl peroxides and FeIV(O) complexes. The interaction of O2 with O1 leads to a significant downshift of the Fe-O1 Raman frequency (815 cm-1) relative to the 903 cm-1 value predicted for the hypothetical [(MeO-PyNMe3)FeV(O)(NCMe)]3+ complex.