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We experimentally demonstrate time-resolved exciton propagation in a monolayer semiconductor at cryogenic temperatures. Monitoring phonon-assisted recombination of dark states, we find a highly unusual case of exciton diffusion. While at 5 K the diffusivity is intrinsically limited by acoustic phonon scattering, we observe a pronounced decrease of the diffusion coefficient with increasing temperature, far below the activation threshold of higher-energy phonon modes. This behavior corresponds neither to well-known regimes of semiclassical free-particle transport nor to the thermally activated hopping in systems with strong localization. Its origin is discussed in the framework of both microscopic numerical and semiphenomenological analytical models illustrating the observed characteristics of nonclassical propagation. Challenging the established description of mobile excitons in monolayer semiconductors, these results open up avenues to study quantum transport phenomena for excitonic quasiparticles in atomically thin van der Waals materials and their heterostructures.

An optimized procedure is described for the acquisition of 785 nm excited SERS spectra of dried bloodstains and shown to offer great potential for rapid, portable, highly sensitive and specific, confirmatory identification for forensic applications. Following extraction in 1 μL of 50% acetic acid, a robust, highly reproducible SERS spectrum is observed from dried bloodstains resulting from a hematin-like heme moiety (ferric, high spin). As anticipated, this blood signature can be classified with 100% specificity and sensitivity with respect to the SERS spectra of other body fluids. High quality SERS spectra can be observed from stains of blood diluted by as much as 105. Dried blood spectra acquired on Au and Ag SERS active substrates exhibit very different relative intensities at this electronically, non-resonant excitation wavelength (785 nm) indicating that a strong chemical effect contributes to the SERS enhancement of this body fluid. DFT calculations further confirm the vibrational band assignments of the features seen in these SERS spectra of dried blood.

Specific features in the formation of Landau levels in a three-dimensional topological insulator based on an 80-nm-thick strained mercury telluride film in an inclined magnetic field are studied. The magnetoresistance in the Hall bar geometry with a gate is measured at a temperature 1.9 K and in an applied magnetic field up to 10 T. The gate allows varying the Fermi level position from the valence to conduction band, passing through the bulk band gap. The samples are mounted on a rotating platform that makes it possible to change arbitrarily the angle between the magnetic field direction and the normal to the sample plane within the range of 0°–90°. It is found that the Shubnikov–de Haas oscillations are formed if the perpendicular magnetic field component exceeds 0.4 T, independent of the applied gate voltage. However, the sensitivity of the system to the parallel magnetic field component exhibits a pronounced dependence on the applied gate voltage. Namely, if the Fermi level is in the bulk band gap and the conductivity is determined by the surface states, the amplitude and position of Shubnikov–de Haas oscillations remain independent of the perpendicular magnetic field component even in the situation where the parallel component is twice as large as the perpendicular one. At high magnetic fields, the amplitude of oscillations is suppressed by the parallel field component. On the contrary, if the Fermi level is in the valence or conduction band, the parallel field component not only affects the amplitude of oscillations but also results even in their qualitative changes, e.g., giving rise to new minima related to Zeeman splitting. The observed behavior of the system is in agreement with the modern concepts concerning the spin polarization of the surface state in three-dimensional topological insulators and the spin degeneracy of charge carriers in the bulk.

A density matrix treatment of plasmon-enhanced (PE) stimulated Raman spectroscopies is developed. Specifically, PE stimulated Raman Gain/Loss (PE-SRG/L) and coherent anti-Stokes Raman scattering (PE-CARS) due to monochromatic excitation and PE femtosecond stimulated Raman spectroscopy (PE-FSRS) are considered. A Lorentz oscillator model is used to explicitly describe the time dependence of plasmon-enhanced optical fields. These temporal characteristics are required for a density matrix based description of all plasmon-enhanced nonlinear molecular spectroscopies. Dispersive vibrational line shapes in PE-SRG/L and PE-FSRS spectra are shown to result primarily from terms proportional to the square of the complex optical field enhancement factor. The dependence on the plasmon resonance, picosecond and femtosecond pulse characteristics, and molecular vibrational properties are evident in the density matrix derived PE-FSRS intensity expression. The difference in signal detection mechanisms accounts for the lack of dispersive line shapes in PE spontaneous Raman spectroscopy. This density matrix treatment of PE-FSRS line shapes is compared with prior coupled wave results.

Surface enhanced Raman spectroscopy (SERS) provides highly specific vibrational signatures identifying dried blood for a variety of forensic applications. SERS spectra on Au nanoparticle substrates excited at 785 nm are found to identify dried stains of human and nonhuman blood from seven animals, and distinguish stains due to menstrual and peripheral blood. In addition, the unique SERS bloodstain spectrum is distinct from the SERS spectra of thirty red-brown stains of potential household substances that could be visually mistaken for bloodstains and from food stains that have been shown to give positive results with presumptive colorimetric blood tests. Finally, a SERS swab procedure has been developed and demonstrates that the substrates that a blood sample dried on does not offer any Raman or fluorescence interference for the SERS identification of dried blood. Such bloodstains on porous and nonporous materials are all identical and exclusively due to the heme moiety of hemoglobin. Optimized selection of the extraction solvent is found to control the chemical composition of molecular components appearing in the SERS spectrum of complex, multicomponent biological mixtures, such as body fluids.

Stimulated Raman scattering (SRS) microscopy allows for high-speed label-free chemical imaging of biomedical systems. The imaging sensitivity of SRS microscopy is limited to ~10 mM for endogenous biomolecules. Electronic pre-resonant SRS allows detection of sub-micromolar chromophores. However, label-free SRS detection of single biomolecules having extremely small Raman cross-sections (~10−30 cm2 sr−1) remains unreachable. Here, we demonstrate plasmon-enhanced stimulated Raman scattering (PESRS) microscopy with single-molecule detection sensitivity. Incorporating pico-Joule laser excitation, background subtraction, and a denoising algorithm, we obtain robust single-pixel SRS spectra exhibiting single-molecule events, verified by using two isotopologues of adenine and further confirmed by digital blinking and bleaching in the temporal domain. To demonstrate the capability of PESRS for biological applications, we utilize PESRS to map adenine released from bacteria due to starvation stress. PESRS microscopy holds the promise for ultrasensitive detection and rapid mapping of molecular events in chemical and biomedical systems., Stimulated Raman scattering (SRS) microscopy enables label-free chemical imaging at high speed, but has been limited by low sensitivity. Here, the authors demonstrate plasmon-enhanced SRS microscopy and achieve single molecule detection sensitivity.

It has been recently suggested [N. E. Marotta and L. A. Bottomley, Appl. Spectrosc. 64, 601-606 (2010)] that previously reported surface-enhanced Raman scattering (SERS) spectra of vegetative bacterial cells are due to residual cell growth media that were not properly removed from samples of the lab-cultured microorganism suspensions. SERS spectra of several commonly used cell growth media are similar to those of bacterial cells, as shown here and reported elsewhere. However, a multivariate data analysis approach shows that SERS spectra of different bacterial species grown in the same growth media exhibit different characteristic vibrational spectra, SERS spectra of the same organism grown in different media display the same SERS spectrum, and SERS spectra of growth media do not cluster near the SERS spectra of washed bacteria. Furthermore, a bacterial SERS spectrum grown in a minimal medium, which uses inorganics for a nitrogen source and displays virtually no SERS features, exhibits a characteristic bacterial SERS spectrum. We use multivariate analysis to show how successive water washing and centrifugation cycles remove cell growth media and result in a robust bacterial SERS spectrum in contrast to the previous study attributing bacterial SERS signals to growth media.

Defined nanoparticle cluster arrays (NCAs) with total lateral dimensions of up to 25.4 microm x 25.4 microm have been fabricated on top of a 10 nm thin gold film using template-guided self-assembly. This approach provides precise control of the structural parameters in the arrays, allowing a systematic variation of the average number of nanoparticles in the clusters (n) and the edge-to-edge separation (Lambda) between 1 < n < 20 and 50 nm < or = Lambda < or = 1000 nm, respectively. Investigations of the Rayleigh scattering spectra and surface-enhanced Raman scattering (SERS) signal intensities as a function of n and Lambda reveal direct near-field coupling between the particles within individual clusters, whose strength increases with the cluster size (n) until it saturates at around n = 4. Our analysis shows that strong near-field interactions between individual clusters significantly affect the SERS signal enhancement for edge-to-edge separations Lambda < 200 nm. The observed dependencies of the Raman signals on n and Lambda indicate that NCAs support a multiscale signal enhancement which originates from simultaneous inter- and intracluster coupling and |E|-field enhancement. The NCAs provide strong and reproducible SERS signals not only from small molecules but also from whole bacterial cells, which enabled a rapid spectral discrimination between three tested bacteria species: Escherichia coli, Bacillus cereus, and Staphylococcus aureus.

An analysis of optical heterodyne detected (OHD) two-photon absorption (TPA) resonant signals in transparent materials is presented. The formal analogies between the vibrationally resonant Raman and electronically resonant TPA P ( 3 ) signals and response functions are given. Fourier transform procedures are shown to allow recovery of the complex TPA response function and eliminate residual Raman contributions from these spatially selected OHD TPA responses. The phase selectivity and wavelength dependence of this technique can be exploited to reveal both the real and imaginary parts of the line shape function describing the solute-solvent response to the two-photon resonant electronic excitation. In-phase and in-quadruture measurements at any resonant wavelength yield the absorption band shape. Intramolecular vibronic structure can be determined for totally diffuse absorption bands. These effects are illustrated by the analysis of two model electronic absorption systems: the MD-simulated methyl iodide B-state origin band in Ar and a multimode Brownian-oscillator-modeled absorption line shape with simple vibronic structure. Electronic excitations less accessible to one-photon resonant ultrafast studies such as UV/VUV transition, electronic excitations in inherently optically dense media, and dipole-forbidden states are well suited for study by this TPA approach.

The frequency selected dichroic ultrafast responses of neat carbon tetrachloride are reported as a function of pump and probe polarization orientations in a standard two beam experimental configuration. The recovery of the Raman spectral density from these experimental observations is demonstrated. The influence of coherent coupling effects, in this one color, electronically nonresonant class of ultrafast pump–probe experiments is considered. While their contribution to Raman spectral densities via birefringent observations can be neglected, these terms can only be eliminated from the dispersed dichroic technique for Raman spectral density recovery when two-color pump–probe studies are performed.

Two-photon absorption (TPA) resonance effects on the ultrafast pump–probe and transient grating (TG) responses of some transparent liquids excited with 400 nm pulses are demonstrated. The contribution of TPA and ground state Raman responses are separated in a TG experiment when the third-order polarization signal is viewed in the appropriate phase-matched direction as shown for the homodyne, TG response of CS 2 . The pump–probe dichroic responses of liquids with (CS 2 and CH 3 I) and without (H 2 O, CCl 4 , and CH 3 OH) TPA resonances at 200 nm are compared. These results are discussed in the context of a perturbative third-order polarization treatment of TPA.

A simple time domain method for the observation of polarization-specific Raman responses in electronically nonresonant materials is demonstrated. When a cutoff filter is placed in the probe beam pa...

The depolarized reduced Raman and corresponding optical Kerr effect (OKE) spectral density of ambient CS2 have been calculated by way of time correlation function (TCF) and instantaneous normal mode (INM) methods and compared with experimental OKE data. When compared in the reduced Raman spectrum form, where the INM spectrum is proportional to the squared polarizability derivative weighted density of states (DOS), the INM results agree nearly quantitatively (at all but the lowest frequencies) with the TCF results. Both are in excellent agreement with experimental measurements. The INM signal has a significant contribution from the imaginary INMs. Within our INM theory of spectroscopy the imaginary INMs contribute like the real modes, at the magnitude of their imaginary frequency. When only the real modes are allowed to contribute, and the spectrum is rescaled to account for the missing degrees of freedom, the results are much poorer, as has been observed previously. When the spectra are compared in their ...

Care must be exercised in the use of Raman spectroscopy for the identification of blood in forensic applications. The Raman spectra of dried whole human blood excited at 785 nm are shown to be exclusively due to oxyhemoglobin or related hemoglobin denaturation products. Raman spectra of whole blood are reported as a function of the incident 785-nm-laser power, and features attributable to heme aggregates are observed for fluences on the order of 10(4) W/cm(2) and signal collection times of 20 s. In particular, the formation of this local-heating-induced heme aggregate product is indicated by a redshifting of several heme porphyrin ring vibrational bands, the appearance of a large broad band at 1,248 cm(-1), the disappearance of the Fe-O2 stretching and bending bands, and the observation of a large overlapping fluorescence band. This denaturation product is also observed in the low-power-excitation Raman spectrum of older ambient-air-exposed bloodstains (2 weeks or more). The Raman spectrum of methemoglobin whole blood excited at 785 nm is reported, and increasing amounts of this natural denaturation product can also be identified in Raman spectra of dried whole blood particularly when the blood has been stored prior to drying. These results indicate that to use 785-nm-excitation Raman spectra as an identification method for forensic applications to maximum effect, incident laser powers need to be kept low to eliminate variable amounts of heme aggregate spectral components contributing to the signal and the natural aging process of hemoglobin denaturation needs to be accounted for. This also suggests that there is a potential opportunity for 785-nm-excitation Raman spectra to be a sensitive indicator of the age of dried bloodstains at crime scenes.

An instantaneous normal mode (INM) analysis of the short‐time solvation dynamics of the B‐state (200 nm) Rydberg excitation of methyl iodide in high pressures of Ar (ρ*=0.08, 0.3, and 0.8) is presented. Solute–solvent interaction potentials for this system have been determined by previous absorption and resonance scattering studies. The B‐state transition energy correlation function (ECF), also known as the solvation correlation function, calculated by the linear coupling INM theory is in good agreement with the ECF given by molecular dynamics simulation at short times (≤150 fs) that are well beyond the so‐called inertial regime (≤100 fs). The shape and peak frequency of the solvation spectra are relatively constant over the wide range of bath densities considered here in contrast to the INM total density of states. This is attributed to the relative density independence of the first peak in the solute–solvent pair distribution function. Similarly, the ECFs are also only modestly dependent on solvent dens...

The polarization of the resonance fluorescence of symmetric top rigid rotors is described by a third‐order density matrix treatment of resonance emission and a sum‐over‐all‐rovibronic states scattering‐tensor invariant framework. Within this theoretical approach the resonance fluorescence depolarization is a function of the excited electronic state population and rovibronic coherence decay rates, as well as the electronic absorption/emission line shapes. This description of the depolarization of resonance fluorescence is contrasted with that of resonance Raman in terms of angular momentum selection rules and dependence on material relaxation parameters. In contrast to resonance Raman emission in solution, the accompanying resonance fluorescence polarization is found to be most sensitive to the resonant excited state lifetime when this population decay time is of the order or less than rotational periods. These effects are demonstrated for excitation resonant with the B‐state origin of CH3I vapor in high p...

In this study we report pump-probe femtosecond spectroscopy on 5CB and 8CB liquid crystals at different phase transitions. Also, molecular dynamic calculations of the liquid crystal vibrational modes will be presented using Gaussian 09 software.

The observed electronic line broadening of the X→B (6s Rydberg) absorption spectrum of CH3I vapor in moderate to high pressures (55–140 atm, reduced densities ∼0.08–0.14) of Ar and CH4 is analyzed via molecular dynamics simulations. Good fits to the absorption line shapes are found in this pressure/density range for a given set of ground and excited state solute–solvent potential parameters in the static limit, i.e., the absorption linewidths and shapes are dominated by inhomogeneous broadening on the time scale of the decay of the dipole correlation function. The pressure dependence of these absorption line shape changes is explained in terms of the shape of the solute–solvent ground‐excited state difference potential. Consistent with the static limit description at these moderate to high pressures, the corresponding transition energy correlation function, a quantity of central importance in stochastic and Brownian oscillator line shape theories, decays on a much longer time scale than the inverse absorp...

We report the use of gold nanorods as solution-based SERS substrates for the detection of ultralow-levels of three different dithiocarbamate fungicides: thiram, ferbam and ziram. Gold nanorods are attractive to use as SERS substrates due to the ability to tune the surface plasmon resonance of the nanoparticles to the laser excitation wavelength of the Raman spectrometer equipped with a 785 nm diode laser. The gold nanorods are synthesized using a seed-mediated growth method and characterized using UV-Visible spectroscopy, zeta potential, and TEM. The gold nanorods have an aspect ratio of 2.19 ± 0.21 and have an average length of 37.81 ± 4.83 nm. SERS spectra are acquired at different concentrations of each fungicide and calibration curves are obtained by monitoring the intensity of the band arising from the ν(C-N) stretching mode coupled to the symmetric δ(CH(3)) motion. The limits of detection and limits of quantitation are obtained for each fungicide. The limits of detection are 11.00 ± 0.95 nM, 8.00 ± 1.01 nM, and 4.20 ± 1.22 nM for thiram, ferbam, and ziram respectively. The limits of quantitation are 34.43 ± 0.95 nM, 25.61 ± 1.01 nM, and 12.94 ± 1.22 nM for thiram, ferbam, and ziram respectively. It can be seen that the three different dithiocarbamates can be detected in the low nM range based on the limits of detection that are achieved.

SERS spectra of whole human blood, blood plasma and red blood cells on Au nanoparticle SiO2 substrates excited at 785 nm have been observed. For the sample preparation procedure employed here, the SERS spectrum of whole blood arises from the blood plasma component only. This is in contrast to the normal Raman spectrum of whole blood excited at 785 nm and open to ambient air, which is exclusively due to the scattering of oxyhemoglobin. The SERS spectrum of whole blood shows a storage time dependence that is not evident in the non-SERS Raman spectrum of whole blood. Hypoxanthine, a product of purine degradation, dominates the SERS spectrum of blood after ~10 – 20 hours of storage at 8 °C. The corresponding SERS spectrum of plasma isolated from the stored blood shows the same temporal release of hypoxanthine. Thus, blood cellular components (red blood cells, white blood cells and/or platelets) are releasing hypoxanthine into the plasma over this time interval. The SERS spectrum of red blood cells (RBCs) excited at 785 nm is reported for the first time and exhibits well known heme group marker bands, as well as other bands that may be attributed to cell membrane components or protein denaturation contributions. SERS, as well as normal Raman spectra, of oxy- and met-RBCs are reported and compared. These SERS results can have significant impact in the area of clinical diagnostics, blood supply management and forensics.

Vibronically specific subrotational period lifetimes of the predissociated 22B2 state of NO2 vapour (250–235 nm) have been determined by spontaneous resonance Raman scattering polarization measurements. The lifetimes of the vibronic symmetric stretching, ν1′, (100), bending, ν2′, (101), and asymmetric stretching, ν3′, (002), overtone levels are 75 ± 10 fs, 2.5 ± 05 ps and 155 ± 15 fs respectively. When one quantum of bending (ν2,′) is excited in combination with one quantum of totally symmetric stretching (ν1′), the lifetime increases from 75 ± 10 to 125 ± 15 fs (110). The 22 B2 state electronic origin lifetime has been previously shown to be 42 ± 5 ps independent of angular momentum quantum numbers. These results are interpreted in terms of the shape of the potential energy surface intersections in the region and the results of earlier quantum yield measurements.

A density matrix treatment of the time evolution of the third order polarization response describing the optical heterodyne detected (OHD) transient birefringence and dichroism excited by ultrafast pulses is given. The relationship between frequency domain (Raman scattering) and time domain (pump–probe) spectroscopies is revealed by this pathway explicit description. Constructive and destructive interferences between time evolution density matrix pathways account for the respective strong birefringent and weak dichroic ground state nuclear response when the pulses are electronically nonresonant. However, for electronically resonant chromophores, the dichroic response is larger than the corresponding birefringent response due to constructive and destructive interferences respectively between density matrix time evolution histories. No such interferences contribute to spontaneous Raman scattering. The relative magnitude of the resonant dichroic and birefringent responses is pulse width dependent in the fast...

The resonance hyper‐Raman (RHR) scattering of CH3I vapor is observed as blue incident radiation is tuned through two‐photon resonance with the ν2 vibronic absorption band of the predissociative B←X Rydberg transition in the UV. In analogy to linear resonance Raman spectroscopy, the RHR band shapes and polarization are found to be a sensitive function of the two‐photon resonant vibronic state lifetime and detuning. The dependence of these scattering characteristics on the two‐photon excited‐state dephasing constant provides a technique for determining subpicosecond predissociation rates via nonlinear spontaneous Raman scattering. Theoretical fits to the observed resonant rovibrational hyper‐Raman depolarization ratio dispersion curves are obtained for a vibronic B‐state lifetime of 0.5±0.2 ps. This lifetime value is in agreement with previous results obtained from linear resonance Raman scattering studies. The polarization analysis of RHR scattering, in particular, provides a technique for measuring excited‐state lifetimes of molecular transitions in the VUV.

The polarization-resolved resonance hyper-Raman (RHR) scattering of CH3I vapor is observed as blue incident radiation is tune through two-photon resonance with the origin absorption band of the predissociative ∼ ← ∼ Rydberg transition in the VUV (183–175 nm). These results demonstrate the ability of this nonlinear Raman technique to probe the ultrafast dynamics of molecular levels in the VUV. The C-state lifetime is determined to be 180±30 fs by fits of theory to the depolarization ratio excitation profile and rovibrational hyper-Raman bandshapes. Hence, predissociation is approximately three times faster on the C-state than the B-state surface of CH3I (τ = 500 fs). Additionally, the depolarization ratio dispersion and RHR bandshapes indicate mode specific predissociation effects, at least for the ν3 methyl iodine vibronic C-state level, just as found for the B state.

A simple and fast electrochemical process for gold surface functionalization aimed to immobilize biomolecules is presented here. Raman spectroscopy was used to characterize the complete system starting from surface functionalization to final structure with immobilized proteins.

ICORS, Boston, Massachusetts, USA, paper BThP11, 2010; International audience

This contribution describes progress towards the development and testing of a functionalized capillary that will provide detection of low‐concentration gas‐phase analytes through SERS. Measurement inside a waveguide allows interrogation of a large surface area, potentially overcoming the short distance dependence of the SERS effect.The possible use of Raman spectroscopy for gas detection is attractive for IR‐inactive molecules or scenarios where infrared technology is inconvenient. However, the weakness of Raman scattering limits the use of the technique to situations where low detection limits are not required or large gas pressures are present.With surface‐enhanced Raman spectroscopy (SERS), signal enhancements of 106 are often claimed, and higher values are seen in specific instances. However, most of the examples of SERS analysis are on liquid‐phase samples, where the molecular density is high, usually combined with some sort of sample concentration at the surface. Neither of these factors is present ...

Hydroxyethylammonium (L) tartrate monohydrate was synthesized and crystals were grown using slow evaporation solution growth technique. Theoretical value of hyperpolarizability was calculated. Factor group analysis was carried out and is associated with the polarized Raman spectra.