Stokes Mode Research Articles

Upconversion emission of fluorescent silver nanoclusters and in situ selective DNA biosensing

Due to their good photostability, high quantum yield and low toxicity, fluorescent silver nanoclusters (Ag NCs) have received much attention as novel fluorophores for sensing applications. In this work, we investigate the upconversion emission of Ag NCs templated by single- and double-stranded DNAs. DNA-templated Ag NCs exhibit upconversion emission at wavelengths identical to those observed for the corresponding Stokes emission. Consequently, the Ag NCs' upconversion behavior can be easily tuned by the used DNA sequences. In addition, the Ag NCs are more stable under such an NIR excitation with the upconversion mode relative to the Stokes mode. As a proof-of-concept application, DNA nucleobase recognition with the in situ formed Ag NCs is realized using the Ag NCs' upconversion emission. We expect that the Ag NCs' upconversion emission is more advantageous than the previously used rare-earth materials, at least with respect to easy modulation of the emission energies by DNA sequences, and could find wide application in sensor design.

Cooperative scattering effect between Stokes and anti-Stokes field stimulated by a stream of atoms

This paper is devoted to the study of the cooperative two-photon scattering processes between two resonator modes stimulated by an excited atomic beam. It has been proved that these collective scattering phenomena between the Stokes and anti-Stokes resonator modes are taking place due to the energy transfer between these fields. The statistical properties of the Stokes and anti-Stokes photons have been described using the photon correlation functions. The numerical solution of this equation describes the statistical transformation of n-Stokes photons into anti-Stokes photons.

Sub-shot noise photon-number correlation in stimulated and spontaneous Raman processes

The light initially prepared in a coherent state runs the stimulated and spontaneous Raman processes in a second-order nonlinear medium. Using analytical expressions of field operators for the light modes, we determine the quantum statistical properties of the modes in the stimulated and spontaneous Raman processes. By a choice of phase, we obtain a boson-pair property correlating the pump and vibration modes and such a property correlating the pump and anti-Stokes modes. The Stokes and vibration modes exhibit the boson-pair property for all initial phase differences between the pump mode and the other ones. In the spontaneous Raman process only the Stokes and vibration modes show such a property.

The Hybrid Kelvin–Edge Wave and Its Role in Tidal Dynamics

Abstract A full set of long waves trapped in the coastal ocean over a variable topography includes a zero (fundamental) mode propagating with the coast on its right (left) in the Northern (Southern) Hemisphere. This zero mode resembles a Kelvin wave at lower frequencies and an edge wave (Stokes mode) at higher frequencies. At the intermediate frequencies this mode becomes a hybrid Kelvin–edge wave (HKEW), as both rotational effects and the variable depth become important. Furthermore, the group velocity of this hybrid mode becomes very small or even zero depending on shelf width. It is found that in midlatitudes a zero group velocity occurs at semidiurnal (tidal) frequencies over wide (∼300 km), gently sloping shelves. This notion motivated numerical experiments using the Regional Ocean Modeling System in which the incident HKEW with a semidiurnal period propagates over a wide shelf and encounters a narrowing shelf so that the group velocity becomes zero at some alongshore location. The numerical experiments have demonstrated that the wave energy increases upstream of this location as a result of the energy flux convergence while farther downstream the wave amplitude is substantially reduced. Instead of propagating alongshore, the wave energy radiates offshore in the form of Poincaré modes. Thus, it is concluded that the shelf areas where the group velocity of the HKEW becomes zero are characterized by an increased tidal amplitude and (consequently) high tidal energy dissipation, and by offshore wave energy radiation. This behavior is qualitatively consistent with the dynamics of semidiurnal tides on wide shelves narrowing in the direction of tidal wave propagation, including the Patagonia shelf and the South China Sea.

All-Optical Control of Gigahertz Acoustic Resonances by Forward Stimulated Interpolarization Scattering in a Photonic Crystal Fiber

We report the observation of a novel nonlinear optoacoustic phenomenon, that we name forward stimulated interpolarization scattering. When two frequency-offset laser signals are colaunched into orthogonally polarized guided modes of a birefringent small-core (1.8 μm diameter) photonic crystal fiber, a pattern of axially moving polarization fringes is produced, with a velocity and spacing that depends on the frequency offset. At values of frequency offset in the few-GHz range, the pattern of moving fringes can perfectly match the phase velocity and axial wavelength (3.9 mm) of the torsional-radial acoustic mode tightly guided in the core. An intense optoacoustic interaction ensues, leading to efficient nonlinear exchange of power from the higher frequency (pump) mode to the orthogonally polarized lower frequency (Stokes) mode. A full-vectorial theory is developed to explain the observations.

Accurate determination of electronic transition energy of carbon nanotubes from the resonant behavior of radial breathing modes and their overtones

The resonant Raman behavior of the radial breathing modes are very useful to analyze the electronic property of carbon nanotubes. We investigated the resonant behaviors of Stokes and anti-Stokes radial breathing mode and its overtone of a metallic nanotube, and show how to accurately determine the electronic transition energy of carbon nanotubes from radial breathing modes and their overtones. Based on the present results, the previously reported resonant Raman behavior of the radial breathing modes of SWNT bundles can be interpreted very well.

First integrals of equations of motion of a droplet in the presence of mass transfer to the carrier medium

Analysis is performed of first integrals in the limiting Stokes and Newtonian modes of motion of a droplet for two cases, namely, that of the droplet moving in its own vapor in the presence of phase transitions and that of the droplet moving in dusty gas without phase transformations but in view of the capture of dust particles. Expressions are found for the lengths over which the processes of condensation, complete evaporation of droplet, and deposition of particles occur. Comparison is made of the lengths and times of complete evaporation of droplet in the Stokes and Newtonian modes of droplet motion. Asymptotic formulas are obtained for these quantities, which correspond to low and high superheating of vapor, as well as formulas for the limiting diameters of droplet in the cases of condensation of vapor or capture of particles.

Steady-state and transient Raman gain in magnetoactive narrow band-gap semiconductors

Using the hydrodynamic model of semiconductor-plasmas and following the coupled-mode approach, a detailed analytical investigation is undertaken to study both steady-state and the transient Raman gain in transversely magnetized narrow band-gap semiconductors arising from electron density perturbations and molecular vibrations of the medium. Using the fact that the origin of stimulated Raman scattering (SRS) lies in the third-order susceptibility of the medium, we obtain an expression for the gain coefficient of the backward Stokes mode in steady-state and transient regimes and study the dependence of the magnetic field and pump pulse duration on its growth rate. The threshold pump intensity and optimum pulse duration for the onset of transient SRS are estimated. An externally applied magnetic field substantially enhances the transient SRS gain coefficient in narrow band-gap semiconductors, which can be of great use in the compression of scattered pulses.

Dependence on geometry of coherent Raman-scattered Stokes mode in weakly polar magnetized semiconductors

Abstract Using electromagnetic treatment, a detailed analytical investigation is made of the coherent Raman scattering (CRS) of the Stokes component of scattered wave in an obliquely magnetized (with respect to the direction of propagation) weakly polar III–V semiconductor. The origin of the CRS process lies in the third-order nonlinear optical susceptibility arising from the induced nonlinear current density (due to finiteness of Szigeti effective charge and differential polarizability) and interaction of pump wave with the density fluctuation generated within the medium. The agreement in magnitude of the third-order Raman susceptibility between our calculated values and experimental as well as other theoretical values is found to be good. Using the coupled mode theory of plasmas the steady-state Raman gain coefficient is determined via the effective susceptibility. The results suggest that magnetic field enhances the CRS gain coefficient by a factor of three when cyclotron frequency is tuned to optical phonon frequency under moderate pump field strengths. For field strengths E0>108 V m−1, the contribution of effective charge is nearly wiped off and the Raman gain spectrum becomes dependent on differential polarizability only. The CRS gain coefficient increases with increasing scattering angle and results in a maximum value for the backscattered mode. The backward Raman gain is found to be nearly 104 times larger than forward gain. The analysis also suggests the possibility of observing optical phase conjugation via CRS in weakly polar III–V semiconductors subjected to an external magnetic field.

Beam quality of the Stokes output of continuous-wave Raman fiber amplifiers using multimode fiber

The beam quality of the Stokes output of a Raman fiber amplifier based on a long multimode graded-index fiber is examined. The beam quality of the Stokes output is modeled by considering the relative gain of the Stokes modes of the fiber; this model predicts that the beam quality of the coupled Stokes beam and the beam quality of the coupled pump beam determine the beam quality of the Stokes output. Experimental investigations validate this model by showing that the beam quality of the Stokes output is similar to the beam quality of the Stokes input.

Aromatic Amino Acids Providing Characteristic Motifs in the Raman and SERS Spectroscopy of Peptides

Raman and surface-enhanced Raman spectroscopies (SERS) are potentially important tools in the characterization of biomolecules such as proteins and DNA. In this work, SERS spectra of three cysteine-containing aromatic peptides: tryptophan-cysteine, tyrosine-cysteine, and phenylalanine-cysteine, bound to Au nanoshell substrates, were obtained, and compared to their respective normal Raman spectra. While the linewidths of the SERS peaks are significantly broadened (up to 70%), no significant spectral shifts (<6 cm (-1)) of the major Stokes modes were observed between the two modalities. We show that the Raman and SERS spectra of penetratin, a cell-penetrating peptide oligomer, can be comprised quite reliably from the spectra of its constituent aromatic amino acids except in the backbone regions where the spectral intensities are critically dependent on the length and conformations of the probed molecules. From this study we conclude that, together with protein backbone groups, aromatic amino acid residues provide the overwhelmingly dominant features in the Raman and SERS spectra of peptides and proteins when present. It follows that the Raman modes of these three small constructed peptides may likely apply to the assignment of Raman and SERS features in the spectra of other peptides and proteins.

Diode-pumped multi-frequency Q-switched laser with intracavity cascade Raman emission

A diode-pumped actively Q-switched mixed Nd:Y(0.3)Gd(0.7)VO(4) laser with an intracavity KTP crystal is developed to produce cascade SRS emission up to the fourth order. With an incident pump power of 14 W and a repetition rate of 50 kHz, the average output powers at the first, second, third and fourth Stokes modes are approximately 0.05 W, 0.61 W, 0.25 W, and 0.11 W, respectively. The maximum peak power is greater than 2 kW.

On the large-scale structure and spectral dynamics of two-dimensional turbulence in a periodic channel

This paper reports on a numerical study of forced two-dimensional turbulence in a periodic channel with flat no-slip walls. Since corners or curved domain boundaries, which are met in the standard rectangular, square, or circular geometries, are absent in this geometry, the (statistical) analysis of the flow is substantially simplified. Moreover, the use of a standard Fourier–Chebyshev pseudospectral algorithm enables high integral-scale Reynolds number simulations. The paper focuses on (i) the influence of the aspect ratio of the channel and (ii) the integral-scale Reynolds number on the large-scale self-organization of the flow. It is shown that for small aspect ratios, a unidirectional flow spontaneously emerges, notably in the absence of a pressure gradient in the longitudinal direction. For larger aspect ratios, the flow tends to organize into an array of counter-rotating vortical structures. The computed energy and enstrophy spectra provide further evidence that the injection of small-scale vorticity at the no-slip walls modify the inertial-range scaling. Additionally, the quasistationary final state of decaying turbulence is interpreted in terms of the Stokes modes of a viscous channel flow. Finally, the transport of a passive tracer material is studied with emphasis on the role of the large-scale flow on the dispersion and the spectral properties of the tracer variance in the presence of no-slip boundaries.

Surface-Enhanced Raman Spectroscopy of DNA

We report a method for obtaining highly reproducible surface-enhanced Raman spectroscopy (SERS) of single and double-stranded thiolated DNA oligomers. Following a protocol that relaxes the DNA into an extended conformation, SERS spectra of DNA oligonucleotides are found to be extremely similar, strongly dominated by the Stokes modes of adenine. A spectral correlation function analysis useful for assessing reproducibility and for quantifying the highly complex changes corresponding to modifications in molecular conformation of the adsorbate molecules is introduced. This approach is used to monitor the interaction of DNA with cisplatin, a chemotherapy agent in widespread use.

Final states of decaying 2D turbulence in bounded domains: Influence of the geometry

Direct numerical simulations of two-dimensional decaying turbulence in wall bounded domains are presented. The Navier–Stokes equations are solved using a Fourier pseudo-spectral method with volume penalization. Starting from random initial conditions, we study the influence of the geometry of the domain on the flow dynamics, in particular on the long time behaviour. Circular, square, triangular and annular domains are considered and we show how the geometry plays a crucial role regarding the decay scenario towards final states. Three stages can be distinguished: formation of coherent vortices from random initial conditions, vortex wall interactions, and finally relaxation towards a quasi-steady structure. The eigenvalues estimated from the decay rate of both energy and enstrophy depend on the geometry and agree well with the theoretical eigenvalues based on the Stokes mode of the corresponding domain. For the final states we find a linear functional relation between vorticity and streamfunction.

THE AMPLITUDE SQUEEZING EFFECTS OF THE ELECTROMAGNETIC FIELD IN SIX-WAVE INTERACTION MODEL

The amplitude squeezing properties of the electromagnetic field is investigated in the fundamental, Stokes (probe) and signal mode in six-wave mixing process under short-time approximation based on a fully quantum mechanical approach. The interaction is looked upon as a process involving absorption of two pump photons and emission of three probe photons and one signal photon at different frequencies. The coupled Heisenberg quantum mechanical equations of motion involving real and imaginary parts of the quadrature operators are established and solved under short-time scale. The occurrence of amplitude squeezing effects in both the quadrature of the radiation field in the fundamental mode as well as in the Stokes (probe) mode is investigated using the required conditions of squeezing in each of the cases. It is also shown that the squeezing in the signal mode is found to be dependent on amplitude-squared squeezing of the initial pump field. This gives a method of converting higher-order (amplitude-squared) squeezing into normal squeezing.

An explanation of SRS beam cleanup in graded-index fibers and the absence of SRS beam cleanup in step-index fibers

Beam cleanup via stimulated Raman scattering in multimode fibers is modeled by numerically considering the competition between the Stokes modes of graded-index and step-index fibers. The relative gain of each Stokes mode is calculated by considering the overlap the various pump and Stokes modes of the fibers. Mode competition in a graded-index fiber favors the LP(01) Stokes mode while mode competition in a step-index fiber does not favor the LP(01) Stokes mode. This model explains why beam cleanup has only been reported for graded-index fibers and not for step-index fibers.

Stimulated Raman Scattering in a Weakly Polar III-VSemiconductor: Effect of dc Magnetic Field and Free CarrierConcentration

Using the hydrodynamic model of semiconductor plasmas, we perform ananalytical investigation of stimulated Raman scattering (SRS) of anelectromagnetic pump wave in a transversely magnetized weakly polarsemiconductor arising from electron-density perturbations and molecularvibrations of the medium both produced at the longitudinal opticalphonon frequency. Assuming that the origin of SRS lies in thethird-order susceptibility of the medium, we investigate the growthrate of Stokes mode. The dependence of stimulated Raman gain on theexternal dc magnetic field strength and free carrier concentration isreported. The possibility of the occurrence of optical phaseconjugation via SRS is also studied. The steady-state Raman gain isfound to be greatly enhanced by the presence of the strong external dcmagnetic field.

Steady-state and transient Brillouin gain in magnetoactive narrow band gap semiconductors

Using electromagnetic treatment, a detailed analytical investigation is made to investigate both the steady-state and transient gains of Brillouin Stokes mode in magnetoactive piezoelectric III–V semiconductors. The Brillouin Stokes mode arises from the nonlinear interaction of an intense pump beam with the internally generated acoustic phonon mode and magnetoacoustic phonon mode in the finiteness of piezoelectricity and electrostriction. Following a coupled-mode approach, the third-order (Brillouin) susceptibility and steady-state and transient regime Brillouin gain of the backward Stokes mode are obtained in the presence and absence of an external static magnetic field. The dependence of the piezoelectricity, an external static magnetic field and pump pulse duration on growth rate of the backward Stokes mode has been analysed. The piezoelectricity and an externally applied static magnetic field substantially enhance both the steady-state and transient gain coefficients in a moderately doped III–V semiconductor. The enhanced transient gain coefficient can be greatly used in the compression of the backward scattered Stokes pulse.

Self-imaging silicon Raman amplifier

We propose a new type of waveguide optical amplifier. The device consists of collinearly propagating pump and amplified Stokes beams with periodic imaging of the Stokes beam due to the Talbot effect. The application of this device as an Image preamplifier for Mid Wave Infrared (MWIR) remote sensing is discussed and its performance is described. Silicon is the preferred material for this application in MWIR due to its excellent transmission properties, high thermal conductivity, high damage threshold and the mature fabrication technology. In these devices, the Raman amplification process also includes four-wave-mixing between various spatial modes of pump and Stokes signals. This phenomenon is unique to nonlinear interactions in multimode waveguides and places a limit on the maximum achievable gain, beyond which the image begins to distort. Another source of image distortion is the preferential amplification of Stokes modes that have the highest overlap with the pump. These effects introduce a tradeoff between the gain and image quality. We show that a possible solution to this trade-off is to restrict the pump into a single higher order waveguide mode.