Sub-nanosecond strong pulse generated by backward Raman scattering

Hundreds picosecond strong short-wavelength pulses have been generate by a backward Raman oscillator amplifier pumped with a 10-J KrF laser from Heaven-1 MOPA system. Not only high power but also high energy laser pulses have been obtained with an energy conversion efficiency up to 17%. 640-picosecond pulse duration was observed in our experiments by a 1.5-GHz-bandwidth oscilloscope corresponding to 34 times of pulse compression rate.     

Time-domain measurements of S-branch N2–N2 Raman linewidths using picosecond pure rotational coherent anti-Stokes Raman spectroscopy

Time-resolved dual-broadband picosecond pure rotational CARS has been applied to measure self-broadened S-branch N₂–N₂ Raman linewidths in the temperature range 294–1466 K. The coherence decays were detected directly in the time domain by following the J-dependent CARS signal decay as a function of probe delay. The rotational Raman N₂–N₂ linewidths were derived from these time-dependent decays and evaluated for thermometric accuracy. Comparisons were made to the energy-corrected sudden (ECS) and modified exponential gap (MEG) dynamical scaling laws, and the results were used to quantify the sensitivity of nanosecond rotational CARS thermometry to the linewidth model employed. The uncertainty based on the linewidth model used in pure N₂ was found to be 2 %. The merits and limitations of this rapid method for the determination of accurate Raman linewidths are discussed.     

Observation of the Higgs Boson of strong interaction via Compton scattering by the nucleon

It is shown that the Quark-Level Linear σ Model (QLLσM) leads to a prediction for the diamagnetic term of the polarizabilities of the nucleon which is in excellent agreement with experimental data. The bare mass of the σ meson is predicted to be m _σ =666 MeV and the two-photon width Γ(σ→γ γ)=(2.6±0.3) keV. It is argued that the mass predicted by the QLLσM corresponds to the $\gamma\gamma\to\sigma\to N\bar{N}$ reaction, i.e. to a t-channel pole of the γ N→N γ reaction. Large-angle Compton scattering experiments revealing effects of the σ meson in the differential cross section are discussed. Arguments are presented that these findings may be understood as an observation of the Higgs boson of the strong interaction while being a part of the constituent quark.     

Study on forward stimulated Brillouin scattering in a backward pumped fiber Raman amplifier

Strong multi-order forward stimulated Brillouin scattering (SBS) has been observed in the backward pumped S-band distributed fiber Raman amplifier (PRA) with tunable narrow signal source (less than 100 MHz) when the pump power of FRA reached the SBS threshold. This does not obey the theory that only weak backward SBS lines exist according to the conservation of energy and momentum and the wave vector selected rule. This is because the sound waveguide characteristic weakens the wave vector rule, and the forward transmitted sound waveguide Brillouin scattering lines are generated and amplified in FRA. When the pump power is further increased, 11 orders of SBS lines and comb-like profile are observed. For the excited line, the frequency is 197.2296 THz and the power is 0 dBm. The even order SBS lines are stronger than odd order SBS lines, the power of the 2nd and 4th order SBS lines is 1.75 dBm, which is 16 dB higher than that of the 1st and 3rd order SBS lines. The odd order SBS lines are named Brillouin R     

Time-resolved spectral characteristics of external-cavity quantum cascade lasers and their application to stand-off detection of explosives

We present the time-resolved spectral analysis of the settling process in a pulsed external-cavity quantum-cascade laser (EC-QC laser) emitting in the wavelength range around 7.4 μm. Due to mode competition between the two cavities formed by the laser-chip on one hand and the external-cavity on the other hand, a time-dependent spectral emission on a ns time-scale can be observed. Depending on the spectral characteristic of the external cavity with respect to the chip gain curve time-delays of the external cavity emission of 15–35 ns within the 100 ns drive pulse can be observed.     

Effects of O2–CO2 polarization beating on femtosecond coherent anti-Stokes Raman scattering (fs-CARS) spectroscopy of O2

Femtosecond coherent anti-Stokes Raman scattering (fs-CARS) spectroscopy has recently emerged as a promising laser-based temperature-measurement technique in flames. In fs-CARS, the broad spectral bandwidths of the pump and Stokes lasers permit the coupling of each ro-vibrational Raman transition via a large number of pump-Stokes photon pairs, creating a strong Raman coherence. However, the broad-bandwidth fs pulses also excite other molecular transitions that are in resonance. The polarization beating between these closely spaced Raman transitions can affect the coherence dephasing rate of the target molecule, making it difficult to extract accurate medium temperature. In a previous study our group investigated N₂/CO polarization beating in N₂ fs-CARS; in the present work we study O₂/CO₂ polarization beating in O₂ fs-CARS. O₂ fs-CARS can be particularly important for thermometry in non-air-breathing combustion in the absence of N₂. The effects of O₂/CO₂ polarization beating are investigated in the temperature range 300–900 K at atmospheric pressure and also at 300 K for pressures up to 10 bar. Unlike in the N₂/CO system, it was observed in the O₂/CO₂ system that the presence of CO₂ can significantly alter the time evolution of the Raman coherence and, hence, affect the measured temperature.     

A numerical simulation of the backward Raman amplifying in plasma

This paper describe a numerical simulation method for the interaction between laser pulses and low density plasmas based on hydrodynamic approximation. We investigate Backward Raman Amplifying （BRA） experiments and their variants. The numerical results are in good agreement with experiments.     

Continuously tunable VUV radiation (129–210 nm) by anti-Stokes Raman scattering in cooled H2

The generation of tunable vacuum ultraviolet radiation by anti-Stokes Raman scattering of tunable ultraviolet dye-laser radiation in cold hydrogen has been investigated. The scattering efficiency of XeCl laser and Nd:YAG laser pumped commercial dye lasers and the influence of different beam profiles has been studied. Up to 12 anti-Stokes orders down to 129 nm were observed with output powers between about 20 kW at =191 nm and somewhat less than 100 W at =129 nm. The efficiency of transversely pumped lasers with an intensity peaked in the center of the beam profile was found to be higher than doughnut shaped intensity distributions. The cooling of the active gas to liquid nitrogen temperatures improved the output pulse energies 3 to 5 times on average. It was found that this intensity increase was caused mainly by the narrowing of the Raman linewidth upon cooling.     

Effect of nonstoichiometry on Raman scattering of VO2 films

We report on Raman scattering of VO2 films prepared by radio frequency magnetron sputtering under different conditions. Our investigations revealed that the dominated Raman peaks shift towards high frequency for both V-rich and O-rich VO2 films, compared with the stoichiometry VO2 films. The experimental evidence is presented and the cause for nonstoichiometry dependence of Raman spectra of VO2 films is discussed.     

Generation of Highly Resilient to Decoherence Macroscopic Quantum Superpositions via Phase-covariant Quantum Cloning

In this paper we analyze the resilience to decoherence of the Macroscopic Quantum Superpositions (MQS) generated by optimal phase-covariant quantum cloning according to two coherence criteria, both based on the concept of Bures distance in Hilbert spaces. We show that all MQS generated by this system are characterized by a high resilience to decoherence processes. This analysis is supported by the results of recent MQS experiments of N=3.5×10⁴ particles.     

Preparation of Highly Squeezed States and Multi—component Entangled Coherent States via the Raman Interaction

A method is presented for generating highly squeezed states of a cavity field via the atom-cavity field interaction of the Raman type.In the scheme a sequence of three-level Λ-type atoms interacts with a cavity field,displaced by a classical source,in a Raman manner.Then the atomic states are measured.By this way the cavity field may collapse onto a superposition of several coherent states,which exhibits strong squeezing.The scheme can also be used to prepare superpositions of many two-mode coherent states for two cavity fields.The coherent states in each mode are on a straight line.This is the first way for preparing multi-component entangled coherent states of this type in cavity QED.     

Generation of picoseconds stimulated Raman scattering in a BaWO4 crystal and frequency up-conversion by difference-frequency generation in BBO

The characters of stimulated Raman scattering of BaWO₄ crystal excited by a picoseconds laser at 1064 nm are studied based on optical parametric amplification (OPA). Up to six-order Stokes components and five-order anti-Stokes components are observed. The SRS components are amplified by an OPA and the wavelength tunable range from 411 to 2594 nm is achieved with a maximum conversion efficiency of 38% using the OPA stage.     

Hartmann phase pick-up method for detection and correction of piston aberrations in a multi-beam coherent combination system

In multi-beam coherent combination laser systems, piston aberration between each sub-beam is often regarded as the dominative factor to deteriorate the combination performance. So as to obtain a favorable combination result, a Hartmann phase pick-up method in combination with a seven-element active segmented mirror is presented to pick up and correct the piston aberrations of a seven-beam coherent combination system. A simulative result indicates that the RMS value of the wavefront can be reduced from 401 nm to 3.7 nm, while the far-field Strehl ratio can be increased from 0.22 to 0.99 after correction.     

Detection of explosives and latent fingerprint residues utilizing laser pointer–based Raman spectroscopy

A modular, compact Raman spectrometer, based on a green laser pointer, an air cooled intensified charged coupled device and a x, y motorized translation stage was developed and applied for point detection. Its performance was tested for measurements of Raman spectra of liquids, trace amounts of explosives and individual particles, as well as for locating individual particles of interest and for chemical imaging of residues of latent human fingerprints. This system was found to be highly sensitive, identifying masses as low as ~1 ng in short times. The point and real-time detection capabilities of the spectrometer, together with the portability that it offers, make it a potential candidate for replacing existing Raman microscopes and for field applications.     

Synthesis of indium–silver bimetallic nanocomposites for surface-enhanced Raman scattering

We report on the synthesis of indium–silver bimetallic nanocomposites by chemical reduction method under atmospheric condition and their activity for surface-enhanced Raman scattering (SERS). It is found that the indium–silver bimetallic nanocomposites have better SERS activity with larger enhancement factors (EF) than pure silver nanoparticles with similar size. The SERS EF can reach 10⁷ for 4-mercaptobenzoic acid and 10⁹ for crystal violet and rhodamine 6G adsorbed on the nanocomposites and the detection limits can be at least down to 10^?7 and 10^?10 M, respectively. The results demonstrate that the indium–silver bimetallic nanocomposites are promising as SERS substrate for a myriad of chemical and biological sensing applications.     

Dependence of surface-enhanced Raman scattering from Calf thymus DNA on anions

Dependence of surface-enhanced Raman scattering (SERS) from Calf thymus DNA on anions is inves- tigated.With the silver colloid,the bands at 732,960 and 1333 cm~(-1) for adenine (A),1466 cm~(-1) for deoxyribose,and 1652 cm~(-1) for the C=O group of thymine (T) are observably enhanced.With the pres- ence of the C1 or SO~-anions,the bands at 732 and 1326/1329 cm-1 for the symmetric stretching and skeletal vibrational modes of adenine (A) are dramatically enhanced,and the enhancement effect with the SO_4~(2-) ion is more than that with the Cl~-ion.The experimental results show that the DNA molecule can be adsorbed on the silver colloid particles through the C_6N and N_7 of adenine (A),the C=O of thymine (T) and deoxyribose.Moreover,the formed hydrogen bonding of the Cl~-or SO_4~(2-) ions to the C_6NH_2 group of adenine (A) can induce larger C_6N electronegativity,which is favor for the C_6N/N_7 cooperative adsorption on the (Ag)_n~ colloid particles.     

Elementary-field modeling of surface-plasmon excitation with partially coherent light

We present a mathematical model and its numerical implementation for the analysis of the interaction of spatially partially coherent electromagnetic fields with micro- and nanostructured objects. The model is based on the decomposition of the incident field into a set of fully coherent but mutually uncorrelated elementary field modes, and the use of the Fourier Modal Method (FMM) with the S-matrix propagation algorithm. We apply the model to studies of the excitation of surface plasmons in thin metallic slabs, nanowires, and resonant structures. We demonstrate, e.g., that the plasmon excitation efficiency is not essentially affected by the degree of spatial coherence. However, certain plasmon interference effects can be efficiently smoothed out by using illumination with reduced coherence.     

Laser-induced synthesis of metal–carbon materials for implementing surface-enhanced Raman scattering

Metal–carbon materials exhibiting surface-enhanced Raman scattering have been synthesized by laser irradiation of colloidal systems consisting of carbon and noble metal nanoparticles. The dependence of the Raman scattering intensity on the material composition and laser irradiation conditions has been investigated. The possibility of recording the Raman spectrum of organic dye rhodamine 6G, deposited in amount of 10^–6 M on the substrate obtained from a colloidal solution is demonstrated.     

Predictive powers of chiral perturbation theory in Compton scattering off protons

We study low-energy nucleon Compton scattering in the framework of baryon chiral perturbation theory (BχPT) with pion, nucleon, and Δ(1232) degrees of freedom, up to and including the next-to-next-to-leading order (NNLO). We include the effects of order p ², p ³, and p ⁴/Δ, with Δ≈300 MeV the Δ-resonance excitation energy. These are all “predictive” powers in the sense that no unknown low-energy constants enter until at least one order higher (i.e., p ⁴). Estimating the theoretical uncertainty on the basis of natural size for p ⁴ effects, we find that uncertainty of such a NNLO result is comparable to the uncertainty of the present experimental data for low-energy Compton scattering. We find an excellent agreement with the experimental cross-section data up to at least the pion-production threshold. Nevertheless, for the proton’s magnetic polarizability we obtain a value of (4.0±0.7)×10^?4 fm³, in significant disagreement with the current PDG value. Unlike the previous χPT studies of Compton scattering, we perform the calculations in a manifestly Lorentz-covariant fashion, refraining from the heavy-baryon (HB) expansion. The difference between the lowest order HBχPT and BχPT results for polarizabilities is found to be appreciable. We discuss the chiral behavior of proton polarizabilities in both HBχPT and BχPT with the hope to confront it with lattice QCD calculations in a near future. In studying some of the polarized observables we identify the regime where their naive low-energy expansion begins to break down, thus addressing the forthcoming precision measurements at the HIGS facility.     

Frequency shifting of sub-100 fs laser pulses by stimulated Raman scattering in a capillary filled with pressurized gas

The technique of Raman conversion of sub-100 fs laser pulses based on excitation of active medium by two orthogonally polarized pulses has been developed for Raman lasers with a glass capillary. 52 fs Stokes pulse at the wavelength of 1200 nm has been generated by stimulated Raman scattering of 48 fs Ti:sapphire laser pulse at the wavelength of 800 nm in hydrogen. 13% energy conversion efficiency has been achieved at pulse repetition rate up to 2 kHz.