DCM荧光增强C3H6O 受激拉曼散射（SRS）的增益特性研究

报道荧光染料DCM对丙酮SRS的高效放大, 研究了DCM中Stokes波的增益特性. 改变DCM乙醇溶 液浓度，变化进入DCM的Stokes光强度以及抽运激光功率，测量并分析了Stokes波增益与上 述条件的关系. 研究结果发现，当DCM浓度为 2×10^-4mol/L时，Stokes波 获得最大功 率增益达到57；抽运激光功率增加或进入DCM的Stokes信号增强时Stokes波输出增强，但 过强的Stokes 信号将导致增益饱和. 关键词： 受激拉曼散射 荧光增强Stokes波 丙酮 DCM     

Transient Raman gain stimulated by a noise pump

We predict a strong transient Raman gain stimulated by a Gaussian noise field in dispersive media. Our theory is based on the method of successive approximations, the first of which is the Markovian approximation. It is shown that the transient behavior will take place near critical pump intensity both for “short” (τ_p > T₂) and “ultrashort” (τ_p < T₂) laser pulses (τ_p is the pump pulse duration, T₂ is the relaxation time of optical phonons). Solutions for average intensities of the Stokes wave and optical phonons are obtained for arbitrary form of the given pump pulse. We show that transient Raman gain allows to form a very steep leading edge of the Stokes pulse.The steady-state increments of the second approximation describe the effects of a strong noise field. We show that after the intensity of the pump has exceeded a value beyond which the framework of validity of the Markovian approximation does not hold anymore, the steady-state Raman gain is saturated.We give a review of experimental results and make estimations for conditions of observing transient scattering in the field of a noise pump. Methods of obtaining the powerful femtosecond pulses based on stimulated Raman scattering of broad-band excimer lasers and supercontinuum emission are suggested.     

Frequency locking of dye lasers by intracavity stimulated Raman scattering

Intracavity stimulated Raman scattering gain is used to force spectral condensation of the dye laser emission to ～ 2 Å wide lines in an untuned dye oscillator longitudinally pumped by a repetitive N₂-laser. Several stimulated Stokes Raman frequencies of the solvents are observed. Interpretation is discussed in terms of spectro-temporal evolution of the dye intensity within an homogeneous band oscillator.     

Off-resonant excitation of molecular vibrations in liquids,investigated by picosecond probe scattering

The coherent excitation of molecular vibrations in liquids via off-resonant stimulated Raman amplification is shown to consist of two parts with vastily different spectral and temporal properties. One component situated at the frequency difference _L –e _S of the driving fields follows almost instantaneously the system response function. The second part at the eigenfrequency e ₀ carries information on the vibrational dephasing. Detecting both frequency components simultaneously by coherent anti-Stokes probe scattering a novel 2-channel method is demonstrated. The vibrational dephasing time of thev ₃-mode of CH₃I is directly measured to beT ₂=2.6±0.15 ps. This result gives experimental support to the deconvolution techniques required in conventional spectroscopy.     

强激光照射下,α-LiIO3晶体中晶格振动的多模激发和模之间的相互作用

本文叙述了用调Q倍频YAG脉冲激光照射α-LiIO₃晶体,在不同的散射配置下,获得了各种散射谱图。实验表明,当用e光入射时,可以比用o光入射激发起更多不同模的晶格振动;获得了E₂模的很强的一阶、二阶斯托克斯散射线和反斯托克斯散射线,E₁模Polariton的一阶、二阶、三阶斯托克斯线和反斯托克斯线,还获得了反斯托克斯频率区一条频移为469cm^-1的新线,认为是不同模的Polariton之间的相互作用引起的。最后,提到实验中初步观察到的晶体拉曼活性的“疲劳现象”。 关键词：     

Observation of intense stokes and anti-stokes lines in CH4 pumped by 355 nm of a Nd:YAG laser

The stimulated Raman scattering (SRS) and four-wave mixing (FWM) processes have been studied in detail in CH₄ gas pumped by the third-harmonic of Nd:YAG laser at =355 nm. The conversion efficiency and average output energy have been measured for the various Stokes and anti-Stokes lines at different experimental conditions. The threshold input energy at various CH₄ pressures has been calculated for the unguided and capillary waveguide Raman cells. The calculated values for the capillary case have been compared with the measured values at different CH₄ pressures and the two results are in good agreement.     

Frequency shifts in stimulated Raman scattering

The nonresonant contributions to the nonlinear susceptibility χ⁽³⁾ produce a frequency chirp during stimulated Raman scattering. In the case of transient stimulated Raman scattering, the spectrum of the generated Stokes pulse is found at higher frequencies than expected from spontaneous Raman data. The frequency difference can be calculated from the theory of stimulated Raman scattering.     

Transient coherent Raman scattering in the time and frequency domain

A new type of Raman spectroscopy is presented: After transient excitation of molecular modes coherently scattered Raman spectra are investigated in a depayed probing experiment. The spectral position of the Raman mode is observed after long delay times. The dephasing time is obtained from the time dependence of the scattered amplitudes. Frequency disturbing non-resonant susceptibilities are eliminated. We report on first experimental results of transient coherent Raman spectroscopy of liquid CH₃CCl₃.     

New far-infrared laser lines from CH3Cl and CH3Br optically pumped with a continuously tunable high pressure CO2 laser

In this paper we report on the detection of new far-infrared laser lines from CH₃Cl and CH₃Br optically pumped with a continuously tunable high pressure CO₂ laser. We found 80 new lines for CH₃Cl and 9 new lines for CH₃Br in the frequency region between 16 cm^–1 and 41 cm^–1, all due to stimulated Raman scattering. For the Raman gain regions bandwidths up to about 700 MHz were found. We also observed high intensity short far-infrared laser pulses of durations in the nanosecond regime.Permanent address: Physics Department, State Pedagogical University, SU-119435 Moscow, USSR     

Control of radiation spatial coherence under synchronous amplification in crystalline LiF:F 2 − amplifier

A technique of spatial coherence control, based on the synchronous amplification of a radiation in LiF crystals with F ₂ ^? color centers, is demonstrated. Spatial radiation distributions of stimulated Raman scattering (SRS) in oxide crystals were investigated under picosecond laser excitation. Low spatial radiation coherence was revealed for both the transient and quasi-stationary SRS. Spatially incoherent SRS was transformed to spatially coherent radiation as a result of phase—locked picosecond synchronous laser pumping of nonlinear Raman and LiF: F ₂ ^? crystals and the Stokes radiation amplification in the color center crystal.     

Stimulated Raman Stokes scattering influenced by all-trans-β -carotene in liquid-core optical fiber

We demonstrate stimulated Raman Stokes (SRS) scattering of carbon disulfide (CS₂) influenced by a natural dye named all-trans-β-carotene in a liquid-core optical fiber (LCOF). These Stokes emissions can acquire the extra effects of seeding or suppression, both of which result from the optical properties of the carotenoids. Then, the efficient control of the selective developments of the Stokes lines by the natural dyes is available in LCOFs. Especially, the low threshold of the high-order Stokes lines, such as the 2nd Stokes line of CS₂, can be more easily obtained through the seeding effect. The saturation intensity of the 1st Stokes line can be suppressed more when the concentration of all-trans-β-carotene in CS₂ becomes high. Simultaneously, the threshold of the 1st Stokes line increases with the addition of the carotenoids. This result is expected to be worthy of new SRS-based fiber light sources for bio-medicine applications such as photodynamic therapy.     

Temporal characteristics of picosecond stimulated Raman scattering in oxide crystals

The temporal characteristics of stimulated Raman scattering (SRS) under 22-ps laser excitation were studied in eight oxide crystals with a T ₂ optical-phonon dephasing time variable by up to two orders of magnitude. The measured SRS pulse width was shortened from 13.8 ps for Ba(NO₃)₂ (T ₂ = 26.5 ps) to 4 ps for the LiNbO₃ (T ₂ = 0.38 ps) crystal. The dependence of SRS pulse width on the dephasing time was analyzed in the framework of the known SRS theory.     

Picosecond stimulated Raman scattering of BaWO4 crystal

In single-pass configuration, a picosecond pulse laser was used to investigate the stimulated Raman scattering in different BaWO₄ crystals. Using the thresholds for the first Stokes line along the a- and c-axis, the Raman gain coefficients of the BaWO₄ crystals were calculated along their respective direction. The higher Stokes (fourth) and anti-Stokes (third) lines were observed. Compared with other well-known Raman crystals, such as KGd(WO₄)₂ and Ba(NO₃)₂, BaWO₄ has favorable properties for scattering the pump radiation with picosecond pulse duration. The Raman gain coefficients of BaWO₄ are different, but do not vary for different transmission directions, which means that this Raman material can be selected more freely.     

Effect of buffer gas on the generation of eye-safe 1.54 μm radiation using the stimulated Raman scattering technique in CH4

Experimental results on the generation of 1.54 μm eye-safe radiation in pure CH₄, CH₄:He and CH₄:Ar mixtures pumped by the fundamental of an Nd:YAG laser, using the stimulated Raman scattering (SRS) technique, are described. A decrease in the energy conversion efficiency and degradation in the beam quality of S₁ was observed in pure CH₄ at high pump energies. This problem was overcome in CH₄:He and CH₄:Ar mixtures. Compared with the first Stokes (1.54 μm) energy conversion efficiency in pure CH₄, at a pump energy of 126 mJ, an enhancement of 50% in energy conversion efficiency was observed in the CH₄:Ar mixture (60% argon concentration) and as much as 100% in the CH₄:He mixture (60% helium concentration). The use of these buffer gas mixtures improved the spatial beam quality of the Stokes radiation considerably and also resulted in raising the pump threshold for optical breakdown of the Raman gain medium.     

Non‐degenerate second‐order scattering and difference combination bands in infrared‐pump/anti‐Stokes resonance Raman‐probe experiments

Non‐degenerate second‐order scattering due to interaction of infrared and ultraviolet pulses is observed in picosecond infrared‐pump/anti‐Stokes Raman‐probe experiments under electronic resonance conditions. We detected resonance hyper‐Rayleigh scattering at the sum frequency of the pulses as well as the corresponding frequency‐down‐shifted resonance hyper‐Raman lines. Nearly coinciding resonance hyper‐Raman and one‐photon resonance Raman spectra indicate conditions of A‐term resonance Raman scattering. Second‐order scattering is distinguished from transient anti‐Stokes Raman scattering of v = 1 to v = 0 transitions and v = 1 to v′ = 1 combination transitions by taking into account their different spectral and temporal behaviour. Separating these processes is essential for a proper analysis of transient vibrational populations. Copyright © 2007 John Wiley & Sons, Ltd.     

Effect of buffer gas on the generation of eye-safe 1.54 μm radiation using the stimulated Raman scattering technique in CH4

Experimental results on the generation of 1.54 μm eye-safe radiation in pure CH₄, CH₄:He and CH₄:Ar mixtures pumped by the fundamental of an Nd:YAG laser, using the stimulated Raman scattering (SRS) technique, are described. A decrease in the energy conversion efficiency and degradation in the beam quality of S₁ was observed in pure CH₄ at high pump energies. This problem was overcome in CH₄:He and CH₄:Ar mixtures. Compared with the first Stokes (1.54 μm) energy conversion efficiency in pure CH₄, at a pump energy of 126 mJ, an enhancement of 50% in energy conversion efficiency was observed in the CH₄:Ar mixture (60% argon concentration) and as much as 100% in the CH₄:He mixture (60% helium concentration). The use of these buffer gas mixtures improved the spatial beam quality of the Stokes radiation considerably and also resulted in raising the pump threshold for optical breakdown of the Raman gain medium.     

采用圆偏振啁啾飞秒激光脉冲操控CS2分子的相干拉曼散射过程

采用两束圆偏振啁啾飞秒激光脉冲,非共线相干激发三原子分子CS₂液体. 在相位匹配的方向上,探测到由CS₂频率为397 cm^-1的振动模式产生的强度对称分布的相干反斯托克斯拉曼散射(CARS)信号和相干斯托克斯拉曼散射(CSRS)信号. 当调整两束激发光的圆偏振状态时,CARS,CSRS信号的强度、偏振、波长均发生规律性的改变:CARS,CSRS信号的强度分布反映了CS₂ 在不同极化状态下的受激拉曼散射截面大小;信号光的 关键词： 啁啾脉冲 相干反斯托克斯拉曼散射(CARS) 相干斯托克斯拉曼散射(CSRS) 2')" href="#">CS₂     

Enhanced stimulated Raman scattering in H<Subscript>2</Subscript> by seeding an SRS line into an H-ɛ Balmer line: Amplification-coupled laser wavelength shifter

The stimulated Raman scattering (SRS) in H₂ gas above the dissociation energy limit was recorded using a 266-nm UV laser. All of the observed Stokes and anti-Stokes SRS lines showed a normal behavior except the third Stokes SRS lines at 397.8 nm, which showed a substantial intensity enhancement of about a 36%-conversion efficiency of the pump energy. This enhancement in the SRS line is attributed to the seeding of the SRS line into the Balmer H-? line at 397 nm in molecular hydrogen. To the best of our knowledge, there is no report of any work on enhanced stimulated Raman scattering in H₂ by the seeding of the H-? Balmer line into the SRS line and attaining a very high intensity at the third Stokes SRS lines at 397.8 nm. The cell pressure and the laser pulse energy dependence of these SRS lines substantiate our explanation.     

Statistical Properties of Stimulated Raman Scattering in a Nonlinear Regime

Statistical properties of the Raman scattering of an intense step pulse are investigated by numerical solution of the equations of a semi-classical theory with assignment of a random Gaussian distribution of the initial polarization over the sample. The probability density functions of the intensity, energy, delay time, and width of a Stokes pulse and also of the position, height, and width of the spectral peaks of the first incidental Stokes component with the preservation of the phase memory of scattering centers are calculated. The influence of collisional dephasing on the fluctuation of these characteristics is considered.     

Picosecond stimulated Raman scattering in crystals

The comparative values of the peak and integral cross sections of spontaneous Raman scattering and the optical dephasing time of molecular vibrations were determined for several oxide crystals by spontaneous Raman spectroscopy. The spectral, time, and energy parameters of stimulated Raman scattering (SRS) were measured for ten crystals using picosecond YLF: Nd laser pumping with a radiation wavelength of 1047 nm. An analysis of the experimental dependence of the threshold energy of pumping SRS on the integral and peak cross sections of spontaneous Raman scattering showed that the SRS gain increment explicitly depended on the integral cross section and was independent of the peak cross section of spontaneous Raman scattering as the ratio between the pumping pulse width (11 ps) and the time of optical dephasing of molecular vibrations changed from 0.42 to 9.3. The gain coefficients of steady-state stimulated Raman scattering under threshold stimulated Raman scattering conditions were determined for all the crystals studied on the basis of the measured threshold SRS pumping energies, the duration and width of the spectrum of pulses, the nonlinear interaction length, the intensity of pumping, and the theoretical dependences that relate the steady-state and transient SRS gain increments. The steady-state SRS gain coefficients obtained in this work fitted well a linear dependence on the peak cross sections of spontaneous Raman scattering, which substantiated the correctness of our analysis and measurements.