One-dimensional single-shot thermometry in flames using femtosecond-CARS line imaging

We report single-laser-shot one-dimensional thermometry in flames using femtosecond coherent anti-Stokes Raman scattering (fs-CARS) line imaging. Fs-CARS enables high-repetition-rate (1-10?kHz), nearly collision-free measurement of temperature and species concentration in reacting flows. Two high-power 800?nm beams are used as the pump and probe beams and a 983?nm beam is used as the Stokes beam for CARS signal generation from the N2Q-branch transitions at ～2330?cm(-1). The probe beam is frequency-chirped for single-laser-shot imaging. All three laser beams are formed into sheets and crossed in a line which forms the probe region. The resulting 1D line-CARS signal at ～675?nm is spatially and spectrally resolved and recorded as a two-dimensional (2D) image. Single-shot temperature measurements are demonstrated in flat-field flames up to temperatures exceeding 2000?K, demonstrating the potential of fs-CARS line imaging for high-repetition-rate thermometry in turbulent flames. Such measurements can provide valuable data to validate complex turbulent-combustion models as well as increase the understanding of the spatio-temporal instabilities in practical combustion devices such as modern gas-turbine combustors and augmentors.     

Broadband coherent anti-Stokes Raman scattering spectroscopy of nitrogen using a picosecond modeless dye laser

Broadband picosecond coherent anti-Stokes Raman scattering (CARS) spectroscopy of nitrogen is demonstrated using 145-ps pump and probe beams and a 115-ps Stokes beam with a spectral bandwidth of 5 nm. This is, to our knowledge, the first demonstration of broadband CARS using subnanosecond lasers. The short temporal envelope of the laser pulses and the broadband spectral nature of the Stokes beam will enable nonresonant-background-free, single-shot, or time-dependent spectroscopy in high-pressure or hydrocarbon-rich environments. Successful correlation of room-temperature broadband picosecond N2 CARS with a theoretical spectrum is presented.     

Efficient inversionless amplification of infrared radiation using quantum dots with quasi-discrete energy levels

A model of inversionless gain on a transition between quasi-discrete excited levels of a quantum system is proposed that minimizes the absorption of a coherent optical pump beam due to the finite lifetimes of the corresponding states. A quantum-dot system with transition frequency between quasi-discrete levels lying in the mid-infrared (mid-IR) range is used as an example to determine an optimal pump frequency at which the pump absorption rate is minimized and to calculate the gain for a mid-IR field. The proposed model system can be considered as a prototype for creating highly efficient mid- and far-IR quantum-dot amplifiers and lasers operating on transitions between quasi-discrete levels at room temperature.     

光纤中基于双宽带抽运的受激布里渊散射增益谱展宽及慢光传输中脉冲失真减小的理论研究

分析了单模光纤中双宽带抽运的受激布里渊散射 （stimulated Brillouin scattering,SBS） 光速减慢效应.模拟了两高斯型宽带抽运光同时作用下的SBS增益谱和损耗谱,提出了一种获得均匀增益谱的方法.由于较强抽运光增益谱的中心频率与较弱抽运光损耗谱的中心频率相重合,通过调节较弱抽运光的功率和谱宽,能够部分地抵消掉较强增益谱的顶部从而构建出平坦的增益谱.研究了该平顶增益情形下信号脉冲畸变的性质.与单宽带抽运下脉冲畸变的对比表明,这种产生平顶增益谱的方法能有效地减小脉冲畸变. 关键词： 慢光 受激布里渊散射（SBS） 单模光纤 脉冲失真     

On the Sub-Doppler Spectroscopy of Optically Excited Atoms in a Thin Gas Cell

This work continues a theoretical investigation of the capabilities of the well-known method based on using a monochromatic probe light beam in combination with optical pumping of atoms (molecules) of a rarefied-gas medium by a broadband radiation in a thin cell the diameter of which is much larger than its internal thickness. In contrast to calculations carried out in the previous publications on this method of spectroscopy, here, we consider the case of arbitrary values of pump intensity and thickness of a cylindrical gas cell. Thus, all the possible mechanisms and specificities of velocity selection of atoms in optically excited levels caused by transit-time relaxation of such atoms in gas cells of this kind are analyzed. Within the framework of this approach, sub-Doppler absorption resonances of the probe light beam corresponding to quantum transitions from the upper level excited by optical pumping are investigated. The obtained results can be used in high-resolution spectroscopy of atoms (molecules), as well as for laser-frequency stabilization to established narrow spectral resonances.     

GaAs掺杂超晶格的光反射谱

不同调制光强下测量了MBE GaAs掺杂超晶格的室温光反射谱。较低光强下观察到一系列对应于禁戒跃迁(△n≠0)的精细结构,因此否定了它是三阶微商谱的可能性。明确提出了掺杂超晶格的光反射为一阶微商谱,并讨论了其产生机制。理论计算结果能较好地解释实验现象。 关键词：     

V型三能级系统中五阶极化拍频的理论研究

从理论上研究了由于五阶极化强度间的干涉所导致的Ｖ型三能级系统相闰共轭超快调制光谱学，考虑了抽运光束为窄带线宽或宽带线宽的情形。发现能级分裂的测量精度决定于光学跃迁的均匀增宽，即这种技术在测量能级分裂时可得到消除多普勒增宽的精度。     

Cs(6P)激发态的辐射及与N2碰撞的能量转移

应用激光吸收和荧光方法,测量了Cs(6P)态与N2碰撞的精细结构转移和碰撞猝灭截面。Cs原子被激光激发到6P3/2态,将与泵浦激光束反向平行的检测激光束调到6PJ→8S1/2的跃迁,测量了6PJ激发态的密度及空间分布,由此计算了6PJ→6S的有效辐射率。在T=337 K(蒸气压公式给出Cs密度N0=1.25×1012cm-3)和N2密度2×1016相似文献   

四能级系统中五阶极化拍频的理论研究

研究了四能级系统中基于五阶极化的拍频效应,考虑了抽运光束为窄带线宽或宽带线宽的情形．发现其对能级分裂的测量精度决定于光学跃迁的均匀增宽．也就是说,这种技术在测量与基态是偶极禁戒跃迁的两激发态之间的能级分裂时,可得到消除多普勒增宽的精度． 关键词：     

Development of high-throughput, high-damage-threshold beam separator for 13 nm high-order harmonics

We demonstrate a high-throughput, high-damage-threshold beam separator for wavelengths shorter than 30 nm, which uses a 10 nm thick niobium nitrogen film prepared on a Si substrate, set at the Brewster angle relative to the pump wavelength. The film was deposited by rf reactive magnetron sputtering on a Si substrate. The beam separator has an attenuation ratio of 0.01 and a damage-threshold intensity of at least 0.8 TW/cm2 for a 26 fs pump pulse. The measured reflectivity of the beam separator exceeded 70% in a wavelength range of 13-18 nm. This broadband beam separator may be used to eliminate energetic laser pulses from longitudinally pumped x ray lasers as well as from high-order harmonics.     

Photoreflectance measurements in GaAs/AlGaAs asymmetric quantum wells

In this work we investigated the optical control of the bidimensional electron gas density in a single asymmetric quantum well using, for the first time, photoreflectance. We performed our measurements at 80 and 300 K as a function of the power density of the pump beam. Under strong illumination, the bidimensional electron gas density is washed out of the quantum well and under a dark condition, it reaches its maximum value. The variation of the optical transitions observed in our photoreflectance spectra was related to the induced changes of the band profile in between these two limiting cases.     

Photoreflectance spectroscopy of GaAs doping superlattices

Room temperature photoreflectance of molecular beam epitaxy GaAs doping superlattices was measured. Additional structures corresponding to forbidden transitions (Δn≠0) were observed at very low pump beam intensity. Experimental results show that the dominant modulation mechanism of photoreflectance of doping superlattices is different from that of bulk materials. We suggest that the photoreflectance spectrum of doping superlattices have mainly first derivative functional lineshapes, which is caused by the subband shift in doping superlattices. The experiments are well explained by this mechanism.     

First-order antiferro-ferromagnetic transition in Fe(49)(Rh(0.93)Pd(0.07))(51) under simultaneous application of magnetic field and external pressure

A magnetic field-pressure-temperature (H-P-T) phase diagram for first-order antiferromagnetic (AFM) to ferromagnetic (FM) transitions in Fe(49)(Rh(0.93)Pd(0.07))(51) has been constructed using resistivity measurements under simultaneous application of magnetic field (up to 8 T) and pressure (up to 20 kbar). The temperature dependence of resistivity (ρ-T) shows that the width of the transition and the extent of hysteresis decreases with pressure and increases with magnetic field. By exploiting opposing trends of dT(N)/dP and dT(N)/dH (where T(N) is the first-order transition temperature), the relative effects of temperature, magnetic field and pressure on disorder-broadened first-order transitions has been studied. For this, a set of H and P values are chosen for which T(N)(H(1),P(1)) = T(N)(H(2),P(2)). Measurements for such combinations of H and P show that the temperature dependence of resistivity is similar, i.e. the broadening (in temperature) of transition as well as the extent of hysteresis remains independent of H and P. Isothermal magnetoresistance measurements under various constant pressures show that even though the critical field required for AFM-FM transition depends on applied pressure, the extent of hysteresis as well as transition width (in magnetic field) remains constant with varying pressure.     

Doppler-free spectroscopy of neon atoms dressed by optical photons in nearly-degenerate four-wave mixing

We have investigated nearly degenerate four-wave mixing in neon on the 607 nm line. We have considered the case where one pump beam saturates the medium while the two other incident beams have a much weaker intensity. The frequency of the most intense pump beam is equal to the frequency of the probe beam. When the frequency of the second pump beam is scanned, narrow resonances are observed. These resonances correspond to transitions between the energy levels of the atom dressed by the photons of the intense pump beam.     

Development of two-color laser system for high-resolution polarization spectroscopy measurements of atomic hydrogen

We have developed a high-spectral-resolution laser system for two-photon pump, polarization spectroscopy probe (TPP-PSP) measurements of atomic hydrogen in flames. In the TPP-PSP technique, a 243-nm laser beam excites the two-photon 1S-2S transition, and excited n=2 atoms are then detected by polarization spectroscopy of the n=2 to n=3 transition using 656-nm laser radiation. The single-frequency-mode 243 and 656-nm beams are produced using injection-seeded optical parametric generators coupled with pulsed dye amplifiers. The use of single-mode lasers allows accurate measurement of signal line shapes and intensities even with significant pulse-to-pulse fluctuations in pulse energies. Use of single-mode lasers and introduction of a scheme to select nearly constant laser energies enable repeatable extraction of important spectral features in atomic hydrogen transitions.     

CARS measurements of temperature and species concentrations in an IC engine

Temperature measurements alone, and also simultaneous CO₂ concentration and temperature measurements, have been performed inside a fired IC engine. To tackle the two main problems encounted when making measurements in an engine (i.e., cycle-to-cycle variations and beam steering by density gradients), the shot-by-shot referencing technique and the in situ referencing technique using N₂ to determine [CO₂] were used. The temperature is deduced from the broadband N₂ CARS signal and narrow band detection is used for CO₂. A representative example of temperature measurements is given which shows a correlation between temperature and knocking intensity in the burnt gas of a knocking engine. The first simultaneous determinations of temperature and [CO₂] are also reported; so far these are reliable only during the compression stoke.     

Analytical modelling of end thermal coupling in a solid-state laser longitudinally bonded by a vertical-cavity top-emitting laser diode

The intrinsic features involving a circularly symmetric beam profile with low divergence, planar geometry as well as the increasingly enhanced power of vertical-cavity surface-emitting lasers (VCSELs) have made the VCSEL a promising pump source in direct end bonding to a solid-state laser medium to form the minimized, on-wafer integrated laser system. This scheme will generate a surface contact pump configuration and thus additional end thermal coupling to the laser medium through the joint interface of both materials, apart from pump beam heating. This paper analytically models temperature distributions in both VCSEL and the laser medium from the end thermal coupling regarding surface contact pump configuration using a top-emitting VCSEL as the pump source for the first time. The analytical solutions are derived by introducing relative temperature and mean temperature expressions. The results show that the end contact heating by the VCSEL could lead to considerable temperature variations associated with thermal phase shift and thermal lensing in the laser medium. However, if the central temperature of the interface is increased by less than 20~K, the end contact heating does not have a significant thermal influence on the laser medium. In this case, the thermal effect should be dominated by pump beam heating. This work provides useful analytical results for further analysis of hybrid thermal effects on those lasers pumped by a direct VCSEL bond.     

用不同强度皮秒脉冲相关技术测量甲酚紫吸收恢复时间

以Ar+离子锁模激光器同步泵浦R6G染料激光器作光源,用不同强度皮秒脉冲相关的泵涌—探测技术测得甲酚紫甲醇溶液的吸收恢复时间为162±3.5ps.并且用三能级模型对这一动力学过程进行了理论分析.     

Measurements of photoinduced refractive index changes in bacteriorhodopsin films

We report the pump-probe measurements of nonlinear refractive index changes in photochromic bacteriorhodopsin films. The photoinduced absorption is caused by pump beam at 532 nm and the accompanying refractive index changes are studied using a probe beam at 633 nm. The proposed technique is based on a convenient and accurate determination of optical path difference using digital interferometry-based local fringe shift. The results are presented for the wild-type as well as genetically modified D96N variant of the bacteriorhodopsin.