基于光强传输方程相位成像的宽场相干反斯托克斯拉曼散射显微背景抑制

相干反斯托克斯拉曼散射(CARS)显微能够对样品的特殊化学组分进行选择性成像,无需荧光标记,在生物医学领域被广泛应用.然而,传统的CARS图像往往存在非共振背景信号.本文将基于光强传输方程的单光束相位成像技术用于CARS显微成像,来抑制CARS的非共振背景信号.该方法通过记录样品在三个相邻平面上的CARS图像,然后利用光强传输方程获取CARS光场的相位分布,最后利用共振CARS信号和非共振背景信号在相位上的差异,实现了对背景噪声的抑制.该方法无需参考光,通过三次测量可完成CARS的背景噪声抑制,具有良好的应用前景.     

Autocorrelation coefficient of the radiation output of a frequency-doubled Nd:YAG laser

The autocorrelation coefficient of a frequency-doubled Nd:YAG laser output has been extracted for various operating conditions from the pulse form measured by a streak camera and from the enhancement of the nonresonant CARS intensity for correlated as against uncorrelated pump beams. The measured values are compared with values in the normalized Gaussian model.     

High-sensitivity vibrational imaging with frequency modulation coherent anti-Stokes Raman scattering (FM CARS) microscopy

We demonstrate a new approach to coherent anti-Stokes Raman scattering (CARS) microscopy that significantly increases the detection sensitivity. CARS signals are generated by collinearly overlapped, tightly focused, and raster scanned pump and Stokes laser beams, whose difference frequency is rapidly modulated. The resulting amplitude modulation of the CARS signal is detected through a lock-in amplifier. This scheme efficiently suppresses the nonresonant background and allows for the detection of far fewer vibrational oscillators than possible through existing CARS microscopy methods.     

Theoretical analysis of non-thermal pump effects in broadband CARS spectroscopy

The effect of finite-bandwidth multimode pump lasers in CARS is normally accounted for by an intensity convolution integral based on gaussian (thermal) pump sources. This paper presents a derivation of the convolution integral for non-gaussian pump sources derived from frequency doubling. The modified convolution integral makes it possible to identify regions of insensitivity to pump stochastic properties.     

High-spectral-resolution coherent anti-Stokes Raman scattering with interferometrically detected broadband chirped pulses

To achieve high-spectral-resolution multiplex coherent anti-Stokes Raman scattering (CARS), one typically uses a narrowband pump pulse and a broadband Stokes pulse. This is to ensure a correspondence between anti-Stokes and vibrational frequencies. We obtain high-resolution CARS spectra of isopropanol, using a broadband chirped pump pulse and a broadband Stokes pulse, by detecting the anti-Stokes pulse with spectral interferometry. With the temporally resolved anti-Stokes signal, we can remove the chirp of the anti-Stokes pulse and restore high spectral resolution while also rejecting nonresonant scattering.     

Intensity convolutions of CARS spectra

A derivation of the convolution integral for finite bandwidth pump and Stokes sources in CARS is given. The resulting “partially coherent” convolution confirms previously published results, and is compared to the widely used incoherent treatment. The effect of intensity convolution choice on multiplex CARS thermometry is evaluated by reducing N₂ data taken in an ethylene-air diffusion flame. N₂ CARS thermometry using multimode pumps is ordinarily not very sensitive to convolution.     

The line shape of Coherent Antistokes Raman Scattering excited under resonance conditions

If Coherent Antistokes Raman Scattering (CARS) is excited by a biharmonic pump in resonance with one photon molecular transition, the Raman line shape is significantly changed compared to the case of nonresonant excitation. This is due to an essential alteration of the 3rd order susceptibility responsible for CARS. Aside from a considerable enhancement of the Raman signal (which makes it possible to detect dissolved molecules in concentration lower than 10^-4 mole/l) the most marked effects are a reversal of the spectral sequence of the Raman maximum and minimum in the CARS-spectrum and for a specially chosen concentration an enhanced contrast of the signal. The factors determining the CARS line shape are discussed theoretically and preliminary experimental results concerning resonance CARS spectra are reported.     

The impact of laser field statistics in determination of temperature and concentration by multiplex USED CARS

Experiments have been performed which demonstrate that the ratio of resonant to nonresonant third-order susceptibilities measured in multiplex USED CARS are affected by the time correlation of the pump fields. Comparing the resonant to nonresonant signal ratio obtained with decorrelated fields to the ratio obtained with correlated fields, a relative increase of 2.5 was measured for the nitrogen Q-branch, whereas a corresponding increase of 1.9 was observed for the hydrogen Q(1)-line. To compare our experimental results with theoretical calculations of the spectral shapes, the nonresonant third-order susceptibilities of a number of gases were re-evaluated by calibrating to the nonresonant susceptibility of argon.     

Interferometric polarization coherent anti-Stokes Raman scattering (IP-CARS) microscopy

We report a novel interferometry-based polarization coherent anti-Stokes Raman scattering (IP-CARS) implementation for effectively suppressing the nonresonant background while significantly amplifying the resonant signal for vibrational imaging. By modulating the phase difference between the two interference CARS signals generated from the same sample and measuring the peak-to-peak intensity of the periodically modulated interference CARS signal, the IP-CARS technique yields a sixfold improvement in the signal-to-background ratio compared with conventional CARS while providing an approximately 20-fold amplification of the resonant CARS signal compared with conventional polarization CARS. We demonstrate this method by imaging 4.69 microm polystyrene beads and unstained human epithelial cells immersed in water.     

Theoretical modeling of single-laser-shot, chirped-probe-pulse femtosecond coherent anti-Stokes Raman scattering thermometry

Chirped-probe-pulse (CPP) femtosecond (fs) coherent anti-Stokes Raman scattering (CARS) spectroscopy for single-laser-shot temperature measurements in flames is discussed. In CPP fs CARS, a giant Raman coherence is created in the medium by impulsive pump-Stokes excitation, and the temperature-dependent temporal decay of this initial coherence is mapped into the frequency of the CARS signal using a CPP. The theory of the CPP fs CARS technique is presented. A computer code has been developed to calculate theoretical CPP fs CARS spectra. The input parameters for the calculation of the theoretical spectra include the temperature, probe time delay, ratio of the resonant and nonresonant susceptibilities, and parameters for characterizing the pump, Stokes and probe pulses. The parameters for characterizing the pump, Stokes and probe pulses are determined from the best fit of theoretical spectra to experimental spectra acquired from calibration flames at a known temperature. For spectra acquired in subsequent measurements, these laser parameters are fixed and temperature is determined as one of the fit parameters from the best fit of theoretical spectra to experimental spectra. For single-laser-shot CPP fs CARS temperature measurements performed in steady, near-adiabatic flames, the best-fit temperature distribution width is typically less than 1.5% of the mean temperature. The mean temperature is accurate to within approximately 3% with respect to the adiabatic flame temperature. The most significant limitation on temperature measurement accuracy is associated with the evaluation of the theoretical laser parameters. Significant improvements in the temperature measurement accuracy are expected once monitoring equipment capable of characterizing the spectrum and phase of each laser pulse is incorporated in the experiments.     

Ultrabroadband background-free coherent anti-Stokes Raman scattering microscopy based on a compact Er:fiber laser system

We demonstrate a scheme for efficient coherent anti-Stokes Raman scattering (CARS) microscopy free of nonresonant background. Our method is based on a compact Er:fiber laser source. Impulsive excitation of molecular resonances is achieved by an 11 fs pulse at 1210 nm. Broadband excitation gives access to molecular resonances from 0 cm(-1) up to 4000 cm(-1). Time-delayed narrowband probing at 775 nm enables sensitive and high-speed spectral detection of the CARS signal free of nonresonant background with a resolution of 10 cm(-1).     

Image formation in CARS and SRS: effect of an inhomogeneous nonresonant background medium

We investigate the role of a spatially inhomogenous nonresonant background medium on several Raman-based imaging modalities. In particular, we consider a small resonant bead submerged in a spatially heterogeneous nonresonant χ(3) background. Using detailed 3D electrodynamic simulations, we compare coherent anti-Stokes Raman scattering (CARS), frequency-modulated CARS, amplitude-modulated stimulated Raman scattering (SRS), and frequency-modulated SRS. We find that only FM-SRS is background-free.     

Time-resolved coherent anti-Stokes Raman scattering with a femtosecond soliton output of a photonic-crystal fiber

We demonstrate time-resolved coherent anti-Stokes Raman scattering (CARS) by using a frequency-tunable femtosecond soliton output of a silica photonic-crystal fiber (PCF) as a Stokes field. This approach allows quantum beats originating from two close Raman modes to be resolved in the time-domain CARS response. The nonresonant CARS background is efficiently suppressed by introducing a delay time between the probe pulse and the pump-Stokes pulse dyad, suggesting a convenient fiber-optic format for the Stokes source in time-resolved CARS and allowing sensitivity improvement in PCF-based CARS spectroscopes and microscopes.     

Experimental study on terahertz radiation

In this letter, we describe a coherent subpicosecond terahertz (THz) spectroscopy system based on nonresonant optical rectification for the generation of THz radiation. We studied the two-photon absorption (TPA) of ZnTe induced by femtosecond laser pulses via THz generation, and its influence on the generation of THz radiation. Experimental results demonstrated that the intensity of pump beam against TPA must be traded off to get an optimum generation of THz radiation. As an example, we measured absorption spectrum of water vapor by time-domain spectroscopy (TDS) in the frequency range from 0.5 to 2.5 THzwith a high overall accuracy.     

Heterodyne coherent anti-Stokes Raman scattering (CARS) imaging

We have achieved rapid nonlinear vibrational imaging free of nonresonant background with heterodyne coherent anti-Stokes Raman scattering (CARS) interferometric microscopy. This technique completely separates the real and imaginary responses of nonlinear susceptibility chi(3) and yields a signal that is linear in the concentration of vibrational modes. We show that heterodyne CARS microscopy permits the detection of weak vibrational resonances that are otherwise overshadowed by the strong interference of the nonresonant background.     

Broadband (1000 cm−1) multiplex CARS spectroscopy: Application to polarization sensitive and time-resolved measurements

A multiplex CARS spectrometer based on a cw diode-pumpedQ-switched Nd: YLF laser, a broadband dye-laser and a multichannel spectrum detection system has been constructed. Excellent mode characteristics of the laser beams and high pulse repetition rate (2 kHz) have resulted in good signal-to-noise ratio achieved with a few seconds accumulation time. A 1000 cm^–1 wide spectral range is covered in a single CARS spectrum with an expanded bandwidth of the dye laser. A thin-jet sampling method is used in order to avoid the phase-matching limitation. The efficient spectral intensity normalization by the reference (CCl₄) nonresonant spectrum and subsequent computer fitting have been implemented. The performance of the system is demonstrated by two different experiments. First, the polarization sensitive measurements (PS-CARS) of cyclohexane show its potential for accurate Raman depolarization ratio determination and for detection of weak (overlapped) Raman bands. Second, the transient resonance CARS measurement of the lowest excited triplet state of all-trans retinal indicate its feasibility to time-resolved CARS spectroscopy of fluorescent excited states.     

Is rotational CARS an alternative to vibrational CARS for thermometry?

Recent developments in rotational CARS thermometry and critical issues when comparing vibrational and rotational CARS thermometry are described. In particular, the development of dual broadband rotational CARS and the noise characteristics of this approach are emphasized. The difficulty with unambiguous temperature determination in vibrational CARS with unknown parameters, in particular the nonresonant background susceptibility, and the lower sensitivity of rotational CARS thermometry at flame temperatures are also discussed.     

Raman-resonance-enhanced composite nonlinearity of air-guided modes in hollow photonic-crystal fibers

Coherent anti-Stokes Raman scattering (CARS) is used to measure relations between the resonant (Raman) and nonresonant (Kerr-type) optical nonlinearities of air-guided modes in a hollow-core photonic-crystal fiber (PCF). We demonstrate that, due to its interference nature, CARS provides a convenient tool for measuring the contribution of the fiber cladding to the total nonlinearity sensed by air-guided modes in hollow PCFs. On a Raman resonance with molecular vibrations in the gas that fills the fiber core, a two-color laser field is shown to induce optical nonlinearities that are several orders of magnitude higher than the nonresonant Kerr-type nonlinearities typical of air-guided PCF modes.     

可调空间分辨USEDCARS技术研究

相干反斯托克斯拉曼散射（CARS）技术是一种非常重要的燃烧诊断技术,该技术具有非常强的抗干扰能力和非常高的测量精度。但空间分辨力不足会使CARS技术产生很强的空间平均效应,引起成CARS光谱畸变,进而造成CARS光谱分析困难,无法通过CARS光谱反演燃烧场参数。针对非稳腔空间增强探测CARS（USEDCARS）技术存在的空间分辨不足以及空间分辨力不易改变的特点,分析了影响USEDCARS技术测量空间分辨力的各种因素,采用一组轴棱锥对USEDCARS系统中的泵浦激光进行环状光束整形,并通过调节轴棱锥之间的距离获得了不同直径的环状光束,在此基础上,建立了空间分辨可调USEDCARS诊断系统。开展了空间分辨力分析实验,获得了CARS信号强度随空间位置的分布数据,以CARS信号总强度95%包含的空间区域代表CARS的纵向空间分辨力,以此计算得到了CARS系统空间分辨力为1.7～6.5 mm连续可调。其中,高分辨力情况,达到了现有BOXCARS技术的空间分辨力。利用所建立的空间分辨可调USEDCARS诊断系统测量了酒精/空气预混火焰温度参数,获得了不同空间分辨条件下的CARS光谱。空间分辨力为1.7 mm时,获得了高质量CARS光谱,通过光谱拟合给出了所测火焰的温度信息。分辨力分别为4.9和6.5 mm时获得了较强的CARS信号,但存在光谱畸变。结果显示,空间分辨力对CARS信号的强度和空间平均效应有很大地影响,提高测量的空间分辨力可以有效消除空间平均效应,获得准确的CARS光谱,增强光谱拟合精度,同时空间分辨可调的特性使该系统能够更好地适应不同实验条件下的诊断工作。     

Measurement of the third-order nonlinear susceptibility ratio by the CARS Maker fringe technique

Using the CARS Maker fringe technique, the nonresonant third-order nonlinear susceptibility was measured in several liquids.