Broadly tunable dual-wavelength light source for coherent anti-Stokes Raman scattering microscopy

The signal and idler beams from a picosecond, synchronously pumped optical parametric oscillator (OPO) provide the two colors necessary for coherent anti-Stokes Raman scattering (CARS) microscopy. The OPO provides a continuously tunable frequency difference between the two beams over a broad range of Raman shifts (100-3700 cm(-1)) by varying the temperature of a single nonlinear crystal. The near-infrared output (900-1300 nm) allows for deep penetration into thick samples and reduced nonlinear photodamage. Applications of this light source to in vivo cell and ex vivo tissue imaging are demonstrated.     

High‐resolution narrowband CARS spectroscopy in the spectral fingerprint region

Coherent anti‐Stokes Raman scattering (CARS) spectroscopy is an important technique for spectroscopy and chemically selective microscopy, but wider implementation requires dedicated versatile tunable sources. We describe an optical parametric oscillator (OPO) based on a magnesium oxide‐doped periodically poled lithium niobate crystal, with a novel variable output coupler, used as a tunable coherent light source. The OPO's signal wavelength ranges from 880 to 1040 nm and its idler wavelength from 1090 to 1350 nm. We use this OPO to demonstrate high‐resolution narrowband CARS spectroscopy on bulk polystyrene from 900 to 3600 cm⁻¹, covering a large part of the molecular fingerprint region. Recording vibrational spectra using narrowband CARS spectroscopy has several advantages over spontaneous Raman spectroscopy, which we discuss. We isolate the resonant part of the CARS spectrum and compare it to the spontaneous Raman spectrum of polystyrene using the maximum entropy method of phase retrieval; we find them to be in extremely good agreement. Copyright © 2009 John Wiley & Sons, Ltd.     

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.     

Test of an optical parametric oscillator (OPO) as a compact and fast tunable stokes source in coherent anti-stokes raman spectroscopy (CARS)

An optical parametric oscillator (OPO), as a novel kind of broadband Stokes source, is employed for coherent anti-Stokes Raman spectroscopy (CARS). Compared to the conventional dye laser configuration OPO-CARS offers practical advantages. The tunable OPO allows a fast and comfortable frequency tuning. The excitation bandwidth of about 35 cm^–1 (FWHM) limits the spectral range of effective and stable single pulse CARS generation but can be used to enhance selected spectral structures.     

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.     

Fourier-transform coherent anti-Stokes Raman scattering microscopy

We report a novel Fourier-transform-based implementation of coherent anti-Stokes Raman scattering (CARS) microscopy. The method employs a single femtosecond laser source and a Michelson interferometer to create two pulse replicas that are fed into a scanning multiphoton microscope. By varying the time delay between the pulses, we time-resolve the CARS signal, permitting easy removal of the nonresonant background while providing high resolution, spectrally resolved images of CARS modes over the laser bandwidth (approximately 1500 cm(-1)). We demonstrate the method by imaging polystyrene beads in solvent.     

Molecularly sensitive optical coherence tomography

Molecular contrast in optical coherence tomography (OCT) is demonstrated by use of coherent anti-Stokes Raman scattering (CARS) for molecular sensitivity. Femtosecond laser pulses are focused into a sample by use of a low-numerical-aperture lens to generate CARS photons, and the backreflected CARS signal is interferometrically measured. With the chemical selectivity provided by CARS and the advanced imaging capabilities of OCT, this technique may be useful for molecular contrast imaging in biological tissues. CARS can be generated and interferometrically measured over at least 600 microm of the depth of field of a low-numerical-aperture objective.     

Three-dimensional nonlinear microspectroscopy and imaging of soft condensed matter

We report on the realization of a sensitive microspectroscopy and imaging approach based on a three-color femtosecond coherent anti-Stokes Raman scattering (CARS) technique with high spectral, time, and spatial resolution. Independently tunable, high-repetition rate optical parametric oscillators were used to attain a dynamic range of 5 orders of magnitude for time-domain CARS signal. The attained sensitivity permitted tracing the decay of weak and structurally complex Raman active modes in soft condensed matter. Application of this approach to imaging of the biological specimen shows a great potential in quantitative characterization of live biological media with an ability to access inter- and intra-molecular interactions.     

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.     

硅甲烷CARS光谱的研究

测量和分析了硅甲烷(SiH_4)v_1带和v_3带的CARS光谱,并采用偏振CARS技术,解除了v_1带的影响,获得了完全的v_3带的CARS谱.偏振测量和理论模拟计算给出了v_1带和v_3带Q支峰值间的频率位移及两者喇曼跃迁截面平方的比值.     

利用准相位匹配光学参量振荡器获得可调谐强度差压缩光

利用106μm全固化单频Nd:YVO4激光器抽运由PPLN构建的三共振光学参量振荡器，在光学参量振荡器近简并运转的情况下：信号光和闲置光的波长相差约为200nm，观察到了信号光和闲置光的强度起伏的关联，实测强度差噪声压缩度达04dB.在此基础上，利用温度调谐了信号光和闲置光的波长，在17nm的范围内都观察到信号光和闲置光的强度起伏的关联. 关键词： 准相位匹配 光学参量振荡 强度差噪声压缩     

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.     

Wide-field coherent anti-Stokes Raman scattering microscopy with non-phase-matching illumination

We have developed and tested a wide-field coherent anti-Stokes Raman scattering (CARS) microscopy technique, which provides the simultaneous imaging of an extended illuminated area without scanning. This method is based on the non-phase-matching illumination of a sample and imaging of a CARS signal with a CCD camera using conventional microscope optics. We have identified a set of conditions on the illumination and imaging optics, as well as on sample preparation. Imaging of test objects proved high spatial resolution and chemical selectivity of this technique.     

Applying the method of Coherent Anti‐stokes Raman microscopy for imaging of carotenoids in microalgae and cyanobacteria

Coherent anti‐stokes Raman scattering microscopy (CARS) was applied to visualize carotenoids in microalgae and cyanobacteria. Nonlinear light–matter interaction utilized in CARS microscopy inevitably induces a number of competing nonlinear processes, such as multiphoton excitation fluorescence. Microalgae and cyanobacteria being an intrinsically well‐fluorescent object generates a strong two‐photon‐excitation fluorescence (TPEF) signal which should be effectively suppressed during the CARS experiment. Using an energetically balanced duel‐wavelength excitation scheme and spectral purification of detecting signal, the TPEF was completely blocked providing a possibility to probe microalgae and cyanobacteria in a fingerprint region of the CARS spectrum. Microspectroscopy experiments were carried out with three species ‐ cyanobacteria Nostoc Commune, Nostoc sp. and Chlorella sp. Distinct bands obtained in CARS spectra of such species were assigned to carotenoids and were taken as spectral markers in the imaging experiment. CARS imaging known as a chemical selective and label‐free technique allows non‐invasion monitoring of accumulation and movement of chemical compound at the subcellular level. Obtained high‐resolution images of carotenoid distribution in algae and cyanobacteria clearly demonstrate the potential of CARS microscopy for spatially resolved analysis of the natural products stored in the microalgae and cyanobacteria cell. Copyright © 2013 John Wiley & Sons, Ltd.     

双光子荧光与相干反斯托克斯拉曼散射显微成像技术的实验研究

双光子荧光与相干反斯托克斯拉曼散射同属于三阶非线性效应,二者之间的差异与联系是一个值得研究的问题.本文基于自行搭建的超连续谱近红外宽带相干反斯托克斯拉曼散射显微成像系统进行光谱成像,同时通过理论与实验对比分析了双光子荧光与相干反斯托克斯拉曼散射图像存在差异的原因.结果表明,具有亚微米以上横向分辨率的相干反斯托克斯拉曼散射成像系统,可以使用较大尺寸的荧光珠进行双光子荧光成像,通过解卷积得到双光子荧光成像的系统分辨率,并将它近似等效于相干反斯托克斯拉曼散射成像系统的当下分辨率.如果需要得到相干反斯托克斯拉曼散射成像系准确的分辨率结果,就必须使用尺寸比相干反斯托克斯拉曼散射成像系统实际分辨率小的球形样品进行实验测量.     

Time‐resolved coherent anti‐Stokes Raman spectroscopy impulsively excited by supercontinuum

Time‐resolved coherent anti‐Stokes Raman scattering (CARS) impulsively excited by a supercontinuum (SC) is investigated. We show that it is critical to optimize the temporal overlapping of the constituent solitons of a SC in order to impulsively excite vibrational modes over a broad frequency range. The cross‐correlation frequency‐resolved optical gating technique is utilized to retrieve the optical response function of molecules under SC impulsive excitation, and background‐free CARS with spectral resolution significantly better than the bandwidth of the probe pulses is achieved. Copyright © 2011 John Wiley & Sons, Ltd.     

Chirped coherent anti-Stokes Raman scattering as a high-spectral- and spatial-resolution microscopy

Coherent anti-Stokes Raman scattering (CARS) microscopy is a promising tool for chemically selective imaging based on molecular vibrations. While CARS is currently used as a biological imaging tool, many variations are still being developed, perhaps the most important being multiplex CARS microscopy. Multiplex CARS has the advantage of comparing images based on different molecular vibrations without changing the excitation wavelengths. Here we demonstrate both high-spectral- and spatial-resolution multiplex CARS imaging of polymer films, using a simple scheme for chirped CARS with a spectral bandwidth of 300 cm(-1).     

用于CARS激发源的全光纤飞秒脉冲谱压缩

进行了基于光纤预啁啾和自相位调制的多模/单模组合式全光纤啁啾谱压缩研究.提出利用多模光纤模式估计群速度色散均值的方法,并将该估计值作为啁啾参量分析的计算参数,仿真计算了50/125μm折射率渐变多模光纤的群速度色散均值及其与单模光纤在不同长度比值下的光谱压缩效果.采用三种折射率渐变多模光纤进行实验,对比分析了折射率渐变多模光纤的芯径大小及其与单模光纤的长度比值对光谱压缩效果的影响.实验结果表明使用50/125μm折射率渐变多模光纤获得光谱最大压缩比为5.796,谱宽为2.243 nm,与理论仿真一致;使用105/125μm折射率渐变多模光纤,可进一步提高压缩比至152.941,输出谱宽为0.085 nm的光脉冲.将此脉冲用于相干反斯托克斯拉曼散射光谱探测,理论光谱分辨率可达1.386 cm~(-1).     

Ultrafast multiplex CARS investigation of vibrational characteristics in chloroform and PMMA

This paper demonstrates the femtosecond time-resolved coherent anti-Stokes Raman scattering by using folded BOXCARS geometry where an ultrashort broadband coherent white light continuum was used as Stokes pulse, and carries out the non-contact detection at long distance. The CARS signal is so easy to be detected that it can be seen even by nude eye. The C--H bonds of chloroform or PMMA were detected and the vibration modes belonging to the side chain and the main chain in PMMA were also compared. Their vibrational characteristics involved decay process and quantum beating were discussed. This modified CARS experimental technique could make up the deficiency of traditional CARS technique.