Fiber four-wave mixing source for coherent anti-Stokes Raman scattering microscopy

We present a fiber-format picosecond light source for coherent anti-Stokes Raman scattering microscopy. Pulses from a Yb-doped fiber amplifier are frequency converted by four-wave mixing (FWM) in normal-dispersion photonic crystal fiber to produce a synchronized two-color picosecond pulse train. We show that seeding the FWM process overcomes the deleterious effects of group-velocity mismatch and allows efficient conversion into narrow frequency bands. The source generates more than 160 mW of nearly transform-limited pulses tunable from 775 to 815 nm. High-quality coherent Raman images of animal tissues and cells acquired with this source are presented.     

High-sensitivity coherent anti-Stokes Raman scattering microscopy with two tightly synchronized picosecond lasers

We demonstrate a significant improvement in signal-to-noise ratio in coherent anti-Stokes Raman scattering (CARS) spectroscopy/microscopy, using two highly synchronized picosecond Ti:sapphire lasers. A temporal jitter between the pulse trains from the two independent commercial lasers is reduced from a few picoseconds to ~21 fs , maintained over several hours. The tight synchronization brings the fluctuation of the CARS signal down to the shot-noise limit, leading to enhanced CARS vibrational images of living cells and polymer beads.     

纳米分辨相干反斯托克斯拉曼散射显微成像

采用附加探测光声子耗尽法来实现超衍射极限相干反斯托克斯拉曼散射显微成像. 此方法引入一束环形分布的附加探测光来消耗点扩展函数周边的相干声子, 实现点扩展函数的改造, 从而达到超越衍射极限的空间分辨率. 为了获得更高的空间分辨率和更佳的相位匹配条件, 通常需采用高数值孔径物镜对抽运光、斯托克斯光和探测光进行聚焦, 此时标量衍射理论不再成立. 基于矢量衍射理论, 分析了线偏振光、圆偏振光先后经过螺旋相位片和高数值孔径物镜后的光强分布, 结果表明: 圆偏振光在高数值孔径物镜后焦平面的光强分布呈中心对称状, 较线偏振环形光更适合作为附加探测光. 此外, 采用全量子理论分析了附加探测光声子耗尽法. 结果表明: 当附加探测光与探测光强度比为80时, 成像系统的横向空间分辨率可以达到45 nm; 继续提高附加探测光强度, 空间分辨将进一步提高.     

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.     

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 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.     

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).     

Use of multimode optical fibers for fiber-based coherent anti-Stokes Raman scattering microendoscopy imaging

A multimode fiber (MMF) was used for both delivery of excitation lasers and collection of returned coherent anti-Stokes Raman scattering (CARS) signals in a CARS microendoscopy prototype imaging system. We demonstrated a polarization-based scheme for suppression of four-wave mixing (FWM) signals in delivery fibers. Our experimental results showed that this polarization-based FWM-suppressing scheme can dramatically reduce FWM signals generated in MMFs, and MMFs can be used to produce CARS images in this microendoscopy system. The proposed MMF-based CARS microendoscopy imaging system with the polarization-based FWM-suppressing scheme offers a potential platform for building fiber-based CARS microendoscopes that can effectively suppress FWM background noises.     

High-contrast chemical imaging with gated heterodyne coherent anti-Stokes Raman scattering microscopy

A novel method is presented which substantially improves the signal-to-background ratio for coherent anti-Stokes Raman scattering (CARS) microscopy. It exploits the fixed phase relation between pump, Stokes and CARS fields together with the strong phase coherence in supercontinua generated by femtosecond lasers. Three phase-locked optical parametric amplifiers are used for the realisation of heterodyne signal detection. Proper pulse timing yields a gating mechanism which nearly completely suppresses solvent background signals. PACS 42.65.Dr; 42.65.Hw; 42.65.Yj     

Tip-enhanced coherent anti-stokes Raman scattering for vibrational nanoimaging

An electric field enhanced by a metallic nanoprobe has locally induced coherent anti-Stokes Raman scattering (CARS) of adenine molecules in a nanometric DNA network structure. Owing to the third-order nonlinearity, the excitation of the CARS polarization is extremely confined to the end of the tip apex, resulting in a spatial resolution far beyond the diffraction limit of light. Our tip-enhanced CARS microscope visualized the DNA network structure at a specific vibrational frequency (approximately 1337 cm(-1)) corresponding to the ring-breathing mode of diazole of adenine molecules.     

All-ultraviolet time-resolved coherent anti-Stokes Raman scattering

We report all-UV coherent anti-Stokes Raman scattering (CARS) in calcite with 250-280 nm pump, Stokes, probe, and anti-Stokes light. UV CARS efficiency is approximately 7x higher than for comparable scattering in the visible, 480-540 nm. Time-resolved UV CARS reveals lengthening of the dephasing time of 1086 cm(-1) CO3(2-) internal vibrations from 4 to 7 ps with increasing vibrational excitation, consistent with a phonon depletion model.     

Tunable light source for coherent anti-Stokes Raman scattering microspectroscopy based on the soliton self-frequency shift

We present a photonic crystal fiber (PCF)-based light source for generating tunable excitation pulses (pump and Stokes) that are applicable to coherent anti-Stokes Raman scattering (CARS) microspectroscopy. The laser employed is an unamplified Ti:sapphire femtosecond laser oscillator. The CARS pump pulse is generated by spectral compression of a laser pulse in a PCF. The Stokes pulse is generated by redshifting a laser pulse in a PCF through the soliton self-frequency shift. This setup allows for probing up to 4000 cm(-1) with a spectral resolution of approximately 25 cm(-1). We characterize the stability and robustness of CARS microspectroscopy employing this light source.     

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.     

Polarization coherent anti-Stokes Raman scattering microscopy

We report polarization coherent anti-Stokes Raman scattering (P-CARS) microscopy that allows vibrational imaging with high sensitivity and spectral selectivity. The nonresonant background signals from both Raman scatterers and the solvent are efficiently suppressed in P-CARS microscopy. We demonstrate P-CARS imaging of unstained cells based on the contrast of the protein amide I band.     

Chemical specificity in three-dimensional imaging with multiplex coherent anti-Stokes Raman scattering microscopy

We demonstrate the three-dimensional (3D) imaging capabilities and chemical specificity of multiplex coherent anti-Stokes Raman scattering microscopy. The simultaneous acquisition of a significant part of the vibrational spectrum at each specimen position permits straightforward differentiation among chemical species. 3D imaging is illustrated with a lipid multilamellar vesicle, and lateral and axial resolutions are determined.     

Tunable picosecond transient stimulated Raman scattering in ethanol

Transient stimulated Raman scattering (TSRS) of picosecond dye laser pulses in ethanol has been investigated experimentally. In particular some temporal features of TSRS from the C-H stretching mode (2928 cm^-1) in ethanol have been studied with a Photochron II streak camera with subpicosecond time resolution. It is shown that the use of TSRS provides a simple method for producing picosecond pulses tunable over the spectral region ≈492–532 nm and 691–772 nm by tuning a passively mode-locked Rhodamine 6G dye laser from 575–630 nm.     

Chirped-pulse adiabatic control in coherent anti-Stokes Raman scattering for imaging of biological structure and dynamics

Two novel control methods based on adiabatic passage are proposed to be implemented in coherent anti-Stokes Raman scattering (CARS) microscopy for noninvasive imaging of biological structure and dynamics. The first method provides optimal pulse-area control of the resonant vibrational transitions by using a pair of equally linear-chirped pulses. The second method, named the 'roof' method, utilizes the chirp sign variation at the central time and gives robust adiabatic excitation of the resonant vibrational mode. Both methods are robust with respect to suppression of the off-resonant transitions. The methods allow one to achieve chemical sensitivity with high resolution and can be used to obtain CARS spectra of biological molecules with efficiently suppressed background.     

相干反斯托克斯拉曼散射显微成像技术研究

基于全量子理论对相干反斯托克斯拉曼散射(CARS)过程进行了分析, 在此基础上搭建了单频CARS显微成像系统, 获得了不同尺寸聚苯乙烯微球高对比度的CARS显微图像. 为了标定成像系统的空间分辨率, 采用逐点扫描方式对直径为110 nm聚苯乙烯微球成像, 从而重构出系统的点扩展函数. 结果表明: 该CARS显微成像系统的横向空间分辨率约为600 nm, 而由阿贝衍射极限决定的理论空间分辨率约为300 nm. 分析了导致分辨率降低的原因, 并提出了解决方案. 为实现纳米分辨的CARS显微成像打下了坚实的基础.     

Compact fibre-based coherent anti-Stokes Raman scattering spectroscopy and interferometric coherent anti-Stokes Raman scattering from a single femtosecond fibre-laser oscillator

We demonstrate a new approach to CARS spectroscopy by efficiently synthesizing synchronized narrow-bandwidth (less than 10 cm⁻¹) pump and Stokes pulses (frequency difference continuously tunable upto ≈3000 cm⁻¹) based on spectral compression together with second harmonic generation (in periodically-poled nonlinear crystals) of femtosecond pulses emitted by a single compact Er-fibre oscillator. For a far better signal to non-resonant background contrast, interferometric CARS (I-CARS) is demonstrated and CARS signal enhancement upto three orders of magnitude is achieved by constructive interference with an auxiliary local oscillator at anti-Stokes field, also synthesized by spectral compression of pulses emitted from the same fibre oscillator.     

Highly sensitive single-beam heterodyne coherent anti-Stokes Raman scattering

Single-beam coherent anti-Stokes Raman-scattering (CARS) microspectroscopy achieves a complete CARS scheme with a femtosecond laser. Here, we introduce heterodyne detection in a simple experimental extension: the optical fields driving the CARS process and the local oscillator used for heterodyning are derived from a single beam of ultrashort laser pulses by pulse shaping. The heterodyne signal is amplified by more than 3 orders of magnitude and is linearly dependent on the concentration of Raman scatterers. This dramatically increases the sensitivity of chemically selective detection at microscopic resolution while maintaining the simplicity of the single-beam setup.