Coherent anti-Stokes Raman scattering microscopy with a photonic crystal fiber based light source

A coherent anti-Stokes Raman scattering microscope based on a Ti:sapphire femtosecond oscillator and a photonic crystal fiber is demonstrated. The nonlinear response of the fiber is used to generate the additional wavelength needed in the Raman process. The applicability of the setup is demonstrated by imaging of micrometer-sized polystyrene beads.     

Near-infrared coherent anti-Stokes Raman scattering microscopy using supercontinuum generated from a photonic crystal fiber

Coherent supercontinuum generated from a photonic crystal fiber is applied to near-infrared coherent anti-Stokes Raman scattering (NIR-CARS) microscopy. A clear CARS image of polystyrene beads has been successfully obtained at a wavenumber difference resonant with the CH-stretching vibrational mode. PACS 87.64.Je; 39.30.+w; 87.64.Vv     

Broadband coherent anti-Stokes Raman scattering spectroscopy using soliton pulse trains from a photonic crystal fiber

Pulse trains of fundamental soliton pulses with different center wavelengths and delay times from a photonic crystal fiber were generated and used as Stokes optical pulses in coherent anti-Stokes Raman scattering (CARS) spectroscopy. The pulse trains were created by shaping optical pulses with a pulse shaper and their waveforms were measured by a cross-correlation frequency-resolved optical gating method. By the use of pulse trains, the time required for obtaining broadband CARS signals was reduced to be about one third compared with our previous study without using pulse trains. With this setup, broadband CARS signals between 500 and 3100 cm⁻¹ of a single polystyrene bead sample have been measured and the most of the Raman peaks in this frequency range of samples have been observed clearly.     

Coherent anti-Stokes resonance Raman scattering

The intensity of coherent anti-Stokes resonance Raman scattering (CARRS) is calculated taking into account the resonant absorption of laser and anti-Stokes waves and changes in the molecule population. The resonant absorption causes an alteration of the phase of the laser wave leading to a decrease and periodic behaviour of the anti-Stokes intensity. This phase change can be compensated by the choice of a definite wave vector mismatchK, which determines the optimum condition for the CARRS. The optimum angle between the Stokes and the laser propagation direction and the optimum cell length are calculated. The line shape of the CARRS is discussed and used for the explanation of the observed dependence of the CARRS spectra on the laser intensity and the frequency and on the molecule number density. In the case of a O-O excitation the vibrational frequencies of the ground and the excited level are generated simultaneously in the CARRS spectra.     

Coherent anti-Stokes Raman scattering microspectroscopy of silicon components with a photonic-crystal fiber frequency shifter

Coherent anti-Stokes Raman scattering (CARS) microspectroscopy of silicon components is demonstrated with pump and probe fields delivered by a mode-locked Cr:forsterite laser and the frequency-shifted soliton output of a photonic-crystal fiber as a Stokes field. CARS microspectroscopy is shown to allow a visualization of microscale features and defects on the surface of silicon wafers, offering much promise for online diagnostics of electronic and photonic silicon chip components.     

Coherent anti-Stokes Raman scattering velocimetry with nearly degenerate pumps

The time evolution of the anti-Stokes signal produced from the non-linear interaction of a short Stokes pulse and two long pump pulses that are nearly degenerate in frequency has been investigated. It is shown that this approach allows us to specify the accuracy of CARS (coherent anti-Stokes Raman scattering) velocimetry and to extend the range of operation of the method. In addition, an original optical scheme capable of delivering short visible pulses with good spatial and spectral properties is reported. The optical bench has been used for the characterisation of a low-pressure laminar Mach-10 flow. Received: 24 October 2001 / Revised version: 8 January 2002 / Published online: 14 March 2002     

Coherent anti-Stokes Raman scattering in silicon nanowire ensembles

In this letter, we, for the first time, report on coherent anti-Stokes Raman scattering (CARS) spectroscopy of an ensemble of silicon nanowires (SiNWs) formed by wet chemical etching of crystalline silicon with a mask of silver nanoparticles. The fabricated SiNWs have diameter ranged from 30 to 200 nm and demonstrate both visible and infrared photolumine cence (PL) and spontaneous Raman signal, with their intensities depending on presence of silver nanoparticles in SiNWs. The efficiency of CARS in SiNW ensembles is found to be significantly higher than that in crystalline silicon. The results of CARS and PL measurements are explained in terms of resonant excitation of the electron states attributed to silicon nanoparticles.     

Coherent anti-Stokes resonance Raman scattering of excited molecules

A theoretical investigation of the coherent anti-Stokes resonance Raman scattering (CARRS) of excited molecules is given. The calculation of the anti-Stokes intensity predicts the simultaneous appearance of the vibrational frequencies of the ground and the excited electronic state in the CARRS spectra. The line shape of these spectra and the optimum conditions for the anti-Stokes amplification are discussed.     

Broad spectral photonic crystal fiber surface enhanced Raman scattering probe

A broad spectral surface enhanced Raman scattering sensor is developed using the solid core holey photonic crystal fiber with silver nanoparticles cluster. This SERS probe offers an operational excitation wavelength range overlaying visible light and near infrared light. The PCF SERS sensing is demonstrated in the detection of the 4-Mercaptobenzoic acid (10⁻⁶ M) solution with 514.5 and 785 nm excitation. In this structure of PCF sensor, the related analysis shows that leakage modes also make an important contribution in the SERS activity not only by the evanescent field way.     

Multiwavelength fiber laser based on a photonic crystal fiber loop mirror with cooperative Rayleigh scattering

In this work, a multiwavelength fiber Raman laser based on a highly birefringent photonic crystal fiber loop mirror is presented. A laser resonator is formed when the Raman amplification with cooperative Rayleigh scattering in a dispersion-compensating fiber is used as a distributed mirror and combined with a photonic crystal fiber loop mirror filtering structure. Stable multiwavelength lasing at room temperature is achieved due to the low temperature sensitivity of the highly birefringent photonic crystal fiber.     

Coherent anti-Stokes Raman scattering and normal anti-Stokes Raman scattering due to LO phonons in GaP using cw diode lasers

Coherent anti-Stokes Raman scattering (CARS) and normal anti-Stokes Raman scattering (NARS) have been measured in (001) GaP at room temperature due to the 403 cm⁻¹ LO phonons using a continuous wave (CW) 785.0 nm fixed-wavelength pump laser and a CW Stokes laser tunable in the 800-830 nm wavelength range. CARS measurements are normally made using pulsed lasers. The use of CW diode lasers allows a more accurate comparison between the measured and calculated values of the CARS signal. The pump and Stokes laser beams were linearly polarized perpendicular to each other, same as the pump and normal Stokes/anti-Stokes scattered light for the GaP sample used in this work. The pump and Stokes laser powers incident upon the GaP sample, located in the focal plane of a 20 mm effective focal length lens, were <20 and 50 mW, respectively. The diameter of the laser beams in the focal plane of the focusing lens was determined to 40±5 μm. The pump and Stokes laser beam intensities incident upon the 0.3 mm thick GaP sample were <2 and 5 kW cm², respectively. The powers of the CARS and NARS signals were measured using a Raman spectrometer. The signal output of the Raman spectrometer was calibrated using the pump laser and several neutral density filters. The Raman linewidth (full-width at half-maximum) of the LO phonons was determined to be 0.95±0.05 cm⁻¹, using the variation of the CARS signal with the wavelength of the Stokes laser. The measured powers of the CARS and NARS signals are about a factor of 5 and 1.5, respectively, smaller than those calculated from the corresponding theoretical expressions.     

光子晶体光纤中高效的反斯托克斯信号产生

基于四波混频的反斯托克斯变换, 被广泛应用于短波辐射高分辨率成像以及直接激发分子的电子跃迁等方面. 为了实现更加高效的反斯托克斯变换, 利用中心波长为810 nm脉冲宽度为120 fs的钛蓝宝石(Ti: sapphire)飞秒激光器作为抽运光源, 在长度为0.5 m和3 m的光子晶体光纤中分别实现了高阶模和基膜的简并四波混频. 实验中, 采用的光子晶体光纤的零色散波长在820 nm附近. 在基模相位匹配条件下, 在560 nm附近实现了高效地反斯托克斯信号的产生, 反斯托克斯信号与残余抽运信号的最大功率比为33:1; 反斯托克斯信号和斯托克斯信号的最大功率比25:1; 反斯托克斯信号最大功率转换效率P_a/P_p0为34%. 抽运波长从790 nm逐渐增加到810 nm过程中, 在长为3 m的光子晶体光纤中相位从不匹配状态转化为高阶模匹配状态后, 再转化为基模匹配状态. 通过实验研究得出了相位匹配程度随抽运功率、波长和光纤长度的变化规律, 同时分析了造成理论计算与实验结果存在差异的主要因素. 本文为研究在光子晶体光纤基模中实现相位匹配和产生高效反斯托克斯信号提供了理论和实验依据.     

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.     

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).     

All fiber supercontinuum light source using photonic crystal fibers pumped by nanosecond fiber laser pulses

A novel compact supercontinuum (SC) source using the single mode photonic crystal fibers (PCF) pumped with an all fiber MOPA fiber laser is demonstrated experimentally. A bandwidth of 700 nm is achieved by operating the pumping fiber laser at a wavelength of 1064 nm, pulse duration of 10 ns, repetition rate of 50 kHz and peak power of 1 kW. The SC generation is initiated through modulation instability (MI) which breakups the nanosecond pump pulses into picosecond or femtosecond pulses, and further broadened through nonlinear effects of PCF.     

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.     

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.     

Broadband coherent anti-Stokes Raman scattering light generation in BBO crystal by using two crossing femtosecond laser pulses

As broad as 12000 cm(-1) coherent anti-Stokes Raman scattering (CARS) light from ultraviolet to infrared was generated in a BBO crystal by using two crossing femtosecond laser pulses with 30% conversion efficiency. More than fifteenth-order anti-Stokes and second-order Stokes Raman sidebands were observed with nice Gaussian spatial mode. The effect of the crossing angle between two input beams on the spectrum and emitting angle of the Raman sidebands was studied in detail. Calculation shows that the phase-matching condition determines the frequencies and angles of the sidebands.     

Ultrahigh efficiency laser wavelength conversion in a gas-filled hollow core photonic crystal fiber by pure stimulated rotational Raman scattering in molecular hydrogen

We report on the generation of pure rotational stimulated Raman scattering in a hydrogen gas hollow-core photonic crystal fiber. Using the special properties of this low-loss fiber, the normally dominant vibrational stimulated Raman scattering is suppressed, permitting pure conversion to the rotational Stokes frequency in a single-pass configuration pumped by a microchip laser. We report 92% quantum conversion efficiency (40 nJ pulses in 2.9 m fiber) and threshold energies (3 nJ in 35 m) more than 1 x 10(6) times lower than previously reported. The control of the output spectral components by varying only the pump polarization is also shown. The results point to a new generation of highly engineerable and compact laser sources.