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.     

Ultrabroadband multiplex CARS microspectroscopy and imaging using a subnanosecond supercontinuum light source in the deep near infrared

Subnanosecond supercontinuum (SC) has been generated by a 1,064 nm microchip laser combined with a photonic crystal fiber. The ultrabroadband (>2,000 cm(-1)) SC has facilitated multiplex coherent anti-Stokes Raman scattering (CARS) microspectroscopy in the spectral range from 1,000 to 3,000 cm(-1) with lateral and depth spatial resolution of 0.9 and 4.6 microm, respectively. A clear CARS image of a Nicotiana tabacum L. cv. Bright Yellow 2 cell has been obtained with high vibrational contrast.     

Ultrabroadband (>2000 cm-1) multiplex coherent anti-Stokes Raman scattering spectroscopy using a subnanosecond supercontinuum light source

We have developed ultrabroadband (>2000 cm(-1)) multiplex coherent anti-Stokes Raman scattering (CARS) spectroscopy using a subnanosecond (sub-ns) microchip laser source. A photonic crystal fiber specifically designed for sub-ns supercontinuum (SC) generation has been used for obtaining ultrabroadband Stokes radiation, which enables us to achieve simultaneous vibrational excitation in the range from 800 to 3000 cm(-1). We have successfully obtained multiplex CARS spectra for several molecular liquids. Since the CARS system using the sub-ns SC is simple and compact, it can be easily applied to ultrabroadband multiplex CARS microspectroscopy.     

Fast three‐dimensional chemical imaging by interferometric multiplex coherent anti‐Stokes Raman scattering microscopy

We report significant improvements in both signal sensitivity and imaging speed of Fourier transform spectral interferometry coherent anti‐Stokes Raman scattering (FTSI‐CARS) microscopy. With a help of an apodization function in the signal retrieval process, background due to the spectral change of nonresonant signals is eliminated. We experimentally verify that the sensitivity of the improved method is nearly shot‐noise‐limited. The current maximum detection sensitivity is ∼10 mM of aqueous sulfate ions, which correspond to ∼10⁶ oscillators in the microscopy focal volume. Operating the charge‐coupled device (CCD) in the crop mode increases the image acquisition speed by more than ten times. A vibrational hyperspectral image of a polymer sample with 100 × 100 pixel can be obtained within 3 s. With the improved sensitivity and speed, we also perform three‐dimensional volume imaging. Superior chemical selectivity is demonstrated with a mixture of two different oil droplets, which have identical vibrational peak positions but different relative peak ratios. Copyright © 2010 John Wiley & Sons, Ltd.     

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.     

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.     

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.     

Quadruplex CARS micro‐spectroscopy

We demonstrate a technique for simultaneous detection of coherent anti‐Stokes Raman scattering (CARS) at four vibrational frequencies, using simple passive optical elements and without spectrally resolved detection. The technique is based on pump and Stokes femtosecond pulses selectively exciting vibrational resonances through spectral focusing. By replicating the pump and Stokes pair into four pairs, each traveling through appropriate glass elements, we simultaneously excite four different vibrational frequencies. The resulting CARS is a periodic train of intensities detected by a single photomultiplier and frequency analyzed to retrieve its Fourier coefficients. We demonstrate detection of methanol and ethanol mixtures in water and quantitative determination of their concentration owing to the improved chemical selectivity of this quadruplex CARS scheme. Copyright © 2012 John Wiley & Sons, Ltd.     

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).     

Microanalytical nonlinear single-beam spectroscopy combining an unamplified femtosecond fibre laser,pulse shaping and interferometry

Nonlinear vibrational spectroscopy using a single beam of femtosecond pulses from an unamplified fibre laser oscillator is demonstrated. To achieve high spectral resolution and sensitive signal detection with the picojoule pulse energies available, pulse shaping and integrated interferometric detection is employed. The spectroscopic technique used is coherent anti-Stokes Raman scattering (CARS), which yields well-resolved spectra of molecular vibrations in the 100–350 cm^-1 domain of halomethane samples in the liquid phase. We explore the implications of phase control for the interferometric detection of weak signals. The presented combination of a fiber laser, pulse shaping and CARS microspectroscopy is a first example of simplified schemes for compact and robust nonlinear spectroscopic detection and sensing, which is demonstrated exemplarily by on-line monitoring of the chemical composition in a microfluidic flow cell. PACS 42.55.Wd; 42.62.Fi; 78.47.Fg; 42.65.Dr; 82.80.Gk; 92.20.cn     

Experimental Investigation on Selective Excitation of Two-Pulse Coherent Anti-Stokes Raman Scattering

Selective excitation of coherent anti-Stokes Raman scattering from the benzene solution is achieved by adaptive pulse shaping based on genetic algorithm, and second harmonic generation frequency-resolved optical gating （SHG-FROG） technique is adopted to characterize the original and optimal laser pulses. The mechanism for two-pulse coherent mode-selective excitation of Raman scattering is experimentally investigated by modulating the pump pulse in the frequency domain, and it is indicated that two-pulse coherent mode-selective excitation of Raman scattering mainly depends on the effective frequency components of the pump pulse related to specific vibrational mode. The experimental results suggest that two-pulse CARS has good signal-to-background ratio and high sensitivity, and it has attractive potential applications in the complicated molecular system.     

Nonlinear interferometric vibrational imaging

Coherent anti-Stokes Raman scattering (CARS) processes are "coherent," but the phase of the anti-Stokes radiation is lost by most incoherent spectroscopic CARS measurements. We propose a Raman microscopy imaging method called nonlinear interferometric vibrational imaging, which measures Raman spectra by obtaining the temporal anti-Stokes signal through nonlinear interferometry. With a more complete knowledge of the anti-Stokes signal, we show through simulations that a high-resolution Raman spectrum can be obtained of a molecule in a single pulse using broad band radiation. This could be useful for identifying the three-dimensional spatial distribution of molecular species in tissue.     

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.     

Time‐resolved investigation of the ν1 ro‐vibrational Raman band of H2CO with fs‐CARS

The technique of femtosecond time‐resolved coherent anti‐Stokes scattering (fs‐CARS) is used to investigate the strongly perturbed ν₁ ro‐vibrational Raman band of formaldehyde (H₂CO). The time‐dependent signal is simulated using a ‘Watson‐’Hamiltonian in A‐type reduction and Raman theory for asymmetric rotors. The results are compared with the experimental data. The fs‐CARS method measures the evolution of the polarization in a molecular ensemble via superposition of many states and is sensitive to spectral irregularities or line shifts of the involved transitions. ‘Coriolis’ interactions play a major role in the analysis of the ν₁ band of formaldehyde. We successfully simulate the fs‐CARS transient signal from the ν₁ band of formaldehyde including a model for multiple ‘Coriolis’ interactions, without the necessity of describing the complete interaction between all the vibrational levels. ‘Coriolis’ coupling coefficients and energy shifts are derived from the experiment by a least‐square fit. The results are discussed and compared to literature values. Copyright © 2006 John Wiley & Sons, Ltd.     

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.     

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.     

Heterodyne interferometric polarization coherent anti‐Stokes Raman scattering (HIP‐CARS) spectroscopy

We demonstrate a new technique that combines polarization sensitivity of the coherent anti‐Stokes Raman scattering (CARS) response with heterodyne amplification for background‐free detection of CARS signals. In this heterodyne interferometric polarization CARS (HIP‐CARS), the major drawbacks of polarization and heterodyne CARS are rectified. Using a home‐built picosecond optical parametric oscillator, we are able to address vibrational stretches between 600 and 1650 cm⁻¹ and record continuous high‐resolution Raman equivalent HIP‐CARS spectra. Copyright © 2010 John Wiley & Sons, Ltd.     

Effects of laser linewidth on coherent antiStokes Raman spectroscopy

Expressions for the generated antiStokes spectral density in coherent antiStokes Raman spectroscopy (CARS) are obtained, which take into account the finite linewidths of laser sources and which may be used to analyse observed spectra. Lorentzian and gaussian laser lineshapes are taken as special cases, which enable further analytic results for single and multiline CARS spectra to be derived. Emphasis is placed on scanning and broadband (multiplex) CARS techniques, and the choice of laser sources discussed from the spectral point of view. As examples of multiline spectra an analytical account of a periodic spectrum is presented and temperature measurement from Q-branch spectra is treated.     

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.