Vibrational spectroscopy has been widely applied in different fields due to its label-free chemical-sensing capability. Coherent anti-Stokes Raman scattering (CARS) provides stronger signal and faster acquisition than spontaneous Raman scattering, making it especially suitable for molecular imaging. Coherently-controlled single-beam CARS simplifies the conventional multi-beam setup, but the vibrational bandwidth and non-trivial spectrum retrieval have been limiting factors. In this work, a coherent supercontinuum generated in an all-normal-dispersion nonlinear fiber is phase-shaped within a narrow bandwidth for broadband vibrational spectroscopy. The Raman spectra can be directly retrieved from the CARS measurements, covering the fingerprint regime up to 1750 cm−1. The retrieved spectra of several chemical species agree with their spontaneous Raman data. The compact fiber supercontinuum source offers broad vibrational bandwidth with high stability and sufficient power, showing the potential for spectroscopic imaging in a wide range of applications.
©2013 Optical Society of America
Vibrational spectroscopy is a label-free technique for investigating the chemical content of samples. Compared to spontaneous Raman scattering, coherent anti-Stokes Raman scattering (CARS) provides coherent signal enhancement, realizing real-time vibrational imaging [1, 2]. In CARS, pump and Stokes fields generate molecular vibration, which is scattered off by a probe field to create a blue-shifted anti-Stokes field. Experimental and computational methods have been developed to retrieve the informative resonant CARS signal and to reject the non-resonant background, especially for spectroscopic applications [3–9]. Among them, pulse shaping enables coherent control of the driving fields and extracting spectral information in a single-beam alignment-insensitive configuration [10, 11]. Several approaches have been proposed to detect molecular vibrational signatures using a single-beam setup, including sinusoidal-phase [12–14], time-resolved pump-probe , spectral focusing , and narrow-band probe methods [17–23]. Currently, two of the major challenges in single-beam CARS spectroscopy include a limiting vibrational bandwidth that can be attained by the laser sources, and the non-trivial extraction and representation of the Raman spectrum.
Fiber supercontinuum (SC) presents an alternative to solid-state lasers with extended vibrational bandwidth. The use of fiber SC for CARS spectroscopy has been demonstrated in a multi-beam configuration [24–27]. For single-beam methods, a fiber continuum is preferred over a broadband (<20 fs) solid-state laser due to several notable advantages: (1) environmental stability of passive extracavity spectral broadening; (2) the absence of a trade-off between broad bandwidth and stable operation; and (3) intrinsic compatibility with alignment-free fiber-based components. Unfortunately, few known fiber SC sources have sufficient coherence, bandwidth, and power for coherent control . Previous works have shown using a fiber SC for sinusoidal-phase or time-resolved CARS measurements [14, 15]. However, the applicability of a narrow-band probe [17–23] using a fiber SC has not been demonstrated, possibly due to more demanding spectral phase stability and coherence. Recently, fiber SCs with high coherence, stability, and power have been generated by use of all-normal-dispersion fibers [29–33]. This development can be beneficial to high-precision phase shaping applications.
In this study, we demonstrate single-beam vibrational spectroscopy by narrow-band phase shaping of a coherent fiber SC generated in an all-normal-dispersion nonlinear fiber. The pulse shaping strategy and direct vibrational spectrum retrieval are presented. The retrieved spectra of several chemical species agree with their spontaneous Raman data across the fingerprint regime. The performance and advantages of the fiber SC are discussed as well.
2. Experimental methods
The experimental setup is illustrated in Fig. 1 . Pulses of 1041 nm, 180 fs (FWHM), and 80 MHz from a compact Yb:KYW laser (femtoTRAIN IC Model-Z, High Q Laser, Austria) were coupled into a nonlinear dispersion-flattened dispersion-decreased all-normal-dispersion (DFDD-ANDi) fiber (NL-1050-NEG-1, NKT Photonics, Denmark). The fiber was pumped at the slow axis with an input power of 600 mW to generate the SC with an output power of 345 mW (~4.3 nJ of pulse energy). Due to weak birefringence of the fiber, a portion of the SC (21%) was depolarized and discarded by a polarizer. The remaining SC was modulated by a pulse shaper (MIIPS Box 640, Biophotonics Solutions Inc.) and then focused by a near-infrared objective (LUMPlanFl/IR 60xW, N.A. = 0.9, Olympus). The spectral phase of the SC at the objective focus was characterized and compensated by the pulse shaper prior to arbitrary pulse shaping [32, 33].
The pulse shaping strategy was simplified from the narrow-band probe approach with impulsive vibrational excitation, as illustrated in Fig. 2 [17, 19, 23]. The SC spectral tail below 900 nm was blocked using a knife-edge at the Fourier plane of the pulse shaper in order to detect CARS signal within this spectral range. A narrow bandwidth centered at the lowest-wavelength peak of the SC spectrum was phase-shifted by ± π/2 radian to serve as the probe pulse. One pixel on the spatial light modulator was modulated, corresponding to a 10 cm−1 spectral resolution in the setup. The CARS data of ± π/2 pulses were subsequently acquired for retrieving the Raman spectra.
For CARS measurements, the SC power was attenuated by a neutral density filter to 10 mW for solvent samples and to 4 mW for the KGW crystal (KGd(WO4)2, potassium gadolinium tungstate) at the objective focus. The forward CARS signal from samples between two coverslips was collected by another objective (LUMPlanFl/IR 40xW, N.A. = 0.8, Olympus) and delivered into a spectrometer equipped with an EMCCD camera (ProEM 1600(2), Princeton Instruments). The exposure time was 1 ms, and 50 acquisitions were averaged. To validate these spectroscopic measurements, corresponding spontaneous Raman spectra were measured on a commercial confocal Raman microscope (LabRAM HR, Horiba) at the power of 8 mW. The integration time using this Raman microscope was 1 s to achieve the equivalent signal-to-noise ratio.
3. Theoretical modeling
The CARS process can be described in two steps,
Where Ep, Es, and Epr are the pump, Stokes, and probe fields, respectively. The term is the third-order nonlinear susceptibility, including resonant, , and non-resonant, , contributions. For the single-beam approach, the shaped pulses act as Ep, Es, and Epr simultaneously. The CARS spectra generated by the ± π/2 pulses described earlier are
Where ϕ(ω) is the phase induced by the vibrational resonance. The imaginary part of the resonant CARS signal can be obtained by calculating
The purely resonant term, , is relatively small and neglected. A constant prefactor is dropped since it does not affect the spectrum. The Raman spectrum, which is the imaginary part of , can be retrieved by correcting the impulsive vibrational excitation,
A simulation of this modeling and processing is shown in Fig. 3 . The experimental parameters, including the SC spectrum and the pulse shapes, were implemented in the simulation. A hypothetical molecule was assumed to possess vibrations modeled by complex Lorentzians evenly distributed from 600 to 1800 cm−1 (Fig. 3(a), blue line). The vibrational excitation spectrum of the compressed SC pulses was calculated to be smooth over the spectral range of interest (Fig. 3(a), gray line). The CARS signal generated by the shaped pulses shows spectral features of on top of the non-resonant CARS spectra, which result from the interference of resonant and non-resonant contributions (Fig. 3(b)). The extracted spectrum recovered the Raman spectrum with only minor distortion (Fig. 3(c)), which originated from the purely resonant contribution neglected in Eq. (3).
4. Experimental results
The retrieved and spontaneous Raman spectra of acetone, isopropanol, and toluene are shown in Fig. 4 . The CARS spectra agree well with their corresponding Raman data within the fingerprint region. For acetone, the highest detected shift at 1702 cm−1 (C = O stretching) is close to the frequency difference between the two furthest peaks in the SC spectrum (1709 cm−1), which dictates the accessible vibrational bandwidth by the SC. The strong C-C stretching vibration of isopropanol at 819 cm−1 is used to estimate the spectral resolution of the system, which is measured to be 13 cm−1 (FWHM). At this spectral resolution, the two close peaks of toluene at 1000 cm−1 and 1027 cm−1 are easily resolved.
The retrieved and spontaneous Raman spectra of a KGW crystal at two orientations (E||Ng, E||Nm: input electric field parallel to the Ng and Nm refractive index axes, respectively) are shown in Fig. 5 . The CARS measurement is sensitive to the orientation of the crystal. In the E||Ng orientation, the CARS data reproduce two major shifts at 767 cm−1 and 900 cm−1 as in the Raman spectrum . In the E||Nm orientation, the peak at 767 cm−1 is observed to be much weaker in both the retrieved and Raman spectra.
The spectral broadening of the SC generated in the DFDD-ANDi fiber is mainly governed by self-phase modulation, as is evident from its fringe-shaped spectrum. The reduced dispersion and definite spectral phase can be easily compensated to higher orders by a pulse shaper at the focus of a high-NA objective and therefore suitable for coherent control applications. The fiber was pumped at the slow axis to reduce the coupling between the slow and the fast axes, which can degrade the SC generation. An input power of 600 mW was used to induce sufficient spectral broadening for CARS spectroscopy in fingerprint regime. However, a higher input power up to 800 mW has been tested with broader spectrum without damaging the fiber. The depolarization was observed to be higher (21%) than that reported in our previous study (7%) because of a different strand of fiber used . The power, the spectrum and the spectral phase of the SC were measured to be consistent for more than 100 hours of operation, showing good long-term stability. For more details regarding the coherent fiber SC, including power dependence, phase characterization, and depolarization effects, readers are referred to [30, 32, 33, 35].
The fringe-shaped spectrum might appear to be unsuitable for single-beam CARS spectroscopy because the vibrational excitation can also be structured. Our simulation demonstrates smooth vibrational excitation, and the experiments retrieve Raman spectra across the fingerprint region with good agreement to their corresponding spontaneous Raman data. The detectable spectral range up to about 1750 cm−1 is limited by the SC bandwidth, which can be further extended by inducing wave-breaking . The full power of the SC is 345 mW at the fiber output. The nJ-order pulse energy is comparable to some lasers used in single-beam CARS. The SC generation setup is simple and low cost, mainly consisting of a compact Yb:KYW laser and a short piece of nonlinear fiber (9 cm). The broad bandwidth of the fiber SC provides sufficient vibrational excitation for CARS while the moderate pulse energy and compactness can potentially be beneficial for biophotonics applications.
A narrow-band probe pulse shaping strategy was utilized for retrieving Raman spectra, and for determining the molecular contents of samples. The ± π/2 phase shifts, as a complex conjugate pair, introduce a small change to the non-resonant background that is used as the local oscillator for homodyne mixing with the resonant signal. Therefore, a signal normalization step that might normally be needed when using 0 and π phase shifts can be relaxed. In addition, the difference spectrum yields the imaginary part of the CARS signal directly without post-processing using the Fourier transform or Kramers-Kronig transformation [19, 20, 37, 38]. Despite the simplicity, this pulse shaping approach has low probe pulse energy and relies heavily on homodyne mixing with the non-resonant signal. The spectra retrieved from the differential measurement can suffer from lower signal-to-noise ratio due to limited dynamic range because the non-resonant signal can easily saturate the detector at low excitation power. By optimizing the detection and pulse shaping scheme, a higher signal-to-noise ratio and faster acquisition could potentially be achieved.
Single-beam broadband vibrational spectroscopy has been performed by pulse shaping of a coherent SC generated from a DFDD-ANDi fiber. The SC provides broadband and smooth Raman excitation up to 1750 cm−1 with nJ-order pulse energy, enabling its use for single-beam CARS spectroscopy in fingerprint regime. Impulsive excitation with narrow-band probe pulses phase-shifted by ± π/2 radian facilitates direct retrieval of Raman spectra. The retrieved spectra of acetone, isopropanol, toluene and a KGW crystal in two orientations agree with their spontaneous Raman measurements. The SC bandwidth can potentially be extended by inducing wave-breaking or by pumping a polarization-maintained DFDD-ANDi fiber. The presented method is promising for spectroscopic imaging of samples in many application areas.
We appreciate the collaborative efforts from NKT Photonics, Inc. and Biophotonic Solutions Inc. We thank Prof. Alfred Leitenstorfer for the discussion about collimating the SC. We are grateful for the discussion with Prof. Rohit Bhargava and Dr. Matthew Schulmerich about Raman spectroscopy. We also thank Darold Spillman for his technical support. This work was supported in part by a grant from the National Institutes of Health (NIH R01CA166309, S.A.B.). Additional information can be found at http://biophotonics.illinois.edu.
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