1 Nature Physics 2007 Vol: 4(2):125-129. DOI: 10.1038/nphys809

Noise autocorrelation spectroscopy with coherent Raman scattering

Coherent anti-Stokes Raman scattering (CARS) with femtosecond laser pulses has become a widespread method in nonlinear optical spectroscopy and microscopy1, 2. As a third-order nonlinear process, femtosecond CARS exhibits high efficiency at low average laser power. High sensitivity to molecular structure enables detection of small quantities of complex molecules3, 4 and non-invasive biological imaging5. Temporal and spectral resolution of CARS is typically limited by the duration of the excitation pulses and their frequency bandwidth, respectively. Broadband femtosecond pulses are advantageous for time-resolved CARS spectroscopy6, 7, but offer poor spectral resolution. The latter can be improved by invoking optical8, 9 or quantum10, 11 interference at the expense of increasing complexity of instrumentation and susceptibility to noise. Here, we present a new approach to coherent Raman spectroscopy in which high resolution is achieved by means of deliberately introduced noise. The proposed method combines the efficiency of a coherent process with the robustness of incoherent light. It does require averaging over different noise realizations, but no temporal scanning or spectral pulse shaping as commonly used by frequency-resolved spectroscopic methods with ultrashort pulses.

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Figures
Figure 1: Interaction scheme for CARS and experimental arrangement for the detection of NASCARS signal.a, Pump and Stokes pulses (black) overlap in time and induce the coherent molecular vibrations. The broadband probe pulse (green) interacting with the vibrational energy levels 1,2 generates the anti-Stokes light (red and blue). In NASCARS, the noise in the probe spectrum is imprinted onto the corresponding spectral regions of the anti-Stokes light. b, Diagrams of various experimental configurations. In (i), the random spectral noise is applied to the probe pulse by the spatial light modulator (SLM) in the pulse-shaper configuration. In (ii), the phase of the probe is randomized owing to the intermodal dispersion of a multi-mode optical fibre. In both cases, the pump and Stokes pulses are generated by the optical parametric amplifier (OPA). In the single-pulse configuration (iii), a diffuser is introduced inside the compressor stage (C) of the regenerative amplifier (RGA) system. Random scattering results in the partial decoherence of the pulse required by NASCARS. The anti-Stokes spectrum is detected by the CCD-based spectrometer, and the autocorrelation is calculated by the computer. LWP: long-wave pass filter; SWP: short-wave pass filter. Figure 2: Numerical simulations of the noise autocorrelation spectroscopy with CARS.a, The frequency bandwidth of the probe pulse, represented by the broad gaussian envelope, is much broader than the width of and the distance between the vibrational resonances of toluene (black curve). White noise is applied to the spectral phase of the probe pulse via random phase jumps of 0–2 radians every 1 cm-1. b, Time-domain representation of the decaying molecular vibrations (black line), the incoherent probe pulse train obtained by applying the spectral phase noise to the broadband probe (green line) and the resulting anti-Stokes pulse train (red line). Being a product of the vibrational amplitude and the probe field amplitude, the output anti-Stokes field acquires spectral correlations owing to the periodic vibrational modulation superimposed onto the uncorrelated input noise. c, Autocorrelation of the calculated noisy anti-Stokes spectrum (shown in the inset, and exhibiting no clear evidence of the Raman modes) with a single realization of the phase noise (red line) and after averaging over 100 noise realizations (blue line). The strong peak at 27 cm-1 and weak peak at 218 cm-1, marked by the dotted lines, correspond to the beating of the vibrational modes of toluene at 782, 1,000 and 1,027 cm-1. Figure 3: Experimental NASCARS spectra (solid lines) compared with the autocorrelation of the spontaneous Raman spectra obtained by the commercial Raman spectrometer (dashed lines).a, Mixture of toluene and ortho-xylene with the relevant Raman shifts of 782, 1,000, 1,027, 982 and 1,049 cm-1. Dotted lines point to the beating signals at 27, 49 and 218 cm-1. The uncorrelated spectral phase noise was introduced to the probe field through the pulse shaper (b(i)). The anti-Stokes spectrum for a single noise realization is shown in the inset. The observed linewidth is determined by the convolution of the real resonance width of 3 cm-1 and the spectral resolution of the pulse shaper (3.5 cm-1) which determines the correlation length of the applied spectral noise. b, Single-pulse NASCARS results for the sample of liquid CBrCl3. Here, a single ultrashort pulse was focused directly in the medium. A thin sheet of a scattering material was placed inside the compressor stage of the laser system (b (iii)) to create partial decoherence of the beam. Vibrational beating at 47, 126 and 173 cm-1 is clearly identifiable at the expected locations (dotted lines). The observed peaks are associated with the three Raman modes of CBrCl3 at 246, 293 and 419 cm-1, shown on the spontaneous Raman spectrum in the inset.
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References
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