Spectrally-broad coherent anti-Stokes Raman scattering hyper-microscopy utilizing a Stokes supercontinuum pumped at 800 nm

Abstract

Abstract:We demonstrate spectral-focusing based coherent anti-Stokes Raman scattering (SF-CARS) hyper-microscopy capable of probing vibrational frequencies from 630 cm 1 to 3250 cm 1 using a single Ti:Sapphire femtosecond laser operating at 800 nm, and a commercially-available supercontinuum-generating fibre module. A broad Stokes supercontinuum with significant spectral power at wavelengths between 800 nm and 940 nm is generated by power tuning the fibre module using atypically long and/or chirped ~200 fs pump pulses, allowing convenient access to lower vibrational frequencies in the fingerprint spectral region. This work significantly reduces the instrumental and technical requirements for multimodal CARS microscopy, while expanding the spectral capabilities of an established approach to SF-CARS.

Multimodal spectral focusing CARS and SFG microscopy with a tailored coherent continuum from a microstructured fiber

“…In typical SF-CARS implementations, fs pulses have been stretched using dispersive glass elements [24,26] and the group delay dispersion (GDD) equalized for the pump and the Stokes by placing extra glass in the Stokes arm. The resolution of the system is dependent on the equalization of GDD which has been investigated analytically [25,27]. The power requirements with SF-CARS are similar to singlefrequency CARS, e.g.…”

“…To avoid the need for a 10 fs oscillator for SF-CARS, methods using Stokes and pump carved from SC pulses generated in so-called microstructured fibers (MOFs) have been developed at significantly lower cost and are now widely used for SC pumped bioimaging [27,[30][31][32]. The SC in fibers such as the femtoWHITE 800 fiber from NKT is ordinarily generated in the anomalous dispersion regime, where the soliton fission threshold, above which the SC pulse breaks into multiple pulses, limits the coherence and reduces the effectiveness for exciting CARS signals [33].…”

“…It has two closely spaced zero dispersion wavelengths (ZDW) to avoid extended propagation in the anomalous dispersion regime thereby increasing the coherence. Such modules have been used for tissue imaging, spectroscopy [27] and CARS for resonances both in the fingerprint region and the CH stretch region [31]. However, the excessive broadening due to the dispersive and nonlinear properties of the fiber limits the useful power available for CARS signal generation, so it has been applied only for CARS detected in the forward (transmission) direction, rather than in the weaker epi-direction (with one notable exception [35]), as would be needed for thick and hence easily prepared clinical samples [21][22][23].…”

“…Users of femtoWHITE CARS modules have, to date, used a pick-off from the fs seed laser as the pump and combined that with just the long wavelength part of the SC as the Stokes, thus losing the energy in the short-wavelength part of the SC and reducing signal power [27,36,37]. We wanted to prevent the waste of that short-wavelength power so, motivated partly by the recently reported technique using intra-pulse frequency combinations for broadband spectrumbased 'hyperspectral' 3-color CARS [13], we have used the full spectrum from an all normal dispersion (ANDi) fiber, NL-1050-NEG-1 from NKT Photonics, to produce a coherent continuum [38] and demonstrated its use for SF-CARS and SHG/SFG imaging.…”