Enhancement of coherent anti-Stokes Raman signal via tailored probing in spectral focusing

Imaging and component analysis of pumpkin stem tissue with simultaneous SF-CARS and TPEF microscopy

A multimodal nonlinear optical imaging platform based on a single femtosecond oscillator is built for simultaneous TPEF and SF-CARS imaging. TPEF microscopy and SF-CARS microscopy is utilized for mapping the distribution of the lignin component and the polysaccharide component, respectively. Visualization of vessel structure is realized. And the relative distribution of lignin and polysaccharide of vessel structure is mapped. Two pumpkin stem tissue areas with different degrees of lignification are observed with simultaneous TPEF and SF-CARS imaging, and two types of cell walls are identified. The different distribution patterns of lignin and polysaccharide in these two types of cell walls, induced by different degrees of lignification, are analyzed in detail.

Raman Techniques: Fundamentals and Frontiers

Driven by applications in chemical sensing, biological imaging and material characterisation, Raman spectroscopies are attracting growing interest from a variety of scientific disciplines. The Raman effect originates from the inelastic scattering of light, and it can directly probe vibration/rotational-vibration states in molecules and materials. Despite numerous advantages over infrared spectroscopy, spontaneous Raman scattering is very weak, and consequently, a variety of enhanced Raman spectroscopic techniques have emerged. These techniques include stimulated Raman scattering and coherent anti-Stokes Raman scattering, as well as surface- and tip-enhanced Raman scattering spectroscopies. The present review provides the reader with an understanding of the fundamental physics that govern the Raman effect and its advantages, limitations and applications. The review also highlights the key experimental considerations for implementing the main experimental Raman spectroscopic techniques. The relevant data analysis methods and some of the most recent advances related to the Raman effect are finally presented. This review constitutes a practical introduction to the science of Raman spectroscopy; it also highlights recent and promising directions of future research developments.

Flexible and broadly tunable infrared light source based on shaped sub-10-fs pulses for a multimodal microscopy setup

We present a versatile approach for mid-infrared spectroscopy through the flexible control of a difference-frequency-generation (DFG) process by femtosecond (fs) pulse shaping and spectral focusing. Based on a broadband sub-10-fs oscillator, the spectral position and spectral resolution can be independently selected within the molecular fingerprint region of more than 2000  cm^-1. A spectral resolution better than 20  cm^-1 can be achieved, which depends solely on the pulse shaper configuration. An absorption experiment on a polystyrene reference sample finally validates the concept and opens the door for an additional modality in nonlinear multimodal microscopy setups.

Resolution of spectral focusing in coherent Raman imaging

In this manuscript, we present a detailed investigation of the impact of dispersion on the spectral resolution achievable by the application of spectral focusing in coherent Raman imaging. Our results reveal the detrimental effect of third order dispersion that limits the resolution for group delay dispersion of 100 000 fs² and more. Experimental examples for the exact determination of the described effects are given as well as a condensed presentation of the known equations. We introduce useful approximations to the latter, which serve to facilitate the straightforward integration of spectral focusing into any multimodal microscope.

Selective Coherent Anti-Stokes Raman Scattering Microscopy Employing Dual-Wavelength Nanofocused Ultrafast Plasmon Pulses

Ultrafast surface plasmon polariton (SPP) nanofocusing on a plasmonic metal tapered tip with femtosecond laser pulses enables background-free localized excitation beyond the diffraction limit. We demonstrate simultaneous nanofocusing of ultrafast SPP pulses at 440 and 800 nm, which were coupled with a common diffraction grating structure fabricated on an aluminum (Al) tapered tip, to the tip apex with a radius of ∼35 nm. We achieved selective coherent anti-Stokes Raman scattering (CARS) microscopy that combined an 800 nm (ω) SPP pump pulse, which achieves selective vibrational excitation by spectral focusing, and a 440 nm (2ω) SPP probe pulse. Raman intensity of this novel 2ω-CARS increased by a factor of 3.96 at the G-band and 4.00 at the 2D-band compared with that with ω-CARS for the monolayer graphene. The 2ω-CARS imaging method was applied for imaging a multiwalled carbon nanotube at the D-, G-, and 2D-bands. This dual-wavelength nanofocusing will open up new nanoscale microspectroscopy and optical excitation at the tip apex, such as sum frequency mixing, two-photon excitation.

Spectral focusing dual-comb coherent anti-Stokes Raman spectroscopic imaging

Coherent Raman microscopy provides label-free imaging by interrogating the intrinsic vibration of molecules. Here, we present a concept in a 100 MHz dual-comb scheme to facilitate high-speed and broadband coherent anti-Stokes Raman spectroscopic imaging based on down-converted, automatically varying delay-time in spectral focusing excitation. A rapid measurement of vibrational microspectroscopy on submicrosecond scale over a spectral span ∼700  cm^-1 with a refresh rate of 1.2 kHz provides access to well-resolved molecular signatures within the fingerprint region. We demonstrate hyperspectral images of a mixture of spatially inhomogeneous distributions of chemical substances.

Quantitative chemical imaging with background-free multiplex coherent anti-Stokes Raman scattering by dual-soliton Stokes pulses

Coherent anti-Stokes Raman microscopy (CARS) is a quantitative, chemically specific, and label-free optical imaging technique for studying inhomogeneous systems. However, the complicating influence of the nonresonant response on the CARS signal severely limits its sensitivity and specificity and especially limits the extent to which CARS microscopy has been used as a fully quantitative imaging technique. On the basis of spectral focusing mechanism, we establish a dual-soliton Stokes based CARS microspectroscopy and microscopy scheme capable of quantifying the spatial information of densities and chemical composition within inhomogeneous samples, using a single fiber laser. Dual-soliton Stokes scheme not only removes the nonresonant background but also allows robust acquisition of multiple characteristic vibrational frequencies. This all-fiber based laser source can cover the entire fingerprint (800-2200 cm^-1) region with a spectral resolution of 15 cm^-1. We demonstrate that quantitative degree determination of lipid-chain unsaturation in the fatty acids mixture can be achieved by the characterization of C = C stretching and CH₂ deformation vibrations. For microscopy purposes, we show that the spatially inhomogeneous distribution of lipid droplets can be further quantitatively visualized using this quantified degree of lipid unsaturation in the acyl chain for contrast in the hyperspectral CARS images. The combination of compact excitation source and background-free capability to facilitate extraction of quantitative composition information with multiplex spectral peaks will enable wider applications of quantitative chemical imaging in studying biological and material systems.

Single-beam heterodyne FAST CARS microscopy

We demonstrate, for the first time, single-beam heterodyne FAST CARS imaging without data post-processing and with nonresonant background subtraction in a simple setup via the real-time piezo modulation of the probe delay. Our fast signal acquisition scheme does not require a spatial light modulator in the pulse shaper, and is suitable for high-resolution imaging and time-resolved dynamics. In addition, the spectral detection of the back-scattered FAST CARS signal is incorporated into the pulse shaper, allowing for a compact and more efficient design. Such epi-detection capability is demonstrated by imaging Si and MoS₂ microstructures.