Tao Wu

Dual-soliton Stokes-based background-free coherent anti-Stokes Raman scattering spectroscopy and microscopy

We propose an all-fiber-generated, dual-soliton, Stokes-based scheme for background-free coherent anti-Stokes Raman scattering (CARS) under the spectral focusing mechanism. Owing to the strong birefringence and high nonlinearity of a polarization-maintaining PCF (PM-PCF), two soliton pulses can be simultaneously emitted along different eigenpolarization axes and both serve as Stokes pulses, while allowing feasible tunability of frequency distance and temporal interval between them. This proposed scheme, based on an all-fiber light source, exploits a unique combination of slight frequency-shift temporal walk-off of these two solitons to achieve efficient suppression of the nonresonant background and beat the inaccessibility and complexity of the excitation source. Capability is experimentally demonstrated by background-free CARS spectroscopy and unambiguous CARS microscopy in the fingerprint region.We propose an all-fiber-generated dual-soliton pulses based scheme for the background-free detection of coherent anti-Stokes Raman spectroscopy under the spectral focusing mechanism. Due to the strong birefringence and high nonlinearity of a polarization-maintaining photonic crystal fiber (PM-PCF), two redshifted soliton pulses can be simultaneously generated relying on high-order dispersion and nonlinear effects along two eigenpolarization axes in the anomalous dispersion regime, while allowing feasible tunability of the frequency distance and temporal interval between them. This proposed scheme, termed as DS-CARS, exploits a unique combination of slight frequency-shift and advisable temporal walk-off of this two soliton pulses to achieve robust and efficient suppression of nonresonant background with compact all-fiber coherent excitation source. Capability of the DS-CARS is experimentally demonstrated by the background-free CARS spectroscopy and unambiguous CARS microscopy of polymer beads in the fingerprint region.

A Raman peak recognition method based automated fluorescence subtraction algorithm for retrieval of Raman spectra of highly fluorescent samples

Intense fluorescence background is a major problem in the application of Raman spectroscopy. An appropriate algorithm which can faithfully retrieve weak tissue Raman signals is required. In this article, we propose a new algorithm for automated and artifact-free recovery of Raman spectra which combines a novel Raman peak recognition method (RPR method) with an improved iterative smoothing method (SG-SR method). The SG-SR method, based on the modified Savitzky–Golay iterative process, substantially improves its convergence speed. By applying a novel negative relaxation factor to the successive relaxation iterative method, automatic recognition of Raman peaks is realized. In the proposed algorithm (RIA-SG-RPR algorithm), a real Raman peak position is first detected by the RPR method to serve as the intrinsic criterion of convergence for the SG-SR method to avoid human interference. Then, real Raman signals are recovered from the iterative procedure of the SG-SR method. This algorithm has been optimized and validated with mathematically simulated Raman spectra as well as experimentally recorded Raman spectra from various fluorescent samples, resulting in a significant improvement in the rejection of both high fluorescence background and direct human intervention. This algorithm drastically avoids false Raman features to benefit the utilization of Raman spectroscopy to characterize molecular specifics in more challenging Raman applications.

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 100xa0MHz 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 ∼700u2009u2009cm-1 with a refresh rate of 1.2xa0kHz 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.

Cascaded Dual-Soliton Pulse Stokes for Broadband Coherent Anti-Stokes Raman Spectroscopy

We report a spectrally and temporally cascaded dual-soliton Stokes excitation source for broadband coherent anti-Stokes Raman spectroscopy (CARS) based on a single Yb-doped fiber laser. The strong birefringence of a polarization-maintaining photonic crystal fiber has the potential to simultaneously generate two solitons along different eigenpolarization axes, allowing for an extra degree of freedom in tuning the properties of the supercontinuum. With well-separated spectral regions of this dual-soliton, the CARS spectral coverage can be greatly extended. Taking advantage of the dispersion characteristic of the two orthogonal polarized modes and applying identical linear frequency chirp on both soliton Stokes pulses further allows that spectral CARS information can be encoded and acquired in time. The chirped pump pulse, in conjunction with dual-soliton Stokes pulses, can potentially yield gapless coverage of Raman transitions from 800 to 1800 cm-1 with a spectral resolution of 15 cm-1. Broadband spectral coverage capability is theoretically verified with simulations and then experimentally demonstrated by CARS spectroscopy with varied polymer materials. We also demonstrate hyperspectral Raman microscopy with molecular contrast of lipid droplets. An all-fiber-based broadband excitation source may greatly simplify CARS implementation and extend the qualitative and even quantitative chemical analysis of CARS microspectroscopy.

Point-specific self-calibration for improved characterization of thickness distribution

Abstract. This work proposes a point-specific self-calibration method to characterize film thickness distribution by exploiting the multiple detection capability of a home-built full-field ellipsometer. The self-calibration method offers a feasible route for retrieving calibration information from the actual real-time sample measurement in conjunction with the ellipsometric parameters, thus leading to error-free data after the elimination of systematic errors and addressing the problem of high time-consumption. With the help of the multiple detection capability of a full-field ellipsometer, we can further implement self-calibration for every point-specific pixel, termed as point-specific self-calibration to achieve a high-accuracy film thickness profile. The synthetic thickness distribution composed of structural-anisotropy pixels with tilted surface is utilized to demonstrate the potential of the proposed approach by retrieving the ellipsometric angles and the calibration parameters of every single pixel. A three orders-of-magnitude improvement in the accuracy of thickness determination was achieved in the simulation. To demonstrate the feasibility of the proposed approach, a SiO2 film deposited on the Si substrate is measured in this work. This approach could be easily extended to implement thickness distribution measurements accurately and rapidly in other rotating-element ellipsometer cases.