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Timestamp: 2019-04-22 13:14:25+00:00

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We developed a Raman optical activity (ROA) spectroscopic system with visible-excited coherent anti-Stokes Raman scattering (CARS). A supercontinuum within the visible region was generated with a photonic crystal fiber pumped with both 532 and 1064 nm excitation, generating a multiplexed CARS-ROA spectrum covering the whole fingerprint region. In visible excitation, the CARS-ROA spectrum of (-)-β-pinene shows a higher contrast ratio of the chirality-induced signal to the achiral background than that of the previously reported near-infrared CARS-ROA spectrum.
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Fig. 1. Schematic diagram of the visible-excited CARS-ROA spectrometer. HWP, half-wave plate; DM, short-pass dichroic mirror (Thorlabs, DMSP805); DM2, long-pass dichroic mirror (Thorlabs, DMLP900); PBS, polarized beam splitter (Thorlabs, PBS201); PCF, custom-made photonic crystal fiber; OD, optical delay; LPF, long-pass filter (Semrock, BLP01-532R-25); GTP, Glan–Taylor prism (Thorlabs, GT5); S, sample; NF, notch filter (Semrock, NF03-532/1064E-25); SPF, short-pass filter (Semrock, SP01-532RU-25).
Fig. 2. Spectra of the SC generated with the different pump schemes. These are measured as nonresonant CARS spectra of water.
Fig. 3. CARS spectra of (+)- β -pinene measured at θ = 0.5 ° (red curve) and θ = − 0.5 ° (blue curve). Dots represent the ratio of these two CARS spectra I ( θ ) / I ( − θ ) and the dashed curve is from the spline fitting.
Fig. 4. (a) CARS spectrum of (−)- β -pinene with 1 h exposure time with the 532 nm setup developed in this research (reduced to 1/10 for comparison). (b) CARS-ROA spectra of (+)- β -pinene (black) and (−)- β -pinene (red) obtained with the 532 nm setup. The exposure time was 1 min, and the excitation power was 100 mW for ω 1 and 20 mW for ω 2 . (c) CARS-ROA spectra of (+) and (−)- β -pinene obtained with 1 h exposure. Measurement conditions other than exposure time are the same as (b). (d) CARS-ROA spectrum of (−)- β -pinene obtained with the previous 1064 nm setup with the 1 h exposure. The excitation power was 200 mW for ω 1 and 70 mW for ω 2 . (e) Calculated CARS-ROA spectrum.
Fig. 5. Comparison of the spectra of Re [ χ 1111 * χ 2111 ] / | χ 1111 | 2 calculated from the VIS- and NIR-CARS-ROA measurements.
(1) I ( θ ) ∝ | sin ( θ − α ) χ 1111 + cos ( θ − α ) χ 2111 | 2 .
(3) θ + α θ − α [ I ( θ ) − ( θ − α θ + α ) 2 I ( − θ ) ] ∝ ∼ θ Re [ χ 1111 * χ 2111 ] .

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