Observation of Raman Optical Activity by Heterodyne-Detected Polarization-Resolved Coherent Anti-Stokes Raman Scattering

We report the first observation of Raman optical activity (ROA) by coherent anti-Stokes Raman scattering. Thanks to the more freedom of polarization configurations in coherent anti-Stokes Raman scattering than in spontaneous Raman spectroscopy, the contrast ratio of the chiral signal to the achiral background has been improved markedly. For (-)-$\beta$-pinene, it is 2 orders of magnitude better than that in the reported spontaneous ROA measurement. This is also the first measurement of ROA signal using a pulsed laser source.

Figure 1

(a) Schematics of the experimental configuration around the sample. $\theta$ represents the angle of the analyzer and ${\theta }_0$ is the angle at which the polarizers before and after the sample are perpendicular. (b) Schematic of our heterodyne-detected polarization-resolved CARS spectroscopic system: LP1, 1064 nm long-pass filter; LP2, 1050 nm long-pass filter; GTP, Gran-Taylor Prisms; NF, 1064 nm notch filter; SP, 1050 nm short-pass filter. The polarizations of the indent and CARS radiation are precisely (extinction ratio $\sim 1:{10}^{-7}$) set by Gran-Taylor prisms.

Figure 2

Circles: A set of observed polarization-resolved CARS spectra of the liquid (-)-$\beta$-pinene [the collection time; 100 s (100 times accumulation of one second exposure) for the spectrum at $\theta =10.0°$, 30 min for the other spectra, the incident power; $\sim 200\mathrm{mW}$ for ${\omega }_1$ and $\sim 70\mathrm{mW}$ for ${\omega }_2$]. The spectra are normalized by the collection time. The spectrum at $\theta =10.0°$ is minified by 1/16 000. Dashed lines: Curves fitted by using Eq. (14) with ${\chi }_{2111}$ and $C_{\mathrm{leak}}$ are fixed to be 0. From the spectrum at $\theta =10.0°$, the parameters were obtained as ${\chi }_{1111}^{\mathrm{NR}}=3.2\times {10}^{-14}[\mathrm{esu}]$, ${\omega }_i=640.39\pm 0.027$, $716.28\pm 0.16$, $766.74\pm 0.80[{\mathrm{cm}}^{-1}]$, $A_i=(17.4\pm 0.1)\times {10}^{-14}$, $(2.2\pm 0.1)\times {10}^{-14}$, $(4.2\pm 0.1)\times {10}^{-14}[\mathrm{esu}]$, ${\Gamma }_i=4.94\pm 0.05$, $3.76\pm 0.25$, $3.49\pm 0.13[{\mathrm{cm}}^{-1}]$ ($i=1$, 2,3, respectively) Solid lines: Curves fitted by using Eq. (14) with free ${\chi }_{2111}$ and $C_{\mathrm{leak}}$. From the spectra at $\theta =-0.075°,-0.050,°\dots ,0.075°$, the parameters were obtained as ${\chi }_{2111}^{\mathrm{NR}}=0\pm 3\times {10}^{-19}\times {10}^{-19}[\mathrm{esu}]$, $C_{\mathrm{main}}=(6.35\pm 0.03)\times {10}^2$, $C_{\mathrm{leak}}=(3.65\pm 0.35)\times {10}^{-6}$, ${\theta }_0=0.016°\pm 0.0006°$, ${\Delta }_1=(0.38\pm 0.03)\times {10}^{-3}$, ${\Delta }_2=(-1.17\pm 0.14)\times {10}^{-3}$, ${\Delta }_3=(0.52\pm 0.08)\times {10}^{-3}$. The absolute magnitudes of the third-order optical susceptibilities are evaluated by using the value of the nonresonant third-order susceptibility of water as a standard [21].

Figure 3

(a): The sum of CARS spectra with weight coefficients: $I_{\mathrm{sum}}=I(0.075°-{\theta }_0)+I(-0.075°-{\theta }_0)\sin (0.075°-{\theta }_0)/\sin (-0.075°-{\theta }_0)$ (b): The difference of CARS spectra with weight coefficients: $I_{\mathrm{dif}}=I(0.075°-{\theta }_0)-I(-0.075°-{\theta }_0)\sin (0.075°-{\theta }_0)/\sin (-0.075°-{\theta }_0)$ (c): Spontaneous ROA spectrum reproduced from Ref. 15.