Coherent Anti-Stokes Hyper-Raman Spectroscopy

Abstract

Coherent Raman scattering spectroscopies have been established as a powerful tool for investigating molecular systems with high chemical specificity. The existing coherent Raman scattering techniques detect only Raman active modes, which are a part of the whole molecular vibrations. Here, we report the first observation of coherent anti-Stokes hyper-Raman scattering (CAHRS) spectroscopy, which allows measuring hyper-Raman active vibrations at high speed. The CAHRS process relies on a fifth-order nonlinear process that combines hyper-Raman scattering with coherent Raman scattering. Observed signals are proven to come from the CAHRS process through various experiments concerning the dependences of the signals on incident laser powers, time-delay, polarizations, and selection rules of molecular vibrations. Comparisons of CAHRS signals with spontaneous hyper-Raman signals from para-nitroaniline solutions and benzene liquid manifest much higher signal-to-noise ratios of CAHRS signals than spontaneous hyper-Raman signals. This study illustrates that CAHRS spectroscopy can offer additional information on molecular vibrations unobtainable from the present coherent Raman techniques at a much higher speed than spontaneous hyper-Raman spectroscopy.

Fig. 1. Concept and experimental layout of CAHRS spectroscopy.

a Energy diagrams for CAHRS, CARS, and spontaneous HR, and possible diagrams for the non-resonant background (NRB) b Phase-matching geometries for CAHRS and CARS satisfying k_CAHRS = 4k₁−k₂ and k_CARS = 2k₁−k₂, where k₁ and k₂ are the wavevectors of the ω₁ and ω ₂ beams, respectively. θ₁ and θ₂ are the angles of the ω ₁ and ω₂ beams with respect to the CAHRS/CARS beam, respectively. c Experimental schematics for CAHRS spectrometer. OPA, pol, HWP, and LP denote optical parametric amplifier, polarizer, half-wave plate, and long-pass filter, respectively.

Fig. 2. CAHRS spectra of PNA solution.

a, b Raw observed spectra from PNA solution (red lines) and neat chloroform solvent (black lines). c, d Intensity-corrected spectra of PNA solution by dividing the raw spectra of PNA solution by those of neat chloroform. e, f Spontaneous HR spectra of the PNA solution. Spectral ranges of a, c, and e are around 1350 cm⁻¹, and b, d, and f are around 850 cm⁻¹. In the CAHRS measurements, the powers of the ω₁ and ω₂ lights were 60 and 5 mW, the exposure times were 1 s, and 10 exposures were averaged. In the spontaneous HR measurements, the power of the excitation light was 50 mW, the exposure time was 10 s, and 10 exposures were averaged.

Fig. 3. Delay-time dependence.

a Intensity-corrected spectra from PNA solution with various Δt from −1.3 to +1.3 ps. b Delay-time dependence of the non-resonant background signals corresponding to the cross-correlation of the ω₁ and ω₂ pulses at 4ω₁−ω₂. The FWHM was determined to be 1.3 ps.

Fig. 4. Incident-power dependence.

Integrated signal intensities from 1200 to 1450 cm⁻¹ were plotted against the laser power of the ω₁ light (a) and against the laser power of the ω₂ light (b). The inset of (a) is the ω₁ intensity dependence in the small laser power range.

Fig. 5. Polarization dependence.

CAHRS spectra from PNA solution of CHCl₃ around 1320 cm⁻¹ under the eight polarization combinations, a VVV, b HHH, c VHV, d HVH, e VHH, f HVV, g VVH, and h HHV. The first letter refers to the polarization of the CAHRS signal, the second letter to that of the ω₁ beam, and the third letter to that of the ω₂ beam, respectively.

Fig. 6. Selection rules verified by measurements of benzene.

CAHRS spectra of neat benzene in 550 ~ 750 cm⁻¹ (a) and in 900 ~ 1100 cm⁻¹ (b). Spectra are intensity corrected by using the non-resonant background signals from neat benzene-d₆. Spontaneous HR spectra of benzene in 550 ~ 750 cm⁻¹ (c) and in 900 ~ 1100 cm⁻¹ (d). In the CAHRS measurements, the powers of the ω₁ and ω₂ lights were 60 and 5 mW, the exposure times was 1 s, and 10 exposures were averaged. In the spontaneous HR measurements, the power of the excitation light was 30 mW, the exposure time was 10 s, and 10 exposures were averaged.