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Infrared electric field sampled frequency comb spectroscopy

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Science Advances  07 Jun 2019:
Vol. 5, no. 6, eaaw8794
DOI: 10.1126/sciadv.aaw8794
  • Fig. 1 Infrared dual frequency comb electric field sampling.

    (A) An MIR electric field (with repetition rate fr + Δfr) induces a nonlinear polarization rotation, φ, on an NIR sampling pulse (with repetition rate fr) that is directly proportional to its amplitude, EMIR. (B) In the frequency domain, the entire MIR spectrum is down-sampled to Δfr and folded into the free-spectral range of the NIR sampling pulse such that it is contained within every Nyquist zone (fr/2). (C) Experimentally, the sampling and MIR pulses are combined at a germanium beam splitter (BS), and the SF interaction occurs in an electro-optic crystal (NLC). The output is spectrally filtered with a bandpass filter (BPF). Ellipsometry using a quarter waveplate (QWP), Wollaston prism (WP), and balanced photodetectors (BPD) yields a signal, s(t) ∝ EMIR.

  • Fig. 2 MIR electric field measurements with high dynamic range.

    (A) A 1.2-cycle MIR electric field, oscillating at 7.6 μm (39 THz, 1316 cm−1), is sampled with a 5-fs resolution. The dual frequency comb measurement enables observation of the trailing molecular free induction decay of atmospheric absorbents, e.g., H2O at 135 ps away from the centerburst (inset, top). The noise level is 10,000 times smaller than the signal with 22 min of averaging (inset, bottom). (B) Ultrabroadband MIR spectra corresponding to the Fourier transform of the measured electric fields from PPLN (blue), OP-GaP (green), and GaSe (purple)—encompassing the functional group and molecular fingerprint regions and facilitating the study of important organic compounds such as proteins. Vibrational spectra of (i) C─H stretches in CH4 and C2H6, (ii) anti-symmetric C─O stretch in CO2, (iii) O─H─O bend in H2O, and (iv) O─C─O bend in CO2 are all seen. (C) Comb-tooth resolution measurement (blue) of a single absorption feature in the R branch is compared against the modeled spectrum (red) for the ν2 vibration (symmetric O─C─O bend) of CO2. (D) The absorption of this CO2 bending mode is shown from 630 to 700 cm–1 at a 500-MHz resolution. The residuals (gray) show quantitative agreement with the model.

  • Fig. 3 High-resolution and broad bandwidth spectroscopy of ammonia.

    (A) Absorption measurements are performed with instantaneous octave-spanning bandwidth from 600 to 1500 cm–1 (6.7 to 16.7 μm) in ambient conditions. (B) The ν2 vibration (A1 symmetric bend) of gas-phase ammonia is measured (blue) between 770 and 1160 cm−1 (8.6 to 13 μm) with a resolution of 0.0033 cm–1 and compared to modeled absorption (red). The averaging time was 88 min. (C) and (D) Quantitative agreement is seen across the entire spectrum for both intensity and phase. The line intensity residuals (gray) are shown offset from zero on the same scale.

  • Fig. 4 Broadband condensed phase absorbers across 500 to 2500 cm–1.

    (A) Liquid-phase R-(-)- carvone over a 15-μm path length is measured at a resolution of 1.2 cm–1. The reference spectrum is shown on the right axis (in logarithmic scale). (B) Infrared absorption spectrum (with a resolution of 4 cm–1) of the monoclonal antibody, NISTmAb, showing the amide I, II, and III bands. The β-sheet structure is shown graphically in the inset.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/6/eaaw8794/DC1

    Section S1. Noise in EOS detection

    Section S2. Noise-equivalent absorption in dual-comb EOS

    Section S3. Atmospheric water vapor in the 585- to 630-cm−1 band

    Fig. S1. EOS response function.

    Fig. S2. Atmospheric water vapor absorption in the 585- to 630-cm−1 band.

    Fig. S3. Phase-matching curves for SFG in GaSe.

    Reference (61)

  • Supplementary Materials

    This PDF file includes:

    • Section S1. Noise in EOS detection
    • Section S2. Noise-equivalent absorption in dual-comb EOS
    • Section S3. Atmospheric water vapor in the 585- to 630-cm−1 band
    • Fig. S1. EOS response function.
    • Fig. S2. Atmospheric water vapor absorption in the 585- to 630-cm−1 band.
    • Fig. S3. Phase-matching curves for SFG in GaSe.
    • Reference (61)

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