Sub-Doppler Double-Resonance Spectroscopy of Methane Using a Frequency Comb Probe

Abstract

We report the first measurement of sub-Doppler molecular response using a frequency comb by employing the comb as a probe in optical-optical double-resonance spectroscopy. We use a 3.3 μm continuous wave pump and a 1.67 μm comb probe to detect sub-Doppler transitions to the 2ν 3 and 3ν 3 bands of methane with ∼1.7 MHz center frequency accuracy. These measurements provide the first verification of the accuracy of theoretical predictions from highly vibrationally excited states, needed to model the high-temperature spectra of exoplanets. Transition frequencies to the 3ν 3 band show good agreement with the TheoReTS line list.

Measurement and assignment of double-resonance transitions to the 8900–9100- cm−1 levels of methane

“…The experimental setup, which was the same as used in Companion Paper [36], is shown in Fig. 2 amplified Er:fiber frequency comb (Menlo Systems, FC1500-250-WG) with 250-MHz repetition rate, spectrally shifted to cover a bandwidth of 55 nm (200 cm −1 ) around 1.67 μm (6000 cm −1 ).…”

“…The entire probe spectrum spans 5900-6100 cm −1 and an example of a spectrum recorded with the pump locked to the R(0) line in the ν 3 fundamental band with perpendicular relative pump-probe polarization is shown in Fig. 3 in Companion Paper [36]. The central part of this spectrum, i.e., lines P(4) through R( 4), is shown in black in Fig.…”

“…In Companion Paper [36], we demonstrated the implementation of optical-optical sub-Doppler resolution molecular double-resonance spectroscopy using a continuous wave (cw) pump and a frequency comb probe and measured transitions from the ν 3 fundamental band of methane to the 3ν 3 overtone band region, as shown schematically in Fig. 1.…”

A methane line list with sub-MHz accuracy in the 1250 to 1380 cm−1 range from optical frequency comb Fourier transform spectroscopy

“…Different spectroscopic techniques have been used to study the rotation-vibration spectrum of methane across the near-and mid-infrared range, such as Fourier-transform infrared (FTIR) absorption spectroscopy [16,18,[20][21][22][23], FTIR emission spectroscopy [24,25], THz synchrotron measurements [26], laser absorption spectroscopy [27,28], cavity ringdown-spectroscopy [29], sub-Doppler spectroscopy [30][31][32], and frequency comb spectroscopy [33][34][35]. However, the available data around 8 μm, a spectral region lying in the atmospheric water window, stem largely from FTIR absorption and emission spectroscopy with limited accuracy and precision.…”

Controlling rotationally-resolved two-dimensional infrared spectra with polarization

“…Lomsadze and Cundiff have demonstrated high-resolution coherent multidimensional spectroscopy in optically thick Rubidium vapors with multiple frequency combs [13]. Meanwhile, others have pursued a hybrid approach performing (incoherent) double-resonance spectroscopy using a combination of a nearinfrared frequency comb and an intense CW laser to saturate particular transitions [14]. This previous nonlinear spectroscopy work has used combs in the near infrared and visible, where frequency comb technology is relatively mature.…”