Abstract
We report a setup for high-resolution two-photon spectroscopy using a mid-infrared continuous wave optical parametric oscillator (CW-OPO) and a near-infrared diode laser as the excitation sources, both of which are locked to fully stabilized optical frequency combs. The diode laser is directly locked to a commercial near-infrared optical frequency comb using an optical phase-locked loop. The near-infrared frequency comb is also used to synchronously pump a degenerate femtosecond optical parametric oscillator to produce a fully stabilized mid-infrared frequency comb. The beat frequency between the mid-infrared comb and the CW-OPO is then stabilized through frequency locking. We used the setup to measure a double resonant two-photon transition to a symmetric vibrational state of acetylene with a sub-Doppler resolution and high signal-to-noise ratio.
Highlights
Spectroscopy methods based on stimulated two-photon transitions, such as two-photon absorption, stimulated emission probing, and stimulated Raman spectroscopy, allow accessing energy states in which typical single photon excitations are forbidden under spectroscopic selection rules [1,2,3]
We have presented a frequency comb assisted setup for measuring infrared-infrared double resonant two-photon transitions
For our near-infrared source, we can directly establish a phase-locked loop to stabilize the beat between the diode laser and the comb by tuning the laser current, and reach relative linewidth below 10 Hz, as limited by the resolution of our spectral analyzer
Summary
Spectroscopy methods based on stimulated two-photon transitions, such as two-photon absorption, stimulated emission probing, and stimulated Raman spectroscopy, allow accessing energy states in which typical single photon excitations are forbidden under spectroscopic selection rules [1,2,3]. Stimulated two-photon transition can be used to measure spectral features at sub-Doppler resolution, without the need for cooling or molecular jets [4]. The inherent subDoppler resolution has been utilized, for example, in probing minute spectral features, such as hyperfine transitions and Zeeman splitting [5, 6], and in laser stabilization [7]. The measurement resolution may become limited by the linewidths of the exciting lasers. Light sources with narrow linewidths and precise wavelength tuning are required to take the full advantage of the sub-Doppler resolution. Achieving these requirements generally calls for active stabilization of the laser wavelengths
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