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arXiv:2510.07455 · Physical Review Letters

Letokhov-Chebotayev Intracavity Trapping Spectroscopy of H 2

Wim Ubachs, Frank M. J. Cozijn, Meissa L. Diouf, Clément Lauzin, Hubert Jóźwiak, Piotr Wcisło

Published Oct 8, 2025• 5 citations
arXivHTMLPDF

Abstract

In the early days of laser spectroscopy, Letokhov and Chebotayev proposed a scheme for measuring narrow spectral lines where the resolution is not restricted to Doppler effects because the molecules are entrained in a standing-wave light field. Now, such one-dimensional trapping in the intensity maxima of an intracavity field, slightly detuned from resonance, is experimentally demonstrated in the measurement of the very weak S(0) (2-0) quadrupole overtone transition in H_{2} at 1189 nm. The trapping manifests as an extremely narrow absorption feature at the predicted zero-recoil position, a 70 kHz shift from the blue-recoil component observed in Lamb-dip spectroscopy. A quantitative analysis of the saturation and trapping conditions supports the findings.

DOI
10.1103/rxcj-6dw6

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In the early days of laser spectroscopy, Letokhov and Chebotayev proposed a scheme for measuring narrow spectral lines where the resolution is not restricted to Doppler effects because the molecules are entrained in a standing-wave light field. Now, such one-dimensional trapping in the intensity maxima of an intracavity field, slightly detuned from resonance, is experimentally demonstrated in the measurement of the very weak S(0) (2-0) quadrupole overtone transition in H_{2} at 1189 nm. The trapping manifests as an extremely narrow absorption feature at the predicted zero-recoil position, a 70 kHz shift from the blue-recoil component observed in Lamb-dip spectroscopy. A quantitative analysis of the saturation and trapping conditions supports the findings.
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The cover picture shows strontium atoms which are laser cooled to a microkelvin temperature and held in an optical lattice trap. The trap consists of a series of microtraps at the antinodes of the optical standing wave. The intense blue light in the background is the fluorescence of the atoms when they are laser cooled or detected. The clock transition is in red. In their Concept article Zelevinsky et al. (page 375) show that an ultrastable laser recovers a high-resolution spectrum of the narrow, forbidden optical clock transition with a linewidth of 2 Hz. Spectroscopy is performed without recoil shifts, Doppler effects, or ac Stark shifts from the trapping light.
We report what we believe to be the first systematic study of Doppler-free, nonlinear absorption by use of cavity ringdown spectroscopy. We have developed a variant of cavity ringdown spectroscopy for the mid-infrared region between 9 and 11 microm, exploiting the intracavity power buildup that is possible with continuous-wave lasers. The infrared source consists of a continuous-wave CO2 laser with 1-mW tunable infrared sidebands that couple into a high-finesse stable resonator. We tune the sideband frequencies to observe a saturated, Doppler-free Lamb dip in the nu7, 11(1,10) <-- 11(2,10) rovibrational transition of ethylene (C2H4). Power studies of the Lamb dip are presented to examine the intracavity effects of saturation on the Lamb-dip linewidth, the peak depth, and the broadband absorption.
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A new competing beam method is proposed for intracavity measurement of weak absorption. In this method two independent lasers with the same active medium are employed. An investigated sample is placed inside the resonator of one of the lasers with a narrow tunable emission line. In contrast to the selective intracavity absorption method, photoelectric recording of the spectra can be used in the proposed method and this can be done for narrow lines as well as for wide absorption bands. Moreover, the new method can be used to determine the two-photon absorption spectra.