Letokhov-Chebotayev intracavity trapping spectroscopy of H$_2$

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.

Figures (4)

• Figure 1: The three optical configurations used for probing the $v_y=0$ velocity class of the S(0) (2-0) quadrupole line of H$_2$ at resonance by the three counter propagating fields. (a) Carrier-Carrier scheme; (b) Blue-Blue sideband scheme; (c) Red-Red sideband scheme; (d) the Doppler profile of H$_2$ at 72 K with a representation of a Lamb dip.
• Figure 2: Saturated absorption (Lamb dip) spectra recorded under application of the three optical configurations as displayed in Fig. \ref{['Sideband-scheme']} at $p=$ 0.5 Pa and $T=72$ K. All three measurements were conducted with 150 W of saturation power, but for the sideband measurements a 2 kW strong detuned carrier wave was also present. The experimental data is fitted with a standard 1st derivative of a dispersive Lorentzian. For the sideband measurements the fit is only constrained in the center (solid line) as the lineshape starts to deviate at the wings, possibly due to effects of the strong carrier wave. Carrier (b), blue-blue-sideband (a), and red-red sideband (c) schemes indicated at the tops. The dashed (blue) vertical line in each of the panels refers to the same $f_0 = 252\,016\,361\,229$ kHz frequency, which is offset by the known modulation frequency $f_{\rm FSR}$, and corresponds to the blue-recoil position (see text).
• Figure 3: Curves displaying the sum of internal energy of ground and excited states, |g$\rangle$ and |e$\rangle$, augmented with kinetic energy $\hbar^2 k^2/2M$. (a) The red recoil component is produced from molecules moving at momentum $p = \pm\hbar k$, pumped into the excited state (solid arrow) and decaying by stimulated emission to the ground state at momentum $p=0$. (b) The blue recoil component is produced from ground state molecules at $p=0$, excited upon absorption into the excited state moving at $p=\pm \hbar k$.
• Figure 4: Linear absorption spectra of the S(0) (2-0) quadrupole line of H$_2$ recorded at low powers at resonance at $p=$ 0.5 Pa, $T=72$ K, with a trapping field of 2 kW shifted by one FSR (405 MHz). (a) The blue-blue sideband absorption feature at 0.5 W power; (b) The red-red sideband feature at 1 W. In both panels the blue-dashed marker represents the Lamb-dip blue recoil position, and the green-dashed marker represents the zero recoil location. A 3-point moving average interpolation confidence band is added for clarity due to the low signal-to-noise ratio and the absence of a quantitative lineshape model. The ordinary $1f$ NICE-OHMS lineshape model is only fitted in the range of the solid line.