SUB-DOPPLER FREQUENCY METROLOGY IN HD FOR TEST OF FUNDAMENTAL PHYSICS

Molecular hydrogen has evolved into a benchmark quantum test system for fundamental physics. Accurate results on the vibrational splitting in hydrogen isotopologues can be exploited to provide a test of QED in the smallest neutral molecule, and open up an avenue to resolve the proton radius puzzle, as well as constrain putative fifth forces and extra dimensions. We will present the first sub-Doppler determination of weak dipole transitions in the (2,0) overtone band of HD at $\lambda\sim{\rm 1.38\,\mu m}$. To saturate and detect the weak absorption we have implemented a technique called Noise-Immune Cavity-Enhanced Optical Heterodyne Molecular Spectroscopy (NICE-OHMS). To obtain an absolute frequency during the measurements, the spectroscopy laser is simultaniously locked onto a Cs-clock referenced optical frequency comb. The obtained Doppler-free linewidth of $\sim{\rm 300 kHz}$ (FWHM) could give access and insight into the underlying hyperfine structure. Our current determination of the obtained transition frequencies is around 30 kHz; a 1000-fold improvement on the previous Doppler-broadened determination

SUB-DOPPLER FREQUENCY METROLOGY IN HD FOR TEST OF FUNDAMENTAL PHYSICS

Molecular hydrogen has evolved into a benchmark quantum test system for fundamental physics. Accurate results on the vibrational splitting in hydrogen isotopologues can be exploited to provide a test of QED in the smallest neutral molecule, and open up an avenue to resolve the proton radius puzzle, as well as constrain putative fifth forces and extra dimensions. We will present the first sub-Doppler determination of weak dipole transitions in the (2,0) overtone band of HD at $\lambda\sim{\rm 1.38\,\mu m}$. To saturate and detect the weak absorption we have implemented a technique called Noise-Immune Cavity-Enhanced Optical Heterodyne Molecular Spectroscopy (NICE-OHMS). To obtain an absolute frequency during the measurements, the spectroscopy laser is simultaniously locked onto a Cs-clock referenced optical frequency comb. The obtained Doppler-free linewidth of $\sim{\rm 300 kHz}$ (FWHM) could give access and insight into the underlying hyperfine structure. Our current determination of the obtained transition frequencies is around 30 kHz; a 1000-fold improvement on the previous Doppler-broadened determination

Network-Based Design of Near-Infrared Lamb-Dip Experiments and the Determination of Pure Rotational Energies of H218O at kHz Accuracy

© 2021 Author(s).Taking advantage of the extreme absolute accuracy, sensitivity, and resolution of noise-immune-cavity-enhanced optical-heterodyne-molecular spectroscopy (NICE-OHMS), a variant of frequency-comb-assisted Lamb-dip saturation-spectroscopy techniques, the rotational quantum-level structure of both nuclear-spin isomers of H218O is established with an average accuracy of 2.5 kHz. Altogether, 195 carefully selected rovibrational transitions are probed. The ultrahigh sensitivity of NICE-OHMS permits the observation of lines with room-temperature absorption intensities as low as 10−27 cm molecule−1, while the superb resolution enables the detection of a doublet with a separation of only 286(17) kHz. While the NICE-OHMS experiments are performed in the near-infrared window of 7000-7350 cm−1, the lines observed allow the determination of all the pure rotational energies of H218O corresponding to J values up to 8, where J is the total rotational quantum number. Both network and quantum theory have been employed to facilitate the measurement campaign and the full exploitation of the lines resolved. For example, to minimize the experimental effort, the transitions targeted for observation were selected via the spectroscopic-network-assisted precision spectroscopy (SNAPS) scheme built upon the extended Ritz principle, the theory of spectroscopic networks, and an underlying dataset of quantum chemical origin. To ensure the overall connection of the ultraprecise rovibrational lines for both nuclear-spin isomers of H218O, the NICE-OHMS transitions are augmented with six accurate microwave lines taken from the literature. To produce absolute ortho-H218O energies, the lowest ortho energy is determined to be 23.754 904 61(19) cm−1. A reference, benchmark-quality line list of 1546 transitions, deduced from the ultrahigh-accuracy energy values determined in this study, provides calibration standards for future high-resolution spectroscopic experiments between 0-1250 and 5900-8380 cm−

Parity-pair-mixing effects in nonlinear spectroscopy of HDO

© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement.A non-linear spectroscopic study of the HDO molecule is performed in the wavelength range of 1.36–1.42 µm using noise-immune cavity-enhanced optical-heterodyne molecular spectroscopy (NICE-OHMS). More than 100 rovibrational Lamb dips are recorded, with an experimental precision of 2–20 kHz, related to the first overtone of the O–H stretch fundamental of HD16O and HD18O. Significant perturbations, including distortions, shifts, and splittings, have been observed for a number of Lamb dips. These spectral perturbations are traced back to an AC-Stark effect, arising due to the strong laser field applied in all saturation-spectroscopy experiments. The AC-Stark effect mixes parity pairs, that is pairs of rovibrational states whose assignment differs solely in the Kc quantum number, where Kc is part of the standard JKaKc asymmetric-top rotational label. Parity-pair mixing seems to be especially large for parity pairs with Ka ≥ 3, whereby their energy splittings become as small as a few MHz, resulting in multi-component asymmetric Lamb-dip profiles of gradually increasing complexity. These complex profiles often include crossover resonances. This effect is well known in saturation spectroscopy, but has not been reported in combination with parity-pair mixing. Parity-pair mixing is not seen in H216O and H218O, because their parity pairs correspond to ortho and para nuclear-spin isomers, whose interaction is prohibited. Despite the frequency shifts observed for HD16O and HD18O, the absolute accuracy of the detected transitions still exceeds that achievable by Doppler-limited techniques

Laser cooling of beryllium ions using a frequency-doubled 626 nm diode laser

We demonstrate laser cooling of trapped beryllium ions at 313 nm using a frequency-doubled extended cavity diode laser operated at 626 nm, obtained by cooling a ridge waveguide diode laser chip to -31°C. Up to 32 mW of narrowband 626 nm laser radiation is obtained. After passage through an optical isolator and beam shaping optics, 14 mW of 626 nm power remains of which 70% is coupled into an external enhancement cavity containing a nonlinear crystal for second-harmonic generation. We produce up to 35 μW of 313 nm radiation, which is subsequently used to laser cool and detect 6 × 1