Abstract
Optical frequency combs have revolutionized the measurement of optical frequencies and improved the precision of spectroscopic experiments. Besides their importance as a frequency-measuring ruler, the frequency combs themselves can excite target transitions (direct frequency comb spectroscopy). The direct frequency comb spectroscopy may extend the optical frequency metrology into spectral regions unreachable by continuous wave lasers. In high precision spectroscopy, atoms/ions/molecules trapped in place have been often used as a target to minimize systematic effects. Here, we demonstrate direct frequency comb spectroscopy of single 25Mg ions confined in a Paul trap, at deep-UV wavelengths. Only one mode out of about 20,000 can be resonant at a time. Even then we can detect the induced fluorescence with a spatially resolving single photon camera, allowing us to determine the absolute transition frequency. The demonstration shows that the direct frequency comb spectroscopy is an important tool for frequency metrology for shorter wavelengths where continuous wave lasers are unavailable.
Highlights
Optical frequency combs have revolutionized the measurement of optical frequencies and improved the precision of spectroscopic experiments
A prominent example is the frequency of the 1S-2S two-photon transition in atomic hydrogen, which has been measured with a fractional uncertainty of 4.1 × 10−151, 2
While commercially available crystals are limited by transparency and phase matching requirements to about λ > 190 nm and not yet commercially available KBBF crystals improve this limit to approximately λ > 150 nm[9, 10], high harmonic generation (HHG) can reach well into the X-ray regime[11]
Summary
The offset frequency is stabilized to a radio frequency reference by feeding back to the rf-drive of an acousto-optic modulator (AOM) that drains power from the pump beam of the Ti:sapphire mode-locked laser. Both comb parameters (fr and fo) are locked to oscillators which are phase-locked to a GPS-calibrated hydrogen-maser, ensuring traceability of the frequency of each comb-line. For data analysis a region of interest (ROI) is defined around each ion and total fluorescence counts are extracted as a function of the absolute frequency The latter is determined with the help of the frequency comb which is referenced to the hydrogen maser.
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