High-accuracy deep-UV Ramsey-comb spectroscopy in krypton

Abstract

and bound-state QED tests based on precision spectroscopy in atoms, molecules and highly charged ions (see, e.g., [5][6][7][8][9][10]). Moreover, in the pursuit of testing QED ever better, substantial efforts have been made to extract fundamental quantities such as the Rydberg constant R ∞ and the proton charge radius. Both can be obtained from spectroscopy of atomic hydrogen, assuming that QED is sufficiently precise. However, when the CREMA collaboration determined the proton charge radius from spectroscopy in muonic hydrogen (consisting of a proton and a muon), it leads to a considerable (7σ) mismatch with the value extracted from normal (electronic) hydrogen [11][12][13]. This mismatch, known as the proton radius puzzle, remains to be explained and requires more spectroscopic measurements, e.g., in systems other than (muonic) hydrogen. Recent results on muonic deuterium [14] reveal that also the deuteron radius is significantly smaller (7.5 σ) than the radius based on normal deuterium spectroscopy.Interesting candidates for precision spectroscopy to solve this puzzle need to be sufficiently simple for precise theoretical treatment. One example is molecular hydrogen, made possible by recent improvements in theory [15]. Another is He + [16], which can be compared to muonic-He + spectroscopy [13]. The experimental challenge is the short wavelengths required for excitation, which ranges from the deep UV (≈ 200 nm) for H 2 to extreme ultraviolet (XUV, < 60 nm) for He + .Such short wavelengths are typically obtained by frequency upconversion of near-infrared lasers in nonlinear crystals or noble gases. One can use a frequency-comb (FC) laser as the fundamental laser and take advantage of its excellent spectral resolution and pulse peak power, to perform direct frequency-comb spectroscopy (DFCS) [17][18][19]. To achieve sufficient upconversion to the UV or XUV range, several approaches have been investigated. AbstractIn this paper, we present a detailed account of the first precision Ramsey-comb spectroscopy in the deep UV. We excite krypton in an atomic beam using pairs of frequency-comb laser pulses that have been amplified to the millijoule level and upconverted through frequency doubling in BBO crystals. The resulting phase-coherent deep-UV pulses at 212.55 nm are used in the Ramseycomb method to excite the two-photon 4p 6 → 4p 5 5p[1/2] 0 transition. For the 84 Kr isotope, we find a transition frequency of 2829833101679(103) kHz. The fractional accuracy of 3.7 × 10 −11 is 34 times better than previous measurements, and also the isotope shifts are measured with improved accuracy. This demonstration shows the potential of Ramsey-comb excitation for precision spectroscopy at short wavelengths.

Deep-Ultraviolet Frequency Metrology of H2 for Tests of Molecular Quantum Theory

“…1). Only two comb pulses are selectively amplified (at a repetition rate of 28 Hz) in a noncollinear optical parametric chirped-pulse amplifier to a pulse energy of 2.4 mJ (for details, see [43][44][45]). The amplified pulses are upconverted to the fourth harmonic by frequency doubling and two stages of sum-frequency mixing to produce 62 μJ of 201.80 radiation; see Fig.…”

Paving the way for fundamental physics tests with singly-ionized helium

“…We selectively amplify two pulses out of the FC pulse train in a noncollinear optical parametric chirpedpulse amplifier (NOPCPA, in the following simply OPA) [28] to a few mJ per pulse. The pump laser for the OPA is similar to the one previously used [29] for the measurements in krypton, hydrogen, and xenon. The pump pulses are created in a Nd:YVO 4 oscillator which is synchronized with the frequency comb.…”

Phase transfer between three visible lasers for coherent population trapping

“…We have therefore implemented a phase-lock via an optical frequency comb (OFC) which covers this wide frequency range [8,9]. Indeed, OFCs have made possible the implementation of novel spectroscopy approaches by connecting optical frequencies to the * Electronic address: martina.knoop@univ-amu.fr microwave frequency range [10][11][12][13].…”