XUV frequency-comb metrology on the ground state of helium

Abstract

The operation of a frequency comb at extreme ultraviolet (xuv) wavelengths based on pairwise amplification and nonlinear upconversion to the 15th harmonic of pulses from a frequency-comb laser in the near-infrared range is reported. It is experimentally demonstrated that the resulting spectrum at 51 nm is fully phase coherent and can be applied to precision metrology. The pulses are used in a scheme of direct-frequency-comb excitation of helium atoms from the ground state to the 1s4p and 1s5p 1 P 1 states. Laser ionization by auxiliary 1064 nm pulses is used to detect the excited-state population, resulting in a cosine-like signal as a function of the repetition rate of the frequency comb with a modulation contrast of up to 55%. Analysis of the visibility of this comb structure, thereby using the helium atom as a precision phase ruler, yields an estimated timing jitter between the two upconverted-comb laser pulses of 50 attoseconds, which is equivalent to a phase jitter of 0.38 (6) cycles in the xuv at 51 nm. This sets a quantitative figure of merit for the operation of the xuv comb and indicates that extension to even shorter wavelengths should be feasible. The helium metrology investigation results in transition frequencies of 5 740 806 993 (10) and 5 814 248 672 (6) MHz for excitation of the 1s4p and 1s5p 1 P 1 states, respectively. This constitutes an important frequency measurement in the xuv, attaining high accuracy in this windowless part of the electromagnetic spectrum. From the measured transition frequencies an eight-fold-improved 4 He ionization energy of 5 945 204 212 (6) MHz is derived. Also, a new value for the 4 He ground-state Lamb shift is found of 41 247 (6) MHz. This experimental value is in agreement with recent theoretical calculations up to order mα 6 and m 2 /Mα 5 , but with a six-times-higher precision, therewith providing a stringent test of quantum electrodynamics in bound two-electron systems.

Hydrogen molecular ions for improved determination of fundamental constants

“…An independent determination of R ∞ , which is strongly correlated to r p and r d , is another way to shed new light on this problem. Experiments on helium atoms and He + ions [17][18][19], as well as highly charged hydrogenlike ions [20] may ultimately achieve this goal. On a more general level, the proton size puzzle exemplifies how improved and independent determinations of fundamental physical constants from different physical systems provide essential cross checks of our understanding of the physical world.…”

Experimental progress in positronium laser physics

“…Precision measurements performed using a wide variety of both hydrogenic and non-hydrogenic atoms, ions, and molecules are, of course, also used for QED tests (e.g. [600][601][602][603]), but Ps has some unique properties in this regard, since it has no constituents with internal structure, and the equal mass of the electron and positron mean that it is very sensitive to recoil effects [146]. All QED corrections to Ps energy levels are now known up to order mα 6 [150][151][152], and some of the higher order terms have also been calculated (e.g.…”

Coherent control and giant enhancement of multiphoton ionization and high-order-harmonic generation driven by intense frequency-comb laser fields: Anab initiotheoretical investigation

“…It was found that a nested comb structure appears within each of the harmonics, * sichu@ku.edu ranging from the first harmonic all the way to the cutoff harmonic [36,37]. More recently, it was shown by several experiments [15,[38][39][40] that the frequency-comb structure and coherence can indeed survive in very high-order harmonics and in the situation in which substantial ionization occurs in the presence of high-intensity laser fields. In these experiments, the femtosecond enhancement cavity technique (for a recent review, see Ref.…”

“…For example, Cingöz et al [39] reported the generation of XUV frequency combs up to 27th harmonic order (wavelengths of 40 nm) by coupling a high-power near-infrared frequency comb to a robust femtosecond enhancement cavity. Alternatively, Kandula et al [15,40] demonstrated the frequency comb structure and phase coherence in 15th harmonics (wavelengths of 51 nm) by employing Ramsey spectroscopy [42,43]. In addition, Son and Chu [44] have recently presented a theoretical extension of the many-mode Floquet theory (MMFT) [45][46][47][48][49] for the nonperturbative treatment of the coherent control of multiphoton resonance dynamics and the enhancement of HHG driven by intense frequency-comb laser fields.…”