We report the experimental observation of an enhancement in the yield of high-order harmonics using an array of gas jets as the source medium. By comparing the experimental outcome for jet arrays of different spacings with the predicted harmonic intensity in the case of slit sources of equivalent lengths, we clearly show how the periodic modulation of the gas density can improve the harmonic yield. This behavior may attributed to a quasi-phase-matching effect which increases the length of coherent harmonic build-up during propagation by partially counteracting the dephasing induced by free electrons.PACS numbers: 32.80. Rm,42.65.Ky,32.80.Fb Keywords: Since its discovery in 1987 [1], the phenomenon of highorder harmonic generation (HHG) has become one of the most interesting topics in the field of highly nonlinear processes. Apart from its fundamental physics interest, HHG is now one of the most promising ways to obtain tunable, short-pulse, narrow-band radiation in the vacuum and extreme ultraviolet (VUV and XUV), and in the soft X-ray regions, where other coherent sources are scarcely available. However, the possibility of using high-order harmonics as a table-top VUV-XUV coherent source for applications is strongly connected to the optimization of the brightness of the source over the broadest spectral range.As in many other nonlinear processes, conversion efficiency in HHG depends on the interplay between the single-atom response [2] and the macroscopic response during propagation in the medium [3]. In particular, a constructive interference of the harmonic field emitted from different locations of the source length can only be obtained over the so-called coherence length L c . For media longer than L c , destructive interference soon depletes the generated field and dramatically limits the conversion efficiency. By an appropriate choice of the interaction parameters it is often possible to make the coherence length longer than the medium, thus reaching the so-called phase-matching conditions. However, when phase-matching is not achievable, one can still artificially beat the coherence length limits by properly modulating the interaction parameters along the field propagation direction. Examples of such quasi-phase-matching (QPM) techniques now abound for low-order nonlinear phenomena in structured crystals, and a few examples have been recently demonstrated in HHG. Many different phenomena contribute to limit the coherence length in HHG. Here, the atomic dispersion and the geometric Guoy phase [4,5], are always accompanied by the dispersion connected to free electrons in the partially ionized medium [6,7], and by the characteristic atomic dipole phase [8,9,10,11,12]. Depending on the gas type and density, and on the level of ionization, the coherence length in HHG is usually so short as to pose a serious limit to significant conversion efficiencies. Differently from QPM in periodically-poled nonlinear crystals [13], where the sign of the nonlinear coefficient is periodically flipped so as to always add c...

We report the first direct measurements of the atomic dipole phase in the process of high-order harmonic generation. Differently from previously reported studies based on frequency chirp measurements, we use extreme ultraviolet interferometry as the most natural and direct way to measure phase shifts. Our approach has the important advantage of allowing us to investigate the effects associated to both the main quantum paths involved in the emission of a particular harmonic, thus offering a particularly clear and simple picture of the underlying electronic dynamics.

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We report the first oscillation achieved on 1 at. % Yb-doped Sc2O3 and Lu2O3 ceramics under quasi-CW pumping at 968 nm. With Sc2O3 we measured a maximum output power of 2.2 W with a slope efficiency of 59% at 1040.5 nm. Comparable results are obtained with Lu2O3 at 1032.5 nm, i.e., 1.8 W with a corresponding slope of 45%, while at 1078 nm we measured 1.5 W with 34% of slope efficiency. Finally, we present the range of tunability, which exceeds 41 nm for both samples. To the best of our knowledge, these are the first measurements reported in literature either in terms of laser emission or continuous tunability range achieved with 1 at. % doping level.

The Yb:Lu2O3 precursor made up of spherical particles was synthesized through the co-precipitation method in the water/ethanol solvent. The 5 at% Yb:Lu2O3 powder is in the cubic phase after calcination at 1100 °C for 4 h. The powder also consists of spherical nanoparticles with the average particle and grain sizes of 96 and 49 nm, respectively. The average grain size of the pre-sintered ceramic sample is 526 nm and that of the sample by hot isostatic pressing grows to 612 nm. The 1.0 mm-thick sample has an in-line transmittance of 81.6% (theoretical value of 82.2%) at 1100 nm. The largest absorption cross-section at 976 nm is 0.96×10−20 cm2 with the emission cross-section at 1033 nm of 0.92×10−20 cm2 and the gain cross sections are calculated with the smallest population inversion parameter β of 0.059. The highest slope efficiency of 68.7% with the optical efficiency of 65.1% is obtained at 1033.3 nm in quasi-continuous wave (QCW) pumping. In the case of continuous wave (CW) pumping, the highest slope efficiency is 61.0% with the optical efficiency of 54.1%. The obtained laser performance indicates that Yb:Lu2O3 ceramics have excellent resistance to thermal load stresses, which shows great potential in high-power solid-state laser applications.

Using a Ramsey-type setup, the lambda-doublet transition in the J = 1, Ω = 1 level of the a 3 Π state of CO was measured to be 394 064 870(10) Hz. In our molecular beam apparatus, a beam of metastable CO is prepared in a single quantum level by expanding CO into vacuum and exciting the molecules using a narrow-band UV laser system. After passing two microwave zones that are separated by 50 cm, the molecules are state-selectively deflected and detected 1 meter downstream on a position sensitive detector. In order to keep the molecules in a single m B J level, a magnetic bias field is applied. We find the field-free transition frequency by taking the average of the m B J = +1 → m B J = +1 and m B J = −1 → m B J = −1 transitions, which have an almost equal but opposite Zeeman shift. The accuracy of this proof-of-principle experiment is a factor of 100 more accurate than the previous best value obtained for this transition.