kobayashi_1998 ============== Y\. Kobayashi, T\. Sekikawa, Y\. Nabekawa and S\. Watanabe, "27\-fs extreme ultraviolet pulse generation by high\-order harmonics", Opt\. Lett\. 23, 64\-66 \(1998\) `Source record `__ · :doc:`Catalogue ` · :doc:`Review register ` DOI: `10\.1364/OL\.23\.000064 `__ **Source kind:** measurement **Recorded technique:** Autocorrelation of the ninth harmonic of a Ti\:sapphire laser \(10 Hz, run at 1 TW with 34\-fs pulses; the harmonics generated in Xe in a 1\-mm gas envelope 15 mm before the focus of a 1\-m radius\-of\-curvature mirror, the beam split into two spatially separated halves in a delay line\) by two\-photon ionisation of He\: the harmonics above the seventh selected by a beam splitter at 70 degrees and an Al mirror \(f = 10 cm\), focused into a pulsed gas jet \(about 5 Torr\) in a time\-of\-flight ion spectrometer, the ions counted by an electron multiplier \(pp\. 64\-65\)\. The ninth harmonic's two\-photon energy is above He's ionisation energy, the seventh's below; its role confirmed by an MgF2\-coated mirror \(which absorbs it\) and by the quadratic dependence of the He\+ yield on its intensity \(slope 2\.0, Fig\. 3\)\. The trace \(Fig\. 4\) fitted with a sech^2 shape gives 27 fs \(p\. 66\)\. Neither the fundamental's wavelength nor the harmonic's photon energy is stated\. **Materials in structured entries:** None recorded. **Dataset quantities:** None recorded. **Contents:** 0 datasets; 0 formulas. Curation notes -------------- Opt\. Lett\. 23\(1\), 64\-66 \(1 January 1998\)\: PDF pages 1\-3 = pp\. 64\-66, a digital text layer with the figures as embedded images\. Identity checked on the first page \(PDF p\. 1\: 'OPTICS LETTERS / Vol\. 23, No\. 1 / January 1, 1998', p\. 64, Y\. Kobayashi, T\. Sekikawa, Y\. Nabekawa and S\. Watanabe, Institute for Solid State Physics, University of Tokyo; received August 1, 1997\) on 2026\-10\-09\. The library's identity is VALIDATED\_WITH\_WARNINGS; the copy is the version of record\. WHY NOTHING IS TAKEN\. The paper prints no two\-photon cross section of He \(or Ar\), neither measured nor quoted from another paper, and uses none\. Figs\. 2\-4 \(raster images\) are the time\-of\-flight spectrum, the number of He\+ ions against the ninth harmonic's intensity in arbitrary units \(slope 2\.0\), and the autocorrelation trace; the one absolute comparison is with a Keldysh\-Faisal\-Reiss estimate of the ion number, not a cross section\: 'If we assume an intensity of 155 GW/cm2 and a pressure of 1 Torr, the number of ions expected per pulse is 60, whereas the number actually detected was ~20\. This factor\-of\-3 difference may be due to the estimation of the size of the focal spot' \(p\. 66; the theory is H\. R\. Reiss, Phys\. Rev\. A 22, 1786 \(1980\), ref\. 13; its two\-photon rate and the focal volume are not given\)\. The photon energy is not stated either \(the ninth harmonic of an unnamed Ti\:sapphire wavelength\)\. NUMBERS IN THE TEXT \(not data of any quantity\)\: the harmonic energies in the ionisation region, fifth 17 nJ, seventh 7 nJ, ninth 13 nJ \(50 nJ generated from 44 mJ of the fundamental; pp\. 64\-65\); 'more than 10 ions per pulse' of He\+ at about 5 Torr \(p\. 64\); the slope 2\.0 of the He\+ \(and Ar\+, seventh or ninth harmonic\) yield against the harmonic's intensity \(p\. 65\); the 27\-fs pulse width \(sech^2\), against the fundamental's 34 \+\- 5 fs and the 11 fs of tau / sqrt\(q\), and about 6 fs from the Fourier transform of the ninth harmonic's spectrum; the autocorrelation contrast 1\.7 against the usual 3 \(p\. 66\)\. An Ar\+ signal was seen with both the seventh and the ninth harmonics \(p\. 65\)\. FURTHER TWO\-PHOTON DATA CITED\: none \(the cited works are on short\-pulse lasers, harmonic generation and cross\-correlation, and Reiss's strong\-field theory\)\. **Recorded bibliographic identity status:** VALIDATED\_WITH\_WARNINGS. This concerns identification of the source; it does not establish the scientific correctness of its contents. Datasets -------- No datasets are recorded. Formulas -------- No formulas are recorded. Corrections ----------- No dataset errata are recorded in this source record. Formula corrections, where recorded, appear with their formulas above. Curation notes may also discuss corrections or unresolved interpretation. Consult the :doc:`review register ` for the checks and decisions.