{'fraction_non_alphanumeric': 0.048196333530455354, 'fraction_numerical': 0.01212300413956239, 'mean_word_length': 4.610281923714759, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Comment on "Coherent Extreme Ultraviolet Light Amplification by Strong-Field-Enhanced Forward Scattering".

In Ref. [1], the response of helium exposed to a twocolor (IRþ XUV) pulse was modeled. Large amplification of extreme ultraviolet (XUV) radiation throughout the spectrum was found, especially in the cutoff region, see also Ref. [2] for a follow-up. Figure 1(a) reproduces these results for three different XUV intensities by solving the time-dependent Schrödinger equation [3] using helium model potential [4]. A Hanning window is used over the duration of the IR pulse [5]. The pulses are identical to Ref. [1] with the XUV centered at a maximum of the IR electric field, for maximal amplification. In Ref. [1], the enhancement is given by RΩþ Ω− E 2 IRþXUVdΩ= RΩþ Ω− E 2 IRdΩ, whereΩ− andΩþ are the photon energies of the harmonics, neighboring the harmonic that coincides with the central wavelength of the XUV pulse and EIRþXUV (EIR) is the high-order harmonic generation dipole power spectrum with (without) the XUV field. Enhancements of 26, 2500, and 2.5 × 10 are calculated for XUV intensities IXUV of 5 × 10, 5 × 10, and 5 × 10 W=cm, respectively. Figure 1(a) also shows the power spectra for the XUVonly cases (dashed lines). This XUV-only response is on top of the XUVþ IR response for which amplification of XUV light was claimed in Ref. [1]. This already suggests that the “amplification” corresponds to XUV-induced increase of the response due to one-photon absorption into the continuum, which however, leads to loss, not gain, of the XUV light. The latter statement is proven in Fig. 1(b). The dipole power spectrum does not differentiate between the gain and loss in the medium. Rather, it is necessary to consider the relative phases of the driving field and the dipole response (see the textbook Ref. [6] and Ref. [7]). Figure 1(b) shows the corresponding XUV transient absorption signal SXUVðΩÞ in the absence (dashed line) and presence (solid line) of the strong IR dressing field. The signals are positive throughout; i.e., only absorption (XUV loss) is present, even in the presence of the IR field. This absorption is slightly damped in the IR dressed case (solid lines) because of small IR-induced depletion of the helium ground state. Also, the signals are independent of the XUV intensity because the XUV intensities lie in the linear response regime. These results do not depend on the timing of the XUV with respect to the IR pulse or on the choice of the binding potential—identical conclusions are reached with a short-range Yukawa-potential (not shown). Hence, the amplification claimed in Ref. [1] is in reality the opposite, i.e., XUV loss. In summary, we have shown that even in the presence of a strong IR field, XUV absorption by the ground-state atoms constitutes the dominant physical process. Any aspiring XUV amplification scheme must be designed to overcome this absorption. Specifically, we find no evidence for amplification of the incident XUV light under the conditions of Ref. [1]. Weak XUV amplification due to stimulated recombination becomes possible in the deep depletion regime (See Ref. [7]); however, this regime is not reached in Ref. [1] or in the very recent related experimental work [8]. Therefore, the physical mechanism behind the exciting and intriguing experimental observations of Ref. [8] remains unexplained.