Enhancing the yield of high-order harmonics with an array of gas jets

Abstract

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...

Extreme ultraviolet and soft X-ray imaging with compact, table top laser plasma EUV and SXR sources

“…This approach has been proved experimentally using gas-filled hollow-core fiber with a modulated inner diameter [41] and coherent superposition of harmonics generated in two successive sources by the same laser pulse [42]. Strong enhancement of HHG has been also observed for an array of gas jets [43] and dual-gas multi-jet arrays [44], however, no characterization measurements of gas density profiles in the laser interaction region have been performed. Thus, to visualize and measure directly the gas density in targets dedicated to HHG, we employed shadowgraphy and proposed modulated density gas puff targets.…”

“…The experiments were performed at 13.5 nm wavelength in the EUV range for study and optimization of low density media. Our object is again a modulated density gaseous target, which might have potential application in phase-matched HHG experiments [39,40,43]. To access information about this type of object indirect interferometric techniques may be applied, [60,61], however, to measure the density directly, EUV light, due to increased absorption in matter, is much more suitable for that purpose.…”

Modification of modulated plasma plumes for the quasi-phase-matching of high-order harmonics in different spectral ranges

“…One of the methods used in gas HHG studies explores the division of extended medium on a few jets where the constructive accumulation of XUV photons was maintained along the whole length of single gas jet, with the following optimal dephasing before entering the next jet. [3][4][5] This approach of QPM was analyzed both theoretically and experimentally by using the modulated gas structures whose pressure and separation were properly controlled. A proofof-concept of the QPM by periodical modulation of the gas density showing the coherent buildup of the harmonic field in two separated gas sources has been demonstrated by Seres et al 3 Another interesting medium for application of the QPM is a low-ionized plasma produced during laser ablation of various materials in vacuum.…”

Characterization of pulsed capillary channel gas puff target using EUV shadowgraphy

“…It is one of the methods to obtain tunable, short-pulse and highly spatially coherent radiation in the extreme ultraviolet (EUV) and soft Xray (SXR) regions [3]. Over the years, HHG targetry has evolved from simple single stream gas jets, hollow-core fibers [4] elongated gas-jet targets [5], or with modulated diameter capillaries for phasematching [6], gas cells [7], coherent superposition of harmonics generated in two successive sources [8], array of gas jets [9], to recently modulated, dual-gas multi-jet gas puff targets [10,11]. The key factor in target development is always related to improvement of harmonic generation efficiency.…”