Soft X-Ray Second Harmonic Generation as an Interfacial Probe

Lam, Royce K.; Raj, Sumana L.; Pascal, Tod A.; Pemmaraju, C. D.; Foglia, Laura; Simoncig, Alberto; Fabris, Nicola; Miotti, Paolo; Hull, C.J.; Rizzuto, Anthony M.; Smith, Jacob W.; Mincigrucci, Riccardo; Masciovecchio, C.; Gessini, Alessandro; Allaria, E.; Ninno, G. De; Diviacco, B.; Roussel, Eléonore; Spampinati, S.; Penco, G.; Mitri, S. Di; Trovò, M.; Danailov, Miltcho; Christensen, Steven T.; Sokaras, Dimosthenis; Weng, Tsu‐Chien; Coreno, Marcello; Poletto, Luca; Drisdell, Walter S.; Prendergast, David; Giannessi, L.; Principi, Emiliano; Nordlund, Dennis; Saykally, Richard J.; Schwartz, Craig P.

doi:10.1103/physrevlett.120.023901

Cited by 73 publications

(66 citation statements)

References 40 publications

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“…However, the observation of coherent emission at harmonics in the range from 84 to 101 indicates the possibility to extend the lasing to wavelengths as short as 2 nm (620 eV) or less in a more optimized setup, either by using EEHG directly, or with a cascade employing both EEHG and HGHG schemes 24 . Such a layout has the potential to set the stage for entirely new experiments using X-ray nonlinear optical techniques [37][38][39][40][41] .…”

Section: Discussionmentioning

confidence: 99%

Coherent soft X-ray pulses from an echo-enabled harmonic generation free-electron laser

et al. 2019

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X-ray free electron lasers (FELs), which amplify light emitted by a relativistic electron beam, are extending nonlinear optical techniques to shorter wavelengths, adding element specificity by exciting and probing electronic transitions from core levels. These techniques would benefit tremendously from having a stable FEL source, generating spectrally pure and wavelength-tunable pulses. We show that such requirements can be met by operating the FEL in the so-called echo-enabled harmonic generation (EEHG) configuration. Here, two external conventional lasers are used to precisely tailor the longitudinal phase space of the electron beam before emission of X-rays. We demonstrate high-gain EEHG lasing producing stable, intense, nearly fully coherent pulses at wavelengths as short as 5.9 nm (~211 eV) at the FERMI FEL user facility. Low sensitivity to electron-beam imperfections and observation of stable, narrow-band, coherent emission down to 2.6 nm (~474 eV) make the technique a prime candidate for generating laser-like pulses in the X-ray spectral region, opening the door to multidimensional coherent spectroscopies at short wavelengths.

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Section: Discussionmentioning

confidence: 99%

Coherent soft X-ray pulses from an echo-enabled harmonic generation free-electron laser

et al. 2019

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“…This is significant, as very few experiments have been able to directly measure the properties of the ablation plume on ultrafast timescales. In future work, we hope to study the scattering over broader angle and time ranges, effecting the direct study of both the growth and structural composition of the nanomaterials in the ablation plumes; this will facilitate a deeper understanding of the factors controlling the properties and morphology of nanomaterials produced by laser ablation, and will provide a valuable complement to the femtosecond second harmonic generation 35 and two-photon absorption 36 experiments that we recently demonstrated with free electron laser sources in the soft X-ray region. The sample is mounted on a diffractometer and aligned at grazing incidence relative to the X-ray pulse.…”

Section: Discussionmentioning

confidence: 99%

Early time dynamics of laser-ablated silicon using ultrafast grazing incidence X-ray scattering

Hull

Raj

Lam

et al. 2019

Chemical Physics Letters

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Controlling the morphology of laser-derived nanomaterials is dependent on developing a better understanding of the particle nucleation dynamics in the ablation plume. Here, we utilize the femtosecond-length pulses from an x-ray free electron laser to perform time-resolved grazing incidence x-ray scattering measurements on a laser-produced silicon plasma plume. At 20 ps we observe a dramatic increase in the scattering amplitude at small scattering vectors, which we attribute to incipient formation of liquid silicon droplets. These results demonstrate the utility of XFELs as a tool for characterizing the formation dynamics of nanomaterials in laser-produced plasma plumes on ultrafast timescales.

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“…This approach is broadly applicable for calculating electronic SHG and SFG spectra from the optical regime through the x-ray energies and is detailed in the Supplemental Material [25]. Figures 3(a) and 3(b) present the linear and SH response, respectively, of an eight-layer slab of AB stacked hexagonal corresponding to the calculated linear spectrum at the C K-edge NEXAFS region calculated with a density-functional-theory-based supercell approach for an eight-layer slab of graphite compared with the experimental linear spectrum (gray, dashed).…”

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confidence: 99%

“…Measurements were made both with and without the sample present to permit subtraction of any remaining second harmonic contamination from the FEL beam. See the Supplemental Material for additional details on the experimental design and analysis procedures [25].…”

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confidence: 99%

X-Ray Probe Targets Interfaces

Nilsson

2018

Physics

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Nonlinear optical processes at soft x-ray wavelengths have remained largely unexplored due to the lack of available light sources with the requisite intensity and coherence. Here we report the observation of soft x-ray second harmonic generation near the carbon K edge (∼284 eV) in graphite thin films generated by high intensity, coherent soft x-ray pulses at the FERMI free electron laser. Our experimental results and accompanying first-principles theoretical analysis highlight the effect of resonant enhancement above the carbon K edge and show the technique to be interfacially sensitive in a centrosymmetric sample with second harmonic intensity arising primarily from the first atomic layer at the open surface. This technique and the associated theoretical framework demonstrate the ability to selectively probe interfaces, including those that are buried, with elemental specificity, providing a new tool for a range of scientific problems. DOI: 10.1103/PhysRevLett.120.023901 Nonlinear optics has recently been extended from visible and near UV wavelengths to new regimes with the development of x-ray free electron lasers (XFELs) capable of delivering x-ray pulses with high brightness, ultrashort pulse duration, and high coherence [1][2][3][4][5][6][7]. One fundamental nonlinear probe is second harmonic generation (SHG), a secondorder process which combines two photons of the same energy to generate a single photon with twice the energy [8]. At infrared, visible, and ultraviolet wavelengths, secondorder nonlinear spectroscopies have become important tools in surface science, as symmetry considerations within the dipole approximation constrain signal generation to regions lacking centrosymmetry, such as surfaces and interfaces [9][10][11][12]. In contrast, at hard x-ray wavelengths, second harmonic and sum frequency generation (SFG) have been observed in centrosymmetric materials with a nonuniform electron density and are essentially bulk probes [1,2]. As soft x-ray wavelengths fall in between the hard x-ray and UV regimes, there has been uncertainty regarding the interface specificity of soft x-ray SHG. Here, by utilizing a recently constructed, highly coherent, soft x-ray free electron laser [13,14], we report the first observation of soft x-ray second harmonic generation near the carbon K edge (∼284 eV). Our experimental results and accompanying theoretical analysis indicate that soft x-ray SHG is an interface-specific probe with symmetry constraints similar to optical SHG or SFG, and is highly sensitive to resonance effects. This enables a powerful new approach for surface and interface analysis with broad applicability to many scientific fields, as it combines the elemental and chemical specificity of x-ray absorption spectroscopy with the rigorous interfacial specificity of second-order nonlinear spectroscopies, while maintaining a fully coherent signal. With several new coherent free electron lasers under development (SwissFEL, SXFEL, LCLS-II, FLASH2020) [15,16], this new technique offers exciting application...

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Soft X-Ray Second Harmonic Generation as an Interfacial Probe

Cited by 73 publications

References 40 publications

Coherent soft X-ray pulses from an echo-enabled harmonic generation free-electron laser

Coherent soft X-ray pulses from an echo-enabled harmonic generation free-electron laser

Early time dynamics of laser-ablated silicon using ultrafast grazing incidence X-ray scattering

X-Ray Probe Targets Interfaces

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