X-Ray Second Harmonic Generation

X-Ray Probe Targets Interfaces

“…This resonance enhancement means that the technique can be used to generate a surface-specific soft x-ray spectrum with sensitivity to chemical composition. The pump intensities used here correspond to peak fields of 10 12 W=cm 2 , which is 4 orders of magnitude less than what was required for hard xray SHG (10 16 W=cm 2 ) [2]. Note that at these power densities, the efficiency of the SHG process is at least an order of magnitude lower than the linear absorption for a single atomic layer [36].…”

“…Prior to the development of free electron lasers (FELs), only parametric downconversion had been observed [17]. While the advent of XFELs has recently enabled second-and third-order nonlinear spectroscopies at hard x-ray energies, including SHG [2], SFG [1], two-photon absorption [4], and inelastic Compton scattering [18], current hard x-ray FELs lack the longitudinal and temporal coherence necessary for efficiently satisfying the phase-matching conditions required for nonlinear spectroscopies, thus, making the exploitation of some of these techniques difficult [19,20]. Furthermore, the shorter hard x-ray wavelengths (λ < 0.2 nm) induce second harmonic and sum frequency generation even within centrosymmetric media, as the observed response depends on material inhomogeneity on the length scale of the x-ray wavelength, similar to how SHG is seen in a plasma, and effectively making this method a bulk probe [21].…”

“…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.…”

“…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].…”

Detecting electronic coherence by multidimensional broadband stimulated x-ray Raman signals

“…Optical Raman techniques have been applied to study electron transfer and nonadiabatic dynamics at conical intersections. Using recently developed FEL and HHG sources [17][18][19], Raman techniques can be further extended to the X-ray regime [20][21][22][23][24] whereby the system is initially prepared in the superposition of valence electronic states and an X-ray Raman probe then reveals information about electronic, rather than vibrational, coherence.…”