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We have examined the transmission of soft X-ray pulses from the FERMI free electron laser through carbon films of varying thickness, quantifying nonlinear effects of pulses above and below the carbon K-edge. At typical of soft X-ray free electron laser intensities, pulses exhibit linear absorption at photon energies above and below the K-edge, ∼308 and ∼260 eV, respectively; whereas two-photon absorption becomes significant slightly below the K-edge, ∼284.2 eV. The measured two-photon absorption cross section at 284.18 eV (∼6 × 10−48 cm4 s) is 7 orders of magnitude above what is expected from a simple theory based on hydrogen-like atoms - a result of resonance effects.

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.

Immunoglobulin G4-related disease (IgG4-RD) is a multisystemic immune-mediated disease, encompassing several conditions previously thought to be unrelated, which can mimic malignant, infectious, and inflammatory disorders. Head and neck IgG4-RD most frequently affects the salivary glands and orbit; however, it can involve virtually any subsite. Diagnosis is challenging and relies on the clinical presentation, serum immunoglobulin G4 levels, and histopathology. Although imaging is nonspecific, growing knowledge of IgG4-RD allows characterization of imaging patterns and description of novel findings. This review focuses on cross-sectional imaging findings of extracranial IgG4-RD in the head and neck, presenting cases in frequent and unusual subsites, with some cases with few descriptions in the radiological literature, such as the pharynx and larynx, temporal bone, paranasal sinuses, and perivascular areas. Learning Objective: To describe the imaging findings of IgG4-RD in the head and neck to suggest the diagnosis and identify the involvement of common and uncommon subsites. [ABSTRACT FROM AUTHOR]