X-ray parametric down-conversion: Challenging previous findings on the basis of improved experimental methods

Abstract

X-ray parametric down-conversion is a fundamental nonlinear effect that promises a wide range of possible applications. Nevertheless, it has been scarcely investigated and unequivocal evidence of the effect is still missing for certain spectral regions. In particular, this is the case for down-conversion into pairs of x-ray and optical photons, which would open pathways to characterise and image valence electron-density fluctuations. In this work, we present a systematic approach to scan the parameter space, wherein we expect to identify the effect based on its characteristic signature. Within the resolution of the experimental setup we cannot establish any evidence for the nonlinear effect. Instead, we trace the measured signals back to elastic scattering contributions, thereby challenging the interpretation of previous studies on the effect of x-ray parametric down conversion producing photons in the visible spectral range. As a benchmark for future investigations of x-ray parametric down-conversion -both experimental and theoretical -we extract an upper bound for the effect's conversion efficiency of 10 −11 within the resolution of our setup.

Towards novel probes for valence charges via X-ray optical wave mixing

“…The characterization (Figure 7a and b) clearly shows that the aimed for improvement of the instrumental function is achieved. Previous experiments 11,37 , conducted with only with a two-bounce monochromator (Si 111) and analyzer (Si 440), yielded additional features, which extended beyond ±100 mdeg along Ω and ±50 mdeg along the scattering dimension 2θ. The setup was dominated by monochromator and analyzer streaks.…”

“…If confirmed, these would mark highly desirable features of a nonlinear probe and thus warrant in-depth analysis. At the same time, however, our first independent investigation of XPDC has raised serious concerns with the aforementionned reports 11 . Now, we put our discussion of XPDC on a broader basis-providing both a theoretical description of the process and novel experimental results.…”

“…Especially the latest reports 9 for XPDC in LiNbO3 mention conversion rates, which deviate by 6 orders of magnitude from our here presented results. A detailed revision 11 of the proof-of-principle study 7 allowed for an identification of the alleged signal as elastic scattering contribution from the experimental apparatus itself (i.e., the instrumental function).…”

Quantum Sensing with Extreme Light

“…[87,88] However, at this point, it should be briefly stated why many of these experiments are currently under discussion. [71,75] As a clear proof for PDC into optical regions, the characteristic scattering signature should be observable, resulting directly from the phasematching condition. However, this signature is not reported in most experiments.…”

“…[78] Up to now, mainly diamond single crystals have been applied to generate biphotons in the optical and the X-ray frequency range. [71,75,78,79,[81][82][83][84][85][86] Although conversion efficiencies of the order of 10 −10 are reported, only a small fraction of the generated optical radiation is accessible due to internal reflection. [78] But as recently demonstrated, other nonlinear crystal materials such as lithium niobate (LiNbO 3 ) and gallium arsenide (GaAs) offer conversion efficiencies several orders of magnitude higher than any other material known to date.…”