arXiv subjects

Ankita Negi

Publications and source records attributed to Ankita Negi.

4 recordsLinked to original sources

arXiv2026

Polariton spectroscopy at the diamond K-edge via X-ray parametric down-conversion

It has recently been shown that x-ray parametric down-conversion (XPDC) provides access to high-energy polaritons, resulting from the hybridization of down-converted photons with electronic excitations in a nonlinear medium. Here, we present a spectrally resolved study of this effect around the K-shell absorption edge in diamond. Our results exhibit pronounced signatures of polaritonic hybridization, which we visualize by introducing a polariton spectral map and analyze by help of theoretical modelling. We find that the hybridization at this absorption edge results in substantially higher coupling strength than previously reported for a non-resonant case and reaches well into the strong-coupling regime. In addition, we demonstrate how our measurements of polaritonic XPDC allow us to extract the refractive index for bulk diamond at high spectral resolution around the carbon K-edge.

physics.ins-det↗

arXiv2025

Energy Time Ptychography for one-dimensional phase retrieval

Phase retrieval is at the heart of adaptive optics and modern high-resolution imaging. Without phase information, optical systems are limited to intensity-only measurements, hindering full reconstruction of object structures and wavefront dynamics essential for advanced applications. Here, we address a one-dimensional phase problem linking energy and time, which arises in X-ray scattering from ultrasharp nuclear resonances. We leverage the Mössbauer effect, where nuclei scatter radiation without energy loss to the lattice, and are sensitive to their magneto-chemical environments. Rather than using traditional spectroscopy with radioactive gamma-ray sources, we measure nuclear forward scattering of synchrotron X-ray pulses in the time domain, providing superior sensitivity and faster data acquisition. Extracting spectral information from a single measurement is challenging due to the missing phase information, typically requiring extensive modeling. Instead, we use multiple energetically overlapping measurements to retrieve both the transmission spectrum and the phase of the scattering response, similar to ptychographic phase retrieval in imaging. Our robust approach can overcome the bandwidth limitations of gamma-ray sources, opening new research directions with modern X-ray sources and Mössbauer isotopes.

physics.optics↗

arXiv2025

Interferometric measurement of nuclear resonant phase shift with a nanoscale Young double waveguide

The phase shift of an electromagnetic wave, imprinted by its interaction with atomic scatterers, is a central quantity in optics and photonics. In particular, it encodes information about optical resonances and photon-matter interaction. While being a routine task in the optical regime, interferometric measurements of phase shifts in the x-ray frequency regime are notoriously challenging due to the short wavelengths and associated stability requirements. As a result, the methods demonstrated to date are unsuitable for nanoscopic systems. Here, we demonstrate a nanoscale interferometer, inspired by Young's double-slit experiment, to measure the dispersive phase shift due to the 14.4 keV nuclear resonance of the Mössbauer isotope $^{57}$Fe coupled to an x-ray waveguide. From the single-photon interference patterns, we precisely extract the phase shifts in the vicinity of the nuclear resonance resolved in photon energy by using Bayesian inference. We find that the combined information from phase shift and absorbance reveals microscopic coupling parameters, which are not accessible from the intensity data alone. The demonstrated principle lays a basis for integrated x-ray interferometric sensors.

physics.optics↗

arXiv2025

Collective nuclear excitation and pulse propagation in single-mode x-ray waveguides

Waveguides offer a means to controllably couple atomic ensembles to the electromagnetic field therein. Here, we demonstrate x-ray propagation in planar thin-film waveguides coupled to Mössbauer nuclei under collective resonant excitation by short pulses of synchrotron radiation. We record x-ray photons that have been emitted into resonant modes of the waveguide. Depending on the geometry and mode of excitation, two fundamentally different signatures of the collective emission are observed, for which we present a unifying theoretical model. Our results form a new platform for waveguide quantum electrodynamics in the hard x-ray regime with the potential to provide a coherent narrowband source of x-rays on the nanometer scale.

quant-ph↗