Non-linear self-driven spectral tuning of Extreme Ultraviolet Femtosecond Pulses in monoatomic materials

Self-action nonlinearity is a key aspect either as a foundational element or a detrimental factor of several optical spectroscopies and photonic devices.Supercontinuum generation,wavelength converters,and chirped pulse amplification are just a few examples.The recent advent of Free Electron Lasers(FEL)fostered building on nonlinearity to propose new concepts and extend optical wavelengths paradigms for extreme ultraviolet(EUV)and X-ray regimes.No evidence for intrapulse dynamics,however,has been reported at such short wavelengths,where the light-matter interactions are ruled by the sharp absorption edges of core electrons.Here,we provide experimental evidence for self-phase modulation of femtosec ond FEL pulses,which we exploit for fine self-driven spectral tun ability by interaction with sub-micrometric foils of selected monoatomic materials.Moving the pulse wavelength across the absorption edge,the spectral profile changes from a non-linear spectral blue-shift to a red-shifted broadening.These findings are rationalized account!ng for ultrafast ionization and delayed thermal response of highly excited electrons above and below threshold,respectively.

Phase-matching-free parametric oscillators based on two-dimensional semiconductors

Optical parametric oscillators are widely used as pulsed and continuous-wave tunable sources for innumerable applications,such as quantum technologies,imaging,and biophysics.A key drawback is material dispersion,which imposes a phase-matching condition that generally entails a complex design and setup,thus hindering tunability and miniaturization.Here we show that the burden of phase-matching is surprisingly absent in parametric microresonators utilizing mono-layer transition-metal dichalcogenides as quadratic nonlinear materials.By the exact solution of nonlinear Maxwell equations and first-principle calculations of the semiconductor nonlinear response,we devise a novel kind of phase-matching-free miniaturized parametric oscillator operating at conventional pump intensities.We find that different two-dimensional semiconductors yield degenerate and non-degenerate emission at various spectral regions due to doubly resonant mode excitation,which can be tuned by varying the incidence angle of the external pump laser.In addition,we show that high-frequency electrical modulation can be achieved by doping via electrical gating,which can be used to efficiently shift the threshold for parametric oscillation.Our results pave the way for the realization of novel ultra-fast tunable micron-sized sources of entangled photons—a key device underpinning any quantum protocol.Highly miniaturized optical parametric oscillators may also be employed in lab-on-chip technologies for biophysics,detection of environmental pollution and security.