Coherence of bulk-generated supercontinuum

We have developed a numerical framework that allows estimation of coherence in spatiotemporal and spatiospectral domains. Correlation properties of supercontinuum(SC) pulses generated in a bulk medium are investigated by means of second-order coherence theory of non-stationary fields. The analysis is based on simulations of individual space–time and space–frequency realizations of pulses emerging from a 5 mm thick sapphire plate,in the regimes of normal, zero, and anomalous group velocity dispersion. The temporal and spectral coherence properties are analyzed in the near field(as a function of spatial position at the exit plane of the nonlinear medium) and as a function of propagation direction(spatial frequency) in the far field. Unlike in fiber-generated SC, the bulk case features spectacularly high degrees of temporal and spectral coherence in both the spatial and spatial-frequency domains, with increasing degrees of coherence at higher pump energies. When operating near the SC generation threshold, the overall degrees of temporal and spectral coherence exhibit an axial dip in the spatial domain, whereas in the far field, the degree of coherence is highest around the optical axis.

Terahertz structured light:nonparaxial Airy imaging using silicon diffractive optics

Structured light–electromagnetic waves with a strong spatial inhomogeneity of amplitude,phase,and polarization–has occupied far-reaching positions in both optical research and applications.Terahertz(THz)waves,due to recent innovations in photonics and nanotechnology,became so robust that it was not only implemented in a wide variety of applications such as communications,spectroscopic analysis,and non-destructive imaging,but also served as a low-cost and easily implementable experimental platform for novel concept illustration.In this work,we show that structured nonparaxial THz light in the form of Airy,Bessel,and Gaussian beams can be generated in a compact way using exclusively silicon diffractive optics prepared by femtosecond laser ablation technology.The accelerating nature of the generated structured light is demonstrated via THz imaging of objects partially obscured by an opaque beam block.Unlike conventional paraxial approaches,when a combination of a lens and a cubic phase(or amplitude)mask creates a nondiffracting Airy beam,we demonstrate simultaneous lensless nonparaxial THz Airy beam generation and its application in imaging system.Images of single objects,imaging with a controllable placed obstacle,and imaging of stacked graphene layers are presented,revealing hence potential of the approach to inspect quality of 2D materials.Structured nonparaxial THz illumination is investigated both theoretically and experimentally with appropriate extensive benchmarks.The structured THz illumination consistently outperforms the conventional one in resolution and contrast,thus opening new frontiers of structured light applications in imaging and inverse scattering problems,as it enables sophisticated estimates of optical properties of the investigated structures.

Multiple conical odd harmonics from filament-inscribed nanogratings

We report on the observation of conical third,fifth,seventh,and ninth harmonics that gradually emerge during the supercontinuum generation by filamentation of femtosecond midinfrared pulses in lithium strontium hexafluoroaluminate crystal.We show that the generation of conical odd harmonics is an optical signature of lightdriven material reorganization in the form of volume nanogratings at the site irradiated by repetitive femtosecond filaments.The angle-resolved spectral measurements demonstrate remarkably broad spectra of individual odd harmonics,benefiting from a spectrally broadened pump pulse(supercontinuum),and reveal that filamentinscribed nanogratings represent photonic structures that are able to provide ultrabroad phase-matching bandwidths covering the wavelength range from the ultraviolet to the near infrared.We propose a scenario that interprets the generation of conical fifth,seventh,and ninth harmonics as nanograting phase-matched cascaded noncollinear four-wave mixing processes.