Ultrabroadband nonlinear Raman-Nath diffraction against femtosecond pulse laser

Nonlinear Raman–Nath diffraction(NRND) offers an effective way to realize multiple noncollinear parametric processes based on the partially satisfied transverse phase-matching conditions in quadratic nonlinear media.Here, the realization of ultrabroadband NRND(UB-NRND) driven by a high-peak-power ultrashort femtosecond pump laser in two types of nonlinear crystals is reported: periodically poled lithium niobate(PPLN) and chirped PPLN(CPPLN). Multi-order ultrabroadband Raman–Nath second-harmonic(SH) signal outputs along fixed diffraction angles are simultaneously observed. This distinguished transversely phase-matched supercontinuum phenomenon is attributed to the synergic action of natural broad bandwidth of an ultrashort femtosecond pump laser and the third-order nonlinear effect induced spectral broadening, in combination with the principal ultrabroadband noncollinear second-harmonic generation processes. The NRND process with multiple quasiphase matching(QPM) interactions from CPPLN leads to the SH output covering a wide range of wavelengths between 389 and 997 nm and exhibiting an energy conversion efficiency several orders of magnitude higher than previous studies. This UB-NRND scheme would bring better techniques and tools for applications ranging from ultrashort pulse characterization and nondestructive identification of domain structures to accurate parameter monitoring of second-and third-order nonlinear susceptibilities within solid-state nonlinear microstructured materials.

White Laser Realized via Synergic Second-and Third-Order Nonlinearities

White laser with balanced performance of broad bandwidth,high average and peak power,large pulse energy,high spatial and temporal coherence,controllable spectrum profile,and overall chroma are highly desirable in various fields of modern science.Here,for the first time,we report an innovative scheme of harnessing the synergic action of both the second-order nonlinearity(2^(nd)-NL)and the third-order nonlinearity(3^(rd)-NL)in a single chirped periodically poled lithium niobate(CPPLN)nonlinear photonic crystal driven by a high-peak-power near-infrared(NIR)(central wavelength~1400 nm,energy~100μJ per pulse)femtosecond pump laser to produce visible to near infrared(vis-NIR,400-900 nm)supercontinuum white laser.The CPPLN involves a series of reciprocal-lattice bands that can be exploited to support quasiphase matching for simultaneous broadband second-and third-harmonic generations(SHG and THG)with considerable conversion efficiency.Due to the remarkable 3rdNL which is due to the high energy density of the pump,SHG and THG laser pulses will induce significant spectral broadening in them and eventually generate bright vis-NIR white laser with high conversion efficiency up to 30%.Moreover,the spectral profile and overall chroma of output white laser can be widely modulated by adjusting the pump laser intensity,wavelength,and polarization.Our work indicates that one can deeply engineer the synergic and collective action of 2^(nd)-NL and 3^(rd)-NL in nonlinear crystals to accomplish high peak power,ultrabroadband vis-NIR white laser and hopefully realize the even greater but much more challenging dream of ultraviolet-visible-infrared full-spectrum laser.