We present a detailed theoretical and numerical analysis on the temporal-spectral-spatial evolution of a high-peak-power femtosecond laser pulse in two sets of systems: a pure lithium niobate (LN) plate and a periodically poled lithium niobate (PPLN) plate. We develop a modified unidimensional pulse propagation model that considers all the prominent linear and nonlinear processes and carried out the simulation process based on an improved split-step Fourier transformation method. We theoretically analyze the synergic action of the linear dispersion effect, the second-order nonlinearity (2nd-NL) second-harmonic generation (SHG) effect, and the third-order nonlinearity (3rd-NL) self-phase modulation (SPM) effect, and clarify the physical mechanism underlying the peculiar and diverse spectral broadening patterns previously reported in LN and PPLN thin plate experiments. Such analysis and discussion provides a deeper insight into the synergetic contribution of these linear and nonlinear effects brought about by the interaction of a femtosecond laser pulse with the LN nonlinear crystal and helps to draw a picture to fully understand these fruitful optical physical processes, phenomena, and laws.

A supercontinuum white laser with ultrabroad bandwidth, intense pulse energy, and high spectral flatness can be accomplished via synergic action of third-order nonlinearity (3rd-NL) and second-order nonlinearity. In this work, we employ an intense Ti:sapphire femtosecond laser with a pulse duration of 50 fs and pulse energy up to 4 mJ to ignite the supercontinuum white laser. Remarkably, we use water instead of the usual solid materials as the 3rd-NL medium exhibiting both strong self-phase modulation and stimulated Raman scattering effect to create a supercontinuum laser with significantly broadened bandwidth and avoid laser damage and destruction. Then the supercontinuum laser is injected into a water-embedded chirped periodically poled lithium niobate crystal that enables broadband and high-efficiency second-harmonic generation. The output white laser has a 10 dB bandwidth encompassing 413 to 907 nm, more than one octave, and a pulse energy of 0.6 mJ. This methodology would open up an efficient route to creating a long-lived, high-stability, and inexpensive white laser with intense pulse energy, high spectral flatness, and ultrabroad bandwidth for application to various areas of basic science and high technology.

Metasurfaces have emerged as a flexible platform for shaping the electromagnetic field via the tailoring phase, amplitude, and polarization at will. However, the chromatic aberration inherited from building blocks’ diffractive nature plagues them when used in many practical applications. Current solutions for eliminating chromatic aberration usually rely on searching through many meta-atoms to seek designs that satisfy both phase and phase dispersion preconditions, inevitably leading to intensive design efforts. Moreover, most schemes are commonly valid for incidence with a specific spin state. Here, inspired by the Rayleigh criterion for spot resolution, we present a design principle for broadband achromatic and polarization-insensitive metalenses using two sets of anisotropic nanofins based on phase change material Ge₂Sb₂Se₄Te₁. By limiting the rotation angles of all nanofins to either 0 deg or 90 deg, the metalens with a suitable numerical aperture constructed by this fashion allows for achromatic and polarization-insensitive performance across the wavelength range of 4–5 μm, while maintaining high focusing efficiency and diffraction-limited performance. We also demonstrate the versatility of our approach by successfully implementing the generation of broadband achromatic and polarization-insensitive focusing optical vortex. This work represents a major advance in achromatic metalenses and may find more applications in compact and chip-scale devices.

We propose a pseudospin-field-dependent waveguide (PFDW) by constructing a sandwiched heterostructure consisting of three magneto-optical photonic crystals (MOPCs) with different geometric parameters. The upper expanded MOPC applied with an external magnetic field has broken time-reversal symmetry (TRS) and an analogous quantum spin Hall (QSH) effect, while the middle standard and the lower compressed ones are not magnetized and trivial. Attributed to the TRS-broken-QSH effect of the upper MOPC, the topological large-area one-way transmission that uniformly distributes over the middle domain is achieved and exhibits the characteristics of a pseudospin-field-momentum-locking; i.e., pseudospin-down (or pseudospin-up) leftward (or rightward) waveguide state when the positive (or negative) magnetic field is applied on the upper MOPC. We further demonstrate the strong robustness of the PFDW against backscattering from various kinds of defects. In addition, a topological beam modulator that can compress or expand the light beam, and a large-area pseudospin beam splitter have been designed. These results have potential in various applications such as sensing, signal processing, and optical communications.

A quantum dot (QD) mode-locked laser as an active comb generator takes advantage of its small footprint, low power consumption, large optical bandwidth, and high-temperature stability, which is an ideal multi-wavelength source for applications such as datacom, optical interconnects, and LIDAR. In this work, we report a fourth-order colliding pulse mode-locked laser (CPML) based on InAs/GaAs QD gain structure, which can generate ultra-stable optical frequency combs in the O-band with 100 GHz spacing at operation temperature up to 100°C. A record-high flat-top optical comb is achieved with 3 dB optical bandwidth of 11.5 nm (20 comb lines) at 25°C. The average optical linewidth of comb lines is measured as 440 kHz. Single-channel non-return-to-zero modulation rates of 70 Gbit/s and four-level pulse amplitude modulation of 40 GBaud/s are also demonstrated. To further extend the comb bandwidth, an array of QD-CPMLs driven at separate temperatures is proposed to achieve 36 nm optical bandwidth (containing 60 comb lines with 100 GHz mode spacing), capable of a total transmission capacity of 4.8 Tbit/s. The demonstrated results show the feasibility of using the QD-CPML as a desirable broadband comb source to build future large-bandwidth and power-efficient optical interconnects.

We present a discovery of an unusual unidirectionally rotating windmill scattering of electromagnetic waves by a magnetized gyromagnetic cylinder via an analytical theory for rigorous solution to fields and charges and an understanding of the underlying mathematical and physical mechanisms. Mathematically, the generation of nonzero off-diagonal components can break the symmetry of forward and backward scattering coefficients, producing unidirectional windmill scattering. Physically, this windmill scattering originates from the nonreciprocal unidirectional rotation of polarized magnetic charges on the surface of a magnetized gyromagnetic cylinder, which drives the scattering field to radiate outward in the radial direction and unidirectionally emit in the tangential direction. Interestingly, the unidirectional electromagnetic windmill scattering is insensitive to the excitation direction. Moreover, we also discuss the size dependence of unidirectional windmill scattering by calculating the scattering spectra of the gyromagnetic cylinder. These results are helpful for exploring and understanding novel interactions between electromagnetic waves and gyromagnetic materials or structures and offer deep insights for comprehending topological photonic states in gyromagnetic systems from the aspect of fundamental classical electrodynamics and electromagnetics.

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 quasi-phase 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.