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The phase-matching bandwidth of nonlinear crystal is of great significance in ultrashort laser pulse characterization. In order to satisfy the phase-matching bandwidth, ultra-thin nonlinear crystals are generally required. However, the significantly reduced conversion efficiency, as well as the machining difficulties, limits its applications. Here, we show that sufficient spectrum bandwidth response can be achieved for a thick crystal when the phase-matching wavelength is tuned outside of the spectral window of the measured pulse. By applying this phenomenon to a single-shot second-harmonic generation frequency resolved optical gating (SHG-FROG) device, we successfully characterized a few-cycle pulse using a 150µmβ-barium borate (BBO) crystal. The accuracy of the method was verified by comparing the conventional pulse retrieving approach with a 5µm BBO crystal, which has a sufficient phase-matching bandwidth.We report the results of experimental and theoretical studies of monochromatic coherent terahertz radiation generated by a short relativistic electron bunch interacting with a metasurface. The metasurface consists of subwavelength metal elements arranged on a dielectric substrate. The constructed theory explains the experimental spectra of Smith-Purcell radiation and grating transition radiation with very high precision. The orientational distribution of transition radiation shows a fine structure, which, as we suppose, may be due to contribution of coupling between the metasurface's elements.Octave-spanning optical frequency combs (OFCs) are essential for various applications, such as precision metrology and astrophysical spectrometer calibration. In this Letter, we demonstrate, for the first time to our knowledge, the generation of octave-spanning Kerr frequency combs ranging from 1150 to 2400 nm in aluminum nitride (AlN) microring resonators, by pumping the TM00 modes at 250 mW on-chip power. By simply adjusting the pump detuning, we observe the transition and coexistence of Kerr OFC and stimulated Raman scattering. For the TE00 mode in the same device, a broadband Raman-assisted frequency comb is demonstrated by adjusting the pump power and tuning. These results indicate a crucial development for the fundamentals of nonlinear dynamics and comb applications in AlN.We systematically studied femtosecond laser-inscribed self-organized nanogratings and geometric phase elements such as a polarization diffraction focusing lens and Q-plate in sapphire crystal. Besides the void structures observed in the focus, nanogratings with periods of 150~300 nm were observed, depending on a nanoslit that took the role of a seeding effect by localized light field enhancement. The non-polarized refractive index change and birefringence were measured with values around 1∼2×10-3 and 6×10-4, respectively. Based on the laser-inscribed form birefringence, a geometric phase lens and Q-plate were successfully demonstrated in sapphire with high imaging and a focusing effect. We expect that our findings may promote the understanding of laser-induced nanogratings in bulk and potential applications in geometric phase elements.Bandwidth-tunable filters are essential in elastic optical networks for dynamic bandwidth allocation. Existing solutions in silicon photonics face challenges to meet requirements in real-world applications due to design trade-offs and fabrication errors. In this Letter, we propose and experimentally demonstrate a silicon photonic tunable add-drop filter in a single-stage, hyperbolic-tangent-apodized contra-directional coupler with a segmented microheater. It allows to create an arbitrary temperature profile along the device for bandwidth tuning in both through and drop responses. We show that the algorithmic operation of the device can effectively compensate local fabrication nonuniformity and improve the out-of-band suppression ratio by 69%. Applying proper temperature offsets and slopes allows to continuously tune the filter's center wavelength over 8 nm and its drop-port 3 dB bandwidth between 14.0 and 22.4 nm.Metasurfaces with orthogonal nano-slit pairs arranged on spirals are proposed to generate vector beams (VBs) of Bell-like states and slanted polarizations. The design of the metasurfaces is based on the theoretically derived parameter condition for manipulation of the two vector vortex modes, which is satisfied by matching the three parameters of rotation order m, the spiral order n, and incident polarization helicity σ. The linear polarization states of the VBs are controlled by the initial orientation angle φ0 of slit pairs. VBs of satisfying quality are experimentally obtained, with the analytical and simulated results validated.We present a new, to the best of our knowledge, scheme of realizing all-optical switching by exploiting the phenomenon of the bound state in the continuum (BIC) supported by an array of slotted silicon disks. The air slot in an individual silicon disk works as a void antenna and can then help to excite the quasi-BIC (QBIC) mode by a linearly polarized plane wave. Thanks to the high quality factor of the QBIC resonance and the associated large local electric field enhancement, the self-switching effect is quite pronounced, and a laser power intensity level less than 1KW/cm2 is required to achieve a change of transmittance from 0 to above 65%.We developed a simple multi-foci-shaped femtosecond pulsed (MFSFP) method for processing circular cross section waveguides in transparent materials. selleck chemical With this flexible processing method, the focus energy distribution can be designed freely and arbitrarily, and single-mode waveguides with cross section circularity better than 96.0% were achieved. The mode shape difference (1.93%) of circular waveguides is smaller than the difference (7.01%) of normal elliptical waveguides. The coupling abilities of the two kinds of waveguides were investigated with three-dimensional (3D) directional couplers in both experiments and theoretical simulations. The coupling coefficient difference of circular waveguides in vertical and horizontal coupling directions was ∼0.01mm-1, which was smaller than 0.33mm-1 of normal waveguides. The circular symmetric waveguides will play an important role in large-scale high-intensity 3D photonic integrated circuits.
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