We present a kind of harmonic mode locking of bound-state solitons in a fiber laser based on molybdenum disulfide (MoS(2)) saturable absorber (SA). The mode locker is fabricated by depositing MoS(2) nanosheets on a D-shaped fiber (DF). In the fiber laser, two solitons form the bound-state pulses with a temporal separation of 3.4 ps, and the bound-state pulses are equally distributed at a repetition rate of 125 MHz, corresponding to 14th harmonics of fundamental cavity repetition rate (8.968 MHz). Single- and multiple-pulses emissions are also observed by changing the pump power and optimizing the DF based MoS(2) SA. Our experiment demonstrates an interesting operation regime of mode-locked fiber laser, and shows that DF based MoS(2) SA can work as a promising high-power mode locker in ultrafast lasers.

We propose a strategy for active control of second harmonic generation (SHG) in a plasmonic Fano structure by electrically doping its underlying monolayer graphene. A detailed theoretical model for the proposed scheme is developed and numerical simulations are carried out to demonstrate the operation. Specifically, we show that a merely 30 meV change in graphene Fermi level can result in 45 times increase in SHG peak intensity, accompanied by a resonance wavelength shift spanning 220 nm. Further analysis uncovers that such tunability in SHG arises from the Fermi-level-modulated graphene permittivity, the real and imaginary parts of which dominate the resonance wavelength and the intensity of SHG, respectively.

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We report a method to tune the second harmonic generation (SHG) frequency of a metallic octamer by employing cylindrical vector beams as the excitation. Our method exploits the ability to spatially match the polarization state of excitations with the fundamental target plasmonic modes, enabling flexible control of the SHG resonant frequency. It is found that SHG of the octamer is enhanced over a broad band (400 nm) by changing the excitation from the linearly polarized Gaussian beam to radially and azimuthally polarized beams. More strikingly, when subjected to an azimuthally polarized beam, the SHG intensity of the octamer becomes 30 times stronger than that for the linearly polarized beam even in the presence of Fano resonance.

Being an enabling technology for applications such as ultrasensitive biosensing and surface enhanced spectroscopy, enormous research interests have been focused on further boosting the local field enhancement at Fano resonance. Here, we demonstrate a plasmonic Fano resonance resulting from the interference between a narrow magnetic dipole mode and a broad electric dipole mode in a split-ring resonator (SRR) coupled to a nanoarc structure. Strikingly, when subjected to an azimuthally polarized beam (APB) excitation, the intensity enhancement becomes more than 60 times larger than that for a linearly polarized beam (LPB). We attribute this intensity enhancement to the improved conversion efficiency between the excitation and magnetic dipole mode along with improved near-field coupling. The APB excited Fano structure is further used as a nanoruler and beam misalignment sensor, due to the high sensitivity of intensity enhancement and scattering spectra to structure irregularities and excitation beam misalignment. Interestingly, we find that, regardless of the presence of structural translations, the proposed structure still maintains over 60 times better intensity enhancement under APB excitation compared to LPB excitation. Moreover, even if the APB excitation is somewhat misaligned, our Fano structure still manages to give a larger intensity enhancement than its counterpart excited by LPB.