| Owing to their short duration and high peak power,pulsed lasers are widely used in medical research,high-precision manufacturing and other relative regimes.Passive mode-locking and passive Q-switched methods are two common schemes for pulsed fiber operation,and their cores lie in the modulation device--saturable absorber.The output of short pulse in laser cavity can be realized by using the saturable absorption characteristics of a saturable absorber.Since gold nanostructures have the characteristics of large third-order nonlinearity,fast response time and wide-band resonant absorption,they are ideal candidates for saturable absorbers.Many studies have demonstrated that gold nanomaterials can be used in mode-locked lasers or Q-switched lasers.However,the mainstream synthesis methods like seedmediated growth method,lead to solution samples containing dissimilar gold nanostructrues with dispersed dimensions and random directions,hindering accurate performance control.Optical metasurfaces,i.e.,thin planar structures with subwavelength metal or dielectric units,can be customized to adjust the propagation of light by tailoring their permittivity and permeability,precisely regulating the amplitude,phase and polarization of light.It has a broad application prospect in the field of linear and nonlinear modulation of light field,and broadens the vision for the realization of new optical components.But their saturable absorption properties and corresponding applications have not been widely studied so far.In this paper,the saturable absorption characteristics of metasurfaces are studied deeply,and their practical applications in mode-locked and Qswitched fiber lasers are presented.The main research achievements and innovations are as follows:(1)The single-rod gold metasurface was designed and fabricated,and its linear and nonlinear characteristics were studied in different polarization conditions.When the polarization angles of the incident polarized light are 0°,30° and 90°,the near-field electric field enhancement characteristics show obvious polarization sensitivity,and the modulation depth corresponding to the nonlinear absorption is 1.70%,0.91%,0.19%.It is applied to the passive mode-locked laser at 1 μm band,and the typical mode-locked pulse is generated.The pulse width is～62.3 ps,the repetition rate is 10.33 MHz,and the signal-to-noise ratio is～74 dB.(2)A novel double-rod gold metasurface is proposed.Numerical calculations show that by introducing Fano resonance and surface lattice resonance,this structure has a narrower full width at half maximum of the transmission spectrum,a larger Q value and a stronger near-field electric field enhancement than the single-rod gold metasurface.The experimental results show that when the polarization angles of the incident polarized light are 0°,30° and 90°,the modulation depths corresponding to the double-rod structure are 7.99%,3.31%and 0.36%,respectively,which are significantly stronger than that of the single-rod structure in similar conditions.The metasurface is applied to the passive Q-switched polarization maintaining fiber laser,and typical Q-switched pulse output is obtained.When the pump power ranges from 105 mW to 160 mW,the repetition frequency increases from 21.82 kHz to 35.76 kHz,and the pulse width decreases from 3.38 μs to 1.92 μs.In this paper,two kinds of gold metasurface saturable absorbers are proposed and their saturable absorption effects in passive mode-locked and Q-switched fiber lasers are studied.This is the first demonstration based on metasurface saturable absorber for mode-locked and Q-switched lasers in the range of 1 μm.At the same time,the structural design method provides enlightenments to improve the nonlinear saturable absorption effect of the metasurface,and expands the application of optical metasurface in nonlinear optics,and integrated optics. |
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