| Pulsed lasers driven by passive modulation method based on saturable absorbers have benefits of compact structure of the laser cavity,system simplicity,fast modulation speed and low cost,and they can generate shorter laser pulses as compared with other modulation mechanisms like acousto-optic or electro-optic modulation.Therefore,this method is recognized as the cheapest solution to realize ultrafast pulses.However,the widely used semiconductor saturable absorber mirrors are characterized by a narrow working bandwidth and slow nonlinear optical responses,and the recently developed saturable absorbers based on new two-dimensional materials suffer from poor stability and the difficulty in mass production.To solve these problems,highly stable metallic titanium nitride nanoparticles enhanced by surface plasmon resonance are used to achieve ultrafast nonlinear optical responses in an ultra-wide near infrared spectrum from 1.0 ?m to 2.0?m.Saturable absorbers based on titanium nitride are utilized as optical switches in the generation of ultrashort pulsed lasers.In this paper,the nonlinear optical properties and application of titanium nitride are studied:Firstly,the morphology,surface state and linear absorption of the vapor-phase deposited titanium nitride nanoparticles and films were characterized.It is found that the size of the titanium nitride nanoparticles ranges from 10 to 20 nm.The resonance peak of the nanoparticles is centered at around 700 nm.In addition,a compact titanium nitride film was also deposited on the surface of a quartz plate by cathodic arc ion plating.The morphological,composition and non-linear optical characterization are performed.Secondly,the nonlinear optical properties of titanium nitride film were characterized by the Z-scan and I-scan methods.It is found that the films based on titanium nitride nanoparticles exhibit strong nonlinear optical absorption.According the calculations based nonlinear optical theory,modulation depth are characterized to be 10.8%,25.4%and 57.5%in 1.0 ?m,1.5 ?m and 2.0 ?m regions,respectively,and the saturable absorption coefficient of the as-prepared TiN NPs films is-37.1 cm/GW.With further transient absorption analyses based on pump-probe tests,it is clarified that the ultrafast nonlinear responses of titanium nitride result from the cooling of hot carriers,with the shortest response time of 3.2 ps.These results suggest that titanium nitride films can be used as saturable absorbers operating in the near infrared spectrum.Finally,based on the strong saturable absorption effect and short photobleaching recovery time,titanium nitride is applied as a passive optical modulation device in pulsed lasers.Titanium nitride nanoparticles are drop-casted onto a gold mirror as a reflective optical switch,which is integrated into a solid-state laser to generate high repetition rate Q-switched laser pulses at 1064 nm.The developed Q-switched pulsed laser shows the highest repetition rate of 590 kHz.Moreover,we use a titanium nitride nanoparticle/polyvinyl alcohol composite film,in the form of transmissive optical switch,in a fiber laser cavity to generate 1.5 ?m mode-locked pulses,and the resulting mode-locked pulsed laser characterize an ultra-short pulse width of 763 fs.The above results show that the developed optical switches based on titanium nitride nanostructure can be applied in optical devices working in the near-infrared spectral regions,and our work provides an experimental basis for photonic applications of transition metal nitrides. |
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