Stabilized gold nanoparticle (AuNP) colloids have been fabricated by nanosecond pulsed laser ablation of a pure gold plate in cyclohexanone. The AuNPs colloid exhibits a UV–Vis absorption spectrum with a surface plasmon absorption peak at about 540 nm. Scanning electron microscopy has shown the formation of spherical AuNPs with average size about 53 nm. The shift of 24 cm−1 is observed in the carbonyl band of the colloid using FTIR spectroscopy. This shift indicates that the monomer carbonyl group of cyclohexanone interacts with the surface of the AuNPs and leads to stabilizing the colloid. A large nonlinear refractive index of −2.92 × 10−7 cm2/W is measured using the Z‐scan technique under continuous wave laser irradiation at 532 nm. Our results show that the large induced nonlinear refraction is attributed to the surface plasmon resonance (SPR) enhancement effect of AuNPs, high thermo‐optic coefficient and low thermal conductivity of cyclohexanone. Observation of far‐field diffraction ring patterns confirm a thermally induced negative lens effect and spatial self‐phase modulation in the laser beam as it traverses the colloids.
Silver sulfide nanoparticles (Ag 2 S NPs) were synthesized by nanosecond pulsed laser ablation of a silver plate in dimethyl sulfoxide. Ultraviolet-visible absorption spectrometry, X-ray diffraction, and transmission electron microscopy were used to characterize the obtained nanocrystal samples. The mechanisms for optical limiting performance of an Ag 2 S NP colloid were investigated under exposure to nanosecond pulsed laser irradiation at 532 nm. The observed aperture dependence of the Ag 2 S NP limiting performance indicates that the nonlinear scattering plays an important role in the limiting action of the sample. The Ag 2 S NPs may enhance the absorption of laser light by the two-photon absorption process and induce a very high rise in the temperature of the sample, which leads to the formation of scattering centers. [17]. This synthetic flexibility of physical and chemical properties is one of the reasons why nanoparticle suspensions are good candidates for nonlinear optical applications like photonic switches, optical limiters, etc. In particular, optical limiters have received significant attention during recent decades due to the ability to protect human eyes and sensors from intense laser pulses. An ideal optical limiter exhibits high transmission for low-intensity light, while it shows low transmission for intense light. This phenomenon can be caused by various nonlinear light-matter interactions, especially nonlinear absorption, nonlinear refraction, and nonlinear scattering. Many organic materials such as phthalocyanines [18], fullerene families [19], and carbon-black suspensions [20] were reported as good limiters due to their excellent nonlinear optical properties.
Optical limiting performance of gold nanoparticles (AuNPs) in castor oil is studied using a low power laser irradiation at 532 nm. Our results suggest that it is possible to tune the optical limiting threshold of the AuNPs by modifying the parameters of the set-up and gold nanoparticle concentration of the colloids. The closed Z-scan measurements reveal that non-local thermal nonlinear refraction process is responsible for the limiting action of the colloids. We have investigated the Z-scan behavior of the samples based on the non-local thermal nonlinear refraction process without using photo-thermal thin lens approximation. It is shown that this model is in good agreement with the experimental results and large enhancement factors are measured for values of nonlinear thermal refractive index and thermo-optical coefficients of the colloids at higher gold nanoparticle concentration compared to the measurement of castor oil.
In nanoparticle colloidal systems, the thermal nonlinearity is affected by the thermal parameters of the surrounding solution. Having a low temperature gradient rate solution may be a key factor in producing high thermal nonlinear properties in colloids. In this manuscript, the effect of the thermal conductivity of the surrounding liquid environment on the thermal nonlinear refraction of gold nanoparticles (AuNPs) synthesized by laser ablation of a gold target in different solutions is investigated. Gold nanoparticles colloids have been fabricated by the nanosecond pulsed laser ablation of a pure gold plate in different liquid environments with a thermal conductivity range of 0.14-0.60 W mK −1 including cyclohexanone, castor oil, dimethyl sulfoxide, ethylene glycol, glycerin and water. The AuNPs colloids exhibit a UV-Vis absorption spectrum with a surface plasmon absorption peak at about 540 ± 20 nm. The thermal nonlinear optical responses of the gold colloids are measured using the Z-scan technique under low power CW laser irradiation at 532 nm near the surface plasmon peak of the nanoparticles. Our results show that the nonlinear refractive index of the nanoparticle colloids is considerably affected by the thermal conductivity of liquid medium. The largest nonlinear refractive index of −3.1 × 10 -7 cm 2 W −1 is obtained for AuNP in cyclohexanone with the lowest thermal conductivity of 0.14 W mK −1 whereas the lowest one of −0.1 × 10 −7 cm 2 W −1 is obtained for AuNP in water with the highest thermal conductivity of 0.60 W mK −1 . This study shows that the nonlinear refractive index value of colloids can be controlled by the thermal conductivity of the used liquid's environment. This allows us to design low threshold optical limiters by choosing a solution with low thermal conductivity for colloidal nanoparticles.
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