Random Laser Behavior Of Quantum Dots Doped With Metal Nanoparticles

The optical feedback of a random laser is provided by multiple scattering in a disordered gain medium.The random laser does not require an additional optical cavity,so it has the advantages of small size,low cost,and simple structure.Semiconductor quantum dots exhibit strong quantum confinement effects due to their ultra-small size.As an optical gain medium,quantum dots have a wide excitation wavelength and high quantum yield,thus the random laser emission with low threshold,high color purity and good environmental stability can achieve.Metal nanoparticles(NP)generate a strong local electric field around the nanoparticles under the excitation of an electromagnetic field.The enhanced local electromagnetic field can greatly increase the photon state density near the metal nanoparticles,not only increasing the excitation rate of the quantum dots,but also increasing the radiation decay rate of the quantum dots.In this paper,based on the local surface plasmon resonance effect(LSPR)of Ag NPs,the random laser emission characteristics of CdSe/ZnS colloidal quantum dots(CQDs)have been investigated.In the random laser experiment of quantum dot films doped with Ag nanoparticles,optical amplification is obtained by a higher volume fraction of the film.The waveguide confinement effect of the thin film structure and random cracks on the surface of the film provide optical feedback for the random laser.In the pure CdSe/ZnS quantum dot films,only amplified spontaneous emission(ASE)with a full width at half maximum of about 4-9 nm can be observed in the spectrum.After doping with Ag NPs,sharp laser peak with a full width at half maximum of less than 0.7 nm appears,and the threshold of laser emission is about 2.22 mJ/cm2.The exit wavelength of the random laser can be adjusted by pumping different positions.In addition,the doping concentration of Ag NPs was changed,and the influence of the concentration of different Ag NPs on the random laser emission intensity and threshold was investigated.In the hollow fiber structure,based on the high refractive index of the quantum dot and the total internal reflection characteristics of the hollow fiber,the random laser emission characteristics of the hollow optical fiber loaded with the CdSe/ZnS QDs solution are studied.Firstly,the CdSe/ZnS quantum dot solution was filled into the hollow optical fiber modified with Ag NPs,and the coherent random laser emission was observed with a threshold of 1.66 mJ/cm2.Moreover,in close-packed optical fiber,self-assembled quantum dot clusters provide optical gain and optical feedback for the random lasers,and the threshold of the random laser is reduced to 0.76 mJ/cm2.By changing the pumping manner,it was found that the random laser emission intensity increased under the stripe beam pumping,but the threshold also increased to 1.14 mJ/cm2.Finally,the direction of random laser emission was recorded in two different pumping manners.The experimental results show that the random fiber laser based on quantum dots has great application prospects in the field of display and sensing.Ag@SiO2-QDs nanoparticles were also prepared and the effect of Ag@SiO2 core-shell nanoparticles on the random laser emission characteristics of quantum dots was studied.The Ag@SiO2 core-shell nanoparticles have a certain enhancement to the absorption and fluorescence spectra of QDs.In the Ag@SiO2-QDs film,the coherent random laser occurs when the pump intensity reaches 1.18 mJ/cm2.When the concentration of Ag@SiO2 was 12 mg/mL,the lowest threshold was obtained.Furthermore,the random lasers based on Ag@SiO2-QDs films also exhibit good spectral stability and optical stability.In order to investigate the role of Ag NPs,the random laser from the SiO2-QDs films was studied.The results show that the local surface plasmon resonance effect of Ag NPs can effectively reduce the threshold of laser emission.Finally,using the quasi-one-dimensional waveguide limiting effect of the hollow optical fiber,the random laser emission spectrum becomes sharper and the threshold intensity is also reduced.