Tunable Random Lasers Based On Plasmonic Feedback

In 1968,Letokhov first reported in theory that light amplification by stimulated emission of radiation can be achieved by scattering feedback in a disordered system without reflecting elements.In 1994,Lawandy first observed laser-like emission in a strongly scattered gain media in experiment.The light amplification of stimulated emission of radiation based on multiple light scattering in a disordered gain medium is defined as a random laser.Since then,random lasers have greatly attracted the attention of scientists as a new type of laser source.Random lasers,characterized by unique spatial distribution,random frequency of emission light,simple structure,flexible design and low cost,have shown the potential applications in speckle free imaging,information technology,sensing illumination and integrated devices.The tunablity of random laser is the pivotal part to determine its practical application.In recent years,various methods have been explored to tune the performances of random lasers,mainly including control of the property and concentration of gain medium,control of the structure and distribution of scatterer,modulation of the pump light,and introduction of the new structures,such as microcavity.Based on these methods,the performances of random lasers have been regulated effectively,such as threshold,wavelength,linewidth,emission direction,and output modes.However,some disadvantages of the current regulation methods show up,such as irreversible changes to the sample,small tunable scope,high cost,inflexible operation,and relatively limited regulation results,impeding the applied diversity of random lasers in fields of imaging and sensing.The strong localized field enhancement can be observed due to the surface plasmonic effect of metal nanoparticles,which can be used to promote the generation and optimization of random lasers.In this dissertation,novel random lasers based on plasmonic feedback are designed using new methods.The random lasing performances are effectively tuned,facilitating the application of random lasers.The main contents and innovations are as follows:1)A flexible random laser is achieved in a gain membrane attached on silver nanoflowers.The surface plasmonic effect of silver nanoflowers and the light confinement of the gain waveguide are main factors for the generation of low-threshold random lasing action.By bending the flexible substrate,the lasing wavelength can be tuned.The one-step solution-phase synthesis of the silver nanoflowers is fast and facile,enabling easy realization of this type of random lasers.The flexible and high-efficiency random lasers would facilitate the development of illumination and display devices.2)A random laser based on wedge-shaped microresonator is fabricated by simple press-coating method.The sample is composed of a dye-doped polymer film embedding disordered silver nanowires.A coherent random lasing action is observed attributed to the localized plasmonic enhancement and the scattering effect of silver nanowires in the wedge-shaped microresonator.The emission wavelength can be tuned continuously from 561 nm to 578 nm by changing the excitation position along the thickness gradient of the wedge shaped microresonator.What's more,as the thickness increased,higher threshold is obtained.The red-shift of lasing wavelength is resulted from the reabsorption of R6 G.The increase of threshold is caused by the loss of the in-active part in the thick region.Our proposed sample can be used for wavelength tuning and mode selecting.A multi-wavelength random laser based on wedge shaped microresonator provides a novel design for random laser,which can promote the diversity of lighting source.3)We report a dual-color plasmonic random laser in a film with the thickness of400 nm.The film is fabricated by binary quantum dots doped with gold nanorods.The ratio of gold nanorods can influence the luminescence lifetime and emission efficiency of gain media.Under optical pumping,we observe the low-threshold random lasing.Green-red random lasing at is effectively enhanced by the transversal and longitudinal surface plasmon resonance of the gold nanorods,respectively.Using the proposed random laser as light source,we achieve the speckle-free dual-color imaging.These properties can provide favorable factors for the development of random lasers in fields of imaging and illumination.4)Here,a flexible and wearable plasmonic random laser is designed.The sample is composed with three layers,including a flexible substrate layer,a humidity-sensitive layer,and a gain layer.The sample is fabricated by transferring a free-standing light-emitting polymer film doped with Ag nanoparticles on the PE/PEDOT: PSS composite films.Under optical excitation,random lasing with low-threshold is achieved due to the strong surface plasmonic effect of the Ag nanoparticles.The emission intensity can be effectively tuned by changing the humidity around the sample.Comparing with the emission intensity at initial RH(34.8%),the peak intensity increases 5.5 times at a RH value of 93.5%.These experiment results can be attributed to the relative refractive index difference between polymer membrane and PEDOT: PSS film at different RH values.Further,the proposed random laser is used as humidity sensor based on emission intensity of random lasing.The emission intensity increases with the increase of RH value,exhibiting a good linear response with a coefficient of 0.997.In our experiment,the PE film used as substrate is nontoxic and flexible,which can be transplanted easily.The proposed random laser is transplanted successfully to fruit slice and washable mask,exhibiting good humidity response.This study will be conducive to the application of random laser in fields of flexible wearable devices and sensing.