Manipulation Of The Threshold Of Distributed Feedback Polymer Lasers

Organic semiconductor materials are the best choice of gain materials for organic semiconductor lasers due to their easy film formation,broad range of fluorescence spectrum,covering all the visible range,excellent flexibility,and high pumping efficiency.Organic semiconductor lasers have significant application prospects in optical communication and medical sensing due to their small size,easy fabrication,low cost,sensitive to the environmental refractive index,and low thresholds.Organic semiconductor microcavity lasers have attracted great attention in the field of integrated and miniaturized lasers due to their simple preparation methods.In order to easily integrate,realize applications in multiple fields,and improve the pumping efficiency,the study on the threshold of the laser is a very important subject.How to achieve ultra-low threshold laseris always a problem.It is a common way to adjust the threshold by changing the gain material,and also by changing the cavity structure,such as introducing a metal structure,to produce a plasmon effect,which can achieve an ultra-low threshold laser.It is also an important research method to control the laser output threshold by adjusting the polarization and the size of the pump spot.Based on the output characteristics of organic semiconductor lasers,there are two methods to manipulate the threshold : one is changing the polarization and size of the pumping spot and another is to introduce metal in the microcavity to generate the plasmon effect:(1)The effect of pump size and polarization on laser performance.A DFB polymer laser was prepared by interference lithography.By adding an x-y slit in the light path,the size of the pump spot along the grating line and the direction of the vertical grating line can be changed,and the laser threshold under different sizes can be obtained.A half-wave plate is employed to change the polarization of the pump laser.When the size of the pump spot changes along the direction of the grid line,the threshold changes faster and the threshold value changes more.And as the polarization direction of the pump light changes from a direction parallel to the grating line to a direction perpendicular to the grating line,the laser threshold gradually increases.Finally,a simulation software is used to simulate the relationship between the output intensity and the pump spot length as the pump size changes alongthe y direction.With the increase of the size of the pump spot along the y direction,the increase of the number of excited grating lines causes an increase in the feedback in the cavity,thereby output intensity increase.When the size of the pump spot exceeds a certain length,the output intensity of the laser light increases significantly.When the polarization angle of the pump changes from 0° to 60°,there will be slight differences between the laser output intensities.(2)Effect of plasmon on the laser threshold.In this structure,the silver film is obliquely vapor-deposited on one side of the grating by thermal evaporation,thereby realizing an asymmetric cavity structure,metal-dielectric.Due to the introduction of the metal film,a plasmon effect is generated at the interface between the metal and the dielectric,and a low threshold laser is achieved.Simulations for the electric field distribution of the dielectric cavity and the metal-dielectric cavity verifies that the performance of the metal-dielectric cavity is superior.And with the increase of the thickness of the silver film,the lasing threshold has an obvious increase,which may be due to the increase of the absorption loss of the metal.And by increasing the thickness of the metal film,different output wavelengths are achieved.