External feedback assisted reduction of the lasing threshold of a continuous wave random laser in a dye doped polymer film and demonstration of speckle free imaging

ABSTRACT The applications of random laser (RL) are still restricted, due to its complexity, high lasing threshold, unstable nature, difficulty in tuning and modulating its light emission characteristics. However, here a low threshold, continuous wave (CW) RL system has been designed in a fluoresecent laser dye (namely DCM) -doped in a polymer film with ZnO micro-cabbages as scatterers. Interestingly, by putting the active medium inside an external feedback (ExFB) system, a narrowed and stable RL emission at ∼598 nm is observed with the subsequent reduction of CW lasing threshold (CWITh) by ∼2.4 times. The effective cavity length (Leff) has been varied by an ingenious approach to demonstrate the role of Leff over the RL emission characteristics and the experimental results are supported by a theoretical explanation. The statistical fluctuation in the position of generated modes over time and the role of pump volume amplification on the emission characteristics of the developed RL has been studied in detailed. Thus, it is found here that due to the presence of ExFB, reinjection of pump light into scattering-formed cavities has taken place which has led to the generation of stable RL modes with the reduction of CWITh. The temporal coherency of the ExFB based RL light is found to be ∼3.2 times less than that of a conventional He-Ne laser light. As a result, we could demonstrate here speckle free imaging with much lowered value of speckle contrast (C) on two different model systems, such as commercially available carbon film-coated TEM grid and cell membrane of onion (Allium cepa). Therefore, it is expected that this report will open up new possibilities for the development of stable and tunable RLs with various other laser dyes for various optoelectronic applications including in bio-imaging.