Research On Performance Improvement Of DFB Organic Semiconductor Laser

Optically pumped organic semiconductor lasers have been investigated extensively due to their unique advantages, such as the supreme fall of lasing threshold, the broad of the gain spectrum, ease of processing from solution and low cost of organic materials. The distributed feedback (DFB) structure is considered to be the most beneficial to reduce the laser threshold. A variety of fabrication schemes have been demonstrated to construct the DFB cavities, such as nanoimprinted lithography, UV embossing, photoisomerization, etc. However most of the technologies have the disadvantages of complex fabrication process and poor parameter controllability, which limits the further applications of organic lasers.To solve these problems, one-dimensional (1-D) DFB lasers are fabricated by direct laser ablating the film of the conjugated polymer (2-methoxy-5-(2’-ethylhexyloxy)-1.4-phenylence vinylene)(MEH-PPV). We spin a thin film of MEH-PPV on a flat glass substrate and then two-beam interference is conducted on the film to ablate the top of MEH-PPV. The MEH-PPV film is400nm thick, and the grating period varied from360to390nm at intervals of10nm. The center wavelength of the laser emission shows redshift with the increasing of the grating period, i.e.,602.91nm,609.24nm,613.26nm,619.01nm. With the grating period of360nm,370nm,380nm, and390nm, respectively. We observe a laser threshold of182uJ.cm-2.pulse-1for the DFB polymer lasers with370nm of the grating period.In order to reduce the threshold of the1-D DFB laser, we evaporated a25nm silver film between the glass substrate and the organic light emitting layer of MEH-PPV, the device structure for Glass/Ag (25nm)/MEH-PPV (400nm). The one-dimensional DFB laser device with a silver film indicates the threshold of33μJ/cm2/pulse, the sharp peak centered at615nm with FWHM of-9nm. Compared with the DFB laser without the silver film, the lasing threshold is dramatically lowered by more than80%. The device with silver film shows a more complex field intensity distribution both into the metal interface and the MEH-PPV layer. Understanding of the lasing threshold lowing mechanism starts from analyzing the fields of the DFB modes by our in-house generated FDTD codes. The numerical simulations and experimental results demonstrate that the lowered threshold should be attributed to the hybrid WG-SPP mode induced field confinement and DFB enhancement.In order to further reduce the threshold of the1-DFB laser device, we fabricated a two-dimensional (2-D) grating structure to replace the1-D grating structure with grating period of370nm. The2-D DFB laser device with a silver film exhibits the threshold of6.3μJ/cm2/pulse, and the sharp peak centered at619nm with FWHM of-5run. Compared to taht of the1-D DFB laser with silver film, the lasing threshold is significantly reduced. The2-D DFB laser device with silver film have two feedback sections XY and XZ, there are the TE mode and TM mode in the device, the two mode can also excited surface plasmon. The1-D DFB lasers with silver film has one feedback section XY, and only TM mode is existed. The period was70nm. The threshold of the2-D DFB laser device with and without silver film were compared with each other, and the former is smaller, which is also attributed to the introduction of surface plasmon mode.In summary, we prepared1-D and2-D DFB lasers by one-step laser ablation of two interference beams. A hybrid WG-SPP mode originated from the coupling between the SPP and WG modes was formed in the DFB structure, which enhanced outcoupling of radiation and light outcoupling efficiency. These findings open a promising way to achieve electrically pumped organic laser.