Low threshold all-organic composite laser

Abstract

Light-emitting polymers (LEPs) and related organic materials are attractive for many applications including optoelectronics and photonics [1]. They exhibit high photoluminescence quantum yields and can be tailored to emit at wavelengths across the whole visible spectrum. Furthermore, thanks to their 'soft matter' nature they offer great processing flexibility and should enable a wide range of novel devices and applications. Consequently, research into photonic components based on LEPs is being keenly pursued. However, organic materials typically suffer from photo-oxidation which can result in an alteration of the conjugation length at the molecular scale and ultimately destroys their light-emitting properties. Encapsulation of the LEPs while maintaining their processability and attractive physical properties is therefore essential to fully leverage this technology. The idea of composites, where a light-emitting material is blended in a protective host matrix, has often been the chosen strategy and many distributed feedback (DFB) lasers based on this approach were reported [2]. However, their laser oscillation threshold often exceeds the thresholds of their "neat" counterparts by more than 10 to 100-fold. In this work, we report a composite based on the incorporation of LEP into a novel polymer matrix which shows low amplified spontaneous emission (ASE) threshold. A mechanically-flexible, surfaceemitting DFB laser with a threshold as low as those typically shown by organic DFB lasers made from neat materials is also demonstrated.The LEP used in our experiments was a π-conjugated polymer based on a poly(p-phenylene vinylene)-core (PPV), called BBEHP-PPV [3]. Fig. 1 gives the absorption and emission spectrum of the polymer diluted in toluene. The polymer matrix is a blend of monomer and initiator which we will be calling here BAPO. It is in a chloroform solution for processing. The BBEHP-PPV powder was directly diluted in a volume of the BAPO solution to achieve the desired load and ultrasonically mixed for 3 minutes. The amplification capability of the composite was demonstrated by pulse-excitation (5 ns pulses / 10 Hz repetition rate) at 355 nm of a 25mg/mL nanocomposite film spin coated on silica. The ASE threshold was found to be 7.3kW/cm 2 (36.5µJ/cm 2 ) (Fig. 2) and the emission spectrum was seen to narrow above such fluence from 30nm down to 5nm. The surface emission DFB laser was realised by first reproducing by soft lithography a 340nm-period silica master grating in a fully transparent polymer, namely 1,4-cyclohexyldimethanol divinyl ether, using a transparency as substrate. A 20mg/mL-concentrated composite was then spin-coated onto the flexible reproduced grating for a targeted thickness below 200nm. Fig. 2 shows the threshold behaviour of the vertical emission of the laser. The laser emits at 522nm with a threshold below 2kW/cm 2 (<10 µJ/cm 2 ). Insets of Fig. 2 show a typical emission spectrum of the DFB laser, with a linewidth below 0.2nm limited by the resolution of our spectrometer, and a picture of ...