Semiconducting polymer distributed feedback lasers

We have made polymer distributed feedback (DFB) lasers by spin casting films of poly(2-butyl, 5-(2´-ethyl-hexyl)-1,4-phenylene vinylene), BuEH-PPV, onto gratings in SiO₂ cladding films on silicon substrates. The gratings had periods of 170-185 nm and depths of up to 30 nm and were made by holographic lithography and reactive ion etching. The lasers were photopumped by 10 nsec pulses of 435 nm light from a doubled Nd:YAG laser equipped with a hydrogen Raman cell. Above a threshold of 60 W/cm², a narrow line emission from DFB lasing was detected at 563 nm, which is the wavelength where Bragg reflections were expected for this laser. Above a threshold of 2200 W/cm², a broader peak characteristic of amplified spontaneous emission (ASE) was detected at 547 nm. When other lasers with different grating periods were studied, similar behavior was observed with the DFB peak always occurring at the wavelength predicted by the Bragg equation. We studied the competition between distributed feedback lasing and amplified spontaneous emission. One of the limitations of these initial semiconducting polymer DFB lasers is that the feedback is so strong that light at the Bragg wavelength cannot travel much further than approximately 10 μm before it is reflected. We will present our current work on making waveguide structures with weaker feedback by introducing an intermediate cladding layer between the grating and gain polymer and on distributed Bragg reflector (DBR) lasers, which have a smooth gain region with short grating reflectors at each end. We will also present work which is in progress for improving the quality of the waveguides