Researchers have presented a promising way to obtain inexpensive distributed feedback (DFB) thin-film based lasers that are tunable over the entire visible spectrum. Typically, the pumping of these organic semiconductor lasers requires rather bulky and expensive pump lasers. However, scientists from three cooperating German institutions have found that the emission wavelength range of DFBs can be controlled by the film thickness of the organic semiconductor in the active medium, permitting lower lasing threshold and thus small, less expensive compact pump lasers.1
The high efficiency and broad-wavelength range of organic semiconductor lasers has prompted activity on optically and electrically pumped organic lasers. Because of the low lasing threshold and broad gain spectra, such lasers are promising for many applications. A team of researchers at Ludwig-Maximilians-Universität (Munich, Germany), Universität Bayreuth (Beyreuth, Germany), and Fraunhofer Institut für Solare Energiesysteme (Freiburg, Germany) has created a very compact all-solid-state pump-laser source to take better advantage of the low laser threshold of organic semiconductor lasers. Such compact pump lasers have the potential to replace pumping by frequency doubled or frequency tripled Nd:YAG lasers, N2 lasers, and femtosecond laser systems.
The chosen film for the active medium was tris (8-hydroxyquinoline) aluminum (Alq) doped with the laser dye 4-dicyanmethylene-2-methyl-6-4H-pyran (DCM). The Alq:DCM system, say the researchers, is one of the most promising materials for organic light-emitting diode applications. The system actively funnels neutral excitations from Alq to the DCM molecules by diffusion and a subsequent energy transfer. The team used a flexible sheet of polyethylene terephtalate (PET) for substrates, and DFB gratings with a periodicity of 400 nm. The resulting organic semiconductor film forms a waveguide because of a higher refractive index than the surrounding air or the substrate.
The experiment involved fabricating several DFB lasers with varying thicknesses of the Alq:DCM film, from df = 120 to 435 nm. For a 120-nm-thick film, the laser wavelength was 604 nm. For a 435-nm-thick film, the laser wavelength was 648 nm. The experimental data correlated closely with the calculated curve thicknesses above 300 nm, but was slightly higher for the thinnest films of 100 to 200 nm thick. The researchers think this may be due to a perturbation in the waveguide structure, or by a wavelength-dependent refractive index.
The low lasing threshold of the devices enables a subnanosecond pulse pump laser with pulse energy of about 100 nJ at 355 nm and 10.8-kHz repetition rate. Because no focusing optics are required, the entire system is only about 10 cm long. Continuous tuning over the entire gain spectrum might be possible on a single substrate, say the researchers, with use of a tapered grating with spatially varying periodicity and/or a graduated film thickness.
- S. Riechel et al., Optics. Lett., 26, 593, 2001.