Organic vertically emitting lasers: toward low-cost, UV-visible sources
Organic solid-state lasers offer the potential of low-cost, compact, and broadly tunable coherent sources over the whole visible spectrum.1 They may find applications in bio/chemo-sensing, spectroscopy, and short-haul data communications. In the 1960s, solid-state dye lasers in the form of bulk dye-doped plastic blocks were developed to replace the inconvenient and toxic solutions used in liquid dye lasers. The recent advent of organic semiconductors and the subsequent boom of organic light-emitting diodes (OLEDs) led to a new idea: the organic semiconductor thin-film laser.
As soon as the laser medium was a thin film, it became extremely inexpensive, compact, and easy to make with simple solution processing techniques, unlike plastic blocks that need to be optically polished before use. Thin-film lasers also brought the promise that they could one day be electrically driven. However, in the face of a large number of challenges to realize electrical driving, researchers are turning to an indirect or hybrid approach that uses a compact light source (a diode laser or an LED) to optically pump the organic film.2
Most recent work deals with laser structures such as distributed feedback resonators or microcavities with low thresholds. They are thought to be prerequisites of the electrically driven organic laser diode. They have a monolithic design (no straightforward tunability, no room for inserting external elements), a generally modest efficiency (around 10%, not often reported), and a diverging or poorly defined output beam. Optically pumped organic lasers are therefore still at an intermediate step of development with impressive achievements, but lacking some key properties that would make them relevant to real-life, practical applications.
We recently demonstrated a new strategy that combines the advantages of the thin-film approach with the inherent benefits of bulk open cavities, that is, good efficiency, excellent beam quality, power-scaling capability, and high flexibility for wavelength control. To this end, we applied to organic thin-film lasers a well-known concept used with inorganic semiconductor lasers, the vertical external cavity. This led to the development of a vertical external cavity surface-emitting organic laser (VECSOL).3 It is composed of a plane mirror coated with a thin film of organic material, poly(methyl methacrylate), doped with a rhodamine dye, and a remote concave mirror closing the cavity (see Figure 1). The laser is pumped in the nanosecond regime by a green diode-pumped, solid-state laser operating at a 10Hz repetition rate. The macroscopic (up to several centimeters) cavity defines the spatial geometry of the laser mode and gives birth to a TEM00 diffraction-limited transverse profile. This end-pumped laser architecture enables optimal overlap between the pump and the laser mode to provide a record optical conversion efficiency for thin-film organic lasers (more than 55%) together with respectable output energies (>30μJ).4 Tunability is observed around a span of 40nm in the red using a Fabry-Pérot etalon effect related to the organic thin film's varying thickness. The laser wavelength can be tuned by a simple mechanical translation of the sample.
The open nature of the resonator, as well as the high peak powers obtained, also allowed us to demonstrate the emission of tunable deep UV radiation (down to 310nm) from an organic laser system. This is thanks to intracavity frequency doubling, which to our knowledge is the organic solid-state, thin-film laser with the lowest operating wavelength to date.5
The simplicity of our structure opens the way to application-ready organic lasers. The threshold is low enough to think of diode pumping, making them potential low-cost compact laser sources with full tunability over the visible and UV spectrum. The efficiency is very high, the output laser beam is perfectly Gaussian, and the output power can be scaled to the millijoule level. Our next step is to see how these laser sources can be useful for practical applications such as sensing or spectroscopy.
Sébastien Chénais received his PhD from Institut d'Optique Graduate School, Paris 11 University, in the solid-state laser group headed by Patrick Georges. Since 2003, he has been an assistant professor in the Laser Physics Laboratory of Paris 13 University, working on organic photonics, OLEDs, and organic lasers.