Researchers at Bell Labs, Murray Hill, N.J., have demonstrated the first electrically powered organic laser using a semiconductor called tetracene. Optically excited lasing and amplified spontaneous emissions have been observed earlier in semiconducting polymers and organic single crystals, but this work underscores the use of charge-injection techniques to drive a laser derived from a flexible material. The results indicate that high-quality single crystals hold promise for injection lasers based on organic semiconductors.
"Previously, researchers in the laser community thought organic materials would never be able to carry the large current necessary for electrically driven plastic lasers," says Bertram Batlogg, director of Bell Labs' solid-state physics research department. Tetracene, though, is among the purest organic semiconductors. Its molecule's four connected benzene rings conduct electricity well, providing the electrical properties needed for an organic solid-state laser. "The tetracene crystal remains transparent just before the intense beams of light are formed, so it absorbs very little light, which enhances the lasing effect."
Organic materials are cheaper than the most commonly used semiconductor materials in today's lasers, like gallium arsenide. So, the scientists feel it may be possible to substantially decrease individual laser costs. Engineers may be able to design several hundred lasers into a machine, such as an optical storage device or laser printer, for the price of one. This would allow more rapid access or display of data.
The demonstration opens up a whole new set of possibilities for electrically driven lasers, notes Federico Capasso, vice president for physical research at Bell Labs. "They not only would be inexpensive to manufacture, but they could be tailor-made to produce a wide range of wavelengths, each of which could have specific applications. They can be driven by today's silicon circuitry and may someday be combined with plastic transistors, which would further reduce production costs and potentially lead to lightweight, flexible products." Since the current configuration of the plastic laser operates at visible wavelengths, though, it isn't suitable for optical communications.
An electrically driven laser is strongly influenced by the transport properties of the semiconductor. So the team chose tetracene, as it exhibits high mobilities for electrons as well as holes. Researchers say that mobilities on the order of 2 cm2/VS can be achieved in tetracene at room temperature—over four orders of magnitude higher than mobilities in materials used in organic LEDs. Plus, tetracene offers higher photoluminescence quantum yield and electroluminescence.
Single-crystal tetracene is grown through vapor-phase techniques in a stream of inert gas. Field-effect device structures are then built on freshly cleaved crystal surfaces. Source and drain electrodes, a gate dielectric layer, and gate electrodes are deposited on the top and bottom surface of the single crystal (Fig. 1). The stucture's FET channel length is 25 µm, with several hundred microns of channel width. An amorphous aluminum-oxide (Al2O3) dielectric layer is sputtered onto the crystal between the gate electrode and the crystal. Also, this design incorporates a transparent aluminum-doped zinc-oxide gate electrode.
Electrons and holes are injected by applying a positive (bottom) and negative (top) source-gate voltage, respectively. In this process, researchers apply approximately 5 V across the crystal between drain 1 and source 2, as well as between source 1 and drain 2. Next, researchers apply up to 50 V as the gate voltage to achieve carrier densities of about 1013 A/cm2.
While the source-gate voltage is modulated between 2 and 50 V, the potential difference across the organic crystal is held at 5 V. As a result, electrons and holes flow to the top and bottom of the tetracene/Al2O3 interface, producing a net current across the device (Fig. 2). Subsequently, with the injection of electrons and holes, excitons are formed and light is emitted. For laser action, reflections at cleaved edges of the crystal provide feedback, resulting in a Fabry-Perot resonator. The light emitted here is yellow-green with a 576-nm wavelength.
For a current density of 1000 A/cm2, the researchers obtained an optical power of about 10 µW at room temperature, corresponding to a conversion efficiency of only a few percent. Increasing the density to about 1600 A/cm2 doubles the output optical power to 20 µW. A pulsed excitation of 10 µs and 100 Hz was used. For a similar excitation at 5K, the team realized a substantially higher optical power. The source-gate voltage modulation in this arrangement affects the changes in the intensity as well as the gain narrowing of the emission spectra.
Since carrier mobility is much higher at lower temperatures, the output power is greater. The transition from amplified spontaneous emission to laser action occurs at a lower current density. Consequently, this transition takes place at a 200-A/cm2 current density at 5K for the organic injection laser, versus 500 A/cm2 at room temperature for the same device. Increasing the injected current density above a certain threshold value narrows the emitted spectral width.
The team intends to optimize the thickness of the crystal and FET dimensions, along with injection parameters. Furthermore, it will employ a low-loss waveguide. This also will incorporate a high-Q cavity resonator to facilitate true room-temperature CW-laser action by reducing the threshold. A commercial device, however, is several years away.
Batlogg's team included Christian Kloc, Hendrik Schon, and Ananth Dodabalapur. Their results were reported in the July 28 issue of Science magazine. For details, go to www.bell-labs.com.