Organic Semiconductor Lasers Offer Novel Properties

By: Yvonne Carts-Powell

Last May, researchers from Princeton University (Princeton, NJ) demonstrated the first organic semiconductor laser at the Conference on Lasers and Electro- Optics (CLEO '97, Baltimore, MD).¹ The laser was as notable for its output properties as for the novel gain medium: When optically pumped by an ultraviolet (UV) nitrogen laser, the device emitted up to 50 W in a bright red beam at 645 nm, with each lasing mode having a linewidth of less than 0.1 nm. In the ensuing months, Vladimir Kozlov and his coworkers have done further design development on the original laser material, as well as developing lasers from other conductive dye/host systems with light emission at blue wavelengths (about 450 nm) through infrared wavelengths (about 700 nm).

The materials currently under investigation are low-molecular-weight organic semiconductors. In the past, solid-state dye lasers have used organic host materials, but these have been electrical insulators -- an important distinction. Using an organic conductor as the host material opens up the possibility of electrically-pumped, rather than optically-pumped, organic semiconductor lasers. For many applications, this property is necessary for commercialization.

Potential applications of electrically-pumped organic semiconductor lasers include a host of applications now served by inorganic diode lasers, such as in fiberoptic communication systems, laser printers, and optical scanners. The lasers could also fill a niche in potential applications that conventional devices have been unable to serve: Next-generation CD technology, for example, is waiting for a reliable blue diode laser. In addition, some properties of these organic lasers, such as the ability to use plastic substrates, could create new markets for the lasers.

Why organic?
Organic semiconductor lasers could offer some benefits over conventional (inorganic) diode lasers now in use. According to Kozlov, ease of integration and temperature independence are the most promising attributes of the organic lasers. In marked contrast to inorganic diode lasers, the differential quantum efficiency, output wavelength and power, and threshold pump energy of organic lasers developed to date are independent of temperature in the range from 0 to 140° C.²

Because inorganic diode lasers are crystalline materials grown on crystalline substrates using high-temperature epitaxy, they are rigid devices requiring substrates with matching crystal lattices, and the high-temperature processing makes them difficult to integrate with other electronics. In contrast, organic semiconductors can be grown as amorphous thin films using relatively inexpensive room-temperature vacuum deposition, which allows both integration with existing electronics and the use of a wide variety of substrates. As a proof of principle, the researchers created a laser from an amorphous film deposited on a plastic transparency sheet. Researcher Vladimir Bulovic´ says that the semiconductor conformed to the edges of the plastic sheet so well that it formed a surface flat enough to create an optical cavity without further processing.

The most mature of the inorganic semiconductor diode materials, gallium arsenide (GaAs), has emission wavelengths limited to the red and infrared. The obvious market potential for blue diode lasers in next-generation optical storage systems has fueled investigations into other inorganic semiconductor materials, but researchers have yet to produce a commercially viable blue diode laser.

In the organic laser, a variety of dyes and organic semiconductors could be used to span the visible spectrum. In the first material system, films of the organic semiconductor tris-(8-hydroxyquinoline) aluminum (Alq₃) are doped with DCM laser dye, creating a classic four-level laser system that emits orange or red wavelengths; the wavelength can be changed by altering the concentration of the dye. Kozlov and others at Princeton and University of Southern California (Los Angeles, CA) recently demonstrated devices based on films of a carbazole derivative doped with Coumarin 47, perylene, and Coumarine 30; the resultant devices lapsed at 460, 485, and 510 nm, respectively (see Figure 1).³

FIGURE 1:
Edge-emitting organic semiconductor laser based Alq₃:DCM emits at red wavelengths, while a version incorporating a carbazole derivative doped with a blue-emitting laser dye emits at blue wavelengths.

Photo courtesy of the Center for Photonics and Optoelectronic Materials, Princeton University

Vertical cavity organic lasers
The lasers mentioned above emit from the edge of the films, but the researchers are also working on organic vertical-cavity surface-emitting lasers (OVCSELs; see Figure 2).⁴ Although Kozlov's group has developed optically-pumped organic lasers with an edge-emitting double-heterostructure design, converting these to electrically-pumped devices may prove difficult. In this waveguided design, the electrical contacts would have to run the length of the guide. Because most organic materials have low carrier mobilities, the active layers would be restricted to layers of less than 200 nm, and absorption losses at the contact layers for such thin structures are sufficiently high to prevent lasing.

Using a vertical cavity design makes forming electrical contacts much easier, because they can also act as the top and bottom mirrors of the cavity. Bulovic, Kozlov, V. B. Khalfin, and Forrest developed a small vertical cavity device with a Fourier-transform-limited linewidth (comparable to inorganic semiconductor lasers with similar mirror reflectivities). As with the edge-emitting lasers, the OVCSELs have demonstrated lifetimes of more than 106 pump-laser pulses. This shows that the materials are robust, although more development will be needed to create reliable and usable devices.

FIGURE 2:
Vertical-cavity surface-emitting laser design uses a silver top mirror and emits through the quartz substrate. The bottom mirror of the optical cavity is formed by the distributed Bragg reflector (DBR). An ultraviolet laser pumps the Alq₃:DCM active region.

Excitation
Due to the mechanism for energy transfer between host and dye, the quantum efficiency of the organic lasers is surprisingly good. In this Förester energy transfer, the host material is pumped in its absorption band, and the energy transfers non-radiatively to the dye by a fast (about 1 ps) dipole transition. In the case of the Alq₃:DCM laser, the ultraviolet wavelengths of the nitrogen laser pump energy into the absorption bands of the Alq₃ host, which transfers energy to the DCM. The excited DCM dye molecules relax back to their ground state by emitting light at orange or red wavelengths. This part of the spectrum is far from the host material's absorption area, so the material is transparent to the DCM emission, an effect that lowers the lasing threshold.

The materials are small organic materials, rather than polymers. Although optically pumped conducting polymers have shown optical gain, there is controversy over whether the polymers are actually lasers. If polymer-based organic lasers are developed, they too should be easy to integrate with electronic devices and show temperature independence.

So far, all the organic semiconductor lasers have been optically pumped by another laser. Although the OVCSEL design eases the placement of the diodes, the devices will need much lower lasing thresholds for electrical pumping to be feasible. Assuming a pulsed peak pump current of 1 kA/cm², the lasing threshold will need to be near 10 µJ/cm²; the first OVCSELs made have a lasting threshold of 300 µJ/cm².

The organic lasers are being developed rapidly, however, with researchers pursuing both OVCSEL and edge-emitting designs. Kozlov says, "With luck we might do it [demonstrate electrical pumping] in a year or two." He adds, "There is still a question about whether light amplification exists in electrically pumped organic films. Once we prove that it does exist, the technological problems can be solved."

References
1. Vladimir G. Kozlov, Paul E. Burrows, and Stephen R. Forrest, "Low threshold, high-peak- power organic semiconductor lasers", CLEO 1997 Postdeadline paper CPD18.

2. Vladimir G. Kozlov, Vladimir Bulovic´, and Stephen R. Forrest, "Temperature independent performance of organic semiconductor lasers", Applied Physics Letters 71[18], p. 2575, (1997).

3. Vladimir G. Kozlov et. al., "Optically Pumped Blue Organic Semiconductor Lasers". Applied Physics Letters 72[2], p. 144, (1998).

4. Vladimir Bulovic´ et. al., "Transform-Limited, Narrow Linewidth Lasing Action in Organic Semiconductor Microcavities", Science 279, 23 January, 1998.

About the author
Yvonne Carts-Powell is a freelance science writer. She can be reached at 49 Gilbert Rd, Belmont, MA 02178. Phone: 617-484-9679; e-mail: vonnie@apocalypse.org.