OPTOELECTRONIC MATERIALS: Laser action in plastic promises all-polymer devices

Nov. 1, 1996

A research group at Cambridge University's Cavendish Laboratory (Cambridge, England) has demonstrated laser-like behavior from a solid conjugated polymer, poly(p-phenylenevinylene), or PPV.

Bridget Marx

FIGURE 1. PPV, or solid conjugated polymer poly(p-phenylenevinylene), is contained as a 100-nm-thick film in a laser cavity comprising a distributed Bragg reflector (DBR)—designed to act as a high reflector across the visible wavelength range—and an output coupler formed of a 60-nm-thick film of silver. The chirped DBR provides a cavity length that is effectively longer for longer wavelengths, which allows several lasing modes (l1 - l3) to be supported in the microcavity.

A research group at Cambridge University’s Cavendish Laboratory (Cambridge, England) has demonstrated laser-like behavior from a solid conjugated polymer, poly(p-phenylenevinylene), or PPV. There are many potential applications for active optoelectronic devices made from polymers. The lower fabrication costs and ease of processing polymers—they can, for example, be patterned with techniques similar to those used for photoresist in the microelectronics industry—compared with silicon and other technologies has led to a drive to include them in optoelectronic integrated circuits. Thus, hybrid devices based on standard diode-laser and detector materials with polymer waveguides are already being evaluated. The Cambridge results open the way for all-polymer devices.

The new research follows the demonstration in 1990 of electroluminescence in PPV and measurements of optical gain in PPV using pump/probe techniques—the latter by the same Cavendish team last year. In the most recent experiments, the polymer was optically pumped by a regeneratively pumped Nd:YAG laser producing 200-ps pulses at 355 nm. The solid polymer film is contained in a high-Q microcavity in which a commercially available distributed Bragg reflector (DBR) is used as the bottom mirror of the cavity and is designed to act as a high reflector across the visible wavelength range.

The DBR consists of various stacks of alternating high and low refractive-index layers, each stack having different layer thicknesses—a chirped DBR (see Fig. 1). The cavity length is effectively longer for longer wavelengths because they are reflected from deeper in the stack. This allows several modes to be supported in the microcavity. A 100-nm-thick layer of a PPV precursor is spun on top of the DBR; PPV is insoluble and cannot be spin-cast from solution. Hence, a chemically altered precursor is used that reverts to PPV on heating. And, finally, a 60-nm-thick film of silver, forming the top mirror, is evaporated on top of the PPV. The Cavendish group works closely with Cambridge Display Technology (CDT; Cambridge, England), which supplied the PPV for the present work.

The device is excited by the UV pump light directed through the DBR—which has a 30% transmission at this wavelength—and the output light is collected through a collimating lens and adjustable aperture. The maximum incident pump energy is 5 µJ, focused to a spot size of 250 µm. The emission spectrum from the PPV is multipeaked at low excitation energies, switching to a spectrum dominated by the 545-nm mode at high excitation energies (see Fig. 2). This dramatic change of spectrum is the key indication that stimulated emission takes over from spontaneous emission at a threshold level and that the device is indeed lasing. The ratio of power in this mode to that in the rest of the spectrum as a function of input energy shows a threshold effect, as does the spectral width of the main mode. There is also an increase in the directionality of the output beam as the input energy increases. The measured mode linewidth is about 1 nm and is limited both by the spectrometer resolution and the dynamic shift of the laser mode wavelength occurring in pulsed mode.

FIGURE 2. Emission spectrum of optically pumped solid poly(p-phenylenevinylene) exhibits several peaks at low excitation energies, but at high power the spectrum is dominated by the mode at 545 nm.

Applications

Solid films of PPV such as those used in the Cambridge experiments are appropriate for direct electrical pumping. The demonstration of laser action from optical pumping encourages development of electrically pumped systems by answering some of the fundamental questions about the nature of the material. Previously, some researchers thought that this material would not lase because most of the excited states were expected to be nonemissive interchain polaron pairs, which contribute only to the absorption of any light produced. This new work directly supports the opposing view, that the main photoexcitation in high-quality PPV is the emissive intrachain exciton. While much work remains to be done on electrode design, it can proceed with the knowledge that PPV is a real contender for an electrically pumped polymer laser.

Nir Tessler of the Cavendish team said, "The present structure is relatively simple and easy to make. Any significant progress would require technological effort both in further improving the polymer and the laser structure. These aspects are at present being evaluated before taking the next step."