LED-Pumped Polymer Laser Emits at 568 nm

Breck Hitz

Semiconducting conjugated polymer materials are easy to fabricate into photonic devices, and their emission spans the visible spectrum. They would be attractive candidates for commercial lasers, but until now, they required pumping by another laser, an added complexity that was a barrier to further development.

Recently, however, researchers at the University of St. Andrews in the UK integrated a pump InGaN LED with a semiconducting polymer and observed visible lasing from the polymer.

Figure 1. The polymer laser is pumped by an InGaN LED. Courtesy of Ying Yang and Georgios Tsiminis, University of St. Andrews.

For the laser gain medium, they spin-coated a layer of semiconducting polymer that was several hundred nanometers thick onto a corrugated silica substrate. They added another polymer layer atop this structure to prevent contamination of the gain medium by water or oxygen. They then contacted the pump LED against the top of the assembly and positioned a dichroic mirror beneath it (Figure 2). The mirror served as both the output coupler (slightly less than 10 percent transmissive at 570 nm) and as a pump mirror to reflect the pump light for a second pass through the gain medium (98 percent reflective at 450 nm).

Figure 2. The LED pumped the semiconducting polymer ADS233YE. The fluorinatedpolymer CYTOP layer provided a transparent barrier to oxygen and water. Reprinted with permission of Applied Physics Letters.

First-order Bragg scattering from the corrugation on the silica substrate, which had a 355-nm period and 70-nm depth, provided horizontal distributed feedback for the laser resonator, while second-order scattering diffracted some of the circulating power to the dichroic-mirror output coupler. The gain medium itself was a thin high-index slab waveguide that supported the lowest-order transverse electric and magnetic modes, TE₀ and TM₀, respectively.

The scientists initially pumped the laser with an optical parametric oscillator rather than with the LED that is shown in Figure 2. They focused the 450-nm pump light to an ~1.7-mm spot on the top of the polymer and observed the narrow spectral peak indicating lasing at pump intensities above 200 W/cm² when the pump light made a single pass through the gain medium, or 150 W/cm² when it was reflected for a second pass.

The next step was to substitute the LED shown in Figure 2 for the optical parametric oscillator, but the commercial LED from Philips Lumileds of San Jose, Calif., could not supply the continuous 150 W/cm² necessary to achieve laser threshold. When the researchers operated the LED in a pulsed mode, however, they observed a sharp emission peak at 568 nm emerging from the background photoluminescence at peak pump currents of more than 140 A (Figure 3a).

Figure 3. The emergence from background photoluminescence of a narrow peak at 568 nm provided strong evidence that the semiconducting polymer laser had reached threshold (a). Further evidence of lasing was provided by the sharp elbow in the output power at 568 nm, corresponding to oscillation of the TE0 mode in the slab waveguide (b). On the other hand, the TM0 mode at 555 nm did not reach threshold under LED pumping. Reprinted with permission of Applied Physics Letters.

This emission peak, together with the sharp elbow in the output power (Figure 3b), were a clear indication of lasing in the resonator’s TE₀ mode. Although the researchers saw a secondary peak corresponding to lasing in the TM₀ mode at 555 nm when pumping with the optical parametric oscillator, no such lasing occurred with LED pumping, as evidenced in Figure 3b.

Applied Physics Letters, April 23, 2008,163306.