Mid-IR VCSEL Operates at Room Temperature
Michael D. Wheeler
An optically pumped vertical-cavity surface-emitting laser (VCSEL) that emits in the mid-IR at above room temperature in pulsed mode could have far-reaching implications for chemical analysis and atmospheric pollution monitoring.
Nearly all gas molecules, including a number of important pollutants, have their strongest absorption in the mid-infrared portion of the spectrum. Therefore, the sensitivity of gas detection using mid-IR radiation is several magnitudes higher than it is using near-IR. No semiconductor laser emitting in the mid-IR, however, comes close to the performance or the stability of its near-IR counterparts.
Researchers have demonstrated an optically pumped vertical-cavity surface-emitting laser that operates in the mid-IR at room temperature. The laser source may find application in the sensing of atmospheric pollutants. Courtesy of Frank Zhao, University of Oklahoma.
Now a team at the University of Oklahoma in Norman is bridging the gap. Zhisheng Shi and his colleagues have demonstrated an optically pumped multiple-quantum-well VCSEL emitting at 4.12 µm with a room-temperature gain peak. By applying the vertical-cavity structure to lead-salt materials, they reduced the laser threshold, improved the heat dissipation and overcame problems such as mode-hopping and multimode operation. And, compared with previous efforts, the gain peak and the cavity mode of the VCSEL are closely matched at 300 K.
This is important because the thickness of the active semiconductor materials determines the possible wavelengths of the laser emission. When the thickness is right, Shi explained, only one cavity mode is allowed, resulting in the emission of a single wavelength. At the same time, a semiconductor material that favors emission at a specific wavelength is said to have a "gain peak" at a specific temperature.
The researchers carefully controlled the thickness of the active semiconductor material in the stack, which they grew by molecular beam epitaxy, so that the VCSEL operated at peak power at room temperature. To enable the pump radiation to reach the active region between the two mirrors, they engineered the lead-salt materials to be transparent to 1-µm radiation from the pump source. In the experiments, an Nd:YAG pumped the VCSEL, but because high-power semiconductor lasers emitting at that wavelength are commercially available, the work promises a compact diode-pumped mid-IR laser system in the not-too-distant future.
Such a system still is most likely two to three years away, but Shi said that the work could usher in a number of improvements for mid-IR lasers, such as continuous-wave operation, spectral purity and superior beam quality. In the meantime, the team will refine the VCSEL structure to reduce threshold and to eliminate transverse modes.
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