16 February 2017

Walter Schottky Institute demonstrates first electrically pumped, single-mode, tunable VCSELs emitting cw up to 4µm

A group of researchers at the Walter Schottky Institute at Germany's Technische Universität München (TUM) has developed a buried tunnel junction VCSEL with a single-stage type-II active region to extend the wavelength coverage of electrically pumped vertical-cavity surface-emitting lasers (VCSELs).

One of the main applications of VCSELs is in gas sensing. Gases each have a unique set of energies they can absorb, derived from their molecular structure. These sets of absorption lines are akin to fingerprints, which enables unambiguous and sensitive detection with a suitable tunable laser such as a tunable VCSEL.

There are several important gases that are detectable with mid-infrared (mid-IR) light with wavelengths of 3-4µm, including methane, carbon dioxide and nitrogen dioxide. Application-grade VCSELs, however, aren't yet available for this wavelength range, but the increasing need for compact, portable and affordable gas sensors is spurring demand for energy-efficient semiconductor sources of mid-IR light.

Addressing this demand, the team of researchers set out to develop a concept to extend the wavelength coverage of VCSELs into this important regime (Ganpath Kumar Veerabathran et al, 'Room-temperature vertical-cavity surface-emitting lasers at 4µm with GaSb-based type-II quantum wells', Appl. Phys. Lett. 110, 071104 (2017)).

Typical VCSELs suffer in performance for the relatively long wavelengths of the mid-IR range, in part due to side effects of heating that disproportionally affect IR wavelengths. However, these effects are minimized by the buried tunnel junction configuration of the VCSELs, where a material barrier is embedded between the standard p- and n-type materials. This structuring results in resistance-like behavior for the device and provides tunability of the optical properties in the desired range.

"The buried tunnel junction VCSEL concept has already yielded high-performance VCSELs within the entire 1.3-3µm wavelength range," says Ganpath K. Veerabathran, a doctoral student at the Walter Schottky Institute. "And so-called type-II 'W' quantum well active regions have been used successfully to make conventional edge-emitting semiconductor lasers with excellent performance within the 3-6µm wavelength range."

By combining the tunnel-junction VCSEL concept with these conventional edge-emitting laser designs, where the beam is emitted in parallel with the bottom surface, in this wavelength regime, the researchers created a buried tunnel junction VCSEL with a single-stage, type-II material active region to extend the wavelength coverage of electrically pumped VCSELs.

The development is reckoned to be the first known demonstration of electrically pumped, single-mode, tunable VCSELs emitting continuous wave up to 4µm. "It marks a significant step from state-of-the-art devices emitting at 3µm in a continuous wave, and up to 3.4µm in pulsed mode, respectively," says Veerabathran. "Further, our demonstration at 4µm paves the way for application-grade VCSELs within the entire 3-4µm wavelength range, because the performance of these VCSELs generally improves at shorter wavelengths."

Although gas-sensing systems within this wavelength range are already available using other types of lasers, they are considered to be power hogs compared to VCSELs, note the researchers. They also tend to be cost-prohibitive, and are mainly used by industries to detect trace gases for safety and monitoring applications, they add.

"The 4µm VCSEL demonstrates that low-power, battery-operated, portable and inexpensive sensing systems are within reach," Veerabathran says. "Once sensing systems become more affordable, there is great potential for deployment by industries such as the auto industry for emission monitoring and control, and these systems may even find uses within our homes."

Next, the group will focus on making improvements "in terms of the maximum operation temperature and optical output power of the VCSELs," Veerabathran says. "In the future, it may be possible to extend this concept to make VCSELs emit further into the mid-infrared region beyond 4µm. This would be beneficial because the absorption strength of gases typically becomes orders of magnitude stronger, even for relatively small wavelength increases," he adds.

Tags: VCSELs

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