Researchers Develop Mid-IR Room-Temperature Laser
WASHINGTON -- Researchers from the US Naval Research Laboratory and from Sarnoff Laboratories and Sensors Unlimited, both of Princeton, N.J., have developed what they say is the world's first room-temperature interband III-V laser diode that emits at a wavelength greater than 3 µm. The diodes are constructed of 10 "W"-shaped quantum wells grown on a GaSb substrate. At an operating temperature of 300 K, the diodes can emit 1-µs pulses of 3.3-µm radiation at a repetition rate of 200 Hz. Instrumentation based on mid-infrared lasers is more sensitive to trace chemical amounts than shorter-wavelength devices. Room-temperature mid-infrared laser diodes will make sensitive chemical detectors portable, convenient and inexpensive.
Researchers have developed a "W"-shaped quantum-well structure for a room-temperature III-V semiconductor laser that enables relatively high temperature operation. The W structure in the electron and hole energy bands confines the electrons and holes, and creates a large wave function overlap. The team built several W wells within an AlGaAsSb broadband waveguide.
To be useful for chemical analysis instrumentation, the diodes must operate continuous wave (CW). Jerry Meyer, the research lab's principal investigator, expects thermoelectrically cooled CW operation within a year, and he is confident that the next development steps will bring further improvements. "We've already operated [continuous wave] at 195 K, higher than any previous III-V semiconductor laser. ... Our devices are nowhere near being fully optimized yet."
The W structure that Meyer's team has constructed consists of a central GaInSb hole well-sandwiched between two InAs electron wells, which are in turn bracketed by two high AlGaAsSb barriers. The conduction band potential profile has a W shape, which is what gives the quantum well its name.
High temp operation
The team has built devices with five and 10 neighboring W quantum wells, which are then confined within an AlGaAsSb structure that forms a broadband waveguide. The unique structure allows excellent electrical confinement and good electron-hole wave function overlap.
These characteristics result in a relatively low threshold current, which in turn makes possible relatively high temperature operation. The team has demonstrated CW semiconductor laser output of 3.6 mW at 195 K. Meyer feels that CW operating temperatures of at least 230 K must be reached to avoid the need for cryogenic cooling in chemical analyzers.
The utility of such laser sources has motivated research for decades. Lead-salt lasers have approached this temperature, but their low output power limits their utility. Another contender is the intersubband quantum cascade laser, which has demonstrated single-mode pulsed operation above room temperature but has not gone above 175 K for CW. Pulsed operation of the first 3.1-µm InAs diode was demonstrated at 77 K in 1963. This work has pushed that temperature up to 310 K, but it has taken 36 years.
"This is still a work-in-progress," said Meyer, "but we have the CW goals in sight."
If the research team's confidence is justified, the W-well devices may be the heart of a new generation of portable pollution monitors, medical diagnostics and chemical weapons site detectors.
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