Microlasers for Terahertz Radiation

 Array of micro fabricated lasers on a single chip with very low power requirements (Source: MIT)

Array of micro fabricated lasers on a single chip with very low power requirements (Source: MIT)

Terahertz radiation has promising appli­cations in security and medical diag­nostics, but such devices will require the development of compact, low-power, high-quality terahertz lasers. Researchers at MIT and Sandia National Labora­tories describe a new way to build tera­hertz lasers that could signifi­cantly reduce their power con­sumption and size, while also enabling them to emit tighter beams, a crucial requirement for most practical applications.

The work also represents a fundamen­tally new approach to laser design, which could have rami­fications for visible-light lasers as well. The researchers’ device is an array of 37 micro­fabricated lasers on a single chip. Its power require­ments are so low because the radiation emitted by all of the lasers is phase locked, meaning that the troughs and crests of its waves are perfectly aligned. The device represents a funda­mentally new way to phase-lock arrays of lasers.

The researchers identified four previous phase-locking techniques, but all have drawbacks at the micro­scale. Some require posi­tioning photonic components so closely together that they’d be difficult to manu­facture. Others require additional off-chip photonic components that would have to be precisely positioned relative to the lasers. Hu and his colleagues’ arrays, by contrast, are mono­lithic, meaning they’re etched entirely from a single block of material.

“This whole work is inspired by antenna engi­neering technology,” says Qing Hu, a professor of electrical engineering and computer science at MIT, whose group led the new work. “We’re working on lasers, and usually people compart­mentalize that as photonics. And microwave engineering is really a different community, and they have a very different mindset. We really were inspired by microwave-engineer technology in a very thoughtful way and achieved something that is totally concep­tually new.”

The researchers’ laser array is based on the same principle that underlies broadcast TV and radio. An electrical current passing through a radio antenna produces an electro­magnetic field, and the electro­magnetic field induces a corres­ponding current in nearby antennas. In Hu and his colleagues’ array, each laser generates an electro­magnetic field that induces a current in the lasers around it, which syn­chronizes the phase of the radiation they emit.

This approach exploits what had previously been seen as a drawback in small lasers. Chip-scale lasers have been an active area of research for decades, for potential appli­cations in chip-to-chip communi­cation inside computers and in environ­mental and biochemical sensing. But as the dimensions of a laser shrink, the radiation the laser emits becomes more diffuse. “This is nothing like a laser-beam pointer,” Hu explains. “It really radiates everywhere, like a tiny antenna.”

If a chip-scale laser is intended to emit radiation in one direction, then any radiation it emits in lateral directions is wasted and increases its power consumption. But Hu and his colleagues’ design recaptures that laterally emitted radiation. In fact, the more emitters they add to their array, the more laterally emitted radiation is recaptured, lowering the power threshold at which the array will produce laser light. And because the laterally emitted radiation can travel long distances, similar benefits should accrue as the arrays grow even larger.

In large part, the energy from the recaptured lateral radiation is re-emitted in the direction perpen­dicular to the array. So the beam emitted by the array is much tighter than that emitted by other experi­mental chip-scale lasers. And a tight beam is essential for most envisioned appli­cations of terahertz radiation. In security applications, for instance, terahertz radia­tion would be directed at a chemical sample, which would absorb some frequencies more than others, producing a charac­teristic absorption finger­print. The tighter the beam, the more radiation reaches both the sample and, sub­sequently, a detector, yielding a clearer signal. (Source: MIT)

Reference: T. Kao et al.: Phase-locked laser arrays through global antenna mutual coupling, Nat. Phot., online 13 June 2016, DOI: 10.1038/nphoton.2016.104

Links: Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, USA • Center of Integrated Nanotechnologies, Sandia National Laboratories, Albuquerque, New Mexico, USA

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