In the Lehigh team’s “antenna feedback” approach, the plasmonic-laser cavity is enclosed between two metal films, with periodic slits on the top film. One SPP wave is confined inside the 10-micron-thick cavity; the other, with a larger spatial extent, is located on top of the cavity and coupled both to it and the far field, allowing a strong, narrow-beam emission. [Image: Sushil Kumar]

Lasers based on coherent surface plasmon polaritons (SPPs)—subwavelength oscillations of electrons that are excited when incident light hits a metal-dielectric interface—hold promise for ultraminiaturized, chip-scale optics, and also as a possible platform for terahertz quantum cascade lasers (QCLs). But there’s a catch: SPP lasers, precisely because of their subwavelength apertures, tend to have divergent radiation patterns, making it tough to produce a sharp, directional beam.

Now, a research team led by Sushil Kumar of Lehigh University, Penn., USA, has devised an “antenna feedback” scheme that reportedly can provide single-mode operation and strong, highly directional far-field coupling in such SPP lasers, bringing them “closer to practical applications” (Optica, doi:10.1364/OPTICA.3.000734). The team’s work includes a proof-of-concept terahertz QCL based on the scheme that, according to the study, achieved the narrowest beam yet reported for such a QCL.

The pros and cons of “spasers”

SPP lasers—also called plasmonic lasers or “spasers”—operate by confining light energy as coherent SPP oscillations in subwavelength cavities (commonly parallel-plate Fabry-Perot-type cavities in with a length greater than the subwavelength cavity width). Their subwavelength dimensions make these lasers intriguing for certain applications in integrated photonics and nanophotonics. Parallel-plate cavities with SPP modes are also used for terahertz QCLs (which have some interesting potential applications in biosensing and standoff detection of dangerous materials), as they can show low-threshold, high-temperature performance at those frequencies.

It turns out, however, that it’s difficult to extract light from the plasmonic energy trapped in the spaser cavity. And when light can be made to leak out, it tends to be low in power and highly divergent, which limits its usefulness in actual applications.

A plasmonic phased array?

Kumar’s team found a potential solution through a distributed-feedback approach that the team has dubbed “antenna feedback,” and that Kumar compares to the action of phased-array antennas in microwave communication systems. The team demonstrated by numerical modeling that a grating of slits on one side of the subwavelength Fabry-Perot resonator, spaced at a specific value, would allow a single SPP mode within the cavity to diffract outside of the cavity in the surrounding medium, through Bragg diffraction. The energy outside of the cavity builds up with positive feedback (again the result of the selection of the grating period).

As a result, a second intense SPP wave develops in the medium outside of the cavity that remains coupled to the cavity’s metal cladding but also can form a highly directional beam outside of it. “The narrow-beam emission,” the team writes, “is due in part to the cavity acting like an end-fire phased-array antenna at microwave frequencies.”

In a proof of concept, the team implemented the antenna-feedback scheme in a terahertz QCL, using a box-shaped cavity consisting of two 100-µm by 1400-µm metallic plates, separated by a distance of 10 µm. The researchers report that the resulting laser showed a beam divergence as small as 4 degrees by 4 degrees—“the narrowest beam reported for any terahertz QCL to date,” according to the study.

Applications in security and elsewhere

The researchers note that terahertz QCLs in particular have some interesting applications in security and standoff detection. At a recent innovation conference, they pointed out that “approximately 80 to 95 percent of explosives, and all commonly used ones, have unique and identifiable terahertz signatures.”

But, while their experiments focused particularly on terahertz QCLs, they stress that the antenna-feedback scheme should be applicable to plasmonic lasers of any operating wavelength that operate with Fabry-Perot cavities. That, in turn, could aid help make other applications of plasmonic lasers, in areas such as nanophotonics, more feasible, according to the scientists.