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Periodic Photonic Structures Focus Spaser Light for Nanoscale Optics

Photonics.com
Aug 2016
BETHLEHEM, Penn., August 4, 2016 — Single-mode operation in plasmonic lasers has been demonstrated using a technique that implements distributed feedback (DFB) in a novel way that couples the resonant surface plasmon polariton (SPP) mode of the laser to a highly directional far-field radiation pattern and integrates hybrid SPPs in the surrounding medium into the laser’s operation.

Researchers at Lehigh University have implemented DFB on a terahertz (THz) quantum cascade laser (QCL), a type of plasmonic laser that emits long-wavelength THz radiation. They have demonstrated single-mode THz QCLs with a beam divergence as small as 4°×4°, which may be the narrowest beam reported for any THz QCL to date.

In this illustration of a terahertz plasmonic laser, the laser cavity is enclosed between two metal films (with periodic slits on the top film).
In this illustration of a terahertz plasmonic laser, the laser cavity is enclosed between two metal films (with periodic slits on the top film). The colors represent coherent SPP light waves. One wave is confined inside the 10-micron-thick cavity. The other, with a large spatial extent, is located on top of the cavity. Courtesy of Sushil Kumar.

To implement DFB in the laser, researchers used periodic structures with broad-area emission of both short wavelength spasers and THz QCLs, and they made periodic gratings in one of the metal claddings that encased the laser’s cavity. The light energy was confined inside the cavity and sandwiched between two metallic plates separated by a distance of 10 µm. The periodicity in the laser cavity provided feedback for sustained laser oscillations in the cavity.

In contrast to the radiative field in conventional photonic band-edge lasers, in which the periodicity follows the integer multiple of half-wavelengths inside the active medium, the researchers’ technique, which they termed antenna-feedback, exceeded the integer limit and enhanced the radiative field of the lasing mode.

“Our technique allows a plasmonic laser to radiate in a narrow beam, very much like a phased-array antenna,” said professor Sushil Kumar. “The period we choose depends on the desired wavelength of light from the laser, the refractive index of the cavity medium, and the refractive index of the surrounding medium.”

A semiconductor laser chip measuring approximately 3mm x 1.5mm contains 10 lasers.

A semiconductor laser chip measuring approximately 3 mm × 1.5 mm contains 10 lasers. A scanning electron microscopy magnification (right) shows one of the laser cavities. Periodic slits in the thin-film top metal layer provide the distributed feedback in the cavity. Courtesy of Sushil Kumar.

The periodicity establishes an intense SPP wave which occupies the surrounding medium of the laser’s cavity while remaining tied to its metal cladding; and propagates in tandem with the SPP wave inside the cavity.

“All plasmonic lasers have SPPs inside their cavities,” said Kumar. “Our laser also generates SPPs in the air, or any other medium that may surround the laser. The large size of the SPP wave in the surrounding medium leads to a highly directive radiation pattern from the plasmonic laser.”

According to the Fraunhofer diffraction formula, when the near field is narrow, the far field is broad and vice-versa. The researchers, by creating a near field with a large spatial extent, have created a far-field, or focused beam.

By giving the laser a periodic structure, the researchers have made it possible for the laser to emit light at just one wavelength, enabling a spectrally pure, single-mode laser application. The periodic structure may also enhance the quality of the laser beam by channeling light intensely into a tight spot. Light can be delivered to a location where it is needed most, shine for long distances, and be redirected more easily to desired locations.

The antenna-feedback technique may be applied to any plasmonic laser with a Fabry-Perot cavity, regardless of its operating wavelength.

Sushil Kumar and Chongzhao Wu have filed a patent application on their invention, which may lead to commercial applications for plasmonic lasers and especially THz QCLs with narrow beams, which could be used for integrated as well as standoff THz spectroscopy and sensing and for ultrafast digital communications.

The research was published in Optica, a journal of The Optical Society (doi: 10.1364/optica.3.000734)


GLOSSARY
photonics
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
quantum cascade laser
A Quantum Cascade Laser (QCL) is a type of semiconductor laser that emits light in the mid- to far-infrared portion of the electromagnetic spectrum. Quantum cascade lasers offer many benefits: They are tunable across the mid-infrared spectrum from 5.5 to 11.0 µm (900 cm-1 to 1800 cm-1); provide a rapid response time; and provide spectral brightness that is significantly brighter than even a synchrotron source. Quantum cascade lasers comprise alternating layers of semiconductor...
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