A Sharper Focus for Plasmonic Lasers

Illustration of a terahertz plasmonic laser: the laser cavity is enclosed between two metal films. The colors represent coherent SPP light waves. (Source: S. Kumar)

Illustration of a terahertz plasmonic laser: the laser cavity is enclosed between two metal films. The colors represent coherent SPP light waves. (Source: S. Kumar)

The ability to generate and amplify light waves into a coherent, mono­chromatic and well-focused beam has yielded appli­cations too numerous to count: laser scanners, laser printers, laser surgery, laser-based data storage, ultrafast data communi­cations via laser light, and the list goes on. In the past decade, researchers have attempted to minia­turize photonic techno­logies for dense inte­gration onto tiny semi­conductor chips. To that end there is intense activity in developing even smaller nano­lasers, of which plasmonic lasers are the tiniest.

The plasmonic laser, says Sushil Kumar, an associate professor of electrical and computer engineering, uses metal films or nanoparticles to confine light energy inside the cavity from which laser light is generated. By storing light energy inside the cavity through a combination of electron oscillations in the integrated metal films or nano particles, plasmonic lasers utilize surface-plasmon-pola­ritons to store energy in dimensions that can be made smaller than the wavelength of light that they generate. This unique ability of plasmonic lasers makes them attractive for potential applications in integrated (on-chip) optics, for trans­porting large swathes of data on-chip and between neigh­boring chips, and for ultrafast digital infor­mation processing.

Several problems need to be solved, however, before plasmonic lasers can be widely used. One of the main issues, says Kumar, is the difficulty of extracting light from the cavity of a plasmonic laser. The lasers are also extremely poor emitters of light, and whatever light does come out is highly divergent rather than focused, which severely limits their usefulness. While most plasmonic lasers emit visible or near-infrared radiation, Kumar’s group develops plasmonic lasers that emit long-wavelength tera­hertz radiation, which are also known as terahertz quantum-cascade lasers, or QCLs. As the brightest solid-state sources of terahertz radiation, says Kumar, QCLs are uniquely poised to find appli­cations in biology and medicine for sensing and spectros­copy of molecular species, in security screening for remote detection of packaged explosives and other illicit materials, and in astro­physics and atmospheric science.

Terahertz QCLs, however, also emit highly divergent beams, which poses an obstacle to commer­ciali­zation. But Kumar and his group have demonstrated that it is possible to induce plasmonic lasers to emit a narrow beam of light by adapting a technique called distributed feedback. They have experimentally implemented a scheme for terahertz plasmonic lasers that emit radiation at extremely long wavelengths of appro­ximately 100 microns. The light energy in their laser is confined inside a cavity sand­wiched between two metallic plates separated by a distance of 10 microns. Using a box-shaped cavity measuring 10 microns by 100 microns by 1,400 microns, the group produced a terahertz laser with a beam divergence angle of just 4 degrees by 4 degrees, the narrowest divergence yet achieved for such tera­hertz lasers.

“There are two main reasons for giving lasers a periodic structure,” says Kumar. “The first is to improve spectral selec­tivity. A laser can emit light in several closely spaced wave­lengths, or colors. But a laser with a periodic structure can be forced to emit light at just one wavelength by the mechanism of spectral filtering. Such a spectrally pure, single-mode laser is often indis­pensable for many applications. A periodic structure can also enhance the quality of the laser beam by channe­ling light intensely into a tight spot. Such narrow beam lasers can deliver light energy to a location where it is needed most. They can shine for long distances, and are easier to mani­pulate and re-direct at a desired location using small optical components.” (Source: Lehigh)

Reference: C. Wu et al.: Terahertz plasmonic laser radiating in an ultra-narrow beam, Optica, 3, 734, DOI: 10.1364/OPTICA.3.000734

Link: Dept. of Electrical and Computer Engineering, Lehigh University, Bethlehem, Pennsylvania, USA 

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