Thinnest Plasmonic Nanolasers Achieved
We use lasers larger and small everyday and everywhere, in your CD or DVD player or when you go through the check out in a supermarket. Those are tiny lasers measured thinner than our hair. But lasers are used in more than these places. Small lasers are now used for optical communications for distances as long as across continents and as short as inside a computer. One of the future applications of lasers is for communication inside a CPU chip. That would solve the on-chip communication bottleneck and help to make even faster computers or internet. But for these applications to happen, the small lasers we have today are too large. The smaller we can make the lasers, the more likely we can integrate with tiny electronics and more lasers we can integrate to make communication faster. But how small you can make a laser to be? Or what is the smallest size limit ultimately?
Standard laser theory says that the size of a laser in any one dimension (for example, the thickness) is ultimately limited by one half of the wavelength involved. For lasers used for optical communication, the wavelength is around 1500 nanometer (nm). 750 nm will be the smallest size in air. In an optically denser medium such as semiconductor, this limit is reduced by a factor of the index of refraction (~ 3.1 for Indium phosphide) of semiconductor, to be about 242nm. This limit is also called diffraction limit, a property associated with any wave such as a light beam. Or in another word, this theory says that you cannot make a laser smaller than this diffraction limit or 242 nm for a semiconductor laser for communication.
Figure 1 left: Schematic of the metal-coated
semiconductor pillar plasmonic nanolaser. Right:
SEM micrographes of the semiconductor
pillar: circular (top) and rectangular (bottom).
And in fact you may beat this limit by using a combination of semiconductors and metals such as gold or silver. It turns out that the electrons excited in the metal can help you to confine a laser that is as small as the 50% of size required by the diffraction limit, as reported by our recent paper  in collaboration with Martin Hill’s group from Technical University of Eindhoven. Or in another word, you can make the laser to be as thin as about one quarter, as opposed to one half, of wavelength.
The team, led by Professor Martin Hill of Eindhoven and Professor Cun-Zheng Ning of Arizona State, used a metal-semiconductor-metal sandwich structure (see Fig.1) and the semiconductor is as thin as 90 nm with 20 nm insulator on each side. The optical thickness of the structure is 359 (nm), The team demonstrated that such a thin layer can actually support a lasing mode, or emit laser light. While the structure worked as a laser, the operating temperature is very low. The next goal of the team is to realize same lasing at room temperature, according to Hill and Ning.
Integrating metallic structures with semiconductors as a possible way of realizing the so called nanolasers has been a highly contested field of research worldwide due to the many technological applications potentially enabled by such lasers. But it is generally believed that the size of a laser is ultimately restricted by the half-wavelength. In a theoretical study, Maslov and Ning  demonstrated that a semiconductor-core metal-shell structure can have an overall positive modal gain, despite the metal loss. A semiconductor-metal core shell structure was indeed shown to be able to lase by Hill et al.  in 2007. Our current work is the first to show that this thinnest limit can actually be broken. This opens a lot of new possibilities of applications in as diverse fields as on-chip integrated communications and single molecule detection and imaging. It represents a significant step in the field of nanophotonics. The knowledge gained in this study can also be used for making ever smaller waveguides using metal-semiconductor combinations.
The research is funded by the Netherlands Organization for Scientific Research under NRC Photonics and by DARPA under a program called NACHOS (Nanoscale Architecture for Coherent Hyper Optical Sources, http://www.darpa.mil/MTO/Programs/nachos/index.html)
 Martin T. Hill, Milan Marell, Eunice S. P. Leong, Barry Smalbrugge, Youcai Zhu, Minghua Sun, Peter J. van Veldhoven, Erik Jan Geluk, Fouad Karouta, Yok-Siang Oei, Richard Nötzel, Cun-Zheng Ning, and Meint K. Smit, OPTICS EXPRESS, Vol. 17, No. 13, P. 11107, http://www.opticsinfobase.org/DirectPDFAccess/0A7B4D8E-BDB9-137E-C5667E774627D931_182907.pdf?da=1&id=182907&seq=0&CFID=28345599&CFTOKEN=83759966