A Nanolaser Shines in All Directions

Researchers in Eind­hoven have developed a new type of low-energy, nanoscale laser that shines in all directions. The key to its omni­directional light emission is the intro­duction of something that is usually highly undesirable in nano­technology: irre­gularities in the materials. The researchers foresee a vast range of potential appli­cations, but first they hope their funda­mental work will inspire others to further improve it and deepen the under­standing.

An experimental sample of the new laser. It contains ten patches that each have their own silver nanoparticle pattern. (Source: A. Halpin, TU/e)

Lack of control of the variables deter­mining the response of a system is usually seen as a curse in science and tech­nology. But what about a slight pinch of imper­fection and disorder? Imper­fections and irregu­larities are unavoidable in nanoscience due to our limited level of control of nanofabri­cation processes. Disorder is potentially detrimental to nano­systems, but if well-contained, disorder might not be an intruder after all, leading to novel physical concepts and applications. Scientists from Eindhoven University of Technology (TU/e) and the Dutch Institute for Funda­mental Energy Research (DIFFER) have inves­tigated the role of imperfections and disorder in nanolasers. By introducing a slight degree of disorder, they have observed a dramatic change: the laser no longer emits in one specific direction, but in all directions.

Develop­ment of nanoscale lasers is a very active field of research. Polariton lasers work on the principle not of cloning photons but making non-identical photons identical in much the same way as water vapor molecules, moving in all direc­tions with different velo­cities, are condensed into a single drop. Conden­sation of photons gives rise to the intense and directional emission charac­teristic of a laser. An important advantage of polariton lasers is that they have a much lower lasing threshold, which makes them excellent candi­dates for many appli­cations.

However, a major problem of polariton lasers has been that they need to operate at very low tempera­tures, but by using organic materials, it is possible to obtain polariton laser emission even at ambient temperature. The Eindhoven researchers demon­sstrated last year that they can realize nanoscale polariton lasers that function at ambient tempera­sture, using metallic nano­sparticles instead of mirrors as in normal lasers. They have now discovered a new kind of polariton laser that consists of a regular pattern of silver nano­sstripes covered with colored PMMA-polymer whose dye comprises organic emitting molecules.

However, the silver stripes deli­sberately have some degree of imperfection and disorder. The emission from this non-perfect nanolaser is omni­sdirectional and mainly is determined by the properties of the organic molecules. This result is not expected in the framework of conden­ssation, as omni­sdirectional emission requires emissions from inde­spendent organic molecules instead of the collective emission that is typical for condensation. The demon­sstration of omni­sdirectional emission defines new boundaries for the development of nanoscale lasers at ambient temperatures.

The researchers think their laser may eventually be applied in many areas. Compared to a LED, the omni­sdirectional laser light is much brighter and better defined. That’s why it is a good candidate for microscopy lighting, which currently uses LEDs. LIDAR is another potential appli­scation. Current LIDAR use one or more lasers and a set of fast moving mirrors in order to cover large areas to image distant objects. An omni­sdirectional laser does not require the moving mirrors, thereby significantly reducing the com­splexity. And also general illu­smination is an option, says lead researcher Jaime Gomez Rivas. “But the research is still very funda­smental. We hope that our results will sti­smulate other researchers to improve them by further reducing the lasing threshold or increasing the range of emitted colors.” (Source: TU Eindhoven)

Reference: M. Ramezani et al.: Nonlinear Emission of Molecular Ensembles Strongly Coupled to Plasmonic Lattices with Structural Imperfections, Phys. Rev. Lett. 121, 243904 (2018); DOI: 10.1103/PhysRevLett.121.243904

Link: Institute for Photonic Integration, Eindhoven University of Technology, Eindhoven, The Netherlands

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