A lab-on-a-chip is the ultimate goal for many researchers, and one important component is a small but efficient laser. Many attempts at creating such a device have been made, and include the use of photonic crystals, ring resonators and plasmonic arrays. However, all of these suffer from either wavelength-limited miniaturization, or high material losses and low quality (Q-) factors. In all cases, radiation damping lowers the Q-factor considerably.

A team of researchers at the Institute of Electronic Structure and Laser, Heraklion, Greece and Ames Laboratory and Department of Physics and Astronomy, Iowa, USA, have found a way around these problems. They create a linear grating of silver strips, where the space between the strips is filled by a high-index dielectric with a gain medium embedded in it. This periodically modulated thin dielectric film supports resonant, dark-bound states. The spacing of the grating is carefully chosen to match the emission band of the gain medium, and to provide the highest Q-factor. This structure is much thinner than the emission wavelength, and does not suffer so much from material damping as it is made of mostly dielectric, not metal.

When the gain medium is pumped by an external light source, the energy accumulates in the gain medium and lasing, i.e., stimulated emission, occurs directly in the dark bound state of the laser. By definition this mode does not radiate to free space, eliminating radiative losses. Then, by including small non-resonant scatterers on the surface of the grating, which collectively act as an electromagnetic metasurface, the energy in the dark mode is scattered to free space in the form of a wave, creating the optical laser beam output. This approach separates the conceptual constituents of lasing action, cavity resonance and out-coupling to the emitted laser-radiation from one another, and allows the researchers to independently optimize the individual components.

Using this concept, first proposed in an earlier paper, the team, led by Costas Soukoulis, took full advantage of the exotic capabilities of metasurfaces (wavelength-scale periodic arrays that manipulate electromagnetic fields) to affect the shape and properties of the emitted laser beam. They fully characterized possible geometries for the laser through extensive simulations. For example, changing the position of the scatterer between the two silver strips allows control of the Q-factor, lasing threshold, directionality and loss channel (whether the energy is radiated away or stored and eventually lost to Joule heating).

The next challenge the researchers will focus on is the fabrication and experimental realization of such a device. The researchers have already outlined some of the fabrication issues that may be faced, along with potential solutions. For example, the effects of radiative and dissipative loss can be balanced by appropriate adjustments to the geometry. Some simplified geometries are explored, taking into account the presence of a substrate and the possibility of layer-by-layer fabrication.

More information can be found in the research paper here, published in Physical Review B.