A nanolaser and a bendable-light material promise to speed up microelectronic devices

March 27, 2015

The ultra-thin semiconductor stretches across the top of the photonic cavity (credit: University of Washington)

University of Washington (UW) scientists have built a new nanometer-sized laser — using the thinnest semiconductor available today — that is energy efficient, easy to build, and compatible with existing electronics.

The UW nanolaser, developed in collaboration with Stanford University, uses a tungsten-based semiconductor only three atoms thick as light emitter.

The technology is described in a paper published in the March 16 online edition of Nature.

Nanolasers — which are so small they can’t be seen with the eye — have the potential to be used in a wide range of applications from next-generation computing to implantable microchips that monitor health problems.

But current nanolaser designs use materials that are either much thicker or that are embedded in the structure of the cavity that captures light. That makes them difficult to build and to integrate with modern electronic circuits and computing technologies.

The UW version, instead, uses a flat sheet that can be placed directly on top of a commonly used optical cavity, a tiny cave that confines and intensifies light. The ultrathin nature of the semiconductor — made from a single layer of a tungsten-based molecule — yields efficient coordination between the two key components of the laser.

The UW nanolaser requires only 27 nanowatts, which is very energy efficient, can be easily fabricated, and it can potentially work with silicon components common in modern electronics.

Researchers at Tianjin University in China, Oak Ridge National Laboratory, the University of Tennessee, Humboldt University in Berlin and the University of Hong Kong where also involved.

Primary funding came from the Air Force Office of Scientific Research. Other funders include the National Science Foundation, the state of Washington through the Clean Energy Institute, the Presidential Early Award for Scientists and Engineers administered through the Office of Naval Research, the U.S. Department of Energy, and the European Commission.

How to bend light around ultra-tiny corners

Another problem with using photonics (light-based electronics), in interconnects between chips, for example, is the difficulty in steering light around corners in tiny spaces.

Sending information on light beams instead of electrical signals allows data to be transmitted thousands of times more quickly, but controlling the light beams without losing their energy has been the challenge. Conventional light waveguides, like optical fibers, can be used to steer light through turns. But the turns must be gradual. If the turn is too quick, the light beams escape and energy is lost.

Spatially variant photonic crystals gently steer light around a 90-degrees corner (credit: Jennefir L. Digaum et al./Optics Express)

So researchers at the University of Texas El Paso (UTEP) and the University of Central Florida (UCF) developed a plastic honeycomb-like device that can steer light beams around tighter curves than ever before possible, opening up new possibilities for high-speed light-based data transmission in computers, smartphones, and other devices.

The device is based on “spatially variant photonic crystals” (SVPCs), in which the orientation of the 3D-printed “honeycomb” cell gradually changes. This allows for directing the flow of infrared light around a 90 degree bend.

The work was published in an open-access article in the journal Optics Express and was supported by NSF CAREER Awards, NSF grants, and DARPA.

Abstract of Monolayer semiconductor nanocavity lasers with ultralow thresholds
Ellis, B. et al. Ultralow-threshold electrically pumped quantum-dot photonic-crystal nanocavity laser. Nature Photon. 5, 297-300 (2011)”>Engineering the electromagnetic environment of a nanometre-scale light emitter by use of a photonic cavity can significantly enhance its spontaneous emission rate, through cavity quantum electrodynamics in the Purcell regime. This effect can greatly reduce the lasing threshold of the emitter, providing a low-threshold laser system with small footprint, low power consumption and ultrafast modulation. An ultralow-threshold nanoscale laser has been successfully developed by embedding quantum dots into a photonic crystal cavity (PCC). However, several challenges impede the practical application of this architecture, including the random positions and compositional fluctuations of the dots, extreme difficulty in current injection, and lack of compatibility with electronic circuits. Here we report a new lasing strategy: an atomically thin crystalline semiconductor—that is, a tungsten diselenide monolayer—is non-destructively and deterministically introduced as a gain medium at the surface of a pre-fabricated PCC. A continuous-wave nanolaser operating in the visible regime is thereby achieved with an optical pumping threshold as low as 27 nanowatts at 130 kelvin, similar to the value achieved in quantum-dot PCC lasers. The key to the lasing action lies in the monolayer nature of the gain medium, which confines direct-gap excitons to within one nanometre of the PCC surface. The surface-gain geometry gives unprecedented accessibility and hence the ability to tailor gain properties via external controls such as electrostatic gating and current injection, enabling electrically pumped operation. Our scheme is scalable and compatible with integrated photonics for on-chip optical communication technologies.

Abstract of Tight control of light beams in photonic crystals with spatially-variant lattice orientation

Spatially-variant photonic crystals (SVPCs), in which the orientation of the unit cell changes as a function of position, are shown to be capable of abruptly controlling light beams using just low index materials and can be made to have high polarization selectivity. Multi-photon direct laser writing in the photo-polymer SU-8 was used to fabricate three-dimensional SVPCs that direct the flow of light around a 90 degree bend. The lattice spacing and fill factor were maintained nearly constant throughout the structure. The SVPCs were characterized at a wavelength of 2.94 μm by scanning the faces with optical fibers and the results were compared to electromagnetic simulations. The lattices were shown to direct infrared light of one polarization through sharp bends while the other polarization propagated straight through the SVPC. This work introduces a new scheme for controlling light that should be useful for integrated photonics.