Reducing the size of photonic and electronic components is important for a wide range of technological applications, from ultrafast data processing to ultradense data storage, and miniaturizing the laser is no exception. Nanoscale coherent light sources are crucial not only for exploring physical phenomena at these tiny dimensions but also for making optical devices that are small enough to beat the diffraction limit. A team led by Teri Odom at Northwestern has now come up with a way to manufacture single laser devices that are around the same size as a virus particle and that can operate at room temperature – a first.

The researchers succeeded in fabricating their nanolasers by making the laser cavity out of two coupled metal nanoparticles. These metal nanostructures (made of gold in this particular case) have a 3D bowtie shape that support so-called localized surface plasmons – collective oscillations of electrons – that have no fundamental size limits when it comes to confining light. The complete nanolaser device consists of a gain medium slab, comprising a dye in the polymer polyurethane that supports an array of gold bowties covered by a dielectric overlayer.

Bowtie advantages

"Compared with previous approaches, the bowtie geometry has two significant advantages when it comes to making plasmon lasers," explained Odom. "First, the bowtie structure provides a well defined electromagnetic 'hot spot' in a nanosized volume thanks to the 'nanoantenna' effect. And second, the structures only lose a small amount of light because of light trapping in the bowtie gap, which is just tens of nanometres across."

Surprisingly, the researchers also found that when arranged in an array, the 3D bowtie resonators could emit light at specific angles. The direction of the emitted light beam is an important characteristic of a laser, explains Odom, and most plasmon-based lasers made to date have poor beam directionality. "What is more, depending on the periodicity of the array, the laser beams produced will be at different angles – something that provides us with an extra degree of control over the laser light generated by nanoscale sources."

Nanoantennas are similar to the radio antennas that we are all familiar with, except that they are 10 million times smaller. These tiny devices couple electromagnetic waves in the optical part of the spectrum into cavities that are tens of nanometres across instead of metres across as is the case for coupling radio waves.

"Our results will hopefully interest the nanotechnology community in general, and in particular those scientists working in the fields of nanoantennas, plasmonics and quantum information processing," Odom told nanotechweb.org. "The photonics and laser industries should also be interested of course."

The new work is reported in Nano Letters.