A new type of nanostructured lasing system called a metalaser emits light with highly tuneable wavefronts – something that had proved impossible to achieve with conventional semiconductor lasers. According to the researchers in China who developed it, the new metalaser can generate speckle-free laser holograms and could revolutionize the field of laser displays.
The first semiconductor lasers were invented in the 1960s and many variants have since been developed. Their numerous advantages – including small size, long lifetimes and low operating voltages – mean they are routinely employed in applications ranging from optical communications and interconnects to biomedical imaging and optical displays.
To make further progress with this class of lasers, researchers have been exploring ways of creating them at the nanoscale. One route for doing this is to integrate light-scattering arrays called metasurfaces with laser mirrors or insert them inside resonators. However, the wavefronts of the light emitted by these metalasers have proven very difficult to control, and to date only a few simple profiles have been possible without introducing additional optical elements.
Not significantly affected by perturbations
In the new work, a team led by Qinghai Song of the Harbin Institute of Technology, Shenzhen, created a metalaser that consists of silicon nitride nanodisks that have holes in their centres and are arranged in a periodic array. This configuration generates bound states in a continuous medium (BICs). Since the laser energy is concentrated in the centre of each nanodisk, the wavelength of the BIC is not significantly affected by perturbations such as tiny holes in the structure.
“At the same time, the in-plane electric fields of these modes are distributed along the periphery of each nanodisk,” Song explains. “This greatly enhances the light field inside the centre of the hole and induces an effective dipole moment there, which is what produces a geometric phase change to the light emission at each pixel.”
By rotating the holes in the nanodisks, Song says that it is possible to introduce specific geometric phase profiles into the metasurface. The laser emission can then be tailored to create focal spots, focal lines and doughnut shapes as well as holographic images.
And that is not all. Unlike in conventional laser modes, the waves scattered from the new metalaser are too weak to undergo resonant amplification. This means that the speckle noise generated is negligibly small, which resolves the longstanding challenge of reducing speckle noise in holographic displays without reducing image quality.
According to Song, this property could revolutionize laser displays. He adds that the physical concept outlined in the team’s work could be extended to other nanophotonic devices, substantially improving their performance in various optics and photonics applications.
“Controlling laser emission at will has always been a dream of laser researchers,” he tells Physics World. “Researchers have traditionally done this by introducing metasurfaces into structures such as laser oscillators. This approach, while very straightforward, is severely limited by the resonant conditions of this type of laser system. With other types of laser, they had to either integrate a metasurface wave plate outside the laser cavity or use bulky and complicated components to compensate for phase changes.”
With the new metalaser, the laser emission can be changed from fixed profiles such as Hermite-Gaussian modes and Laguerre-Gaussian modes to arbitrarily customized beams, he says. One consequence of this is that the lasers could be fabricated to match the numerical aperture of fibres or waveguides, potentially boosting the performance of optical communications and optical information processing.
Developing a programmable metalaser will be the researchers’ next goal, Song says.
The new metalaser design is described in Nature.
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