The might of metasurfaces

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The beam shape of the metasuface based external cavity laser can be fully controlled to project a complex hologram, such as the Harvard shield. (Image: Christina Spägele/Harvard SEAS)

Harvard researchers have developed a metasurface that can effectively tune the properties of laser light without the need of additional optical components

One of many aspects of paramount importance to science and technology is the efficient transformation and shaping of light.

Many modern versatile optical devices demand several functions that result in complex systems when implemented with bulk optics. In contrast, it is possible to achieve compact, lightweight and customised optical devices when such bulk optics are replaced with metasurfaces.

Metasurfaces are artificial planar metamaterials that consist of arrays of subwavelength-spaced nanostructures – often referred to as metaatoms – which can locally manipulate the amplitude, phase, and polarisation of light to achieve a variety of optical functions such as lensing, structured light, enhanced cameras and optical computing.

However, according to Harvard researchers, the efficiency and the angular range of current approaches to create multifunctional metasurfaces are limited, thus preventing their widespread application. They say that there is currently no optical component with the flexibility of metasurfaces in beam shaping, which also implements different functions with large difference in deflection angle.

In a recent Nature Communications paper1, the researchers describe their development of a single metasurface that can effectively tune the different properties of laser light, including wavelength, without the need of additional optical components.

The metasurface can split light into multiple beams and control their shape and intensity in an independent, precise and power-efficient way.

They say that their research could aid the development of lightweight and efficient optical systems for a range of applications, from quantum sensing to VR/AR headsets. ‘Our approach paves the way to new methods to engineer the emission of optical sources and control multiple functions, such as focusing, holograms, polarisation, and beam shaping, in parallel in a single metasurface,’ said Federico Capasso, senior author of the paper and professor at the Harvard John A Paulson School of Engineering and Applied Sciences (SEAS).

Souping up external cavity lasers Unlike previous metasurfaces, which relied on a network of individual pillars to control light, the researchers’ surface uses so-called supercells, groups of pillars that work together to control different aspects of light. By combining one of these metasurfaces with a laser diode, the researchers were able to develop a wavelength-tuneable metasuface based external cavity laser (MECL).

While metasurfaces have already been used as intracavity devices to obtain orbital angular momentum lasing or to redirect the emission from solid state lasers or as their gain medium, their use for external laser cavities has not yet been considered.

The incident light can be split into three independent beams, each with different properties – a conventional beam (right), a beam known as a Bessel beam (centre) and an optical vortex (left). (Image: Christina Spägele/Harvard SEAS)

The supercell reflector, which is tilted with respect to the diode laser source, splits the light from the diode in two beams. One beam is then focused back on the facet of the laser diode, creating a laser cavity between the diode and the metasurface (cavity feedback), which enables laser operation. The other is the output beam, which can be collimated or shaped arbitrarily. According to the researchers, this functionality would otherwise be impossible to achieve using standard metasurfaces or external cavity designs. The lasing wavelength can then be controlled by moving the supercell-metasurface reflector with respect to the laser diode, without changing the direction of the output beam.

‘When light hits the metasurface, different colours are deflected in different directions,’ said Christina Spägele, a graduate student at SEAS and first author of the paper. ‘We harnessed this effect and design it so that only the wavelength that we selected has the correct direction to enter back into the diode, enabling the laser to operate only at that specific wavelength.’

The researchers report that their system overcomes the shortcomings of two common ECL configurations based on gratings. In the first configuration, a change in wavelength can be achieved by rotating the grating and hence causes a change in direction of the output beam, which makes this configuration problematic for many applications.

The second configuration mitigates this problem by introducing a mirror into the set-up. Light is deflected by a grating towards the mirror, which reflects it back to the grating. For one specific wavelength, light is orthogonal to the mirror and is therefore reflected back on the same path and focused on the laser facet, providing feedback at that wavelength only. The lasing wavelength can be selected by rotating the mirror instead of the grating, which leaves the direction of the outgoing light unchanged. However, part of the light reflected by the mirror is lost as a specular reflection from the grating, thereby reducing efficiency to about 50 per cent of the first configuration.

The researchers’ system not only combines the strengths of both common ECL configurations by enabling a change in wavelength while neither introducing a change in propagation direction nor a power loss, but has several additional appealing qualities. With no need for additional optical components, the researchers’ system is compact and easy to align. ‘The design is more compact and simpler than existing wavelength-tunable lasers, since it does not require any rotating component,’ said Michele Tamagnone, former postdoctoral fellow at SEAS and co-author of the paper. In addition, the change in wavelength of the MECL is controlled by simple translation of the metasurface, while an ECL is dependent on both rotation and translation. This further eases the control of the laser. Lastly, the approach enables an arbitrary control of the laser beam shape.

Harvard holography

The researchers showed that the shape of the laser beam can be fully controlled to the point where it can be used to project a complex hologram – in this case the complex, century-old Harvard shield. The team also demonstrated the ability to split the incident light into three independent beams, each with different properties – a conventional beam, an optical vortex and a beam known as a Bessel beam, which looks like a bullseye and is used in many applications including optical tweezing.

‘In addition to controlling any type of laser, this ability to generate multiple beams in parallel and directed at arbitrary angles, each implementing a different function, will enable many applications from scientific instrumentation to augmented or virtual reality and holography,’ said Capasso.

The researchers’ experimental realisations are based on simple supercells containing up to two rectangular pillars. They explain, however, that combining their approach with freeform, inverse-designed nanostructures will enable the realisation of metasurfaces with unprecedented capabilities. ‘For instance, the creation of efficient wide-angle holograms in far field can be achieved by splitting the far field radiation pattern of the hologram into smaller portions having a smaller solid angle,’ they say.

‘Then, each portion can be implemented independently by a different diffraction order. Multiple diffraction orders can work synergistically to implement a single, wide hologram.’ The researchers conclude that their ECL concept with arbitrary beam control capabilities cannot be realised by existing metasurface and metagrating implementations. With the concept potentially outperforming standard ECLs, it could pave the way to new active optoelectronic devices. Lasers based on other gain media and light emitting systems can be controlled using the same method, say the researchers.

Reference

[1] Spägele, C., Tamagnone, M., Kazakov, D. et al. Multifunctional wide-angle optics and lasing based on supercell metasurfaces. Nat Commun 12, 3787 (2021). https://doi. org/10.1038/s41467-021-24071-2

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