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Articles

https://doi.org/10.1038/s41566-020-0623-z

1School of Physics, University of the Witwatersrand, Wits, South Africa. 2Harvard John A. Paulson School of Engineering and Applied Sciences, Harvard

University, Cambridge, MA, USA. 3Department of Electrical and Computer Engineering, National University of Singapore, Singapore, Singapore. 4CSIR

National Laser Centre, Pretoria, South Africa. 5Applied Physics, Vrije Universiteit Brussel, Brussels, Belgium. 6Center for Nanoscale Systems, Harvard

University, Cambridge, MA, USA. 7CNST – Fondazione Istituto Italiano di Tecnologia Via Giovanni Pascoli, Milan, Italy. 8Present address: Molecular

Chirality Research Center, Chiba University, Inage-ku, Chiba, Japan. ✉e-mail: andrew.forbes@wits.ac.za

In recent years it has become possible to tailor light in what is com-

monly referred to as structured light1, suppporting applications

including high-bandwidth optical communication2–5, access to

high-dimensional quantum states6,7, enhanced resolution in imag-

ing8 and microscopy9 and control of matter by optical trapping and

tweezing10,11. Foremost among the family of structured light fields are

those related to chiral light, which carries spin angular momentum

and orbital angular momentum (OAM)12, the latter characterized

by a helical phase

expð

i‘ϕ

Þ

I

about the azimuth (ϕ) with helicity ℓ.

Driven by the many applications these beams have spurred13, much

attention has been focused on their efficient creation.

Scalar OAM modes are easily created by dynamic phase

approaches14, while geometric phase is a convenient mechanism for

creating vector combinations of spin and OAM15–21, forming cylin-

drical vector vortex beams that are rotationally symmetric complex

states of light with an internal intensity null due to a polarization

singularity22. They are conveniently expressed on the higher-order

Poincaré sphere (HOPS)23,24, where the poles are combinations of

left- and right-circular spin angular momentum states (±σ) com-

bined with symmetrical left- and right-helicity OAM states (±ℓ).

This vector addition has opened many exciting possibilities for new

applications that exploit chiral control of both spin angular momen-

tum and OAM degrees of freedom of light25–27.

An ongoing challenge is to control light’s chirality, spin and

orbital at source28–31. So far, advances have been limited, in part due

to fundamental symmetry restrictions when using geometric phase

and topological photonics and in part due to implementation restric-

tions, for example, in regard to the physical size and spatial resolu-

tion of the optical elements. Such advances include the generation

of symmetric OAM states via the geometric phase32,33, as integrated

on-chip devices34–40, in organic lasers41 and as fibre lasers42. Despite

these impressive advances, breaking the symmetry of the spin and

orbital states for arbitrary angular momentum control of light

at source has remained elusive. Arbitrary angular momentum con-

trol requires the ability to produce any desired spin–orbital chiral

state of light, including arbitrary, differing and non-symmetric

OAM values coupled to user-defined polarizations, an infinitely

larger set than the special case of symmetric OAM states, allow-

ing access to super-chiral light with high angular momentum. In

contrast, OAM lasers so far have been demonstrated with only

symmetric superpositions of ±ℓ and ±σ, which add to a total

angular momentum of zero, and with modest OAM values of up

to ∣ℓ∣ = ±10 (ref. 32). However, super-chiral light with high angu-

lar momentum is known to be important in many fundamental

and applied studies, for example, in quantum studies with Bose–

Einstein condensates, remote sensing with structured light, accurate

rotation measurements, metrology of chiral media and for larger

photon information capacity43.

Here, we report a laser with an intracavity metasurface for

control of light’s angular momentum at source. We design custom

metasurfaces for arbitrary OAM coupling to linear polarization

states, including a metasurface with an extreme imbued helicity of

up to ℓ = 100. By doing so, we are able to produce new chiral states

of light from a laser, including simultaneous lasing across vastly dif-

fering and non-symmetric OAM values that are up to Δℓ = 90 apart,

an extreme violation of previous symmetric spin–orbit (SO) lasing

devices, and demonstrate some intriguing lasing phenomena, for

example, vortex splitting inside a laser medium and coherent lasing

across modes with no spatial overlap. By designing a cavity for mode

metamorphosis, our laser is able to generate ultrahigh-purity OAM

modes with orders of magnitude enhanced purity over their exter-

nally created counterparts, which we show with OAM modes up to

ℓ = 100. Importantly, the coupling to linear polarization states facili-

tates a compact design with a reduction in complexity and number

of optical elements over previous geometric phase lasers, the latter

of course restricted to only symmetric states. In addition to these

High-purity orbital angular momentum states

from a visible metasurface laser

Hend Sroor1, Yao-Wei Huang 2,3, Bereneice Sephton1, Darryl Naidoo1,4, Adam Vallés 1,8, Vincent Ginis2,5,

Cheng-Wei Qiu 3, Antonio Ambrosio 6,7, Federico Capasso 2 and Andrew Forbes 1 ✉

Orbital angular momentum (OAM) from lasers holds promise for compact, at-source solutions for applications ranging from

imaging to communications. However, conjugate symmetry between circular spin and opposite helicity OAM states (±ℓ) from

conventional spin–orbit approaches has meant that complete control of light’s angular momentum from lasers has remained

elusive. Here, we report a metasurface-enhanced laser that overcomes this limitation. We demonstrate new high-purity OAM

states with quantum numbers reaching ℓ = 100 and non-symmetric vector vortex beams that lase simultaneously on indepen-

dent OAM states as much as Δℓ = 90 apart, an extreme violation of previous symmetric spin–orbit lasing devices. Our laser

conveniently outputs in the visible, producing new OAM states of light as well as all previously reported OAM modes from

lasers, offering a compact and power-scalable source that harnesses intracavity structured matter for the creation of arbitrary

chiral states of structured light.

NATURE PHOTONICS | VOL 14 | AUGUST 2020 | 498–503 | www.nature.com/naturephotonics

498

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