theradiusofthehole,wepreservetheQ
factor of mode 1 throughout the operational
range. This mode represents the topology-
protected BIC with an embedded polarization
vortex, which has been discussed previously
( 20 , 23 – 26 ).
The perovskite metasurfaces were fabricated
from MAPbBr 3 with a combined process of
electron-beam lithography and reactive ion
etching ( 27 ). Figure 2A shows the top-view
scanning electron microscope (SEM) image
of the sample. To attain the designed BIC vor-
tex lasers, we optically pump the perovskite
metasurface with a frequency-doubled Ti:
sapphire laser at room temperature. Figure 2B
shows the evolution of the emission spectrum
at different pumping densities. A broad, spon-
taneous emission peak of lead halide centered
at 520 nm is observed at low pumping den-
sity. With an increase of pumping power, a
single peak appears at 552 nm (the resonant
wavelength of mode 1 in the normal direction)
and quickly dominates the emission spectrum
at higher pumping fluence. Figure 2C shows
the output laser intensity as a function of the
pumping density, and it confirms the onset
of lasing with the characteristic S-shaped
curve.
The vortex characteristics of the emission
are studied from the far-field angular distribu-
tions by the back focal plane imaging technique
( 27 ) (Fig. 2). The intensity of the perovskite
laser emission is spatially distributed in a donut
shape with a dark zone at the center (Fig. 2D).
The dominant bright ring corresponds to the
far-field angle ofqFF=2°.Thedarkcenteris
caused by a topological singularity at the beam
axis. Figure 2D shows the self-interference
pattern of a donut beam ( 27 ), where a fork-
shaped interference pattern can be seen. In
the profiles of the donut beam behind a linear
polarizer (Fig. 2, E to H), two lobes are ob-
served, and their direction follows the axis of
the linear polarizer, demonstrating the radial
polarization. These experimental results for
the beam profiles, self-interference patterns,
and polarization states confirm the onset of
the vector vortex lasing at the BIC mode ( 28 ).
The BIC vortex microlasers are robust to
global changes. These observations have been
reproduced in more than 10 samples ( 27 ), and
they have proven to be robust to a change ofthe excitation power. The donut-shaped laser
beams and fork-shaped interference patterns
are well preserved from the laser threshold to
gain saturation (Fig. 2D).
Vortex emission is typically produced by
real-space chiral structures ( 9 ), which are ab-
sent in our experiment. Instead, in our system,
the topology that protects the BICs manifests
itself as the rotation of polarization along
the beam axis in the far field ( 28 , 29 ). Acting
alone, it affects an OAM in the cross-polarized
transmission of circularly polarized beams
( 29 ). Together with the transverse spin angu-
lar momentum introduced in real samples,
we observe the emergence of the effective
Pancharatnam-Berry phase ( 27 ).
Notably, the laser emission at the symmetry-
protected BICs can be all-optically controlled.
Although the BIC lasers are robust to a
global change, they are extremely sensitive to
symmetry-breaking perturbations ( 14 , 21 )and,
thus, are easily controllable. In passive systems,
such control is realized via a deformation of
nanostructures ( 26 ) or the Kerr nonlinearity,
but these options are not suitable for post-
fabrication control or require strong opticalHuanget al.,Science 367 , 1018–1021 (2020) 28 February 2020 2of4
Fig. 2. Demonstration of vortex perovskite microlasers.(A) SEM image of the
fabricated perovskite metasurface, following the design of Fig. 1. Scale bar,
500 nm. (B) Evolution of the normalized emission spectrum with pumping
density. a.u., arbitrary units. (C) Integrated output intensity as a function of
pumping density. The S-shaped curve shows a laser threshold at ~4.2mJ/cm^2.
Inset shows the laser polarization within the plane of the device. TE, transverse
electric polarization with E in-plane. (D) Far-field patterns and corresponding
self-interference at different pumping densities. Numbers 3 to 5 denote the same
spectra plotted in (B) and (C). (EtoH) Measured intensity distribution of the
vortex laser beam after a linear polarizer with polarization orientations along 0°,
45°, 90°, and 135°. The scale and color bars are the same across the panels 3 to
5 in (D). White arrows denote the direction of the linear polarizer.RESEARCH | REPORT