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REPORTS
◥
OPTICS
Orbital angular
momentum microlaser
Pei Miao,
1
*Zhifeng Zhang,
1
*Jingbo Sun,
1
*Wiktor Walasik,
1
Stefano Longhi,
2
Natalia M. Litchinitser,
1
†Liang Feng
1
†
Structured light provides an additional degree of freedom for modern optics and practical
applications. The effective generation of orbital angular momentum (OAM) lasing, especially
at a micro- and nanoscale, could address the growing demand for information capacity.
By exploiting the emerging non-Hermitian photonics design at an exceptional point, we
demonstrate a microring laser producing a single-mode OAM vortex lasing with the ability
to precisely define the topological charge of the OAM mode.The polarization associated
with OAM lasing can be further manipulated on demand, creating a radially polarized vortex
emission. Our OAM microlaser could find applications in the next generation of integrated
optoelectronic devices for optical communications in both quantum and classical regimes.
Light typically consists of a stream of linearly
polarized photons, traveling in a straight
line and carrying a linear momentum. How-
ever, it was recognized that beyond the
linear momentum, circularly polarized light
carries angular momentum (1). The angular mo-
mentum associated with the polarization degree
of freedom, or spin angular momentum (SAM),
cantakeonlyoneoftwovaluesTℏ.Inadditionto
the SAM, it was also demonstrated that a light
beam can carry orbital angular momentum (OAM)
(2). Such beams possess helical phase fronts so
that the Poynting vector within the beam is
twisted with respect to the principal axis. This
fundamental discovery of an OAM opened a new
branch of optical physics, facilitating studies rang-
ingfromrotaryphotondrag(3), angular uncer-
tainty relationships (4), and rotational frequency
shifts (5), to spin-orbital coupling (6). The OAM
degree of freedom has enabled technological ad-
vances, for example, edge-enhanced microscopy
(7). Moreover, in contrast to the SAM that can
take only two values, the OAM is unbounded.
OAM beams are thus being considered as poten-
tial candidates for encoding information in both
quantum and classical systems. The combined
use of spin and orbital angular momenta is
expected to enable the implementation of entirely
new high-speed secure optical communication
and quantum teleportation systems in a multi-
dimensional space (8), satisfying the exponen-
tially growing demand worldwide for network
capacity.
To date, most of the light sources only produce
relatively simple light beams with spatially hom-
ogeneous polarization and planar wavefront. Gen-
eration of the complex OAM beams usually relies
on either bulk devices, such as spiral phase plates,
spatial light modulators, and computer-generated
holograms (1), or recently developed planar op-
tical components, including phase modulation–
based metasurfaces (9–14), q-plates (15), and silicon
resonators (16). Although the science of the OAM
light beams on the micro- and nanoscale is still
in its early days, it is likely to advance our
knowledge of light interaction with conventional
and artificial atoms (e.g., quantum dots) pro-
vided that the OAM beam is focused to sub-
wavelength dimensions (17), facilitating on-chip
functionalities for micromanipulation and micro-
fluidics. Nevertheless, it remains a grand chal-
lenge to integrate the existing approaches for
OAM microlasers on-a-chip. For an ultimate min-
iaturized optical communication platform, there
is a necessity of independent micro- and nano-
scalelasersources(18) emitting complex vector
beams carrying the OAM information.
One approach to creating an OAM laser (19)
is based on combining a conventional bulk laser
with additional phase-front shaping components.
Despite being straightforward, this approach re-
lies on rather different device technologies and
material platforms, and therefore it is not easily
scalable and integratable. On the contrary, here
we integrate the advantages of semiconductor
microlasers with the pronounced changes in light
propagation at the exceptional point to realize a
fundamentally new, compact, active OAM source
on a complementary metal-oxide-semiconductor
(CMOS) compatible platform. We consider a mic-
roring cavity that supports whispering gallery
modes (WGMs). These modes circulate inside
thecavityandcarrylargeOAM.However,because
of the mirror symmetry of a ring cavity, clockwise
and counterclockwise eigen-WGMs can be simul-
taneously excited, and their carried OAMs con-
sequently cancel each other. This is evidenced by
the quantized phase, taking values of either 0 or
p, azimuthally distributed in the ring, which re-
sults from the interference between two counter-
propagating WGMs (fig. S1) (20). To observe the
OAM of an individual WGM, it is essential to
RESEARCH
464 29 JULY 2016 •VOL 353 ISSUE 6298 sciencemag.org SCIENCE
1
Department of Electrical Engineering, The State University
of New York at Buffalo, Buffalo, NY 14260, USA.
2
Dipartimento di Fisica, Politecnico di Milano and Istituto di
Fotonica e Nanotecnologie del Consiglio Nazionale delle
Ricerche, Piazza L. da Vinci 32, Milano I-20133, Italy.
*These authors contributed equally to this work. †Corresponding
author. Email: fengl@buffalo.edu (L.F.); natashal@buffalo.edu (N.M.L.)
Fig. 1. Design of OAM microlaser. (A) Schematic of the OAM microlaser on an InP substrate.The diam-
eter of the microring resonator is 9 mm, the width is 1.1 mm, and the height is 1.5 mm(500nmofInGaAsP
and 1 mm of InP). Thirteen-nanometer Ge single-layer and 5-nm Cr/11-nm Ge bilayer structures are
periodically arranged in the azimuthal direction on top of the InGaAsP/InP microring, mimicking real index
and gain/loss parts of an EP modulation at n′¼n″¼0:01 to support unidirectional powe r circulati on. The
designed azimuthal order is N¼56 at the resonant wavelength of 1472 nm. Equidistant sidewall scatters
with a total number of M¼57 are introduced to couple the lasing emission upward, creating an OAM vortex
emission with a helical wavefront. Its topological charge is defined by l¼N−M¼−1. (B) Simulated phase
distribution of emitted light. A spiral phase map for an OAM charge-one vortex is clearly demonstrated.
on July 29, 2016http://science.sciencemag.org/Downloaded from
introduce a mechanism of robust selection of
either clockwise or counterclockwise mode. In
conventional bulk optics, unidirectional ring lasers
have been demonstrated by implementing a non-
reciprocal isolator in the light path. The optical
isolator breaks the reciprocity between counter-
propagating waves, facilitating the desired uni-
directional flow. This approach, however, is not
feasible at the micro- and nanoscale, as the reali-
zation of micrometer-sized isolators is extremely
challenging.
To overcome this fundamental limitation, we
realize the unidirectional power circulation by
introducing complex refractive-index modulations
to form an exceptional point (EP) (Fig. 1A). Driven
by non-Hermiticity (i.e., gain and loss in optics)
(21,22), an EP occurs when multiple eigenstates
coalesce into one (23–26). In our device, EP ope-
ration is essential to obtaining OAM laser emis-
sion (20). The microring laser resonator is designed
with 500-nm-thick InGaAsP multiple quantum
wells on an InP substrate. The complex refractive-
index grating is achieved by placing on top of
InGaAsP along the azimuthal direction (q)pe-
riodically alternate single-layer Ge and bilayer
Cr/Ge structures, corresponding to the refrac-
tive index (n′) and gain/loss (n″) in the cavity,
respectively:
Dn¼
in″for 2pp=N<q<2ppþ1
4
=N
n′for 2ppþ3
8
=N<q<2ppþ5
8
=N
8
>
>
<
>
>
:ð1Þ
where Ndenotes the azimuthal number of the
targeted WGM and ptakes integer values from
the set {0, N–1}. An EP is obtained when the
amplitudes of index and gain/loss gratings are
set equal (i.e., n′¼n″). At EP, the Fourier trans-
form of the complex refractive-index modulation
is one-sided, yielding one-way distributed feedback
(27–29) and robust unidirectional laser emission
above threshold, as shown by a detailed semicon-
ductor rate equation analysis (20). As a result,
the counterclockwise WGM unidirectionally cir-
culates in the cavity carrying large OAM through
the azimuthally continuous phase evolution (figs.
S2 and S3) (20).
The OAM associated with the unidirectional
power flow is extracted upward into free space
by introducing sidewall modulations periodically
arranged along the microring perimeter (16). The
azimuthal phase dependence of the targeted uni-
directional Nth WGM is given by φ¼Nq.The
sidewall modulations coherently scatter light,
with the phase continuously varying in azimuthal
direction, defined by the locations of the scatters
(Fig. 1A, inset). For Mequidistant scatters, the
locations of the scatters are given by qs¼2ps=M,
where s∈f0;M−1g, resulting in the extracted
phase qs¼2psN =Mthat carries OAM. Because
thephysicallymeaningfulphaseismeasured
modulo 2p,wecansubtract2psfrom each of the
extracted phases and derive
φs¼2psðN−MÞ=Mð2Þ
Equation 2 shows that the extracted phase
increases linearly from 0 to 2pðN−MÞ,thereby
creating a vortex beam with topological charge
l¼N−M.Figure1Bshowsthemodelingresult
of the vortex laser emission from our OAM mic-
rolaser, where N¼56 and M¼57. The phase of
SCIENCE sciencemag.org 29 JULY 2016 •VOL 353 ISSUE 6298 465
Fig. 3. Characterization of OAM lasing. (A) Evolution of the light emission spectrum from PL, to ASE, and
to lasing at 1474 nm, as the peak power density of pump light was increased from 0.63, to 0.68, to 2.19 GW m
−2
,
respectively. (B) Input-output laser curve, showing a lasing threshold of ~1 GW m
−2
.(C) Far-field intensity
distribution of the laser emission exhibiting a doughnut-shaped profile, where the central dark core is due
to the phase singularity at the center of the OAM vortex radiation. (D) Off-center self-interference of the
OAM lasing radiation, showing two inverted forks (marked with arrows) located at two phase singularities.
Originating from the superposition of central helical and outer quasiplanar phases intrinsically associated
with OAM, the double-fork pattern confirms the OAM vortex nature of the laser radiation.
Fig. 2. Scanning electronmicroscope images of OAM microlaser.The OAM microlaser was fabricated
on the InGaAsP/InP platform. Alternating Cr/Ge bilayer and Ge single-layer structures were periodically
implemented in the azimuthal direction on top of the microring, presenting, respectively, the gain/loss
and index modulations required for unidirectional power circulation.
RESEARCH |REPORTS
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the electric field changes by 2pupon one full
circle around the center of the vortex. The phase
is continuous everywhere except for the center
of the emission path, presenting a topological
phase singularity point at the beam axis. The
topological charge of the vortex emission can
be viewed as the number of twists done by the
wavefront in one wavelength, exhibiting OAM
lasing of charge l¼−1.
The OAM microlaser with the EP modula-
tion by periodically arranged Ge and Cr/Ge (Fig.
2) was fabricated by means of overlay electron
beam lithography (20). The unidirectional power
flow oscillating in the cavity eliminates the un-
desired spatial hole-burning effect that would
be created by the interference pattern of two
counterpropagating WGMs. The preferential
gain saturation in the antinodes of the interfer-
ence pattern would cause spatial gain inhomo-
geneity, leading to a decrease in the laser slope
efficiency, multilongitudinal mode operation,
and unstable laser emission. In our OAM mic-
rolaser, unidirectional power flow forced at the
EP modulation (fig. S3) (20) enables efficient and
stable single-mode lasing with a sideband sup-
pression ratio of ~40 dB (Fig. 3A). In the tran-
sition from broadband photoluminescence (PL),
to amplified spontaneous emission (ASE), and
finally to lasing (Fig. 3, A and B), the emission
peak stabilized at the same resonant wavelength,
demonstrating the avoidance of multimode osci-
llation typically existing in a microring cavity.
The OAM characteristics, such as the vortex
nature and the phase singularity, were charac-
terized by analyzing the spatial intensity profile
of lasing emission and its self-interference (fig.
S4) (20). In the far field, we observed the inten-
sity of lasing emission spatially distributed in a
doughnut shape with a dark core in the center
(Fig. 3C). The observed dark center is due to
the topological phase singularity at the beam
axis where the phase becomes discontinuous,
as predicted in Fig. 1B. The presence of the OAM
was then validated by the self-interference of two
doughnut-shaped beams split from the same
lasing emission. In each doughnut beam, be-
cause of its OAM, optical phase varies more
markedly with a helical phase front close to the
central singularity area, whereas the outer dough-
nut area is of a relatively uniform quasiplanar
phase front. At the observation plane, we inten-
tionally created a horizontal offset between two
doughnut beams, so that the dark center of one
beam overlapped with the bright doughnut area
of the other, and vice versa. The resulting in-
terference patterns between the helical and
quasiplanar phase fronts revealed two inverted
forks (Fig. 3D), as the quasiplanar and helical
phases were reversed at the centers of two
doughnuts. For both of them, the single fringe
split into two at the fork dislocation, evidently
confirming that the radiation from our OAM
laser was an optical vortex of topological charge
l¼−1.
The polarization properties of the demonstra-
ted OAM microlaser can be designed on demand.
In particular, radially polarized beams, charac-
terized by a nonuniform spatial distribution of
their polarization vector, have enabled unique
functionalities, such as high–spatial resolution
microscopy by their sharp focusing (30). Although
the conventional schemes require external op-
tical components, such as geometric phase–based
diffraction elements (9), radially polarized beams
can be directly produced from our OAM micro-
laser. In a microring cavity, the resonant mode
can be designed to be either quasi–transverse
magnetic (TM) or quasi–transverse electric (TE).
The radially polarized component of the quasi-
TM mode is tightly confined at the microring
perimeter and sensitive to sidewall modulations,
facilitating the outcoupling of this mode from the
laser (fig. S5) (20). Therefore, in our microring
cavity, the dominant oscillating mode is designed
to be a quasi-TM mode, and its scattering by the
sidewall modulation results in the radially polar-
ized OAM lasing. In experiments, the polarization
state of the OAM lasing was validated. After trans-
mission through a linear polarizer, the doughnut
profile splits into two lobes aligned along the
orientation of the polarizer (Fig. 4). The two
lobes remained parallel to the polarization axis
regardless of the rotation of the polarizer, man-
ifesting pure radially polarized OAM lasing.
Additionally, in contrast to linearly polarized
OAM modes that are not compatible with op-
tical fibers, fibers can support radially polar-
ized OAM eigenmodes.
We have demonstrated a microring OAM laser
producing an optical vortex beam with an on-
demand topological charge and vector polariza-
tionstates.Thisisenabledthroughcombined
index and gain/loss modulations at an EP, which
breaks the mirror symmetry in the lasing gen-
eration dynamics and facilitates the unidirec-
tional power oscillation. Finally, OAM vector
laser beams might offer novel degrees of freedom
for the next generation of optical communica-
tions in both classical and quantum regimes.
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466 29 JULY 2016 •VOL 353 ISSUE 6298 sciencemag.org SCIENCE
Fig. 4. Polarization state of OAM lasing. Measured intensity distributions of the OAM lasing radiation passing through a linear polarizer with different polar-
ization orientations indicated by arrows: (A)0°,(B)90°,(C) 45°, and (D)–45°. The two-lobe structure rotated with the rotation of the polarizer in the same fashion,
confirming radially polarized OAM lasing.
RESEARCH |REPORTS
on July 29, 2016http://science.sciencemag.org/Downloaded from
ACKNOWL EDGMENTS
We acknowledge funding from the U.S. Army Research Office
Award (W911NF-15-1-0152) that enabled the design, modeling, and
topological char ge characterization of the EP-based OAM laser,
Department of Energy Award (DE-SC0014485) that was used to perform
the analysis and characterization of the spectral properties of OAM lasing,
and National Science Foundation Award (DMR-1506884) that facilitated
the fabrication of the device and optimization of the EP modulation.
SUPPLEMENTARY MATERIALS
www.sciencemag.org/content/353/6298/464/suppl/DC1
Supplementary Text
Figs. S1 to S5
References (31–34)
10 April 2016; accepted 30 June 2016
10.1126/science.aaf8533
ELECTROCHEMISTRY
Nanostructured transition metal
dichalcogenide electrocatalysts for
CO
2
reduction in ionic liquid
Mohammad Asadi,
1
Kibum Kim,
1,2
*Cong Liu,
3
*Aditya Venkata Addepalli,
1
Pedram Abbasi,
1
Poya Yasaei,
1
Patrick Phillips,
4
Amirhossein Behranginia,
1
José M. Cerrato,
5
Richard Haasch,
6
Peter Zapol,
3
Bijandra Kumar,
1,7
Robert F. Klie,
4
Jeremiah Abiade,
1
Larry A. Curtiss,
3
†Amin Salehi-Khojin
1
†
Conversion of carbon dioxide (CO
2
) into fuels is an attractive solution to many energy and
environmental challenges. However, the chemical inertness of CO
2
renders many
electrochemical and photochemical conversion processes inefficient. We report a transition
metal dichalcogenide nanoarchitecture for catalytic electrochemical CO
2
conversion to carbon
monoxide (CO) in an ionic liquid.We found that tungsten diselenide nanoflakes show a current
density of 18.95 milliamperes per square centimeter, CO faradaic efficiency of 24%, and CO
formation turnover frequency of 0.28 per second at a low overpotential of 54 millivolts. We also
applied this catalyst in a light-harvesting artificial leaf platform that concurrently oxidized water
in the absence of any external potential.
Electrochemical or photochemical reduction
of carbon dioxide (CO
2
) could in principle
conveniently recycle the greenhouse gas back
into fuels (1–6). However, existing catalysts
are too inefficient in practice (7–11): Either
weak binding interactions between the reaction
intermediatesandthecatalystgiverisetohigh
overpotentials, or slow electron transfer kinetics
result in low exchange current densities. Both of
these metrics depend not only on the intrinsic
electronic properties of the catalyst, but also
on the solvent and the catalyst morphology. Re-
cently, we reported that three-dimensional (3D)
bulk molybdenum disulfide (MoS
2
) catalyzes CO
2
reduction to CO at an extremely low overpo-
tential (54 mV) (12) in an ionic liquid (IL). Here,
we report 2D nanoflake (NF) architectures of
this and other transition metal dichalcogenides
(TMDCs)thatmanifestmuchhigherperformance
for electrocatalytic CO
2
reduction in the IL 1-ethyl-
3-methylimidazolium tetrafluoroborate (EMIM-BF
4
).
CO
2
reduction activities of similarly sized
(~100 nm) TMDC NFs including MoS
2
,WS
2
,
MoSe
2
, and WSe
2
were tested using a rotating
disc electrode. All TMDCs were grown using a
chemical vapor transport technique (13). Fig-
ure 1A shows cyclic voltammetry (CV) results of
WSe
2
NFs, and bulk MoS
2
as well as Ag nano-
particles (Ag NPs) and bulk Ag as a representa-
tive noble-metal catalyst. All experiments were
performed inside a two-compartment, three-
electrode electrochemical cell (fig. S6) using an
electrolyte of 50 volume percent (vol %) EMIM-
BF
4
and 50 vol % deionized water; this compo-
sition gives the maximum CO
2
reduction activity
(13). The polarization curves of all studied cata-
lysts were obtained by sweeping potential be-
tween +0.8 and –0.764 V versus RHE (reversible
hydrogen electrode; all potentials reported here
are based on RHE) with a scan rate of 50 mV s
−1
(Fig. 1A and fig. S8). We also performed chrono-
amperometry at different applied potentials for
WSe
2
NFs. The results indicate that the obtained
current densities for all applied potentials are
10 to 20% less than the CV results with 50 mV/s
scan rate (fig. S9). The difference is attributed to
the charging current (capacitive behavior) in the
CV measurements.
The CO
2
reduction began at –0.164 V (over-
potential of 54 mV) for WSe
2
NFs, as confirmed
by faradaic efficiency (FE) measurements (Fig.
1B). At this potential (overpotential of 54 mV), a
current density of 18.95 mA/cm
2
(normalized
on the basis of geometrical surface area) was ob-
tained for WSe
2
NFs; by comparison, current den-
sities were 0.19 mA/cm
2
for bulk Ag, 1.57 mA/cm
2
for Ag NPs, and 3.4 mA/cm
2
for bulk MoS
2
.The
CO formation FEs for WSe
2
NFs (Fig. 1B) and
bulk MoS
2
(12) were 24% and 3%, respectively.
However, the Ag NPs and bulk Ag did not reduce
CO
2
at this overpotential. At –0.764 V potential,
the recorded current density for WSe
2
NFs was
330 mA cm
−2
, versus 3.3 mA cm
−2
for bulk Ag,
11 mA cm
−2
for Ag NPs, and 65 mA cm
−2
for bulk
MoS
2
. The CO formation turnover frequency (TOF)
(Fig. 1C) (13), a measure of per-site activity of
catalysts to produce CO, was 0.28 s
−1
for WSe
2
NFs versus 0.016 s
−1
for bulk MoS
2
.However,this
valuewaszeroforAgNPs,astheycouldnot
produce CO at this overpotential (54 mV). Figure
1C also shows that the CO formation TOF of
WSe
2
was approximately three orders of mag-
nitude higher than that of Ag NPs in the over-
potential range of 150 to 650 mV.
Gas chromatography and differential electro-
chemical mass spectroscopy analyses indicated
that CO and H
2
weretheonlygas-phaseproducts
(5,11,12,14–16)inthepotentialrangeof0to
–0.764 V (13). The measured FE for WSe
2
NFs/IL
(Fig. 1B) showed that this system is highly sel-
ective for CO formation at high potentials (–0.2
to –0.764 V). However, at smaller potentials (–0.164
to –0.2 V), it produces a mixture of CO and H
2
(synthesis gas). Figure S13 shows the selectivity
(FE) results of all TMDCs tested in this study (13).
The catalytic performance of TMDC NFs was
compared with that of other reported catalysts
(Fig. 1D) by multiplying current density (activity)
by CO formation FE (selectivity). At 100 mV
overpotential, the performance of WSe
2
NFs ex-
ceeded that of bulk MoS
2
and Ag NPs tested under
identical conditions in an ionic liquid by a factor
of nearly 60. The performance of WSe
2
NFs also
exceeds those of Au NPs (17)andCuNPs(18)by
three orders of magnitude. Additionally, at this
overpotential, the performance of WSe
2
exceeded
that of WS
2
and MoSe
2
NFs by factors of 3 and 2,
respectively (Fig. 1D). We also performed chro-
noamperometry experiments to examine the elec-
trochemical stability of WSe
2
NFs in 50:50 vol
% IL/deionized water. At the applied potential of
–0.364 V (0.254 V overpotential), a small decay
(10%) was observed after 27 hours of continuous
operation of the three-electrode two-compartment
cell (fig. S14) (13).
The photochemical performance of WSe
2
/IL
was also studied using a custom-built wireless
setup. This artificial leaf mimics the photosynthesis
process in the absence of any external applied
potential. The cell (Fig. 2A) (13) is composed of
three major segments: (i) two amorphous silicon
triple-junction photovoltaic (PV-a-si-3jn) cells in
series to harvest light, (ii) the WSe
2
/IL cocatalyst
SCIENCE sciencemag.org 29 JULY 2016 •VOL 353 ISSUE 6298 467
1
Department of Mechanical and Industrial Engineering,
University of Illinois, Chicago, IL 60607, USA.
2
Department
of Mechanical Engineering, Chungbuk National University,
Cheongju 361-763, South Korea.
3
Materials Science Division,
Argonne National Laboratory, Argonne, IL 60439, USA.
4
Department of Physics, University of Illinois at Chicago,
Chicago, IL 60607, USA.
5
Department of Civil Engineering,
University of New Mexico, Albuquerque, NM 87131, USA.
6
Materials Research Laboratory, University of Illinois at
Urbana-Champaign, Urbana, IL 61801, USA.
7
Conn Center for
Renewable Energy Research, University of Louisville,
Louisville, KY 40292, USA.
*These authors equally contributed to this work. †Corresponding
author. Email: salehikh@uic.edu (A.S.-K.); curtiss@anl.gov (L.A.C.)
RESEARCH |REPORTS
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(6298), 464-467. [doi: 10.1126/science.aaf8533]353Science
Longhi, Natalia M. Litchinitser and Liang Feng (July 28, 2016)
Pei Miao, Zhifeng Zhang, Jingbo Sun, Wiktor Walasik, Stefano
Orbital angular momentum microlaser
Editor's Summary
, this issue p. 464Science
the rate of information transmission. increaseThese microlasers could find application in telecommunication and information technologies to
controlled amount of optical angular momentum is generated internally to the designed device structure.
demonstrate a possible route for an integrated optics approach in which a twisted-light source with a
et al.optical angular momentum, or twist, has usually been achieved with bulk optic devices. Miao
encode information for optical communications. Creating light beams with the desired amount of
Structured light, in the form of helical wavefronts, provides an additional degree of freedom to
Microlasers with a twist
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