Zero-index metamaterials (photonic materials and artificial media with a near-zero refractive index in different parts of the electromagnetic spectrum) make possible novel forms of light–matter interactions where light behaves in a qualitatively different way than in conventional materials. Owing to the “stretching” of the wavelength as a consequence of the refractive index being near zero, the exceptional nature of electromagnetic fields in such zero-index metamaterials reveals itself in the form of spatially static but temporally dynamic field distributions, which possess unusual propagation, radiation, and nonlinear properties.

During the last several years, there has been an upsurge of exciting discoveries of exotic wave phenomena in these materials, leveraging the impact of their near-zero refractive index on several photonic technologies, including nonlinear optics, thermal emitters, reconfigurable optical devices, integrated photonics, and quantum technologies. Interest in zero-index metamaterials has exploded in recent years, bridging nanophotonics, electrical engineering, materials science, nanofabrication, and quantum physics. In this collection, we feature 15 exciting contributions to this field of research. The papers span nonlinear optics, quantum optics, heat transfer, integrated photonics, photonic crystals, and bound states, and they demonstrate the breadth of applications made possible by the exciting light–matter interactions associated with zero-index metamaterials. In the following, we give a brief overview of this collection, as an introduction to the papers.

Epsilon-near-zero (ENZ) materials, a subclass of zero-index metamaterials characterized by a near-zero relative permittivity, are known to exhibit strong and ultrafast nonlinearities, even within subwavelength distances. Therefore, research on the nonlinear response of ENZ materials has become a very active field of research, cutting across from materials science to nanophotonic engineering. Three articles in this collection are good examples of the multidisciplinary efforts in the field. Secondo et al.1 establish a novel framework to compute the performance of nonlinear ENZ materials. Their method allows for the prediction of both inter- and intraband effects, and its applicability is validated by studying several ENZ materials. Britton et al.2 use post-deposition annealing processes to modify the structural properties of ENZ nanolayers. The studied thin films present an enhanced nonlinear susceptibility, confirming the importance on structural control and secondary phase formation in the nonlinear response of zero-index metamaterials. Ebrahim et al.3 investigate the temporal dynamics in strongly coupled ENZ plasmonic systems, illustrating the dynamics of field enhancement processes and highlighting the impact of dissipation loss.

The applications of zero-index metamaterials also extend to the field of quantum optics. Specifically, wavelength expansion benefits long-range entanglement, allowing for longer entanglement distances and robustness against inaccuracies in the positioning of the emitters. Several articles in this collection discuss the role of zero-index metamaterials in entanglement generation. Li and Argyropoulos4 explore this idea with three- and four-level emitters as well as their application in high fidelity two-qubit quantum phase gates. Issah and Caglayan5 propose that rolled-up ENZ waveguides provide high long-range entanglement as compared to the more conventional rectangular waveguides. Mello et al.6 investigate many-body configurations in diamond metamaterials, observing radiative power enhancements associated with collective super-radiant effects.

Dirac-cone photonic crystals have recently emerged as a promising platform in which to explore zero-index concepts. Dirac cone photonic crystals are low-loss all-dielectric structures, compatible with integrated photonic circuits, a leading technology in modern communication, sensing, and computation devices. Furthermore, designing the geometry of the unit-cell of the photonic crystal adds extra functionalities to zero-index metamaterials. For example, Song et al.7 demonstrate that the geometry of the photonic crystal empowers great control over the quality factor of zero-index modes as well as their angular and spectral selectivity. Jamilan et al.8 show that anisotropic photonic crystals enable collimation of the fields and nondiffractive propagation. Moreover, Xu et al.9 show that the judicious combination of loss and gain further widens the scope of zero-index photonic crystals to non-Hermitian photonics with applications to angular sensors, coherent perfect absorbers, and lasers. The fine control of the radiation from zero-index photonic crystals is likely to find technological applications, as evidenced by the isotropic radiators designed by Liu et al.10 

Zero-index metamaterials can also offer interesting opportunities to control radiative heat transfer due to the associated enhanced light–matter interactions. Hajian et al. demonstrate strong near-field heat transfer enhancement in a hybrid parallel-plate structure made of polaritonic materials operating near their zero-permittivity frequency.11 

Finally, as another opportunity of zero-index metamaterials, ultimate control on trapping and guiding of light waves has been demonstrated in various platforms. As examples of these opportunities, Prudencio and Silveirinha12 and Castaldi et al.13 discuss embedded eigenstates within the continuum of radiation in nanospheres and layered media featuring zero-index materials. Beruete et al.14 demonstrate experimentally the unusual waveguiding features of zero-permittivity structures with applications for enhanced sensing, and in parallel, Nicolussi et al.15 discuss unusual unidirectional transparency in waveguides featuring zero-permittivity materials obeying parity-time symmetry.

We hope that you may enjoy this special topic collection and the excellent contributions by the authors and that this can serve as a snapshot of the latest discoveries in this rapidly growing field of research, inspiring new opportunities and research directions. We thank the contributing authors for their articles, Professor Lesley Cohen, Editor-in-Chief of Applied Physics Letters, and the editorial board and staff for their support in putting together this collection.

1.
R.
Secondo
,
A.
Ball
,
B.
Diroll
,
D.
Fomra
,
K.
Ding
,
V.
Avrutin
,
Ü.
Özgür
,
D. O.
Demchenko
,
J. B.
Khurgin
, and
N.
Kinsey
, “
Deterministic modeling of hybrid nonlinear effects in epsilon-near-zero thin films
,”
Appl. Phys. Lett.
120
,
031103
(
2022
).
2.
W. A.
Britton
,
F.
Sgrignuoli
, and
L.
Dal Negro
, “
Structure-dependent optical nonlinearity of indium tin oxide
,”
Appl. Phys. Lett.
120
,
101901
(
2022
).
3.
M. H.
Ebrahim
,
A.
Marini
,
V.
Bruno
,
N.
Kinsey
,
J. B.
Khurgin
,
D.
Faccio
, and
M.
Clerici
, “
Temporal dynamics of strongly coupled epsilon near-zero plasmonic systems
,”
Appl. Phys. Lett.
119
,
221101
(
2021
).
4.
Y.
Li
and
C.
Argyropoulos
, “
Multiqubit entanglement and quantum phase gates with epsilon-near-zero plasmonic waveguides
,”
Appl. Phys. Lett.
119
,
211104
(
2021
).
5.
I.
Issah
and
H.
Caglayan
, “
Qubit–qubit entanglement mediated by epsilon-near-zero waveguide reservoirs
,”
Appl. Phys. Lett.
119
,
221103
(
2021
).
6.
O.
Mello
,
Y.
Li
,
S. A.
Camayd-Muñoz
,
C.
DeVault
,
M.
Lobet
,
H.
Tang
,
M.
Lonçar
, and
E.
Mazur
, “
Extended many-body superradiance in diamond epsilon near-zero metamaterials
,”
Appl. Phys. Lett.
120
,
061105
(
2022
).
7.
A. Y.
Song
,
A. R. K.
Kalapala
,
R.
Gibson
,
K. J.
Reilly
,
T.
Rotter
,
S.
Addamane
,
H.
Wang
,
C.
Guo
,
G.
Balakrishnan
,
R.
Bedford
,
W.
Zhou
, and
S.
Fan
, “
Controllable finite ultra-narrow quality-factor peak in a perturbed Dirac-cone band structure of a photonic-crystal slab
,”
Appl. Phys. Lett.
119
,
031105
(
2021
).
8.
C.
Xu
,
M.
Farhat
, and
Y.
Wu
, “
Non-Hermitian electromagnetic double-near-zero index medium in a two-dimensional photonic crystal
,”
Appl. Phys. Lett.
119
,
224102
(
2021
).
9.
S.
Jamilan
,
M.
Danyal
, and
E.
Semouchkina
, “
Collimation effects controlled by near-zero refractive indices in highly anisotropic dielectric photonic crystals: Simulation and experiment
,”
Appl. Phys. Lett.
119
,
251901
(
2021
).
10.
Z.
Liu
,
Z.
Zhou
,
Y.
Li
, and
Y.
Li
, “
Integrated epsilon-near-zero antenna for omnidirectional radiation
,”
Appl. Phys. Lett.
119
,
151904
(
2021
).
11.
H.
Hajian
,
I. D.
Rukhlenko
,
V.
Erçağlar
,
G.
Hanson
, and
E.
Ozbay
, “
Epsilon-near-zero enhancement of near-field radiative heat transfer in BP/hBN and BP/α-MoO3 parallel-plate structures
,”
Appl. Phys. Lett.
120
,
112204
(
2022
).
12.
F. R.
Prudêncio
and
M. G.
Silveirinha
, “
Monopole embedded eigenstates in nonlocal plasmonic nanospheres
,”
Appl. Phys. Lett.
119
,
261101
(
2021
).
13.
G.
Castaldi
,
M.
Moccia
, and
V.
Galdi
, “
Synthesizing quasi-bound states in the continuum in epsilon-near-zero layered materials
,”
Appl. Phys. Lett.
119
,
171110
(
2021
).
14.
M.
Beruete
,
N.
Engheta
, and
V.
Pacheco-Peña
, “
Experimental demonstration of deeply subwavelength dielectric sensing with epsilon-near-zero (ENZ) waveguides
,”
Appl. Phys. Lett.
120
,
081106
(
2022
).
15.
M.
Nicolussi
,
J. A.
Riley
, and
V.
Pacheco-Peña
, “
Unidirectional transparency in epsilon-near-zero based rectangular waveguides induced by parity-time symmetry
,”
Appl. Phys. Lett.
119
,
263507
(
2021
).