Topological photonics and polaritonics
1
Alexander Khanikaev1,2
1 Department of Electrical Engineering and Department of Physics,
The City College of New York, NY, USA2Department of Physics, The City College and the Graduate Center of CUNY, NY, USA
PhD students and postdocs: Xiang Ni, Sriram Guddala, Filipp Komisasrenko, Mengyao Li, Matthew Weiner, Kai Chen, Anton Vakulenko, Svetlana Kiriushechkina, Yuma Kawaguchi
Collaborators: Andrea AlΓΉ, Yuri Kivshar, Vinod Menon, Dmitriy Kryzhanovskiy, Anton Samusev
TEXAS TECH University, 23 March 2021.
Scattering of waves
WOLFRAM Demonstrations Project
Conventional Topological
Nature 461, 772-775 (2009).
Topological roadmap: From Insulators to Quantum spin Hall state and Topological insulators
3
One-way edge states Spin-locked one-way edge states
Kane, C. L. & Mele, E. J., PRL. 95, 146802 (2005).
Spin-orbit coupling can have an effect similar to that of magnetic field without breaking the time-reversal symmetry.
π» = π»0 β ππ΅πΊ β π© π» = π»0 β ππππΊ β π³
ROBUST!
FDM HaldaneNobel prize 2016
v. Klitzing, Nobel prize 1985
M. KΓΆnig, et al. Science 318, 766-770 (2007).K. v. Klitzing, et al., PRL 45 (6): 494β497 (1980).
From condensed matter to photonics:photonic crystals β semiconductors of light
4
Joannopoulos, J. D., Johnson, S. G., Winn, J. N. & Meade, R. D., Photonic Crystals: Molding the Flow of Light, 2nd ed. (Princeton University Press, Princeton , 2008).
Periodically stacking materials with different dielectric or magnetic properties
Infinite in all other directions
Topological order for photons
Classification of photonic topological systems
+Non-Hermitian (parity-time) + unconventional symmetries (supersymmetry/square root)?
Topological order for photons
6
Nature Phys. 7, 907β912 (2011).Nature Photon. 7, 1001β1005 (2013)
arXiv:1507.00337 (2015)Nature Physics (2016)
Nature Photon. 6, 782β787 (2012)
Nature Mater. 12, 233β239 (2012)Nature 496, 196β200, (2013)
Nature 461, 772-775 (2009).
Nature Comm. 5, 5782, (2014)
Phys. Rev. Lett. 114, 223901 (2015)arXiv:1401.1276 (2012)
Broken TR symmetry and Floquet Preserved TR symmetry
PRL 100, 013904, (2008)
Topological invariants for photonicsMathematical foundation of topological photonics & Schrodinger form of Maxwellβs equations.
π»-effective Hamiltonian
Effective Hamiltonian Description
+ ]π¬π =[ π¬π
π π€ = β¨π¬π π« π»π π¬π β©
πΆ =1
2ππΰΆ±
BZ
π2π π»π Γπ π§ =12
π―π||z
K
πΆ = πΆK + πΆKβ² = 1
Haldane, Raghu, PRL 100, 013904, (2008)
Topological roadmap: From Insulators to Quantum spin Hall state and Topological insulators
10
One-way edge states Spin-locked one-way edge states
Kane, C. L. & Mele, E. J., PRL. 95, 146802 (2005).
Spin-orbit coupling can have an effect similar to that of magnetic field without breaking the time-reversal symmetry.
π» = π»0 β ππ΅πΊ β π© π» = π»0 β ππππΊ β π³
ROBUST!
FDM HaldaneNobel prize 2016
v. Klitzing, Nobel prize 1985
M. KΓΆnig, et al. Science 318, 766-770 (2007).K. v. Klitzing, et al., PRL 45 (6): 494β497 (1980).
Role of Symmetries: topological systems with the time-reversal symmetry
β = β0 + πππ
Preserved TR symmetry ensures the presence of Kramerβs TR partners (two spins/helicities) in fermionic systems but not in bosonic.
π―π β π―πβ1 = β and π―π
2 = β1
β0 - unperturbed fermionic lattice potential
πππ = βππππΊ β π³ - gauge (SO) potential inducing topological transition (band crossing)
Kane, C. L. & Mele, E. J., Phys. Rev. Lett. 95, 146802 (2005).Hasan, M. Z. & Kane, C. L., Rev. Mod. Phys. 82, 3045-3067 (2010).Qi, X.-L. & Zhang, S.-C., Rev. Mod. Phys. 83, 1057-1110 (2011).
π―π β π―πβ1 = β and π―π
2 = 1
Fermions Classical/Bosons β = β0 + ππππ’ππ
β0 - unperturbed bosonic lattice potential
ππππ’ππ =Photonic βgaugeβ potential
(SO or pseudo-magnetic) inducing topological transition
Consequence: TR alone is not sufficient for topological order for bosons, i.e. no topological phase analogous to fermionic TR phase is possible.
Robustness is insured by TR symmetry (no magnetic defects are allowed). Doublets generated by TR are locked to theirpropagation directions β spin-locking.
Solution: non-TR symmetry protected phases.
απΆπ β απΆπβ1 = β and απΆπ
2 = β1
Where απΆπ is a spatial or internal symmetry operator generating a doublet state βpseudo-spin degree of freedom.
π―ππβ β π = π―ππ
β β βπ
Role of Symmetry and Gauge Potentials in Topological Phases
Photonic topological insulator: I. Duality of EM field as the pseudo-spin generating symmetry
πΒ± π; π = πΈπ§(π; π) Β± π»π§(π; π)
Duality in free space follows by the symmetry of Maxwell equations with respect to electric and
magnetic fields: π· π¬,π― β βπ―,π¬
Broken by materials response ΖΈπβ ΖΈπ, it can be restored by (meta-)materialβs design.
In dual material ππ§π§ =ππ§π§ , πβ₯ =πβ₯ , duality transformation operator, which satisfies π·2 = β1, allows emulating spin degree of freedom.
π―π πΒ± π; π = πβ π;βπ
π = Β±1 dipolar degeneracy
Khanikaev et al., Nat. Mater. 12, 233 (2012).
Role of Symmetry and Gauge Potentials in Topological Phases
Photonic topological insulator: II. Bianisotropy as the gauge field
π« = ΖΈππ¬ + π ΖΈππ― and π© = ΖΈππ― β π ΖΈπππ¬, where ΖΈπ =
0 ππ₯π¦ 0
βππ₯π¦ 0 0
0 0 0
Khanikaev et al., Nat. Mater. 12, 233 (2012).
π = Β±1 dipolar degeneracy
β = π£π· ΖΈπ0 ΖΈπ 0ΰ·πβ₯ β πΏπβ₯ +π ΖΈπ3 ΖΈπ 3 ΰ·π3
14
πΆπΒ± = Β±
1
2πΰΆ±BZ
π2π πππ₯π΄π¦πΒ± π β πππ¦π΄π₯
πΒ± π
π¨πΒ± = βπβ¨ππΒ± π€ π»π€ π
πΒ±(π€)β©
Topological invariants of the photonic topological insulator
π+ = π¬π§ +π―π§ πβ = π¬π§ βπ―π§
πΆ1+ = 1
πΆ2+ = β1
πΆ1β = β1
πΆ2β = 1
Khanikaev et al., Photonic Topological Insulators, Nat. Mater. 12, 233 (2012).
ββ/β = π£π· ΖΈπ0 ΖΈπ 0ΰ·πβ₯ β πΏπβ₯ +π ΖΈπ3 ΖΈπ 3 ΰ·π3 πΆπ’/πβ = Β±
π
πand πΆπ’/π
β = βπ
π
Reconfigurable photonic topological insulator
Effective Kane-Mele-like Hamiltonian Non-vanishing spin-Chern numbers
X. Cheng, C. Jouvaud, X. Ni, S. H. Mousavi, A. Z. Genack, and A. B. Khanikaev, Robust propagation along reconfigurable pathways within a photonic topological insulator, Nature Materials 15, 542β548 (2016).
Topological edge states
Reconfigurable domain wall Static non-reconfigurable interface
X. Cheng et al., Nature Materials 15, 542β548 (2016).
Reconfigurable guiding along arbitrarily shaped pathways
Frequency (GHz)
Two 60 deg. bends Two 90 deg. bends
Two 120 deg. bends
Frequency (GHz)
Frequency (GHz)
Exponential localization of the edge states
Transmission through the bulk
Transmission through the domain wall
X. Cheng et al., Nature Materials 15, 542β548 (2016).
3D Photonic Topological Insulatorsβ = π0 + π£|| ΖΈπ 0 πΏππ₯ ΰ·ππ₯ + πΏππ¦ ΰ·ππ¦ + π£π§ ΖΈπ π¦ ΰ·ππ§πΏππ§ Β±π ΖΈπ π§ ΰ·ππ§
Ω± = Β± π2 + π£πΉ2 ππ₯
2 + ππ¦2 + π£π§
2ππ§2Jackiw-Rebbi-like surface states:
Slobozhanyuk, A. et al. 11, 130, Nature Photonics (2017) Yang, Y., et al., Nature (2019).
Lattice/sublattice symmetry protected photonic structures
Wu & Hu, Phys. Rev. Lett. 114, 223901 (2015).
Barik, Science 359, 666 (2018).Gorlach, Nature Comm. 9 , 909 (2018).
Examples of symmetry protected topological systems
Science 357, 61-66 (2017).
Nature 555, 342β345 (2018).
K
Ξ
Kβ²
M
K and Kβ² valleys as pseudo-spin: valley Chern insulator
K = 0,4
3
π
π0
Kβ² = 0,β4
3
π
π0
Shrinking/Expanding the βhexamerβ leads to folding of K and Kβ points to Ξ point of the new lattice.
Ξ
Ξ
K
Kβ²
Ξ
KKβ
As a result of such symmetry reduction, valleys/pseudo-spins mix leading to the topological transition
ππ
ππ
ππ
ππ
Lattice symmetry protected photonic topological metasurfaces
Phys. Rev. Lett. 114, 223901 (2015)
Experimental realizations:
Barik, Science 359, 666 (2018).
Gorlach et al., Nature Comm. 9, 909 (2018)
SOS (Silicon on Sapphire) implementation of all-dielectric PTM: modeling
Gorlach et al., Nature Comm. 9, 909 (2018)
πΒ± = ππ₯ Β± πππ¦
πΒ± = ππ₯π¦ Β± πππ₯2βπ¦2
πΒ± = ππ₯ Β± πππ¦
πΒ± = ππ₯π¦ Β± πππ₯2βπ¦2
SOS (Silicon on Sapphire) implementation and spectroscopy
(Nano-fabrication and SEM imaging was performed at the Center for Functional Nanomaterials of Brookhaven National Laboratory, spectroscopy was carried out at AK lab at City College)
Shrunk (topologically trivial)
Expanded (topologically nontrivial)
Gorlach et al., Nature Comm. 9, 909 (2018)
πΒ± = ππ₯ Β± πππ¦
πΒ± = ππ₯π¦ Β± πππ₯2βπ¦2
πΒ± = ππ₯ Β± πππ¦
πΒ± = ππ₯π¦ Β± πππ₯2βπ¦2
Far field imaging of the edge states
10 periods 10 periods10 periods
Trivial TrivialTopological
~30 repetitions
Gorlach et al., Nature Comm. 9, 909 (2018)
Higher-order photonic topological insulators as a platform for manipulating electromagnetic waves
HOTIs β topological insulators supporting states with dimensionality lower by two and more than the dimensionality of the system
Hierarchy of HOT states in 3D PHOTI
Expanding applications: not just guiding!
Class I: Multipolar higher-order topological insulators
Class II: Wannier-type (dipolar polarization) topological insulators
Benalcazar, et al.,
Quantized electric multipole
insulators. Science 357,61 (2017).
2D: Mittal et al., Nat. Photon. 13, 692
(2019).
Barik, et al.,Science 359, 666 (2018).
3D: Ni et al., Nat. Comm. 2019 (in
press)
Requires complicated phase engineering on the lattice
Wannier type - protected by generalized chiral symmetries
Ni, et al, New J. Phys. 19, 055002 (2017).
Ezawa, Phys. Rev. Lett. 120, 026801 (2018).
Ni, et al., Nat. Mater. 18, 113 (2019).
Li, et al., Nat. Photon. 14 (2), 89-94 (2020).
Higher-order states: simple explanation1D: SuβSchriefferβHeeger model
π» π = π + πΎ cos π ΰ·ππ₯ + πΎ sin π ΰ·ππ¦
π~ΰΆ±π΅π
β¨π(π)|1
π
π
ππ|π(π)β© ππ
Bulk dipole polarizationβ 0
2D: Dimerized (trimerized) Kagome lattice
The βbulk polarizationβ ππ βΆ
ππ = 1/ππ Οπ,ππ‘π£π π(ππ‘) ,
where ππ π(ππ‘) is the eigenvalue (Wannier center)
of the Wilson loop, which is defined asπ2π+ππ βππ ,ππ‘ =
π’2π+ππ ,ππ‘ π’2π+ππ βπΏπ,ππ‘ β¦ π’ππ +πΏπ,ππ‘ π’ππ ,ππ‘
Ni, et al, New J. Phys. 19, 055002 (2017).
Ezawa, Phys. Rev. Lett. 120, 026801 (2018).
Ni, et al., Nat. Mater. 18, 113 (2019).
Excitation of bulk and corner modes at the same frequency!
Experimental design and results: embedded corner states in the bulk continuum
Zero energy states are embedded into the bulk continuum
The generalized chiral symmetry Ξ3 of
the effective Hamiltonian H0:
Ξ3H0Ξ3β1 = H1,
Ξ3H1Ξ3β1 = H2,
H0 + H1 + H2 = 0.
It ensures that π1 + π2 + π3 = 0,and the presence of βzero energyβcorner states.
Note: compare to regular chiral symmetry ΞH0Ξβ1 = βH0, which ensures Ο΅1 + Ο΅2=0, or Ο΅2 = βΟ΅1
X. Ni, M. Weiner, A. Alu, A. B. Khanikaev, Nature Materials 12 (2018).
Two different Kagome unit cell design
Silicon pillars
Photonic implementation: New Physics
M. Li, et al. Nature Photonics 2020 (doi:s41566-019-0561-9)
Two different Kagome unit cell design
Silicon pillars Holes in silicon
Optical domain implementation
Photonic implementation: higher order 0D corner states
A. Vakulenko et al, under review.
Simulations results for the finite triangular shaped domain of topological (expanded) Kagome lattice surrounded by the shrunken (trivial) lattice.
Topological boundary modes of a higher-order topological metasurface. a, Schematic of the 2D HOTI crystal (inset) with diamond
shaped holes, and its bandstructure calculated by finite-element method (FEM) for topological (red solid lines) and gapless cases (black
dashed lines). b, c, d, show field profiles for edge, Type-I and Type-II corner modes, respectively, calculated for a finite topological lattice
surrounded by the trivial lattice. π0 is the lattice constant. Parameters used: the long size of the diamond hole is 0.71π0 the short one is
0.41π0. The structure is expanded or shrunken by 10%. The numbers on top of b, c, d are the eigenfrequencies of the boundary modes
(relative to the Dirac-point frequency).
DARPA NLM Quarterly Review β Polaritonic Metamaterials 31
Higher-order states induced by far-field interactionsβ = β0 + βNNN
β0 =
0 πΎ + π½ππ(12ππ₯+
32ππ¦)a0 πΎ + π½πβπ(
12ππ₯β
32ππ¦)a0
πΎ + π½πβπ(12ππ₯+
32ππ¦)a0 0 πΎ + π½πβπππ₯a0
πΎ + π½ππ(12ππ₯β
32ππ¦)a0 πΎ + π½ππππ₯a0 0
βNNN = π½2 β
0 ππππ₯a0 + πβπ(12ππ₯β
32ππ¦)a0 ππ(
12ππ₯+
32ππ¦)a0 + πβπππ₯a0
πβπππ₯a0 + ππ(12ππ₯β
32ππ¦)a0 0 πβπ(
12ππ₯β
32ππ¦)a0 + πβπ(
12ππ₯+
32ππ¦)a0
πβπ(12ππ₯+
32ππ¦)a0 + ππππ₯a0 ππ(
12ππ₯β
32ππ¦)a0 + ππ(
12ππ₯+
32ππ¦)a0 0
Energy spectrum of finite Kagome triangle structure with growing NNN coupling π½2. Type II corner states
clearly split from edge band as π½2 increases.
Next-nearest neighbor correction
M. Li, et al. Nature Photonics 2019 (doi:s41566-019-0561-9)
Air
Si - Kagomelayer
Substrate(glass n=1.45)
ππππ=1.91
Freq
uen
cy (
Hz)
Dirac cone
Light line - Air
Light line β Substrate (SiO2)
Design of HOTI SOI metasurface for optical domain
Wavenumber k (1/m)
Freq
uen
cy (
Hz)
Topological bandgap
Light line - Air
Light line β Substrate (SiO2)
364 nm
165 nm
110nm
Si π=12
Photonic metasurfacetopological band gap
a0 = 520 nm
364 nm
150 nm
78
nm
210.2 nm
Photonic metasurface with Dirac cone
Photonic implementation and probing
The Dirac cone and the topological transition takes place below the light cone; therefore the near-field solid-state immersion spectroscopy
is required for the extraction of the band structure
The shrinkage (bringing holes together) or expansion (bringing them art) leads to opening trivial and topological bandgaps, respectively, in place of the Dirac cone
Experimental observation of the topological transition
Solid-state immersion angle resolved spectroscopy
Theory Experiment
Dirac cone and itβs opening induced by dimerization of the photonic Kagome metasurface. Experiment was performed by solid state immersion spectroscopy with ZnSe lens (n~2.5).
Experimental observation of photonic higher-order states in far-field
Experimental setup Finite triangular-shaped topological crystal surrounded by the trivial domain
Co
rne
r st
ate
sEd
ge s
tate
s
Experimental observation of photonic higher-order states in near-field
a, SEM image of a metasurface showing the topological domain surrounded by the trivial domain. Yellow dashed triangle
shows the domain wall separating topological (yellow unit cell) and trivial (purple unit cell) domains. b, near-field
amplitude distribution of the topological edge states at 6.24 Β΅m, c, near-field images of the Type-I corner state, and, d, near-
field of the Type-II topological corner state induced by the long-range interactions at ~6.2 Β΅m.
Near-field imaging of first- and of the two types of higher-order
topological boundary states in mid-IR.
A. Vakulenko et al, under review.
Tapping-mode atomic force microscope
(AFM) was used, where the incident p-
polarized Mid-IR light is focused onto a
commercial tip with a PtIr5 coating
(NanoWorld) that enables the light
interaction with the local near field of the
metasurface.
Excitation souce β MirCAT QCL.
The near-field profiles are plotted (in color)
over AFM topographic (grayscale) images,
and all were obtained from s-SNOM signals
demodulated at the 3rd harmonics of the
tapping frequency to subtract the
background signal.
Phase measurement in experiment
Near-field phase profile of the lower energy bulk state at 6.55 Β΅m wavelength.Purple dashed line shows the interface between shrunken and expanded domains. White dashed linesshow boundaries of unit cells.
Local phase accumulated is ~ to delay time photonsexperience in the system, which, in its turn, is ~ tothe local density of states
π~ππππππ¦~ππβ
Direct observation of the Wannier center shift enabled by the near-field measurements
Trivial Topological
Mixing light with matter β topological polaritonicsPolaritons β half-light half-matter quasi-excitaions in strongly coupled systems
β Quantum phenomena:BE condensation due to small massβ Enhanced nonlinear optical effects (solitons) β Polaritonic lasers
+Topology
S. Klembt, T. H. Harder, O. A. Egorov, K. Winkler, R. Ge, M. A. Bandres, M. Emmerling, L. Worschech, T. C. H. Liew, M. Segev, C. Schneider & S. HΓΆfling, Nature 562, 552β556 (2018).
Skolnik, Shelysh, Amo,β¦
Topological polaritonic states Excitonic (L/R) bands
Photonic states
Photonic states
Topological light strongly interaction with excitons in 2D materials/photonic metasurfaces heterostructures
High-Q topological photonic metasurface
Highly confined topologicalexciton-polaritonic edge states
Topological excitons polaritons in 2D TMDCs
2D semiconductors TMDCs(MoS2, MoSe2, WS2, WSe2)
Topological charge/curvature transfer
Valley excitons in MoSe2
Topological Photonic bands
Photonic BS Polaritonic BS
Berry curvature/spin-Chern number is transferred to upper polariton from topological photonic band due to strong coupling
Gives rise to (photonic) pseudo-spin and valley polarized (excitonic) polaritonic topological boundary modes
Experimental observation of topological exciton-polaritons in MoSe2
Topological photonic metasurface hBN encapsulated MoSe2 on top of metasurface
Summary and Outlook
β’ Photonics offers an ideal platform for emulating topological states of condensed matter and for controlling light with synthetic gauge fields.
β’ Topological states envision a broad range of applications such as robust waveguides with controllable routing along the domain walls and integrated into classical and quantum optical systems where interaction among optical/RF elements has robust character.
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