Metamaterial research at Aalto
University
Aalto University
School of Electrical Engineering
Department of Radio Science and Engineering
Sergei Tretyakov and colleagues
Outline
• University, Department, SMARAD Center of Excellence,
research group
• Overview of recent research
– Electromagnetics of complex media
– Invisibility and cloaking
– Isotropic artificial magnetics in the visible
– Some other topics
About myself and my visit to Jena...
Optics and photonics
Jena, Germany
Electromagnetics,
microwave engineering
Espoo, Finland
Page 4
Aalto University
(former Helsinki University of Technology)
• The oldest and the biggest technical university in Finland – About 15000 students and 250 professors
• School of Electrical Engineering (former faculty of Electronics, Communications, and Automation)
• Department of Radio Science and Engineering – Electromagnetics, circuit theory, microwave to terahertz
techniques (antennas, devices) and measurements, space technology, radio wave propagation, advanced artificial electromagnetic materials
• Research groups lead by professors
About geography
There is no such place called ”Aalto”
• Campuses of the Aalto University are in Espoo and
Helsinki
How it looks like?
Aalto is growing:
SIX Tenure Track PROFESSOR Positions
The School of Electrical Engineering seeks experts in the following fields:
• Electrical engineering, Power and energy, Automatic control, Embedded systems, Smart living environment;
• Radio science and engineering, Radio astronomy, Space science and engineering, Optoelectronics;
• Communications and networking engineering.
The positions are open at all levels from assistant professor to full tenured professor.
Application deadline is September 30, 2012.
The RAD department was established in the beginning of 2008 by integrating four laboratories of TKK (TKK is today Aalto University School of Science and Technology), namely Radio Laboratory, Electromagnetics Laboratory, Circuit Theory Laboratory, and Laboratory of Space Technology, into a single unit.
Personnel:
– 8 professors
– Over 20 other researchers with a doctor degree
– About 30 doctoral students
– Total number of employees about 100
RAD department is involved in SMARAD CoE in research selected by the Academy of Finland for 2002–2007 (as Smart and Novel Radios Research Unit) and 2008–2013 (as Finnish Centre of Excellence in Smart Radios and Wireless Research).
RAD department is also involved in the MilliLab (Millimetre Wave Laboratory of Finland), which has the status of External Laboratory of the European Space Agency (ESA). MilliLab is a joint research institute between Aalto and VTT.
Department of Radio Science and Engineering (RAD)
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The Virtual Institute for Artificial Electromagnetic Materials
and Metamaterials (”Metamorphose VI”) is a non-for-
profit international association whose purposes are the
research, the study and the promotion of artificial
electromagnetic materials and metamaterials.
About 20 European universities and institutions are members.
METAMORPHOSE Virtual Institute www.metamorphose-vi.org
6th International Congress
on Advanced Electromagnetic Materials in
Microwaves and Optics
Metamaterials 2012 St. Petersburg, Russia
17-22 September 2012
congress2012.metamorphose-vi.org
Distributed European Doctoral
School on Metamaterials
• Led by Dr. Carsten Rockstuhl, U. Jena
• www.school.metamorphose-eu.org
• METAMORPHOSE Virtual Institute
www.metamorphose-vi.org
The next EUPROMETA school
• Reconfigurable and tunable metamaterials
– September 21-22, 2012, in conjunction with the
Metamaterials'2012 Conference
– St. Petersburg, Russia
Artificial electromagnetic materials and applications
Research group
4 senior researchers (doctoral degree)
4 doctoral students
visitors + undergraduate students
Page 15
Research group: Main current research
directions
• Artificial materials (metamaterials) with unusual and extreme
electromagnetic properties – engineering materials for applications
• Nano-scale composite materials and artificial surfaces
• New designs of antennas and microwave devices
• New designs for terahertz and optical devices (imaging and sensing, nano-
scale optics...)
• New applications (cloaks, superlenses, solar cells, thermophotovoltaics...)
Classification of electromagnetic
nanostructured materials
Not yet
investigated, but
possible
Metawaveguides1D, linear
nanostructured
optically dense
surfaces without
useful and unusual
electromagnetic
properties
Artificial
impedance surfaces
with long inclusions
or slots
Plasmonic
diffraction grids,
optical band-gap
surfaces and
optical frequency
selective surfaces
Metasurfaces /
metafilms
2D, surface
Bulk nanostructured
materials without
useful and unusual
electromagnetic
properties
Wire media,
multilayer optical
fishnet structures,
alternating solid
plasmonic and
dielectric
nanolayers
Photonic crystals
and quasi-
crystals,
Optically sparse
random
composites
Bulk MTM of small
inclusions
3D, bulk
Optically dense in
one direction, while
either optically
sparse or with
extended inclusions
in other
direction(s)
Optically sparse
(q·a >1)
Optically dense
(q·a<1)
Nanostructures
Not yet
investigated, but
possible
Metawaveguides1D, linear
nanostructured
optically dense
surfaces without
useful and unusual
electromagnetic
properties
Artificial
impedance surfaces
with long inclusions
or slots
Plasmonic
diffraction grids,
optical band-gap
surfaces and
optical frequency
selective surfaces
Metasurfaces /
metafilms
2D, surface
Bulk nanostructured
materials without
useful and unusual
electromagnetic
properties
Wire media,
multilayer optical
fishnet structures,
alternating solid
plasmonic and
dielectric
nanolayers
Photonic crystals
and quasi-
crystals,
Optically sparse
random
composites
Bulk MTM of small
inclusions
3D, bulk
Optically dense in
one direction, while
either optically
sparse or with
extended inclusions
in other
direction(s)
Optically sparse
(q·a >1)
Optically dense
(q·a<1)
Nanostructures
q
a
leff=2p/q
Bianisotropic constitutive relations
HEB
HED
=
=
j
j TT
=
=
arbitary,0)j(j :Reciprocal
real,)j(j :Lossless
TTTT
*TTTT*
===
==
(Ari Sihvola)
Classification of bianisotropic materials
Symmetric part:
6 parameters
(RECIPROCAL)
Dielectric crystal
Magnetic medium
Chiral medium
Tellegen (Cr2O3)
Anti-symmetric part
3 parameters
(NON-RECIPROCAL)
Magneto-plasma
Biased
ferrite
Omega medium
Moving medium
(Ari Sihvola)
Material optimized for the purpose?
The ”optimal” spiral The resonant frequency (the total length of the wire) is fixed.
For which shape the particle has the max energy in a given external
field?
I.V. Semchenko, S.A. Khakhomov, and A. L. Samofalov, The Optimal Shape
of a Spiral: Equality of Dielectric,Magnetic and Chiral Properties, NATO
Advanced Research Workshop META’08, Marrakesh, Morocco, May 7-10, 2008.
Page 20
Optimal metamaterials Chiral media
”Optimal” spiral inclusions: n=0, κ=1 – ”extreme parameter values”
Negative refraction, negative reflection, ”standing spirals”,... S. Tretyakov, I. Nefedov, A. Sihvola, S. Maslovski, C. Simovski, Waves and energy in chiral nihility,
Journal of Electromagnetic Waves and Applications, vol. 17, no. 5, pp. 695-706, 2003.
E. Saenz, I. Semchenko, S. Khakhomov, K. Guven, R. Gonzalo, E. Ozbay, S. Tretyakov,
Modelling of spirals with equal dielectric, magnetic and chiral susceptibilities, Electromagnetics,
vol. 28, pp. 476–493, 2008.
I.V. Semchenko, S.A. Khakhomov and S.A. Tretyakov, Chiral metamaterial with unit negative
refraction index, The European Physical Journal - Applied Physics, vol. 46, no. 3, p. 32607, 2009.
Optimal metamaterials for linear polarization Omega media
Omega coupling is a very general phenomenon
Particle energy (omega particle)
But for evanescent waves the impedance is imaginary
and we can optimize the particle shape to maximize the particle energy.
Bianisotropic materials optimized for strong interactions with
electromagnetic fields
Total absorption in thin bianisotropic layers
Example: Total absorption
in an omega-layer
General approach to design of thin layers for
any desired electromagnetic response
Example: Twist polarizer
Electromagnetic cloaking
• Cloaking: Reduction of an object’s total scattering cross section (SCS)
• Total scattering cross section = ∫ (σ) dΩ
• In the case of infinitely high structures we speak about the
total scattering width: The integration is done over one angle in a plane
Broadband cloaking devices/approaches developed at
Aalto University
• Transmission-line cloak
• Metal-plate cloak
P. Alitalo and S. Tretyakov, “Electromagnetic cloaking with metamaterials,” Materials Today,
vol. 12, no. 3, pp. 22-29, 2009.
P. Alitalo and S. A. Tretyakov, “Broadband electromagnetic cloaking realized with transmission-line and
waveguiding structures,” Proc. IEEE, vol. 99, no. 10, pp. 1646-1659, 2011.
5 6 7 8 9 10 11 12 13 14 150
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
f [GHz]
No
rma
lize
d to
tal sca
tte
rin
g w
idth
Transmission-line cloak
• Designed for TE polarization
• HFSS simulations of E-fields at 10 GHz
(complete structure, no homogenization!):
P. Alitalo, et al., “Experimental characterization of a broadband transmission-line cloak in free space,”
IEEE Trans. Antennas Propagat., accepted for publication. (arXiv: 1110.2353)
Metal-plate cloak
5 6 7 8 9 10 11 12 13 14 150
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
f [GHz]
No
rma
lize
d to
tal sca
tte
rin
g w
idth
P. Alitalo, et al., “Bistatic scattering characterization of a three-dimensional broadband cloaking
structure,” J. Appl. Phys., vol. 111, p. 034901, 2012.
• Designed for TE polarization
• HFSS simulations of E-fields at 10 GHz
(complete structure, no homogenization!):
Bistatic measurements
• Cloak: metal-plate cloak
• Object to be hidden: metal cylinder (diameter 30mm, height 184mm)
1λ at 10 GHz
Bistatic X-band measurement setup at DLR
• Bistatic measurements can be
carried out in the xy-plane for
angles 22 - 180 deg
• Frequency range: X-band,
specifically, 8.2 GHz – 12.4 GHz
• The measured object is placed
at the origin
Normalized total scattering widths
• Solid lines: HFSS simulations (full integration)
• Dashed lines: HFSS simulations (integration from 22 … 180)
• Dotted lines: measurements (integration from 22 … 180)
Challenge: Isotropic magnetic material in the visible
Another design idea
Silver core – plasmonic electric response
Si shell – Mie magnetic response
Individual polarizabilities
r1 = 30 nm, r2 = 130 nm
Maxwell Garnett
Periodic 3D array
Random 3D array
Effective material parameters:
Plot PER
Plot RAND
Field distributions
λ = 1153 nm λ = 1071 nm
Field distributions
λ = 877 nm λ = 833 nm
Field distributions
λ = 750 nm
Refraction indices
retrieved from all
these pictures
are values
between those
predicted by plots
PER and RAND:
Our interpretation:
Compensation of
radiative losses of
a particle is
incomplete
Arrays of resonant nanoparticles
Regular vs random arrays
Theory
Enhancement of micron-gap
thermophotovoltaic systems
Radiative heat transfer spectral density q’’l across a 1 m gap normalized to the black-body heat transfer density q’’ pl. Blue line – with CNT. Red line – without them. Hot surface - T=300ºK, cold surface - T=0.
Modification of the micron gap between the hot
and photovoltaic surfaces of a thermophotovoltaic
system by inserting Carbon NanoTubes or nanowires.
The interdigital arrangement
1. transforms the gap into an indefinite medium layer
2. prevents the phonon transfer
(which would suppress the photovoltaic operation).
Super-doped n-Si (”cold”)
Super-doped n-Si (’’hot’’)
2r
a
Light-trapping structures for thin-film solar cells
The results of the simulation of EM fields in a unit cell of the array
of Au nanoantennas illuminated by a normally incident plane wave
at 370 THz. It is seen that the nanoantenna produces hot spots
which are fully located in the spacings between metal
nanoelements and do not heat them. The transmittance through
the photoabsorbing layer averaged over the frequency range is
equal to 0.08: the heating of the substrate is also suppressed.
Without light-trapping
structure but with ARC
With light-trapping
structure
The enhancement of photo-absorption
in the presence of nanoantennas versus
frequency (simulations). Nanoantennas
are far from the plasmon resonance
So-called domino modes help to create
a broadband cavity between
the substrate and the superstrate.
CIGS 110 nm
SiO2 10 nm
Fused
silica
240 nm
Flexible
substrate
Flexible
super-
strate
320 nm
W
Electromagnetic characterization of
metasurfaces
x
z
Incidence
plane
r
k
Meta-
surface
r=12 (Q=45o) r=12 (Q=45o)
Substrate-induced biansiotropy
appears when r>>1
An ultra-broadband electromagnetically
indefinite medium formed by aligned carbon
nanotubes
Thank you!
http://users.tkk.fi/~sergei