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Deliverable 12: Report on fabrication issues and
optical characterization of bulk Photonic
Metamaterials
The strategy of PHOME towards achieving truly three-dimensional (and bulk) photonic
metamaterials has been to follow two fabrication routes in parallel, namely, planar
electron-beam lithography plus stacking of several layers on the one hand (KIT and
Bilkent) and inherently three-dimensional direct-laser-writing (DLW) optical lithography
on the other hand (KIT). The previous status has been reported on in D10, which shall
not be repeated here. Meanwhile it has become clear that the DLW approach – which
seemed like the more risky one at the start – is actually advantageous in terms of
fabrication complex structures within reasonable time.
However, a major drawback of DLW optical lithography has been its limited spatial
resolution, which has not been anywhere near that of state-of-the-art electron-beam
lithography. This aspect has limited further progress by the FORTH and KIT partners
regarding three-dimensional chiral metamaterials as well as further progress with respect
to three-dimensional transformation-optics architectures (e.g., invisibility cloaks) that the
collaboration between KIT and Imperial introduced in a publication in Science
magazine in early 2010. This contrasted the theoretical progress made in PHOME
[D12:1] [D12:2] [D12:3].
Thus, the KIT partner has put tremendous effort on systematically improving the spatial
resolution of DLW optical lithography in all three dimensions by combining [D12:5]
[D12:6] it with the concept of stimulated-emission-depletion (STED) known from
fluorescence microscopy. The underlying idea is illustrated in Fig.D12.1.
Figure D12.1: (a) Scheme of stimulated-emission-depletion (STED) direct-laser-writing (LW) optical
lithography. A red laser focus exposes the photoresist, while a green laser depletes (or de-excites or
“erases”) from all sides using a bottle beam. (b) Foci measured by scanning a gold bead through the focus
in three dimensions. Taken from Ref.[D12:5].
Using this approach, we were able to miniaturize our 2010 result by more than a factor of
two in all three spatial directions. This step has brought the operation frequency from the
infrared all the way to the visible part of the electromagnetic spectrum. In particular,
optical microscopy revealed excellent cloaking in three dimensions and for any
polarization of light at 700-nm wavelength [D12:4]. In this publication, we also
systematically studied the wavelength dependence. Only at wavelengths below 600 nm
does the light field “feel” the underlying periodicity of the metamaterial. Hence,
deviations from the effective-medium approximation occur. Comparison of the
experiments by the KIT partner with ray-tracing modelling along the lines of
Ref.[D12:1] revealed only minor imperfections with respect to the carpet-cloak design
introduced by the Imperial partner in 2008.
Figure D12.2: Scheme of the imaging Michelson interferometer allowing for directly measuring the phase
front reconstruction by a carpet invisibility cloak. Taken from Ref.[D12:8].
In a broad sense, invisibility cloaking based on photonic metamaterials can be viewed as
a particularly demanding example of aberration corrections, which are of interest in many
optical systems. Here, however, not only correction of the light amplitude but also of the
phase of the light wave is mandatory. While the two are connected by the Maxwell
equations, not a single far-field optical experiment had previously actually shown that
such invisibility cloaks also properly reconstruct the phase of the wave. To test this
aspect, the KIT partner has built a dedicated imaging interferometer for detailed optical
characterization (see Fig.D12.2).
Using this refined characterization set-up, the phase images shown in Fig.D12.3 have
been obtained. Obviously, the bump at the top (compare T. Ergin et al., Science 328, 337
(2010)) shows up as a phase hill for the reference. The phase distortion almost
completely vanishes for the case of the cloaking structure shown at the bottom.
The same three-dimensional STED-DLW lithography shall also allow for further
miniaturizing and improving our previously introduced gold-helix photonic
metamaterials that act as broadband circular polarizers. The corresponding status is
reported in D11. However, STED-DLW optical lithography is presently restricted to
negative-tone photoresist, whereas electroplating for gold helices required a positive-tone
photoresist. Thus, an additional inversion step is necessary. We are pursuing atomic-layer
deposition of a sacrificial dielectric before electroplating with gold. Encouraging progress
has recently been made by the KIT partner. The same technology can also be applied to
fabricate the corrugated-wire metamaterials theoretically introduced by the FORTH
partner to achieve bulk three-dimensional negative-index metamaterials with improved
angular dependence.
Another promising route is to replace the traditional metals (e.g., gold or silver) by very
highly doped semiconductors. This was, e.g., recently suggested by Alexandra Boltasseva
and Harry Atwater (Science 331, 290 (2011)) for planar metamaterial structures. In
PHOME, the KIT partner has successfully fabricated [D12:7] three-dimensional
architectures based on Al-doped ZnO grown by atomic-layer deposition. Plasma
frequencies up to visible frequencies have been achieved at reasonably small losses
[D12:7]. Thus, this approach appears to be very attractive for three-dimensional infrared
metamaterials and transformation-optics architectures.
Figure D12.3: Measured phase images (compare Fig.D12.2) on carpet-cloak photonic metamaterial
structures made by STED-DLW optical lithography and measured at 700-nm wavelength of light. Taken
from Ref.[D12:8].
The other novel path in facing losses is based on a combination of electromagneticallyinduced transparency (EIT) with non-linearity and gain components. EIT in
metamaterials is based on two elements—the active and the dark; one of the elements,
according to our proposed design, incorporates the gain component and transfers the
energy non-linearly to the other element. Under certain conditions, the spectral response
of such a coupled structure can be significantly different from the mere superposition of
the two independent resonances. Recently, we have fabricated two EIT structures [D12:9,
D12:10] that showed low absorption and slow light velocity. Figure D12.4 shows the
photograph of the sample and the schematic representation of our structure [D12:9].
Fig. D12.4. (a) Photograph of the sample. (b)
Schematic representation of our structure. Ref.
[D12:9].
Fig. D12.5. All spectra exhibit the typical features of EITwith low absorption inside a broader resonance. Ref. [D12:9].
Therefore, the absorption spectrum develops a very narrow transmission window in the
broader Lorentzian-like absorption peak associated with the transition to the excited state
(see Fig. D12.4).
Slow Light: Another application of the EIT is to slow the light by a large factor. At the
resonance frequency, the anomalous dispersion profile, normally observed for a two-level
resonance, is transformed into an extremely steep normal dispersion. This may slow
down light pulses by many orders of magnitude [D12:10]. Most of the EIT experimental
work has not seen a strong absorption dip, and the reduction of the speed of light is very
small—a factor of five to ten. So there is a strong need to experimentally demonstrate
that metamaterials can reduce the speed of light dramatically. We have collaborated with
Prof. Steven Anlage’s group, from the Univ. of Maryland, which has expertise in
fabricating superconducting wires. We have fabricated [D12:10] these EIT structures
with metals and superconductors, to verify that the experimental results will agree with
our numerical simulations and reduce the speed of light by a factor of 500. Therefore, we
will have a strong reduction in the speed of light and these new samples will be useful to
construct light-slowing devices. In addition, by manipulation of the superconducting
properties of the dark resonators through temperature or magnetic field, the EIT effects
are tunable to an unprecedented extent [D12:10].
References D12
[D12:1] T. Ergin, J.C. Halimeh, N. Stenger, and M. Wegener, Optical microscopy of 3D
carpet cloaks: ray-tracing simulations, Opt. Express 18, 20535 (2010)
[D12:2] R. Schmied, J.C. Halimeh, and M. Wegener, Conformal carpet and grating
cloaks, Opt. Express 18, 24361 (2010)
[D12:3] J.C. Halimeh, R. Schmied, and M. Wegener, Newtonian photorealistic ray
tracing of grating cloaks and correlation-function-based cloaking-quality assessment,
Opt. Express 19, 6078 (2011)
[D12:4] J. Fischer, T. Ergin, and M. Wegener, Three-dimensional polarizationindependent visible-frequency carpet invisibility cloak, Opt. Lett. 36, 2059 (2011)
[D12:5] J. Fischer and M. Wegener, Three-dimensional direct laser writing inspired by
stimulated-emission-depletion microscopy, Opt. Mater. Express 1, 614 (2011)
[D12:6] T.J.A. Wolf, J. Fischer, M. Wegener, and A.-N. Unterreiner, Pump-probe
spectroscopy on photoinitiators for stimulated-emission-depletion optical lithography,
Opt. Lett., in press (2011)
[D12:7] A. Frölich and M. Wegener, Spectroscopic characterization of highly doped ZnO
by atomic-layer deposition for three-dimensional infrared metamaterials,
Opt. Mater. Express, in press (2011)
[D12:8] T. Ergin, J. Fischer, and M. Wegener, Optical phase cloaking of 700-nm light
waves in the far field by a three-dimensional carpet cloak, submitted (2011);
arXiv:1107.4288v1
[D12:9] R. Zhao, Lei Zhang, J. Zhou, Th. Koschny and C. M. Soukoulis, “Conjugated
gammadion chiral metamaterials with optical activity and negative refractive index,”
Phys. Rev B 83, 035105 (2011).
[D12:10] C. Kurter, P. Tassin, Lei Zhang, Th. Koschny, A. P. Zhuravel, A. V. Ustinov, S.
M. Anlage and C. M. Soukoulis, “Classical analogue of Electromagnetic Induced
Transparency with a metal/superconductor hybrid metamaterial,” Phys. Rev. Lett. 107,
043901 (2011).