University of Tehran College of Engineering

School of Electrical and Computer Engineering

University of Tehran College of Engineering

School of Electrical And Computer Engineering

Using Metamaterials

As Optical Perfect Absorbers

Review Report

Sepehr Ahmadzadeh

810188299

Spring 2012 - 2013

Submitted June 23, 2013

All Rights Received

Contents I. Introduction 3 II. EM Wave Absorber 3 II.a Resonant Absorbers 4 II.b BroadBand Absorbers 4 II.c Near Unity Absorber 4 III. EM Wave Absorber Theory 5

III.a Fresnel Equations 6 IV. Metamaterial Perfect Absorbers 9

IV.a Simulation 9

IV.b Fabrication 10

IV.c Characterization 10

IV.d Applications 10 V. Recent Papers 11

VI. Conclusion 22

VII. References 23

Submitted June 23, 2013

All Rights Received

Metamaterial as Optical

Perfect Absorber

Sepehr Ahmadzadeh – 810188299 Photonic Research Lab Dept. Communication University of Tehran

Tehran, Iran se.ahmadzadeh@ut.ac.ir

Abstract – The following report describes the history, theory, implementation and characterization of metamaterial perfect absorbers (MPAs). The motivation for studying MPAs comes mainly from their use in

potential applications. These applications briefly include: emitters, sensors, spatial light modulators, IR camouflage, use in thermophotovoltaics, and wireless communication. MPAs also provide insight into the

theory of metamaterials (MMs) as an effective medium where the designer can control the electromagnetic properties by engineering the geometry. Also using artificial materials as perfect optical absorbers are

discussed and some recent papers in the area have been introduced. Keywords – Optical perfect absorber, Metamaterial, perfect absorber, Effective medium, Electromagnetic,

MPA

I. INTRODUCTION

Metamaterials are artificial materials engineered for specific electric and magnetic responses. Since

the first attempts for designing metamaterials new applications of such materials have been proposed.

For example, Absorbers are devices in which incident radiation is absorbed at the operating frequency.

Near unity absorption is one the hot topics of this modern world because it has several applications such

as solar cell materials, photodetectors, selectivee thermal emmiters, detection and sensing,

microbolometers, integrated photonic circuits and other things. In the following sections, first some

basics about EM wave absorbers and types of them are discussed. Then in section III, the basic theory of

EM wave absorption is discussed and then in section IV, metamaterial perfect absorbers will be

introduced and finally, some recent papers in the area will be covered.

II. EM WAVE ABSORBER

Electromagnetic (EM) wave absorbers can be categorized into two types: resonant absorbers and

broadband absorbers [1]. Resonant absorbers rely on the material interacting with the incident radiation

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in a resonant way at a specific frequency, 흎ퟎ (where the wavelength corresponding to 흎ퟎ is 흀ퟎ = ퟐ흅풄흎ퟎ

and c is the speed of light in vacuum). Broadband absorbers generally rely on materials whose properties

are frequency independent and therefore can absorb radiation over a large bandwidth.

2.1 Resonant Absorbers

Resonant absorbers have utilized, for the most part, multiple layers separated by a quarter of the

operation wavelength. In transmission line theory, a metal plate acts like a short circuit, and when it is

placed 흀ퟎퟒ

behind any sort of “load,” will act like an open circuit at the resistive sheet (i.e. conductance G

= 0). Therefore, the incident wave sees just the admittance of the resistive sheet. When the load

impedance matches free space, the reflectivity goes to zero.With the addition of loss, high absorption can

be achieved. Initial interests in electromagnetic wave absorbers were largely in the microwave range.

The usefulness of absorbers in both improving radar performance and providing concealment against

others’ radar systems was utilized as a military technique.

Two well known scientists who developed EM absorbers are W. W. Salisbury and J. Jaumann,

who independently created similar devices. One such device, known as the Salisbury screen, is a basic

example of the resonant absorber mentioned above. A resistive sheet is placed 흀ퟎퟒ

in front of a metal

ground plane, usually separated by some lossless dielectric. The effective open circuit creates R(흎ퟎ ) =

ퟎ off the resistive layer.

The Jaumann absorber can conceptually be considered an extension of the Salisbury screen which

consists of two or more resistive sheets in front of a single ground plane. All sheets are designed to

operate at a distinct wavelength, and thus each sheet is separated by approximately λ/4, producing

multiple reflection minimums around some center frequency 흀ퟎ. The effect is that it acts as a resonant

absorber over multiple wavelengths, achieving a broadband response.

The Dällenbach layer employs a different mechanism than the Salisbury screen; its design consists of

a homogeneous layer in front of a ground plane. The homogeneous layer is selected for particular loss

values resulting from the imaginary portions of the electric permittivity and the magnetic permeability.

The idea is to impedance match to free space as to minimize the reflection of the surface and then utilize

the loss in the homogeneous layer to absorb the incident radiation.

Another type of resonant electromagnetic wave absorber, known as the crossed grating absorber, uses

a reflective metal plane with an etched shallow periodic grid which is shown in Fig.1.

Fig. 1. Crossed grating Absorbers

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A resonance is created due to the interaction between the periodic grid and incident radiation, creating

a period of anomalous diffraction. [2] It was shown that anomalous diffraction is correlated to periods of

enhanced absorption.

2.2 Broadband Absorbers

One example of a broadband absorber is a geometric transition absorber. These devices are most

commonly used in anechoic chambers. The idea is to create a slowly varying transition from free space

into a lossy material using shapes such as pyramids or wedges loaded with lossy material which is

shown if Fig.2.

Fig.2 Geometric Transition Absorber

This way reflectivity is minimized and the wave is gradually absorbed over the length of the shaped

geometry. By using a thick layer of this material, one can generate enough loss to create high

absorption.[1]

2.3 Near Unity Absorber (Perfect Absorbers)

A near unity absorber is a device in which all incident radiation is absorbed at the operating

frequency– transmissivity, reflectivity, scattering and all other EM propagation channels are disabled.

What happens in optical frequency regime? One of the most important issues in this frequency band is

material’s transparency against light. Therefore, scientists seek a way to change the material in order to

have a perfect absorption and zero transmission or reflection. Artificial materials are engineerd structures

which can be used for this purpose. Hence, in this report we introduce metamaterial perfect absorbers

and the theory behind them. But first, let me explain a little about the theory of EM wave absorption in

general.

III. Electromagnetic Wave Absorption Theory

We begin by considering all possible ways in which electromagnetic energy can propagate at an

interface. Electromagnetic waves incident upon a boundary or surface may be reflected, transmitted,

absorbed, scattered, or may excite surface electromagnetic waves (SEWs). Let us consider wavelengths

in the range of 휆 , and assume that the surface has an average roughness that is much smaller than the

wavelength, 푅 ≪ 휆 such that we may ignore scattering effects. The surface may also support plasmons

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or, more generally, surface electromagnetic waves, which may be explored by considering their

propagation length, often described as L = , where k is the imaginary part of the complex

wavevector k|| = k + ik . It is clear that it is impossible to clarify if an external electromagnetic wave

can couple to a surface and propagate as a SEW even if the plasmon propagation length L is known.

Usually, researchers propose a figure of merit which is shown below:

Obviously the figure of merit presented describes how much our incident wavevector matches the

dispersion of our surface k , and considers the loss k of the surface for propagation of the SEW.

Clearly, W reduces to L if k = k .

If k is sufficiently large, generation of surface electromagneticnetic waves may be a form of

loss but the SEW may re-radiate the wave if, e.g. our surface is curved. Thus, assuming we have a flat

surface such that any SEWs or plasmons die out before re-scattering, we may then finally resolve that a

wave may be reflected (R), transmitted (T), or absorbed (A), with their relationship given as A = 1–T–

R.

3.1 Fresnel Equations

Let us consider two cases; (1) a slab of thickness d of magneto-dielectric medium described by both

the magnetic permeability 휇(휔) = 휇 휇 (휔) and the electric permittivity 휀(휔) = 휀 휀 (휔) and backed

by a highly conductive opaque metallic ground plane. (2) A slab of thickness d of a magneto-dielectric

medium embedded in a vaccum.

For Case (1), since we consider a highly conductive metallic ground plane, transmission can be

neglected and we begin by considering the reflectivity (R) and reflection coefficient (r) of an interface,

for transverse electric (TE) and transverse magnetic (TM) polarized waves as,

푅 = |푟 | = 푐표푠 휃 − 휇 √푛 − 푠푖푛 휃푐표푠 휃 + 휇 √푛 − 푠푖푛 휃

푅 = |푟 | = 휀 푐표푠 휃 − √푛 − 푠푖푛 휃휀 푐표푠 휃 + √푛 − 푠푖푛 휃

Where θ is the angle of incidence, and 푛 is the index of refraction of the magneto-dielectric

medium. If we restrict our incident electromagnetic wave to normal, i.e. θ = 0°, equations above reduce

to:

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Where Z is the impedance of the magneto-dielectric material and 푍 is the impedance of free space.

However, as mentioned, our impedance matched condition above is only valid for the reflectivity of an

interface and thus incident radiation may still be transmitted through the medium. If our material is not

of sufficient thickness (d) and loss (푘 ) then the wave will be reflected from the conducting metallic

plane and may be reflected back into free space. A simple unit cell of this type absorber is shown in

Fig.3. This structure is suggested by Smith [3] in 2008 and it is a unit cell which is

operating at microwave regime.

Fig.3 magneto-electric unit cell consisting of an ERR combined with a metallic ground plane–polarization

In general, we can assume absorbers in two cases. Therefore, it is logical to present their

absorption, transmission and reflection coefficients in the way shown in Fig.4. [4]. In the figure we plot

results of Case (i): the reflectivity (green curve), transmissivity (blue curve), and the absorptivity (red

curve); in this figure the magneto-dielectric layer is shown. As can be observed 푇 is zero everywhere,

but 푅 is small and thus the absorptivity is near unity.

The second case is magneto dielectric material in air and without any ground plane. The reflection and

transmission coefficients for this matieral are given by

푍 is the relative impedance of the medium which is . Note that in this case there is no reflection due

to matching between the material and free space but unlike case (1) there is transmission. Therefore, we

can write equations as shown below:

8

As you can see in this figure, absorption and transmission is shown in figure ( c ) which is related to

magneto-dielectric material embedded in the vaccum. But there is no reflection due to impedance

matching between the material and the free space. As it mentioned above 푇 is related to 푘 and 푑.

Hence, by assuming large 푘 and 푑 we have a little transmission in a desired frequency. It is obvious

that, in order to have a perfect absorber in optical regime, it is necessary to have small values of 푑

because 푘 is big and their multiplication has to be a small value.

In both cases, the absorptivity is narrow band and out of this band the electromagnetic wave is

reflected in case 1 or transmitted in case 2.

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IV. Metamaterial Perfect Absorbers

Using Metamaterials as perfect absorbers is a logical option approximately in all frequency regimes.

Because of their subwavelength engineerd structure and their ability to design effective constitutive

parameters, they can be used to achieve desired results.

In order to have a wide band absorption, using lossy materials is necessary. Thus, finding a way to

design a wide band perfect absorber is one the most crucial characteristics which attract researchers. For

example, solar cells could be more practical if their absorbing material functions in a wider band of

frequency. Using multistructures as a resonant part of a design, can help the metamaterials to resonance

at various frequencies and absorb better in a wider band.

Another performance flexibility which is important for scientists is polarization of the incident

wave which absorbed by the material. Nowadays, independency of the polarization is the goal.

Polarization independent magnetic metamaterials were first proposed for near infrared frequencies[5] as

a means to eliminate bianisotropy by appealing to racemic mixtures of unit cells. Another work proposed

that chiral metamaterials could achieve polarization independent absorption.

Angle of incidence is in a high priority too. Wide angle absorption is a characteristic which can help

engineers to design high performance solar cells, accurate sensors and so on. Most of the studies noted

that a monotonic decrease in the absorptivity at resonance for TE modes as a function of incidence angle

for those above roughly 40°, whereas there was little change for the TM mode–at least below ∼80 °. It

was stated in multiple studies[6] that this is due to the fact that, as the incident angle increases, the

parallel magnetic field component approaches zero and thus can no longer effectively induce antiparallel

currents in the top MM layer and the back metal structure resulting in a drop in the magnetic flux.

4.1. Simulation

Metamaterial perfect absorbers, similar to other metamaterials, are composed of repeating unit cells

arranged in two or three dimensional periodic structures. The periodic array can be precisely modeled by

simulation of one unit cell with the knowledge of material properties and the assignment of appropriate

excitations (i.e., ports) and boundary conditions. One advantage of MPA simulation is that an optimized

structure can be designed and the behavior predicted without unnecessary fabrication iterations. Also,

due to the accuracy of the simulation techniques, there is generally a good match between simulated and

experimental results if the material properties are well known. Among the simulation programs, CST

Microwave studio, HFSS, and Comsol are some of the most common. Metal is one critical part of MPAs

which affects the resonating behavior. Therefore, good knowledge of metal properties in simulation is

essential to obtain trustable results. At low frequencies, such as microwaves, metals such as gold and

copper are modeled as good conductors with a particular value for the conductivity. However, when

simulating metamaterials at higher frequencies, such as infrared or optical, metals tend to be lossier and

the Drude model is often used to reproduce their frequency dependent optical properties. Simulations

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also provide the phase information of 푆 and 푆 . Together with the amplitude, the effective permittivity

and permeability can be calculated for a MPA.

4.2 Fabrication

MPAs that operate in the microwave frequency range are normally fabricated using the printed

circuit board (PCB) method in which a certain thickness of copper is deposited on both sides of a

photosensitized board, FR-4 being a common example. Since, in this frequency range, the sizes of

metamaterial resonators are relative large, i.e. on the order of millimeters with the smallest

dimensionapproximately 100 μ m, a photo mask can simply be printed on a transparency using a high

resolution printer. After exposing to light, developing, and the post etching process, MPAs with a

patternon one or both sides can be fabricated.[7] Since the resonance of a metamaterial scales with the

size of the operational wavelength, by moving to a higher frequency the size of the resonator becomes

smaller, which requires higher precision fabrication techniques. Many studies on MPAs have been

carried out in the THz range due to the many interesting properties and possible applications at these

frequencies. In this range, metamaterial resonators are on the order of tens of microns with a smallest

feature size of several microns. For these sizes, photolithography is the most effective manner of

fabrication.[8] Fabricating the MPA at higher frequencies surpasses the capability of photolithography

and thus requires a technique with a higher resolution. It has been demonstrated that MPAs operating in

the infrared and visible range are best fabricated using techniques such as e-beam lithography and

focused ion beam (FIB). These methods are capable of making structures with sizes that are on the order

of tens of nanometers.

4.3 Characterization

Different techniques are used to characterize the performance of MPAs at different frequencies. In the

microwave range, characterization is usually carried out in a microwave anechoic chamber where horn

antennas, connected to a vector network analyzer, detect reflected and transmitted microwaves from a

sample. Terahertz time domain spectroscopy (TDS) is another powerful tool to characterize

theperformance of MPAs, especially at THz frequencies. By Fourier transforming the time pulse from

the sample and reference, both amplitude and phase information can be obtained. Fourier transform

infrared (FTIR) spectroscopy is the most frequently used method to characterize MPAs working in

ranges higher than microwave and covers an extremely broad spectrum ranging from THz to visible.[9]

4.4. Applications

Other than their rich ability as a platform to study fundamental electromagnetic wave theory,

MPAs offer a wide variety of practical applications.

Because MPAs are tunable with respect to their operational wavelength, they can be used as

spectrally sensitive detectors or sensors. Much work has done in both integrating MPAs into existing

designs and creating novel devices based on MPAs to provide detection and sensing throughout the

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electromagnetic spectrum. Microbolometers are a type of thermal detector in which incident

electromagnetic radiation is absorbed by a material and then sensed by a thermometer.[10]

There are a multitude of other applications for MPAs. absorbers in the millimeter range could be

used for radar sensors for adaptive cruise control.[11] MPAs have been postulated to be useful in

integrated photonic circuits, spectroscopy and imaging and so on.

V. Recent Papers

In this section we discuss about some recent papaers and researches about metamaterial perfect

absorbers and then cover the performance flexibility and some other issues which was mentioned before

and it was crucial in designing EM wave absorbers.

5.1 Wide-angle infrared absorber based on negative index plasmonic metamaterial [12]

In this paper, the author proposed an approach to design a perfect absorber which has wide angle

absorption in infrared frequency band. It is shown analytically that a sub-wavelength in all three

dimensions enables absorption of close to 100% for incidence angles up to 45 deg to the normal.

The structure proposed in this paper is useful for wavelength-selective infrared and THz detection

which is important for thermal imaging, night vision systems and non-destructive detection. Wide-angle

power absorption efficiency is desirable for miniaturizing photodetectorsor microbolometers down to the

wavelength size.

Consider a semi-infinite slab of a lossy metamaterial with engineered dielectric permittivity and

magnetic permeability tensors ε and μ. Radiation is assumed to be incident in the x−z plane at an angle θ

with respect to the vacuum-material interface normal. Therefore, relevant components of constitutive

parameters are 휇 , 휇 and 휀 . For our semi-infinite slab (assuming that the metamaterial’s thickness

퐿 is sufficient to absorb all transmitted radiation), absorptivity A is limited only by reflection. A

straightforward calculation yields the reflection 푟 and absorption.

Note that Lossy material causes that elimination of transmission in this structure. By assuming 휃 = 0

it is obvious that absorber’s material impedance is equal to με

. Hence, if in a specific wavelength

휀 = 휇 and if we can consider 휇 = 1 then A ≈ 1− tan (θ). Therefore, it can be assumed that

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between 0 and the absorption is more than 97%. Matching between the structure and air causes zero

reflectivity and also by using taylor series it is obvious that between a specific angles, aborptivitty is

high. Their proposed structure is shown in Fig.5. Each unit cell consists of two parallel layers separated

by the distance ℎ . Electromagnetic resonances in effective permittivity and magnetic permeability of

plasmonic composites are unambiguously related to the electrostatic surface plasmon resonances of the

appropriate symmetry (electric dipole and magnetic dipole, correspondingly). Wire-strip loops

participate in magnetic field and both of them also participate in Electrical field.

Fig.5. PIMNIM Structure. Unit cell for electromagnetic and electrostatic simulations isinside the dashed rectangles.

Angular dependence of the absorption coefficient is shown in the Fig.7. This structure was only

simulated by commercial programs such as HFSS or CST. In this paper they use CST for their

simulation. And their result for absorption, transmission and reflection and constitutive parameters is

shown below:

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Fig.6 Constitutive Parameters

Fig.7 Angular dependence of the absorption coefficient for the idealized structure

14

Fig.8 Absorption, Reflection and Transmission Coefficients

Also you can find the dimensions of the structure here:

5.2 Optically thin composite resonant absorber at the near infrared band: a polarization independent and spectrally broadband configuration [13]

The proposed absorber in this paper electrically and magntecialy is perfectly matched to free space.

Therefore, there is no reflection from the surface to the air. Mainly, it consists of 4 layers which is a

metal back plate, dielectric spacer and two artificial layers respectively. Hence, it can be assumed as a

case (1) EM wave absorber which was mentioned in part II. The most important characteristic of this

structure is the broad band performance and independency of polarization. Also due to the

subwavelength unit cell, incident wave support wide angles. This kind of absorber can be used in

thermal photovoltaic, sensors and camouflage applications.

The structure is shown in Fig.9.

15

Fig.9 Geometry and schematic of the absorber design. The absorber consists of an array of magnetic resonators placed on top of a thin dielectric. The wave vector (k) of the incident field is in the ̶ z-direction and the electric field (E) is in the y-

direction

They fabricated this structure and test it with spectrometer with free space method and the result

which was reported in the paper is shown in Fig.10. The numerical simulation was performed using CST

Microwave Studio.

The resonators which are used were from gold and up to this point the normal incidence and

single polarization is supported. But they modified their structure and add this capability to support

various angles and polarizations.

It is also stated that the magnetic response of the metamaterial layer is independent of the back

metal and high absorbance is present for multiple dielectric spacer thicknesses that may be desired for

specific applications. As we increased the dielectric layer thickness, the absorption magnitude shows an

oscillatory behavior, and the maxima and minima depend on the surface impedance variation of the

metamaterial layer. The magnetic resonance frequency of the individual SRRs strongly depend on

the resonator arm length (L). By changing L, we can change the resonance frequency considerably,

which is one of the reasons for the wide bandwidth response in the experiments as large parametric

variations for L were present at the fabricated samples.

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Fig.10 Numerical and experimental data of absorbance derived from scattering parameters. The blue dotted line

corresponds to gold-only SRR layer performance. The SEM image of a section of the printed area and an example SRR are shown on the right

By modifying the sample and rotate nano structures same as the Fig.11. it is possible to have wide-

angle absorption and polarization independency. They added a resistive sheet (thin titanium) layer

between the metamaterial and dielectric layers. The three layer configuration composed of resistive

sheet, dielectric spacer, and back metal itself behaves as a resonant absorber. By changing the thickness

of the titanium layer, its electrical surface impedance can be tuned. In order to obtain wide bandwidth

operation, the resistive sheet resonant absorption wavelength and metamaterial layer resonant

absorption wavelength can be combined. They merged the two structures and the composite absorber

thereby had a larger bandwidth than the two individual cases. In order to achieve polarization

independence, we changed the unit cell so that it is now composed of 4 SRRs. There are elements

parallel to the y-direction and other elements that are parallel to the x-direction. They saw that the

simulated absorption spectra for the incident wave polarization of 45° is the same as the polarization

angles 0°and 90°, which clearly proves the polarization independent response.

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Fig.9 Polarization independent response and corresponding unit cell

Fig.10. Simulated absorption response of the SRR based metamaterial absorber for several incidence angles

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For the oblique illumination they investigated the incidence angles of 20°, 40°, 60°, and 80° in the x-z

and y-z planes. Fig.10. shows the spectral response for several angles of incidence: the peak absorption

frequency changes and remains more than 70%, and up to a 60° angle of incidence. For oblique

illumination, the excitation of SRRs is partially electrically and partially magnetically originated. There

was a slight shift of the operation frequency that slightly decreased the operation bandwidth. Even

though the operation frequency of the absorber changed slightly, the absorption values remained large

for up to 60° at the x-y and y-z planes.

5.3 Optical metamaterial absorber based on leaf-shaped cells [14]

In this paper the authors presented the model of an infrared metamaterialabsorber composed of

metallic leaf-shaped cells, dielectric substrate, and continuous metallic film which has absorptivity more

than 99.3 % at the 126.7 Thz and support different incident angles and radiation modes.

Note that this structure is kind of random nano structures which can be fabricated only by

electrochemical decomposition techniques which is a chemical process. According to author postulate, it

can be used in applications such as IR imaging systems, thermal bolometers, and optical bi-stable

switches. Fig.11. shows the schematic illustration of leaf-shaped metamaterial absorber. From the

inspiration of natural existed leaf as shown in Fig.11(c), they design and fabricate a similar structure

shown in Fig.11(a). For EM wave normal incidence (Fig.11), the continuous trunks of the leafshaped

cells behave as an array of periodical wires [15], supplying the electric coupling to incidentEfield. The

magnetic coupling is created by the antiparallel currents between the metal leaf and metal film response

to the incident Hfield. We will show, in this paper, these two resonances could be well overlapped in the

given frequency range if appropriately modulating the geometrical parameters, and it may be able to

realize almost complete absorption to the incident electromagnetic wave. In the paper first they simulate

and experiment the structure for microwave regime then they fabricate the nano structre for optical

regime. Due to self-scalable ability of metamaterials it is a correct decision. The results is shown in

Fig.12. and Fig.13. If the dimensions of the proposed structure are reduced down to nanoscale, the

structure will give perfect absorption at optical frequency. Figure 11(d) shows the infrared metamaterial

prepared with a chemical deposition method. Metamaterial absorber could be prepared with the similar

method that combines with a thin silver film.

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Fig 11. (a) The schematic illustration of the leaf-shaped based metamaterial absorber model, (b) the unit cell of the leaf shaped configuration, and definition of the geometry parameters, (c) scheme of natural existed leaf, (d) the silver leaf-

shaped cell fabricated with electrochemical deposition

Fig 12. TheS11(a) and absorptivity (b) of the microwave metamaterial absorber from simulations and microwave

experiments

c

db a

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Fig 13. TheSparameter (a) and absorptivity (b) of the infrared metamaterial absorber from simulation

The S parameters of the optical metamaterial are numerically simulated. In Fig.13 (a), you could find

that the S11 has a sudden dip near the frequency of 126.7 THz, the minimum of which is -25.1dB. The

S21 (the amplitude of transmission) is all below -20 dB at the whole wavelength reign, and get -24.6dB

at 126.7 THz.

In this case, the absorptivity could be calculated as A(ω) = 1-10×exp(S11/10)-10×exp(S21/10), and

the result is shown in Fig.13(b). We could find the absorptivity as high as 99.3% and is achieved for this

metamaterial close to the frequency of 126.7 THz. The result shows clearly the viability of using silver

leaf-shaped cells to build metamaterial absorber at infrared frequencies if it combines with additional

continuous silver film.

6.3 Perfect absorbers on curved surfaces and their potential applications [16]

In this paper, the author presented a curved surface metamaterial absorber which it can be used in

applications such as suppression of back-scattered light from covered objects and clocked it in

reflection, optical black holes and suppression of spurious back-scattered light for example in

Radar absorbers. In this paper just simulation is done and the structure is assumed from flexible

polymer film. The structure is periodic in y-direction with periodicity P and is infinitely extended in

z-direction. We assumed that the dielectric deposited onto the metal is characterized by 휀 = 2.25 reflecting the properties of SiO2. The ground plate and the metallic wires are assumed to be made

from silver. The structure is shown in Fig.14. Since the operational domain should be in the near-

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infrared, the thickness of the ground plate is set to be 200nm. The perfect absorber is optimized such

that reflection is negligible (R~0) when an antisymmetric resonance is excited in the coupled system

made from the nanowire and the ground plate. Therefore, the absorption of the perfect absorber is

close to unity (A=1−T−R~1).

Fig.14. Geometry and illumination under consideration(a) Schematic of a planar perfect metamaterial absorber. (b) Schematic of the perfect metamaterial absorber on a curved surface. Geometrical parameters are chosen according to dfilm=200 nm,tgap=10 nm, twire=10 nm, L=125 nm,P=200 nm, Rdie=8.2μm. The structure illuminated by TM polarized plane wave and the magnetic filed is always along the infinite nanowires i.e. z direction.

To quantify the optical response, the two-dimensional scattering cross sections (SCSs) of the cylinder

with and without perfect absorbing cover are shown in Fig.15(d). The total SCS of the cylinder coincides

with the absorption cross section at resonance frequency as suggested by the principle of critical

coupling. The back-scattering cross section of the covered cylinder compared to the dielectric cylinder is

significantly suppressed at resonance and even indistinguishable from zero on a linear scale. Hence, the

proposed device can reduce the back-scattering significantly at the design frequency. Obviously, the

total SCS at resonance frequency is then equal to the forward SCS that can be easily extracted while

subtracting the backward SCS from the total SCS.

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Fig.15. Optical response of the perfect metamaterial absorber on planar and curved surface.(a) Hz-

component and Jy- component at resonance frequency f =232 (b) Absorption of the planar absorberas a function of frequency and the angle of incidence. (c)Hz- component at resonance for a plane wave incident at an absorber on curved surface. (d) Cross sections per unit length of the absorber on curved surface (solid lines) and a referential dielectric cylinder (dashed lines). The figure shows the total and the backward scattering cross section as well as the absorption cross section (all per unit length).

VI. Conclusion In this review report first we introduce EM wave absorbers and describe the necessity of using them

and name some applications. Then we explain the mechanism of absorption and types of absorber

matieral. We paid attention to some crucial issues in the design and try to solve them by using

metamaterial perfect absorber. Then we said that optical perfect absorbers is just like others but with

very smaller dimensions. At the end we explaind four recent papers about using metmaterial as optical

perfect absorbers and solving some important problems in absorber’s performance.

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VII. References

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OPTICS EXPRESS

[17] Jiaming Hao, Jing Wang, Xianliang Liu, Willie J. Padilla, Lei Zhou, and Min Qiu,High performance optical absorber