2024年3月18日发(作者：360儿童手表10x)

Ultra-broadband Light Absorption by a

Sawtooth Anisotropic Metamaterial Slab

Yanxia Cui

1, 2, 3

, Kin Hung Fung

1,4

, Jun Xu

1,4

, Hyungjin Ma

1

, Yi Jin

2

,

Sailing He

2

, and Nicholas X. Fang

1,4,

*

1

Department of Mechanical Science and Engineering and Beckman Institute of Advanced Science and

Centre for Optical and Electromagnetic Research, State Key Laboratory of Modern Optical

Technology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA

2

Instrumentation; Joint Research Centre of Photonics of the Royal Institute of Technology (Sweden) and

Zhejiang University, Zhejiang University, Hangzhou 310058, China

3

Department of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan 030024,

Department of Mechanical, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139,

China

4

USA

*Corresponding author:nicfang@

Abstract

We present an ultra broadband thin-film infrared absorber made of saw-toothed

anisotropic metamaterial. Absorbtivity of higher than 95% at normal incidence is supported

in a wide range of frequencies, where the full absorption width at half maximum is about

86%. Such property is retained well at a very wide range of incident angles too. Light of

shorter wavelengths are harvested at upper parts of the sawteeth of smaller widths, while

light of longer wavelengths are trapped at lower parts of larger tooth widths. This

phenomenon is explained by the slowlight modes in anisotropic metamaterial waveguide.

Our study can be applied in the field of designing photovoltaic devices and thermal emitters.

1

Metamaterials (MMs) are artificial materials engineered to exhibit extraordinary electromagnetic

properties that are not available in nature.

1,2

Potential applications of MMs are diverse and include

superlenses,

3

invisible cloaks,

4

highly sensitive sensors,

5

ultrafast modulators,

6

antenna systems.

7

In

most applications, the absorption loss in MMs often degrades the performance; however, for artificial

light absorbers, the absorption loss becomes useful and can be significantly enhanced by proper designs

of MMs.

8,9

In 2008, Landy

et. al

proposed a single-wavelength perfect absorber consisting of metallic split ring

resonators and cutting wires.

10

Later, some improvement works were followed to make the absorbers

insensitive to incident angle and polarization.

11,12

Unfortunately, those past efforts suffer common

disadvantage of narrow bandwidth, which will reflect a fairly large amount of total incident energy and

could not be employed to adequately improve the solar energy harvesting efficiency.

8,9

One may suggest

adding various different resonances in order to broaden the absorption band,

13-20

but the strong coupling

among resonators often put more limitations so that the designed absorbers often perform much poorer

in comparison with a black body which absorbs all incident electromagnetic radiation. We note that

Yang

et. al

have designed an ultra-broadband absorber based on an array of metallic nanogrooves of

different depths.

21

However, it is almost impractical to obtain metallic grooves with parameters like 10

nm width and 5

μ

m depth using current fabrication technologies. Therefore, MM structures of simple

schematics which can absorb light efficiently in a broadband are very demanding.

In this letter, based on slowlight waveguide modes of weakly coupled resonances in a MM slab, we

design an ultra-broadband thin film infrared absorber for TM-polarized light. The slowlight waveguide

can be obtained by etching a MM slab into sawtooth shape with the tooth widths increasing gradually

from top to bottom, Figure 1. Our broadband absorber can be regarded as a group of ultra-short vertical

waveguides which support slowlight modes at different frequencies so that the incident light at different

wavelengths can be captured at positions of different tooth widths. In detail, we employ an anisotropic

MM (AMM) consisting of alternating layers of flat metal and dielectric plates. Metal plates are made of

gold with thickness

t

m

= 15 nm; dielectric plates are made of germanium with thickness

t

d

= 35 nm. The

2