Simulation, Fabrication and Characterization of THz Metamaterial Absorbers

Summary

This protocol outlines the simulation, fabrication and characterization of THz metamaterial absorbers. Such absorbers, when coupled with an appropriate sensor, have applications in THz imaging and spectroscopy.

Abstract

Metamaterials (MM), artificial materials engineered to have properties that may not be found in nature, have been widely explored since the first theoretical¹ and experimental demonstration² of their unique properties. MMs can provide a highly controllable electromagnetic response, and to date have been demonstrated in every technologically relevant spectral range including the optical³, near IR⁴, mid IR⁵ , THz⁶ , mm-wave⁷ , microwave⁸ and radio⁹ bands. Applications include perfect lenses¹⁰, sensors¹¹, telecommunications¹², invisibility cloaks¹³ and filters^14,15. We have recently developed single band¹⁶, dual band¹⁷ and broadband¹⁸ THz metamaterial absorber devices capable of greater than 80% absorption at the resonance peak. The concept of a MM absorber is especially important at THz frequencies where it is difficult to find strong frequency selective THz absorbers¹⁹. In our MM absorber the THz radiation is absorbed in a thickness of ~ λ/20, overcoming the thickness limitation of traditional quarter wavelength absorbers. MM absorbers naturally lend themselves to THz detection applications, such as thermal sensors, and if integrated with suitable THz sources (e.g. QCLs), could lead to compact, highly sensitive, low cost, real time THz imaging systems.

Introduction

This protocol describes the simulation, fabrication and characterization of single band and broadband THz MM absorbers. The device, shown in Figure 1, consists of a metal cross and a dielectric layer on top of a metal ground plane. The cross-shaped structure is an example of an electric ring resonator (ERR)^20,21 and couples strongly to uniform electric fields, but negligibly to a magnetic field. By pairing the ERR with a ground plane, the magnetic component of the incident THz wave induces a current in the sections of the ERR that are parallel to the direction of the E-field. The electric and magnetic response can then be tuned ind....

Protocol

1. Simulation of a Single Band THz Metamaterial Absorber

A 3D view of the simulation set-up is shown in Figure 2.

1. Lumerical FDTD is used to optimize the transmission, reflection and absorption characteristics of the THz metamaterial absorber. All units are given in μm.
2. Define the THz polyimide material properties by left clicking Materials, Add (n,k) material and inputting 1.68 as the n and 0.06 as the k. Double left click on "new material 1" and rename it as "polyimide". Note that if theoretical or experimental refractive index data is available one can "add sampled dat....

Results

Figure 5(a) shows the experimentally obtained and simulated absorption spectra for a MM absorber with a 3.1 μm thick polyimide dielectric spacer. This MM structure has a repeat-period of 27 μm and dimensions K = 26 μm, L = 20 μm, M = 10 μm and N = 5 μm. Experimental measurements were also performed on samples with no ERR layer to confirm that absorption was a consequence of the MM structure and not of the dielectric. The 7.5 μm thick polyimide sample with no ERR structure has a maximum abso.......

Discussion

This protocol describes the simulation, fabrication and characterization of THz metamaterial absorbers. It is essential such sub-wavelength structures are accurately simulated before any effort is committed to costly fabrication procedures. Lumerical FDTD simulations provide information on not only the MM absorption spectrum but also the location of the absorption, essential knowledge to aid placement of a transducer and obtain the maximum response. In addition the optimization algorithm in Lumerical can be implem.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Lumerical FDTD	Lumerical
Silicon wafer	IDB technologies		Single sided polished
Plassys 450 MEB evaporator	Plassys Bestek
VM651 Primer	Dupont
PI2545	Dupont
Methyl Isobutyl Ketone	Sigma-Aldrich
Isopropanol	Sigma-Aldrich
Plasmaprep5 barrel Asher	Gala Instrumente
VB6 UHR EWF electron beam writer	Vistec
Tanner L-Edit	Tanner Inc.
Layout Beamer	GenISys Inc.
Polymethyl methacrylate (PMMA)	Sigma-Aldrich	293261 Sigma-Aldrich
IFV 66v/s FTIR	Bruker
Pike 30spec reflection unit	Pike Technologies
Hg arc lamp	Bruker
Au mirror	Thor Labs	PF05-03-M01
Leica INM20 Optical Microscope	Leica microsystems
6 mm Mylar Beamsplitter	Bruker