Nano Focus: Thermal tunability in terahertz metamaterial achieved on strontium titanate single crystals
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in order to explore the adsorption and diffusion of small molecules in a subset of their zeolite database as well as how their predicted zeolites can be used for carbon sequestration. The researchers said that their approach “may serve as a guide for construction of analogous databases for
other materials such as metal-organic frameworks or crystal hydrates. Diversity in structures and possible functionality among these classes of materials could reveal themselves through such efforts.” Steven Trohalaki
Nano Focus a
Thermal tunability in terahertz metamaterial achieved on strontium titanate single crystals
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MRS BULLETIN
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VOLUME 36 • MAY 2011
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1.0
295 K
120
200 K
60 metamaterial reference 0
Refractive index n
6
30
d
30
150 K 200 K 295 K 409 K 0.15
0.20 0.25 Frequency (THz)
0.30
0.25
0.075
0.20
0.060
0.15
0.045
150 K
25 200 K 20
295 K 409 K
15 10 0.10
0.0 0.10
36
35
0.4 0.2
150 K
12 18 24 Time delay (ps)
0.6
0.15
0.20 0.25 Frequency (THz)
0.30
1/n
0
c
Transmission
409 K
180
Resonance frequency (THz)
THz signal (a.u.)
0.8
A
lthough split-ring resonator (SRR) based metamaterials are attractive for use in devices with novel functionalities over a large electromagnetic spectral domain, devices incorporating SRRs fall short on performance due to lack of dynamic control over their resonances. R. Singh, H.-T. Chen, and co-researchers at the Center for Integrated Nanotechnologies at the Los Alamos National Laboratory, hypothesized that in most cases the frequency tuning of metamaterial resonance is accompanied with a large variation in resonance strength, which is undesirable and caused by the damping from the materials integrated in metamaterials. As reported in the April 7th issue of Optics Letters (DOI: 10.1364/ OL.36.001230; p. 1230), the researchers fabricated a planar square array of subwavelength 200-nm-thick gold electric SRRs on a 533-μm-thick single crystal (100) oriented strontium titanate (STO) substrate. They measured the resonant behavior in the THz frequency range of the metamaterial as a function of temperature using a time-domain spectroscopy (TDS) system incorporated with a continuous flow liquid helium cryostat. The researchers observed a 43% shift in resonance frequency after
b
240
0.10 100
0.030 200 300 Temperature (K)
400
At various temperatures, (a) THz pulse in the time-domain after transmission through the metamaterial and a bare strontium titanate (STO) substrate, (b) normalized THz transmission amplitude spectra through the metamaterial, (c) refractive index of the STO substrate as a function of THz frequency, and (d) a comparison of resonance frequency and inverse of STO refractive index. Inset to (a) is the microscopy image of a metamaterial unit cell. Reproduced with permission from Opt. Lett. 36 (7) (2011) DOI: 10.1364/OL.36.001230; p. 1230. © 2011 Optical Society of America.
cooling the metamaterial from 409 K to 150 K with less disparity in resonance strength. They attributed this behavior to the temperature-dependant dielectric
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