1. Ultra broadband plasmonic absorbers for terahertz waves
Aditi Upadhyay,1* Withawat Withayachumnankul,1,2 Madhu Bhaskaran, 1 Sharath Sriram1 1 Functional Materials and Microsystems Research Group, School of Electrical and Computer Engineering,
RMIT University, Melbourne, Australia
2 School of Electrical and Electronic Engineering, The University of Adelaide, Adelaide, Australia *
ABSTRACT
In this work, a plasmonic perfect absorber for terahertz waves has been designed and experimentally validated. The absorber created from a patterned silicon substrate achieved a bandwidth of 90% in this untapped frequency regime. The broadband performance and structural simplicity are ideal for various emerging applications.
INTRODUCTION
Perfect absorbers can absorb electromagnetic waves with near-unity absorbance, which are promising for applications in terahertz imaging and detection. In this work, we show that distinctive plasmonic modes in silicon-based cavities can greatly enhance the absorption bandwidth for ultra broadband operation.
B. Simulations and testing
The numerical and experimental results are in good agreement, with a slight blue shift in the spectral response observed in the experiment. Small deviation is caused by tolerances in fabrication, measurement and the limit of the system dynamic range at higher frequencies.
SCHEMATICS
The plasmonic absorber is in the form of a two-dimensional array of cross structures etched into a moderately doped silicon substrate. A phosphorous-doped (n-type) silicon wafer having material resistivity between 0.02 and 0.05 Ω cm is utilized. Either of the two dotted areas in the 2D array can be regarded as a unit cell of the plasmonic absorber. The unit cell defined by the etched middle region (shown by the red dotted box) acts as a resonant cavity, which supports different LSPRs in the terahertz frequency range.
Figure 3: Field distributions of the plasmonic absorber at different resonance frequencies: 0.80 THz (top panel) and 1.41 THz (bottom panel). (a, b) surface current density distributions on the cross structure. (c, d) Instantaneous electric fields between the adjacent cross structures, and (e, f) power loss density distributions.
Figure 1: A two-dimensional array from top view and (b) a unit cell of the cross structure carved from a silicon substrate. The parameters of the geometry are: px = py = 200 μm, tb = 65 μm, a = 60 μm, l = 160 μm, ts = 200 μm.
Figure 4: Numerically and experimentally resolved spectra. (a) Reflectance R(ω) and absorbance A(ω) for the 2D cross array absorber. (b) Reflectance R(ω) and absorbance A(ω) for the bare, doped silicon substrate. The shaded area marks the limit of the system dynamic range.
RESULTS
Scanning electron micrographs
FABRICATION
Microfabrication as a combination of photolithography and plasma-assisted deep reactive ion etching was undertaken to realize the structures in Figure 1(a). Samples 35 × 35 mm2 in area were produced, containing 30,625 identical cross structures. Following microfabrication, reflection-mode terahertz time-domain spectroscopy (THz-TDS) was performed on the sample at normal incidence.
CONCLUSION and FUTURE WORK
Experimental absorbance of 98.22% and 99.85% at resonance frequencies of 0.86 THz and 1.49 THz, and over 90% absorbance from 0.67 THz to 1.78 THz was achieved, which corresponds to a relative bandwidth of 90%.
This design yielded broadband near-perfect absorption over a wide range of incidence angles for both TE and TM waves.
The bandwidth can be extended further with alternative designs to accommodate mode resonance modes.
RELATED PUBLICATION
Y. Z. Cheng, W.Withayachumnankul, A. Upadhyay, D. Headland, Y. Nie, R.Z. Gong, M. Bhaskaran, S. Sriram, and D. Abbott, Advanced Optical Materials (2015).
Figure 2: Scanning electron micrographs of the fabricated cross structure with a 35° tilt angle. (a) Magnified view of a single cross. (b) Partial view of the cross arrays. .
Acknowledgements