Abstract-Broadband terahertz absorption enabled by coating an ultrathin antireflection film on doped semiconductor

Hongxing Wu, Fenghua Shi, and Yihang Chen
https://www.osapublishing.org/oe/abstract.cfm?uri=oe-24-18-20663

We show that perfect absorption of terahertz wave can be achieved in a compact system where an ultrathin film of lossless dielectric is coated on a doped semiconductor substrate. Due to the nontrivial reflection phase shift at the interface between the two media, strong resonant behavior and the concomitant antireflection occur at wavelengths that are much larger than the thickness of the dielectric film, resulting in strong absorption of the incident wave in a wide frequency range. Using this mechanism, we design a broadband terahertz absorber by coating a Ge film on a highly doped GaAs substrate. We show that such a system not only has a perfect absorption peak, but also exhibits high absorptance (over 0.9) within a fractional bandwidth of over 20%. By varying the free carrier density in the GaAs substrate, the central frequency of the absorption band can be tuned from 1.79 to 2.69 THz. In addition, the absorption performance of the proposed system is shown to be insensitive to both incident angle and polarization. Our results offer a low-cost way for the design of absorption-based THz devices.

© 2016 Optical Society of America

Full Article  |  PDF Article

Abstract-Broadband, Spectrally Flat, Graphene-based Terahertz Modulators

1. 1. 
      
2. 1. 
      
   1. 
      
   1. 
      
   1. 
      
   1. 
      
   1. 
      
   1. 
      
   1. 
      
   1. 
      
   1. 
      
   1. 
      
   
   1. 
      
   Fenghua Shi¹, 
3. Yihang Chen^1,*, 
4. Peng Han¹ and
5. Philippe Tassin^2,*

Article first published online: 8 OCT 2015

DOI: 10.1002/smll.201502036

© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
http://onlinelibrary.wiley.com/doi/10.1002/smll.201502036/abstract

Advances in the efficient manipulation of terahertz waves are crucial for the further development of terahertz technology, promising applications in many diverse areas, such as biotechnology and spectroscopy, to name just a few. Due to its exceptional electronic and optical properties, graphene is a good candidate for terahertz electro-absorption modulators. However, graphene-based modulators demonstrated to date are limited in bandwidth due to Fabry–Perot oscillations in the modulators’ substrate. Here, a novel method is demonstrated to design electrically controlled graphene-based modulators that can achieve broadband and spectrally flat modulation of terahertz beams. In our design, a graphene layer is sandwiched between a dielectric and a slightly doped substrate on a metal reflector. It is shown that the spectral dependence of the electric field intensity at the graphene layer can be dramatically modified by optimizing the structural parameters of the device. In this way, the electric field intensity can be spectrally flat and even compensate for the dispersion of the graphene conductivity, resulting in almost invariant absorption in a wide frequency range. Modulation depths up to 76% can be achieved within a fractional operational bandwidth of over 55%. It is expected that our modulator designs will enable the use of terahertz technology in applications requiring broadband operation.