Abstract-Large Cavity-Optomechanical Coupling with Graphene at Infrared and Terahertz Frequencies

Ian A.D. Williamson, S. Hossein Mousavi, and Zheng Wang

http://pubs.acs.org/doi/abs/10.1021/acsphotonics.6b00553?journalCode=apchd5

Graphene exhibits many unusual elastic properties, making it an intriguing material for mechanical measurement and actuation at the quantum limit. We theoretically examine the viability of graphene for cavity optomechanics from near-infrared to terahertz wavelengths, fully taking into account its large optical absorption and dispersion. A large optomechanical coupling coefficient, on the same order of that observed in state-of-the-art optomechanical materials, can be realized in the mid-infrared spectrum with highly doped graphene, a high optical quality factor, and optimal positioning of graphene. Around 100 THz, the dispersive coupling coefficient reaches 180 MHz/nm and 500 MHz/nm in the resolved and unresolved sideband regimes, respectively. We find that predominantly dispersive coupling requires a high graphene Fermi level and mid-infrared excitation, while predominantly dissipative coupling favors a moderate graphene Fermi level and near-infrared excitation.

Abstract-Extraordinary wavelength reduction in terahertz graphenecladded photonic crystal slabs

Ian A. D. Williamson1 , S. Hossein Mousavi1 , Zheng Wang1,*

1Microelectronics Research Center, Department of Electrical and Computer Engineering, The University of Texas at Austin, Austin, TX 78758 US
*Corresponding author: zheng.wang@austin.utexas.edu
http://arxiv.org/ftp/arxiv/papers/1509/1509.00443.pdf


Photonic crystal slabs have been widely used in nanophotonics for light confinement, dispersion engineering, nonlinearity enhancement, and other unusual effects arising from their structural periodicity. Sub-micron device sizes and mode volumes are routine for silicon-based photonic crystal slabs, however spectrally they are limited to operate in the near infrared. Here, we show that two single-layer graphene sheets allow silicon photonic crystal slabs with submicron periodicity to operate in the terahertz regime, with an extreme 100x wavelength reduction and excellent out-of-plane confinement. The graphene-cladded photonic crystal slabs exhibit band structures closely resembling those of ideal two-dimensional photonic crystals, with broad twodimensional photonic band gaps even when the slab thickness approaches zero. The overall photonic band structure not only scales with the graphene Fermi level, but more importantly scales to lower frequencies with reduced slab thickness. Just like ideal 2D photonic crystals, graphenecladded photonic crystal slabs confine light along line defects, forming waveguides with the propagation lengths on the order of tens of lattice constants. The proposed structure opens up the possibility to dramatically reduce the size of terahertz photonic systems by orders of magnitude. 

Abstract-Terahertz direct-detection behavior of niobium nitride superconducting tunnel junctions above liquid helium temperature

Zheng Wang1,2, Dong Liu1,2, Shao-liang Li1,2, Jing Li1,2 and Sheng-Cai Shi1,2

zhwang@pmo.ac.cn http://iopscience.iop.org/0953-2048/27/7/075003scshi@pmo.ac.cn

1 Millimeter & Submillimeter Wave Lab, Purple Mountain Observatory, CAS, Nanjing, Jiangsu, 210008, People's Republic of China
2 Key Laboratory of Radio Astronomy, CAS, Nanjing, Jiangsu, 210008, People's Republic of China 

Niobium nitride (NbN) superconducting tunnel junctions (STJs) have an energy gap nearly double that of niobium STJs, making them potential sensitive detectors that operate at higher frequencies and temperatures. In this paper, we investigate the direct-detection behavior (i.e. the spectral response, current responsivity, noise characteristics, and noise equivalent power (NEP)) of a 500 GHz NbN STJ detector at temperatures from approximately 5 to 9 K. The detector shows an uncorrected NEP of 3.8 × 10−13W/around 5 K and 1.5 × 10−12 W/at 9 K. Its performance can be further improved by adopting a cryogenically cooled readout circuit and fabricating the device with a wiring layer of higher critical-transition temperatures.