We demonstrated that the electric and magnetic resonances of metamaterials can be separately switches off and on by positioning the metamaterials along a standing wave, while both resonances are present in travelling-wave spectra

Fang, X.

Tsai, D.P.

Tseng, M.L.

Zheludev, N.I.

English

Southampton (e-Prints Soton)

Coherent Excitation-Selective Spectroscopy in Planar Metamaterials  Xu Fang,1 Ming Lun Tseng,2,3 Din Ping Tsai,2,3,4 and Nikolay I. Zheludev1,5 1Optoelectronics Research Centre and Centre for Photonic Metamaterials, University of Southampton, Southampton SO17 1BJ, UK 2Graduate Institute of Applied Physics, National Taiwan University, Taipei 106, Taiwan 3 Department of Physics, National Taiwan University, Taipei 106, Taiwan 4 Research Center for Applied Sciences, Academia Sinica, Taipei 115, Taiwan 5Centre for Disruptive Photonic Technologies, Nanyang Technological University, Singapore 637371, Singapore x.fang@soton.ac.uk; niz@orc.soton.ac.uk  We demonstrated that the electric and magnetic resonances of metamaterials can be separately switches  off  and  on  by  positioning  the  metamaterials  along  a  standing  wave,  while  both resonances are present in travelling-wave spectra. OCIS codes: (160.3918) Metamaterials; (300.0300) Spectroscopy  We  demonstrate  an  excitation-selective spectroscopy,  in  which  the  optical  transitions  of  different  nature  in nanoscale thick films can be manipulated by using a standing wave formed by two counter-propagating coherent waves. By placing the thin films at the electric or magnetic antinodes of the standing wave, one can selectively emphasize the electric or the magnetic dipole transitions correspondingly. In a proof-of-principle experiment with periodic arrays of slit nano-antennas and “magnetic wire” metamaterials exhibiting respectively electric dipolar and magnetic dipolar resonances, we demonstrated that the electric and magnetic resonances can be separately switches off and on by positioning the metamaterials along a standing wave, while both resonances are present in travelling-wave spectra. Our demonstration could be generalized to  spectroscopic application when a new degree of freedom, the position in the standing wave, is used for the purpose of excitation-selective spectroscopy in sub-wavelength thick films.       Fig. 1. Schematic of coherent excitation-selective spectroscopy.    The  origin  of the  excitation-selective spectroscopy  in  the  field  of  a  standing  wave (Fig.  1)  can  be understood by considering 𝐻?𝑛𝑡 , the Hamiltonian of interaction between electromagnetic radiation and matter, which can written as 𝐻?𝑛𝑡 = 1??? ̂ ?𝑨?𝑡−1?(?𝑞 ̂???𝑡− ??????? ̂ ?)∇?𝑨? where 𝑨 is the vector-potential of the electromagnetic field, and ?? ̂, ?? ̂, and ?? ̂ are operators of electric dipole, electric quadrupole and magnetic dipole of the medium,  respectively [1]. If two linearly polarized coherent counter-propagating  waves 𝑨? = 𝐴0 cos(?𝑡 − 𝑘𝑧) + 𝐴0 cos(?𝑡 + 𝑘𝑧) oscillating  at frequency  ω  form  a standing wave along z-direction, the time dependent Hamiltonian of interaction can be reduced to 𝐻?𝑛𝑡 = −2𝐴0𝑘[?? ̂? sin(?𝑡)cos(𝑘𝑧) − (?? ̂ ? +1???? ̂???𝑡)cos(?𝑡)sin(𝑘𝑧)] Hence, if a subwavelength thick layer of matter is placed at the electric nodes of the wave where cos(kz)=0, the electric  dipole  interaction  vanishes  while  magnetic  dipole  and  electric  quadrupole  interactions  become  the dominant  terms  of  the  Hamiltonian. On  the  contrary,  if  the  layer  is  placed  at  the  magnetic  nodes where sin(kz)=0, the magnetic dipole and the electric quadrupole interactions vanish while the electric dipole interaction becomes the dominant term of the interaction.   Fig. 2. Excitation-selective spectra of two types of metamaterials. (a) A planar array of slit nano-antennas working as electric dipoles. (b) A planar array of nanowires working as magnetic dipoles.  We  have  experimentally  demonstrated  coherent  excitation-selective  spectroscopy  in  two  types  of metamaterials.  In  our  experiments  we  used  a Ti:sapphire  tunable  laser  to  characterize  travelling  wave  and standing wave absorption using an interferometry arrangement [2].  In the first case the metamaterial is a planar array of slit nano-antennas (Fig. 2a), the optical response of which is dominated by an electric dipolar resonance. The metamaterial is fabricated from a Au/Si3N4 film using focused  ion  beam  (FIB)  milling.  The  thickness  of  the  whole  structure  is  80  nm.  An  absorption  peak  at approximately  870  nm  is  observed  in  its  travelling-wave  spectrum,  which  is  associated  with  the  electric resonance arising from the dipolar surface charge oscillation of the nano-antennas. Figure 2a shows absorption spectra of the metamaterial in a standing wave. At the electric antinode (E-antinode), the absorption peak at 870 nm is almost twice as strong as that of the travelling-wave. No substantial absorption is seen at the magnetic antinode (H-antinode). In  the  second  case,  the  metamateraisl  is  a  three-layered  structure,  the  optical  response  of  which  is dominated by the magnetic dipolar resonance (Fig. 2b). The metamaterial is fabricated from a Au/Si3N4/Au film using FIB milling. The thickness of the  whole structure is 110nm. When the metamaterial is placed at the magnetic antinode of the standing wave, it exhibits strong absorption at 890 nm. In comparison, the absorption at the electric antinode is significantly lower. Our demonstration could be generalized to spectroscopic application when a new degree of freedom, the position in the standing wave, is used. This excitation-selective spectroscopy will facilitate detection of weak resonances in sub-wavelength thick films.  [1] Yu. P. Svirko and N. I. Zheludev, Polarization of Light in Nonlinear Optics (Wiley, Chichester, 1998), p. 182. [2] J. Zhang, K. F. MacDonald & N. I. Zheludev. “Controlling light-with-light without nonlinearity,” Light: Sci. Appl 1, e18 (2012). 

Coherent excitation-selective spectroscopy in planar metamaterials

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