Frequency Agile Plasmonic Antennas and Sensors

Developing an integrated method for assembly and characterization of individual nanostructures will facilitate the exploration of new design principles for plasmonic photonic devices.

Calculation demonstrating how the near-field coupling modifies the far-field scattering spectra of a NP-QD hybrid structure placed on a glass substrate. (a) The scattering spectra of a NP-QD hybrid excited by the unpolarized evanescent wave coming from the glass substrate side in all azimuthal angles. The angle 4>_A indicates the orientation of the analyzer in the path of the scattered light. Fano feature is the most (least) prominent when the orientation of the analyzer is parallel (perpendicular) with the in-plane component of the Fano axis, which connects the QD and MNP centers. (b) and (c) shows the field near the NP-QD hybrid at 500 nm and 552 nm, respectively, as indicated by the black arrows in the scattering spectrum at 4>_A = 0 in (a). The scattering signal at these two wavelengths is the same (indicated by the dotted line on the blue curve) while the MNP is excited much more strongly in Fig 1c. This wavelength dependence proves that the presence of the QD indeed controls the MNP’s scattering and anisotropy resonantly.

April 1, 2021

Army Research Office, Research Triangle Park, North Carolina

As demand for faster and smaller electronic and photonic devices increases, plasmonic technology, which shows promise for controlling light at length scales well below the optical diffraction limit, has emerged. The small length scale of plamsmonic devices, however, brings serious challenges in assembling, designing, and characterizing. The objective of this research is to develop an integrated method for assembly and characterization of individual nanostructures, to explore new design principles for plasmonic photonic devices, and to demonstrate prototypical devices to verify the effectiveness of both theoretical simulation and experimental approaches.

The nanomanipulation method was used, which is based on atomic force microscopy (AFM), to assemble plasmonic nanophotonic device in a reconfigurable manner and to characterize these devices using optical dark-field scattering spectra. In addition to the development of the methodology, the expected outcome of the project included the demonstration of two novel photonic devices. The first device is a frequency agile nanoantenna. Different from plasmonic waveguides, which have short propagation lengths limited by material loss, antennas may serve as an alternative approach to transfer energy/optical signals in plasmonic circuits via free-space radiation.

The second device is a novel sensor based on optical dark modes in nanorods. Dark modes can effectively enhance fields and store energy; therefore, they may find applications in sensing as well as lasing and switching. The design of these novel plasmonic devices take advantage of optical circuit element concepts and explore unique near-field couplings between metallic nanostructures.

As a result of this research, it was experimentally demonstrated for the first time that a single semiconductor quantum dot placed in nanometer-scale proximity of a plasmonic cavity can be used to control the scattering spectrum and anisotropy of the latter. Many quantum network and information processing schemes require the enhanced light- matter interaction between a single quantum emitter and a cavity, enabling the effective conversion between photonic and matter-based quantum states. Those cavity-quantum electrodynamics (QED) effects require a high Purcell factor F_P∝Q/V, where Q is the quality factor, and V is the volume of the cavity mode.

Prior experiments exploring cavity QED effects associated with single emitters coupled to plasmonic cavities or waveguides focused almost exclusively on the observations of reducing the emitter’s lifetimes. The possibility of controlling the scattering of a plasmonic nanocavity by a single (and inherently quantum and nonlinear) two-level system has also been proposed but never experimentally observed.

The strongly coupled MNP-QD hybrid structure was assembled into a well-controlled geometry using the technique of AFM nanomanipulation. The strong coupling between the MNP and QD is experimentally confirmed by measuring the exciton lifetime. Analyzing the polarization and spectral properties of light scattered by the MNP-QD hybrid, it was observed that the overall plasmonic cavity scattering is significantly modified over a broad spectral range. A Fano resonance spectrally aligned with the QD’s quantized exciton resonance is clearly identified when the polarization of the scattered photon is along the Fano axis connecting the MNP’s center with the QD. The anisotropic scattering spectrum observed in the experiments suggests that a polarization-controlled, versatile quantum light source may be realized in this simple QD-MNP cavity system.

The calculated polarization-resolved scattering spectra by the QD-MNP (diameters: 2r_QD = 6nm and 2_rMNP = 30nm) hybrid are shown in Fig.1(a) for three polarization angles cpA of the analyzer placed in the collection path of the scattering signal to mimic the experimental setup. In the absence of the QD, all scattering spectra from a single MNP are independent of cp_A and possess a single broad peak at }._MNP ≈ 520nm corresponding to the plasmonic dipole resonance of the MNP. The introduction of a QD under the MNP, with the separation gap of g = 1nm, modifies the scattering spectrum: a sharp Fano feature emerges at the exciton transition wavelength }._QD = 550nm.

This work was done by Xiaoqin (Elaine) Li, Gennady Shvets, and Andrea Alu of the University of Texas-Austin for Army Research Office. For more information, download the Technical Support Package (free white paper) below. ARL-0236