Efficient Generalized Mie Theory Analysis of Nano Silver Dimers for Optical Field Enhancement in the Plasmonic Photoconductive Thz Antenna

Generalized Mie theory (GMT) is a rigorous analytical method to investigate the optical properties of a cluster of nano-spheres, providing useful optical information [1]. We propose a nano-dimer configuration, for which a GMT... [ view full abstract ]

Generalized Mie theory (GMT) is a rigorous analytical method to investigate the optical properties of a cluster of nano-spheres, providing useful optical information [1]. We propose a nano-dimer configuration, for which a GMT matrix equation with size of S=4n(n+2) should be solved, where n indicates the number of modes. To simplify this method for symmetrical nano-spheres, we can remove half the mode coefficients due to the symmetry of configuration. If we investigate each optical mode participation to generate the final field coefficients precisely, it will reveal that by eliminating weak mode coefficients the matrix equation size will reduce. It should be noted that decreasing the distance between the nano-spheres and increasing the size of them leads to a larger number of significant modes. Also, in the polarization parallel to the symmetry axis the number of the effective modes are larger than for the perpendicular polarization. Figure 1 shows how this approach matches with the GMT.

Photoconductive antennas exhibit significant application such as high-efficiency THz generation, THz detection, THz time-domain spectroscopy, and THz imaging. The main problem in this area is optical to THz wave conversion efficiency [2]. The idea is to design a symmetrical nano-dimer to resonate at the λ=800nm, which is a commercial laser wavelength. By applying a cluster of silver nano-dimers between the electrodes of a PC antenna, plasmons will be stimulated at such configuration and intensify the local electrical field of the laser pump beam. This approach enhances the ratio of the generated THz photocurrent power to the optical power in the PC material, which is the optical-to-electrical efficiency. The accuracy and efficiency of the GMT allows us to design improved configurations, optimizing the size of the nano-spheres, inter-particles distance, material, the angle of the incident plane wave, and the permittivity of surrounding medium. Figure 2 depicts the electrical field distribution for a silver nano-dimer configuration that resonates at λ=800nm. The physical properties presented in table 1.

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2. C.W. Berry, et al. Nat. Commun. 4, 1622 (2013)
3. P. B. Johnson, et al. Physical Review 6 (12), 4370-4379 (1972)