Recent technological advances of whispering-gallery mode resonators have opened up a wide area of potential applications such as in spectral switches and filters, optical delay lines, lasers and sensors. Moreover, their intriguing properties have made them part and parcel of cutting-edge research in the optical regime. Unfortunately, the whispering-gallery mode resonators still remain highly unexplored in the terahertz frequency range, albeit their interesting characteristics. The little that has been done has pointed out that the coherent detection scheme in the terahertz frequency domain can allow one to study the amplitude and phase of the whispering-gallery modes, thereby providing complementary information about the resonators. To be more specific, the phase information has potential to be utilized to definitely determine the coupling state of the whispering-gallery modes. Therefore, there is need for a comprehensive study on both the amplitude and phase of Fano resonances in a high-quality spherical whispering-gallery mode.
To this note, Dominik Walter and Rainer Leonhardt from the Dodd-Walls Centre for Photonic and Quantum Technologies at The University of Auckland in New Zealand pioneered a comprehensive study on both the amplitude and phase of Fano resonances in a high-quality spherical whispering-gallery mode resonator coupled to a multi-mode waveguide in the terahertz regime. Their work is currently published in the journal, Optics Letters.
The researchers utilized a high-resistivity float zone silicon waveguide which was fabricated by dicing a 4-inch diameter 0.35 mm thick high-resistivity float zone silicon wafer with a resistivity of greater than 20 kΩcm. The spherical resonator had a diameter of 6mm, and was also made from high-resistivity silicon. They used a continuous wave THz system for their experiments. Afterward they proceeded to spectrally analyze the Fano resonances in the system by comparing the transmitted signal from the waveguide with and without the resonator in close proximity to the waveguide.
The authors observed that the measured fundamental modes and the first higher-order radial whispering-gallery modes were uniquely identifiable using the analytical model based on Mie–Debye–Aden–Kerker theory. Furthermore, they observed a frequency dependent asymmetric line shape of the Fano resonances, which could be qualitatively explained by the waveguide dispersion of the multi-mode waveguide that was used to couple into the whispering-gallery modes of the resonator.
Dominik Vogt and Rainer Leonhardt have successfully studied the Fano resonances in a high-quality whispering-gallery mode spherical resonator coupled to a multi-mode waveguide in the terahertz frequency range (highest Q factor ever so far). They showed that the asymmetric line shape and phase of the Fano resonances detected with coherent continuous wave terahertz spectroscopy measurements are in excellent agreement with the analytical model. To this end, the step-function-like behavior in the phase profile of the resonances at critical coupling is therefore well suited for sensing purposes.
Dominik Walter Vogt, Rainer Leonhardt. Fano resonances in a high-Q terahertz whispering-gallery mode resonator coupled to a multi-mode waveguide. Vol. 42, No. 21 / November 1 2017 / Optics Letters
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