Detection of wavelength shift of near-infrared laser using mechanical microresonator-based sensor with Si-covered gold nanorods as optical absorber

Abstract This study presents a micromechanical resonator to detect near-infrared (NIR) laser wavelength shifts for dense wavelength division multiplexing (DWDM) systems. A feedback control of laser wavelength detecting the wavelength shift is required to keep a spacing of adjacent wavelength channel. The conventional detection method with an etalon filter has the detectable limit of wavelength shift. In this regard, we integrate a micromechanical resonator with plasmonic gold nanorod structures to produce the optical absorption change due to the wavelength shift. The wavelength shift can be detected via the resonant frequency shift based on the thermal stress change in the resonator. The gold nanorod array is fabricated on a gold thin film and subsequently covered by a Si thin film, which is used for controlling the refractive index of the surrounding medium. This causes absorption peaks of plasmonic resonance to be generated in the NIR region. We develop a theoretical equation for the relative resonant frequency that is in proportion to the laser intensity and the beam length cubed. The experimental tendency of relative resonant frequency shifts is observed to be in good agreement with the theoretical equation. The maximum relative resonant frequency shift is 69 Hz/nm, whereas the resolution is estimated to be 0.37 pm, taking into account the thermomechanical noise. Therefore, the fabricated resonator is confirmed to be useful for measuring the laser wavelength with high resolution for large-capacity DWDM systems.