Optical Switching Between Resonant Wavelengths of a Gallium Phosphide (GaP) Cavity

Gallium phosphide <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\text{GaP})$</tex> is an indirect-bandgap semiconductor used for solid-state optical applications due to its optical properties with high refractive index, non-linear properties and large electronic band-gap [1], [2]. This paper investigates the properties and advantages of optomechanical cavities (OMCs) fabricated on low-cost complementary metal-oxide-semiconductor CMOS-compatible photonic wafers using GaP [3]. One particularity of this research is that the OMC design provides two resonances in the wavelength range from 1520 to 1640 nm in very compact devices (in the range of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mu \mathrm{m}^{2}$</tex>). This enables advanced applications such as ultra-fast switching between the two resonant wavelengths of the OMC in GaP, in particular at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\lambda_{1}=1567.6$</tex> nm and at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\lambda_{2}=1636.5$</tex> nm. In this work, the experimental study comprises the analysis of the two optical resonances of the cavity, including its quality figure measurement (Q- factor) and the evaluation of the thermo-optical effect for different input power levels. The switch operates so that output 1 corresponds to the wavelength of the first resonance (in this case at 1567.6 nm) and output 2 to that of the second resonance (at 1636.5 nm) as depicted in Fig. 1(a). Figure 1(b) shows the experimental setup developed to test the OMC switching capability, using two lasers and an Erbium-doped fiber amplifier (EDFA) operating in the C-band with saturation power of + 13 dBm as well as the measured optical resonances at low power. Figure 1 (c) shows the variation of the center wavelength of resonance <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\lambda_{2}$</tex> when the power is fed into <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\lambda_{2}$</tex>, which accounts to the thermo-optical effect (in blue) for higher power levels. In contrast, when we monitor the variation of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\lambda_{2}$</tex> when higher input power is injected using the EDFA in <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\lambda_{1}$</tex>, the second resonance results in a non-linear high-speed wavelength shift that could be used as an optical switch. As it can be observed in Fig. 1(c), the center resonance wavelength experiences a shift in the opposite direction of optical spectrum (to the left) compared with the shift due to the thermo-optical effect.