We propose novel double-notch-shaped microdisk resonator-based devices with gapless waveguide-to-microdisk and inter-cavity coupling via the two notches of the microdisk. Both finite-difference time-domain simulations and experimental demonstrations reveal the high-quality-factor multimode resonances in such microdisks. Using such double-notch microdisk resonators, we experimentally demonstrate the many-element linearly cascaded-microdisk resonator devices with up to 50 elements on a silicon chip.