An ultrabroadband add-drop filter/switch circuit is designed and demonstrated by integrating a pair of subwavelength grating waveguides in a 2\times 2 Mach-Zehnder interferometer configuration using silicon photonics technology. The subwavelength grating is designed such that its stopband and passband are distinguished by a band-edge wavelength \lambda -{\text{edge}} \sim1565 nm, separating C and L bands. The stopband (\lambda < lambda {\text{edge}}) is filtered at the drop port of the device, whereas the passband ( \lambda >\lambda -{\text{edge}}) is extracted either in cross port or in bar port. The device is designed to operate only in TE polarization. Experimental results exhibit a nearly flat-Top band exceeding 40 nm for both stopband and passband. The stopband extinction at cross-and bar ports are measured to be > 35 dB with a band-edge roll-off exceeding 70 dB/nm. Wavelength independent directional coupler design and integrated optical microheaters at different locations of the Mach-Zehnder arms for thermo-optic phase detuning are the key for stopband filtering at the drop port and switching of passband between cross-and bar ports with flat top response. Though the insertion loss of fabricated subwavelength grating waveguides are negligibly small, the observed passband insertion loss is \sim 2 dB, which is mainly due to the combined excess loss of two directional couplers. Experimental results also reveal that the passband switching between cross-and bar ports of the device has been possible with an extinction of >15 dB by an electrical power consumption of P-\pi \sim 54 mW. A switching time of 5 \mus is estimated by analyzing the transient response of the device. The passband edge could also be detuned thermo-optically at a rate of 22 pm/mW. © 1995-2012 IEEE.