Optical integrated sensors, such as Mach-Zehnder Interferometers (MZI), offer remarkable advantages in terms of sensitivity and compactness for label-free bio-sensing applications [1-2], although they need from additional structures and long arms to perform the sensing. In this work, we report a new interferometric sensing method encompassing the benefits of MZIs while reducing the footprint of the integrated device. This new approach is based on the interference between two Bloch modes propagating through a single-channel and one-dimensional photonic crystal, where the slow-wave effect takes place.

The principle of operation is the following: a single-mode waveguide working in TE polarization excites two modes, which interfere with each other by an abrupt discontinuity into the second single-mode waveguide at the output. For a given variation of the cladding refractive index unit (RIU), the propagation constant of the guided modes changes, producing an increase in the phase shift. Due to the slow-wave phenomenon whereby light travels slower than in other structures, good values of sensitivity are obtained for short device lengths in comparison to other interferometers and without the necessity of long modal sections to achieve enough phase shift (dimensions around 35 microns) . In summary, this approach could result in novel interferometric sensing structures for lab-on-a-chip integrated devices and other bio-sensing applications.

[1] Q. Liu, X. Tu, K. W. Kim, J. S. Kee, Y. Shin, K. Han, Y. J. Yoon, G. Q. Lo, and M. K. Park, Sensors Actuators, B Chem. 188, 681 (2013).

[2] Q. Kun, H. Shuren, and S. T., Opt. Lett. 1 (2016).

[3] K. E. Zinoviev, A. B. González-Guerrero, C. Domínguez, and L. M. Lechuga, J. Light. Technol. 29, 1926 (2011).