Integrated photonic studies of silicon meandering distributed feedback structures: A model system for biological ligands

Integrated photonics structures such as ring resonators, distributed Bragg reflectors (DBRs), Mach-Zehnder interferometers (MZIs), as well as their cascaded versions have been extensively studied and successfully tested for the biosensing of samples such as viruses, protein biomarkers, deoxyribonucleic acid (DNA), and microribonucleic acid (microRNA). We propose novel silicon-on-insulator (SOI) meandering distributed feedback (MDFB), antisymmetric MDFB (AMDFB), and symmetric MDFB (SMDFB) structures for biosensing applications. A single transverse electrically (TE) polarized silicon waveguide is used to design these monolithic structures in contrast with ring resonators, where a bus waveguide is necessary to couple with the rings. The base structure is a meandering loop mirror (MLM). Three identical MLMs are serially connected to form an MDFB structure, which shows spectral mode splitting in its spectral response. The AMDFB structure has four interlaced identical MLMs. AMDFB spectra shows spectral mode splitting at a coupling constant of C = 0.09 and electromagnetic induced transparency (EIT)-like peaks from 0.27 < C < 0.51. The SMDFB structure has five interlaced identical MLMs. SMDFB spectra shows spectral mode splitting with Fano resonance, when C = 0.24; and EIT-like peaks, when 0.78 < C < 0.94. The SMDFB structure's experimental spectrum with Fano lineshape shows an extinction ratio of 26 dBm and a slope ratio of 368 dBm/nm. The intrinsic limit of detection of a resonant sensor can be expressed as λ/QS, where λ is the free space wavelength, Q is the Q-factor of the resonator, and S is the sensitivity. The spectrally split peaks of MDFB structures, and the EIT-like peaks in AMDFB and SMDFB structures show Q-factors on the order of 50000; so that at λ = 1550 nm, and S = 50 nm/RIU the theoretical intrinsic limit of our silicon photonic structures is on the order of 0.0006 [RIU], without taking into consideration the biomolecule absorption at 1550 nm. In addition to the existing integrated photonics structures, our novel monolithic SOI based photonic structures show promise for biosensing applications.