Strongly influenced by the advances in the semiconductor industry, the miniaturization and integration of optical circuits into smaller devices has stimulated considerable research efforts in recent decades. Among other structures, integrated interferometers play a prominent role in the development of photonic devices for on-chip applications ranging from optical communication networks to point-of-care analysis instruments. However, it has been a long-standing challenge to design extremely short interferometer schemes, as long interaction lengths are typically required for a complete modulation transition. Several approaches, including novel materials or sophisticated configurations, have been proposed to overcome some of these size limitations but at the expense of increasing fabrication complexity and cost. Here, we demonstrate for the first time slow light bimodal interferometric behaviour in an integrated single-channel one-dimensional photonic crystal. The proposed structure supports two electromagnetic modes of the same polarization that exhibit a large group velocity difference. Specifically, an over 20-fold reduction in the higher-order-mode group velocity is experimentally shown on a straightforward all-dielectric bimodal structure, leading to a remarkable optical path reduction compared to other conventional interferometers. Moreover, we experimentally demonstrate the significant performance improvement provided by the proposed bimodal photonic crystal interferometer in the creation of an ultra-compact optical modulator and a highly sensitive photonic sensor.

Over the recent years, subwavelength grating (SWG) structures have increasingly attracted attention in the area of evanescent-field photonic sensors. In this Letter, for the first time to the best of our knowledge, we demonstrate experimentally the real-time refractive index (RI) sensing using the SWG bimodal interferometric structures. Two different configurations are considered to compare the effect of the nonlinear phase shift, obtained between the two first transverse electromagnetic propagating modes, in the measured bulk sensitivity. Very high experimental values up to 2270 nm/RIU are reached, which perfectly match the numerical simulations and significantly enhance other existing SWG and spectral-based sensors. By measuring the spectral shift, the obtained experimental sensitivity does not depend on the sensor length. As a result, a highly sensitive and compact single-channel interferometer is experimentally validated for refractive index sensing, thus opening new paths in the field of optical integrated sensors.

Porous silicon seems to be an appropriate material platform for the development of high-sensitivity and low-cost optical sensors, as their porous nature increases the interaction with the target substances, and their fabrication process is very simple and inexpensive. In this paper, we present the experimental development of a porous silicon microcavity sensor and its use for real-time in-flow sensing application. A high-sensitivity configuration was designed and then fabricated, by electrochemically etching a silicon wafer. Refractive index sensing experiments were realized by flowing several dilutions with decreasing refractive indices, and measuring the spectral shift in real-time. The porous silicon microcavity sensor showed a very linear response over a wide refractive index range, with a sensitivity around 1000 nm/refractive index unit (RIU), which allowed us to directly detect refractive index variations in the 10⁻⁷ RIU range.

A highly sensitive photonic sensor based on a porous silicon ring resonator was developed and experimentally characterized. The photonic sensing structure was fabricated by exploiting a porous silicon double layer, where the top layer of a low porosity was used to form photonic elements by e-beam lithography and the bottom layer of a high porosity was used to confine light in the vertical direction. The sensing performance of the ring resonator sensor based on porous silicon was compared for the different resonances within the analyzed wavelength range both for transverse-electric and transverse-magnetic polarizations. We determined that a sensitivity up to 439 nm/RIU for low refractive index changes can be achieved depending on the optical field distribution given by each resonance/polarization.

Switches are essential components in several optical applications, in which reduced footprints are highly desirable for mass production of densely integrated circuits at low cost. However, most conventional solutions rely on making long switching structures, thus increasing the final device size. Here, we propose and experimentally demonstrate an ultra-compact 2x2 optical switch based on slow-light-enhanced bimodal interferometry in one-dimensional silicon photonic crystals. By properly designing the band structure, the device exhibits a large group index contrast between the fundamental even mode and a higher order odd mode for TE polarization. Thereby, highly dispersive and broadband bimodal regions for high-performance operation are engineered by exploiting the different symmetry of the modes. Two configurations are considered in the experiments to analyze the dimensions influence on the switching efficiency. As a result, a photonic switch based on a bimodal single-channel interferometer with a footprint of only 63 μm ² , a power consumption of 19.5 mW and a crosstalk of 15 dB is demonstrated for thermo-optic tunability.

A novel configuration of photonic sensors based on a single-channel bimodal interferometer is proposed. The design consists of a subwavelength grating (SWG) periodic structure supporting two dispersive TE-like modes that interfere at the output to create fringes in the transmission spectrum. Dispersion relations of the bimodal periodic structures have been computed in order to study the sensing performance, obtaining a theoretical bulk sensitivity of ~1300nm/RIU and a surface sensitivity of ~6.1nm/nm. Finite-Difference Time Domain (FDTD) analysis has been also carried out in order to confirm the previously obtained sensitivity results, thus showing a perfect agreement between theoretical modelling and simulation.