Fundamentals and applications of on-chip interferometers based on deep-etched silicon-air multilayer reflectors

Deep reactive ion etching (DRIE) of silicon can be used to fabricate vertical (i.e. in-plane) silicon-air multilayer mirrors. In comparison with out-of-plane reflectors fabricated by thin film deposition, in-plane multilayer assemblies can be monolithically integrated with a variety of useful structures such as passive optical fiber alignment grooves, microfluidic systems, waveguides, and microelectromechanical (MEMS) actuators. However, all previously reported devices suffered from high insertion losses (> 10 dB) which translated, in most cases, in weak light confinement abilities (e.g. low finesses in the case of Fabry-Perot cavities). The first objective of this work is therefore to investigate the sources of loss and the technological limitations that affect interferometers based on deep-etched multilayer reflectors. Theoretical models for the prediction of losses—due to Gaussian beam divergence, surface roughness at silicon-air material interfaces, imperfect verticality of the etch profiles, and misalignment between input and output coupling optical fibers—are provided. Of these four loss mechanisms, the first three are demonstrated to be generally significant. For the devices presented in the current thesis, however, verticality deviation of the etch profiles (etch angle error ~ 0.04°) is found to be negligible compared with the measured contributions of surface roughness (30 nm RMS) and Gaussian beam divergence. The fourth loss mechanism (fiber misalignment) is found to be essentially negligible in all cases. These theoretical models are demonstrated to correspond remarkably well with our experimental results, such that we are able to state clear boundaries on the possibilities and limitations of interferometers based on deep-etched silicon-air multilayer reflectors. Within these boundaries, three new devices—with potential applications in biomedical sensing, chemical sensing, and optical fiber telecommunications—are investigated.

Firstly, a deep-etched Fabry-Perot interferometer is monolithically integrated with a silicon microfluidic system and is used to measure the refractive index of homogenous liquids. The refractive index sensitivity of this interferometer (907 nm/RIU) is found to be considerably high and, interestingly, to be independent of insertion losses. A refractive index resolution among the highest reported, for volumetric sensing in microfluidic systems, is consequently achieved (1.7×10 ^–5 RIU), even if high insertion losses (~ 25 dB) and low resonance finesse (< 10) affect the interferometer. This sensor performs measurements in volumes (~ pL) similar to those of single living cells, and allows great flexibility in the design of monolithically integrated microfluidic systems. One of its main expected applications is consequently in-flow characterization of cell populations on-chip.

Secondly, Fabry-Perot interferometers, similar to those used as refractive index sensors, are functionalized with polydimethylsiloxane (PDMS) based polymers and are used to detect two different volatile organic compounds (VOCs), i.e. m-xylene and cyclohexane. In this case, mechanical deformation of the interferometers, induced by polymer swelling upon VOCs absorption, is found to be the main sensing mechanism. Refractive index variations inside the polymers also contribute to the sensitivity, but more modestly, yielding a 10 times lower sensitivity than mechanical deformations. A 1.6 ppm resolution is reached when detecting mxylene vapor using phenyl-doped PDMS as the absorbent polymer. This limit of detection is similar to what was achieved with other micromechanical sensors that currently find applications in artificial olfaction systems. Our proposed sensor—being mass producible, simple to fabricate (two conventional photolithography steps), and simple to package (integrated optical alignment features)—could therefore potentially compete with these technologies. One of its main competitive advantages would be passive remote interrogation using only conventional optical fibers, while other technologies usually require on-site electrical power supply for each sensor head.

Finally, Gires-Tournois interferometers (GTIs) are, for the first time, implemented in an in-plane waveguided configuration. These interferometers reach insertion losses below 2 dB at off-resonance wavelengths, which is significantly better, for example, than the typical insertion losses achieved with deep-etched Fabry-Perot filters (> 10 dB). At resonance wavelengths, however, insertion losses remain high (10 - 15 dB) due to a still too strong interaction of light with surface roughness in the cavity back mirror. The proposed Gires-Tournois could therefore find applications in optical fiber telecommunications but only in devices, such as Michelson-GTI interleavers, that exploit primarily the off-resonance part of the GTI spectra. In this particular case, the proposed in-plane configuration could be advantageous compared with out-of-plane designs. For example, in-plane implementation allows interferometer lengths greater than 800 µm, which are required for free spectral ranges that match the 25 or 50 GHz channel spacing of dense wavelength multiplexed (DWDM) networks. Such lengths are not typically achievable using out-of-plane devices that are limited by their silicon substrate thickness (~ 500 - 600 µm). Calculations show that Michelson-GTI interleavers based on the reported GTIs could yield insertion losses below 2 dB and channel isolation in the order of 20 dB.