Sensitivity enhancement in evanescent optical waveguide sensors

Abstract

Abstract-It is shown, that the sensitivity of the effective refractive index on the cladding index in evanescent optical waveguide sensors, can be larger than unity. This implies that the attenuation of a guided wave propagating in a waveguide immersed in an absorptive medium can be made larger than that of a free-space wave propagating through the same medium. The conditions and physical explanation for this puzzling behavior are identified and as a practical application, an absorption sensor, based on a suspended silicon slab waveguide, is proposed where the sensitivity is enhanced by a factor of 1.35.

First-principle derivation of gain in high-index-contrast waveguides

“…In deriving the confinement factors presented here we have studied gain as a perturbation of the imaginary part of the refractive index over a given region of the guided mode. The same formalism holds true for perturbations to the real part of the refractive index and therefore the confinement factors for gain presented in the paper can also be used as confinement factors for refractive index sensing [10,18] and have shown good agreement with experiment [21].…”

“…This means that it is possible to achieve more gain per unit length in a guided structure than would be possible in bulk. This phenomenon has been noted before for modal absorption [18] and results from the fact that light spends more time in a structure with a large group index, and thus interacts more with the gain material per unit length.…”

“…In general there will be several loss mechanisms which can be written as sum of terms of the form Eq. (18). To achieve lasing, the threshold condition requires the modal gain per unit length be greater than the modal loss per unit length:…”

Evanescent Field Based Photoacoustics: Optical Property Evaluation at Surfaces

“…Advancements in the understanding of optical materials 1,3,4,6,7,10,[13][14][15][16] have provided new insights into the creation of thin film materials for a host of optical devices, including antireflection coatings on lenses, high extinction ratio optical filters, and highly absorbing slab waveguides 17 . These advancements would not be possible without the use of many characterization techniques, such as ellipsometry 4,6,18 , contact angle measurement, atomic force microscopy 7,11,19 , and scanning/transmission electron microscopy, which assist in the iterative improvement of these technologies by providing direct measures or indirect estimates of fundamental optical material properties.…”

“…With such a small optical penetration depth, the evanescent field is able to interact with the environment very close to the interface of the two materials, and well below the optical and acoustic diffraction limits. The optical properties of materials or particles within this range may perturb the field or otherwise alter its generation, which interaction can be detected by a variety of methods 3,5,6,10,15,17,18,21,23,[25][26][27][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][84][85][86][87][88][89][90][91][92][93][94][95] .…”