Strain-tunable photonic bandgap microcavity waveguides in silicon at 1.55 μm

Abstract

The majority of photonic crystals developed till-date are not dynamically tunable, especially in silicon-based structures. Dynamic tunability is required not only for reconfiguration of the optical characteristics based on user-demand, but also for compensation against external disturbances and relaxation of tight device fabrication tolerances. Recent developments in photonic crystals have suggested interesting possibilities for static small-strain modulations to affect the optical characteristics 1-3 , including a proposal for dynamic strain-tunability 4 . Here we report the theoretical analysis, device fabrication, and experimental measurements of tunable silicon photonic band gap microcavities in optical waveguides, through direct application of dynamic strain to the periodic structures 5 . The device concept consists of embedding the microcavity waveguide 6 on a deformable SiO 2 membrane. The membrane is strained through integrated thin-film piezoelectric microactuators. We show a 1.54 nm shift in cavity resonances at 1.56 µm wavelengths for an applied piezoelectric strain of 0.04%. This is in excellent agreement with our modeling, predicted through first-order semi-analytical perturbation theory 7 and finite-difference time-domain calculations. The measured microcavity transmission shows resonances between 1.55 to 1.57 µm, with Q factors ranging from 159 to 280. For operation at infrared wavelengths, we integrate X-ray and electron-beam lithography (for critical 100 nm feature sizes) with thin-film piezoelectric surface micromachining. This level of integration permits realizable silicon-based photonic chip devices, such as high-density optical filters and spontaneous-emission enhancement devices with tunable configurations.Tuning the Optical Response of Photonic Bandgap Structures, edited by Philippe M. Fauchet, Paul V.

Sensing micrometer-scale deformations via stretching of a photonic crystal

“…There have been only a small number of reports on strain measurement via stretching of a PC. Wong et al [13] analytically examined wavelength shifts at strains of up to 0.2%, while Stomeo et al [14] predicted 5.82 nm of wavelength shift for every GPa of applied unidirectional pressure. Ying et al [15] demonstrated a wavelength shift of 212 nm under 20 kPa of applied stress in thin photonic band gap composite films, corresponding to more than 20% and 30% of tensile and compressive strains, respectively.…”

“…Deformation can be induced in a variety of ways, including mechanical [2,3], thermal, and electrostatic actuation [10] as well as through the use of piezoelectric material [12,13]. There have been only a small number of reports on strain measurement via stretching of a PC.…”

On the role of symmetries in the theory of photonic crystals

“…This type of perturbation has been studied theoretically [RH08; EG13; DL14a] and models effects such as uneven thermal [DWN+11; DLT+04] or uneven strain tuning [WYR+04]. Quite naturally, the first task in the study of the perturbed Maxwell operator M λ = M w(λ) is to derive effective dynamics, i. e. to relate the perturbed to the unperturbed dynamics if one knows something about the initial states.…”

Effective Light Dynamics in Perturbed Photonic Crystals

“…The small dimensionless parameter λ 1 relates the scale of the lattice to the scale of the macroscopic modulation. The scalar modulation functions τ and τ µ describe an isotropic perturbation of the matrix-valued material weights and µ which may be induced by uneven thermal [DWN+11; DLT+04] or strain tuning [WYR+04].…”