Onur Kilic

Angular and polarization properties of a photonic crystal slab mirror

It was recently demonstrated that a photonic crystal slab can function as a mirror for externally incident light along a normal direction with near-complete reflectivity over a broad wavelength range. We analyze the angular and polarization properties of such photonic crystal slab mirror, and show such reflectivity occurs over a sizable angular range for both polarizations. We also show that such mirror can be designed to reflect one polarization completely, while allowing 100% transmission for the other polarization, thus behaving as a polarization splitter with a complete contrast. The theoretical analysis is validated by comparing with experimental measurements.

Photonic crystal slabs demonstrating strong broadband suppression of transmission in the presence of disorders.

We characterize the transmission spectra of out-of-plane, normal-incidence light of two-dimensional silicon photonic crystal slabs and observe excellent agreement between the measured data and finite-difference time-domain simulations over the 1050-1600-nm wavelength range. Crystals that are 340 nm thick and have holes of 330-nm radius on a square lattice of 998-nm pitch show 20-dB extinction in transmission from 1220 to 1255 nm. Increasing the hole radius to 450 nm broadens the extinction band further, and we obtain >85% extinction from 1310 to 1550 nm. Discrepancies between simulation and measurement are ascribed to disorder in the photonic lattice, which is measured through image processing on high-resolution scanning electron micrographs. Analysis of crystal imperfections indicates that they tend to average out narrowband spectral features, while having relatively small effects on broadband features.

External fibre Fabry?Perot acoustic sensor based on a photonic-crystal mirror

We demonstrate an acoustic sensor based on a Fabry–Perot interferometer formed by a single-mode fibre and an external silicon photonic-crystal mirror. Measurements in air indicate that this sensor has a relatively uniform frequency response up to at least 50 kHz, and detects pressures as low as 18 μPa Hz−1/2. This limit is four orders of magnitude lower than in similar types of acoustic fibre sensors. The sensor response agrees with mechanical calculations to within a few dB. We predict that this sensor has the potential to detect pressures as low as 600 nPa Hz−1/2, corresponding to the ambient thermal noise level in air at room temperature.

Controlling uncoupled resonances in photonic crystals through breaking the mirror symmetry

We show that modes in a photonic crystal slab that are uncoupled to outside radiation in a symmetric structure can be excited by breaking the mirror symmetry through introducing a protrusion on the side of the photonic crystal holes. We show that coupling to these resonances can be controlled by the strength of this asymmetry, and that it is also possible to choose among modes to couple to, through the shape of the asymmetry introduced. We provide simple theoretical arguments that explain the effect, and present eigenmode simulations and time-domain simulations. We confirm this predicted behavior with measurements on a photonic crystal with a broken mirror symmetry that exhibits an additional sharp resonant feature with a linewidth of 0.5 nm, in agreement with both calculated and simulated predictions.

Asymmetrical Spectral Response in Fiber Fabry–PÉrot Interferometers

We demonstrate that the reflection spectrum of a fiber Fabry–PÉrot formed by a single-mode fiber and an external mirror does not exhibit the standard Lorentzian profile of a conventional bulk-optic Fabry-PÉrot, but is instead highly asymmetric. Measurements indicate that the sign of this asymmetry is different for dielectric and metal mirrors. We show that this asymmetry is due to one of two mechanisms, namely the beam diffraction in the cavity and the complex phase upon reflection from a metal layer. We present an analytical approach to accurately model these spectra, and provide simple analytical formulas, useful in the design and optimization of fiber Fabry-PÉrot-based sensors. Specifically, we present expressions for the maximum finesse and the condition to obtain full contrast in a fiber Fabry-PÉrot. These results are widely applicable, in particular to both low- and high-finesse interferometers.

Analysis of guided-resonance-based polarization beam splitting in photonic crystal slabs

We present an analysis of the phase and amplitude responses of guided resonances in a photonic crystal slab. Through this analysis, we obtain the general rules and conditions under which a photonic crystal slab can be employed as a general elliptical polarization beam splitter, separating an incoming beam equally into its two orthogonal constituents, so that half the power is reflected in one polarization state, and half the power is transmitted in the other state. We show that at normal incidence a photonic crystal slab acts as a dual quarter-wave retarder in which the fast and slow axes are switched for reflection and transmission. We also analyze the case where such a structure operates at oblique incidences. As a result we show that the effective dielectric constant of the photonic crystal slab imposes the Brewster angle as a boundary, separating two ranges of angles with different mechanisms of polarization beam splitting. We show that the diattenuation can be tuned from zero to one to make the structure a circular or linear polarization beam splitter. We verify our analytical analysis through finite-difference time-domain simulations and experimental measurements at infrared wavelengths.

Miniature photonic-crystal hydrophone optimized for ocean acoustics

This work reports on an optical hydrophone that is insensitive to hydrostatic pressure, yet capable of measuring acoustic pressures as low as the background noise in the ocean in a frequency range of 1 Hz to 100 kHz. The miniature hydrophone consists of a Fabry-Perot interferometer made of a photonic-crystal reflector interrogated with a single-mode fiber and is compatible with existing fiber-optic technologies. Three sensors with different acoustic power ranges placed within a sub-wavelength sized hydrophone head allow a high dynamic range in the excess of 160 dB with a low harmonic distortion of better than -30 dB. A method for suppressing cross-coupling between sensors in the same hydrophone head is also proposed. A prototype was fabricated, assembled, and tested. The sensitivity was measured from 100 Hz to 100 kHz, demonstrating a sound-pressure-equivalent noise spectral density down to 12 μPa/Hz(1/2), a flatband wider than 10 kHz, and very low distortion.

Photonic-crystal-diaphragm-based fiber-tip hydrophone optimized for ocean acoustics

We report a miniature fiber hydrophone that consists of a Fabry-Perot interferometer made of a photonic-crystal reflector embedded on a compliant silicon diaphragm placed at the tip of a single-mode fiber. A model was developed to show that after proper optimization to ocean acoustics, this sensor has a minimum detectable pressure that follows the minimum ambient noise of the ocean (reaching a minimum of ~10 μPa/Hz1/2 at ~30 kHz) in the bandwidth of 1 Hz-100 kHz. By placing several such sensors with different acoustic power ranges within a single hydrophone head, the hydrophone is able of exhibiting a dynamic range in the excess of 200 dB. A prototype was fabricated, assembled, and tested that confirmed this high sensitivity and bandwidth.

Fiber-optical acoustic sensor based on a photonic-crystal diaphragm

This paper reports on a highly sensitive fiber-optical acoustic sensor. The miniature acoustic sensor consists of a Fabry-Perot interferometer made of a photonic-crystal reflector embedded on a compliant silicon diaphragm placed at the tip of a single-mode fiber. A high-reflectivity multilayer dielectric mirror is deposited on the tip of the single-mode fiber. When combined with the high reflection from the photonic-crystal mirror, this second mirror forms a Fabry-Perot with a relatively high finesse (~15), giving rise to a high pressure sensitivity over the 10–50 kHz range. A minimum detectable pressure of 30 µPa/Hz 1/2 at 15 kHz is reported.

High-sensitivity thermally stable acoustic fiber sensor

A new design of miniature fiber acoustic sensors with high sensitivity and greatly improved thermal stability and reproducibility of the operating wavelength is presented. It consists of a high-finesse Fabry-Perot (FP) made of a photonic-crystal (PC) reflector fabricated on a compliant Si membrane and placed in close proximity to the reflective end of a fiber. An incident acoustic wave deflects the membrane, which shifts the FP reflection spectrum. This shift is detected as a change in the power reflected by the FP at a fixed wavelength. The reported improvements over an earlier design include (1) the use of photolithography to fabricate the PC diaphragm (higher accuracy and yield), (2) making the sensor chip out of silica instead of Si (higher thermal stability and reproducibility of the FP spectrum), and (3) using silicate bonding to assemble the sensor (higher thermal stability). An experimental fiber acoustic sensor with a finesse of ∼6 is shown to measure pressures in air with a resolution from 180 to 27 µPa/√Hz from ∼700 Hz to ∼8.6 kHz, and as low as 5.6 µPa/√Hz at 12.5 kHz. This last value is ∼4 times better than previously reported. The measured thermal stability is ∼70 times better than that of the earlier Si-based design.