Finite-size limitations on Quality factor of guided resonance modes in 2D photonic crystals.

High-Q guided resonance modes in two-dimensional photonic crystals, enable high field intensity in small volumes that can be exploited to realize high performance sensors. We show through simulations and experiments how the Q-factor of guided resonance modes varies with the size of the photonic crystal, and that this variation is due to loss caused by scattering of in-plane propagating modes at the lattice boundary and coupling of incident light to fully guided modes that exist in the homogeneous slab outside the lattice boundary. A photonic crystal with reflecting boundaries, realized by Bragg mirrors with a band gap for in-plane propagating modes, has been designed to suppress these edge effects. The new design represents a way around the fundamental limitation on Q-factors for guided resonances in finite photonic crystals. Results are presented for both simulated and fabricated structures.

Detection of single nano-defects in photonic crystals between crossed polarizers.

We investigate, by simulations and experiments, the light scattering of small particles trapped in photonic crystal membranes supporting guided resonance modes. Our results show that, due to amplified Rayleigh small particle scattering, such membranes can be utilized to make a sensor that can detect single nano-particles. We have designed a biomolecule sensor that uses cross-polarized excitation and detection for increased sensitivity. Estimated using Rayleigh scattering theory and simulation results, the current fabricated sensor has a detection limit of 26 nm, corresponding to the size of a single virus. The sensor can potentially be made both cheap and compact, to facilitate use at point-of-care.

Reflectivity and polarization dependence of polysilicon single-film broadband photonic crystal micro-mirrors.

We report on the fabrication of 2-D photonic crystal (PC) micro-mirrors, and Finite Difference Time Domain (FDTD) simulations and measurements of their reflectance spectra and polarization dependence at normal incidence. The PC mirrors were fabricated in free-standing thin polysilicon membranes supported by silicon nitride films for stress compensation. Greater than 90% reflectivity is measured over a wavelength range of 35 nm from 1565 nm to 1600 nm with small polarization dependence. Our FDTD simulations show that fabrication errors on the order of tens of nanometers can strongly affect the reflection spectra. When the fabrication errors are kept below this level, FDTD simulations on perfectly periodic structures accurately predict the reflection spectra of the fabricated PC mirrors, despite their sensitivity to the fabrication errors.

Photonic-crystal membranes for optical detection of single nano-particles, designed for biosensor application.

A sensor designed to detect bio-molecules is presented. The sensor exploits a planar 2D photonic crystal (PC) membrane with sub-micron thickness and through holes, to induce high optical fields that allow detection of nano-particles smaller than the diffraction limit of an optical microscope. We report on our design and fabrication of a PC membrane with a nano-particle trapped inside. We have also designed and built an imaging system where an optical microscope and a CCD camera are used to take images of the PC membrane. Results show how the trapped nano-particle appears as a bright spot in the image. In a first experimental realization of the imaging system, single particles with a radius of 75 nm can be detected.

Wood's anomalies and spectral uniformity of focusing diffractive optical elements.

We report on simulations and measurements of focusing diffractive optical elements, fabricated as two-level binary optics. The diffractive optical elements are designed to separate and focus four specific wavelengths in the infrared. The simulations are based on a local linear grating model, and predict anomalies similar to Wood's anomalies known from grating diffraction theory. The anomalies are also seen in the measurements, and are excited at the DOE locations predicted by the simulations. The given examples illustrate the usefulness of the model for evaluation of DOE designs. We also present a comparison of the response and spectral uniformity between two different versions of the four-wavelength diffractive optical elements. In the first version, the optical functions for all the four wavelengths are incorporated into the same surface pattern, covering the whole patterned area. In the second version the pattern f each wavelength is kept separate, and cover one fourth of the area, forming a mosaic of the four individual patterns.

2D back-side diffraction grating for improved light trapping in thin silicon solar cells.

Light-trapping techniques can be used to improve the efficiency of thin silicon solar cells. We report on numerical investigation of a light trapping design consisting of a 2D back-side diffraction grating in combination with an aluminum mirror and a spacing layer of low permittivity to minimize parasitic absorption in the aluminum. The light-trapping design was compared to a planar reference design with antireflection coating and back-side aluminum mirror. Both normally and obliquely incident light was investigated. For normal incidence, the light trapping structure increases the short circuit current density with 17% from 30.4 mA/cm(2) to 35.5 mA/cm(2) for a 20 microm thick silicon solar cell. Our design also increases the current density in thinner cells, and yields higher current density than two recently published designs for cell thickness of 2 and 5 microm, respectively. The increase in current may be attributed to two factors; increased path length due to in-coupling of light, and decreased parasitic absorption in the aluminum due to the spacing layer.

Spectral uniformity of two- and four-level diffractive optical elements for spectroscopy.

We present simulations and characterization of gold coated diffractive optical elements (DOEs) that have been designed and fabricated in silicon for an industrial application of near-infrared spectroscopy. The DOE design is focusing and reflecting, and two-level and four-level binary designs were studied. Our application requires the spectral response of the DOE to be uniform over the DOE surface. Thus the variation in the spectral response over the surface was measured, and studied in simulations. Measurements as well as simulations show that the uniformity of the spectral response is much better for the four-level design than for the two-level design. Finally, simulations and measurements show that the four-level design meets the requirements of spectral uniformity from the industrial application, whereas the simulations show that the physical properties of diffraction gratings in general make the simpler tw level design unsuitable.

Evanescent modes in out-of-plane band structure for two-dimensional photonic crystals.

Reflection, diffraction and transmission of optical waves at the interface between a photonic crystal and the surrounding air can be described by propagating and evanescent Bloch modes. We have found such modes for one of the canonical two-dimensional photonic crystals, identical circular cylinders in a square pattern. We present computed out-of-plane band diagrams for propagating as well as evanescent modes, obtained with a numerical method based on Fourier-Bessel expansions. For a given frequency, all the modes are evanescent, except for a few low-order propagating modes. We find that most of the evanescent modes have a purely imaginary z-component of the Bloch wave vector, but many of the modes have a complex z-component.

Algorithms for the synthesis of complex-value spectral filters with an array of micromechanical mirrors.

We study optical spectral filter synthesis with arrays of piston-actuated micro-mirrors. We propose two algorithms for the calculation of the positions of the micro-mirrors, giving us control of both the amplitude and phase of the synthetic filter. Both algorithms for filter synthesis are explored in an analytic version and in numerical searches for the least deviations between the target and the synthesized filter. We measure the quality of the filter both in terms of the deviations and in filter transmissivity, and present results of numerical simulations for a wide selection of target filters. We find that numerical searches can sometimes yield considerable improvement in the filter synthesis compared to the analytic approximation.

Establishing a high-accuracy spectral response scale in the near infrared with digital filters.

Spectrally invariant detectors are commonly used to interpolate or extrapolate the responsivity of InGaAs detectors in the infrared from absolute calibrations at a few wavelengths. The random noise in such detectors limits the accuracy that can be achieved in a narrowband, double-monochromator setup. We propose the application of a dedicated digital filter, which reduces the uncertainty by 30%, and combine it by calibrating a group of three detectors. The uncertainties are propagated from the observed variance in the relative measurement to the combined uncertainty of 0.4% (2sigma) in the responsivity values of the InGaAs detectors in the range of 1010-1640 nm.

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.