Jon Olav Grepstad

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.

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.

Nanostructuring of free-standing, dielectric membranes using electron-beam lithography

Nanostructured dielectric membranes are used in several applications ranging from de Broglie matter-wave optical elements to photonic crystals. Precise pattern transfer and high aspect ratio structures are crucial for many applications. The authors present an improved method for direct patterning on free-standing, dielectric membranes using electron-beam (e-beam) lithography. The method is based on an advanced etchmask that both reduces charging and allows for tuning of the etch mask thickness to support high aspect ratios even for small structures. The authors etched structures as small as 50 nm radius holes in a 150 nm thick membrane and achieved aspect ratios of up to 1.3 for this structure size range. The etch mask thickness can be tuned to achieve the required aspect ratio. The etchmask is composed of a three layer stack consisting of poly(methyl methacrylate), SiO2 and an antireflective coating polymer. Scanning-electron micrographs of membranes produced with the fabrication method are presented.

Optical Imaging System Designed for Biosensing using a Photonic Crystal Membrane to Detect Nanoparticles

We have built an optical microscope for a new biosensor application, incorporating a 2D photonic crystal membrane enabling detection of particles with a radius less than 50 nm. The microscope has been characterized experimentally.

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.

Enhanced scattering from nano-particles trapped in photonic crystal membranes

We present simulations showing how light scattered by isolated particles increases by a factor 10, when they are placed in photonic crystal membranes. Exploiting this effect, single nano-particles have been detected with an optical microscope.

Does evanescent gain exist

We have investigated the situation where light incident from a passive high-refractive-index medium is totally re ected o an in nite half space with gain. The question of whether or not evanescent gain can prevail in this case, has been at issue for 40 years. We argue that the controversy can be resolved for week gain media using the Laplace transform, combined with a detailed analysis of analytic and global properties of the permittivity function of the active medium.

Total internal reflection and evanescent gain

Total internal reflection occurs for large angles of incidence, when light is incident from a high-refractive-index medium onto a low-index medium. We consider the situation where the low-index medium is active. By invoking causality in its most fundamental form, we argue that evanescent gain may or may not appear, depending on the analytic and global properties of the permittivity function. For conventional, weak gain media, we show that there is an absolute instability associated with infinite transverse dimensions. This instability can be ignored or eliminated in certain cases, for which evanescent gain prevails.

Neutral-helium-atom diffraction from a micron-scale periodic structure: Photonic-crystal-membrane characterization

Surface scattering of neutral helium beams created by supersonic expansion is an established technique for measuring structural and dynamical properties of surfaces on the atomic scale. Helium beams have also been used in Fraunhofer and Fresnel diffraction experiments. Due to the short wavelength of the atom beams of typically 0.1nm or less, Fraunhofer diffraction experiments in transmission have so far been limited to grating structures with a period (pitch) of up to 200nm. However, larger periods are of interest for several applications, for example for the characterization of photonic crystal membrane structures, where the period is typically in the micron/high sub-micron range. Here we present helium atom diffraction measurements of a photonic crystal membrane structure with a two dimensional square lattice of 100x100 circular holes. The nominal period and hole radius were 490nm and 100nm respectively. To our knowledge this is the largest period that has ever been measured with helium diffraction. The helium diffraction measurements are interpreted using a model based on the helium beam characteristics. It is demonstrated how to successfully extract values from the experimental data for the average period of the grating, the hole diameter and the width of the virtual source used to model the helium beam.

Single nano-particle sensing exploiting crossed polarizers to improve the signal-to-noise ratio

Crossed polarized excitation and detection has been used to improve signal-to-noise ratio in an optical nano-particle sensor exploiting guided-resonance-modes in photonic crystal membranes. The sensor can detect particles with a diameter less than 40 nm.