Aasmund Sudbø

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.

Comparison of periodic light-trapping structures in thin crystalline silicon solar cells

Material costs may be reduced and electrical properties improved by utilizing thinner solar cells. Light trapping makes it possible to reduce wafer thickness without compromising optical absorption in a silicon solar cell. In this work we present a comprehensive comparison of the light-trapping properties of various bi-periodic structures with a square lattice. The geometries that we have investigated are cylinders, cones, inverted pyramids, dimples (half-spheres), and three more advanced structures, which we have called the roof mosaic, rose, and zigzag structure. Through simulations performed with a 20 μm thick Si cell, we have optimized the geometry of each structure for light trapping, investigated the performance at oblique angles of incidence, and computed efficiencies for the different diffraction orders for the optimized structures. We find that the lattice periods that give optimal light trapping are comparable for all structures, but that the light-trapping ability varies considerably between th...

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.

Numerically stable formulation of the transverse resonance method for vector mode-field calculations in dielectric waveguides

A new formulation of the eigenvalue problem obtained with the old mode-matching or transverse resonance method for mode-filed calculations in dielectric waveguides is presented. The formulation is numerically stable for a large class of waveguides of practical importance, and vector mode fields may be calculated much faster and more accurately than with sophisticated methods like finite element methods.<<ETX>>

8-channel wavelength division multiplexer based on multimode interference couplers

An 8-channel wavelength division multiplexer with 2-nm channel spacing at 1546 nm is proposed. The device is based on the self-imaging effect in multimode waveguides, and design analysis is carried out in a material system with refractive index contrast equal to 1.96%. Simulated loss is less than 0.75 dB for all channels, and the -1-dB and -3-dB full width are 0.75 nm and 1.3 nm, respectively. Crosstalk is below -13 dB in a 0.75-nm-wide window at the neighboring channels. Polarizing change corresponds to a wavelength shift of less than 0.1 nm.<<ETX>>

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.

Optical microphone based on a modulated diffractive lens

The authors present an optical microphone based on a diffractive lens. The distance between the diffractive lens and a reflecting microphone membrane determines the light intensity at the focal point of the lens. With this design, an integrated micromachined microphone transducer can be mass produced at low cost. They describe the sensing principle, calculate the intensity at the focal point, and show how it depends on the membrane position. The authors present results from measurements with a prototype setup which proves the measurement principle and has excellent properties when compared to an expensive condenser microphone.

Two-Dimensional Photonic Crystals Fabricated in Monolithic Single-Crystal Silicon

We describe the fabrication of photonic crystals (PCs) in monolithic single-crystal Si on standard wafers without the need for silicon-on-insulator materials, and we demonstrate two types of PC devices based on this technology. The first is a PC mirror that exhibits broadband high reflectivity in the telecommunication wavelength band (> 90% from 1420 to 1520 nm). The second device shows large changes in reflectivity over narrow wavelength regions and has a sharp (3.5 nm wide) reflection minimum around 1550 nm, making the PC well suited for sensor applications. In both types of PC devices, guided resonances in the PC slab are exploited to obtain the desired reflection response. The PCs are fabricated in a process consisting of a series of oxidation and etch steps, with only one initial lithographic mask. The resulting PC slabs are monolithic structures, with the advantages of very thin and flat membranes with low internal stress, no temperature expansion coefficient mismatch, and compatibility with microelectromechanical systems and complementary metal-oxide-semiconductor processing.

Monolithic Photonic Crystals

A method for making monolithic 2-D silicon photonic crystals is introduced. We demonstrate a structure with broad-band reflectivity (> 90% from 1420 to 1520 nm), and a structure with a sharp (3.5 nm) reflection minimum.

Reconfigurable near-infrared optical filter with a micromechanical diffractive Fresnel lens

We have fabricated and characterized a micromechanical Fresnel lens that serves as a reconfigurable filter for near-infrared spectrometry, for the analysis of gas, liquids, or solids. The lens consists of silicon segments that can be individually actuated. Moving the segments vertically controls the spectrum of the light that is focused on a detector. The holographic pattern on top of the segments permits both focusing of the light and spectral control. We have demonstrated experimentally the desired spectral control of the filter.