Physical Review B

We have designed and fabricated an out-of-plane coupler for butt-coupling from fiber to compact planar waveguides. The coupler is based on a short second-order grating or photonic crystal, etched in a waveguide with a low-index oxide cladding. The coupler is optimized using mode expansion-based simulations. Simulations using a 2-D model show that up to 74% coupling efficiency between single-mode fiber and a 240-nm-thick GaAs-AlO/sub x/ waveguide is possible. We have measured 19% coupling efficiency on test structures.

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An out-of-plane grating coupler for efficient butt-coupling between compact planar waveguides and single-mode fibers

Sub-wavelength dielectric gratings have emerged recently as a promising alternative to distributed Bragg reflection dielectric stacks for broadband, high-reflectivity filtering applications. Such a grating structure composed of a single dielectric layer with the appropriate patterning can sometimes perform as well as 30 or 40 dielectric distributed Bragg reflection layers, while providing new functionalities such as polarization control and near-field amplification. In this Letter, we introduce an interesting property of grating mirrors that cannot be realized by their distributed Bragg reflection counterpart: we show that a non-periodic patterning of the grating surface can give full control over the phase front of reflected light while maintaining a high reflectivity. This new feature of dielectric gratings allows the creation of miniature planar focusing elements that could have a substantial impact on a number of applications that depend on low-cost, compact optical components, from laser cavities to CD/DVD read/write heads.

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Flat dielectric grating reflectors with focusing abilities

Black silicon (BSi) represents a very active research area in renewable energy materials. The rise of BSi as a focus of study for its fundamental properties and potentially lucrative practical applications is shown by several recent results ranging from solar cells and light-emitting devices to antibacterial coatings and gas-sensors. In this paper, the common BSi fabrication techniques are first reviewed, including electrochemical HF etching, stain etching, metal-assisted chemical etching, reactive ion etching, laser irradiation and the molten salt Fray-Farthing-Chen-Cambridge (FFC-Cambridge) process. The utilization of BSi as an anti-reflection coating in solar cells is then critically examined and appraised, based upon strategies towards higher efficiency renewable solar energy modules. Methods of incorporating BSi in advanced solar cell architectures and the production of ultra-thin and flexible BSi wafers are also surveyed. Particular attention is given to routes leading to passivated BSi surfaces, which are essential for improving the electrical properties of any devices incorporating BSi, with a special focus on atomic layer deposition of Al2O3. Finally, three potential research directions worth exploring for practical solar cell applications are highlighted, namely, encapsulation effects, the development of micro-nano dual-scale BSi, and the incorporation of BSi into thin solar cells. It is intended that this paper will serve as a useful introduction to this novel material and its properties, and provide a general overview of recent progress in research currently being undertaken for renewable energy applications.

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Black silicon: fabrication methods, properties and solar energy applications

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Superlattices and microstructures

We describe the design, fabrication, and evaluation of highly directional grating outcouplers, incorporated in grating-coupled surface-emitting semiconductor lasers. In particular, we demonstrate the use of grating-duty cycle variations to control the radiation factor into air, with maintained directionality of the outcoupling. This is accomplished by using rectangular gratings in combination with a multilayer-substrate reflector. The performance of the outcouplers is compared with numerical calculations based on a perturbation method. Controlled variations of the radiation factor enable advanced holographic-wavefront conversion, making grating-coupled surface emitters equipped with such outcouplers attractive in a large number of applications, both as single-element devices and in optical-integration schemes.

Highly directional grating outcouplers with tailorable radiation characteristics

We present the theory, design, fabrication and evaluation of off-plane computer-generated waveguide holograms (OP-CGWHs). The hologram structure is composed of an array of rectangular elements each containing a waveguide grating coupler. The function of the rectangular elements is twofold: outcoupling the guided optical wave, and introducing phase shift. The phase shift of each rectangular element can be determined by controlling the dislocation of the grating grooves along the guided wave propagation direction. In addition, the phase pattern of the OP-CGWH can be designed using many existing algorithms for computer-generated holograms (CGH), such as the iterative Fourier transform algorithm (IFTA). A design example is presented for a waveguide array generator, which outcouples a Gaussian-like incident guided wave and simultaneously produces an array of spots in free space. Such an OP-CGWH device, based on an AlGaAs-GaAs waveguide, was fabricated using electron-beam lithography and reactive ion beam etching. Experimental results are presented that demonstrate the effectiveness of the proposed idea. The uniformity error and the power efficiency of the fabricated OP-CGWH array generator were measured to be approximately 8% and 30%, respectively.

Off-plane computer-generated waveguide hologram

The physical properties of black silicon (b-Si) formed on Si wafers by reactive ion etching in chlorine plasma are reported in an attempt to clarify the formation mechanism and the origin of the observed optical and electrical phenomena, which are promising for a variety of applications. The b-Si consisting of high density and high aspect ratio sub-micron length whiskers or pillars with tip diameters of well under 3 nm exhibits strong photoluminescence (PL) both in the visible and the infrared, which is interpreted in conjunction with defects, confinement effects and near band-edge emission. Structural analysis indicates that the whiskers are all crystalline and encapsulated by a thin Si oxide layer. The infrared vibrational spectrum of Si-O-Si bondings in terms of transverse-optic (TO) and longitudinal-optic (LO) phonons indicates that disorder induced LO-TO optical mode coupling can be an effective tool in assessing the structural quality of the b-Si. The same phonons are likely coupled to electrons in visible region PL transitions. Field emission properties of these nanoscopic features are demonstrated indicating the influence of the tip shape on the emission. Overall properties are discussed in terms of the surface morphology of the nanowhiskers.

https://iopscience.iop.org/article/10.1088/0957-4484/22/23/235307/pdf

Black silicon with high density and high aspect ratio nanowhiskers

An on-chip internal stress-based testing device has been developed in order to deform silicon nanoribbons and nanowires up to large strains enabling high throughput of data. The fracture strain and survival probability distribution have been generated for 50-nm-thick and 50- or 500-nm-wide specimens with lengths varying between 2.5 and 10 . Fracture strains reaching up to 5% are attained in the smallest specimens, whereas 90% of the specimens survive 2.5% deformation. This testing platform opens an avenue to investigate and use electromechanical couplings appearing under large mechanical stress or large deformation.

High-Throughput On-Chip Large Deformation of Silicon Nanoribbons and Nanowires

We demonstrate highly efficient grating coupled surface emitting lasers with single-grating outcouplers. An outcoupling efficiency of 94% into air was achieved using a nonresonant rectangular second-order grating in combination with a multilayer reflector below the waveguide. This resulted in an external differential quantum efficiency as high as 50%, comparable to that of conventional edge-emitting lasers, limited by absorptive loss in the passive grating sections. The lasers have separated oscillators and outcouplers, which facilitates the fabrication of surface emitters that possess beam-shaping capabilities through holographic wavefront conversion.

M. Hagberg

Papers

Highly directional grating outcouplers with tailorable radiation characteristics

Off-plane computer-generated waveguide hologram

Black silicon with high density and high aspect ratio nanowhiskers

High-Throughput On-Chip Large Deformation of Silicon Nanoribbons and Nanowires

Highly efficient grating-coupled surface-emitters with single outcoupling elements