We consider light trapping in photonic crystals. Using temporal coupled-mode theory and assuming that the active material is weakly absorbing, we show that the upper bound of the angle-integrated light trapping absorption enhancement is proportional to the photonic density of states. The tight bound can be reached if all the modes supported by the structure are coupled to external radiation. We discuss the roles of van Hove singularity, effective medium theory, and periodicity. By appropriate design, the angle-integrated absorption enhancement could surpass the conventional limit substantially in two dimension and marginally in three dimension.
Radiation damage is a topic typically sidestepped in formal discussions of characterization techniques utilizing ionizing
radiation. Nevertheless, such damage is critical to consider when planning and performing experiments requiring large
radiation doses or radiation sensitive samples. High resolution, in situ transmission X-ray microscopy of Li-ion batteries
involves both large X-ray doses and radiation sensitive samples. To successfully identify changes over time solely due to
an applied current, the effects of radiation damage must be identified and avoided. Although radiation damage is often
significantly sample and instrument dependent, the general procedure to identify and minimize damage is transferable.
Here we outline our method of determining and managing the radiation damage observed in lithium sulfur batteries
during in situ X-ray imaging on the transmission X-ray microscope at Stanford Synchrotron Radiation Lightsource.
Light-field cameras capture the intensity, position, and angular information of light from a scene, enabling after-the-fact
focusing and 3D rendering from a single exposure. The sensitivity, pixel density, and directional resolution of light-field
sensors could be increased by taking advantage of the unique photonic effects present in nanoscale and microscale
structures. We demonstrate that semiconductor nanoshell whispering gallery resonators are a versatile platform for
dense, ultra-thin photosite arrays. We show experimentally that an array of nanocrystalline silicon shells only 50 nm
thick is as absorptive as a micron-thick planar film. We further show that we can tune the separation and size of the
nanoshells by etching the underlying nanosphere template and that we can readily transfer the array onto a flexible
substrate. Next, we describe the phenomenon of photonic nanojets emanating from small dielectric microlenses and
microlens arrays. We devise a sensor architecture that uses the super-resolution foci formed by these nanojets to separate
light into different nanoshell photosites depending on the angle of incidence. The proximity of the microlenses to the
photosites corresponds to a small effective f-number, which enables main camera optics with very large apertures for
light collection. In optical simulations, we demonstrate directional resolution in the integrated light-field sensor at
acceptance angles of up to 35 degrees from normal incidence.
State of art III-V multi-junction solar cells have demonstrated a record high efficiency of 43.5%. However, these cells
are only applicable to high concentration systems due to their high cost of substrates and epitaxial growth. We
demonstrate thin film flexible nanostructure arrays for III-V solar cell applications. Such nanostructure arrays allow
substrate recycling and much thinner epitaxial layer thus could significantly reduce the cost of traditional III-V solar
cells. We fabricate the GaAs thin film nanostructure arrays by conformally growing GaAs thin film on nanostructured
template followed by epitaxial lift-off. We demonstrate broadband optical absorption enhancement of a film of GaAs
nanostructure arrays over a planar thin film with equal thickness. The absorption enhancement is about 300% at long
wavelengths due to significant light trapping effect and about 30% at short wavelengths due to antireflection effect from
tapered geometry. Optical simulation shows the physical mechanisms of the absorption enhancement. Using thin film
nanostructure arrays, the III-V solar system cost could be greatly reduced, leading to low $/W and high kW/kg flexible
solar systems.
Conference Committee Involvement (6)
Nanoepitaxy: Homo- and Heterogeneous Synthesis, Characterization, and Device Integration of Nanomaterials II
1 August 2010 | San Diego, California, United States
Nanoepitaxy: Homo and Heterogeneous Synthesis, Characterization, and Device Integration of Nanomaterials
3 August 2009 | San Diego, California, United States
Micro- and Nanotechnology Sensors, Systems, and Applications
15 April 2009 | Orlando, Florida, United States
Nanosensing: Materials, Devices, and Systems III
11 September 2007 | Boston, MA, United States
Nanomaterials Synthesis, Interfacing, and Integrating in Devices, Circuits, and Systems II
9 September 2007 | Boston, MA, United States
Nanomaterial Synthesis and Integration for Sensors, Electronics, Photonics, and Electro-Optics
1 October 2006 | Boston, Massachusetts, United States
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