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.
We present an evaluation of new terahertz sources for biomedical imaging based upon quantum cascade lasers (QCL)
and orientation patterned gallium arsenide (OP-GaAs) optical parametric oscillator (OPO). The recently developed
terahertz quantum cascade laser emits a peak output power of up to 40mW at 3.7 THz (&lgr;=81&mgr;m). Utilizing coherent
terahertz radiation greatly improves the signal to noise ratio of the detection, where it provides a relatively large
dynamic range and high spatial resolution. We demonstrated biomedical imaging of malignant tissue contrast in an
image of a mouse liver with developed tumors with a THz imaging system based on a QCL. In addition, images of
various tissues, such as lung, liver, and brain sections from the laboratory mouse were also obtained. We also explored
distinct images from fat, muscle and tendon and measured the absorption coefficient and compared this with FTIR
measurements. Another recent technological advance in THz source is based on cascaded optical down-conversion in an
OP-GaAs OPO, which provides a tunable THz source over a broad wavelength range with an average power of 1mW at
room temperature (RT). The tunability of the OPO source provides additional imaging modes through the ability to
excite molecular vibrations and obtain biochemical and structural information in addition to normal absorption or
reflectivity contrast.
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