467 research outputs found
Multiple-scales analysis of photonic crystal waveguides
The multiple-scales method is used to derive a scalar differential equation that describes the envelopes of photonic crystal waveguide modes. For a photonic crystal heterostructure waveguide and an air core photonic crystal waveguide, the mode frequencies calculated from the envelope approximation and full numerical simulations agree to 9% in the worst case when compared to the frequency difference of the band edges. The single-mode and cutoff width conditions for a photonic crystal waveguide are predicted and verified
Colloidal quantum dot photodetectors
a b s t r a c t We review recent progress in light sensors based on solution-processed materials. Spin-coated semiconductors can readily be integrated with many substrates including as a post-process atop CMOS silicon and flexible electronics. We focus in particular on visible-, near-infrared, and short-wavelength infrared photodetectors based on size-effect-tuned semiconductor nanoparticles made using quantum-confined PbS, PbSe, Bi 2 S 3 , and In 2 S 3 . These devices have in recent years achieved room-temperature D Ã values above 10 13 Jones, while fully-depleted photodiodes based on these same materials have achieved MHz response combined with 10 12 Jones sensitivities. We discuss the nanoparticle synthesis, the materials processing, integrability, temperature stability, physical operation, and applied performance of this class of devices
Multiple-scales analysis of photonic crystal waveguides
The multiple-scales method is used to derive a scalar differential equation that describes the envelopes of photonic crystal waveguide modes. For a photonic crystal heterostructure waveguide and an air core photonic crystal waveguide, the mode frequencies calculated from the envelope approximation and full numerical simulations agree to 9% in the worst case when compared to the frequency difference of the band edges. The single-mode and cutoff width conditions for a photonic crystal waveguide are predicted and verified
Advancing the speed, sensitivity and accuracy of biomolecular detection using multi-length-scale engineering
Rapid progress in identifying disease biomarkers has increased the importance of creating high-performance detection technologies. Over the last decade, the design of many detection platforms has focused on either the nano or micro length scale. Here, we review recent strategies that combine nano- and microscale materials and devices to produce large improvements in detection sensitivity, speed and accuracy, allowing previously undetectable biomarkers to be identified in clinical samples. Microsensors that incorporate nanoscale features can now rapidly detect disease-related nucleic acids expressed in patient samples. New microdevices that separate large clinical samples into nanocompartments allow precise quantitation of analytes, and microfluidic systems that utilize nanoscale binding events can detect rare cancer cells in the bloodstream more accurately than before. These advances will lead to faster and more reliable clinical diagnostic devices
Wavelength dependence and figures of merit of ultrafast third-order optical nonlinearity of a conjugated 3,3-bipyridine derivative
The wavelength dependence and figures of merit of the third-order optical nonlinearity of a conjugated 3,3Ј-bipyridine derivative, a designed nondipolar structure of the donor-acceptor-acceptor-donor type, are reported. Z scans reveal positive nonlinear refractive-index changes for wavelengths longer than the one-photon absorption wavelength. Although the value of nonlinear Kerr coefficient n 2 decreases from 6.0͑Ϯ0.2͒ ϫ 10 Ϫ6 cm 2 ͞GW at 750 nm to 4.6͑Ϯ0.7͒ ϫ 10 Ϫ6 cm 2 ͞GW at 1550 nm, the value of nonlinear absorption  decreases from 0.084 cm͞GW at 750 nm to a negligible value at 1550 nm, giving rise to excellent nonlinearity-to-loss figures of merit at telecommunications wavelengths
Size dependence of carrier dynamics and carrier multiplication in PbS quantum dots
The time dynamics of the photoexcited carriers and carrier-multiplication efficiencies in PbS quantum dots (QDs) are investigated. In particular, we report on the carrier dynamics, including carrier multiplication, as a function of QD size and compare them to the bulk value. We show that the intraband 1P - \u3e 1S decay becomes faster for smaller QDs, in agreement with the absence of a phonon bottleneck. Furthermore, as the size of the QDs decreases, the energy threshold for carrier multiplication shifts from the bulk value to higher energies. However, the energy threshold shift is smaller than the band-gap shift and, therefore, for the smallest QDs, the threshold approaches 2.35 E(g), which is close to the theoretical energy conservation limit of twice the band gap. We also show that the carrier-multiplication energy efficiency increases with decreasing QD size. By comparing to theoretical models, our results suggest that impact ionization is sufficient to explain carrier multiplication in QDs
Inorganic Tin Perovskites with Tunable Conductivity Enabled by Organic Modifiers
Achieving control over the transport properties of charge-carriers is a
crucial aspect of realizing high-performance electronic materials. In
metal-halide perovskites, which offer convenient manufacturing traits and
tunability for certain optoelectronic applications, this is challenging: The
perovskite structure itself, poses fundamental limits to maximum dopant
incorporation. Here, we demonstrate an organic modifier incorporation strategy
capable of modulating the electronic density of states in halide tin
perovskites without altering the perovskite lattice, in a similar fashion to
substitutional doping in traditional semiconductors. By incorporating organic
small molecules and conjugated polymers into cesium tin iodide (CsSnI3)
perovskites, we achieve carrier density tunability over 2.7 decades, transition
from a semiconducting to a metallic nature, and high electrical conductivity
exceeding 200 S/cm. We leverage these tunable and enhanced electronic
properties to achieve a thin-film, lead free, thermoelectric material with a
near room-temperature figure-of-merit (ZT) of 0.21, the highest amongst all
halide perovskite thermoelectrics. Our strategy provides an additional degree
of freedom in the design of halide perovskites for optoelectronic and energy
applications
Advances in solution-processed near-infrared light-emitting diodes
Near-infrared light-emitting diodes based on solution-processed semiconductors, such as organics, halide perovskites and colloidal quantum dots, have emerged as a viable technological platform for biomedical applications, night vision, surveillance and optical communications. The recently gained increased understanding of the relationship between materials structure and photophysical properties has enabled the design of efficient emitters leading to devices with external quantum efficiencies exceeding 20%. Despite considerable strides made, challenges remain in achieving high radiance, reducing efficiency roll-off and extending operating lifetime. This Review summarizes recent advances on emissive materials synthetic methods and device key attributes that collectively contribute to improved performance of the fabricated light-emitting devices
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