Alexander N. Cartwright

Fingerprint enhancement using STFT analysis

Contrary to popular belief, despite decades of research in fingerprints, reliable fingerprint recognition is still an open problem. Extracting features out of poor quality prints is the most challenging problem faced in this area. This paper introduces a new approach for fingerprint enhancement based on short time Fourier transform (STFT) Analysis. STFT is a well-known technique in signal processing to analyze non-stationary signals. Here we extend its application to 2D fingerprint images. The algorithm simultaneously estimates all the intrinsic properties of the fingerprints such as the foreground region mask, local ridge orientation and local ridge frequency. Furthermore we propose a probabilistic approach of robustly estimating these parameters. We experimentally compare the proposed approach to other filtering approaches in literature and show that our technique performs favorably.

Efficient Heterojunction Photovoltaic Cell Utilizing Nanocomposites of Lead Sulfide Nanocrystals and a Low-Bandgap Polymer

Hybrid polymer–inorganic nanocrystal (NC) solar cells consisting of a hole-conducting conjugated polymer and inorganic semiconducting quantum dots, like cadmium selenide (CdSe), lead sulfi de (PbS) and lead selenide (PbSe), have attracted considerable research attention, since the advantages of two classes of materials can be effectively combined. Some of the potential advantages offered by this hybrid approach include: a) low fabrication costs due to easy solution-based processing; b) high optical absorbance and fl exible substrate manufacturing of the conjugated polymer matrix; and c) high electron conductance, tunable optical bandgap, and carrier multiplication due to the semiconducting NCs. [ 1–14 ] The performance of these hybrid devices depends on energy band matching, blend fi lm morphology, composition ratio, and NC shape, etc . [ 15–20 ] In addition, it is crucial to control interfacial contact at nanometer length scales in hybrid blend fi lms to ensure effi cient exciton generation and charge transfer. Generally, it is desirable to perform ligand exchange on the as-prepared NCs since they typically are shielded by long alkyl chain ligands such as trioctylphosphine oxide (TOPO) and oleic acid (OA) that act as insulating layers. For example, ligand exchange with shorter ligands like pyridine and butylamine have been shown to result in better charge photogeneration and effi cient charge transport. [ 16 , 19 , 21 , 22 ] Such pre-fabrication ligand exchange processing requires that good dispersibility of the NCs is maintained for fi lm fabrication. This method has been successfully applied to CdSe/poly(3-hexyl thiophene) (P3HT) hybrid devices to show signifi cantly improved performance. [ 15 ] Recently, the use of a low-bandgap polymer with CdSe NCs resulted in power conversion effi ciency of 3.2% with a broad absorption spectrum from UV to ∼ 800 nm. [ 23 ] To achieve better light harvesting including nearand mid-infrared (IR) wavelengths, PbS and PbSe NCs have been investigated in hybrid polymer solar cells. [ 24–28 ] While these materials will require appropriate encapsulation and proper disposal to avoid environmental contamination, they represent the best choice

Enhancement of the photovoltaic performance in PbS nanocrystal:P3HT hybrid composite devices by post-treatment-driven ligand exchange

A methodology for achieving versatile and facile ligand exchange by post-fabrication chemical treatment in PbS nanocrystal:poly(3-hexylthiophene) (P3HT) hybrid composite photovoltaic devices is demonstrated. We report a considerable improvement of the photovoltaic performance after post-fabrication chemical treatment using acetic acid to produce PbS nanocrystals surrounded by short-length ligands. Annealing induced morphological and photovoltaic performance changes in the resulting composite devices were investigated as a function of the annealing time.

K-plet and coupled BFS: a graph based fingerprint representation and matching algorithm

In this paper, we present a new fingerprint matching algorithm based on graph matching principles. We define a new representation called K-plet to encode the local neighborhood of each minutiae. We also present CBFS (Coupled BFS), a new dual graph traversal algorithm for consolidating all the local neighborhood matches and analyze its computational complexity. The proposed algorithm is robust to non-linear distortion. Ambiguities in minutiae pairings are solved by employing a dynamic programming based optimization approach. We present an experimental evaluation of the proposed approach and showed that it exceeds the performance of the NIST BOZORTH3 [3] matching algorithm.

Time-resolved spectroscopy of recombination and relaxation dynamics in InN

Subpicosecond resolution differential transmission measurements of an InN epilayer have been employed to probe the carrier recombination dynamics and hot carrier relaxation processes in these materials at room temperature. We observed a fast initial hot carrier cooling followed by a slower recombination process with characteristic decay times of 300–400 ps. At short times after pulsed excitation, modeling of the observed relaxation suggests that the dominant energy relaxation process is longitudinal optical phonon scattering modified by a strong hot phonon effect. At longer times, a redshift of the peak energy in the differential transmission spectra was observed. This redshift is consistent with a reduction of the bandfilling effect that occurs as the photoexcited carriers recombine.

Moiré interferogram phase extraction: a ridge detection algorithm for continuous wavelet transforms

We present a procedure using continuous wavelet transforms (CWTs) to extract the phase information from moiré interferograms. The relationship between precise ridge detection of the two-dimensional CWT magnitude map and accurate phase extraction is detailed. A cost function is introduced for the adaptive selection of the ridge, and a computationally inexpensive implementation of the cost function ridge detection algorithm is explored with dynamic programming optimization. The results of the proposed ridge detection algorithm on actual interferograms are illustrated. Moreover, the resulting extracted phase is demonstrated to be smooth and accurate. As a result, the sensitivity of the moiré interferometry method is improved to obtain a pixel-by-pixel in-plane strain distribution map.

Thermomechanical behavior of micron scale solder joints under dynamic loads

Recent trends in reliability and fatigue life analysis of electronic devices have involved developing structural integrity models for predicting the operating lifetime under vibratory and thermal environmental exposure. Solder joint reliability is the most critical issue for the structural integrity of surface mounted electronics. Extensive research has been done on thermal behavior of solder joints, however, dynamic loading effects to solder joint fatigue life have not been thoroughly investigated. The physics of solder joint failure under vibration is still not very clear. This paper presents a test program which was performed to study inelastic behavior of solder joints of BGA packages. A concurrent loading unit is used which consists of a thermal environmental chamber and an electrodynamic shaker. Laser Moire Interferometry was used to measure the whole deformation field of the prepared specimen surface. The corresponding inelastic strain field is then calculated. It is found that at elevated temperature, vibration and shock can cause the accumulation of inelastic strains and damage in solder joints. In this paper, contrary to the popular belief that all vibration-induced strains are elastic, it is shown that vibration can cause significant inelastic strains.

Multiple exciton generation and electrical extraction from a PbSe quantum dot photoconductor

Multiple exciton generation and subsequent electrical extraction from a thin film photoconductive device constructed from PbSe nanocrystal quantum dots are demonstrated. The hydrazine treatment of the PbSe film drastically improves the conductivity of the film while maintaining excellent optical and structural film quality. The effects of multiple exciton generation and electrical extraction (electrons collected per photon absorbed) were quantified as a function of incident photon energy from 1.55to3.1eV. The multiple carrier extraction (>100%) was observed at photon energies greater than 2.8 times of the quantum dot bandgap with ∼210% measured at 4.4 times the bandgap.

Tunable two-photon pumped lasing using a holographic polymer-dispersed liquid-crystal grating as a distributed feedback element

A holographic polymer-dispersed liquid-crystal (H-PDLC) grating film was employed as an angle-dependent and narrow spectral-band feedback control element for two-photon pumped lasing in a dye solution, 4-[N-(2-hydroxyethyl)-N-(methyl)amino phenyl]-4′-(6-hydroxyhexyl sulfonyl) stilbene (APSS) in dimethyl sulphoxide. The grating film contained about 80 layers of liquid-crystal domains periodically dispersed in an ∼15 μm thick polymer film, featuring a maximum reflectance of 75% at 561 nm position with an ∼9 nm spectral bandwidth. The output lasing wavelength could be tuned from 561.5 to 548.5 nm and the lasing bandwidth changed from 5 to 3 nm when the incidence angle on the grating film varied from 0° to 22°. The overall lasing efficiency was measured to be 10%.

High contrast switching of distributed-feedback lasing in dye-doped H-PDLC transmission grating structures

Electrically switched distributed-feedback (DFB) lasing action is presented in a Pyrromethene 580 lasing dye-doped holographic polymer dispersed liquid crystal (H-PDLC) transmission grating structure. This design, when compared with the previously utilized H-PDLC reflection grating structure, has the advantage of a greatly enlarged gain length (10 mm) and a low concentration of liquid crystal (20%) while maintaining sufficient refractive index modulation. The experimental results demonstrate that the emitted laser bandwidth (~5 nm) can be obtained with a pump energy threshold of ~0.3 mJ at three different wavelengths, 561 nm, 569 nm and 592 nm, corresponding to three different grating spacings. The near- and far-field measurements have shown a high directionality of the lasing output. The lasing can be electrically switched off by an applied field of 30V/mum. The temporal, spectral, and output/input properties of the laser output are also presented.