Current Issue : April - June Volume : 2013 Issue Number : 2 Articles : 7 Articles
Bulk heterojunction (BHJ) based on a donor (D) polymer and an acceptor (A) fullerene derivative is a promising organic photovoltaics\r\n(OPV). In order to improve the incident photon-to-current efficiency (IPCE) of the BHJ solar cell, a comprehensive understanding\r\nof the ultrafast dynamics of excited species, such as singlet exciton (D*), interfacial charge-transfer (CT) state, and\r\ncarrier (D+), is indispensable. Here, we performed femtosecond time-resolved spectroscopy of two prototypical BHJ blend films:\r\npoly(3-hexylthiophene) (P3HT)/[6,6]-phenyl C61-butyric acid methyl ester (PCBM) blend film and poly(9,9-dioctylfluorene-cobithiophene)\r\n(F8T2)/[6,6]-phenyl C71-butyric acid methyl ester (PC70BM) blend film. We decomposed differential absorption\r\nspectra into fast, slow, and constant components via two-exponential fitting at respective probe photon energies. The decomposition\r\nprocedure clearly distinguished photoinduced absorptions (PIAs) due to D*, CT, and D+. Based on these assignments, we\r\nwill compare the charge dynamics between the F8T2/PC70BM and P3HT/PCBM blend films....
We review the temporal dynamics of the laser output spectrum and polarization state of 1.55 �µm wavelength single-mode (SM)\nvertical-cavity surface-emitting lasers (VCSELs) induced by external optical beam injection. Injection of an external continuouswave\nlaser beam to a gain-switched SM VCSEL near the resonance wavelength corresponding to its main polarization-mode output\nwas critical for improvement of its laser pulse generation characteristics, such as pulse timing-jitter reduction, linewidth narrowing,\npulse amplitude enhancement, and pulse width shortening. Pulse injection of pulse width shorter than the cavity photon lifetime\ninto the SM VCSEL in the orthogonal polarization direction with respect to its main polarization mode caused temporal delay\nof the polarization recovery after polarization switching (PS), and its delay was found to be the minimum at an optimized bias\ncurrent. Polarization-mode bistability was observed even in the laser output of an SM VCSEL of a standard circularly cylindrical\nshape and used for all-optical flip-flop operations with set and reset injection pulses of very low pulse energy of order of the\n3.5~4.5 fJ....
We first calculated the diffraction intensity distributions of the Dammann gratings illuminated by Gaussian light wave. The\r\nempirical equations were deduced by numerical calculations to calculate the parameters, such as the spatial period, the maximum\r\nintensity, and the maximum intensity gradient, of the optical trap array composed by a set of Dammann gratings and a focus lens.\r\nThus, a novel type of optical trap array for trapping cold atoms (or molecules) was proposed and its features were discussed. The\r\nresults showed the optical trap array with very short period could be generated. High optical dipole potential could be presented\r\nso as to have strong attractive force to the atoms to form atomic optical lattices of high lattice density. Compared with the optical\r\nlattices formed by standing wave interferences of CO2 laser, there are many unique advantages of which are formed by Dammann\r\ngratings....
By solving Helmholtz equations, relationships to describe propagating modes in an arbitrary graded-index planar waveguide are\r\nderived. We show that in the quadratic- and secant-index waveguides a minimal mode width is 0.4?/n, where ? is the wavelength\r\nin free space and n is the refractive index on the fiber axis. By modeling in FullWAVE, we show that the high-resolution imaging\r\ncan be achieved with half-pitch graded-index Mikaelian microlenses (ML) and Maxwellââ?¬â?¢s ââ?¬Å?fisheyeââ?¬Â lenses. It is shown that using\r\na 2D ML, the point source can be imaged near the lens surface as a light spot with the full width at half maximum (FWHM) of\r\n0.12?. This value is close to the diffraction limit for silicon (n = 3.47) in 2D media FWHM = 0.44?/n = 0.127?.We also show that\r\nhalf-pitch ML is able to resolve at half-maximum two close point sources separated by a 0.3? distance....
The present work is dedicated to the development of formation and diagnostics methods of water colloids of noble metals (Au,\r\nAu, Pt). As anoble nanoparticles formation method, the laser synthesis at air conditions is proposed. By the implantation of noble\r\nnanoparticles into water media, the colloidal systems of noble metals can be obtained. For the aims of investigation of noble colloid\r\nparameters, the complex diagnostics method is used. Such approach deals with direct methods (scanning electron microscopy and\r\nthe characteristic radiation registration) and indirect methods (absorption spectroscopy and extinction modeling by Mie theory)....
Gold nanoparticles have been widely used during the past few years in various technical and biomedical applications. In particular,\r\nthe resonance optical properties of nanometer-sized particles have been employed to design biochips and biosensors used as\r\nanalytical tools. The optical properties of nonfunctionalized gold nanoparticles and core-gold nanoshells play a crucial role for\r\nthe design of biosensors where gold surface is used as a sensing component. Gold nanoparticles exhibit excellent optical tunability\r\nat visible and near-infrared frequencies leading to sharp peaks in their spectral extinction. In this paper, we study how the optical\r\nproperties of gold nanoparticles and core-gold nanoshells are changed as a function of different sizes, shapes, composition, and\r\nbiomolecular coating with characteristic shifts towards the near-infrared region.We show that the optical tenability can be carefully\r\ntailored for particle sizes falling in the range 100ââ?¬â??150 nm. The results should improve the design of sensors working at the detection\r\nlimit....
We study binary axicons of period 4, 6, and 8 �µm fabricated by photolithography with a 1 �µm resolution, 500nm depth, and 4mm\r\ndiameter. Near-field diffraction focal spots varying in diameter from 3.5? to 4.5? (for the axicon of period T = 4 �µm) and from 5?\r\nto 8? (for the axicon with T = 8 �µm) are experimentally found on the optical axis at a distance of up to 40 �µm from the axicon for\r\nthe wavelength ? = 0.532 �µm. The first focal spot is found at distance 2 �µm (T = 4 �µm), with the period of the focal spots being\r\n2 �µm (T = 4 �µm) and 4 �µm (T = 8 �µm). Diffraction of linearly polarized plane and diverging waves is simulated using FullWAVE\r\n(RSoft) and a proprietary program BOR-FDTD, which implement finite-difference schemes to solve three-dimensional Maxwellâ��s\r\nequations in the Cartesian and cylindrical coordinates. The numerically simulated values for diameters of the near-field focal spots\r\nfor the axicon of period T = 4 �µm are in good agreement with the experimental values....
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