Low Optical Efficiency Research Articles

Two-photon absorption processes in semiconductor quantum dots

The two-photon absorption process is a nonlinear phenomenon that shows very low optical efficiency in bulk semiconductors. For this reason the detection of these processes becomes very difficult from the experimental point of view. Nevertheless, when dealing with semiconductor QD's with few nanometer radii, these transitions are enhanced and the detection is possible. In this contribution we present analytical calculations for the absorption coefficient in CdSe spherical QD's subjected to these second order processes, as a function of the characteristic dot parameters. The intensities of the absorption peaks, as well as the statistical treatment involving QD's ensembles, are also reported.

Optical poling of several halogen derivatives of pyrazoloquinoline

Paper deals with optical absorption and second-order optical poling effect in a new synthesized halogen derivatives of 1H-pyrazolo[3,4-b]quinoline. The experimental study and quantum chemical simulations are presented. In optical poling experiment (fundamental wavelength λ=1.76 μm) we have found that the maximal output of second-order susceptibility (up to 1.53 pm/V) is observed for the chromophore possessing two methyl groups and fluor. The quantum chemical analysis reveals similarity in the absorption spectra of methyl-containing halogen derivatives which are characterized by four or five strong absorption bands in the spectral range 200–500 nm. A substitution of the methyl groups by the phenyl group causes the substantial changes of the absorption spectra mainly in the spectral range 240–370 nm. The comparison of measured and calculated absorption spectra manifests rather good agreement, namely in the part regarding the spectral positions of the first oscillator (absorption threshold). At the same time, the measured spectra reveal the considerable broadening almost of all absorption bands and even complete damping some of them in the case of phenyl derivatives. Semi-empirical PM3 method demonstrate substantially better agreement with the experimental values compared to the AM1 method. The lowest magnitude of the nonlinear optical susceptibility is revealed for Br-containing [PQ]-derivative. It is assumed that Br leads here to a suppressing of the charge transfers mechanism what is the reason for a relatively low nonlinear optical efficiency.

Evaluation of the imaging properties of two generations of a CCD-based system for digital chest radiography.

Two generations of a CCD-based detector system with lens-based optical coupling for digital chest radiography were evaluated in terms of presampling MTF, NPS, NEQ, DQE, linearity in response, and SNR over the detector area. Measurements were performed over a wide exposure range and at several different beam qualities. Neither the presampling MTF nor the DQE showed any general strong beam quality dependence, whereas the NPS and NEQ did when compared at specific entrance air kerma values. The exposure dependency for the DQE was found to be considerable, with the detectors showing low DQE at low exposures, and higher DQE at higher exposures. It was found that the second generation has been substantially improved compared to its predecessor regarding all the relevant parameters. The DQE(0) at an entrance air kerma of 5 microGy increased from 9% to 15%, mainly due to a better system gain (including optical coupling efficiency and matching of the energy of the emitted light photons to the sensitivity of the CCD camera). The first generation of detectors was found to have problems with bad peripheral resolution [MTF(muN/2) <0.1]. This problem was nonexistent for the second generation for which uniform resolution has been obtained [MTF(muN/2)=0.3]. A theoretical calculation of the DQE of two model systems similar to the ones evaluated was also performed, and the results were comparable to the experimentally determined data at high exposures. The model shows that both systems suffer from low optical coupling efficiency due to the large demagnification used. The main conclusion is that although the second generation has been improved, there is still a problem with low system gain leading to relatively modest DQE values, especially at low exposures.

Photoluminescence properties of nanocrystalline Y2O3:Eu3+ in different environments

In this study the optical properties of nanocrystalline europium doped yttria Y2O3:Eu3+ were investigated in dependence on different environments such as a silica based molecular sieve MCM-41, with a pore sizes of 2.7nm, a free nanopowder on air and in aqueous solution. All these results were compared to a commercial lamp phosphor powder with a grain size of about 5μm. Investigated optical properties are the photoluminescence emission spectra, the excitation spectra, the lifetimes and the quantum efficiencies. Heavy differences of the change transfer process will be reported and discussed with respect to the structure. A low optical quantum efficiency of the nanoparticles is explained by changes in the crystal field.

Sol-gel coating of scintillating crystals

We propose a new method for the coating of scintillating crystals that combines low thickness and optical efficiency. In this method, the air-gap between crystal and wrapping is replaced by a sol-gel optical coating of very low refractive index (<1.30). The external wrapping is replaced by a diffusive or reflective layer, which can also be made by the sol-gel technique. This leads to a thin coating of a few microns, with light collection properties comparable to usual wrapping, good mechanical and radiation resistance.

Solar collectors with colored absorbers

The integration of solar collectors in buildings should be compatible with the architectural design, and solar collectors with colored absorbers would be aesthetically preferable. In our laboratory we constructed and tested flat plate solar collectors with colored absorbers for water heating applications. The study includes collectors in their typical form with the protective glazing, and also collectors without glazing. Unglazed solar collectors are not widely used, although they are cost effective solar devices, suitable for low temperature thermal applications. We tested outdoors the constructed models, glazed and unglazed, with black, blue and red brown absorbers. In order to overcome the high thermal losses of the unglazed collectors and the low optical efficiency of the colored absorbers, we used flat booster reflectors. The additional solar radiation input from the reflectors increases the thermal energy output of the collectors, improving their performance. Theoretical steady state efficiency curves are also given for collectors with or without glazing. The presented experimental and theoretical results determine the range of the effective operation of the proposed solar collector types, which can be used in a variety of applications, instead of glazed or unglazed solar collectors with a black absorber.

High-redshift galaxies, their active nuclei and central black holes

We demonstrate that the luminosity function of the recently detected population of actively star-forming galaxies at redshift z = 3 and the B-band luminosity function of quasi-stellar objects (QSOs) at the same redshift can both be matched with the mass function of dark matter haloes predicted by standard variants of hierarchical cosmogonies for lifetimes of optically bright QSOs anywhere in the range 106 to 108 yr. There is a strong correlation between the lifetime and the required degree of non-linearity in the relation between black hole and halo mass. We suggest that the mass of supermassive black holes may be limited by the back-reaction of the emitted energy on the accretion flow in a self-gravitating disc. This would imply a relation of black hole to halo mass of the form Mbh ∝ v5halo ∝ M5/3halo and a typical duration of the optically bright QSO phase of a few times 107 yr. The high integrated mass density of black holes inferred from recent black hole mass estimates in nearby galaxies may indicate that the overall efficiency of supermassive black holes for producing blue light is smaller than previously assumed. We discuss three possible accretion modes with low optical emission efficiency: (i) accretion at far above the Eddington rate, (ii) accretion obscured by dust, and (iii) accretion below the critical rate leading to an advection-dominated accretion flow lasting for a Hubble time. We further argue that accretion with low optical efficiency might be closely related to the origin of the hard X-ray background and that the ionizing background might be progressively dominated by stars rather than QSOs at higher redshift.

High-redshift galaxies, their active nuclei and central black holes

We demonstrate that the luminosity function of the recently detected population of star-forming galaxies and the QSO luminosity function at z=3 can be matched with the mass function of dark matter haloes predicted by hierarchical cosmogonies for lifetimes of optically bright QSOs in the range 10^6 to 10^8 yr. We suggest that the mass of supermassive black holes may be limited by the back-reaction of the emitted energy on the accretion flow in a self-gravitating disc. This would imply a relation of black hole to halo mass of the form M_bh ~(v_halo)^5 and a typical duration of the optically bright QSO phase of a few times 10^7 yr. The high integrated mass density of black holes inferred from recent black hole mass estimates in nearby galaxies may indicate that the overall efficiency of supermassive black holes for producing blue light is smaller than previously assumed. We discuss three possible accretion modes with low optical emission efficiency: (i) accretion at far above the Eddington rate, (ii) accretion obscured by dust, and (iii) accretion below the critical rate leading to an advection dominated accretion flow lasting for a Hubble time. We further argue that accretion with low optical efficiency might be closely related to the origin of the hard X-ray background and that the ionizing background might be progressively dominated by stars rather than QSOs at higher redshift.

Effects of saturation on the nonlinear incoherent-erasure joint-transform correlator

A photorefractive nonlinear joint-transform correlator based on the incoherent-to-coherent conversion is presented and analyzed. The nonlinearity of this incoherent-erasure joint-transform correlator (IEJTC) is tunable from the classical-matched to the phase-extraction limit. Correlation peak intensity, sharpness, and discrimination ability increase with the incoherent beam intensity. At easily achievable incoherent-to-coherent beam intensity ratios the IEJTC has its optimal performance, at which the IEJTC approaches the performance of the inverse filter for clean inputs and surpasses the inverse filter performance for noisy inputs. We examine this nonlinearity by using the transform method of analysis and computer simulations. Our study focuses on the effect of saturation on the correlation ability. Our results provide an explanation of why extending the severity of saturation by increasing the incoherent-to-coherent intensity ratio beyond a turning point results in lower optical efficiency, degraded correlation peak, and increased higher-order harmonics.

Transient excitation and emission in hexagonal ZnS:Mn

Pulsed excitation and emission data are reported for ZnS : Mn using UV wavelenghts below and above the bandgap. The data are interpreted to be consistent with donor-acceptor (DA) recombination proceeding simultaneously with Mn intra-atomic emission. Mn excitation occurs primarily through resonance energy transfer. The temperature dependence of the emission spectra shows dramatic differences depending on the UV photon energy relative to the bandgap. Luminescence decay is not exponential for both DA and Mn emission, and it is observed that DA emission is extinguished well before the Mn emission. The dependence of the emission on pump intensity is found to be nonlinear both below and above the bandgap for high pump fluence, and linear for low pump fluence. With the crystal placed in a resonator, no evidence of stimulated emission was observed. This is interpreted as being primarily due to the low optical conversion efficiency of the pump light to 580 nm emission.

Modification of the electrochromic response of WO3 thin films by oxygen backfilling

The effect of oxygen backfilling on WO3 electrochromic thin films has been studied. Previously it was shown that O2 backfilling reduced the corrosion rate in H2SO4. In this phase of the study it has been shown that O2 backfilling also affects the EC response. For example, the coloration speed and optical efficiency were decreased while bleaching speed and self-erasure rate were increased. The current injection during coloration was found to be controlled by the same mechanism as the films without backfilling (i.e., WO3 resistivity and Helmholtz double layer). However, the magnitude of the current passed was lowered due to increased film resistivity from backfilling. The optical absorption maximum remained unchanged while the thermal hopping energy increased with oxygen backfilling. These observations suggest that a decreased density of optically active absorption centers resulted in the lower optical efficiency.

Image storage in a ferroelectric-photoconductor device structure using single-crystal Pb5Ge3O11

Results obtained on an exploratory optically read ferroelectric-photoconductor storage device that utilizes the optical activity of domains in a lead-germanate crystal are reported. The device has been fabricated and tested, and good resolution and contrast were obtained. The major weakness of the device is its relatively low optical readout efficiency.